Initial Mass Function Variations Research Articles

The Near-Infrared Ca ii Triplet- Relation for Bulges of Spiral Galaxies

We present measurements of the near-infrared Ca II triplet (CaT, CaT*), Paschen (PaT), and magnesium (Mg I) indices for a well-studied sample of 19 bulges of early to intermediate spiral galaxies. We find that both the CaT* and CaT indices decrease with central velocity dispersion σ with small scatter. This dependence is similar to that recently found by Cenarro for elliptical galaxies, implying a uniform CaT*-σ relation that applies to galaxies from ellipticals to intermediate-type spirals. The decrease of CaT and CaT* with σ contrasts with the well-known increase of another α-element index, Mg2, with σ. We discuss the role of Ca underabundance ([Ca/Fe] < 0) and initial mass function variations in the onset of the observed relations.

Near-infrared line-strengths in elliptical galaxies: evidence for initial mass function variations?

We present new relations between recently defined line-strength indices in the near-infrared (CaT*, CaT, PaT, MgI and sTiO) and central velocity dispersion (sigma(0) ) for a sample of 35 early-type galaxies, showing evidence for significant anti-correlations between Ca ii triplet indices (CaT* and CaT) and log sigma(0). These relations are interpreted in the light of our recent evolutionary synthesis model predictions, suggesting the existence of important Ca underabundances with respect to Fe and/or an increase of the dwarf to giant stars ratio along the mass sequence of elliptical galaxies.

M82-F: a doomed super star cluster?

We present high dispersion echelle spectroscopy of the very luminous, young super star cluster (SSC) `F' in M82, obtained with the 4.2-m William Herschel Telescope (WHT), for the purpose of deriving its dynamical mass and assessing whether it will survive to become an old globular cluster. We measure a stellar velocity dispersion of 13.4 +/- 0.7 km/s, a projected half-light radius of 2.8 +/- 0.3 pc from archival HST/WFPC2 images, and derive a dynamical mass of 1.2 +/- 0.1 x 10^6 solar masses, demonstrating that M82-F is a very massive, compact cluster. We determine that the current visual luminosity-to-mass ratio for M82-F is 45 +/- 13. Comparison with spectral synthesis models shows that the visual luminosity-to-mass ratio is a factor of 5 higher than that predicted for a standard Kroupa (2001) initial mass function (IMF) at the well-determined age for M82-F of 60 +/- 20 Myr. This high value of the visual luminosity-to-mass ratio indicates a deficit of low mass stars in M82-F; the current mass function (MF) evidently is `top-heavy'. We find that a lower mass cutoff of 2-3 solar masses is required to match the observations for a MF with a slope of 2.3. Since the cluster apparently lacks long-lived low mass stars, it will not become an old globular cluster. We also derive up-dated luminosity-to-mass ratios for the younger SSCs NGC 1569A and NGC 1705-1. We discuss the implications of our findings in the context of large scale IMF variations; with the present data the top-heavy MF could reflect a local mass segregation effect during the birth of the cluster.

The Distribution of Heavy Elements in Spiral and Elliptical Galaxies

ABSTRACTIn large disk and spheroidal galaxies, spatially resolved abundance information can be extracted by analysis of either emission lines, absorption lines, or both, depending on the situation. This review recaps significant results as they apply to nondwarf galaxies, including the Milky Way, spiral disks and bulges, and elliptical and lenticular galaxies. Methods for determining abundances are explained in appendices.Conclusions that span the galaxy types treated here are as follows. All galaxies, on average, have heavy‐element abundances (metallicities) that systematically decrease outward from their galactic centers while their global metallicities increase with galaxy mass. Abundance gradients are steepest in normal spirals and are seen to be progressively flatter going in order from barred spirals to lenticulars to ellipticals. The distribution of abundances N(Z) versus Z is strongly peaked compared with simple closed‐box model predictions of chemical enrichment in all galaxy types. That is, a “G dwarf problem,” commonly known in the solar cylinder, exists for all large galaxies.For spiral galaxies, local metallicity appears to be correlated with total (disk+bulge) surface density. Examination of N/O versus O/H in spiral disks indicates that production of N is dominated by primary processes at low metallicity and secondary processes at high metallicity. Carbon production increases with increasing metallicity. Abundance ratios Ne/O, S/O, and Ar/O appear to be universally constant and independent of metallicity, which argues either that the initial mass function (IMF) is universally constant or that these ratios are not sensitive to IMF variations. In the Milky Way, there is a rough age‐metallicity trend with much scatter, in the sense that older stars are more metal poor.In elliptical galaxies, nuclear abundances are in the range [Z/H] = 0.0–0.4, but the element mixture is not scaled‐solar. In large elliptical galaxies [Mg/Fe] is in the range 0.3–0.5, decreasing to ≈0 in smaller elliptical galaxies. Other light elements track the Mg enhancement, but the heavier Ca tracks Fe. Velocity dispersion appears to be a key parameter in the modulation of [Mg/Fe], but the cause of the connection is unclear.

A possible theoretical explanation of metallicity gradients in elliptical galaxies

ABSTRA C T Models of chemical evolution of elliptical galaxies taking into account different escape velocities at different galactocentric radii are presented. As a consequence of this, the chemical evolution develops differently in different galactic regions; in particular, we find that the galactic wind, powered by supernovae (of Type II and I) starts, under suitable conditions, in the outer regions and successively develops in the central ones. The star formation is assumed to stop after the onset of the galactic wind in each region. The main result found in the present work is that this mechanism is able to reproduce metallicity gradients, namely the gradients in the Mg2 index, in good agreement with observational data. We also find that in order to honour the constant ΩhMg=Feiy ratio with galactocentric distance, as inferred from metallicity indices, a variable initial mass function as a function of galactocentric distance is required. This is only a suggestion, as trends on abundances inferred purely from metallicity indices are still uncertain.

The Evolution of Galaxies. III. Metal-Enhanced Star Formation

The problem of the paucity of low-metal-abundance low-mass stars is discussed. One alternative to the variable-initial-mass-function (VIMF) solution is proposed. It is shown that this solution - metal-enhanced star formation - satisfies the classical test which prompted the VIMF hypothesis. Furthermore, with no additional parameters it provides improved fits to other tests - e.g., inhomogeneities in the abundances in young stars, concordance of all nucleo-cosmochronologies, and a required yield of heavy-element production which is consistent with current stellar evolution theory. In this model the age of the Galaxy is 18.6 plus or minus 5.7 b.y.