LOCAL CLUES

Course: Galaxy formation and evolution: clues in the local Universe

Prof. Ken Freeman
Mt. Stromlo Observatory
AUSTRALIA

When studying high redshift galaxies one should never forget that nearby galaxies contain fossil remnants of their past star formation and thus valuable cosmological clues. Many of these galaxies have a higher current star formation rate than their time average, and are evidence for dramatic local effects similar to those sought by astronomers in old, distant galaxies. As Prof. Freeman, from the Mount Stromlo Observatory, Australia, expresses it in the following interview, "high redshift studies give us some broad features of early galaxy evolution, but a fuller understanding needs more detailed investigations that are possible only in nearby galaxies."

At the present time when the HDF and other deep surveys are providing a direct view of galaxies when the Universe was 5-10% of its current age, what does the study of nearby galaxies contribute to our knowledge of galaxy evolution? Is it still worth while to look at nearby galaxies, or should all young researchers focus on the distant Universe?

"I think the high-z studies and study of nearby galaxies are complementary: we need both. At high redshifts, we can see directly some of the events that will define the properties of nearby galaxies, like mergers and the early star formation history. But there are many things that we cannot study well at high redshift, even with the large telescopes.

For example, say one is interested in understanding the dark matter content of galaxies. For this, rotation curves and velocity dispersions are needed. This is now becoming possible with difficulty at z = 1 but this is not really an interesting redshift for our problem. It would be nice to know the properties of dark halos at z > 3 but this is some way off. Useful dark matter studies can only be done out to quite modest redshifts.

Another example is the chemical evolution of galaxies. Here we are interested to understand what processes produced the chemical enrichment at different times. This kind of work needs very detailed high resolution spectroscopy of the oldest stars in the galaxy (which themselves probably formed at quite high redshifts), to measure the ratios of elements produced in different processes. These observations can only be done for stars in our Galaxy.

There are many examples one could think of in answer to this question. In summary, I think that high redshift studies give us some broad features of early galaxy evolution, but a fuller understanding needs more detailed investigations that are possible only in nearby galaxies."

Do we know what the Milky Way looked like at the age sampled by the HDF’s z=3 galaxies? Do we know (a) whether it was a single galaxy or a group on its way to merging? (b) whether it had a bulge?(c) whether it had a disk? (d) whether it had a (stellar) halo? (e) whether the disk or the bulge have grown significantly since then?

"(a) I think our Galaxy was very likely a multiple system at z = 3. The stellar content of the galactic halo indicates an extended history of minor mergers. Some people argue that there have been more substantial mergers in the past, based on the presence of high metallicity young stars in the halo. The fact that the galactic bulge is not large argues against a major merger at early times. So I would think that our Galaxy had a collection of companions, maybe of the mass of the LMC and smaller, at z = 3.

(b) Our galaxy has a fairly classic small boxy bulge. The current view is that these boxy bulges are bars (and we know from other arguments that the galactic bulge is barlike). These boxy bulge/bars are now believed to originate from instabilities of the disk. Few bars are seen at high redshift, so my guess is that the Galaxy did not have a bulge at z = 3.

(c) I suspect that the Galaxy was starting to assemble its disk at z = 3 but that there would not yet have been time for the disk to settle.

(d) Like most people in this field, I think that the metal poor stellar halo of the Galaxy formed from small accreted satellites. If this is correct, then halo building probably had started at z = 3: we know that it is still going on now.

(e) In the picture described above for the bulge, the bulge is something that would have formed from the disk, after the disk had settled enough to become unstable. And star formation in the disk has been going on at roughly the same rate for the last 10 Gyr. So the bulge and the disk would have grown significantly since z =3."

Nearby galaxy NGC 1300
(image information)

Australia has made key contributions to the study of galaxies at optical and radio wavelengths, but competition is becoming fiercer with the VLT coming into operation. What are the plans for the future of Australian astronomy?

"At optical wavelengths, we are minor partners in Gemini and have hopes for other sources of large telescope time. Without this, the future does not look good for us. At (lower frequency) radio wavelengths, we are very active participants in the SKA project."

In the past decade, an amazing wealth of new information on distant galaxies has been gathered. In your view, does the emerging picture of galaxy formation fit in a cold dark matter cosmology? Do we need a new cosmology, or can we amend CDM to make it fit the new data?

"I am not the right person to say much on this subject. From what I read, CDM does need some adjustment to get the structure (and maybe substructure) of dark halos right."

PROFILE

Ken Freeman was born in Perth, Australia, in 1940.

He studied Mathematics at the University of Western Australia and Theoretical Astrophysics at the University of Cambridge (UK), followed by a postdoctoral year at McDonald Observatory (University of Texas) with G. de

Vaucouleurs, and another year in Cambridge as a fellow of Trinity

College. In 1967 he moved to Mount Stromlo Observatory where he is now Professor.

His research interests are in the formation and dynamics of galaxies and globular star clusters, and he is particularly interested in the problem of dark matter in galaxies: he was one of the first to point out that spiral galaxies contain a large fraction of dark matter.

Freeman worked for a year in the Kapteyn Institute in Groningen (1976) and was the 1990 Aaronson Lecturer at the University of Arizona and the

1994 Oort Professor at Leiden University. In 1997 he was a visiting fellow of Merton College, Oxford. He holds an appointment as Distinguished Visiting Scientist at the Space Telescope Science

Institute in Baltimore. He is very interested in graduate students and has supervised the thesis work of more than 30 PhD students in Australia, the US and the Netherlands.

Ken Freeman is President of Commission 33 (Galactic Structure) and Division VII of the International Astronomical Union. He is a Fellow of the Australian Academy of Science and the Royal Society of London, and won the Dannie Heineman prize of the American Institute of Physics and the American Astronomical Society for 1999.