Sombrero Galaxy
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| Galaxy | List of galaxies |
|---|---|
| Image:M104 ngc4594 sombrero galaxy hi-res.jpg
The Sombrero Galaxy (M104) as observed by the | |
| Observation data (Epoch J2000) | |
| Constellation | Virgo |
| Right ascension | 12h 39m 59.4s<ref name="ned" /> |
| Declination | -11° 37′ 23″<ref name="ned" /> |
| Redshift | 1024 ± 5 km/s<ref name="ned" /> |
| Distance | 30 Mly<ref name="fordetal1996">H. C. Ford, X. Hui, R. Ciardullo, G. H. Jacoby, K. C. Freeman (1996). "The Stellar Halo of M104. I. A Survey for Planetary Nebulae and the Planetary Nebula Luminosity Function Distance". Astrophysical Journal 458: 455-466.</ref><ref name="ajharetal1997">E. A. Ajhar, T. R. Lauer, J. L. Tonry, J. P. Blakeslee, A. Dressler, J. A. Holtzman, M. Postman (1997). "Calibration of the Surface Brightness Fluctution Method for use with the Hubble Space Telescope". Astronomical Journal 114: 626-634.</ref> |
| Type | SA(s)a<ref name="ned">NASA/IPAC Extragalactic Database. Results for NGC 4594. Retrieved on 2006-11-22.</ref> |
| Apparent dimensions (V) | 8′.7 × 3′.5<ref name="ned" /> |
| Apparent magnitude (V) | 9.0<ref name="ned" /> |
| Notable features | |
| Other designations | |
| PGC 42407,<ref name="ned" /> UGCA 293<ref name="ned" /> | |
The Sombrero Galaxy (also known as M104 or NGC 4594) is an unbarred Sa spiral galaxy in the constellation Virgo. It has a bright nucleus, an unusually large central bulge, and a prominent dust lane in its disk. The dark dust lane and the bulge give this galaxy the appearance of a sombrero. This 8th magnitude galaxy can easily be seen with small telescopes.
Contents |
[edit] History
[edit] Discovery
The Sombrero Galaxy was discovered in May 1781 by Pierre Méchain,<ref name="NSOG">Kepple, George Robert, Glen W. Sanner (1998). The Night Sky Observer's Guide, Volume 2. Willmann-Bell, Inc., 451. ISBN 0-943396-60-3.</ref> who described the object in a May 1783 letter to J. Bernoulli that was later published in the Berliner Astronomisches Jahrbuch. Charles Messier made a hand-written note about this and five other objects (now collectively recognized as M104 - M109) to his personal list of objects now known as the Messier Catalogue, but was not "officially" included until 1921. William Herschel independently discovered the object in 1784 and additionally noted the presence of a "dark stratum" in the galaxy's disk, what is now called a dust lane.<ref name="NSOG" /><ref name="jonesbook1991" /> Later astronomers were able to connect Méchain's and Herschel's observations.<ref name="jonesbook1991" />
[edit] Designation as a Messier object
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 the New General Catalogue, and Flammarion declared that it should be included in the Messier Catalogue. Since this time, the Sombrero Galaxy has been known as M104.<ref name="jonesbook1991" />
[edit] Early redshift measurements
In 1912, Vesto Slipher discovered that M104 has a large redshift. From this it was calculated that M104 is moving away from Earth at about 1,000 kilometers per second - a speed too high for any object residing in the Milky Way. This was one of the first solid clues that M104 was not a nebula, as was thought at the time, and that the universe is expanding in all directions.
[edit] Distance
The best distance measurements to M104 have been made by two groups using techniques involving planetary nebulae<ref name="fordetal1996" /> and surface brightness fluctuations<ref name="ajharetal1997" />. The average of these distance measurements generally place the galaxy at 9.2 Mpc, or approximately 30 Mly.
[edit] Dust ring
As noted above, this galaxy's most striking feature is the dust lane that crosses in front of the bulge of the galaxy. This dust lane is actually a symmetric ring that encloses the bulge of the galaxy.<ref name="bendoetal2006">G. J. Bendo, B. A. Buckalew, D. A. Dale, B. T. Draine, R. D. Joseph, R. C. Kennicutt Jr., K. Sheth, J.-D. T. Smith, F. Walter, D. Calzetti, J. M. Cannon, C. W. Engelbracht, K. D. Gordon, G. Helou, D. Hollenbach, E. J. Murphy, H. Roussel (2006). "Spitzer and JCMT Observations of the Active Galactic Nucleus in the Sombrero Galaxy (NGC 4594)". Astrophysical Journal 645: 134-147.</ref> Most of the cold atomic hydrogen gas<ref name="bajajaetal1984">E. Bajaja, G. van der Burg, S. M.; Faber, J. S. Gallagher, G. R. Knapp, W. W. Shane (1984). "The distribution of neutral hydrogen in the Sombrero galaxy, NGC 4594". Astronomy and Astrophysics 141: 309-317.</ref> and the dust<ref name="bendoetal2006" /> lies within this ring. The ring might also contain most of the Sombrero Galaxy's cold molecular gas,<ref name="bendoetal2006" /> although this is an inference based on observations with low resolution and weak detections.<ref name="bajajaetal1988">E. Bajaja, E. Hummel, R. Wielebinski, R.-J. Dettmar (1988). "The large-scale radio continuum structure of the Sombrero galaxy (NGC 4594)". Astronomy and Astrophysics 202: 35-40.</ref><ref name="youngetal1995">J. S. Young, S. Xie, L. Tacconi, P. Knezek, P. Viscuso, L. Tacconi-Garman, N. Scoville, S. Schneider, F. P. Schloerb, S. Lord, A. Lesser, J. Kenney, Y.-L. Huang, N. Devereux, M. Claussen, J. Case, J. Carpenter, M. Berry, L. Allen (1995). "The FCRAO Extragalactic CO Survey. I. The Data". Astrophysical Journal Supplement 98: 219-257.</ref> Additional observations are needed to confirm that the Sombrero galaxy's molecular gas is constrained to the ring. Based on infrared spectroscopy, the dust ring is the primary site of star formation within this galaxy.<ref name="bendoetal2006" />
[edit] Nucleus
The nucleus of the Sombrero galaxy is classified as a low ionization nuclear emission region (LINER).<ref name="hoetal1997">L. C. Ho, A. V. Filippenko, W. L. W. Sargent (1997). "A Search for "Dwarf" Seyfert Nuclei. III. Spectroscopic Parameters and Properties of the Host Galaxies". Astrophysical Journal Supplement 112: 315-390.</ref> These are nuclear regions where ionized 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 in star formation regions, whereas other LINER nuclei may be powered by active galactic nuclei (highly energetic regions that contain supermassive black holes). Infrared spectroscopy 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 for the weak ionization of gas in the Sombrero Galaxy.<ref name="bendoetal2006" />
[edit] Central supermassive black hole
In the 1990's, a research group led by John Kormendy demonstrated that a supermassive black hole is present within the Sombrero Galaxy.<ref name="kormendyetal1996">J. Kormendy, R. Bender, E. A. Ajhar, A. Dressler, S. M. Faber, K. Gebhardt, C. Grillmair, T. R. Lauer, D. Richstone, S. Tremaine (1996). "Hubble Space Telescope Spectroscopic Evidence for a 1 X 10 9 Mʘ Black Hole in NGC 4594". Astrophysical Journal Letters 473: L91-L94.</ref> Using spectroscopy data from both the CFHT and the Hubble Space Telescope, the group showed that the speed of rotation of the stars within the center of the galaxy could not be maintained unless a mass 1 billion times the mass of the Sun, or 109Mʘ, is present in the center. This is among the most massive black holes measured in any nearby galaxies.
[edit] Synchrotron emission
At radio and X-ray wavelengths, the nucleus is a strong source of synchrotron emission.<ref name="debruynetal1996">A. G. de Bruyn, P. C. Crane, R. M. Price, J. B. Carlson (1976). "The radio sources in the nuclei of NGC 3031 and NGC 4594". Astronomy and Astrophysics 46: 243-251.</ref><ref name="hummeletal1984">E. Hummel, J. M. van der Hulst, J. M. Dickey (1984). "Central radio sources in spiral galaxies - Starburst or accretion". Astronomy and Astrophysics 134: 207-221.</ref><ref name="theanetal2000">A. Thean, A. Pedlar, M. J. Kukula, S. A. Baum, C. P. O'Dea (2000). "High-resolution radio observations of Seyfert galaxies in the extended 12-μm sample - I. The observations". Monthly Notices of the Royal Astronomical Society 314: 573-588.</ref><ref name="dimatteoetal2001">T. Di Matteo, C. L. Carilli, A. C. Fabian (2001). "Limits on the Accretion Rates onto Massive Black Holes in Nearby Galaxies". Astrophysical Journal 547: 731-739.</ref><ref name="pellegrinietal2002">S. Pellegrini, G. Fabbiano, F. Fiore, G. Trinchieri, A. Antonelli (2002). "Nuclear and global X-ray properties of LINER galaxies: Chandra and BeppoSAX results for Sombrero and NGC 4736". Astronomy and Astrophysics 383: 1-13.</ref><ref name="pellegrinietal2003">S. Pellegrini, A. Baldi, G. Fabbiano, D.-W. Kim (2003). "An XMM-Newton and Chandra Investigation of the Nuclear Accretion in the Sombrero Galaxy (NGC 4594)". Astrophysical Journal 597: 175-185.</ref><ref name="krauseetal2006">M. Krause, R. Wielebinski, M. Dumke (2006). "Radio polarization and sub-millimeter observations of the Sombrero galaxy (NGC 4594). Large-scale magnetic field configuration and dust emission". Astronomy and Astrophysics 448: 133-142.</ref> Synchrotron emission is produced when high velocity electrons oscillate as they pass through regions with strong magnetic fields. This emission is actually quite common for active galactic nuclei. Although radio synchrotron emission may vary over time for some active galactic nuclei, the luminosity of the radio emission from the Sombrero Galaxy only varies 10-20%.
[edit] Unidentified submillimeter emission
In 2006, two groups published measurements of the submillimeter radiation from the nucleus of the Sombrero Galaxy at a wavelength of 850 micrometres.<ref name="krauseetal2006" /><ref name="bendoetal2006" /> This submillimeter emission was found not to originate from the thermal emission from dust (which is commonly seen at infrared and submillimeter wavelengths), synchrotron emission (which is commonly seen at radio wavelengths), bremsstrahlung emission from hot gas (which is uncommonly seen at millimeter wavelengths), or molecular gas (which commonly produces submillimeter spectral lines).<ref name="bendoetal2006" /> The source of the submillimeter emission remains unidentified.
[edit] Globular clusters
The Sombrero Galaxy has a relatively large number of globular clusters. Observational studies of globular clusters in the Sombrero Galaxy have produced estimates of the population in the range of 1200 to 2000.<ref name="wakamatsu1977">K.-I. Wakamatsu (1977). "Radial distribution and total number of globular clusters in M104". Publications of the Astronomical Society of the Pacific 89: 267-270.</ref><ref name="harrisetal1984">W. E. Harris, H. C. Harris, G. L. H. Harris (1984). "Globular clusters in galaxies beyond the local group. III NGC 4594 (the Sombrero)". Astronomical Journal 89: 216-223.</ref><ref name="bridgeshaynes1992">T. J. Bridges, D. A. Hanes (1992). "The globular cluster system of NGC 4594 (the Sombrero)". Astronomical Journal 103: 800-814.</ref> The ratio of the number of globular clusters to the total luminosity of the galaxy is high compared to the Milky Way and similar galaxies with small bluges, but the ratio is comperable to other galaxies with large bulges. These results have been repeatedly used to demonstrate that the number of globular clusters in galaxies is thought to be related to the size of the galaxies' bulges. The surface density of the globular clusters generally follows the light profile of the bulge except for near the center of the galaxy.<ref name="wakamatsu1977" /><ref name="bridgeshaynes1992" /><ref name="">S. S. Larsen, D. A. Forbes, J. P. Brodie (2001). "Hubble Space Telescope photometry of globular clusters in the Sombrero galaxy". Monthly Notices of the Royal Astronomical Society 327: 1116-1126.</ref>
[edit] Nearby galaxies and galaxy group information
The Sombrero Galaxy lies within a complex, filament-like cloud of galaxies that extends to the south of the Virgo Cluster.<ref name="nbg">R. B. Tully (1988). Nearby Galaxies Catalog. Cambridge: Cambridge University Press. ISBN 0-521-35299-1.</ref> However, it is unclear as to whether the Sombrero Galaxy is part of a formal galaxy group. Hierarchical methods for identifying groups, which determine group membership by considering whether individual galaxies belong to a larger aggragate of galaxies, typically produce results showing that the Sombrero Galaxy is part of a group that includes NGC 4487, NGC 4504, NGC 4802, UGCA 289, and possibly a few other galaxies.<ref name="nbg" /><ref name="fouqueetal1992">P. Fouque, E. Gourgoulhon, P. Chamaraux, G. Paturel (1992). "Groups of galaxies within 80 Mpc. II - The catalogue of groups and group members". Astronomy and Astrophysics Supplement 93: 211-233.</ref><ref name="giuricinetal2002">G. Giuricin, C. Marinoni, L. Ceriani, A. Pisani (2000). "Nearby Optical Galaxies: Selection of the Sample and Identification of Groups". Astrophysical Journal 543: 178-194.</ref> However, results that rely on the percolation method (i.e. 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<ref name="garcia1993">A. Garcia (1993). "General study of group membership. II - Determination of nearby groups". Astronomy and Astrophysics Supplement 100: 47-90.</ref> or that it may only be part of a galaxy pair with UGCA 287.<ref name="giuricinetal2002" />
[edit] See also
- NGC 1291 - a galaxy with an outer dust ring
- NGC 4725 - a similar galaxy with an extended dust ring
- NGC 7742 - a spiral galaxy with a similar dust ring
[edit] External links
- Messier 104, SEDS Messier pages
- HubbleSite: Hubble mosaic of the majestic Sombrero Galaxy
- NightSkyInfo.com - M104, the Sombrero Galaxy
[edit] References
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