The Stellar Halo Metallicity - Luminosity Relationship for Spiral Galaxies

The stellar halos of spiral galaxies bear important chemo-dynamical signatures of galaxy formation. We present here the analysis of 89 semi-cosmological spiral galaxy simulations, spanning ~ 4 magnitudes in total galactic luminosity. These simulations sample a wide variety of merging histories and show significant dispersion in halo metallicity at a given total luminosity - more than a factor of ten in metallicity. Our preliminary analysis suggests that galaxies with a more extended merging history possess halos which have younger and more metal rich stellar populations than the stellar halos associated with galaxies with a more abbreviated assembly. A correlation between halo metallicity and its surface brightness has also been found, reflecting the correlation between halo metallicity and its stellar mass. Our simulations are compared with recent Hubble Space Telescope observations of resolved stellar halos in nearby spirals.Comment: 5 pages, 4 figures. MNRAS Letters, in pres

Galactic Chemical Evolution

The primary present-day observables upon which theories of galaxy evolution are based are a system's morphology, dynamics, colour, and chemistry. Individually, each provides an important constraint to any given model; in concert, the four represent a fundamental (intractable) boundary condition for chemodynamical simulations. We review the current state-of-the-art semi-analytical and chemodynamical models for the Milky Way, emphasising the strengths and weaknesses of both approaches.Comment: 16 pages, 9 figures, 1 table, LaTeX (emulatepasa.sty). Invited Review, accepted for publication in the Publications of the Astronomical Society of Australia, Vol 20, Num 4 (2003

Galactic chemical evolution

We analyze the evolution of oxygen abundance radial gradients resulting from our chemical evolution models calculated with different prescriptions for the star formation rate (SFR) and for the gas infall rate, in order to assess their respective roles in shaping gradients. We also compare with cosmological simulations and confront all with recent observational datasets, in particular with abundances inferred from planetary nebulae. We demonstrate the critical importance in isolating the specific radial range over which a gradient is measured, in order for their temporal evolution to be useful indicators of disk growth with redshift

Photometric Properties of White Dwarf Dominated Halos

Using stellar population synthesis techniques, we explore the photometric signatures of white dwarf progenitor dominated galactic halos, in order to constrain the fraction of halo mass that may be locked-up in white dwarf stellar remnants. We first construct a 10^9 M_sun stellar halo using the canonical Salpeter initial stellar mass distribution, and then allow for an additional component of low- and intermediate-mass stars, which ultimately give rise to white dwarf remnants. Microlensing observations towards the Large Magellanic Cloud, coupled with several ground-based proper motion surveys, have led to claims that in excess of 20% of the dynamical mass of the halo (10^12 M_sun) might be found in white dwarfs. Our results indicate that (1) even if only 1% of the dynamical mass of the dark halo today could be attributed to white dwarfs, their main sequence progenitors at high redshift (z ~ 3) would have resulted in halos more than 100 times more luminous than those expected from conventional initial mass functions alone, and (2) any putative halo white dwarf progenitor dominated initial mass function component, regardless of its dynamical importance, would be virtually impossible to detect at the present-day, due to its extremely faint surface brightness.Comment: 4 pages, Refereed contribution to the 5th Galactic Chemodynamics conference held in Swinburne, July 2003. Accepted for publication in PAS

On the origin of fluorine in the Milky Way

The main astrophysical factories of fluorine (19F) are thought to be Type II supernovae, Wolf-Rayet stars, and the asymptotic giant branch (AGB) of intermediate mass stars. We present a model for the chemical evolution of fluorine in the Milky Way using a semi-analytic multi-zone chemical evolution model. For the first time, we demonstrate quantitatively the impact of fluorine nucleosynthesis in Wolf-Rayet and AGB stars. The inclusion of these latter two fluorine production sites provides a possible solution to the long-standing discrepancy between model predictions and the fluorine abundances observed in Milky Way giants. Finally, fluorine is discussed as a possible probe of the role of supernovae and intermediate mass stars in the chemical evolution history of the globular cluster omega Centauri.Comment: 7 pages, 4 figures. MNRAS in pres

The formation of stellar halos in late-type galaxies

Near–field observations may provide tight constraints - i.e. 'boundary conditions' - on any model of structure formation in the Universe. Detailed observational data have long been available for the Milky Way (e.g., Freeman and Bland-Hawthorn 2002) and have provided tight constraints on several Galaxy formation models (e.g.: Abadi et al. 2003; Bekki andChiba 2001). An implicit assumption still remains unanswered though: is the Milky Way a “normal” spiral? Searching for directions, it feels natural to look at our neighbour: Andromeda. An intriguing piece of the “puzzle” is provided by contrasting its stellar halo with that of our Galaxy, even more so since Mouhcine et al. (2005) have suggested that a correlation between stellar halo metallicity and galactic luminosity is in place and would leave the Milky Way halo as an outlier with respect to other spirals of comparable luminosities. Further questions hence arise: is there any stellar halo–galaxy formation symbiosis? Our first step has been to contrast the chemical evolution of the MilkyWay with that of Andromeda by means of a semi–analytic model. We have then pursued a complementary approach through the analysis of several semi–cosmological late–type galaxy simulations which sample a wide variety of merging histories. We have focused on the stellar halo properties in the simulations at redshift zero and shown that - at any given galaxy luminosity - the metallicities of the stellar halos in the simulations span a range in excess of 1 dex, a result which is strengthened by the robustness tests we have performed. We suggest that the underlying driver of the halo metallicity dispersion can be traced to the diversity of galactic mass assembly histories inherent within the hierarchical clustering paradigm

The evolution of fluorine in galactic systems

No abstract available