Sagittarius Dwarf Galaxy
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The theory on the formation of galaxies currently having the widest acceptance implies that dwarf galaxies were the first to form in the Universe. Many of these then either went on to clump together to form larger galaxies or were gradually swallowed up by larger galaxies that continued to grow in this way. This process means the destruction of dwarf galaxies, such as is observed directly or indirectly in distant systems. It was suspected that our own Milky Way Galaxy also took part in this mechanism; in other words, that, being large galaxy, it had acquired a goodly portion of its mass by "devouring" smaller galaxies. In 1994, a new object now commonly referred to as the , Sagittarius Dwarf Galaxy was discovered very close to the Milky Way and located on the diametrically opposite side of the Galactic Centre from the Sun (Ibata et al, 1994). This satellite is the nearest known neighbor to the Milky Way, being presently situated within the outermost diffuse limits of the Milky Way as cloase as 49,000 lighty-yeras or 15,000 parsecs (pc) from the galactic center (1 kpc is equal to 3262 light years).
l'Observatory de Paris
(Permission being sought)
From the perspective of the Solar System,
the Sagittarius Dwarf is located on the
far side of the center of Milky Way (more).
About 0,000 ly in width, it can be found in (, ICRS 2000) Constellation Sagittarius, the -- . The Sagittarius dwarf galaxy is the Milky Way's nearest neighbor but was discovered only recently in 1994 because it was hidden from observers on Earth by the density of the Milky Way's foreground stars and dust. Located at only 78,000 ly (or 24,000 pc) from the Solar System on the far side of the Galactic Center (at about 52,000 ly or 16,000 pc from the Center), it is the nearest known satellite of the Milky Way. This companion galaxy to the Milky Way is actually on the far side of the Galactic Center. This little galaxy cannot be noticed on images of the central regions of the Milky Way, due to its extremely low surface brightness; there are too few stars that are members of this galaxy, compared to the many stars along the line-of sight that are in the Milky Way Galaxy.
V. Johnston, Chris Mihos,
Van Kleck Observatory/Wesleyan University
(Permission being sought)
Larger and similar simulation images.
The Sagittarius dwarf may have begun as a ball mass of stars,
at top center, before falling towards the Milky Way along the
dashed line and being ripped apart into long streamers along
the path (more on the Sagittarius dwarf and similar simulations).
This little galaxy was discovered in 1994 by Ibata, Gilmore and Irwin, during a study of stars in the central regions of our own Galaxy, the Milky Way. These astronomers studied stars in the little boxes that run parallel and perpendicular to the Galactic PLane in this image. They found that some of the stars in the lines-of-sight that run through the outline of the Sagittarius dwarf on the image were not moving as they should if they were in the central regions of the Milky Way. They realised they had found a little galaxy, hiding.
The morphology of the Sagittarius dwarf galaxy appears to be severely distorted. It may have begun as a ball of stars falling towards the Milky Way and then was torn apart by immense tidal forces. Numerical simulations suggest that stars ripped from the dwarf would spread out in long streamers along its path, as has been observed.
From the first, it was thought that the Sagittarius galaxy had reached an advanced state of destruction, and that a large part of its original matter now formed part of the Milky Way. It now seemed possible to view directly the destruction of a dwarf galaxy (Sagittarius) as it was engulfed by a large galaxy (the Milky Way); in other words, an opportunity had arisen to study in detail and at first hand the mechanism governing the formation of large galaxies. If this were the case, it should then be possible to find stars that originally formed part of the dwarf galaxy, and that would now be strewn along its entire orbit, thereby constituting two streams encircling the Milky Way. The problem was that these streams would extremely diffuse; so much so that they might turn out to be completely indistinguishable, even at short distances from the centre of Sagittarius itself.
In 1998, investigators from the University of Michigan found the apparent remains of one of the streams, that extending to the south west, and could trace it out to 34o from the centre of Sagittarius. Theoretical models predict the presence of another symmetric stream, extending to the north west, that could be so long as to encircle our Galaxy completely. However, this stream would be even more difficult to identify since it would cross the disc of the Milky Way and be hidden by the Galactic Centre. Using the 2.5 m Isaac Newton Telescope at Roque de los Muchachos Observatory on La Palma and relying on evidence supplied by their own dynamical models of Sagittarius and on preliminary results from the sky survey now being carried out by the international Sloan Digital Sky Survey team, researchers from the IAC have been the first to identify an excess of young stars belonging to a stellar system located at 56 kpc from the centre of the Milky Way. Its position in the sky indicates that we are probably dealing with debris belonging to the northwest current of Sagittarius 60o (equivalent to 65 kpc measured along the orbit of Sagittarius) from the centre of the dwarf galaxy. These remants are the furthest from the centre of a progenitor galaxy ever detected and confirm that the Sagittarius galaxy has formed an arc that completely surrounds our Galaxy, just as predicted by theoretical models. (More from IAC press release and David Martínez, Antonio Aparicio y Ricardo Carrera, and María Ángeles Gómez Flechoso, 2001.)
Born of the Magellanic Clouds?
Since the discovery of the Sagittarius dwarf galaxy, researchers have noticed that some of its younger stars are strikingly similar to stars in the Large Magellanic Cloud, another satellite galaxy that sits just a bit further out in space. Now a study led by Patrick Cseresnjes of the Paris Observatory shows strong similarities in a certain class of old stars seen in both of these satellite galaxies. Cseresnjes thinks the evidence may point to a common ancestor, a larger galaxy that was ripped apart to form both the Large Magellanic Cloud and the nearer Sagittarius dwarf galaxy, or Sgr as astronomers call it. The study required rooting out stars that could be clearly identified as belonging to Sgr. "In the direction of Sagittarius, most of the stars are located in our own galaxy, and there is no easy way to determine their distances," Cseresnjes said in an e-mail interview. "Finding a star of the Sagittarius dwarf galaxy is like picking a needle out of a haystack." Cseresnjes examined what are called RR Lyrae stars, about which enough is now known to separate them from the intervening thicket of Milky Way stars. The RR Lyrae stars are ancient, more than 10 billion years old, Cseresnjes explained, and so they provide clues about the environments from which they originated. Most important, these stars vary in brightness. In studying the period of this variation, Cseresnjes found a significant similarity in the distribution of stars with similar periods in both the Sgr and the Large Magellanic Cloud, known as LMC. The fraction of RR Lyraes in a given period range are the same in Sgr and the LMC, he said. He also compared the variable stars in these two galaxies to those in our other neighboring galaxies. "There are no two other dwarf galaxies showing such a high similarity," he said. Cseresnjes figures there are two possible explanations. "First, Sgr and the LMC could have been part of a larger galaxy which broke up into several pieces after colliding with the Milky Way," he said. It's unclear how such a collision could leave the two galaxies in their present configuration, however. The orbital plane of one is perpendicular to the other.
Fortunately, Sgr contains a fair amount of RR Lyrae stars. These variable stars have characteristic light curves and can easily be detected and separated from Galactic stars. Indeed, once their type is identified by their light curve, their absolute luminosity is derived, and the measure of their apparent luminosity gives their distance. Using two series of photographic plates, taken at La Silla (ESO) and digitized by the MAMA (operated at the CAI), Patrick Cseresnjes and his collaborators detected about 2000 RR Lyrae stars in Sgr spread over 50 square degrees. The spatial distribution of these stars allows to map the northern extension of Sgr, where the Galactic stars outnumber those of Sgr by a factor up to a thousand. Compared to other satellites of the Milky Way, Sgr seems to be much more massive and extended, even if one considers only the minor axis which is almost insensitive to Galactic tides.
Stellar evolution theory indicates that RR Lyraes are more than 10 Gigayears old. A catalogue of such stars offers therefore an unique opportunity to determine the progenitor of Sgr. The most obvious information available is the period which is very accurate and independent of crowding and extinction, allowing robust comparisons between different systems. Figure 3 compares the period distribution of RR Lyrae stars in Sgr with those of all other dwarf galaxies with a known RR Lyrae population. The similarity with the Large Magellanic Cloud (LMC) clearly stands out. This similarity is even more striking when one considers that there are no two other couple of distributions showing such a high correlation. Statistical tests show that an identical parent distribution for Sgr and the LMC cannot be ruled out, in spite of the high resolution provided by the large size of the samples in both systems.
he period of an RR Lyrae star is a complex function of its luminosity, temperature, metallicity and mass, leading to a high degeneracy between these different parameters. It is however possible to restrict the comparison to the mass-metallicity plane using RR Lyrae of a special type (type d, or RRd) which are pulsating simultaneously in the fundamental (P0) and first overtone (P1) radial modes. The position of an RRd star in the Petersen diagram (a plot of P1/P0 as a function of P0) is almost independent of its luminosity and temperature.
The Petersen diagrams for all RRd stars detected to date are shown in figure 4. For most systems, RRd stars are clumped in a specific region of the plot, reflecting an homogeneous population, as expected for simple systems such as globular clusters or dwarf spheroidal galaxies. However, this is not the case for Sgr which presents a large spread, showing that this system is more complex than a typical dwarf spheroidal galaxy. Again, there is a strong similarity between Sgr and the LMC.
The similarity between Sgr and the LMC is not restricted to RR Lyrae stars, but has also been observed through other populations like Carbon stars (Whitelock 1998) or Red Giant Branch stars (Cole 2001). These similarities strongly suggest that both systems have similar stellar populations. Numerical simulations show that a dwarf spheroidal galaxy can not survive more than a couple of Gigayears on such a low orbit, unless the progenitor is given an uncomfortable high concentration, inconsistent with observations. This result is in contradiction with the presence of a substantial number of RR Lyrae stars. This contradiction could be solved if Sgr is a debris pulled out of the LMC after a collision and has been injected on its present orbit only recently. Possible configurations are a collision between the LMC and the Galaxy or the Small Magellanic Cloud. This scenario, though attractive, raises many questions which need to be addressed. When did the collision occur? What happened to the gas? How can the present orbital planes of Sgr and the LMC seem to be perpendicular to each other? Future numerical simulations will assess the feasibility of this scenario.
More information and images of Andromeda may be available at NASA and IPAC's Extragalactic Database. See Professor Rosemary R.G. Wyse's presentation on "Galaxies in the Local Universe: The Fossil Record" and "The Merging History of the Milky Way Disk."
Up-to-date technical summaries on the Sagittarius dwarf galaxy may be available at: NASA's ADS Abstract Service for the Astrophysics Data System; the SIMBAD Astronomical Database mirrored from CDS, which may require an account to access; and the NSF-funded, arXiv.org Physics e-Print archive's search interface.
Sagittarius is Latin for "archer," often represented as a centaur wielding a bow and arrow since ancient times. The constellation also contains the Great Sagittarius Star Cloud, where a vast milky swarm of millions of stars mark the way to the center of the galaxy. For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Sagittarius. For another illustration, see David Haworth's Sagittarius.
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