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Register a new userThe kinematics of massive quiescent galaxies at $1.4 < z < 2.1$: dark matter fractions, IMF variation, and the relation to local early-type galaxies
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We study the dynamical properties of massive quiescent galaxies at $1.4 < z < 2.1$ using deep Hubble Space Telescope WFC3/F160W imaging and a combination of literature stellar velocity dispersion measurements and new near-infrared spectra obtained using KMOS on the ESO VLT. We use these data to show that the typical dynamical-to-stellar mass ratio has increased by $sim$0.2 dex from $z = 2$ to the present day, and investigate this evolution in the context of possible changes in the stellar initial mass function (IMF) and/or fraction of dark matter contained within the galaxy effective radius, $f_mathrm{DM}$. Comparing our high-redshift sample to their likely descendants at low-redshift, we find that $f_mathrm{DM}$ has increased by a factor of more than 4 since $z approx 1.8$, from $f_mathrm{DM}$ = $6.6pm1.0$% to $sim$24%. The observed increase appears robust to changes in the methods used to estimate dynamical masses or match progenitors and descendants. We quantify possible variation of the stellar IMF through the offset parameter $alpha$, defined as the ratio of dynamical mass in stars to the stellar mass estimated using a Chabrier IMF. We demonstrate that the correlation between stellar velocity dispersion and $alpha$ reported among quiescent galaxies at low-redshift is already in place at $z = 2$, and argue that subsequent evolution through (mostly minor) merging should act to preserve this relation while contributing significantly to galaxies overall growth in size and stellar mass.
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We contrast the gas kinematics and dark matter contents of $z=2$ star-forming galaxies (SFGs) from state-of-the-art cosmological simulations within the $Lambda$CDM framework to observations. To this end, we create realistic mock observations of massive SFGs ($M_*>4times10^{10} M_{odot}$, SFR $>50~M_{odot}$ yr$^{-1}$) from the TNG50 simulation of the IllustrisTNG suite, resembling near-infrared, adaptive-optics assisted integral-field observations from the ground. Using observational line fitting and modeling techniques, we analyse in detail the kinematics of seven TNG50 galaxies from five different projections per galaxy, and compare them to observations of twelve massive SFGs by Genzel et al. (2020). The simulated galaxies show clear signs of disc rotation but mostly exhibit more asymmetric rotation curves, partly due to large intrinsic radial and vertical velocity components. At identical inclination angle, their one-dimensional velocity profiles can vary along different lines of sight by up to $Delta v=200$ km s$^{-1}$. From dynamical modelling we infer rotation speeds and velocity dispersions that are broadly consistent with observational results. We find low central dark matter fractions compatible with observations ($f_{rm DM}^v(<R_e)=v_{rm DM}^2(R_e)/v_{rm circ}^2(R_e)sim0.32pm0.10$), however for disc effective radii $R_e$ that are mostly too small: at fixed $R_e$ the TNG50 dark matter fractions are too high by a factor of $sim2$. We speculate that the differences in gas kinematics and dark matter content compared to the observations may be due to physical processes that are not resolved in sufficient detail with the numerical resolution available in current cosmological simulations.
The chemical composition of galaxies has been measured out to z~4. However, nearly all studies beyond z~0.7 are based on strong-line emission from HII regions within star-forming galaxies. Measuring the chemical composition of distant quiescent galaxies is extremely challenging, as the required stellar absorption features are faint and shifted to near-infrared wavelengths. Here, we present ultra-deep rest-frame optical spectra of five massive quiescent galaxies at z~1.4, all of which show numerous stellar absorption lines. We derive the abundance ratios [Mg/Fe] and [Fe/H] for three out of five galaxies; the remaining two galaxies have too young luminosity-weighted ages to yield robust measurements. Similar to lower-redshift findings, [Mg/Fe] appears positively correlated with stellar mass, while [Fe/H] is approximately constant with mass. These results may imply that the stellar mass-metallicity relation was already in place at z~1.4. While the [Mg/Fe]-mass relation at z~1.4 is consistent with the z<0.7 relation, [Fe/H] at z~1.4 is ~0.2 dex lower than at z<0.7. With a [Mg/Fe] of 0.44(+0.08,-0.07) the most massive galaxy may be more alpha-enhanced than similar-mass galaxies at lower redshift, but the offset is less significant than the [Mg/Fe] of 0.6 previously found for a massive galaxy at z=2.1. Nonetheless, these results combined may suggest that [Mg/Fe] in the most massive galaxies decreases over time, possibly by accreting low-mass, less alpha-enhanced galaxies. A larger galaxy sample is needed to confirm this scenario. Finally, the abundance ratios indicate short star-formation timescales of 0.2-1.0 Gyr.
Using new long-slit spectroscopy obtained with X-Shooter at ESO-VLT, we study, for the first time, radial gradients of optical and Near-Infrared IMF-sensitive features in a representative sample of galaxies at the very high-mass end of the galaxy population. The sample consists of seven early-type galaxies (ETGs) at $zsim0.05$, with central velocity dispersion in the range $300<sigma<350$km/s. Using state-of-art stellar population synthesis models, we fit a number of spectral indices, from different chemical species (including TiOs and Na indices), to constrain the IMF slope (i.e. the fraction of low-mass stars), as a function of galactocentric distance, over a radial range out to $sim4$kpc. ETGs in our sample show a significant correlation of IMF slope and surface mass density. The bottom-heavy population (i.e. an excess of low-mass stars in the IMF) is confined to central galaxy regions with surface mass density above $sim 10^{10} M_odot kpc^{-2}$, or, alternatively, within a characteristic radius of $sim2$~kpc. Radial distance, in physical units, and surface mass density, are the best correlators to IMF variations, with respect to other dynamical (e.g. velocity dispersion) and stellar population (e.g. metallicity) properties. Our results for the most massive galaxies suggest that there is no single parameter} that fully explains variations in the stellar IMF, but IMF radial profiles at z$sim$0 rather result from the complex formation and mass accretion history of galaxy inner and outer regions.
We investigate the cosmic evolution of the internal structure of massive early-type galaxies over half of the age of the Universe. We perform a joint lensing and stellar dynamics analysis of a sample of 81 strong lenses from the SL2S and SLACS surveys and combine the results with a hierarchical Bayesian inference method to measure the distribution of dark matter mass and stellar IMF across the population of massive early-type galaxies. Lensing selection effects are taken into account. We find that the dark matter mass projected within the inner 5 kpc increases for increasing redshift, decreases for increasing stellar mass density, but is roughly constant along the evolutionary tracks of early-type galaxies. The average dark matter slope is consistent with that of an NFW profile, but is not well constrained. The stellar IMF normalization is close to a Salpeter IMF at $log{M_*} = 11.5$ and scales strongly with increasing stellar mass. No dependence of the IMF on redshift or stellar mass density is detected. The anti-correlation between dark matter mass and stellar mass density supports the idea of mergers being more frequent in more massive dark matter halos.
Observational studies are showing that the galaxy-wide stellar initial mass function are top-heavy in galaxies with high star-formation rates (SFRs). Calculating the integrated galactic stellar initial mass function (IGIMF) as a function of the SFR of a galaxy, it follows that galaxies which have or which formed with SFRs > 10 Msol yr^-1 would have a top-heavy IGIMF in excellent consistency with the observations. Consequently and in agreement with observations, elliptical galaxies would have higher M/L ratios as a result of the overabundance of stellar remnants compared to a stellar population that formed with an invariant canonical stellar initial mass function (IMF). For the Milky Way, the IGIMF yields very good agreement with the disk- and the bulge-IMF determinations. Our conclusions are that purely stochastic descriptions of star formation on the scales of a pc and above are falsified. Instead, star formation follows the laws, stated here as axioms, which define the IGIMF theory. We also find evidence that the power-law index beta of the embedded cluster mass function decreases with increasing SFR. We propose further tests of the IGIMF theory through counting massive stars in dwarf galaxies.
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