Halos of Spiral Galaxies. III. Metallicity Distributions

(Abriged) We report results of a campaign to image the stellar populations in the halos of highly inclined spiral galaxies, with the fields roughly 10 kpc (projected) from the nuclei. We use the F814W (I) and F606W (V) filters in the Wide Field Planetary Camera 2, on board the Hubble Space telescope. Extended halo populations are detected in all galaxies. The color-magnitude diagrams appear to be completely dominated by giant-branch stars, with no evidence for the presence of young stellar populations in any of the fields. We find that the metallicity distribution functions are dominated by metal-rich populations, with a tail extending toward the metal poor end. To first order, the overall shapes of the metallicity distribution functions are similar to what is predicted by simple, single-component model of chemical evolution with the effective yields increasing with galaxy luminosity. However, metallicity distributions significantly narrower than the simple model are observed for a few of the most luminous galaxies in the sample. It appears clear that more luminous spiral galaxies also have more metal-rich stellar halos. The increasingly significant departures from the closed-box model for the more luminous galaxies indicate that a parameter in addition to a single yield is required to describe chemical evolution. This parameter, which could be related to gas infall or outflow either in situ or in progenitor dwarf galaxies that later merge to form the stellar halo, tends to act to make the metallicity distributions narrower at high metallicity.Comment: 20 pages, 8 figures (ApJ, in press

Origin of Two Distinct Populations in Dwarf Spheroidal Galaxies

We study the chemical and kinematic properties of the first galaxies which formed at a high redshift, using high resolution cosmological numerical simulations, and compared them with the recent observational results for the Sculptor dwarf spheroidal galaxy by Tolstoy et al., who found two distinct stellar populations: the lower metallicity stars are more spatially extended and possess a higher velocity dispersion than the higher metallicity stars. Our calculations reproduce these observations as the result of a steep metallicity gradient, within a single populations, induced by dissipative collapse of the gas component. We also predict strong [N/O] enhancements in the lowest metallicity stars in dwarf spheroidals, due to the preferential retention of ejected gas from intermediate mass stars, compared to Type II supernovae.Comment: 11 pages, 10 figures, accepted for publication in Ap

A Near-Infrared Stellar Census of the Blue Compact Dwarf Galaxy VII~Zw~403

We present near-infrared single-star photometry for the low-metallicity Blue Compact Dwarf galaxy VII~Zw~403. We achieve limiting magnitudes of F110W~$\approx$~25.5 and F160W~$\approx$~24.5 using one of the NICMOS cameras with the HST equivalents of the ground-based J and H filters. The data have a high photometric precision (0.1 mag) and are $>95$% complete down to magnitudes of about 23, far deeper than previous ground-based studies in the near-IR. The color-magnitude diagram contains about 1000 point sources. We provide a preliminary transformation of the near-IR photometry into the ground system...Comment: Accepted for publication by the AJ, preprint has 49 pages, 2 tables, and 16 figure

A Stellar Population Gradient in VII Zw 403 - Implications for the Formation of Blue Compact Dwarf Galaxies

We present evidence for the existence of an old stellar halo in the Blue Compact Dwarf galaxy VII Zw 403. VII Zw 403 is the first Blue Compact Dwarf galaxy for which a clear spatial segregation of the resolved stellar content into a "core-halo" structure is detected. Multicolor HST/WFPC2 observations indicate that active star formation occurs in the central region, but is strikingly absent at large radii. Instead, a globular-cluster-like red giant branch suggests the presence of an old (> 10 Gyr) and metal poor (=-1.92) stellar population in the halo. While the vast majority of Blue Compact Dwarf galaxies has been recognized to possess halos of red color in ground-based surface photometry, our observations of VII Zw 403 establish for the first time a direct correspondence between a red halo color and the presence of old, red giant stars. If the halos of Blue Compact Dwarf galaxies are all home to such ancient stellar populations, then the fossil record conflicts with delayed-formation scenarios for dwarfs.Comment: Accepted for publication in the Ap

The Stellar Content of NGC 6789, A Blue Compact Dwarf Galaxy in the Local Void

We find that NGC6789 is the most nearby example of a Blue Compact Dwarf galaxy known to date. With the help of WFPC2 aboard the Hubble Space Telescope, we resolve NGC6789 into over 15,000 point sources in the V and I bands. The young stars of NGC6789 are found exclusively near the center of the galaxy. The red giant population identified at large galacticentric radii yields a distance of about 3.6 Mpc, a stellar metallicity [Fe/H] of about -2, and a minimum age of about 1 Gyr. Despite its isolated location in the Local Void,its low metallicity, and its active star formation, the properties of NGC6789 are clearly not those of a galaxy in formation.Comment: 8 pages, 4 figures, ApJL Accepte

Galactic Cosmic Rays from Superbubbles and the Abundances of Lithium, Beryllium, and Boron

In this article we study the galactic evolution of the LiBeB elements within the framework of a detailed model of the chemical evolution of the Galaxy that includes galactic cosmic ray nucleosynthesis by particles accelerated in superbubbles. The chemical composition of the superbubble consists of varying proportions of ISM and freshly supernova synthesized material. The observational trends of 6 LiBeB evolution are nicely reproduced by models in which GCR come from a mixture of 25% of supernova material with 75% of ISM, except for 6 Li, for which maybe an extra source is required at low metallicities. To account for 7 Li evolution several additional sources have been considered (neutrino-induced nucleosynthesis, nova outbursts, C-stars). The model fulfills the energetic requirements for GCR acceleration.Comment: 25 pages, 9 figures. Accepted for publication in the Astrophysical Journa

Integrated Spectroscopy of Bulge Globular Clusters and Fields. II. Implications for stellar population models and elliptical galaxies

Synthetic Lick indices (e.g. Mg_2, Fe, etc.) of Simple Stellar Population (SSP) models are calibrated for the first time up to solar metallicity with a sample of Milky Way globular clusters (GCs) which includes the metal rich GCs of the Galactic bulge. This metallicity range is relevant to elliptical galaxies. It is shown that the Bulge GCs and integrated light follow the same correlation between Mg and Fe indices of elliptical galaxies, showing weaker Fe indices at given Mg indices with respect to models that assume solar-scaled abundances. This similarity is the robust empirical evidence for enhanced alpha/Fe ratios in the stellar populations of elliptical galaxies, since the globular clusters are independently known to be alpha-enhanced. The uniqueness of this alpha-overabundance solution is checked by exploring the whole range of model ingredients. We argue that the standard models reproduce the Mg-Fe correlation at low metallicities because the stellar templates used in the synthesis are the alpha-enhanced stars of the galactic Halo. These same models, however, fail to recover the Mg-Fe pattern of Bulge clusters and ellipticals at high metallicities because the high-metallicity templates are disk stars, which are not alpha-enhanced. The new SSP models by Thomas, Maraston & Bender (2002) which incorporate the dependence on alpha/Fe reproduce the Mg and Fe indices of GCs at all metallicities, with alpha/Fe=+0.3, which is in agreement with spectroscopic abundance determinations. The Balmer indices (Hbeta, Hdelta, Hgamma) are very well calibrated, provided the Horizontal Branch morphology is taken into account. In particular, we reproduce the Balmer lines of NGC 6388 and NGC 6441, which are metal-rich GCs with a tail of warm Horizontal Branch stars. {Abridged}Comment: 19 pages, 13 figures, Astronomy and Astrophysics in press. Only minor changes after the referee repor

The epochs of early-type galaxy formation as a function of environment

The aim of this paper is to set constraints of the epochs of early-type galaxy formation through the 'archaeology' of the stellar populations in local galaxies. Using our models of absorption line indices that account for variable abundance ratios, we derive the stellar population parameters of 124 early-type galaxies in high and low density environments. We find that all three parameters age, metallicity, and alpha/Fe ratio are correlated with velocity dispersion. We further find evidence for an influence of the environment on the stellar population properties. Massive early-type galaxies in low-density environments appear on average ~2 Gyrs younger and slightly more metal-rich than their counterparts in high density environments. No offsets in the alpha/Fe ratios, instead, are detected. We translate the derived ages and alpha/Fe ratios into star formation histories. We show that most star formation activity in early-type galaxies is expected to have happened between redshifts 3 and 5 in high density and between redshifts 1 and 2 in low density environments. We conclude that at least 50 per cent of the total stellar mass density must have already formed at z 1, in good agreement with observational estimates of the total stellar mass density as a function of redshift. Our results suggest that significant mass growth in the early-type galaxy population below z 1 must be restricted to less massive objects, and a significant increase of the stellar mass density between redshifts 1 and 2 should be present caused mainly by the field galaxy population. The results of this paper further imply vigorous star formation episodes in massive objects at z 2-5 and the presence of evolved ellipticals around z 1, both observationally identified as SCUBA galaxies and EROs.Comment: 20 pages, 10 figures, plus appendix, accepted by Ap

UV-optical colors as probes of early-type galaxy evolution

We have studied ∼2100 early-type galaxies in the SDSS DR3 which have been detected by the GALEX Medium Imaging Survey (MIS), in the redshift range O <z <0.1.1. Combining GALEXUV photometry with corollary optical data from the SDSS, we find that, at a 95% confidence level, at least ∼30% of galaxies in this sample have UV to optical colors consistent with some recent star formation within the last Gyr. In particular, galaxies with an NUV - r color less than 5.5 are very likely to have experienced such recent star formation, taking into account the possibility of a contribution to NUV flux from the UV upturn phenomenon. We find quantitative agreement between the observations and the predictions of a semianalytical ACDM hierarchical merger model and deduce that early-type galaxies in the redshift range 0 <z <0.11 have ∼ 1 % -3 % of their stellar mass in stars less than 1 Gyr old. The average age of this recently formed population is ∼300-500 Myr. We also find that "monolithically" evolving galaxies, where recent star formation can be driven solely by recycled gas from stellar mass loss, cannot exhibit the blue colors (NUV - r <5.5) seen in a significant fraction (∼30%) of our observed sample.Peer reviewe

The Iron Discrepancy in Elliptical Galaxies after ASCA

We present estimates for the iron content of the stellar and diffused components of elliptical galaxies, as derived respectively from integrated optical spectra and from ASCA X-ray observations. A macroscopic discrepancy emerges between the expected iron abundances in the hot interstellar medium (ISM) and what is indicated by the X-ray observations, especially when allowance is made for the current iron enrichment by Type Ia supernovae. This strong discrepancy, that in some extreme instances may be as large as a factor of $\sim 20$, calls into question our current understanding of supernova enrichment and chemical evolution of galaxies. We discuss several astrophysical implications of the inferred low iron abundances in the ISM, including the chemical evolution of galaxies and cluster of galaxies, the evolution of gas flows in ellipticals, and the heating of the intracluster medium. Some of the consequences appear hard to accept, and in the attempt to avoid some of these difficulties we explore ways of hiding or diluting iron in the ISM of ellipticals. None of these possibilities appears astrophysically plausible, and we alternatively rise the question of the reliability of iron-L line diagonostic tools. Various thin plasma emission models are shown to give iron abundances that may differ significantly, especially at low temperatures (kT \lsim 1 keV). From a collection of ASCA and other X-ray observatory data, it is shown that current thin plasma codes tend to give very low iron abundances when the temperature of the objects is below $\sim 1$ keV. We conclude that -- besides rethinking the chemical evolution of galaxies -- one should also consider the possibility that existing thin plasma models may incorporate inaccurate atomic physics for the ions responsible for the iron-L complex.Comment: 39 pages, TeX file, 5 figures, Accepted for publication in the Ap