A. Ferguson on "The Structure of Galaxies at Faint Levels"

Why does one want to look at the hard-to-study faint outskirts of galaxies? The stars in the halo of galaxies and tidal tails from dwarf galaxies give clues about how galaxies are built up. It is quite certain that large galaxies form hierarchically from smaller ones and since new star formation in the center makes relics harder to study, it is in the outer parts of the galaxies where one can look for evidence.

The amount and structure of tidal debris depends on the cosmological structrue, so modelling this with different parameters and comparing it to observations, can yield another test of lambda-CDM cosmology.

The problem is that this stuff is really faint and we are not talking about many stars. Only at surface brightness levels below 30 to 35 mag/arcsec^2, it shows up clearly, and these levels are rarely studied. The speaker presents a survey of the surroundings of M31 which shows a large stream of stars stretching to very large radii. While tidal streams indicate rather recent accretion events, but the stars coming from galaxies that have fallen in long ago have settled smothly into the halo.

Let me also mention that a collegue from Uppsala, Erik Zackrisson, studies the outskirts of Blue Compact Galaxies and finds a red excess in the near-infrared colors that is very hard to explain. Many explanations for this have been ruled out and what seems to be remaining is a bottom-heavy IMF, i.e. an unusually high number of small stars compared to big ones.