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Register a new userFurther evidence for a time-dependent initial mass function in massive early-type galaxies
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I. Ferreras
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Spectroscopic analyses of gravity-sensitive line strengths give growing evidence towards an excess of low-mass stars in massive early-type galaxies (ETGs). Such a scenario requires a bottom-heavy initial mass function (IMF). However, strong constraints can be imposed if we take into account galactic chemical enrichment. We extend the analysis of Weidner et al. and consider the functional form of bottom-heavy IMFs used in recent works, where the high-mass end slope is kept fixed to the Salpeter value, and a free parameter is introduced to describe the slope at stellar masses below some pivot mass scale (M<MP=0.5Msun). We find that no such time-independent parameterisation is capable to reproduce the full set of constraints in the stellar populations of massive ETGs - resting on the assumption that the analysis of gravity-sensitive line strengths leads to a mass fraction at birth in stars with mass M<0.5Msun above 60%. Most notably, the large amount of metal-poor gas locked in low-mass stars during the early, strong phases of star formation results in average stellar metallicities [M/H]<-0.6, well below the solar value. The conclusions are unchanged if either the low-mass end cutoff, or the pivot mass are left as free parameters, strengthening the case for a time-dependent IMF.
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We present new evidence for a variable stellar initial mass function (IMF) in massive early-type galaxies, using high-resolution, near-infrared spectroscopy from the Folded-port InfraRed Echellette spectrograph (FIRE) on the Magellan Baade Telescope at Las Campanas Observatory. In this pilot study, we observe several gravity-sensitive metal lines between 1.1 $mu$m and 1.3 $mu$m in eight highly-luminous ($L sim 10 L_*$) nearby galaxies. Thanks to the broad wavelength coverage of FIRE, we are also able to observe the Ca II triplet feature, which helps with our analysis. After measuring the equivalent widths (EWs) of these lines, we notice mild to moderate trends between EW and central velocity dispersion ($sigma$), with some species (K I, Na I, Mn I) showing a positive EW-$sigma$ correlation and others (Mg I, Ca II, Fe I) a negative one. To minimize the effects of metallicity, we measure the ratio $R$ = [EW(K I) / EW(Mg I)], finding a significant systematic increase in this ratio with respect to $sigma$. We then probe for variations in the IMF by comparing the measured line ratios to the values expected in several IMF models. Overall, we find that low-mass galaxies ($sigma sim 100$ km s$^{-1}$) favor a Chabrier IMF, while high-mass galaxies ($sigma sim 350$ km s$^{-1}$) are better described with a steeper (dwarf-rich) IMF slope. While we note that our galaxy sample is small and may suffer from selection effects, these initial results are still promising. A larger sample of galaxies will therefore provide an even clearer picture of IMF trends in this regime.
We studied the stellar populations, distribution of dark matter, and dynamical structure of a sample of 25 early-type galaxies in the Coma and Abell 262 clusters. We derived dynamical mass-to-light ratios and dark matter densities from orbit-based dynamical models, complemented by the ages, metallicities, and alpha-elements abundances of the galaxies from single stellar population models. Most of the galaxies have a significant detection of dark matter and their halos are about 10 times denser than in spirals of the same stellar mass. Calibrating dark matter densities to cosmological simulations we find assembly redshifts z_{DM} approx 1-3. The dynamical mass that follows the light is larger than expected for a Kroupa stellar initial mass function, especially in galaxies with high velocity dispersion sigma_{eff} inside the effective radius r_{eff}. We now have 5 of 25 galaxies where mass follows light to 1-3 r_{eff}, the dynamical mass-to-light ratio of all the mass that follows the light is large (approx 8-10 in the Kron-Cousins R band), the dark matter fraction is negligible to 1-3 r_{eff}. This could indicate a massive initial mass function in massive early-type galaxies. Alternatively, some of the dark matter in massive galaxies could follow the light very closely suggesting a significant degeneracy between luminous and dark matter.
We determine an absolute calibration of the initial mass function (IMF) of early-type galaxies, by studying a sample of 56 gravitational lenses identified by the SLACS Survey. Under the assumption of standard Navarro, Frenk & White dark matter halos, a combination of lensing, dynamical, and stellar population synthesis models is used to disentangle the stellar and dark matter contribution for each lens. We define an IMF mismatch parameter alpha=M*(L+D)/M*(SPS) as the ratio of stellar mass inferred by a joint lensing and dynamical models (M*(L+D)) to the current stellar mass inferred from stellar populations synthesis models (M*(SPS)). We find that a Salpeter IMF provides stellar masses in agreement with those inferred by lensing and dynamical models (<log alpha>=0.00+-0.03+-0.02), while a Chabrier IMF underestimates them (<log alpha>=0.25+-0.03+-0.02). A tentative trend is found, in the sense that alpha appears to increase with galaxy velocity dispersion. Taken at face value, this result would imply a non universal IMF, perhaps dependent on metallicity, age, or abundance ratios of the stellar populations. Alternatively, the observed trend may imply non-universal dark matter halos with inner density slope increasing with velocity dispersion. While the degeneracy between the two interpretations cannot be broken without additional information, the data imply that massive early-type galaxies cannot have both a universal IMF and universal dark matter halos.
The observed stellar initial mass function (IMF) appears to vary, becoming bottom-heavy in the centres of the most massive, metal-rich early-type galaxies. It is still unclear what physical processes might cause this IMF variation. In this paper, we demonstrate that the abundance of deuterium in the birth clouds of forming stars may be important in setting the IMF. We use models of disc accretion onto low-mass protostars to show that those forming from deuterium-poor gas are expected to have zero-age main sequence masses significantly lower than those forming from primordial (high deuterium fraction) material. This deuterium abundance effect depends on stellar mass in our simple models, such that the resulting IMF would become bottom-heavy - as seen in observations. Stellar mass loss is entirely deuterium-free and is important in fuelling star formation across cosmic time. Using the EAGLE simulation we show that stellar mass loss-induced deuterium variations are strongest in the same regions where IMF variations are observed: at the centres of the most massive, metal-rich, passive galaxies. While our analysis cannot prove that the deuterium abundance is the root cause of the observed IMF variation, it sets the stage for future theoretical and observational attempts to study this possibility.
The Initial Mass Function (IMF) of early-type galaxies (ETGs) has been found to feature systematic variations by both dynamical and spectroscopic studies. In particular, spectral line strengths, based on gravity-sensitive features, suggest an excess of low-mass stars in massive ETGs, i.e. a bottom-heavy IMF. The physical drivers of IMF variations are currently unknown. The abundance ratio of alpha elements, such as [Mg/Fe], has been suggested as a possible driver of the IMF changes, although dynamical constraints do not support this claim. In this letter, we take advantage of the large SDSS database. Our sample comprises 24,781 high-quality spectra, covering a large range in velocity dispersion (100<sigma0<320 km/s) and abundance ratio (-0.1<[Mg/Fe]<+0.4). The large volume of data allows us to stack the spectra at fixed values of sigma0 and [Mg/Fe]. Our analysis -- based on gravity-sensitive line strengths -- gives a strong correlation with central velocity dispersion and a negligible variation with [Mg/Fe] at fixed sigma0. This result is robust against individual elemental abundance variations, and seems not to raise any apparent inconsistency with the alternative method based on galaxy dynamics.
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