It has a bright nucleus, an unusually large central bulge, and a prominentdust lane in its outer disk, which from Earth is viewed almost edge-on. The dark dust lane and the bulge give it the appearance of asombrero hat (thus the name). Astronomers initially thought the halo was small and light, indicative of a spiral galaxy; but theSpitzer Space Telescope found that the halo was significantly larger and more massive than previously thought, indicative of a giantelliptical galaxy.[7]
The galaxy has anapparent magnitude of +8.0,[4] making it easily visible with amateur telescopes, and is considered by some authors to be the galaxy with the highestabsolute magnitude within a radius of 10 megaparsecs of the Milky Way.[8] Its large bulge, centralsupermassive black hole, and dust lane all attract the attention of professional astronomers.
The Sombrero Galaxy was discovered on May 11, 1781 byPierre Méchain, who described the object in a May 1783 letter to J. Bernoulli that was later published in theBerliner Astronomisches Jahrbuch.[9][10]Charles Messier made a handwritten note about this and five other objects (now collectively recognized as M104 – M109) to his personal list of objects now known as theMessier Catalogue, but it was not "officially" included until 1921.[10]William Herschel independently discovered the object in 1784 and additionally noted the presence of a "dark stratum" in the galaxy's disc, what is now called a dust lane.[9][10] Later astronomers were able to connect Méchain's and Herschel's observations.[10]
In 1921,Camille Flammarion found Messier's personal list of the Messier objects including the hand-written notes about the Sombrero Galaxy. This was identified with object 4594 in theNew General Catalogue, and Flammarion declared that it should be included in the Messier Catalogue. Since this time, the Sombrero Galaxy has been known asM104.[10]
As noted above, this galaxy's most striking feature is thedust lane that crosses in front of the bulge of the galaxy. This dust lane is actually a symmetrical ring that encloses the bulge of the galaxy.[11] Most of the cold atomic hydrogen gas[12] and the dust[11] lie within this ring. The ring might also contain most of the Sombrero Galaxy's cold molecular gas,[11] although this is an inference based on observations with low resolution and weak detections.[13][14] Additional observations are needed to confirm that the Sombrero galaxy's molecular gas is constrained to the ring. Based oninfraredspectroscopy, the dust ring is the primary site of star formation within this galaxy.[11]
The nucleus of the Sombrero Galaxy is classified as alow-ionization nuclear emission-line region (LINER).[15] These are nuclear regions whereionized gas is present, but the ions are only weakly ionized (i.e. the atoms are missing relatively few electrons). The source of energy for ionizing the gas in LINERs has been debated extensively. Some LINER nuclei may be powered by hot, young stars found instar formation regions, whereas other LINER nuclei may be powered byactive galactic nuclei (highly energetic regions that containsupermassive black holes).Infraredspectroscopy observations have demonstrated that the nucleus of the Sombrero Galaxy is probably devoid of any significant star formation activity. However, a supermassive black hole has been identified in the nucleus (as discussed in the subsection below), so this active galactic nucleus is probably the energy source that weakly ionizes the gas in the Sombrero Galaxy.[11]
In the 1990s, a research group led by John Kormendy demonstrated that asupermassive black hole is present within the Sombrero Galaxy.[16] Usingspectroscopy data from both theCFHT and theHubble Space Telescope, the group showed that the speed of revolution of the stars within the center of the galaxy could not be maintained unless a mass 1 billion times that of theSun, 109M☉, is present in the center.[16] This is among the most massive black holes measured in any nearby galaxy, and is the nearest billion-solar-mass black hole to Earth.
Atradio andX-ray wavelengths, the nucleus is a strong source ofsynchrotron radiation.[17][18][19][20][21][22][23] Synchrotron radiation is produced when high-velocity electrons oscillate as they pass through regions with strongmagnetic fields. This emission is quite common foractive galactic nuclei. Although radio synchrotron radiation may vary over time for some active galactic nuclei, the luminosity of the radio emission from the Sombrero Galaxy varies only 10–20%.[17]
In 2006, two groups published measurements of theterahertz radiation from the nucleus of the Sombrero Galaxy at a wavelength of850 μm.[11][23] This terahertz radiation was found not to originate from the thermal emission from dust (which is commonly seen atinfrared and submillimeter wavelengths),synchrotron radiation (which is commonly seen atradio wavelengths),bremsstrahlung emission from hot gas (which is uncommonly seen at millimeter wavelengths), or molecular gas (which commonly produces submillimeter spectral lines).[11] The source of the terahertz radiation remains unidentified.
The Sombrero Galaxy has a relatively large number ofglobular clusters, observational studies of which have produced population estimates in the range of 1,200 to 2,000.[24][25][26] The ratio of globular clusters to the galaxy's total luminosity is high compared to theMilky Way and similar galaxies with small bulges, but comparable to other galaxies with large bulges. These results have often been used to demonstrate that the number of a galaxy's globular clusters is thought to be related to the size of its bulge. The surface density of the globular clusters generally follows the bulge's light profile, except near the galaxy's center.[24][26][27]
At least two methods have been used to measure the distance to the Sombrero Galaxy.
The first method relies on comparing the measuredfluxes from the galaxy'splanetary nebulae to the knownluminosity of planetary nebulae in theMilky Way. This method gave the distance to the Sombrero Galaxy as 29 ± 2 Mly (8,890 ± 610 kpc).[28]
The second method is thesurface brightness fluctuations method, which uses the grainy appearance of the galaxy's bulge to estimate the distance to it. Nearby galaxy bulges appear very grainy, while more distant bulges appear smooth. Early measurements using this technique gave distances of 30.6 ± 1.3 Mly (9,380 ± 400 kpc).[29] Later, after some refinement of the technique, a distance of 32 ± 3 Mly (9,810 ± 920 kpc) was measured.[30] This was even further refined in 2003 to 29.6 ± 2.5 Mly (9,080 ± 770 kpc).[31]
The average distance measured through these two techniques is 29.3 ± 1.6 Mly (8,980 ± 490 kpc).[a]
The mass of M104 is estimated to be 800 billion solar masses[32]
The galaxy'sabsolute magnitude (in the blue) is estimated as −21.9 at 30.6 Mly (9,400 kpc) (−21.8 at the average distance of above)—which, as stated above, makes it the brightest galaxy in a radius of 32.6 Mly (10,000 kpc) around the Milky Way.[8]
The Sombrero Galaxy lies within a complex,filament-like cloud of galaxies that extends to the south of theVirgo Cluster.[33] However, it is unclear whether it is part of a formalgalaxy group. Hierarchical methods for identifying groups, which determine group membership by considering whether individual galaxies belong to a larger aggregate of galaxies, typically produce results showing that the Sombrero Galaxy is part of a group that includesNGC 4487,NGC 4504,NGC 4802,UGCA 289, and possibly a few other galaxies.[33][34][35] However, results that rely on thepercolation method (also known as the friends-of-friends method), which links individual galaxies together to determine group membership, indicate that either the Sombrero Galaxy is not in a group[36] or that it may be only part of a galaxy pair withUGCA 287.[35]
The Sombrero Galaxy is 11.5° west ofSpica[10] and 5.5° north-east ofEta Corvi.[38] Although it is visible with 7×35binoculars or a 4-inch (100 mm) amateur telescope,[38] an 8-inch (200 mm) telescope is needed to distinguish the bulge from the disk,[10] and a 10- or 12-inch (250 or 300 mm) telescope to see the dark dust lane.[10]
^Bajaja, E.; Van Der Burg, G.; Faber, S. M.; Gallagher, J. S.; et al. (1984). "The distribution of neutral hydrogen in the Sombrero galaxy, NGC 4594".Astronomy & Astrophysics.141:309–317.Bibcode:1984A&A...141..309B.
^Bajaja, E.; Dettmar, R.-J.; Hummel, E.; Wielebinski, R. (1988). "The large-scale radio continuum structure of the Sombrero galaxy (NGC 4594)".Astronomy & Astrophysics.202:35–40.Bibcode:1988A&A...202...35B.
^Fouque, P.; Gourgoulhon, E.; Chamaraux, P.; Paturel, G. (1992). "Groups of galaxies within 80 Mpc. II – The catalogue of groups and group members".Astronomy & Astrophysics.93:211–233.Bibcode:1992A&AS...93..211F.
^Garcia, A. (1993). "General study of group membership. II – Determination of nearby groups".Astronomy & Astrophysics.100:47–90.Bibcode:1993A&AS..100...47G.
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