Galaxy clusters are expected to form hierarchically in a cold dark matter (CDM) universe, growing primarily through mergers with lower mass clusters and the continual accretion of group-mass haloes. Galaxy clusters assemble late, doubling their masses since z ∼ 0.5, and so the outer regions of clusters should be replete with accreting group-mass systems. We present an XMM-Newton survey to search for X-ray groups in the infall regions of 23 massive galaxy clusters (M 200 ∼ 10 15 M) at z ∼ 0.2, identifying 39 X-ray groups that have been spectroscopically confirmed to lie at the cluster redshift. These groups have mass estimates in the range 2 × 10 13-7 × 10 14 M , and group-to-cluster mass ratios as low as 0.02. The comoving number density of X-ray groups in the infall regions is ∼25× higher than that seen for isolated X-ray groups from the XXL survey. The average mass per cluster contained within these X-ray groups is 2.2 × 10 14 M , or 19 ± 5 per cent of the mass within the primary cluster itself. We estimate that ∼10 15 M clusters increase their masses by 16 ± 4 per cent between z = 0.223 and the present day due to the accretion of groups with M 200 ≥ 10 13.2 M. This represents about half of the expected mass growth rate of clusters at these late epochs. The other half is likely to come from smooth accretion of matter not bound within haloes. The mass function of the infalling X-ray groups appears significantly top heavy with respect to that of 'field' X-ray systems, consistent with expectations from numerical simulations, and the basic consequences of collapsed massive dark matter haloes being biased tracers of the underlying large-scale density distribution.
Many distant objects can only be detected, or become more scientifically valuable, if they have been highly magnified by strong gravitational lensing. We use eagle and bahamas, two recent cosmological hydrodynamical simulations, to predict the probability distribution for both the lens mass and lens redshift when point sources are highly magnified by gravitational lensing. For sources at a redshift of 2, we find the distribution of lens redshifts to be broad, peaking at z ≈ 0.6. The contribution of different lens masses is also fairly broad, with most high-magnification lensing due to lenses with halo masses between 1012 and $10^{14} \mathrm{\, M_\odot }$. Lower mass haloes are inefficient lenses, while more massive haloes are rare. We find that a simple model in which all haloes have singular isothermal sphere density profiles can approximately reproduce the simulation predictions, although such a model overpredicts the importance of haloes with mass $\lt 10^{12} \mathrm{\, M_\odot }$ for lensing. We also calculate the probability that point sources at different redshifts are strongly lensed. At low redshift, high magnifications are extremely unlikely. Each z = 0.5 source produces, on average, 5 × 10−7 images with magnification greater than 10; for z = 2, this increases to about 2 × 10−5. Our results imply that searches for strongly lensed optical transients, including the optical counterparts to strongly lensed gravitational waves, can be optimized by monitoring massive galaxies, groups, and clusters rather than concentrating on an individual population of lenses.
We report direct evidence of pre-processing of the galaxies residing in galaxy groups falling into galaxy clusters drawn from the Local Cluster Substructure Survey (LoCuSS). 34 groups have been identified via their X-ray emission in the infall regions of 23 massive ( M 200 = 10 15 M ) clusters at 0.15 < z < 0.3. Highly complete spectroscopic coverage combined with 24 µm imaging from Spitzer allows us to make a consistent and robust selection of cluster and group members including star forming galaxies down to a stellar mass limit of M = 2 × 10 10 M . The fraction f SF of star forming galaxies in infalling groups is lower and with a flatter trend with respect to clustercentric radius when compared to the rest of the cluster galaxy population. At R ≈ 1.3 r 200 the fraction of star forming galaxies in infalling groups is half that in the cluster galaxy population. This is direct evidence that star formation quenching is effective in galaxies already prior to them settling in the cluster potential, and that groups are favourable locations for this process.
Context. We investigate the possible presence of active galactic nuclei (AGN) in dwarf galaxies and other nearby galaxies to identify candidates for follow-up confirmation and dynamical mass measurements. Aims. We identify candidate active central massive black holes (CMBH) using their mid-infrared emission, verify their nature using existing catalogues and optical line emission diagnostics, and study the relationship between their mass and the mass of their host galaxy. Methods. We use the Wide-field Infrared Survey Explorer (WISE) All-Sky Release Source Catalog and examine the infrared colours of a sample of dwarf galaxies and other nearby galaxies in order to identify both unobscured and obscured candidate AGN by applying the infrared colour diagnostic. Stellar masses of galaxies are obtained using a combination of three independent methods. Black hole masses are estimated using the bolometric luminosity of the AGN candidates and computed for three cases of the bolometric-toEddington luminosity ratio. Results. We identify 303 candidate AGN, of which 276 were subsequently found to have been independently identified as AGN via other methods. The remaining 9% require follow-up observations for confirmation. The activity is detected in galaxies with stellar masses from ∼ 10 6 to 10 9 M ; assuming the candidates are AGN, the black hole masses are estimated to be ∼ 10 3 − 10 6 M , adopting L bol = 0.1L Edd . The black hole masses probed are several orders of magnitude smaller than previously reported for centrally located massive black holes. We examine the stellar mass versus black hole mass relationship in this low galaxy mass regime. We find that it is consistent with the existing relation extending linearly (in log-log space) into the lower mass regime. Conclusions. These findings suggest that CMBH are present in low-mass galaxies and in the Local Universe, and provide new impetus for follow-up dynamical studies of quiescent black holes in local dwarf galaxies.
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