Research School of Astronomy & Astrophysics


SCIENTIFIC HIGHLIGHTS

Dwarf Spheroidal Galaxies and the Formation of the Galactic Halo

At the present time our Milky Way Galaxy has nine small satellite galaxies. They are classified as dwarf spheroidal (dSph) systems and are named after the constellation in which they are found on the sky. The words 'at the present time' are appropriate because it is unclear whether these galaxies represent most, or only a small fraction, of the dSph satellites that existed early in the life of the Galaxy. For example, the closest dSph satellite, Sagittarius, is currently being disrupted by the gravitational force of the Milky Way. Within a billion years or so this dSph will no longer be recognizable as a distinct object - it will have merged with the general population of the galactic halo.

Y (kpc)

X (kpc) galactic center (x,y,z)=(0,0,0)

Figure 1: Model Halo. A computer simulation of how the disruption of a large number of dwarf spheroidal galaxies might contribute to the formation of the Galaxy's halo. Each set of points represents stars originally in a dSph galaxy. Disruption of the dSph by the gravitational field of the Milky Way spreads the stars into 'star streams' which lie predominantly along the dSph's orbit around the Galaxy. Searches are currently underway to ascertain if any such star streams, other than that from the Sagittarius dSph, are present in the galactic halo. A kpc (kiloparsec) is 3 x 1016 km. The Sun's distance from the centre of the Galaxy is 8 kpc. Figure courtesy of Paul Harding, Steward Observatory, University of Arizona.

The discovery of this dSph disruption event has led to the speculation that perhaps most of the stars in the galactic halo have come from disrupted dSph satellites. If this is the case then an external view of the galactic halo might look something like the illustration depicted in Figure 1. It shows the many 'star streams' that result from the disruption of a large number of dSph galaxies that have been assumed to exist early in the life of the Galaxy. Observational searches involving RSAA astronomers are currently underway to find such 'star streams', if they exist.

The properties of the current dSph galaxies which, if the above scenario is correct, are the survivors from an initially large population of such systems, were reviewed by Da Costa in his contribution at the Third Stromlo Symposium. While much work remains to be done, it appears that at least some stars in the current dSph galaxies have spectral


Annual Report 1998


signatures that are different from similar stars in the general field of the galactic halo. For example, some dSph red giants show correlated anomalies in the abundances of the elements C, N, O, Na, Al and Mg which are not seen among field halo red giants. It might then be possible to use these signatures as an alternative way to assess the fraction of the general galactic halo that has come from the disruption of dSph galaxies.

Da Costa also reviewed recent results for the ages of the oldest stars in the satellite dSphs. Comparing them with similar data for the oldest stars in Galactic globular clusters (both nearby clusters and one in the extremity of the halo), and with globular clusters in the Large Magellanic Cloud (LMC), produced a very interesting and surprising outcome. It seems that over the entire volume occupied by the proto-Galaxy, which is many times larger than the Galaxy's current size and which contained the objects that became the globular clusters, the dSph satellites and the LMC, the initiation of star formation among these disparate objects was extremely well synchonized. How this synchronization, which is analogous to 'turning all the lights on at once', was achieved remains a puzzle awaiting a solution.

The Magellanic System

Recent data from the HI Parkes All-Sky Survey (HIPASS) have revealed that the gravitational forces from the Milky Way are tearing apart our neighbouring galaxies, the Magellanic Clouds. HIPASS is a survey for neutral hydrogen across the southern sky (Figure 2), which is being completed with the Parkes Radio Telescope equipped with a new multibeam receiver.

Figure 2: Putman, Gibson and Staveley-Smith, together with the Multibeam Working Group, have examined the first results from the HI Parkes All-Sky Survey (HIPASS) in the region of the South Celestial Pole (SCP), and find that it provides a new and spectacular view of the neutral hydrogen distribution in the vicinity of the Magellanic Clouds and the southern Milky Way. The features found in this figure (labelled Leading Arm and Stream) suggest that our Galaxy (labelled the Galactic Plane) is tearing apart its companion galaxies (the LMC and SMC) with gravitational tides.


Research School of Astronomy & Astrophysics


Putman, Gibson and Staveley-Smith*, together with the Multibeam Working Group, imaged a large region of the HIPASS data about the South Celestial Pole and discovered a leading stream of gas which emanates from the Magellanic Clouds. This 'Leading Arm' is the counterpart to the trailing Magellanic Stream and is the missing link in the attempt to unravel the Milky Way's role in the Magellanic Clouds' demise.

The trailing Magellanic Stream, discovered by Mathewson twenty-five years ago, showed that the Milky Way was disrupting the Clouds, but it was unclear if the Stream had been stripped from the Clouds in a wake as they passed through the Galactic halo or if it had been pulled out with gravitational tides. The newly discovered leading stream is a natural consequence of tidal forces pulling on both sides of the Clouds; it is not possible to have a leading wake of gas. Thus HIPASS, with its unbiased and dense spatial sampling, has resolved the controversy over the Milky Way's favourite destruction mechanism and uncovered the origin of the Magellanic Stream, the largest neutral hydrogen feature in the sky.

This discovery emphasises the fact that our local universe is a violent place. The Magellanic Clouds are not peacefully orbiting our Galaxy, but are being destroyed by the gravitational potential of the Milky Way. Interaction among galaxies is a common phenomenon and by studying our own interaction in detail, we will hopefully gain insight into galaxy interactions throughout the universe.

Dark Matter in Galaxies

Kormendy* and Freeman have discovered that the properties of the dark matter "halos" that surround galaxies correlate with galaxy luminosity. The least luminous galaxies have lower densities of stars but much higher densities of dark matter, up to 100 times denser than the dark matter in the brightest galaxies. Their dark halos are smaller in size and the particles in them move more slowly. Also, the faintest galaxies have the largest fraction of dark matter, with 99% of their mass dark, and only 1% in stars.

Figure 3. Carina dwarf spheroidal galaxy


Annual Report 1998


Fig. 4. Sextans dwarf spheroidal galaxy

Figures 3 and 4 show two examples of very faint galaxies with very high dark matter densities. While the Carina dwarf is just visible on this very deep image from a composite of UK Schmidt plates, the Sextans dwarf is very difficult to detect without advanced computer processing These images were provided by David Malin of the Anglo-Australian Observatory.

These correlations provide new insights into how dark matter affects galaxy formation. The very high densities of the dark matter in the very faint galaxies mean that they probably formed very early in the life of the Universe, when the Universe was only 1/3000 of its present age and was itself very dense. They are pristine relics of the early Universe.

The faintest galaxies are the most numerous in the Universe, and many even darker galaxies are left to be discovered. They will be exceedingly difficult to find, and computerized searches for very low star densities will be needed. The properties of nearly invisible small galaxies suggest that even darker galaxies are likely to have sizes of a few hundred light years and masses of about 10 million suns. They may avoid bright galaxies because, when they formed, the gas that was destined to make the stars in the big galaxy would probably have provided enough gas for small nearby galaxies to form stars and become visible. Also, dark galaxies that formed close to big ones may now have been eaten by the larger galaxies.