Central Dark Matter Fraction Research Articles

Inferring galaxy dark halo properties from visible matter with machine learning

ABSTRACT Next-generation surveys will provide photometric and spectroscopic data of millions to billions of galaxies with unprecedented precision. This offers a unique chance to improve our understanding of the galaxy evolution and the unresolved nature of dark matter (DM). At galaxy scales, the density distribution of DM is strongly affected by feedback processes, which are difficult to fully account for in classical techniques to derive galaxy masses. We explore the capability of supervised machine learning (ML) algorithms to predict the DM content of galaxies from ‘luminous’ observational-like parameters, using the TNG100 simulation. In particular, we use photometric (magnitudes in different bands), structural (the stellar half-mass radius and three different baryonic masses), and kinematic (1D velocity dispersion and the maximum rotation velocity) parameters to predict the total DM mass, DM half-mass radius, and DM mass inside one and two stellar half-mass radii. We adopt the coefficient of determination, R2, as a metric to evaluate the accuracy of these predictions. We find that using all observational quantities together (photometry, structural, and kinematics), we reach high accuracy for all DM quantities (up to R2 ∼ 0.98). This first test shows that ML tools are promising to predict the DM in real galaxies. The next steps will be to implement the observational realism of the training sets, by closely selecting samples that accurately reproduce the typical observed ‘luminous’ scaling relations. The so-trained pipelines will be suitable for real galaxy data collected from Rubin/Large Synoptic Survey Telescope (LSST), Euclid, Chinese Survey Space Telescope (CSST), 4-metre Multi-Object Spectrograph Telescope (4MOST), Dark Energy Spectroscopic Instrument (DESI), to derive e.g. the properties of their central DM fractions.

The kinematics and dark matter fractions of TNG50 galaxies at z = 2 from an observational perspective

ABSTRACT 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 ΛCDM framework to observations. To this end, we create realistic mock observations of massive SFGs ($M_*\gt 4\times 10^{10} \, \mathrm{M}_{\odot}$, SFR >50 M⊙ 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 modelling 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 1D velocity profiles can vary along different lines of sight by up to Δv = 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 ($f_{\rm DM}^v(\lt R_e)=v_{\rm DM}^2(R_e)/v_{\rm circ}^2(R_e)\sim 0.32\pm 0.10$), however for disc effective radii Re that are mostly too small: at fixed Re the TNG50 dark matter fractions are too high by a factor of ∼2. 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.

Early-type galaxy density profiles from IllustrisTNG – I. Galaxy correlations and the impact of baryons

ABSTRACT We explore the isothermal total density profiles of early-type galaxies (ETGs) in the IllustrisTNG simulation. For the selected 559 ETGs at z = 0 with stellar masses $10^{10.7}\, \mathrm{M}_{\odot } \leqslant M_{\ast } \leqslant 10^{11.9}\, \mathrm{M}_{\odot }$, the total power-law slope has a mean of 〈γ′〉 = 2.011 ± 0.007 and a scatter of $\sigma _{\gamma ^{\prime }} = 0.171$ over the radial range 0.4–4 times the stellar half-mass radius. Several correlations between γ′ and galactic properties including stellar mass, effective radius, stellar surface density, central velocity dispersion, central dark matter fraction, and in situ-formed stellar mass ratio are compared to observations and other simulations, revealing that IllustrisTNG reproduces many correlation trends, and in particular, γ′ is almost constant with redshift below z = 2. Through analysing IllustrisTNG model variations, we show that black hole kinetic winds are crucial to lowering γ′ and matching observed galaxy correlations. The effects of stellar winds on γ′ are subdominant compared to active galactic nucleus (AGN) feedback, and differ due to the presence of AGN feedback from previous works. The density profiles of the ETG dark matter haloes are well described by steeper than NFW profiles, and they are steeper in the full physics (FP) run than their counterparts in the dark matter-only (DMO) run. Their inner density slopes anticorrelate (remain constant) with the halo mass in the FP (DMO) run, and anticorrelate with the halo concentration parameter c200 in both the types of runs. The dark matter haloes of low-mass ETGs are contracted whereas high-mass ETGs are expanded, suggesting that variations in the total density profile occur through the different halo responses to baryons.

The inner structure of early-type galaxies in the Illustris simulation

Early-type galaxies provide unique tests for the predictions of the cold dark matter cosmology and the baryonic physics assumptions entering models for galaxy formation. In this work, we use the Illustris simulation to study correlations of three main properties of early-type galaxies, namely, the stellar orbital anisotropies, the central dark matter fractions and the central radial density slopes, as well as their redshift evolution since $z=1.0$. We find that lower-mass galaxies or galaxies at higher redshift tend to be bluer in rest-frame colour, have higher central gas fractions, and feature more tangentially anisotropic orbits and steeper central density slopes than their higher-mass or lower-redshift counterparts, respectively. The projected central dark matter fraction within the effective radius shows a very mild mass dependence but positively correlates with galaxy effective radii due to the aperture effect. The central density slopes obtained by combining strong lensing measurements with single aperture kinematics are found to differ from the true density slopes. We identify systematic biases in this measurement to be due to two common modelling assumptions, isotropic stellar orbital distributions and power-law density profiles. We also compare the properties of early-type galaxies in Illustris to those from existing galaxy and strong lensing surveys, we find in general broad agreement but also some tension, which poses a potential challenge to the stellar formation and feedback models adopted by the simulation.

The co-evolution of total density profiles and central dark matter fractions in simulated early-type galaxies

We present evidence from cosmological hydrodynamical simulations for a co-evolution of the slope of the total (dark and stellar) mass density profile, gamma_tot, and the dark matter fraction within the half-mass radius, f_DM, in early-type galaxies. The relation can be described as gamma_tot = A f_DM + B for all systems at all redshifts. The trend is set by the decreasing importance of gas dissipation towards lower redshifts and for more massive systems. Early-type galaxies are smaller, more concentrated, have lower f_DM and steeper gamma_tot at high redshifts and at lower masses for a given redshift; f_DM and gamma_tot are good indicators for growth by "dry" merging. The values for A and B change distinctively for different feedback models, and this relation can be used as a test for such models. A similar correlation exists between gamma_tot and the stellar mass surface density Sigma_*. A model with weak stellar feedback and feedback from black holes is in best agreement with observations. All simulations, independent of the assumed feedback model, predict steeper gamma_tot and lower f_DM at higher redshifts. While the latter is in agreement with the observed trends, the former is in conflict with lensing observations, which indicate constant or decreasing gamma_tot. This discrepancy is shown to be artificial: the observed trends can be reproduced from the simulations using observational methodology to calculate the total density slopes.

The stellar and dark matter distributions in elliptical galaxies from the ensemble of strong gravitational lenses

We derive the average mass profile of elliptical galaxies from the ensemble of 161 strong gravitational lens systems selected from several surveys, assuming that the mass profile scales with the stellar mass and effective radius of each lensing galaxy. The total mass profile is well fitted by a power-law \rho(r) \propto r^\gamma with best-fit slope \gamma = -2.11+/-0.05. The decomposition of the total mass profile into stellar and dark matter distributions is difficult due to a fundamental degeneracy between the stellar initial mass function (IMF) and the dark matter fraction f_DM. We demonstrate that this IMF-f_DM degeneracy can be broken by adding direct stellar mass fraction measurements by quasar microlensing observations. Our best-fit model prefers the Salpeter IMF over the Chabrier IMF, and a smaller central dark matter fraction than that predicted by adiabatic contraction models.

AN INVENTORY OF THE STELLAR INITIAL MASS FUNCTION IN EARLY-TYPE GALAXIES

Given a flurry of recent claims for systematic variations in the stellar initial mass function (IMF), we carry out the first inventory of the observational evidence using different approaches. This includes literature results, as well as our own new findings from combined stellar-populations synthesis (SPS) and Jeans dynamical analyses of data on $\sim$~4500 early-type galaxies (ETGs) from the SPIDER project. We focus on the mass-to-light ratio mismatch relative to the Milky Way IMF, \dimf, correlated against the central stellar velocity dispersion, \sigs. We find a strong correlation between \dimf\ and \sigs, for a wide set of dark matter (DM) model profiles. These results are robust if a uniform halo response to baryons is adopted across the sample. The overall normalization of \dimf, and the detailed DM profile, are less certain, but the data are consistent with standard cold-DM halos, and a central DM fraction that is roughly constant with \sigs. For a variety of related studies in the literature, using SPS, dynamics, and gravitational lensing, similar results are found. Studies based solely on spectroscopic line diagnostics agree on a Salpeter-like IMF at high \sigs, but differ at low \sigs. Overall, we find that multiple independent lines of evidence appear to be converging on a systematic variation in the IMF, such that high-\sigs\ ETGs have an excess of low-mass stars relative to spirals and low-\sigs\ ETGs. Robust verification of super-Salpeter IMFs in the highest-\sigs\ galaxies will require additional scrutiny of scatter and systematic uncertainties. The implications for the distribution of DM are still inconclusive.

Structural evolution of massive early-type galaxies

AbstractWe use a large sample of cosmological re-simulations of individual massive galaxies to investigate the origin of the strong increase in sizes and weak decrease of the stellar velocity dispersions since z = 2. At the end of a rapid early phase of star-formation, where stars are created from infalling cold gas, our simulated galaxies are all compact with projected half-mass radii of ≲ 1 kpc and central line-of-sight velocity dispersions of ≈ 262 km s−1. At lower redshifts (z < 2) those galaxies grow predominantly by the accretion of smaller stellar systems and evolve towards the observed local mass-size and mass-velocity dispersion relations. This loss of compactness is accompanied with an increase of central dark matter fractions. We find that the structural evolution of massive galaxies can be explained by frequent minor stellar mergers, which is the dominant mode of accretion for our simulated galaxies.

DEEP ABSORPTION LINE STUDIES OF QUIESCENT GALAXIES ATz∼ 2: THE DYNAMICAL-MASS–SIZE RELATION AND FIRST CONSTRAINTS ON THE FUNDAMENTAL PLANE

We present dynamical and structural scaling relations of quiescent galaxies at z = 2, including the dynamical-mass–size relation and the first constraints on the fundamental plane (FP). The backbone of the analysis is a new, very deep Very Large Telescope/X-shooter spectrum of a massive, compact, quiescent galaxy at z = 2.0389. We detect the continuum between 3700 and 22,000 Å and several strong absorption features (Balmer series, Ca H+K, G band) from which we derive a stellar velocity dispersion of 318 ± 53 km s−1. We perform detailed modeling of the continuum emission and line indices and derive strong simultaneous constraints on the age, metallicity, and stellar mass. The galaxy is a dusty (AV = 0.77+0.36−0.32) solar metallicity (log(Z/Z☉) = 0.02+0.20−0.41) post-starburst galaxy, with a mean-luminosity-weighted log(age/yr) of 8.9 ± 0.1. The galaxy formed the majority of its stars at z > 3 and currently has little or no ongoing star formation. We compile a sample of three other z ∼ 2 quiescent galaxies with measured velocity dispersions, two of which are also post-starburst like. Their dynamical-mass–size relation is offset significantly less than the stellar-mass–size relation from the local early-type relations, which we attribute to a lower central dark matter fraction. Recent cosmological merger simulations agree qualitatively with the data, but cannot fully account for the evolution in the dark matter fraction. The z ∼ 2 FP requires additional evolution beyond passive stellar aging to be in agreement with the local FP. The structural evolution predicted by the cosmological simulations is insufficient, suggesting that additional, possibly non-homologous, structural evolution is needed.

Red mergers and the assembly of massive elliptical galaxies: the fundamental plane and its projections

Several recent observations suggest that gas-poor (dissipationless) mergers of elliptical galaxies contribute significantly to the build-up of the massive end of the red sequence at z≲ 1. We perform a series of major merger simulations to investigate the spatial and velocity structure of the remnants of such mergers. Regardless of orbital energy or angular momentum, we find that the stellar remnants lie on the fundamental plane defined by their progenitors, a result of virial equilibrium with a small tilt due to an increasing central dark matter fraction. However, the locations of merger remnants in the projections of the fundamental plane – the Faber–Jackson and Re–M* relations – depend strongly on the merger orbit, and the relations steepen significantly from the canonical scalings (L∝σ4e and Re∝M0.6*) for mergers on radial orbits. This steepening arises because stellar bulges on orbits with lower angular momentum lose less energy via dynamical friction on the dark matter haloes than do bulges on orbits with substantial angular momentum. This results in a less tightly bound remnant bulge with a smaller velocity dispersion and a larger effective radius. Our results imply that the projections of the fundamental plane – but not necessarily the plane itself – provide a powerful way of investigating the assembly history of massive elliptical galaxies, including the brightest cluster galaxies at or near the centres of galaxy clusters. We argue that most massive ellipticals are formed by anisotropic merging and that their fundamental plane projections should thus differ noticeably from those of lower mass ellipticals even though they should lie on the same fundamental plane. Current observations are consistent with this conclusion. The steepening in the L–σe relation for luminous ellipticals may also be reflected in a corresponding steepening in the MBH–σe relation for massive black holes.