Sagittarius Dwarf Elliptical Galaxy

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Sagittarius Dwarf Elliptical Galaxy[1]
Observation data (J2000 epoch)
Constellation Sagittarius
Right ascension 18h 55m 19.5s[2]
Declination -30° 32′ 43″[2]
Redshift 140 ± ? km/s[2]
Distance 65 ± 7 kly (20 ± 2 kpc)[3][4]
Type dSph(t)[2]
Apparent dimensions (V) 450′.0 × 216′.0[2]
Apparent magnitude (V) 4.5[2]
Notable features Heading for a collision
with the Milky Way
Other designations
SagDEG,[5] Sgr dSph,[2] Sagittarius Dwarf Spheroidal,[2] Sgr I Dwarf[2]
See also: Galaxy, List of galaxies

The Sagittarius Dwarf Elliptical Galaxy (SagDEG) is an elliptically looped shaped satellite galaxy of the Milky Way Galaxy. The main cluster which, in 1994, was the first to be discovered, is roughly 10,000 light-years in diameter, and is currently about 70,000 light-years from Earth and traveling in a polar orbit at a distance of about 50,000 light-years from the core of the Milky Way (about 1/3 the distance of the Large Magellanic Cloud). Sag DEG should not be confused with Sag DIG, the Sagittarius Dwarf Irregular Galaxy, a small galaxy over 4 million light-years distant.

Contents

[edit] Features

Further discoveries by astrophysics teams from both the University of Virginia and the University of Massachusetts, drawing upon the 2MASS Two-Micron All Sky Infrared Survey data, revealed the entire loop-shaped structure. In 2003 with the aid of infrared telescopes, and super computers, Steven Majewski, Michael Skrutskie, and Martin Weinberg were able to help create a new star map distinguishing the full Sagittarius Dwarf presence, position, and looping shape out from the mass of background stars and found this smaller galaxy to be at a near right angle to the plane of the Milky Way.[6]

Although it is one of the closest companion galaxies to the Milky Way, the main parent cluster is on the opposite side of the galactic core from Earth, and consequently is very faint, although it covers a large area of the sky. Officially discovered in 1994, by Rodrigo Ibata, Mike Irwin, and Gerry Gilmore, Sag DEG was immediately recognized as being the nearest known neighbor to our Milky Way at the time. Sag DEG appears to be an older galaxy, with little interstellar dust and composed largely of Population II stars, older and metal-poor, as compared to the Milky Way.

Based on its current trajectory, the Sag DEG main cluster is poised to pass through the galactic disc of the Milky Way within the next 100 million years, while the extended looped shaped ellipse is actually even now extended around and through our local space and on through the Milky Way galactic disc in process of slowly being absorbed into the larger galaxy, calculated at 10,000 times the mass of Sag DEG.

[edit] Globular clusters

Sag DEG has four known globular clusters with one, M54, apparently residing at its core. It is also dynamically linked to the "young" globular Terzan 7 as well as to Terzan 8 and Arp 2.[7] Additionally, Palomar 12 is now generally thought to also be associated with Sag DEG (Cohen 2004, Sbordone et al. 2006) as well as as Whiting 1 (Carraro et al. 2007).[8]

[edit] Metallicity

Sag DEG has multiple stellar populations, ranging in age from the oldest globular clusters (15 Gyr) to trace populations as young as several hundred Myr. It also exhibits an age-metallicity relationship, in that its old populations are metal poor ([Fe/H]=-1.6 ± 0.1) while its youngest populations have super-solar abundances.[8][9]

[edit] Geometry

At first, many astronomers thought that Sag DEG had already reached an advanced state of destruction, so that a large part of its original matter was already mixed with that of the Milky Way. However, Sag DEG still has coherence as a dispersed elongated ellipse, and appears to move in a roughly polar orbit around the Milky Way as close as 50,000 light-years from the galactic core. Although it may have begun as a ball of stars before falling towards the Milky Way, Sag DEG is now being torn apart by immense tidal forces over hundreds of millions of years. Numerical simulations suggest that stars ripped out from the dwarf would be spread out in a long stellar stream along its path, which were subsequently detected.

However, some astronomers contend that Sag DEG has been in orbit around the Milky Way for some billions of years, and has already orbited it around ten times. Its ability to retain some coherence despite such strains would indicate an unusually high concentration of dark matter within that galaxy.

From its discovery until 2003, it was considered to be the closest outside galaxy to Earth, but since then has been overtaken by the newly discovered Canis Major Dwarf Galaxy.

In 1999, Johnston et al. concluded that SagDEG has orbited the Milky Way for at least 1 Gyr and that during that time its mass has decreased by a factor of 2 or 3. Its orbit is found to have galactocentric distances that oscillate between ~13 and ~41 kpc with a period of 550 to 750 Myr. The last perigalacticon was ~50 Myr ago. Also in 1999, Jiang & Binney found that it may have started its infall into the Milky Way at a point more than 200 kpc away if its starting mass was as large as ~1011 M☉. The models of both its orbit and the Milky Way's potential field could be improved by proper-motion observations of SagDEG's stellar debris. No neutral hydrogen gas related to SagDEG was found by Burton & Lockman in 1999.[10]

[edit] See also

[edit] References

  1. ^ SIMBAD Astronomical Database. Results for Sagittarius Dwarf Spheroidal. Retrieved on 2006-11-28.
  2. ^ a b c d e f g h i NASA/IPAC Extragalactic Database. Results for Sagittarius Dwarf Spheroidal. Retrieved on 2006-11-28.
  3. ^ I. D. Karachentsev, V. E. Karachentseva, W. K. Hutchmeier, D. I. Makarov (2004). "A Catalog of Neighboring Galaxies". Astronomical Journal 127: 2031-2068. doi:10.1086/382905. 
  4. ^ Karachentsev, I. D.; Kashibadze, O. G. (2006). "Masses of the local group and of the M81 group estimated from distortions in the local velocity field". Astrophysics 49 (1): 3-18. doi:10.1007/s10511-006-0002-6. 
  5. ^ Sagittarius Dwarf Elliptical Galaxy / SagDEG
  6. ^ http://www.astro.virginia.edu/~mfs4n/sgr/sgr_big.jpg
  7. ^ Sbordone, L.; Bonifacio, P.; Marconi, G.; Buonanno, R. & Zaggia, S. (July 3, 2005), “Family ties: Abundances in Terzan 7, a Sgr dSph globular cluster”, Astronomy and Astrophysics 437 (3): 905-910, <http://adsabs.harvard.edu/abs/2005A%26A...437..905S> 
  8. ^ a b Geisler, Doug; Wallerstein, George; Smith, Verne V. & Casetti-Dinescu, Dana I. (September 2007), “Chemical Abundances and Kinematics in Globular Clusters and Local Group Dwarf Galaxies and Their Implications for Formation Theories of the Galactic Halo”, The Publications of the Astronomical Society of the Pacific 119 (859): 939-961, <http://adsabs.harvard.edu/abs/2007PASP..119..939G> 
  9. ^ Siegel, Michael & et al. (September 2007), “The ACS Survey of Galactic Globular Clusters: M54 and Young Populations in the Sagittarius Dwarf Spheroidal Galaxy”, Astrophysical Journal Letters 667 (1): 57-60, <http://adsabs.harvard.edu/abs/2007ApJ...667L..57S> 
  10. ^ van den Bergh, Sidney (April 2000), “Updated Information on the Local Group”, The Publications of the Astronomical Society of the Pacific 112 (770): 529-536, <http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2000PASP..112..529V> 

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