The non-universality of the low-mass end of the IMF is robust against the choice of SSP model

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 public versions of the SSP models are restricted to either solar abundance patterns or solar metallicity, too restrictive if one aims to disentangle elemental enhancements, metallicity changes and IMF variations in massive early-type galaxies (ETGs) with star formation histories different from the solar neighborhood. We define response functions (to metallicity and \alpha-abundance) to extend the parameter space of each set of models. We compare these extended models with a sample of Sloan Digital Sky Survey (SDSS) ETGs spectra with varying velocity dispersions. We measure equivalent widths of optical IMF-sensitive stellar features to examine the effect of the underlying model assumptions and ingredients, such as stellar libraries or isochrones, on the inference of the IMF slope down to ~0.1 solar masses. We demonstrate that the steepening of the low-mass end of the Initial Mass Function (IMF) based on a non-degenerate set of spectroscopic optical indicators is robust against the choice of the stellar population model. Although the models agree in a relative sense (i.e. both imply more bottom-heavy IMFs for more massive systems), we find non-negligible differences on the absolute values of the IMF slope inferred at each velocity dispersion by using the two different models. In particular, we find large inconsistency in the quantitative predictions of IMF slope variations and abundance patterns when sodium lines are used. We investigate the possible reasons for these inconsistencies.Comment: 16 pages, 9 figures, 2 tables, accepted for publication on Ap

Damped Ly{\alpha} Absorption Systems in Semi-Analytic Models with Multiphase Gas

We investigate the properties of damped Ly{\alpha} absorption systems (DLAs) in semi-analytic models of galaxy formation, including partitioning of cold gas in galactic discs into atomic, molecular, and ionized phases with a molecular gas-based star formation recipe. We investigate two approaches for partitioning gas into these constituents: a pressure-based and a metallicity-based recipe. We identify DLAs by passing lines of sight through our simulations to compute HI column densities. We find that models with "standard" gas radial profiles - where the average specific angular momentum of the gas disc is equal to that of the host dark matter halo - fail to reproduce the observed column density distribution of DLAs. These models also fail to reproduce the distribution of velocity widths {\Delta}v, overproducing low {\Delta}v relative to high {\Delta}v systems. Models with "extended" radial gas profiles - corresponding to gas discs with higher specific angular momentum - are able to reproduce quite well the column density distribution of absorbers over the column density range 19 < log NHI < 22.5 in the redshift range 2 < z < 3.5. The model with pressure-based gas partitioning also reproduces the observed line density of DLAs, HI gas density, and {\Delta}v distribution at z < 3 remarkably well. However all of the models investigated here underproduce DLAs and the HI gas density at z > 3. If this is the case, the flatness in the number of DLAs and HI gas density over the redshift interval 0 < z < 5 may be due to a cosmic coincidence where the majority of DLAs at z > 3 arise from intergalactic gas in filaments while those at z < 3 arise predominantly in galactic discs. We further investigate the dependence of DLA metallicity on redshift and {\Delta}v, and find reasonably good agreement with the observations, particularly when including the effects of metallicity gradients (abbrv.).Comment: 27 pages, 15 figures, submitted to MNRA

Stellar populations, neutral hydrogen and ionised gas in field early-type galaxies

We present a study of the stellar populations of a sample of 39 local, field early-type galaxies whose HI properties are known from interferometric data. Our aim is to understand whether stellar age and chemical composition depend on the HI content of galaxies. As a by-product of our analysis, we also study their ionised gas content and how it relates to the neutral hydrogen gas. Stellar populations and ionised gas are studied from optical long-slit spectra. We determine stellar age, metallicity and alpha-to-iron ratio by analysing a set of Lick/IDS line-strength indices measured from the spectra after modelling and subtracting the ionised-gas emission. We do not find any trend in the stellar populations parameters with M(HI). However, we do find that, at stellar velocity dispersion below 230 km/s, 2/3 of the galaxies with less than 100 million solar masses of HI are centrally rejuvenated, while none of the HI-richer systems are. Furthermore, none of the more massive (velocity-dispersion>230 km/s) objects are centrally rejuvenated independently on their HI mass. Concerning the ionised gas, we detect emission in 60% of the sample. This is generally extended and always carachterised by LINER-like emission-line ratios at any radius. We find that a large HI mass is necessary (but not sufficient) for a galaxy to host bright ionised-gas emission. A plausible interpretation of our results is that gas-rich mergers play a significant role in E/S0 formation, especially at lower mass. Within this picture, HI-poor, centrally-rejuvenated objects could form in mergers where gas angular-momentum removal (and therefore inflow) is efficient; HI-rich galaxies with no significant age gradients (but possibly uniformly young) could be formed in interactions characterised by high-angular momentum gas.Comment: Accepted for publication in Astronomy & Astrophysics, 17 pages, 10 figures, 5 tables, 1 appendi

An Unexpectedly High AGN Fraction in Red Cluster Galaxies

As part of a program to study the evolution of active galactic nuclei (AGN) in clusters of galaxies, we present our results for Abell 2104. A deep Chandra observation of this massive, z = 0.154 cluster reveals a significant X-ray point source excess over the expectations of blank fields, including eight X-ray counterparts with R<20 mag. Our spectroscopy shows that all six X-ray sources associated with red counterparts are cluster members and their X-ray properties are consistent with all of them being AGN. Only one of the six has the emission lines characteristic of optically selected AGN; the remaining five would not have been classified as AGN based on their optical spectra. This suggests the existence of a large population of obscured, or at least optically unremarkable, AGN in clusters of galaxies. These six sources correspond to a lower limit of ~5% of the AGN fraction in cluster galaxies with R<20 mag (rest-frame M_V = -19.5 mag) and is comparable to the blue galaxy fraction in the cluster. Such an obscured AGN population in clusters of galaxies has many implications for cluster galaxy evolution, the hidden growth of their central, supermassive black holes, estimates of the star formation rate at infrared and radio wavelengths, and the observed variance in the hard X-ray background.Comment: ApJL Accepted, 12 pages including 3 embedded ps figures. Version with higher resolution figure 1 available at http://www.ociw.edu/~martini/pubs

The Deepest Hubble Space Telescope Color-Magnitude Diagram of M32:Evidence for Intermediate-age Populations

We present the deepest optical color-magnitude diagram (CMD) to date of the local elliptical galaxy M32. We have obtained F435W and F555W photometries based on Hubble Space Telescope (HST) Advanced Camera for Surveys/High-Resolution Channel images for a region 110 '' from the center of M32 (F1) and a background field (F2) about 320 '' away from M32 center. Due to the high resolution of our Nyquist-sampled images, the small photometric errors, and the depth of our data (the CMD of M32 goes as deep as F435W similar to 28.5 at 50% completeness level), we obtain the most detailed resolved photometric study of M32 yet. Deconvolution of HST images proves to be superior than other standard methods to derive stellar photometry on extremely crowded HST images, as its photometric errors are similar to 2x smaller than other methods tried. The location of the strong red clump in the CMD suggests a mean age between 8 and 10 Gyr for [Fe/H] = -0.2 dex in M32. We detect for the first time a red giant branch bump and an asymptotic giant branch (AGB) bump in M32 which, together with the red clump, allow us to constrain the age and metallicity of the dominant population in this region of M32. These features indicate that the mean age of M32's population at similar to 2' from its center is between 5 and 10 Gyr. We see evidence of an intermediate-age population in M32 mainly due to the presence of AGB stars rising to M-F555W similar to -2.0. Our detection of a blue component of stars (blue plume) may indicate for the first time the presence of a young stellar population, with ages of the order of 0.5 Gyr, in our M32 field. However, it is likely that the brighter stars of this blue plume belong to the disk of M31 rather than to M32. The fainter stars populating the blue plume indicate the presence of stars not younger than 1 Gyr and/or BSSs in M32. The CMD of M32 displays a wide color distribution of red giant branch stars indicating an intrinsic spread in metallicity with a peak at [Fe/H] similar to -0.2. There is not a noticeable presence of blue horizontal branch stars, suggesting that an ancient population with [Fe/H] <-1.3 does not significantly contribute to the light or mass of M32 in our observed fields. M32's dominant population of 8-10 Gyr implies a formation redshift of 1 less than or similar to z(f) less than or similar to 2, precisely when observations of the specific star formation rates and models of "downsizing" imply galaxies of M32's mass ought to be forming their stars. Our CMD therefore provides a "ground truth" of downsizing scenarios at z = 0. Our background field data represent the deepest optical observations yet of the inner disk and bulge of M31. Its CMD exhibits a broad color spread of red giant stars indicative of its metallicity range with a peak at [Fe/H] similar to -0.4 dex, slightly more metal-poor than M32 in our fields. The observed blue plume consists of stars as young as 0.3 Gyr, in agreement with previous works on the disk of M31. The detection of bright AGB stars reveals the presence of intermediate-age population in M31, which is, however, less significant than that in M32 at our field's location

On the Age and Metallicity Estimation of Spiral Galaxies Using Optical and Near-Infrared Photometry

In integrated-light, some color-color diagrams that use optical and near-infrared photometry show surprisingly orthogonal grids as age and metallicity are varied, and they are coming into common usage for estimating the average age and metallicity of spiral galaxies. In this paper we reconstruct these composite grids using simple stellar population models from several different groups convolved with some plausible functional forms of star formation histories at fixed metallicity. We find that the youngest populations present (t<2 Gyr) dominate the light, and because of their presence the age-metallicity degeneracy can be partially broken with broad-band colors, unlike older populations. The scatter among simple stellar population models by different authors is, however, large at ages t<2 Gyr. The dominant uncertainties in stellar population models arise from convective core overshoot assumptions and the treatment of the thermally pulsing asymptotic giant branch phase and helium abundance may play a significant role at higher metallicities. Real spiral galaxies are unlikely to have smooth, exponential star formation histories, and burstiness will cause a partial reversion to the single-burst case, which has even larger model-to-model scatter. Finally, it is emphasized that the current composite stellar population models need some implementation of chemical enrichment histories for the proper analysis of the observational data.Comment: 33 pages, 15 figures. Accepted to ApJ (Apr 2007). The major surgery was Fig. 1