arXiv subjects

Chervin F. P. Laporte

Publications and source records attributed to Chervin F. P. Laporte.

59 records · Page 4Linked to original sources

arXiv2014

Under the sword of Damocles: plausible regeneration of dark matter cusps at the smallest galactic scales

We run controlled N-body experiments to study the evolution of the dark matter (DM) halo profiles of dwarf galaxies driven by the accretion of DM substructures. Our initial conditions assume that supernova feedback erases the primordial DM cusps of haloes with $10^{9}-10^{10} \rm{M_{\odot}}$ at $z=0$. The orbits and masses of the infalling substructures are borrowed from the {\it Aquarius} simulations. Our experiments show that a fraction of haloes that undergo 1:3 down to 1:30 mergers are susceptible to reform a DM cusp by $z\approx 0$. Cusp regrowth is driven the accretion of DM substructures that are dense enough reach the central regions of the main halo before being tidally disrupted. The infall of substructures with a mass ratio above 1:6 on the mean of the reported mass-concentration relation systematically lead to cusp regrowth. Between 1:6 to 1:8, and 1:8 to 1:30 substructures need to be located one and two-sigma above the mean, respectively. The merging timescales of these dense, low-mass substructures is relatively long $(5-11 \rm{Gyrs})$, which may pose a timescale problem for the longevity of DM cores in dwarf galaxies. These results suggest that a certain level of scatter in the central density slopes of galactic haloes acted-on by feedback is to be expected given the stochastic mass accretion histories of low-mass haloes and the diverse star formation histories observed in the Local Group dwarves.

astro-ph.GA↗

arXiv2014

The re-distribution of matter in the cores of galaxy clusters

We present cosmological N-body resimulations of the assembly of the Brightest Cluster Galaxies (BCGs) in rich clusters. At $z=2$ we populate dark matter subhalos with self-gravitating stellar systems whose abundance and structure match observed high-redshift galaxies. By $z=0$, mergers have built much larger galaxies at cluster centre. Their dark matter density profiles are shallower than in corresponding dark-matter-only simulations, but their total mass density profiles (stars + dark matter) are quite similar. Differences are found only at radii where the effects of central black holes may be significant. Dark matter density slopes shallower than $γ=1.0$ occur for $r/r_{200} < 0.015$, close to the half-light radii of the BCGs. Our experiments support earlier suggestions that NFW-like profiles are an attractor for the hierarchical growth of structure in collisionless systems -- total mass density profiles asymptote to the solution found in dark-matter-only simulations over the radial range where mergers produce significant mixing between stars and dark matter. Simulated dark matter fractions are substantially higher in BCGs than in field ellipticals, reaching 80\% within the half-light radius. We also estimate that supermassive black hole mergers should create BCG cores as large as $r_{c}\sim 3 \,\mathrm{kpc}$. The good agreement of all these properties with recent observational studies of BCG structure suggests that dissipational processes have not played a dominant role in the assembly of the observed systems.

astro-ph.GA↗

arXiv2013

Measuring the slopes of mass profiles for dwarf spheroidals in triaxial CDM potentials

We generate stellar distribution functions (DFs) in triaxial haloes in order to examine the reliability of slopes $Γ\equiv Δ{\rm log} M / Δ{\rm log} r$ inferred by applying mass estimators of the form $M\propto R_eσ^2$ (i.e. assuming spherical symmetry, where $R_e$ and $σ$ are luminous effective radius and global velocity dispersion, respectively) to two stellar sub-populations independently tracing the same gravitational potential. The DFs take the form $f(E)$, are dynamically stable, and are generated within triaxial potentials corresponding directly to subhaloes formed in cosmological dark-matter-only simulations of Milky Way and galaxy cluster haloes. Additionally, we consider the effect of different tracer number density profiles (cuspy and cored) on the inferred slopes of mass profiles. For the isotropic DFs considered here, we find that halo triaxiality tends to introduce an anti-correlation between $R_e$ and $σ$ when estimated for a variety of viewing angles. The net effect is a negligible contribution to the systematic error associated with the slope of the mass profile, which continues to be dominated by a bias toward greater overestimation of masses for more-concentrated tracer populations. We demonstrate that simple mass estimates for two distinct tracer populations can give reliable (and cosmologically meaningful) lower limits for $Γ$, irrespective of the degree of triaxiality or shape of the tracer number density profile.

astro-ph.GA↗

arXiv2013

The Growth in Size and Mass of Cluster Galaxies since z=2

We study the formation and evolution of Brightest Cluster Galaxies starting from a $z=2$ population of quiescent ellipticals and following them to $z=0$. To this end, we use a suite of nine high-resolution dark matter-only simulations of galaxy clusters in a $Λ$CDM universe. We develop a scheme in which simulation particles are weighted to generate realistic and dynamically stable stellar density profiles at $z=2$. Our initial conditions assign a stellar mass to every identified dark halo as expected from abundance matching; assuming there exists a one-to-one relation between the visible properties of galaxies and their host haloes. We set the sizes of the luminous components according to the observed relations for $z\sim2$ massive quiescent galaxies. We study the evolution of the mass-size relation, the fate of satellite galaxies and the mass aggregation of the cluster central. From $z=2$, these galaxies grow on average in size by a factor 5 to 10 of and in mass by 2 to 3. The stellar mass growth rate of the simulated BCGs in our sample is of 1.9 in the range $0.3<z<1.0$ consistent with observations, and of 1.5 in the range $0.0<z<0.3$. Furthermore the satellite galaxies evolve to the present day mass-size relation by $z=0$. Assuming passively evolving stellar populations, we present surface brightness profiles for our cluster centrals which resemble those observed for the cDs in similar mass clusters both at $z=0$ and at $z=1$. This demonstrates that the $Λ$CDM cosmology does indeed predict minor and major mergers to occur in galaxy clusters with the frequency and mass ratio distribution required to explain the observed growth in size of passive galaxies since $z=2$. Our experiment shows that Brightest Cluster Galaxies can form through dissipationless mergers of quiescent massive $z=2$ galaxies, without substantial additional star formation.

astro-ph.CO↗

arXiv2012

Shallow Dark Matter Cusps in Galaxy Clusters

We study the evolution of the stellar and dark matter components in a galaxy cluster of $10^{15} \, \rm{M_{\odot}}$ from $z=3$ to the present epoch using the high-resolution collisionless simulations of Ruszkowski & Springel (2009). At $z=3$ the dominant progenitor halos were populated with spherical model galaxies with and without accounting for adiabatic contraction. We apply a weighting scheme which allows us to change the relative amount of dark and stellar material assigned to each simulation particle in order to produce luminous properties which agree better with abundance matching arguments and observed bulge sizes at $z=3$. This permits the study of the effect of initial compactness on the evolution of the mass-size relation. We find that for more compact initial stellar distributions the size of the final Brightest Cluster Galaxy grows with mass according to $r\propto M^{2}$, whereas for more extended initial distributions, $r\propto M$. Our results show that collisionless mergers in a cosmological context can reduce the strength of inner dark matter cusps with changes in logarithmic slope of 0.3 to 0.5 at fixed radius. Shallow cusps such as those found recently in several strong lensing clusters thus do not necessarily conflict with CDM, but may rather reflect on the initial structure of the progenitor galaxies, which was shaped at high redshift by their formation process.

astro-ph.CO↗