The Sagittarius Dwarf Elliptical Galaxy, SagDEG
Dwarf Elliptical Galaxy SagDEG
in Sagittarius
Right Ascension | 18 : 55.1 (h : m) |
---|---|
Declination | -30:29 (deg : m) |
Distance | 80.0 (kly) |
Apparent Dimension | 190×490 (arc min) |
In 1994, R. Ibata, M. Irwin, and G. Gilmore found this small
Local Group galaxy by stellar brightness
density investigations (see 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
which was considered to be our closest companion until than.
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
(Sag. Dwarf Irregular Galaxy).
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: 5×10 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 80,000 light years), so probably
M54 is the first extragalactic globular ever discovered
(by Charles Messier in 1778).
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
It was claimed that the Nordic Optical Telescope should have obtained a
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.
Last Modification: 16 Feb 1998, 20:35 MET