We perform a direct comparison of two state-of-the art single stellar population (SSP) models that have been used to demonstrate the non-universality of the low-mass end of the Initial Mass Function (IMF) slope. The two publi
We use deep HST ACS/HRC observations of a field within M32 (F1) and an M31 background field (F2) to determine the star formation history (SFH) of M32 from its resolved stellar population. We find that 2-5Gyr old stars contribute som40%+/- 17% of M32s
mass, while 55%+/-21% of M32s mass comes from stars older than 5 Gyr. The mass-weighted mean age and metallicity of M32 at F1 are <Age>=6.8+/-1.5 Gyr and <[M/H]>=-0.01+/-0.08 dex. The SFH additionally indicates the presence of young (<2 Gyr old), metal-poor ([M/H]sim-0.7) stars, suggesting that blue straggler stars contribute ~2% of the mass at F1; the remaining sim3% of the mass is in young metal-rich stars. Line-strength indices computed from the SFH imply a light-weighted mean age and metallicity of 4.9 Gyr and [M/H] = -0.12 dex, and single-stellar-population-equivalent parameters of 2.9+/-0.2 Gyr and [M/H]=0.02+/-0.01 dex at F1 (~2.7 re). This contradicts spectroscopic studies that show a steep age gradient from M32s center to 1re. The inferred SFH of the M31 background field F2 reveals that the majority of its stars are old, with sim95% of its mass already acquired 5-14 Gyr ago. It is composed of two dominant populations; sim30%+/-7.5% of its mass is in a 5-8 Gyr old population, and sim65%+/-9% of the mass is in a 8-14 Gyr old population. The mass-weighted mean age and metallicity of F2 are <Age>=9.2+/-1.2 Gyr and <[M/H]>=-0.10+/-0.10 dex, respectively. Our results suggest that the inner disk and spheroid populations of M31 are indistinguishable from those of the outer disk and spheroid. Assuming the mean age of M31s disk at F2 (sim1 disk scale length) to be 5-9 Gyr, our results agree with an inside-out disk formation scenario for M31s disk.
We measure black hole masses for 71 BL Lac objects from the Sloan Digital Sky Survey with redshifts out to z~0.4. We perform spectral decompositions of their nuclei from their host galaxies and measure their stellar velocity dispersions. Black hole m
asses are then derived from the black hole mass - stellar velocity dispersion relation. We find BL Lac objects host black holes of similar masses, ~10^{8.5} M_sun, with a dispersion of 0.4 dex, similar to the uncertainties on each black hole measurement. Therefore, all BL Lac objects in our sample have the same indistinguishable black hole mass. These 71 BL Lac objects follow the black hole mass - bulge luminosity relation, and their narrow range of host galaxy luminosities confirm previous claims that BL Lac host galaxies can be treated as standard candles. We conclude that the observed diversity in the shapes of BL Lac object spectral energy distributions is not strongly driven by black hole mass or host galaxy properties.
We study the metallicities and abundance ratios of early-type galaxies in cosmological semi-analytic models (SAMs) within the hierarchical galaxy formation paradigm. To achieve this we implemented a detailed galactic chemical evolution (GCE) model an
d can now predict abundances of individual elements for the galaxies in the semi-analytic simulations. This is the first time a SAM with feedback from Active Galactic Nuclei (AGN) has included a chemical evolution prescription that relaxes the instantaneous recycling approximation. We find that the new models are able to reproduce the observed mass-metallicity (M*-[Z/H]) relation and, for the first time in a SAM, we reproduce the observed positive slope of the mass-abundance ratio (M*-[$alpha$/Fe]) relation. Our results indicate that in order to simultaneously match these observations of early-type galaxies, the use of both a very mildly top-heavy IMF (i.e., with a slope of x=1.15 as opposed to a standard x=1.3), and a lower fraction of binaries that explode as Type Ia supernovae appears to be required. We also examine the rate of supernova explosions in the simulated galaxies. In early-type (non-star forming) galaxies, our predictions are also consistent with the observed SNe rates. However, in star-forming galaxies, a higher fraction of SN Ia binaries than in our preferred model is required to match the data. If, however, we deviate from the classical model and introduce a population of SNe Ia with very short delay times, our models simultaneously produce a good match to the observed metallicities, abundance ratios and SN rates.
