[M54]

The Sagittarius Dwarf Elliptical Galaxy, SagDEG

Dwarf Elliptical Galaxy SagDEG (also Sagittarius dSph) in Sagittarius

[SagDEG, contour map]

Right Ascension 18 : 55.1 (h : m)
Declination -30:29 (deg : m)
Distance 88.0 (kly)
Apparent Dimension 190x490 (arc min)
In 1994, R. Ibata, M. Irwin, and G. Gilmore found this small Local Group galaxy by stellar brightness density investigations (see also e.g. the August 1994 issues of Astronomy or Sky & Telescope or the German Sterne und Weltraum). This galaxy was immediately recognized as being the nearest known neighbor to our Milky Way, significantly closer than the Large Magellanic Cloud which was considered to be our closest companion until than. It held the title of our nearest intergalactic neighbor for nine years, but lost it in November 2003 to the then newly discovered Canis Major Dwarf.

This dwarf galaxy is called SagDEG (for Sagittarius Dwarf Elliptical Galaxy), or sometimes Sagittarius Dwarf Spheroidal Galaxy; don't confuse it with another member, SagDIG (Sagittarius Dwarf Irregular Galaxy). It is strongly recommended to avoid misleading designations such as "Sagittarius Dwarf" (which is an older designation for SagDIG), "Sagittarius I Dwarf", or similar ambiguous names for this galaxy, although they occasionally occur in websites, databases, articles and papers.

These are two minor galaxies in the same constellation Sagittarius, which are of different type: The difference between these types is that dwarf irregulars still have interstellar matter and/or young stars while the dwarf elliptical have only an old yellowish stellar population. From its stellar contents, it is resembling other low surface brightness members of the local group such as the Sculptor dwarf galaxy, but it is so highly obscured that it was hidden up to the 1994 investigation.

SagDEG is one of the most recently discovered members of the Local Group, and is currently in a very close encounter to our Milky Way galaxy. It is apparently in process of being disrupted by tidal gravitational forces of its big massive neighbor in this encounter. Nevertheless it is apparently big: 5x10 degrees in the sky.

Globular cluster M54 coincides with one of the galaxy's two bright knots, and is also receding at about the same velocity. It may also be at the same distance (about 88,000 light years), so probably M54 is the first "extragalactic" globular ever discovered (by Charles Messier in 1778), or a recent immigrant to the globular cluster system of our Milky Way galaxy. When SagDEG will be disrupted after the current encounter, M54 and the other at least three globulars of this dwarf (Arp 2, Terzan 7 and Terzan 8, which are all much fainter than M54) will be the "remnants", while the other stars will be spread over the galactic halo, or escape as intergalactic travelers. The globulars will perhaps be captured and find their place in the halo of the Milky Way galaxy. There are several other Milky Way globular clusters which are suspected to have been captured from SagDEG: NGC 4147, Palomar 2, Palomar 12, and recently discovered Whiting 1 (Van den Bergh 2007).

Because of the extreme intrinsic luminosity of M54 in comparison to the other globular clusters associated with SagDEG, it has been speculated early that M54 may be the nucleus of this dwarf galaxy, or the remnant of its nucleus (Bassino and Muzzio, 1995).

In February 1998, a team of astronomers headed by Rosemary Wyse of John Hopkins University found that SagDEG orbits the Milky Way Galaxy in less than one billion years. Because it must have passed the dense central region of our Galaxy at least about ten times, it is surprising that the dwarf has not been disrupted for so far. Astronomers suspect that this fact is an indication for significant amounts of dark matter within this small galaxy, which ties the stars stronger to the galaxy by its gravity. We have their press release here, or you can read their original report online.

References:

  • SIMBAD data for SagDEG
  • NED data for SagDEG
  • Publications on SagDEG (NASA ADS)

    Hartmut Frommert
    Christine Kronberg
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    Last Modification: May 2, 2007