The Sagittarius Dwarf Elliptical Galaxy, SagDEG

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

Large Magellanic Cloud

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

Milky Way galaxy.

It was claimed that the Nordic Optical Telescope should have obtained a

photographic image of SagDEG.

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