Initial Mass Function In Galaxies Research Articles

Downsizing revised: Star formation timescales for elliptical galaxies with an environment-dependent IMF and a number of SNIa

Previous studies of the stellar mean metallicity and [Mg/Fe] values of massive elliptical (E) galaxies suggest that their stars were formed over a very short timescale that cannot be reconciled with estimates from stellar population synthesis (SPS) studies and with hierarchical assembly. Applying the previously developed chemical evolution code, GalIMF, which allows an environment-dependent stellar initial mass function (IMF) to be applied to the integrated galaxy initial mass function theory instead of an invariant canonical IMF, the star formation timescales (SFT) of E galaxies are re-evaluated. The code’s uniqueness lies in it allowing the galaxy-wide IMF and associated chemical enrichment to evolve as the physical conditions in the galaxy change. The calculated SFTs become consistent with the independent SPS results if the number of type Ia supernovae (SNIa) per unit stellar mass increases for more massive E galaxies. This is a natural outcome of galaxies with higher star formation rates producing more massive star clusters, spawning a larger number of SNIa progenitors per star. The calculations show E galaxies with a stellar mass ≈109.5 M⊙ to have had the longest mean SFTs of ≈2 Gyr. The bulk of more massive E galaxies were formed faster (SFT ≈ 1 Gyr) leading to domination by M dwarf stars and larger dynamical mass-to-light ratios as observed, while lower mass galaxies tend to lose their gas supply more easily due to their shallower potential and therefore also have similarly-short mean SFTs. This work achieves, for the first time, consistency of the SFTs for early-type galaxies between chemical-enrichment and SPS modelling. Equally, it leads to an improved understanding of how the star formation environment may affect the total number of SNIa per unit stellar mass formed.

The evolution of CNO isotopes: a new window on cosmic star formation history and the stellar IMF in the age of ALMA

We use state-of-the-art chemical models to track the cosmic evolution of the CNO isotopes in the interstellar medium (ISM) of galaxies, yielding powerful constraints on their stellar initial mass function (IMF). We re-assess the relative roles of massive stars, asymptotic giant branch (AGB) stars and novae in the production of rare isotopes such as 13C, 15N, 17O and 18O, along with 12C, 14N and 16O. The CNO isotope yields of super-AGB stars, novae and fast-rotating massive stars are included. Having reproduced the available isotope enrichment data in the solar neighbourhood, and across the Galaxy, and having assessed the sensitivity of our models to the remaining uncertainties, e.g. nova yields and star-formation history, we show that we can meaningfully constrain the stellar IMF in galaxies using C, O and N isotope abundance ratios. In starburst galaxies, where data for multiple isotopologue lines are available, we find compelling new evidence for a top-heavy stellar IMF, with profound implications for their star-formation rates and efficiencies, perhaps also their stellar masses. Neither chemical fractionation nor selective photodissociation can significantly perturb globally-averaged isotopologue abundance ratios away from the corresponding isotope ones, as both these processes will typically affect only small mass fractions of molecular clouds in galaxies. Thus the Atacama Large Millimetre Array now stands ready to probe the stellar IMF, and even the ages of specific starburst events in star-forming galaxies across cosmic time unaffected by the dust obscuration effects that plague optical/near-infrared studies.

Can we infer the Initial Mass Function of galaxies at z ~ 2?

AbstractRecent studies show increasing evidence for the possibility of the Initial Mass Function (IMF) of galaxies to vary as a function of numerous physical parameters. These studies mainly focus on the low redshift galaxies (at z≤0.1) and is not backed by any studies at higher redshifts. Using MOSFIRE data from KECK, we use the Hα Equivalent-Width (EW) as a proxy for the IMF to study in situ IMF of galaxies at z ~ 2. In this proceedings we focus on the underlying physics of this method and present our initial findings. More work is needed to interpret the results accurately and will be presented in our future papers.

IAU Focus Meeting 7: Stellar Physics in Galaxies throughout the Universe

AbstractA 3-day Focus Meeting entitled “Stellar Physics in Galaxies throughout the Universe” was held during the IAU XXIX General Assembly. The meeting brought together astrophysicists from the stellar physics, extragalactic astrophysics and cosmology communities to discuss how current and future results can foster progress in these disjoint science areas. Areas covered include stellar evolution of single and binary stars from the zero-age main-sequence to the terminal stage, the feedback of stars to the interstellar medium via radiation, dust production and chemical enrichment, and the properties of the most massive stars and of cosmologically significant stellar phases such as AGB and Wolf-Rayet stars. The limitations of our understanding of the physics of local stars and their effects on, e.g., ages, chemical composition and the initial mass function of galaxies at low to high redshift were evaluated.

Massive Elliptical Galaxies: BH Scouring or a Bottom-Heavy IMF?

AbstractWe present indirect constraints on the stellar initial-mass-function (IMF) in nine massive elliptical galaxies with σ ≈ 300 km/s, via a comparison of dynamical and stellar-population based stellar masses. We use adaptive-optics assisted, high resolution kinematical data from the SINFONI Search for Supermassive Black Holes that allow us to constrain the dynamical stellar mass-to-light ratio in the very centre of each galaxy. Hence we measure the IMF in a galaxy region where the stellar mass dominates over dark matter, minimising any potential degeneracy between the two mass components. In six of our galaxies – those which have depleted stellar cores – we find an IMF consistent with the one measured in the Milky-Way via direct star counts. The three remaining, power-law galaxies have instead stellar masses about a factor of two times larger than expected from a Milky-Way type IMF, indicating either a more bottom-heavy IMF (like, e.g., the Salpeter IMF) or a dark-matter distribution that is degenerate with the stellar mass down to the very centres of these galaxies. The bottom-light IMF in our core galaxies is surprising in view of previous studies that suggested a systematic IMF variation where early-type galaxies with σ ≈ 300 km/s have a Salpeter or even more dwarf-dominated IMF. Core galaxies are particularly important since their unique central orbital structure offers an independent crosscheck for the dynamical models. Our models with a bottom-light IMF are consistent with the distribution of orbits predicted by SMBH-binary core-formation models. This indicates that spatially well resolved central kinematical data are important for determining unbiased dynamical stellar mass-to-light ratios. Our results imply either that the IMF in massive galaxies varies over a wider range than previously anticipated, and is not the same in core and power-law ellipticals, or else that there are systematic variations in the distribution of dark matter among massive early-type galaxies.

The [ /Fe] ratios of very metal-poor stars within the integrated galactic initial mass function theory

The aim of this paper is to quantify the amplitude of the predicted plateau in [alpha/Fe] ratios associated with the most metal-poor stars of a galaxy. We assume that the initial mass function in galaxies is steeper if the star formation rate (SFR) is low -- as per the integrated galactic initial mass function (IGIMF) theory. A variant of the theory, in which the IGIMF depends upon the metallicity of the parent galaxy, is also considered. The IGIMF theory predicts low [alpha/Fe] plateaus in dwarf galaxies, characterised by small SFRs. The [alpha/Fe] plateau is up to 0.7dex lower than the corresponding plateau of the Milky Way. For a universal IMF one should expect instead that the [alpha/Fe] plateau is the same for all the galaxies, irrespective of their masses or SFRs. Assuming a strong dependence of the IMF on the metallicity of the parent galaxy, dwarf galaxies can show values of the [alpha/Fe] plateau similar to those of the Milky Way, and almost independent on the SFR. The [Mg/Fe] ratios of the most metal-poor stars in dwarf galaxies satellites of the Milky Way can be reproduced either if we consider metallicity-dependent IMFs or if the early SFRs of these galaxies were larger than we presently think. Present and future observations of dwarf galaxies can help disentangle between these different IGIMF formulations.

PERSPECTIVES ON INTRACLUSTER ENRICHMENT AND THE STELLAR INITIAL MASS FUNCTION IN ELLIPTICAL GALAXIES

Stars formed in galaxy cluster potential wells must be responsible for the high level of enrichment measured in the intracluster medium (ICM); however, there is increasing tension between this truism and the parsimonious assumption that the stars in the generally old population studied optically in cluster galaxies emerged from the same formation sites at the same epochs. We construct a phenomenological cluster enrichment model to demonstrate that ICM elemental abundances are underestimated by a factor >2 for standard assumptions about the stellar population -- a discrepancy we term the "cluster elemental abundance paradox". Recent evidence of an elliptical galaxy IMF skewed to low masses deepens the paradox. We quantify the adjustments to the star formation efficiency and initial mass function (IMF), and SNIa production efficiency, required to resolve this while being consistent with the observed ICM abundance pattern. The necessary enhancement in metal enrichment may, in principle, originate in the observed stellar population if a larger fraction of stars in the supernova-progenitor mass range form from an initial mass function (IMF) that is either bottom-light or top-heavy, with the latter in some conflict with observed ICM abundance ratios. Other alternatives that imply more modest revisions to the IMF, mass return and remnant fractions, and primordial fraction, posit an increase in the fraction of 3-8 solar mass stars that explode as SNIa or assume that there are more stars than conventionally thought -- although the latter implies a high star formation efficiency. We discuss the feasibility of these various solutions and the implications for the diversity of star formation in the universe, the process of elliptical galaxy formation, and the origin of this "hidden" source of ICM metal enrichment.

A METHOD FOR MEASURING VARIATIONS IN THE STELLAR INITIAL MASS FUNCTION

We present a method for investigating variations in the upper end of the stellar Initial Mass Function (IMF) by probing the production rate of ionizing photons in unresolved, compact star clusters with ages <~10 Myr and with different masses. We test this method by performing a pilot study on the young cluster population in the nearby galaxy NGC5194 (M51a), for which multi-wavelength observations from the Hubble Space Telescope are available. Our results indicate that the proposed method can probe the upper end of the IMF in galaxies located out to at least ~10 Mpc, i.e., a factor ~200 further away than possible by counting individual stars in young compact clusters. Our results for NGC5194 show no obvious dependence of the upper mass end of the IMF on the mass of the star cluster down to ~1000 M_sun, although more extensive analyses involving lower mass clusters and other galaxies are needed to confirm this conclusion.

Constraining the solutions of an inverse method of stellar population synthesis

In three previous papers (Pelat 1997, MNRAS, 284, 365; Pelat 1998, MNRAS, 299, 877; Moultaka & Pelat 2000, MNRAS, 314, 409), we set out an inverse stellar population synthesis method that uses a database of stellar spectra. Unlike other methods, this one provides full knowledge of all possible solutions as well as a good estimation of their stability; moreover, it provides the unique approximate solution, when the problem is overdetermined, using a rigorous minimization procedure. In Boisson et al. (2000, A&A, 357, 850), this method was applied to 10 active and 2 normal galaxies. In this paper we analyse the results of the method after constraining the solutions. Adding a priori physical conditions to the solutions constitutes a good way to regularize the synthesis problem. As an illustration we introduce physical constraints on the relative number of stars taking into account our present knowledge of the initial mass function in galaxies. To avoid biases on the solutions due to such constraints, we use constraints involving only inequalities between the number of stars, after dividing the H-R diagram into various groups of stellar masses. We discuss the results for a well-known globular cluster of the galaxy M 31 and discuss some of the galaxies studied in Boisson et al. (2000, A&A, 357, 850). We find that, given the spectral resolution and the spectral domain, the method is very stable according to such constraints (i.e. the constrained solutions are almost the same as the unconstrained one). However, additional information can be derived about the evolutionary stage of the last burst of star formation, but the precise age of this particular burst seems to be questionable.

Stellar Mass‐to‐Light Ratios and the Tully‐Fisher Relation

We have used a suite of simplified spectrophotometric spiral galaxy evolution models to argue that there are substantial variations in stellar mass-to-light ratios (M/Ls) within and among galaxies, amounting to factors of between 3 and 7 in the optical, and factors of 2 in the near-infrared. Under the assumption of a universal spiral galaxy IMF, our models show a strong and robust correlation between stellar M/L and the optical colors of the integrated stellar populations. Using observed maximum disk M/Ls we conclude that a Salpeter Initial Mass Function (IMF) has too many low-mass stars per unit luminosity, but that an IMF similar to the Salpeter IMF at the high-mass end with less low-mass stars (giving stellar M/Ls 30% or more lower than the Salpeter value) is consistent with the maximum disk constraints. We apply the model trends in stellar M/L with color to the Tully-Fisher (TF) relation. We find that the stellar mass TF relation is relatively steep and has modest scatter, and is independent of the passband and color used to derive the stellar masses. The difference in slope between the optical (especially blue) and near-infrared TF relations is due to the combined effects of dust attenuation and stellar M/L variations with galaxy mass. We find that the baryonic TF relation has a slope of 3.5 (with random and systematic errors of +/- 0.2 each). Since we have normalized the stellar M/L to be as high as can possibly be allowed by maximum disk constraints, the slope of the baryonic TF relation will be somewhat shallower than 3.5 if all disks are substantially sub-maximal. [Abridged]