Carlo Nipoti

THE SL2S GALAXY-SCALE LENS SAMPLE. IV. THE DEPENDENCE OF THE TOTAL MASS DENSITY PROFILE OF EARLY-TYPE GALAXIES ON REDSHIFT, STELLAR MASS, AND SIZE

We present optical and near infrared spectroscopy obtained at Keck, VLT, and Gemini for a sample of 36 secure strong gravitational lens systems and 17 candidates identified as part of the SL2S survey. The deflectors are massive early-type galaxies in the redshift range z_d=0.2-0.8, while the lensed sources are at z_s=1-3.5. We combine this data with photometric and lensing measurements presented in the companion paper III and with lenses from the SLACS and LSD surveys to investigate the cosmic evolution of the internal structure of massive early-type galaxies over half the age of the universe. We study the dependence of the slope of the total mass density profile gamma (rho(r)propto r^{-gamma}) on stellar mass, size, and redshift. We find that two parameters are sufficent to determine gamma with less than 6% residual scatter. At fixed redshift, gamma depends solely on the surface stellar mass density partial gamma/ partial Sigma_*=0.38pm 0.07, i.e. galaxies with denser stars also have steeper slopes. At fixed M_* and R_{eff}, gamma depends on redshift, in the sense that galaxies at a lower redshift have steeper slopes (partial gamma / partial z = -0.31pm 0.10). However, the mean redshift evolution of gamma for an individual galaxy is consistent with zero dgamma/dz=-0.10pm0.12. This result is obtained by combining our measured dependencies of gamma on z,M_*,R_{eff} with the evolution of the R_{eff}-M_* taken from the literature, and is broadly consistent with current models of the formation and evolution of massive early-type galaxies. Detailed quantitative comparisons of our results with theory will provide qualitatively new information on the detailed physical processes at work.

The SL2S galaxy-scale lens sample. V. dark matter halos and stellar IMF of massive early-type galaxies out to redshift 0.8

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 Strong Lensing Legacy Survey and Sloan ACS Lens Survey and combine the results with a hierarchical Bayesian inference method to measure the distribution of dark matter mass and stellar initial mass function (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 a Navarro-Frenk-White 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.

Do globular clusters possess dark matter haloes? A case study in NGC 2419

We use recently published measurements of the kinematics, surface brightness and stellar mass-to-light ratio of the globular cluster NGC 2419 to examine the possibility that this Galactic halo satellite is embedded in a low-mass dark matter halo. NGC 2419 is a promising target for such a study, since its extreme Galactocentric distance and large mass would have greatly facilitated the retention of dark matter. A Markov-Chain Monte Carlo approach is used to investigate composite dynamical models containing a stellar and a dark matter component. We find that it is unlikely that a significant amount of dark matter ( < � 6% of the luminous mass inside the tidal limit of the cluster) can be present if the stars follow an anisotropic Michie model and the dark matter a double power law model. However, we find that more general models, derived using a new technique we have developed to compute non-parametric solutions to the spherical Jeans equation, suggest the presence of a significant dark matter fraction (approximately twice the stellar mass). Thus the presence of a dark matter halo around NGC 2419 cannot be fully ruled out at present, yet any dark matter within the 10 ′ visible extent of the cluster must be highly concentrated and cannot exceed 1.1 × 10 6 M⊙ (99% confidence), in stark contrast to expectations for a plausible progenitor halo of this structure.

PURELY DRY MERGERS DO NOT EXPLAIN THE OBSERVED EVOLUTION OF MASSIVE EARLY-TYPE GALAXIES SINCE z ∼ 1

Several studies have suggested that the observed size evolution of massive early-type galaxies (ETGs) can be explained as a combination of dry mergers and progenitor bias, at least since z ~ 1. In this paper we carry out a new test of the dry-merger scenario based on recent lensing measurements of the evolution of the mass density profile of ETGs. We construct a theoretical model for the joint evolution of the size and mass density profile slope γ driven by dry mergers occurring at rates given by cosmological simulations. Such dry-merger model predicts a strong decrease of γ with cosmic time, inconsistent with the almost constant γ inferred from observations in the redshift range 0 99% CL. We thus suggest a scenario where the outer regions of massive ETGs grow by accretion of stars and dark matter, while small amounts of dissipation and nuclear star formation conspire to keep the mass density profile constant and approximately isothermal.

The imprint of dark matter haloes on the size and velocity dispersion evolution of early-type galaxies

Early-type galaxies (ETGs) are observed to be more compact, on average, at z ≳ 2 than at z ≃ 0, at fixed stellar mass. Recent observational works suggest that such size evolution could reflect the similar evolution of the host dark matter halo density as a function of the time of galaxy quenching. We explore this hypothesis by studying the distribution of halo central velocity dispersion (� 0) and half-mass radius (rh) as functions of halo mass M and redshift z, in a cosmological � -CDM N-body simulation. In the range 0 ≲ z ≲ 2: 5, we find � 0 / M 0: 31 0: 37 and rh / M 0: 28 0: 32 , close to the values expected for homologous virialized systems. At fixed M in the range 10 11 M⊙ ≲ M ≲ 5: 5 � 10 14 M⊙ we find � 0 / (1 + z) 0: 35 and rh / (1 + z) 0: 7 . We show that such evolution of the halo scaling laws is driven by individual haloes growing in mass following the evolutionary tracks � 0 / M 0: 2 and rh / M 0: 6 , consistent with simple dissipationless mergingmodels in which the encounter orbital energyis accounted for. We compare the N-body data with ETGs observed at 0 ≲ z ≲ 3 by populating the haloes with a stellar component under simple but justified assumptions: the resulting galaxies evolve consistently with the observed ETGs up to z ≃ 2, but the model has difficulty reproducing the fast evolution observed at z ≳ 2. We conclude that a substantial fraction of the size evolution of ETGs can be ascribed to a systematic dependence on redshift of the dark matter haloes structural properties.

Gas rotation in galaxy clusters: signatures and detectability in X-rays

We study simple models of massive galaxy clusters in which the intracluster medium (ICM) rotates differentially in equilibrium in the cluster gravitational potential. We obtain the X-ray surface brightness maps, evaluating the isophote flattening due to the gas rotation. Using a set of different rotation laws, we put constraint on the amplitude of the rotation velocity, finding that rotation curves with peak velocity up to sim 600 km s^-1 are consistent with the ellipticity profiles of observed clusters. We convolve each of our models with the instrument response of the X-ray Calorimeter Spectrometer on board the ASTRO-H to calculate the simulated X-ray spectra at different distance from the X-ray centre. We demonstrate that such an instrument will allow us to measure rotation of the ICM in massive clusters, even with rotation velocities as low as sim 100 km s^-1

The effect of tides on the Fornax dwarf spheroidal galaxy

The authors thank M. Breddels for providing Fornaxs LOS velocity-dispersion profile measured in elliptical annuli and the referee, M. Wilkinson, for useful comments. GB thanks the INAF - Bologna for the hospitality during part of this work and gratefully acknowledges support through a Marie-Curie action Intra European Fellowship, funded by the European Union Seventh Framework Programme (FP7/2007-2013) under Grant agreement number PIEF-GA-2010-274151, as well as the financial support by the Spanish Ministry of Economy and Competitiveness (MINECO) under the Ramon y Cajal Programme (RYC-2012-11537). AS acknowledges financial support from PRIN INAF 2011 ‘Multiple populations in globular clusters: their role in the Galaxy assembly’ (PI E. Carretta). CN thanks the Instituto de Astrofisica de Canarias for the hospitality and acknowledges financial support from PRIN MIUR 2010-2011, project ‘The Chemical and Dynamical Evolution of the Milky Way and Local Group Galaxies’, prot. 2010LY5N2T.

UNDERSTANDING THE UNIQUE ASSEMBLY HISTORY OF CENTRAL GROUP GALAXIES

Central galaxies (CGs) in massive halos live in unique environments with formation histories closely linked to that of the host halo. In local clusters, they have larger sizes (R-e) and lower velocity dispersions (sigma) at fixed stellar mass M-*, and much larger R-e at a fixed sigma than field and satellite galaxies (non-CGs). Using spectroscopic observations of group galaxies selected from the COSMOS survey, we compare the dynamical scaling relations of early-type CGs and non-CGs at z similar to 0.6 to distinguish possible mechanisms that produce the required evolution. CGs are systematically offset toward larger R-e at fixed sigma compared to non-CGs with similar M-*. The CG R-e-M-* relation also shows differences, primarily driven by a subpopulation (similar to 15%) of galaxies with large R-e, while the M-*-sigma relations are indistinguishable. These results are accentuated when double Sersic profiles, which better fit light in the outer regions of galaxies, are adopted. They suggest that even group-scale CGs can develop extended components by these redshifts that can increase total R-e and M-* estimates by factors of similar to 2. To probe the evolutionary link between our sample and cluster CGs, we also analyze two cluster samples at z similar to 0.6 and z similar to 0. We find similar results for the more massive halos at comparable z, but much more distinct CG scaling relations at low-z. Thus, the rapid, late-time accretion of outer components, perhaps via the stripping and accretion of satellites, would appear to be a key feature that distinguishes the evolutionary history of CGs.

On the nature of local instabilities in rotating galactic coronae and cool cores of galaxy clusters

A long-standing question is whether radiative cooling can lead to local condensation of cold gas in the hot atmospheres of galaxies and galaxy clusters. We address this problem by studying the nature of local instabilities in rotating, stratified, weakly magnetized, optically thin plasmas in the presence of radiative cooling and anisotropic thermal conduction. For both axisymmetric and nonaxisymmetric linear perturbations, we provide general equations which can be applied locally to specific systems to establish whether they are unstable and, in case of instability, to determine the kind of evolution (monotonically growing or overstable) and the growth rates of the unstable modes. We present results for models of rotating plasmas representative of Milky-Way-like galaxy coronae and cool-cores of galaxy clusters. We show that the unstable modes arise from a combination of thermal, magnetothermal, magnetorotational, and heat-flux-driven buoyancy instabilities. Local condensation of cold clouds tends to be hampered in cluster cool cores, while it is possible under certain conditions in rotating galactic coronae. If the magnetic field is sufficiently weak, then the magnetorotational instability is dominant even in these pressure-supported systems.

A fast and accurate method to compute the mass return from multiple stellar populations

The mass returned to the ambient medium by aging stellar populations over cosmological times sums up to a significant fraction (20% - 30% or more) of their initial mass. This continuous mass injection plays a fundamental role in phenomena such as galaxy formation and evolution, fueling of supermassive black holes in galaxies and the consequent (negative and positive) feedback phenomena, and the origin of multiple stellar populations in globular clusters. In numerical simulations the calculation of the mass return can be time consuming, since it requires at each time step the evaluation of a convolution integral over the whole star formation history, so the computational time increases quadratically with the number of time-steps. The situation can be especially critical in hydrodynamical simulations, where different grid points are characterized by different star formation histories, and the gas cooling and heating times are shorter by orders of magnitude than the characteristic stellar lifetimes. In this paper we present a fast and accurate method to compute the mass return from stellar populations undergoing arbitrarily complicated star formation histories. At each time-step the mass return is calculated from its value at the previous time, and the star formation rate over the last time-step only. Therefore in the new scheme there is no need to store the whole star formation history, and the computational time increases linearly with the number of time-steps.