Radiation astronomy/Galaxies

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This Galaxy Evolution Explorer (GALEX) image of the spiral galaxy Messier 81 is in ultraviolet light. Credit: NASA/JPL-Caltech/J. Huchra (Harvard-Smithsonian CfA).

The radiation astronomy of galaxies generally is about the galaxy as a radiated or radiation emitting astronomical object. The stellar aspects of individual galaxies are in galaxies of stars.

A galaxy is often perceived as a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.[1][2] Galaxies range in size from dwarfs with just a few hundred million (108) stars to giants with one hundred trillion (1014) stars,[3] each orbiting its galaxy's center of mass.

Galaxies are categorized according to their visual morphology as elliptical,[4] spiral, or irregular.[5]

The number of galaxies in the observable universe has increased from a previous estimate of 200 billion (200,000,000,000)[6] to a suggested 2 trillion (2,000,000,000,000) or more,[7][8] containing more stars than all the grains of sand on planet Earth.[9]

Galaxy clusters[edit]

This is a map of the Boötes Supercluster. Credit: Atlas of the Universe.{{free media}}

"There are a couple of [prominent] superclusters in Bootes over 800 million light years away but this region of the sky is more famous for the large Boötes Void that lies next to them."[10]

The Boötes Supercluster is a super cluster of galaxies located in the direction of the Boötes constellation bordering the Super Corona of the Northern Crown with which it is probably connected by a filament of galaxies, and with the Void of Boötes, an area of the universe with a minimum concentration of galaxies (less than one hundred have been identified) with a diameter of about 300 million light years. In Boötes there are two concentrations of galaxy clusters called SCL 349 and SCL 351, placed respectively at 830 million and 1 billion light years from Earth.[11][12]

Some clusters of galaxies from Boötes Supercluster
Name of cluster (Abell) R.A. Dec. Redshift (z) Distance (million light years) Richness class
Abell 1775 13h 41m 9s +26° 22′ ″ 0,0705 950 2
Abell 1781 13h 4m 5s +29° 51′ ″ 0,0606 820 0
Abell 1795 13h 49m 0s +26° 35′ ″ 0,0619 840 2
Abell 1800 13h 49m 7s +28° 04′ ″ 0,0743 1000 0
Abell 1825 13h 58m 0s +20° 39′ ″ 0,0583 790 0
Abell 1827 13h 58m 2s +21° 42′ ″ 0,0642 870 1
Abell 1828 13h 58m 4s +18° 23′ ″ 0,0611 840 1
Abell 1831 13h 59m 2s +27° 59′ ″ 0,0603 815 1
Abell 1861 14h 07m 5s +27° 49′ ″ uncertain uncertain 1
Abell 1873 14h 11m 7s +28° 09′ ″ 0,0764 1025 0
Abell 1898 14h 20m 6s +25° 09′ ″ 0,0762 1025 1

High-velocity galaxies[edit]

The irregular galaxy NGC 1427A is passing through the Fornax cluster at nearly 600 kilometers per second (400 miles per second). Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).

"The irregular galaxy NGC 1427A is a spectacular example of the resulting stellar rumble. Under the gravitational grasp of a large gang of galaxies, called the Fornax cluster, the small bluish galaxy is plunging headlong into the group at 600 kilometers per second or nearly 400 miles per second."[13]

"Galaxy clusters, like the Fornax cluster, contain hundreds or even thousands of individual galaxies. Within the Fornax cluster, there is a considerable amount of gas lying between the galaxies. When the gas within NGC 1427A collides with the Fornax gas, it is compressed to the point that it starts to collapse under its own gravity. This leads to formation of the myriad of new stars seen across NGC 1427A, which give the galaxy an overall arrowhead shape that appears to point in the direction of the galaxy's high-velocity motion."[13]

Active galactic nuclei[edit]

The image shows NGC 1700, the Elliptical Galaxy and Rotating Disk by the Chandra X-ray Observatory. Credit: Thomas S. Statler, Brian R. McNamara.{{free media}}

NGC 1700 is an elliptical galaxy of the Hubble type E4 pec in the constellation Eridanus south of the ecliptic. The galaxy has an angular extent of 3.3 '× 2.1', an apparent brightness of +11.2 mag. It is about 180 million light years away from the solar system and has a diameter of about 215,000 light years.[14] It has an active galactic nucleus.

Radiation astronomy[edit]

Galaxy rotation curve is of a typical spiral galaxy: predicted based on the visible matter (A) and observed (B). The distance is outward from the galactic core. Credit: PhilHibbs.{{free media}}

"The rotation of galaxies was discovered in 1914, when Slipher (1914) detected inclined absorption lines in the nuclear spectra of M31 and the Sombrero galaxy, and Wolf (1914) detected inclined lines in the nuclear spectrum of M81."[15]

In the diagram on the right, expected (A) and observed (B) visible matter velocities as a function of distance from the galactic center are plotted.

"At the dedication of the McDonald Observatory in 1939, Oort’s (1940) comment that “...the distribution of mass [in NGC 3115] appears to bear almost no relation to that of the light” seems from the view in 2000 to have attracted little attention. His conclusion concerning the mass distribution in NGC 3115 is worth quoting, even 60 years later. “In the outer parts of the nebula the ratio f of mass density to light density is found to be very high; and this conclusion holds for whatever dynamical model we consider. The spectrum of the nebula shows the characteristics of G-type dwarfs. Since f cannot be much larger than 1 for such stars, they can account for roughly only 1/2 percent of the mass; the remainder must consist either of extremely faint dwarfs having an average ratio of mass to light of about 200 to 1 or else of interstellar gas and dust”. From a reanalysis of the (scattered) velocities for M31, Schwarzschild (1954) concluded that the approximately flat rotation curve was “not discordant with the assumption of equal mass and light distribution.”"[15]

"Rotation curves are tools for several purposes: for studying the kinematics of galaxies; for inferring the evolutionary histories and the role that interactions have played; for relating departures from the expected rotation curve Keplerian form to the amount and distribution of dark matter; for observing evolution by comparing rotation curves in distant galaxies with galaxies nearby."[15]

"Although Hα, [NII], and [SII] emission lines have traditionally been employed, the Seyfert galaxy NGC 1068 has become the first galaxy whose velocity field has been studied from the IR [Si VI] line (Tecza et al. 2000)."[15]

"For a limited number of nearby galaxies, rotation curves can be produced from velocities of individual HII regions in galactic disks (Rubin & Ford, 1970, 1983; Zaritsky et al. 1989, 1990, 1994)."[15]

"The HI line is a powerful tool to obtain kinematics of spiral galaxies, in part because its radial extent is often greater, sometimes 3 or 4 times greater, than that of the visible disk. Bosma’s thesis (1981a, b; van der Kruit & Allen 1978) played a fundamental role in establishing the flatness of spiral rotation curves."[15]

"While comparison of the inner velocity rise for NGC 3198 showed good agreement between the 21-cm and the optical velocities (van Albada et al. 1985; Hunter et al. 1986), the agreement was poor for Virgo spirals observed at low HI resolution (Guhathakurta et al. 1988; Rubin et al. 1989)."[15]

"The rotational transition lines of carbon monoxide (CO) in the millimeter wave range [e.g., 115.27 GHz for 12
CO
(J = 1 − 0) line, 230.5 GHz for J = 2 − 1] are valuable in studying rotation kinematics of the inner disk and central regions of spiral galaxies, for extinction in the central dusty disks is negligible at CO wavelengths (Sofue 1996, 1997). Edge-on and high-inclination galaxies are particularly useful for rotation curve analysis in order to minimize the uncertainty arising from inclination corrections, for which extinction-free measurements are crucial, especially for central rotation curves."[15]

"CO lines are emitted from molecular clouds associated with star formation regions emitting the Hα line. Hence, CO is a good alternative to Hα and also to HI in the inner disk, while HI is often weak or absent in the central regions. The Hα, CO, and HI rotation curves agree well with each other in the intermediate region disks of spiral galaxies (Sofue 1996; Sofue et al. 1999a, b). Small displacements between Hα and CO rotation curves can arise in the inner regions from the extinction of the optical lines and the contamination of the continuum star light from central bulges."[15]

Interferometric "observations have achieved sub- or one-arcsec resolution (Sargent and Welch 1993; Scoville et al. 1993; Schinnerer et al. 2000; Sofue et al. 2000), comparable to, or sometimes higher than, the current optical measurements [...]. Another advantage of CO spectroscopy is its high velocity resolution of one to several km s−1."[15]

"Radial velocity observations of maser lines, such as SiO, OH and H2O lines, from circum-stellar shells and gas clouds allow us to measure the kinematics of stellar components in the disk and bulge of our Galaxy (Lindqvist et al. 1992a, b; Izumiura 1995, 1999; Deguchi et al. 2000). VLBI astrometry of SiO maser stars’ proper motion and parallax as well as radial velocities will reveal more unambiguous rotation of the Galaxy in the future. VLBI measurements of water masers from nuclei of galaxies reveal circumnuclear rotation on scales of 0.1 pc around massive central black holes, as was successfully observed for NGC 4258 (Miyoshi et al. 1995; see Section 4.4)."[15]

"A simple “rotation curve” is an approximation as a function of radius to the full velocity field of a disk galaxy. As such, it can be obtained only by neglecting small scale velocity variations, and by averaging and smoothing rotation velocities from both sides of the galactic center. Because it is a sim- ple, albeit approximate, description of a spiral velocity field, it is likely to be valuable even as more complex descriptions become available for many galaxies."[15]

"An extreme case of a nuclear warp is counterrotation. Rotating nuclear disks of cold gas have been discovered in more than 100 galaxies, types E through Sc (Bertola & Galletta 1978; Galletta 1987, 1996; Bertola et al. 1990; Bertola et al. 1992; Rubin 1994b; Garcia-Burillo et al. 1998); counterrotation is not especially rare. Simulations of disk interactions and mergers which include gas and stellar particles (Hernquist & Barnes 1991; Barnes & Hernquist 1992) reveal that a kinematically distinct nuclear gas disk can form; it may be counterrotating. Simulation of galactic-shock accretion of nuclear gas disk in an oval potential, such as a nuclear bar, produces highly eccentric streaming motion toward the nucleus, some portion being counterrotating (Wada et al. 1999). Kinematically decoupled stellar nuclear disks are also observed in early type galaxies (Jedrzejewski & Schechter 1989; Franx et al. 1991). Counter rotating nuclear disks can result from merger, mass exchange and/or inflow of intergalactic clouds. In addition to forming the central disk, an inflow of counterrotating gas would also be likely to promote nuclear activity."[15]

"A disk rotation curve manifests the distribution of surface mass density in the disk, attaining a broad maximum at a radius of about twice the scale radius of the exponential disk. For massive Sb galaxies, the rotation maximum appears at a radius of 5 or 6 kpc, which is about twice the scale length of the disk. Beyond the maximum, the rotation curve is usually flat, merging with the flat portion due to the massive dark halo. Superposed on the smooth rotation curve are fluctuations of a few tens of km s−1 due to spiral arms or velocity ripples. For barred spirals, the fluctuations are larger, of order 50 km s−1, arising from non circular motions in the oval potential."[15]

"Universal rotation curves reveal the following characteristics. Most luminous galaxies show a slightly declining rotation curves in the outer part, following a broad maximum in the disk. Intermediate galaxies have nearly flat rotation from across the disk. Less luminous galaxies have monotonically increasing rotation velocities across the optical disk. While Persic et al. conclude that the dark-to-luminous mass ratio increases with decreasing luminosity, mass deconvolutions are far from unique."[15]

"Only a handful of galaxies are presently known to have counterrotating components over a large fraction of their disks (Rubin 1994b). The disk of E7/S0 NGC 4550, (Rubin et al. 1992; Kenney & Faundez 2000) contains two hospital stellar populations, one orbiting programmed, one retrograde. This discovery prompted modification of computer programs which fit only a single Gaussian to integrated absorption lines in galaxy spectra (Rix et al. 1992). In NGC 7217 (Sab), 30% of the disk stars orbit retrograde (Merrifield & Kuijen 1994). The bulge in NGC 7331 (Sbc) may (Prada et al. 1996) or may not (Mediavilla et al. 1998) counterrotate with respect to the disk. Stars in NGC 4826 (Sab; the Black Eye or Sleeping Beauty) orbit with a single sense. Gas extending from the nucleus through the broad dusty lane rotates prograde, but reverses its sense of rotation immediately beyond; radial infall motions are present where the galaxy velocities reverse (Rubin et al. 1965; Braun et al. 1994; Rubin 1994a; Walterbos et al. 1994; Rix et al. 1995; Sil’chenko 1996)."[15]

"Only recently have rotation curves been obtained for distant galaxies, using HST and large-aperture ground-based telescopes with sub-arc second seeing. We directly observe galaxy evolution by studying galaxies closer to their era of formation. Rotation velocities for moderately distant spirals, z≈ 0.2 to 0.4, (Bershady 1997, et al. 1999, Simard & Prichet 1998, Kelson et al. 2000a) have already been surpassed with Keck velocities reaching z≈1 (Vogt et al. 1993, 1996, 1997; Koo 1999), for galaxies whose diameters subtend only a few seconds of arc. The rotation properties are similar to those of nearby galaxies, with peak velocities between 100 to 200 km s−1, and flat outer disk velocities."[15]

"The maximum rotation velocities for Sa galaxies are higher than those of Sb and Sc galaxies with equivalent optical luminosities. Median values of Vmax decreases from 300 to 220 to 175 km s−1 for the Sa, Sb, and Sc types, respectively (Rubin et al. 1985)."[15]

"Large-scale rotation properties of SBb and SBc galaxies are generally similar to those of non-barred galaxies of Sb and Sc types."[15]

Barred "galaxies show velocity jumps from ± ∼ 30 − 40 km s−1 to ≥ 100 km s−1 on the leading edges of the bar, R ∼ 2 − 5 kpc, whereas some barred galaxies show flat rotation (e.g., NGC 253: Sorai et al. 2000)."[15]

"Until the last decade, observations of rotational kinematics were restricted to spirals with average or high surface brightness. Only within the past decade have low surface brightness (LSB) galaxies been found in great numbers (Schombert & Bothun 1988; Schombert et al. 1992); many are spirals. Their kinematics were first studied by de Blok et al. (1996) with HI, who found slowly rising curves which often continued rising to their last measured point. However, many of the galaxies are small in angular extent, so observations are subject to beam smearing. Recent optical rotation curves (Swaters 1999, 2001; Swaters et al. 2000; de Blok et al. 2001) reveal a steeper rise for some, but not all, of the galaxies studied previously at 21-cm."[15]

"Dwarf galaxies, galaxies of low mass, are often grouped with low surface brightness galaxies, either by design or by error. The two classes overlap in the low surface brightness/low mass region. However, some low surface brightness galaxies are large and massive; some dwarf galaxies have high surface brightness. Early observations showed dwarf galaxies to be slowly rotating, with rotation curves which rise monotonically to the last measured point (Tully et al. 1978; Carignan & Freeman 1985; Carignan & Puche 1990a,b; Carignan & Beaulieu 1989; Puche et al. 1990, 1991a, b; Lake et al. 1990; Broeils 1992)."[15]

Theoretical galaxies[edit]

Def. "one of a collection of stars, galactic dust, black hole etc. of the billions which are found in the universe"[16] is called a galaxy.

Def. "a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy"[17] is called an active galaxy.

Def. Any "galaxy, considerably smaller than the Milky Way, that has only several billions of stars"[18] is called a dwarf galaxy.

Morphological classifications[edit]

The composite image shows a classification of galaxies. Credit: Ville Koistinen.{{free media}}

Probably the earliest classification of galaxies "is based on the forms of the photographic images."[19]

Def. a "galaxy having a smooth, featureless light-profile"[20] is called an elliptical galaxy.

Def. a "galaxy which has no spirals[21] and is not elliptical"[22] is called an irregular galaxy

Def. a "galaxy that like spiral galaxies has a flat disk but unlike them has lost most of its interstellar matter and therefore has no spirals; considered a transitional form between spirals and elliptical galaxies"[23] is called a lenticular galaxy.

Def. a "galaxy having a number of arms of younger stars that spiral out from the centre containing older ones"[24] is called a spiral galaxy, or a whirlpool galaxy.

"About 3 per cent are irregular, but the remaining nebulae fall into a sequence of type forms characterized by rotational symmetry about dominating nuclei."[19]

"The sequence is composed of two sections, the elliptical nebulae and the spirals, which merge into each other."[19]

"The classification of these nebulae is based on structure, the individual members of a class differing only in apparent size and luminosity."[19]

The "forms divide themselves naturally into two groups:

  1. those [nebulae] found in or near the Milky Way and
  2. those in moderate or high galactic latitudes."[19]

For the elliptical nebulae [galaxies], the classification En, where "n=1, 2, .... , 7 indicates the ellipticity of the image without the decimal point".[19]

For example, NGC 3379 is E0, NGC 221 is E2, NGC 4621 is E5 and NGC 2117 is E7.[19]

The spirals are divided into two types:

  1. Normal spirals (S) of Early (Sa), Intermediate (Sb), and Late (Sc) and
  2. Barred spirals (SB) of Early (SBa), Intermediate (SBb), and Late (SBc).[19]

The irregular galaxies are put into that structure form with "Irr".[19]

Examples are

  1. Sa - NGC 4594,
  2. Sb - NGC 2841,
  3. Sc - NGC 5457,
  4. SBa - NGC 2859,
  5. SBb - NGC 3351,
  6. SBc - NGC 7479, and
  7. Irr - NGC 4449.[19]

Circumgalactic medium[edit]

"Most galaxies are surrounded by a haze called the circumgalactic medium, or CGM [...]. [The] CGM [may contain] more mass than the galaxy’s stars and controls a galaxy’s life cycle. But because the CGM doesn’t give off much light of its own, it’s hard to study."[25]

Fast radio bursts "FRBs could light up a CGM as well. Both quasars and FRBs emit light in a range of wavelengths, which are slowed along their path to Earth by charged particles en route. Because quasars shine continuously, it’s harder to tell how much the light is slowed. But the brief pulse of an FRB can reveal how much stuff is in the way. So there are characteristics of the CGM that FRBs can reveal directly, such as the gas’s density and magnetic fields, that longer-lasting quasars can’t."[25]

"We can resolve these physical properties of the CGM that are pretty much impossible with other techniques."[26]

On "November 12, 2018, when the Australian Square Kilometer Array Pathfinder detected an FRB pulse lasting less than 40 microseconds, [the Pathfinder] tracked the FRB to a galaxy in the constellation Indus."[25]

"At first, [it appeared that] the FRB came from a bright galaxy about 4 billion light-years away. But the distance measured for the FRB was closer to 5 billion light-years. [There] were two galaxies in a row — just the fortuitous lineup [...] hoped for."[25]

"The foreground galaxy’s density and magnetization both were unexpectedly low, [...]. The density of CGM gas was less than 0.1 atoms per cubic centimeter, at least a factor of 10 lower than expected from previous studies. The magnetic field was less than 0.8 microgauss, a billion times weaker than a refrigerator magnet, suggesting that the gas doesn’t experience much turbulence."[25]

"One of the great hopes for FRBs is using them as tools for probing everything that lies along their paths to Earth."[27]

Clouds[edit]

This shows the Milky Way edge on with the Large Magellanic Cloud. Credit: ESO/S. Brunier.{{free media}}

Two clouds occur in the image above: the Large Magellanic Cloud, second lowest object on the right below the galaxy, and the lowest diffuse object on the right, the Small Magellanic Cloud. Passing your cursor over image on Commons finds these two clouds.

Cosmic rays[edit]

The flux of cosmic-ray particles is a function of their energy. Credit: Sven Lafebre, after Swordy.[28]
Lots of cosmic rays in this one had to be removed manually even after running automatic noise removal over it. Credit: Judy Schmidt.{{free media}}

At right is an image indicating the range of cosmic-ray energies. The flux for the lowest energies (yellow zone) is mainly attributed to solar cosmic rays, intermediate energies (blue) to galactic cosmic rays, and highest energies (purple) to extragalactic cosmic rays.[28]

There is "a correlation between the arrival directions of cosmic rays with energy above 6 x 1019 electron volts and the positions of active galactic nuclei (AGN) lying within ~75 megaparsecs."[29]

The "propagation of relativistic electrons is sufficiently well constrained that the observed distribution may have direct bearing on the distribution of cosmic rays. Observations near 5 GHz trace cosmic ray electrons that propagate 1 to 3 kpc from their sources before losing their energy. Supernova remnants are plausible candidates for cosmic ray production given their expected energy outputs and their surface number densities in galactic disks."[30]

"The sample of galaxies [includes] NGC 5005 [shown on the left]."[30]

Protons[edit]

The secondary antiprotons (p) then propagate through the galaxy, confined by the galactic magnetic fields. Their energy spectrum is modified by collisions with other atoms in the interstellar medium. The antiproton cosmic ray energy spectrum is now measured reliably and is consistent with this standard picture of antiproton production by cosmic ray collisions.[31]

Electrons[edit]

As of December 5, 2011, "Voyager 1 is about ... 18 billion kilometers ... from the [S]un [but] the direction of the magnetic field lines has not changed, indicating Voyager is still within the heliosphere ... the outward speed of the solar wind had diminished to zero in April 2010 ... inward pressure from interstellar space is compacting [the magnetic field] ... Voyager has detected a 100-fold increase in the intensity of high-energy electrons from elsewhere in the galaxy diffusing into our solar system from outside ... [while] the [solar] wind even blows back at us."[32]

X-rays[edit]

This is a Chandra X-ray observatory image of NGC 5813. Credit: NASA.
Chandra X-ray Observatory image is NGC 5846 with superimposed contours of Hα+[N ii] emission. Credit: .
A beautiful new image of two colliding galaxies has been released by NASA's Great Observatories. Credit: NASA, ESA, SAO, CXC, JPL-Caltech, and STScI.{{free media}}

NGC 5846 is the foremost galaxy of the large galaxy group known as the NGC 5846 group which includes NGC 5813, NGC 5831, NGC 5845, and NGC 5854.[33] The group has two subgroups, one centered around the elliptical NGC 5813 and the other around NGC 5846, lying at a projected separation of 600 kpc.[34] The group is part of the Virgo III Groups, a very obvious chain of galaxy groups on the left side of the Virgo cluster, stretching across 40 million light years of space.[35]

In the image on the left of NGC 5846, white crosses mark the detected CO cloud positions.[36]

The galaxy has complex X-ray morphology[37] that is considered to be the result of AGN outflows. Two inner bubbles in the hot gas, at a distance of 600 pc from the center and filled with radio emission, are clear indications of recent AGN feedback. A weak radio source, elongated in the NE–SW direction, connects the inner cavities. X-ray-bright rims surround the inner X-ray bubbles.[38] Many X-ray knots are visible, suggesting cooling sites. The scenario indicated by the Chandra observation is that of an AGN outflow, compressing and cooling the gas[39] in the central ~2 kpc (20" at the distance of NGC 5846).[36]

"A beautiful new image of two colliding galaxies [second down on the right] has been released by NASA's Great Observatories. The Antennae galaxies, located about 62 million light-years from Earth, are shown in this composite image from the Chandra X-ray Observatory (blue), the Hubble Space Telescope (gold and brown), and the Spitzer Space Telescope (red). The Antennae galaxies take their name from the long antenna-like "arms," seen in wide-angle views of the system. These features were produced by tidal forces generated in the collision."[40]

"The collision, which began more than 100 million years ago and is still occurring, has triggered the formation of millions of stars in clouds of dust and gas in the galaxies. The most massive of these young stars have already sped through their evolution in a few million years and exploded as supernovas."[40]

"The X-ray image from Chandra shows huge clouds of hot, interstellar gas that have been injected with rich deposits of elements from supernova explosions. This enriched gas, which includes elements such as oxygen, iron, magnesium, and silicon, will be incorporated into new generations of stars and planets. The bright, point-like sources in the image are produced by material falling onto black holes and neutron stars that are remnants of the massive stars. Some of these black holes may have masses that are almost one hundred times that of the Sun."[40]

"The Spitzer data show infrared light from warm dust clouds that have been heated by newborn stars, with the brightest clouds lying in the overlapping region between the two galaxies."[40]

"The Hubble data reveal old stars and star-forming regions in gold and white, while filaments of dust appear in brown. Many of the fainter objects in the optical image are clusters containing thousands of stars."[40]

"The Chandra image was taken in December 1999. The Spitzer image was taken in December 2003. The Hubble image was taken in July 2004 and February 2005."[40]

Ultraviolets[edit]

This beautiful galaxy is M81, or NGC 3031. Credit: Hubble data: NASA, ESA, and A. Zezas (Harvard-Smithsonian Center for Astrophysics); GALEX data: NASA, JPL-Caltech, GALEX Team, J. Huchra et al. (Harvard-Smithsonian Center for Astrophysics); Spitzer data: NASA/JPL/Caltech/S. Willner (Harvard-Smithsonian Center for Astrophysics.

M81 is the beautiful galaxy tilted at an oblique angle on to our line of sight, giving a "birds-eye view" of the spiral structure, in the image on the right.

Messier 81 aka NGC 3031 or Bode's Galaxy is a spiral galaxy about 12 million light-years away, with a diameter of 90,000 light years, about half the size of the Milky Way, in the constellation Ursa Major.[41]

Close to Earth, M81 has a large size, and an active galactic nucleus (which harbors a 70 million M[42] supermassive black hole. The galaxy's large size and relatively high brightness also makes it a popular target for amateur astronomers.[43]

Messier 81 is located approximately 10° northwest of Alpha Ursae Majoris along with several other galaxies in the Messier 81 Group.[43][44]

Messier 81 and Messier 82 can both be viewed easily using binoculars and small telescopes.[43][44] The two objects are generally not observable to the unaided eye, although highly experienced amateur astronomers may be able to see Messier 81 under exceptional observing conditions with a very dark sky.[43] Telescopes with apertures of 8 inches (20 cm) or larger are needed to distinguish structures in the galaxy.[44]

Opticals[edit]

The telescope photograph of the Great Andromeda Nebula is taken around 1899. Credit: Isaac Roberts.{{free media}}
NGC 5846 (right) and NGC 5850 (left) are by the Schulman Telescope at Mount Lemmon SkyCenter. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona.{{free media}}

Still much further away from the Earth than the Sun or Neptune are the many stars and nebulae that make up the Milky Way. Beyond the confines of our galaxy is the Andromeda Galaxy shown at the top of the page.

Of the Local Group, “[i]ts two dominant galaxies, the Milky Way and Andromeda (M31), are separated by a distance of ~700 kpc and are moving toward each other with a radial velocity of about -117 km s-1 (Binney & Tremaine 1987, p. 605).”[45] making Andromeda one of the few blueshifted galaxies. The Andromeda Galaxy and the Milky Way are thus expected to collide in about 4.5 billion years, although the details are uncertain since Andromeda's tangential velocity with respect to the Milky Way is only known to within about a factor of two.[46] A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy.[47] Such events are frequent among the galaxies in galaxy groups. The fate of the Earth and the Solar System in the event of a collision are currently unknown. If the galaxies do not merge, there is a small chance that the Solar System could be ejected from the Milky Way or join Andromeda.[48]

NGC 5846 is an elliptical galaxy (type E0-1[49]) located in the constellation Virgo at a distance of 93 ± 32 Mly (28.5 ± 9.8 Mpc)[49] from Earth, which, given its apparent dimensions, means that NGC 5846 is about 110,000 light years across. It lies near 110 Virginis and is part of the Herschel 400 Catalogue.[50]

NGC 5846 is a giant elliptical galaxy with a round shape and a low luminosity active galactic nucleus, whose categorisation is ambiguous, having features that are observed both in LINER and HII regions.[51] NGC 5846 apparently harbors a supermassive black hole with estimated mass 1.1±0.1×109
 M_solar
based on the central velocity dispersion.[38][52]

NGC 5846 harbors a large number of globular clusters; over 1,200 have been detected in images by Hubble Space Telescope.[53] The specific frequency of occurrence is similar to other elliptical galaxies in groups as is the metallicity with bimodial distribution, roughly of [Fe/H]=-1.2 and -0.2.[54] Their typical effective radii are in the range of 3 - 5 pc, with the largest clusters located in the central regions; seven of the globular clusters have X-ray counterparts, which are among the most luminous X-ray sources in NGC 5846, and they are mostly in the central region, optically luminous, compact and belong to the red subpopulation.[55]

Blues[edit]

Messier 106 is one of the brightest and nearest spiral galaxies to our own. Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA), and R. Gendler (for the Hubble Heritage Team).{{free media}}

Messier 106 (NGC 4258) is an intermediate spiral galaxy in the constellation Canes Venatici at a distance of about 22 to 25 million light-years, contains an active nucleus classified as a Type 2 Seyfert, and the presence of a central supermassive black hole demonstrated from radio astronomy observations of the rotation of an accretion disk of molecular gas orbiting within the inner light-year around the black hole.[56]

M106 has a water vapor megamaser (the equivalent of a laser operating in microwave instead of visible light and on a galactic scale) that is seen by the 22-GHz line of ortho-H2O that evidences dense and warm molecular gas that give M106 its characteristic purple color.[57] Water masers are useful to observe nuclear accretion disks in active galaxies, enabling the first case of a direct measurement of the distance to a galaxy, thereby providing an independent anchor for the cosmic distance ladder.[58][59] M106 has a slightly warped, thin, almost edge-on Keplerian disc which is on a subparsec scale that surrounds a central area with mass 4 × 107 M.[60]

It is one of the largest and brightest nearby galaxies, similar in size and luminosity to the Andromeda Galaxy.[61] The supermassive black hole at the core has a mass of 39,000,000±0.1 solar mass.[62]

M106 has also played an important role in calibrating the cosmic distance ladder: Cepheid variables from other galaxies could not be used to measure distances since they cover ranges of metallicities different from the Milky Way's, but M106 contains Cepheid variables similar to both the metallicities of the Milky Way and other galaxies' Cepheids, by measuring the distance of the Cepheids with metallicities similar to our galaxy, recalibration of the other Cepheids with different metallicities, a key fundamental step in improving quantification of distances to other galaxies in the universe, was possible.[63]

Cyans[edit]

The Pinwheel Galaxy, a.k.a. Messier 101 or NGC 5457, is a face-on spiral galaxy about 27 million light-years away in the constellation Ursa Major. Credit: Jim Keller.{{free media}}
This is one of the largest and most detailed photo of a spiral galaxy NGC 5457 that has been released from Hubble. Credit: European Space Agency & NASA.{{free media}}

For this image on the right, the DSS red channel was mapped as red, and the DSS blue channel was mapped as cyan. North is to the left, and the field of view is approximately 43x43 arcminutes.

The Pinwheel Galaxy is a face-on spiral galaxy distanced 21 million light-years (six megaparsecs)[64] away from Earth in the constellation Ursa Major.

M101 is a large galaxy, with a diameter of ~170,000 ly in diameter[65] It has around a trillion stars, twice the number in the Milky Way.[66] It has a disk mass on the order of 100 billion solar masses, along with a small central bulge of about 3 billion solar masses.[67]

M101 has a high population of H II regions, many are very large and bright, usually accompanying the enormous clouds of high density molecular hydrogen gas contracting under their own gravitational force where stars form, are ionized by large numbers of extremely bright and hot young stars; those in M101 are capable of creating hot superbubbles.[68] In a 1990 study, 1264 H II regions were cataloged in the galaxy.[69] Three are prominent enough to receive New General Catalogue numbers - NGC 5461, NGC 5462, and NGC 5471.[70]

M101 is asymmetrical due to the tidal forces from interactions with its companion galaxies. These gravitational interactions compress interstellar hydrogen gas, which then triggers strong star formation activity in M101's spiral arms that can be detected in ultraviolet images.[71]

M101 has five prominent companion galaxies: NGC 5204, NGC 5474, NGC 5477, NGC 5585, and Holmberg IV.[72] As stated above, the gravitational interaction between M101 and its satellites may have triggered the formation of the grand design pattern in M101. M101 has also probably distorted the companion galaxy NGC 5474.[72] M101 and its companion galaxies comprise most or possibly all of the M101 Group.[73][74][75][76]

Greens[edit]

The galaxy lies 13 million light-years away in the southern constellation Circinus. Credit: NASA, Andrew S. Wilson (University of Maryland); Patrick L. Shopbell (Caltech); Chris Simpson (Subaru Telescope); Thaisa Storchi-Bergmann and F. K. B. Barbosa (UFRGS, Brazil); and Martin J. Ward (University of Leicester, U.K.).{{free media}}

"Resembling a swirling witch's cauldron of glowing vapors, the black hole-powered core of a nearby active galaxy appears in this colorful NASA Hubble Space Telescope image. The galaxy lies 13 million light-years away in the southern constellation Circinus."[77]

"This galaxy is designated a type 2 Seyfert, a class of mostly spiral galaxies that have compact centers and are believed to contain massive black holes. Seyfert galaxies are themselves part of a larger class of objects called Active Galactic Nuclei or AGN. AGN have the ability to remove gas from the centers of their galaxies by blowing it out into space at phenomenal speeds. Astronomers studying the Circinus galaxy are seeing evidence of a powerful AGN at the center of this galaxy as well."[77]

"Much of the gas in the disk of the Circinus spiral is concentrated in two specific rings - a larger one of diameter 1,300 light-years, which has already been observed by ground-based telescopes, and a previously unseen ring of diameter 260 light-years."[77]

"In the Hubble image, the smaller inner ring is located on the inside of the green disk. The larger outer ring extends off the image and is in the plane of the galaxy's disk. Both rings are home to large amounts of gas and dust as well as areas of major "starburst" activity, where new stars are rapidly forming on timescales of 40 - 150 million years, much shorter than the age of the entire galaxy."[77]

"At the center of the starburst rings is the Seyfert nucleus, the believed signature of a supermassive black hole that is accreting surrounding gas and dust. The black hole and its accretion disk are expelling gas out of the galaxy's disk and into its halo (the region above and below the disk). The detailed structure of this gas is seen as magenta-colored streamers extending towards the top of the image."[77]

"In the center of the galaxy and within the inner starburst ring is a V-shaped structure of gas. The structure appears whitish-pink in this composite image, made up of four filters. Two filters capture the narrow lines from atomic transitions in oxygen and hydrogen; two wider filters detect green and near-infrared light. In the narrow-band filters, the V-shaped structure is very pronounced. This region, which is the projection of a three-dimensional cone extending from the nucleus to the galaxy's halo, contains gas that has been heated by radiation emitted by the accreting black hole. A "counter-cone," believed to be present, is obscured from view by dust in the galaxy's disk. Ultraviolet radiation emerging from the central source excites nearby gas causing it to glow. The excited gas is beamed into the oppositely directed cones like two giant searchlights."[77]

"Located near the plane of our own Milky Way Galaxy, the Circinus galaxy is partially hidden by intervening dust along our line of sight. As a result, the galaxy went unnoticed until about 25 years ago. This Hubble image was taken on April 10, 1999 with the Wide Field Planetary Camera 2."[77]

Yellows[edit]

This image shows a cluster of yellow galaxies near the middle of the photograph. Credit: W.N. Colley and E. Turner (Princeton University), J.A. Tyson (Bell Labs, Lucent Technologies) and STScl/NASA.

"This Hubble Space Telescope image [at right] shows several blue, loop-shaped objects that actually are multiple images of the same galaxy. They have been duplicated by the gravitational lens of the cluster of yellow, elliptical and spiral galaxies - called 0024+1654 - near the photograph's center. The gravitational lens is produced by the cluster's tremendous gravitational field that bends light to magnify, brighten and distort the image of a more distant object. How distorted the image becomes and how many copies are made depends on the alignment between the foreground cluster and the more distant galaxy, which is behind the cluster."[78]

"In this photograph, light from the distant galaxy bends as it passes through the cluster, dividing the galaxy into five separate images. One image is near the center of the photograph; the others are at 6, 7, 8, and 2 o'clock. The light also has distorted the galaxy's image from a normal spiral shape into a more arc-shaped object. Astronomers are certain the blue-shaped objects are copies of the same galaxy because the shapes are similar. The cluster is 5 billion light-years away in the constellation Pisces, and the blue-shaped galaxy is about 2 times farther away."[78]

"Though the gravitational light-bending process is not new, Hubble's high resolution image reveals structures within the blue-shaped galaxy that astronomers have never seen before. Some of the structures are as small as 300 light-years across. The bits of white imbedded in the blue galaxy represent young stars; the dark core inside the ring is dust, the material used to make stars. This information, together with the blue color and unusual "lumpy" appearance, suggests a young, star-making galaxy."[78]

"The picture was taken October 14, 1994 with the Wide Field Planetary Camera-2. Separate exposures in blue and red wavelengths were taken to construct this color picture."[78]

Oranges[edit]

The sky image of NGC 5831 is obtained by the Sloan Digital Sky Survey, DR14 with SciServer. Credit: Sloan Digital Sky Survey.{{free media}}
Response of SDSS filters are for the imaging camera. Credit: Mamoru Doi, Masayuki Tanaka, Masataka Fukugita, James E. Gunn, Naoki Yasuda, Zeljko Ivezic, Jon Brinkmann, Ernst de Haars, Scott J. Kleinman, Jurek Krzesinski, and R. French Leger.{{free media}}
The image shows the arrangement of the CCDs and filters on the SDSS-III camera. Credit: Mamoru Doi, Masayuki Tanaka, Masataka Fukugita, James E. Gunn, Naoki Yasuda, Zeljko Ivezic, Jon Brinkmann, Ernst de Haars, Scott J. Kleinman, Jurek Krzesinski, and R. French Leger.{{free media}}

The SDSS imaging camera collects photometric imaging data using an array of 30 SITe/Tektronix 2048 by 2048 pixel CCDs arranged in six columns of five CCDs each, aligned with the pixel columns of the CCDs themselves.[79]

SDSS-III used the same filter system as the original SDSS with central wavelengths of the six filters:

  1. u 3551 Å (ultraviolet)
  2. g 4686 Å (green)
  3. r 6166 Å (red)
  4. i 7480 Å (infrared)
  5. z 8932 mÅ (Z band).[79]

Reds[edit]

This is an image of H 0323+022 using the red (R) filter. Credit: Renato Falomo, ESO NTT.

A BL Lacertae object or BL Lac object is a type of active galaxy with an active galactic nucleus (AGN) and is named after its prototype, BL Lacertae. In contrast to other types of active galactic nuclei, BL Lacs are characterized by rapid and large-amplitude flux variability and significant optical polarization.

All known BL Lacs are associated with core dominated radio sources, many of them exhibiting superluminal motion.

QSO B0323+022 is a BL Lacertae object. The image at right is taken with the ESO New Technology Telescope (NTT) using the R filter.

Infrareds[edit]

This is a three-color far-infrared image of M51, the Whirlpool Galaxy. Credit: ESA and the PACS Consortium.{{free media}}
NASA/ESA Hubble Space Telescope and NASA's Spitzer Space Telescope joined forces to create this striking composite image of Messier 104. Credit: NASA/JPL-Caltech and The Hubble Heritage Team (STScI/AURA).{{free media}}

Huge, cold clouds of gas and dust in our own galaxy, as well as in nearby galaxies, glow in far-infrared light. This is due to thermal radiation of interstellar dust contained in molecular clouds.

"One of the most interesting discoveries made by IRAS was of galaxies with far-infrared luminosities of 1011 - 1012 L, and LIR/LB ~ 10 - 100 (Soifer et al. 1984). [...] Only a few examples of this type of object, namely Arp 220, NGC 3690, Mrk 231, and NGC 6240, have ever been studied from the ground in detail."[80]

"Three-color far-infrared image of M51, the Whirlpool Galaxy." includes "Red, green and blue correspond to the 160-micron, 100-micron and 70-micron wavelength bands of the Herschels Photoconductor Array Camera and Spectrometer, PACS instruments."[81]

"Glowing light from clouds of dust and gas around and between the stars is visible clearly. These clouds are a reservoir of raw material for ongoing star formation in this galaxy. Blue indicates regions of warm dust that is heated by young stars, while the colder dust shows up in red."[81]

In the image on the left, "NASA/ESA Hubble Space Telescope and NASA's Spitzer Space Telescope joined forces to create this striking composite image of one of the most popular sights in the universe. Messier 104 is commonly known as the Sombrero galaxy because in visible light, it resembles the broad-brimmed Mexican hat. However, in Spitzer's striking infrared view, the galaxy looks more like a "bull's eye"."[82]

"Spitzer's full view shows the disk is warped, which is often the result of a gravitational encounter with another galaxy, and clumpy areas spotted in the far edges of the ring indicate young star-forming regions."[82]

"The Sombrero galaxy is located some 28 million light-years away. Viewed from Earth, it is just six degrees south of its equatorial plane. Spitzer detected infrared emission not only from the ring, but from the center of the galaxy too, where there is a huge black hole, believed to be a billion times more massive than our Sun."[82]

"The Spitzer picture is composed of four images taken at 3.6 (blue), 4.5 (green), 5.8 (orange), and 8.0 (red) microns. The contribution from starlight (measured at 3.6 microns) has been subtracted from the 5.8 and 8-micron images to enhance the visibility of the dust features."[82]

"The Spitzer picture [of] four images [was] taken [in] June 2004 and January 2005."[83]

Submillimeters[edit]

The ALMA observations — shown here in red, pink and yellow — were tuned to detect carbon monoxide molecules. Credit: ALMA (ESO/NAOJ/NRAO). Visible light image: the NASA/ESA Hubble Space Telescope.

Submillimetre astronomy or submillimeter astronomy is the branch of observational astronomy that is conducted at submillimetre wavelengths of the electromagnetic spectrum. Astronomers place the submillimetre waveband between the far-infrared and microwave wavebands, typically taken to be between a few hundred micrometres and a millimetre and using submillimetre observations, astronomers examine molecular clouds and dark cloud cores with a goal of clarifying the process of star formation from earliest collapse to stellar birth.

These wavelengths are sometimes called Terahertz radiation, since they have frequencies of the order of 1 THz.

"The Antennae Galaxies (also known as NGC 4038 and 4039) are a pair of distorted colliding spiral galaxies about 70 million light-years away, in the constellation of Corvus (The Crow). This view combines ALMA observations, made in two different wavelength ranges during the observatory’s early testing phase, with visible-light observations from the NASA/ESA Hubble Space Telescope."[84]

"The Hubble image is the sharpest view of this object ever taken and serves as the ultimate benchmark in terms of resolution. ALMA observes at much longer wavelengths which makes it much harder to obtain comparably sharp images. However, when the full ALMA array is completed its vision will be up to ten times sharper than Hubble."[84]

Microwaves[edit]

The two galaxies shown here, imaged by the Hubble Space Telescope, are named MCG+01-38-004 (the upper, red-tinted one) and MCG+01-38-005 (the lower, blue-tinted one). Credit: NASA Hubble Space Telescope.{{free media}}

"The [microwave] detection of interstellar formaldehyde provides important information about the chemical physics of our galaxy. We now know that polyatomic molecules containing at least two atoms other than hydrogen can form in the interstellar medium."[85]

"Phenomena across the universe emit radiation spanning the entire electromagnetic spectrum — from high-energy gamma rays, which stream out from the most energetic events in the cosmos, to lower-energy microwaves and radio waves."[86]

"Microwaves, the very same radiation that can heat up your dinner, are produced by a multitude of astrophysical sources, including strong emitters known as masers (microwave lasers), even stronger emitters with the somewhat villainous name of megamasers, and the centers of some galaxies. Especially intense and luminous galactic centers are known as active galactic nuclei. They are in turn thought to be driven by the presence of supermassive black holes, which drag surrounding material inwards and spit out bright jets and radiation as they do so."[86]

"The two galaxies shown here, imaged by the Hubble Space Telescope, are named MCG+01-38-004 (the upper, red-tinted one) and MCG+01-38-005 (the lower, blue-tinted one). MCG+01-38-005 is a special kind of megamaser; the galaxy’s active galactic nucleus pumps out huge amounts of energy, which stimulates clouds of surrounding water. Water’s constituent atoms of hydrogen and oxygen are able to absorb some of this energy and re-emit it at specific wavelengths, one of which falls within the microwave regime. MCG+01-38-005 is thus known as a water megamaser!"[86]

"Astronomers can use such objects to probe the fundamental properties of the universe. The microwave emissions from MCG+01-38-005 were used to calculate a refined value for the Hubble constant, a measure of how fast the universe is expanding. This constant is named after the astronomer whose observations were responsible for the discovery of the expanding universe and after whom the Hubble Space Telescope was named, Edwin Hubble."[86]

Radios[edit]

Image is in radio (pink) and X-Ray (cyan) of 3C 75. Credit: X-Ray: NASA / CXC / D.Hudson, T.Reiprich et al. (AIfA); Radio: NRAO / VLA/ NRL.{{free media}}

3C 75 may be X-ray source 2A 0252+060 (1H 0253+058, XRS 02522+060).[87]

"What's happening at the center of active galaxy 3C 75? The two bright sources at the center of this composite x-ray (blue)/ radio (pink) image are co-orbiting supermassive black holes powering the giant radio source 3C 75. Surrounded by multimillion degree x-ray emitting gas, and blasting out jets of relativistic particles the supermassive black holes are separated by 25,000 light-years. At the cores of two merging galaxies in the Abell 400 galaxy cluster they are some 300 million light-years away."[88]

Superluminals[edit]

Centaurus A in X-rays shows the relativistic jet. Credit: NASA.

"The structure of relativistic jets in [active galactic nuclei] AGN on scales of light days reveals how energy propagates through jets, a process that is fundamental to galaxy evolution."[89]

Their lengths can reach several thousand[90] or even hundreds of thousands of light years.[91] The hypothesis is that the twisting of magnetic fields in the accretion disk collimates the outflow along the rotation axis of the central object, so that when conditions are suitable, a jet will emerge from each face of the accretion disk. If the jet is oriented along the line of sight to Earth, relativistic beaming will change its apparent brightness. The mechanics behind both the creation of the jets[92][93] and the composition of the jets[94] are still a matter of much debate in the scientific community; it is hypothesized that the jets are composed of an electrically neutral mixture of electrons, positrons, and protons in some proportion.

A relativistic jet emitted from the AGN of M87 is traveling at speeds between four and six times the speed of light.[90]

"The term 'superluminal motion' is something of a misnomer. While it accurately describes the speeds measured, scientists still believe the actual speed falls just below the speed of light."[90]

"It's an illusion created by the finite speed of light and rapid motion".[90]

"Our present understanding is that this 'superluminal motion' occurs when these clouds move towards Earth at speeds very close to that of light, in this case, more than 98 percent of the speed of light. At these speeds the clouds nearly keep pace with the light they emit as they move towards Earth, so when the light finally reaches us, the motion appears much more rapid than the speed of light. Since the moving clouds travel slightly slower than the speed of light, they do not actually violate Einstein's theory of relativity which sets light as the speed limit."[90]

Milky Way[edit]

The Milky Way is seen on edge when viewed from within. Credit: Digital Sky LLC.
A new map of the Milky Way made with Cepheid stars reveals the warped shape of the galaxy. Credit: J. Skowron/Ogle/Astronomical Observatory/University of Warsaw.{{fairuse}}

The Milky Way is a name for the galaxy we live in.

It is a member of the Local Group of galaxies.

"By using a balloon borne telescope, an extensive survey of the [CII] line emission of the Galaxy has been undertaken. To minimize the instrumental emission, an off-axis telescope with a Newtonian-Nasmyth focus was used in conjunction with a liquid helium cooled Fabry-Perot spectrometer. The beam size of the telescope was 12' in diameter and the spectral resolution of the spectrometer was 175 km/s in velocity scale."[95]

"The balloon flights were made from Palestine, TX in 1991 and from Alice Springs, Australia in 1992. By both observations, the major part of the galactic plane in the northern sky and the southern sky has been scanned. As a result, a complete map of the [CII] line intensity distribution has been constructed for the region from—100° to 80° in galactic longitude and within ±4° in galactic latitude. In addition to the general scan of the galactic plane, the observations were extended to some individual sources, such as Cyg-X region, p-Oph dark cloud and Large Magellanic Cloud."[95]

"The observed maps reveal strong [CII] line emission extensively distributed in the galactic plane, as well as many discrete sources associated with HII regions and/or molecular clouds. The distribution is more or less correlated with far infrared continuum and CO line intensity distributions."[95]

"Rather than being flat as a Frisbee, the Milky Way’s star-studded disk is twisted and warped, [if] viewed from the side, the spiral arms girdling our galaxy’s bulging core would resemble a record bent into an S shape [...]"[96]

"Stretching some 120,000 light-years from tip to tip, the Milky Way [four] large arms wind around its core, with our sun parked along a minor arm some 26,000 light-years from the center."[96]

"The galaxy’s disk of stars and gas is mostly thin and flat toward the middle. But at roughly the sun’s distance from the core, the galaxy begins to bend, flexing upward in one direction and flopping down in the other."[96]

"Near the edges, it gets kind of sloppy: The disk flares, expanding in width from 500 light-years to more than 3,000 light-years, and the warp is even more prominent, with stars living as many as 5,000 light-years above or below the galactic plane."[96]

"We think the warp may have been caused by interactions with satellite galaxies. The Milky Way today is surrounded by a swarm of dwarf galaxies."[97]

Milky Way theory[edit]

Here's a theoretical definition:

Def. a large spiral galaxy that includes the Sun is called the Milky Way.

Def. a "faint galaxy, devoid of gas, having a higher than normal proportion of dark matter; especially those that orbit the Milky Way and Andromeda[98] is called a dwarf spheroidal galaxy.

Def. originating "outside the Milky Way galaxy"[99] or "outside of a galaxy"[99] is called extragalactic.

Galaxies[edit]

The pseudo-colour image is of the large-scale radio structure of the FRII radio galaxy 3C98. Lobes, jet and hotspot are labelled. Credit: .
Another pseudo-colour image is of the large-scale radio structure of the FRI radio galaxy 3C31. Jets and plumes are labelled. Credit: .

"Over the past 30 years, radioastronomy has revealed a rich variety of molecular species in the interstellar medium of our galaxy and even others."[100]

These regions are non-luminous, save for emission of the 21-cm (1,420 MHz) region spectral line. ... Mapping H I emissions with a radio telescope is a technique used for determining the structure of spiral galaxies.

"In 1974, radio sources were divided into two classes Fanaroff and Riley Class I (FRI), and Class II (FRII).[101]

The distinction was originally made based on the morphology of the large-scale radio emission (the type was determined by the distance between the brightest points in the radio emission): FRI sources were brightest towards the centre, while FRII sources were brightest at the edges.

There is a reasonably sharp divide in luminosity between the two classes: FRIs were low-luminosity, FRIIs were high luminosity.[101]

The morphology turns out to reflect the method of energy transport in the radio source. FRI objects typically have bright jets in the centre, while FRIIs have faint jets but bright hotspots at the ends of the lobes. FRIIs appear to be able to transport energy efficiently to the ends of the lobes, while FRI beams are inefficient in the sense that they radiate a significant amount of their energy away as they travel.

The FRI/FRII division depends on host-galaxy environment in the sense that the FRI/FRII transition appears at higher luminosities in more massive galaxies.[102] FRI jets are known to be decelerating in the regions in which their radio emission is brightest,[103]

The hotspots that are usually seen in FRII sources are interpreted as being the visible manifestations of shocks formed when the fast, and therefore supersonic, jet (the speed of sound cannot exceed c/√3) abruptly terminates at the end of the source, and their spectral energy distributions are consistent with this picture.[104]

NGC 628[edit]

ESO's PESSTO survey has captured this view of Messier 74. Credit: ESO/PESSTO/S. Smartt.{{free media}}

"ESO's PESSTO survey has captured this view of Messier 74, a stunning spiral galaxy with well-defined whirling arms. However, the real subject of this image is the galaxy's brilliant new addition from late July 2013: a Type II supernova named SN2013ej that is visible as the brightest star at the bottom left of the image."[105]

"Such supernovae occur when the core of a massive star collapses due to its own gravity at the end of its life. This collapse results in a massive explosion that ejects material far into space. The resulting detonation can be more brilliant than the entire galaxy that hosts it and can be visible to observers for weeks, or even months."[105]

"PESSTO (Public ESO Spectroscopic Survey for Transient Objects) is designed to study objects that appear briefly in the night sky, such as supernovae. It does this by utilising a number of instruments on the NTT (New Technology Telescope), located at ESO's La Silla Observatory in Chile. This new picture of SN2013ej was obtained using the NTT during the course of this survey."[105]

"SN2013ej is the third supernova to have been observed in Messier 74 since the turn of the millennium, the other two being SN 2002ap and SN 2003gd. It was first reported on 25 July 2013 by the KAIT telescope team in California, and the first "precovery image" was taken by amateur astronomer Christina Feliciano, who used the public access SLOOH Space Camera to look at the region in the days and hours immediately before the explosion."[105]

"Messier 74, in the constellation of Pisces (The Fish), is one of the most difficult Messier objects for amateur astronomers to spot due to its low surface brightness, but SN2013ej should still be visible to careful amateur astronomers over the next few weeks as a faint and fading star."[105]

Radial velocity (cz) = 657 km/s.[49]

Morphological type = Sc.[49][106]

NGC 2841[edit]

Image created using the Aladin Sky Atlas software from the Astronomical Data Center of Strasbourg and SDSS public data in FIT format. Credit: Donald Pelletier.{{free media}}

NGC 2841 is an inclined, unbarred spiral galaxy exhibiting a prominent inner ring structure in the constellation Ursa Major discovered on 9 March 1788 by William Herschel.[107] Initially thought to be about 30 million light years distant, a 2001 Hubble Space Telescope survey of the galaxy's Cepheid variables determined that it was approximately 14.1 megaparsecs or 46 million light years distant.[108]

NGC 2841 is a giant spiral galaxy with properties similar to those of the Andromeda Galaxy.[108] It is a prototypical flocculent spiral galaxy, a type of spiral galaxy whose arms are patchy and discontinuous.[109]

NGC 2841 is home to large population of young blue stars, and few H II regions.[110]

NGC 2841 contains a low-ionization nuclear emission-line region (LINER), a type of region that is characterized by spectral line emission from weakly ionized atoms.[111]

New General Catalogue, NGC 2841 is a morphological type: Sa with a LINER-type Active Galaxy Nucleus.[112]

Radial velocity (cz) = 569.6 km/s.[49]

NGC 4594[edit]

This image of the Sombrero Galaxy is a mosaic of six images taken by the Hubble Space Telescope's Advanced Camera for Surveys in May and June 2003 (exposition time: 10.2 hours). Credit: NASA/ESA and The Hubble Heritage Team (STScI/AURA).{{free media}}

"NASA's Hubble Space Telescope has trained its razor-sharp eye on one of the universe's most stately and photogenic galaxies, the Sombrero galaxy, Messier 104 (M104). The galaxy's hallmark is a brilliant white, bulbous core encircled by the thick dust lanes comprising the spiral structure of the galaxy. As seen from Earth, the galaxy is tilted nearly edge-on. We view it from just six degrees north of its equatorial plane. This brilliant galaxy was named the Sombrero because of its resemblance to the broad rim and high-topped Mexican hat."[113]

"At a relatively bright magnitude of +8, M104 is just beyond the limit of naked-eye visibility and is easily seen through small telescopes. The Sombrero lies at the southern edge of the rich Virgo cluster of galaxies and is one of the most massive objects in that group, equivalent to 800 billion suns. The galaxy is 50,000 light-years across and is located 28 million light-years from Earth."[113]

"Hubble easily resolves M104's rich system of globular clusters, estimated to be nearly 2,000 in number - 10 times as many as orbit our Milky Way galaxy. The ages of the clusters are similar to the clusters in the Milky Way, ranging from 10-13 billion years old. Embedded in the bright core of M104 is a smaller disk, which is tilted relative to the large disk. X-ray emission suggests that there is material falling into the compact core, where a 1-billion-solar-mass black hole resides."[113]

"In the 19th century, some astronomers speculated that M104 was simply an edge-on disk of luminous gas surrounding a young star, which is prototypical of the genesis of our solar system. But in 1912, astronomer V. M. Slipher discovered that the hat-like object appeared to be rushing away from us at 700 miles per second. This enormous velocity offered some of the earliest clues that the Sombrero was really another galaxy, and that the universe was expanding in all directions."[113]

"The Hubble Heritage Team took these observations in May-June 2003 with the space telescope's Advanced Camera for Surveys. Images were taken in three filters (red, green, and blue) to yield a natural-color image. The team took six pictures of the galaxy and then stitched them together to create the final composite image. One of the largest Hubble mosaics ever assembled, this magnificent galaxy has an apparent diameter that is nearly one-fifth the diameter of the full moon."[113]

NGC 4594 is a morphological type: Sa galaxy with a LINER-type Active Galaxy Nucleus.[114]

Radial velocity (cz) = 1024 km/s.[49]

See also[edit]

References[edit]

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External links[edit]