[en] Void galaxies, residing within the deepest underdensities of the Cosmic Web, present an ideal population for the study of galaxy formation and evolution in an environment undisturbed by the complex processes modifying galaxies in clusters and groups, as well as provide an observational test for theories of cosmological structure formation. We have completed a pilot survey for the H I imaging aspects of a new Void Galaxy Survey (VGS), imaging 15 void galaxies in H I in local (d < 100 Mpc) voids. H I masses range from 3.5 x 10⁸ to 3.8 x 10⁹ M_sun, with one nondetection with an upper limit of 2.1 x 10⁸ M_sun. Our galaxies were selected using a structural and geometric technique to produce a sample that is purely environmentally selected and uniformly represents the void galaxy population. In addition, we use a powerful new backend of the Westerbork Synthesis Radio Telescope that allows us to probe a large volume around each targeted galaxy, simultaneously providing an environmentally constrained sample of fore- and background control samples of galaxies while still resolving individual galaxy kinematics and detecting faint companions in H I. This small sample makes up a surprisingly interesting collection of perturbed and interacting galaxies, all with small stellar disks. Four galaxies have significantly perturbed H I disks, five have previously unidentified companions at distances ranging from 50 to 200 kpc, two are in interacting systems, and one was found to have a polar H I disk. Our initial findings suggest void galaxies are a gas-rich, dynamic population which present evidence of ongoing gas accretion, major and minor interactions, and filamentary alignment despite the surrounding underdense environment.

[en] We use the current sample of ∼10,000 zCOSMOS spectra of sources selected with I_AB < 22.5 to define the density field out to z ∼ 1, with much greater resolution in the radial dimension than has been possible with either photometric redshifts or weak lensing. We present the new algorithm that we have developed (ZADE) to incorporate objects not yet observed spectroscopically by modifying their photometric redshift probability distributions using the spectroscopic redshifts of nearby galaxies. We present a number of tests on mock catalogs used to justify this approach. The ZADE algorithm allows us to probe a broader range of galaxy environments and reduce the Poisson noise in the density field. The reconstructed overdensity field of the 10k zCOSMOS galaxies consists of cluster-like patterns surrounded by void-like regions, extending up to z ∼ 1. Some of these structures are very large, spanning the ∼50 h ^-1 Mpc transverse direction of the COSMOS field and extending up to Δz ∼ 0.05 in redshift. We present the three-dimensional overdensity maps and compare the reconstructed overdensity field to the independently identified virialized groups of galaxies and clusters detected in the visible and in X-rays. The distribution of the overdense structures is in general well traced by these virialized structures. A comparison of the large-scale structures in the zCOSMOS data and in the mock catalogs reveals an excellent agreement between the fractions of the volume enclosed in structures of all sizes above a given overdensity between the data and the mocks in 0.2 < z < 1, although in the data these overdense regions are in generally larger contiguous structures.

[en] The impact of environment on active galactic nucleus (AGN) activity up to z ∼ 1 is assessed by utilizing a mass-selected sample of galaxies from the 10k catalog of the zCOSMOS spectroscopic redshift survey. We identify 147 AGN by their X-ray emission as detected by XMM-Newton from a parent sample of 7234 galaxies. We measure the fraction of galaxies with stellar mass M_* > 2.5 x 10¹⁰ M _sun that host an AGN as a function of local overdensity using the 5th, 10th, and 20th nearest neighbors that cover a range of physical scales (∼1-4 Mpc). Overall, we find that AGNs prefer to reside in environments equivalent to massive galaxies with substantial levels of star formation. Specifically, AGNs with host masses between 0.25 and 1 x 10¹¹ M _sun span the full range of environments (i.e., field to group) exhibited by galaxies of the same mass and rest-frame color or specific star formation rate. Host galaxies having M_* > 10¹¹ M _sun clearly illustrate the association with star formation since they are predominantly bluer than the underlying galaxy population and exhibit a preference for lower-density regions analogous to Sloan Digital Sky Survey studies of narrow-line AGN. To probe the environment on smaller physical scales, we determine the fraction of galaxies (M_* > 2.5 x 10¹⁰ M _sun) hosting AGNs inside optically selected groups, and find no significant difference with field galaxies. We interpret our results as evidence that AGN activity requires a sufficient fuel supply; the probability of a massive galaxy to have retained some sufficient amount of gas, as evidence by its ongoing star formation, is higher in underdense regions where disruptive processes (i.e., galaxy harassment, tidal stripping) are lessened.

[en] We study the evolution of galaxies inside and outside of the group environment since z = 1 using a large well-defined set of groups and galaxies from the zCOSMOS-bright redshift survey in the COSMOS field. The fraction of galaxies with early-type morphologies increases monotonically with M_B luminosity and stellar mass and with cosmic epoch. It is higher in the groups than elsewhere, especially at later epochs. The emerging environmental effect is superposed on a strong global mass-driven evolution, and at z ∼ 0.5 and log(M_*/M_sun) ∼ 10.2, the 'effect' of the group environment is equivalent to (only) about 0.2 dex in stellar mass or 2 Gyr in time. The stellar mass function of galaxies in groups is enriched in massive galaxies. We directly determine the transformation rates from late to early morphologies, and for transformations involving color and star formation indicators. The transformation rates are systematically about twice as high in the groups as outside, or up to three to four times higher correcting for infall and the appearance of new groups. The rates reach values as high as 0.3-0.7 Gyr^-1 in the groups (for masses around the crossing mass 10^10.5 M_sun), implying transformation timescales of 1.4-3 Gyr, compared with less than 0.2 Gyr^-1, i.e., timescales >5 Gyr, outside of groups. All three transformation rates decrease at higher stellar masses, and must also decrease at lower masses below 10¹⁰ M_sun which we cannot probe well. The rates involving color and star formation are consistently higher than those for morphology, by a factor of about 50%. Our conclusion is that the transformations that drive the evolution of the overall galaxy population since z ∼ 1 must occur at a rate two to four times higher in groups than outside of them.

[en] We use the zCOSMOS galaxy overdensity field to study the biasing of galaxies in the COSMOS field. By comparing the probability distribution function of the galaxy density contrast δ_g to the lognormal approximation of the mass density contrast δ, we obtain the mean biasing function b(δ, z, R) between the galaxy and matter overdensity fields and its second moments b-hat and b-tilde. Over the redshift interval 0.4 < z < 1 the conditional mean function (δ_g|δ) = b(δ, z, R)δ is of a characteristic shape, requiring nonlinear biasing in the most overdense and underdense regions. Taking into account the uncertainties due to cosmic variance, we do not detect any significant evolution in the (δ_g|δ) function, but we do detect a significant redshift evolution in the linear biasing parameter b-hat from 1.23 ± 0.11 at z ∼ 0.55 to 1.62 ± 0.14 at z ∼ 0.75, for a luminosity-complete sample of M_B < -20 - z galaxies. The b-hat parameter does not change significantly with smoothing scale between 8 and 12 h^-1 Mpc, but increases systematically with luminosity (at 2σ-3σ significance between the M_B < -20.5 - z and M_B < -20 - z samples). The nonlinearity parameter b-tilde/b-hat is offset from unity by at most 2%, with an uncertainty of the same order. The b-tilde / b-hat parameter does not show any significant redshift evolution, dependence on the smoothing scale or on the luminosity. By matching the linear bias of galaxies to the halo bias, we infer that the M_B < -20 - z galaxies reside in dark matter halos with a characteristic mass of about (2.6 - 5.6) x 10¹² M_sun with a small dependence on the adopted bias-mass relation. Our detailed error analysis and comparison with previous studies lead us to conclude that cosmic variance is the main contributor to the differences in the linear bias measured from different surveys. While our results support the general picture of biased galaxy formation up to z ∼ 1, the fine-tuning of the galaxy formation models is still limited by the restrictions of the current spectroscopic surveys at these redshifts.

[en] Numerical simulations of cosmological structure formation show that the universe’s most massive clusters, and the galaxies living in those clusters, assemble rapidly at early times ($2.5<<!--\; <\; -->z<<!--\; <\; -->4$). While more than 20 proto-clusters have been observed at $z\gtrsim <!--\; ???\; -->2$ based on associations of 5–40 galaxies around rare sources, the observational evidence for rapid cluster formation is weak. Here we report observations of an asymmetric filamentary structure at z = 2.47 containing 7 starbursting, submillimeter-luminous galaxies and 5 additional active galactic nuclei (AGNs) within a comoving volume of 15,000 Mpc³. As the expected lifetime of both the luminous AGN and starburst phase of a galaxy is ∼100 Myr, we conclude that these sources were likely triggered in rapid succession by environmental factors or, alternatively, the duration of these cosmologically rare phenomena is much longer than prior direct measurements suggest. The stellar mass already built up in the structure is ∼10¹² $M_{\odot <!--\; ???\; -->}$ and we estimate that the cluster mass will exceed that of the Coma supercluster at $z\sim <!--\; ???\; -->0$. The filamentary structure is in line with hierarchical growth simulations that predict that the peak of cluster activity occurs rapidly at $z><!--\; >\; -->2$.

[en] We present a galaxy group catalog spanning the redshift range 0.1 ∼< z ∼< 1 in the ∼ 1.7 deg² COSMOS field, based on the first ∼10,000 zCOSMOS spectra. The performance of both the Friends-of-Friends (FOF) and Voronoi-Delaunay method (VDM) approaches to group identification has been extensively explored and compared using realistic mock catalogs. We find that the performance improves substantially if groups are found by progressively optimizing the group-finding parameters for successively smaller groups, and that the highest fidelity catalog, in terms of completeness and purity, is obtained by combining the independently created FOF and VDM catalogs. The final completeness and purity of this catalog, both in terms of the groups and of individual members, compares favorably with recent results in the literature. The current group catalog contains 102 groups with N ≥ 5 spectroscopically confirmed members, with a further ∼700 groups with 2 ≤ N ≤ 4. Most of the groups can be assigned a velocity dispersion and a dark-matter mass derived from the mock catalogs, with quantifiable uncertainties. The fraction of zCOSMOS galaxies in groups is about 25% at low redshift and decreases toward ∼15% at z ∼ 0.8. The zCOSMOS group catalog is broadly consistent with that expected from the semianalytic evolution model underlying the mock catalogs. Not least, we show that the number density of groups with a given intrinsic richness increases from redshift z ∼ 0.8 to the present, consistent with the hierarchical growth of structure.

[en] We use ∼8600 COSMOS galaxies at mass scales >5 x 10¹⁰ M _sun to study how the morphological mix of massive ellipticals, bulge-dominated disks, intermediate-bulge disks, disk-dominated galaxies, and irregular systems evolves from z = 0.2 to z = 1. The morphological evolution depends strongly on mass. At M > 3 x 10¹¹ M _sun, no evolution is detected in the morphological mix: ellipticals dominate since z = 1, and the Hubble sequence has quantitatively settled down by this epoch. At the 10¹¹ M _sun mass scale, little evolution is detected, which can be entirely explained by major mergers. Most of the morphological evolution from z = 1 to z = 0.2 takes place at masses 5 x 10¹⁰-10¹¹ M _sun, where (1) the fraction of spirals substantially drops and the contribution of early types increases. This increase is mostly produced by the growth of bulge-dominated disks, which vary their contribution from ∼10% at z = 1 to >30% at z = 0.2 (for comparison, the elliptical fraction grows from ∼15% to ∼20%). Thus, at these masses, transformations from late to early types result in diskless elliptical morphologies with a statistical frequency of only 30%-40%. Otherwise, the processes which are responsible for the transformations either retain or produce a non-negligible disk component. (2) The disk-dominated galaxies, which contribute ∼15% to the intermediate-mass galaxy population at z = 1, virtually disappear by z = 0.2. The merger rate since z = 1 is too low to account for the disappearance of these massive disk-dominated systems, which most likely grow a bulge via secular evolution.