Astronomy Picture of the Day
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2003 September 30 
Posted on 09/29/2003 10:02:23 PM PDT by petuniasevan
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2003 September 30
Explanation: Is our Milky Way Galaxy out to lunch? Recent wide field images and analyses now indicate that our home galaxy is actually still in the process of devouring its closest satellite neighbor. This unfortunate neighbor, the Sagittarius Dwarf galaxy, is now seen to be part of a larger Sagittarius Tidal Stream, a loose filament of stars, gas, and possibly dark matter that entangles the Milky Way. An artist's depiction of the stream is shown above. Speculation also holds that the Sagittarius Dwarf was once pulled through the Milky Way disk very close to our Sun's current location. An important resulting realization is that galaxies contain a jumble of clumps and filaments of both dim and dark matter.
![[SagDEG, contour map]](http://www.seds.org/messier/Pics/Jpg/sagdeg.jpg)
| Right Ascension | 18 : 55.1 (h : m) |
|---|---|
| Declination | -30:29 (deg : m) |
| Distance | 80.0 (kly) |
| Apparent Dimension | 190x490 (arc min) |
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). 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 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. There is already one Milky Way globular cluster which is suspected to have been captured from SagDEG: Palomar 12.
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:
"Lumps, Rings, and Tails in the Milky Way Galaxy"
Recent results from the Sloan Digital Sky Survey show that density distribution of the stars the Milky Way halo is anything but smooth. The Sagittarius dwarf galaxy is currently being shredded by the Milky Way's gravity, and tossing its stars across the Galaxy. Recent detection of a "Ring of Stars" around the Galaxy suggest there might be a second such structure in the process of being disbursed. These mergers "caught in the act" have strong implications for the formation mechanism of our galaxy, and galaxies in general. The tidal debris will be used to trace the dark matter potential of the Milky Way, and may even affect the experiments which are attempting to directly detect dark matter here on Earth.
A very interesting topic!
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