Priyamvada Natarajan

A Wide-Field Hubble Space Telescope Study of the Cluster Cl 0024+16 at z = 0.4. I. Morphological Distributions to 5 Mpc Radius

We describe a new wide-field Hubble Space Telescope survey of the galaxy cluster Cl 0024+16 (z ≈ 0.4) consisting of a sparsely sampled mosaic of 39 Wide Field Planetary Camera 2 images that extends to a cluster radius of ~5 Mpc. Together with extensive ground-based spectroscopy taken from the literature, augmented with over a hundred newly determined redshifts, this unique data set enables us to examine environmental influences on the properties of cluster members from the inner core to well beyond the virial radius (~1.7 Mpc). We catalog photometric measures for 22,000 objects to I 25 and assign morphological types for 2181 to I = 22.5, of which 195 are spectroscopically confirmed cluster members. We examine both the morphology-radius (T-R) and morphology-density (T-Σ) relations and demonstrate sensitivities adequate for measures from the core to a radius of ~5 Mpc, spanning over 3 decades in local projected density. The fraction of early-type galaxies declines steeply from the cluster center to 1 Mpc radius and more gradually thereafter, asymptoting toward the field value at the periphery. We discuss our results in the context of three distinct cluster zones, defined according to different physical processes that may be effective in transforming galaxy morphology in each. By treating infalling galaxies as isolated test particles, we deduce that the most likely processes responsible for the mild gradient in the morphological mix outside the virial radius are harassment and starvation. Although more data are needed to pin down the exact mechanisms, starvation seems more promising in that it would naturally explain the stellar and dynamical homogeneity of cluster E/S0s. However, we find significant scatter in the local density at any given radius outside ~0.5 Mpc and that the same T-Σ relation holds in subregions of the cluster, independent of location. In this hitherto unprobed region, where the potential of the cluster is weak, galaxies apparently retain their identities as members of infalling subgroups whose characteristic morphological properties remain intact. Only upon arrival in the central regions is the substructure erased, as indicated by the tight correlation between cluster radius and Σ.

Supermassive black hole formation during the assembly of pre-galactic discs

In this paper, we discuss the evolution of gravitationally unstable pre-galactic discs that result from the collapse of haloes at high redshift z ≈ 10 or so, which have not yet been enriched by metals. In cases where molecular hydrogen formation is suppressed, the discs are maintained at a temperature of a few thousand Kelvin. However, when molecular hydrogen is present, cooling can proceed down to a few hundred Kelvin. Analogous to the case of the larger-scale protogalactic discs, we assume that the evolution of these discs is mainly driven by angular momentum redistribution induced by the development of gravitational instabilities in the disc. We also properly take into account the possibility of disc fragmentation. We thus show that this simple model naturally predicts the formation of supermassive black holes in the nuclei of such discs and provides a robust determination of their mass distribution as a function of halo properties. We estimate that roughly 5 per cent of discs resulting from the collapse of haloes with M ≈ 10 7 M ⊙ should host a massive black hole with a mass M BH ≈ 10 5 M ⊙ . We confirm our arguments with time-dependent calculations of the evolution of the surface density and of the accretion rate in these primordial discs. The luminosity of the outer, colder disc is expected to be very low (in the range of a few thousand L ⊙ ), while the formation of the black hole is expected to produce a burst with a luminosity of a few times 10 9 L ⊙ . This mechanism offers an efficient way to form seed black holes at high redshift. The predicted masses for our black hole seeds enable the comfortable assembly of 10 9 -M ⊙ black holes powering the luminous quasars detected by the Sloan Digital Sky Survey at z = 6 for a concordance cosmology.

Spin-induced Galaxy Alignments and Their Implications for Weak-Lensing Measurements

Large-scale correlations in the orientations of galaxies can result from alignments in their angular momentum vectors. These alignments arise from the tidal torques exerted on neighboring protogalaxies by the smoothly varying shear field. We compute the predicted amplitude of such ellipticity correlations using the Zeldovich approximation for a realistic distribution of galaxy shapes. Weak gravitational lensing can also induce ellipticity correlations, since the images of neighboring galaxies will be distorted coherently. On comparing these two effects that induce shape correlations, we find that for current weak-lensing surveys with a median redshift of zm = 1, the intrinsic signal is of the order of 1%-10% of the measured signal. However, for shallower surveys with zm ≤ 0.3, the intrinsic correlations dominate over the lensing signal. The distortions induced by lensing are curl-free, whereas those resulting from intrinsic alignments are not. This difference can be used to disentangle these two sources of ellipticity correlations.

A WIDE-FIELD HUBBLE SPACE TELESCOPE STUDY OF THE CLUSTER Cl 0024+1654 AT z = 0.4. II. THE CLUSTER MASS DISTRIBUTION

We present a comprehensive lensing analysis of the rich cluster Cl0024+1654 (z=0.395) based on panoramic sparse-sampled imaging conducted with the WFPC2 and STIS cameras on board the Hubble Space Telescope. By comparing higher fidelity signals in the limited STIS data with the wider field data available from WFPC2, we demonstrate an ability to detect reliably weak lensing signals to a cluster radius of ≃5 h −1 65 Mpc where the mean shear is around 1%. This enables us to study the distribution of dark matter with respect to the cluster light over an unprecedented range of cluster radius and environments. The projected mass distribution reveals a secondary concentration representing 30% of the overall cluster mass, which is also visible in the distribution of cluster member galaxies. We develop a method to derive the projected mass profile of the main cluster taking into account the influence of the secondary clump. We normalize the mass profile determined from the shear by assuming that background galaxies selected with 23 2.4. An isothermal mass profile is therefore strongly rejected, whereas a NFW profile with M200= 6.1 +1.2 1.1 10 14 h −1 65 M⊙ provides a good fit to the lensing data. We isolate cluster members according to their optical-near infrared colors; the red cluster light closely traces the dark matter with a mean mass-to-light ratio of M/LK= 40±5 h65 M⊙/L⊙. Similar profiles for mass and light on 1-5 Mpc scales are expected if cluster assembly is largely governed by infalling groups. Subject headings: cosmology: observations — gravitational lensing — cluster of galaxies: individual (Cl 0024+1654)

The evolution of massive black hole seeds

We investigate the evolution of high-redshift seed black hole masses at late times and their observational signatures. The massive black hole seeds studied here form at extremely high redshifts from the direct collapse of pre-galactic gas discs. Populating dark matter haloes with seeds formed in this way, we follow the mass assembly of these black holes to the present time using a Monte Carlo merger tree. Using this machinery, we predict the black hole mass function at high redshifts and at the present time, the integrated mass density of black holes and the luminosity function of accreting black holes as a function of redshift. These predictions are made for a set of three seed models with varying black hole formation efficiency. Given the accuracy of present observational constraints, all three models can be adequately fitted. Discrimination between the models appears predominantly at the low-mass end of the present-day black hole mass function which is not observationally well constrained. However, all our models predict that low surface brightness, bulgeless galaxies with large discs are least likely to be sites for the formation of massive seed black holes at high redshifts. The efficiency of seed formation at high redshifts has a direct influence on the black hole occupation fraction in galaxies at z = 0. This effect is more pronounced for low-mass galaxies. This is the key discriminant between the models studied here and the Population III remnant seed model. We find that there exist a population of low-mass galaxies that do not host nuclear black holes. Our prediction of the shape of the M BH -σ relation at the low-mass end is in agreement with the recent observational determination from the census of low-mass galaxies in the Virgo cluster.

Accretion during the Merger of Supermassive Black Holes

We study the evolution of disk accretion during the merger of supermassive black hole binaries in galactic nuclei. In hierarchical galaxy formation models, the most common binaries are likely to arise from minor galactic mergers and have unequal-mass black holes. Once such a binary becomes embedded in an accretion disk at a separation of a ~ 0.1 pc, the merger proceeds in two distinct phases. During the first phase, the loss of orbital angular momentum to the gaseous disk shrinks the binary on a timescale of ~107 yr. The accretion rate onto the primary black hole is not increased, and can be substantially reduced, during this disk-driven migration. At smaller separations, gravitational radiation becomes the dominant angular momentum loss process, and any gas trapped inside the orbit of the secondary is driven inward by the inspiralling black hole. The implied accretion rate just prior to coalescence exceeds the Eddington limit, so the final merger is likely to occur within a common envelope formed from the disrupted inner disk and to be accompanied by high-velocity (~104 km s-1) outflows.

Discriminating weak lensing from intrinsic spin correlations using the curl-gradient decomposition

The distortion field defined by the ellipticities of galaxy shapes as projected on the sky can be uniquely decomposed into a gradient and a curl component. If the observed ellipticities are induced by weak gravitational lensing, then the distortion field is curl-free. Here we show that, in contrast, the distortion field resulting from intrinsic spin alignments is not curl-free. This provides a powerful discriminant between lensing and intrinsic contributions to observed ellipticity correlations. We also show how these contributions can be disentangled statistically from the ellipticity correlations or computed locally from circular integrals of the ellipticity field. This allows for an unambiguous detection of intrinsic galaxy alignments in the data. When the distortions are dominated by lensing, as occurs at high redshifts, the decomposition provides a valuable tool for understanding properties of the noise and systematic errors. These techniques can be applied equally well to the polarization of the microwave background, where it can be used to separate curl-free scalar perturbations from those produced by gravity waves or defects.

The observed concentration-mass relation for galaxy clusters

The properties of clusters of galaxies offer key insights into the assembly process of structure in the universe. Numerical simulations of cosmic structure formation in a hierarchical, dark matter dominated universe suggest that galaxy cluster concentrations, which are a measure of a halos central density, decrease gradually with virial mass. However, cluster observations have yet to confirm this correlation. The slopes of the run of measured concentrations with virial mass are often either steeper or flatter than that predicted by simulations. In this work, we present the most complete sample of observed cluster concentrations and masses yet assembled, including new measurements for 10 strong-lensing clusters, thereby more than doubling the existing number of strong-lensing concentration estimates. We fit a power law to the observed concentrations as a function of virial mass, and find that the slope is consistent with the slopes found in simulations, though our normalization factor is higher. Observed lensing concentrations appear to be systematically larger than X-ray concentrations, a more pronounced effect than that found in simulations. We also find that at a fixed mass, the bulk of observed cluster concentrations are distributed lognormally, with the exception of a few anomalously high concentration clusters. We examine the physical processes likely responsible for the discrepancy between lensing and X-ray concentrations, and for the anomalously high concentrations in particular. The forthcoming Millennium simulation results will offer the most comprehensive comparison set to our findings of an observed concentration-mass power law relation.

The Mass-to-Light Ratio of Early-Type Galaxies: Constraints from Gravitational Lensing in the Rich Cluster AC 114*

We present a new wide-field image of the distant cluster AC 114 (z = 0.31) obtained with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. This image considerably extends our knowledge of the lensing properties of the cluster beyond that derived earlier from a single WFPC1 pointing. In conjunction with published ground-based spectroscopy, we utilize several newly discovered multiple images to construct an improved mass model for the central regions of the cluster. Using this model, we apply the methodology introduced by Natarajan & Kneib to interpret local perturbations to the cluster shear field on small scales resulting from mass associated with individual cluster galaxies. We use the lensing signal to place new constraints on the average mass-to-light ratio and spatial extents of the dark matter halos associated with morphologically classified early-type cluster members. We find that the total mass of a fiducial L* cluster spheroidal galaxy is largely contained within ~15 kpc radius halo (~8-10Re), with a mass-to-light ratio M/LV~15−4+10 (90% confidence level) in solar units within this radius. Comparisons with similar estimates for field galaxies suggest that the cluster galaxies in AC 114 may possess less extensive and less massive halos. Additionally, we find some indication that, at a fixed luminosity, S0 galaxies are less extended than ellipticals, suggesting a difference in the efficiency of tidal stripping of different galaxy types. We discuss the consequences of our results in the context of models for the dynamical evolution of cluster galaxies and the observational prospects for extending such analyses.

STAR FORMATION IN THE EARLY UNIVERSE: BEYOND THE TIP OF THE ICEBERG

We present late-time Hubble Space Telescope (HST) imaging of the fields of six Swift gamma-ray bursts (GRBs) lying at 5.0 lsim z lsim 9.5. Our data include very deep observations of the field of the most distant spectroscopically confirmed burst, GRB 090423, at z = 8.2. Using the precise positions afforded by their afterglows, we can place stringent limits on the luminosities of their host galaxies. In one case, that of GRB 060522 at z = 5.11, there is a marginal excess of flux close to the GRB position which may be a detection of a host at a magnitude J AB ≈ 28.5. None of the others are significantly detected, meaning that all the hosts lie below L* at their respective redshifts, with star formation rates (SFRs) lsim 4 M ☉ yr-1 in all cases. Indeed, stacking the five fields with WFC3-IR data, we conclude a mean SFR <0.17 M ☉ yr-1 per galaxy. These results support the proposition that the bulk of star formation, and hence integrated UV luminosity, at high redshifts arises in galaxies below the detection limits of deep-field observations. Making the reasonable assumption that GRB rate is proportional to UV luminosity at early times allows us to compare our limits with expectations based on galaxy luminosity functions (LFs) derived from the Hubble Ultra-Deep Field and other deep fields. We infer that an LF, which is evolving rapidly toward steeper faint-end slope (α) and decreasing characteristic luminosity (L*), as suggested by some other studies, is consistent with our observations, whereas a non-evolving LF shape is ruled out at gsim 90% confidence. Although it is not yet possible to make stronger statements, in the future, with larger samples and a fuller understanding of the conditions required for GRB production, studies like this hold great potential for probing the nature of star formation, the shape of the galaxy LF, and the supply of ionizing photons in the early universe.