Galaxies halos

The accretion history of a parent galaxy is constructed using a semi-analytical code. The full phase-space evolution during each accretion event is then followed separately with numerical simulations [2]. Star-formation and chemical evolution models are implemented within each satellite. The star formation prescription matches the number and luminosity of present-day galaxies in the Local Group, whereas the chemical evolution model takes into account the metal enrichment of successive stellar populations as well as feedback processes. Below we present results of a sample of four such simulated galaxy halos, denoted as Halos HI, H2, H3 and H4. 

The metallicity distribution function (MDF) in galaxy halos contains other important clues about the process of galaxy formation. In recent years, a wealth of MDF data have been collected for the Milky Way (eg. [3], [4]), M3I (eg. [I]),... 

Future work [7] will include a modeling of phase-space variations of a elements in galaxy halos and their satellites, using the chemical enrichment code of [10]. 

Perhaps connected with the cusp problem is the satellite problem, which is a mismatch between the observed and the simulated numbers of satellites in a galaxy halo. Too many satellites are predicted by the simulations. In reality, observations can detect only the luminous satellites while simulations contain all satellites, including the dark ones. It may be that many dark satellites do not shine, thus solving the satellite problem, but how and which satellites become luminous is not understood yet. 

Abundance results for additional clusters are currently underway and include analyses of the neutron-capture elements (in order to trace the onset of contributions from low-mass Type II SNe as well as AGB stars). Combined with their ages, the nucleosynthetic histories of the outer halo clusters will better constrain the timescales of formation and construction of the Galaxy. 

In the central region of Sculptor we have high resolution spectra providing direct abundance measurements for numerous elements (Hill et al., in prep). In Fig. 3 we show the preliminary results for the a-elements (Ca, Mg Ti) compared with similar observations of stars in our Galaxy. It is clear that the overall distribution of [a/Fe] versus [Fe/H] in Scl does not match our Galaxy, except for a small number of the most metal poor stars in Scl which overlap with Galactic halo stars. 

How will we identify the extra astrophysics required to reconcile the properties of CDM dark haloes with those of luminous galaxies We can start by developing knowledge of the evolutionary history of at least one place in at least one galaxy. We would be unlucky if that place were far from the norm alternatively, any theory that predicts such a history to be very unusual might be suspect -the galaxian Copernican principle. Kinematics and current spatial location are of course critical parameters, so that traditional stellar populations analyses are... 

That is, the straightforward interpretation of abundance data for Galactic field stars in terms of stellar populations is feasible only because the Galaxy apparently acquired its gas early, or at a rate which was well-matched to the star formation rate across the whole volume now sampled by local halo stars, and kept this gas well-mixed and because the stellar IMF is (close to) invariant over time and metallicity. Neither deduction was obvious, nor is the underlying physics understood. However, these two deductions apply so well they have become assumed authors use any violation to rule out some possible Galaxy merger histories, as in the Venn et al. analysis from which Figure 1 is taken. 

The halo globular cluster system also provides valuable information, since accurate distances, and hence reliable ages, can be derived. Mackey Gilmore (2004) recently acquired and compiled a new, nearly complete, internally consistent set of photometric studies of the globular cluster population in both the Milky Way and its satellite galaxies they deduce, from analysis of HB morphol-ogy, age, abundance and structural information, somewhat more relaxed limits... 

A New Class of Type la Supernovae Inferred from Abundance Patterns of Halo Stars and High-z Galaxies... 

Based on the ACDM concordance cosmological model and adopting the maximum circular velocity as the rotational velocity at the last measured point, we have obtained the DM mass of the galaxy (3.4 x 1010Mq). We use the recipe for the halo mass assembly history described by van den Bosch (2002) and the effect that reionization has on the infalling gas (Kravtsov et al., 2004) to derive the rate at which gas is accreted by the galaxy. 

It is currently very fashionable to identify new fragments of accretion events in the outer Galactic halo, each of which becomes evidence to support some concept of accretion onto the galaxy as an important process. Yet there has been strong evidence for many years, now made irrefutable by the abundance... 

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