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We study the population of active galaxies in void environment in the Sloan Digital Sky Survey. We use optical spectroscopic information to analyse characteristics of the emission lines of galaxies, accomplished by WHAN and BPT diagrams. Also, we study Wide-field Infrared Survey Explorer(WISE) mid-IR colours to assess active galactic nucleus (AGN) activity. We investigate these different AGN classification schemes, both optical and mid-IR, and their dependence on the spatial location with respect to the void centres. To this end, we define three regions: void, the spherical region defined by voidcentric distance relative to void radius (distance/rvoid) smaller than 0.8, comprising overdensities lesser than −0.9, an intermediate/transition shell region (namely, void-wall) 0.8 < distance/rvoid < 1.2, and a region sufficiently distant from voids, the field: distance/rvoid > 2. We find statistical evidence for a larger fraction of AGN and star-forming galaxies in the void region, regardless of the classification scheme addressed (either BPT, WHAN, or WISE). Moreover, we obtain a significantly stronger nuclear activity in voids compared to the field. We find an unusually large fraction of the most massive black holes undergoing strong accretion when their host galaxies reside in voids. Our results suggest a strong influence of the void environment on AGN mechanisms associated with galaxy evolution.

We present a detailed study of the shapes and alignments of different galaxy cluster components using hydrodynamical simulations. We compute shape parameters from the Dark Matter (DM) distribution, the galaxy members and the intra-cluster light (ICL). We assess how well the DM cluster shape can be constrained by means of the identified galaxy member positions and the ICL. Further, we address the dilution factor introduced when estimating the cluster elongation using weak-lensing stacking techniques, which arises due to the misalignment between the total surface mass distribution and the distribution of luminous tracers. The dilution is computed considering the alignment between the DM and the Brightest Cluster Galaxy, the galaxy members and the ICL. Our study shows that distributions of galaxy members and ICL are less spherical than the DM component, although both are well aligned with the semi-major axis of the later. We find that the distribution of galaxy members hosted in more concentrated subhalos is more elongated than the distribution of the DM. Moreover, these galaxies are better aligned with the dark matter component compared to the distribution of galaxies hosted in less concentrated subhalos. We conclude that the positions of galaxy members can be used as suitable tracers to estimate the cluster surface density orientation, even when a low number of members is considered. Our results provide useful information for interpreting the constraints on the shapes of galaxy clusters in observational studies., Comment: Submitted MNRAS. 12 pages, 7 figures (+ appendix)

We explore the persistence of the alignment of brightest cluster galaxies (BCGs) with their local environment. We find that a significant fraction of BCGs do not coincide with the centroid of the X-ray gas distribution and/or show peculiar velocities (they are not at rest with respect to the cluster mean). Despite this, we find that BCGs are generally aligned with the cluster mass distribution even when they have significant offsets from the X-ray centre and significant peculiar velocities. The large offsets are not consistent with simple theoretical models. To account for these observations BCGs must undergo mergers preferentially along their major axis, the main infall direction. Such BCGs may be oscillating within the cluster potential after having been displaced by mergers or collisions, or the dark matter halo itself may not yet be relaxed., In press, MNRAS

We present the results of the optical follow-up, conducted by the TOROS collaboration, of gravitational wave events detected during the Advanced LIGO-Virgo second observing run (Nov 2016 -- Aug 2017). Given the limited field of view ($\sim100\arcmin$) of our observational instrumentation we targeted galaxies within the area of high localization probability that were observable from our sites. We analyzed the observations using difference imaging, followed by a Random Forest algorithm to discriminate between real and bogus transients. For all three events that we respond to, except GW170817, we did not find any bona fide optical transient that was plausibly linked with the observed gravitational wave event. Our observations were conducted using telescopes at Estaci\'{o}n Astrof\'{\i}sica de Bosque Alegre, Cerro Tololo Inter-American Observatory, and the Dr. Cristina V. Torres Memorial Astronomical Observatory. Our results are consistent with the LIGO-Virgo detections of a binary black hole merger (GW170104) for which no electromagnetic counterparts were expected, as well as a binary neutron star merger (GW170817) for which an optical transient was found as expected., Comment: 9 pages, 4 figures

We compute the galaxy-galaxy correlation function of low-luminosity SDSS-DR7 galaxies $(-20 < M_{\rm r} - 5\log_{10}(h) < -18)$ inside cosmic voids identified in a volume limited sample of galaxies at $z=0.085$. To identify voids, we use bright galaxies with $M_{\rm r} - 5\log_{10}(h) < -20.0$. We find that structure in voids as traced by faint galaxies is mildly non-linear as compared with the general population of galaxies with similar luminosities. This implies a redshift-space correlation function with a similar shape than the real-space correlation albeit a normalization factor. The redshift space distortions of void galaxies allow to calculate pairwise velocity distributions which are consistent with an exponential model with a pairwise velocity dispersion of $w \sim 50-70$ km/s, significantly lower than the global value of $w \sim 500$ km/s. We also find that the internal structure of voids as traced by faint galaxies is independent of void environment, namely the correlation functions of galaxies residing in void-in-void or void-in-shell regions are identical within uncertainties. We have tested all our results with the semi-analytic catalogue MDPL2-\textsc{Sag} finding a suitable agreement with the observations in all topics studied., Submitted to MNRAS, comments are welcome

Galaxy group masses are important to relate these systems with the dark matter halo hosts. However, deriving accurate mass estimates is particularly challenging for low-mass galaxy groups. Moreover, calibration of bservational mass-proxies using weak-lensing estimates have been mainly focused on massive clusters. We present here a study of halo masses for a sample of galaxy groups identified according to a spectroscopic catalogue, spanning a wide mass range. The main motivation of our analysis is to assess mass estimates provided by the galaxy group catalogue derived through an abundance matching luminosity technique. We derive total halo mass estimates according to a stacking weak-lensing analysis. Our study allows to test the accuracy of mass estimates based on this technique as a proxy for the halo masses of large group samples. Lensing profiles are computed combining the groups in different bins of abundance matching mass, richness and redshift. Fitted lensing masses correlate with the masses obtained from abundance matching. However, when considering groups in the low- and intermediate-mass ranges, masses computed according to the characteristic group luminosity tend to predict higher values than the determined by the weak-lensing analysis. The agreement improves for the low-mass range if the groups selected have a central early-type galaxy. Presented results validate the use of mass estimates based on abundance matching techniques which provide good proxies to the halo host mass in a wide mass range., Comment: 13 pages, 8 figures, plus appendices. Accepted in MNRAS

Context. Studies of galaxy pairs can provide valuable information to jointly understand the formation and evolution of galaxies and galaxy groups. Consequently, taking the new high-precision photo-z surveys into account, it is important to have reliable and tested methods that allow us to properly identify these systems and estimate their total masses and other properties. Aims. In view of the forthcoming Physics of the Accelerating Universe Survey (PAUS), we propose and evaluate the performance of an identification algorithm of projected close isolated galaxy pairs. We expect that the photometrically selected systems can adequately reproduce the observational properties and the inferred lensing mass-luminosity relation of a pair of truly bound galaxies that are hosted by the same dark matter halo. Methods. We developed an identification algorithm that considers the projected distance between the galaxies, the projected velocity difference, and an isolation criterion in order to restrict the sample to isolated systems. We applied our identification algorithm using a mock galaxy catalog that mimics the features of PAUS. To evaluate the feasibility of our pair finder, we compared the identified photometric samples with a test sample that considers that both members are included in the same halo. Taking advantage of the lensing properties provided by the mock catalog, we also applied a weak-lensing analysis to determine the mass of the selected systems. Results. Photometrically selected samples tend to show high purity values, but tend to misidentify truly bounded pairs as the photometric redshift errors increase. Nevertheless, overall properties such as the luminosity and mass distributions are successfully reproduced. We also accurately reproduce the lensing mass-luminosity relation as expected for galaxy pairs located in the same halo., This project has received funding from the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie grant agreement No 734374. This work has been supported by MINECO grants AYA2015-71825 & ESP2015-66861. IEEC is partially funded by the CERCA program of the Generalitat de Catalunya. This work was also partially supported by Agencia Nacional de Promoción Científica y Tecnoóogica (PICT 2015-3098), the Con-sejo Nacional de Investigaciones Científicas y Técnicas (CONICET, Argentina) and the Secretaría de Ciencia y Tecnología de la Universidad Nacional de Córdoba (SeCyT-UNC, Argentina). MS has been supported by the European Union’s Horizon 2020 research and innovation programme under the Maria Skłodowska-Curie grant agreement No 754510 and National Science Centre (grant UMO-2016/23/N/ST9/02963). MM acknowledges support from the Beat-riu de Pinos fellowship (2017-BP-00114).

We present the results of prompt optical follow-up of the electromagnetic counterpart of GW170817 by the Transient Optical Robotic Observatory of the South Collaboration (TOROS). We detected highly significant dimming in the light curves of the counterpart over the course of only 80 minutes of observations obtained ~35 hr after the trigger with the T80-South telescope. A second epoch of observations, obtained ~59 hr after the event with the EABA 1.5m telescope, confirms the fast fading nature of the transient. The observed colors of the counterpart suggest that this event was a “blue kilonova” relatively free of lanthanides.

We study non-parametric morphologies of mergers events in a cosmological context, using the Illustris project. We produce mock g-band images comparable to observational surveys from the publicly available Illustris simulation idealized mock images at $z=0$. We then measure non parametric indicators: asymmetry, Gini, $M_{20}$, clumpiness and concentration for a set of galaxies with $M_* >10^{10}$ M$_\odot$. We correlate these automatic statistics with the recent merger history of galaxies and with the presence of close companions. Our main contribution is to assess in a cosmological framework, the empirically derived non-parametric demarcation line and average time-scales used to determine the merger rate observationally. We found that 98 per cent of galaxies above the demarcation line have a close companion or have experienced a recent merger event. On average, merger signatures obtained from the $G-M_{20}$ criteria anticorrelate clearly with the elapsing time to the last merger event. We also find that the asymmetry correlates with galaxy pair separation and relative velocity, exhibiting the larger enhancements for those systems with pair separations $d < 50$ h$^{-1}$ kpc and relative velocities $V < 350$ km s$^{-1}$. We find that the $G-M_{20}$ is most sensitive to recent mergers ($\sim0.14$ Gyr) and to ongoing mergers with stellar mass ratios greater than 0.1. For this indicator, we compute a merger average observability time-scale of $\sim0.2$ Gyr, in agreement with previous results and demonstrate that the morphologically derived merger rate recovers the intrinsic total merger rate of the simulation and the merger rate as a function of stellar mass., 19 pages, 20 figures. Published in MNRAS

Evidence shows properties of dark matter haloes may vary with large-scale environment. By studying the halo occupation distribution in cosmic voids it is possible to obtain useful information that can shed light on the subject. The history of the formation of the haloes and galaxies residing in these regions is likely to differ from the global behaviour given their extreme environment. We use two public access simulated galaxy catalogues constructed with different methods: a semi-analytical model and a hydrodynamic simulation. In both, we identify cosmic voids and we measure the halo occupation distribution inside these regions for different absolute magnitude thresholds. We compare these determinations with the overall results and we study the dependence of different characteristics of the voids. Also, we analyze the stellar content and the formation time of the haloes inside voids and confront the general halo population results. Inside the voids, we find a significantly different halo occupation distribution with respect to the general results. This is present in all absolute magnitude ranges explored. We obtain no signs of variation related to void characteristics indicating that the effects depend only on the density of the large-scale environment. Additionally, we find that the stellar mass content also differs within voids, which host haloes with less massive central galaxies (10%) as well as satellites with significantly lower stellar mass content (30%). Finally, we find a slight difference between the formation times of the haloes which are younger in voids than the average population. These characteristics indicate that haloes populating voids have had a different formation history, inducing significant changes on the halo occupation distribution., 11 pages and 12 figures. Submitted to A&A. Comments are welcome

Aims. With the aim of performing a suitable comparison of the internal process of galactic bars with respect to the external effect of interactions on driving gas toward the inner most region of the galaxies, we explored the efficiency of both mechanisms on central nuclear activity in active galactic nuclei (AGN) in spiral galaxies. Methods. We selected samples of barred AGN and active objects residing in pair systems, derived from the Sloan Digital Sky Survey (SDSS). In order to carry out a reliable comparison of both samples (AGNs in barred hosts in isolation and in galaxy pairs), we selected spiral AGN galaxies with similar distributions of redshift, magnitude, stellar mass, color and stellar age population from both catalogs. With the goal of providing an appropriate quantification of the influence of bars and interactions on nuclear activity, we also constructed a suitable control sample of unbarred spiral AGNs with similar host properties than the other two samples. Results. We found that barred AGNs show an excess of nuclear activity (as derived from the $Lum[OIII]$) and accretion rate ($\cal R$) with respect to AGN in pairs. In addition, both samples show an excess of high values of $Lum[OIII]$ and $\cal R$ with respect to unbarred AGNs in the control sample. We also found that the fractions of AGNs with powerful nuclear activity and high accretion rates increase toward more massive hosts with bluer colors and younger stellar populations. Moreover, AGNs with bars exhibit a higher fraction of galaxies with powerful $Lum[OIII]$ and efficient $\cal R$ with respect to AGNs inhabiting pair systems. Regarding to AGN belonging to pair systems, we found that the central nuclear activity is remarkably dependent on the galaxy pair companion features., Comment: 13 pages, 9 figures. Accepted for publication in A&A

We compute the bulk motions of cosmic voids, using a $\Lambda$CDM numerical simulation considering the mean velocities of the dark matter inside the void itself and that of the haloes in the surrounding shell. We find coincident values of these two measures in the range $\sim$ 300-400 km/s, not far from the expected mean peculiar velocities of groups and galaxy clusters. When analysing the distribution of the pairwise relative velocities of voids, we find a remarkable bimodal behaviour consistent with an excess of both systematically approaching and receding voids. We determine that the origin of this bimodality resides in the void large scale environment, since once voids are classified into void-in-void (R-type) or void-in-cloud (S-type), R-types are found mutually receding away, while S-types approach each other. The magnitude of these systematic relative velocities account for more than 100 km/s, reaching large coherence lengths of up to 200 h$^{-1}$ Mpc . We have used samples of voids from the Sloan Digital Sky Survey Data Release 7 (SDSS-DR7) and the peculiar velocity field inferred from linear theory, finding fully consistent results with the simulation predictions. Thus, their relative motion suggests a scenario of a sparkling Universe, with approaching and receding voids according to their local environment., Comment: 6 pages, 4 Figures, accepted for publication in MNRAS Letters

We present a weak lensing analysis of a sample of SDSS Compact Groups (CGs). Using the measured radial density contrast profile, we derive the average masses under the assumption of spherical symmetry, obtaining a velocity dispersion for the Singular Isothermal Spherical model, $\sigma_V = 270 \pm 40 \rm ~km~s^{-1}$, and for the NFW model, $R_{200}=0.53\pm0.10\,h_{70}^{-1}\,\rm Mpc$. We test three different definitions of CGs centres to identify which best traces the true dark matter halo centre, concluding that a luminosity weighted centre is the most suitable choice. We also study the lensing signal dependence on CGs physical radius, group surface brightness, and morphological mixing. We find that groups with more concentrated galaxy members show steeper mass profiles and larger velocity dispersions. We argue that both, a possible lower fraction of interloper and a true steeper profile, could be playing a role in this effect. Straightforward velocity dispersion estimates from member spectroscopy yields $\sigma_V \approx 230 \rm ~km~s^{-1}$ in agreement with our lensing results., Comment: 12 pages, 10 figures. Submitted to MNRAS

Both meteorology and optical conditions are crucial for selecting the best site to host extremely large telescopes such as the Thirty Meter Telescope (TMT) and the European project (E-ELT). For the E-ELT, a year-long meteorological campaign was performed at our two reference sites, the Observatorio del Roque de los Muchachos (ORM) and Cerro Ventarrones (very close to the VLT site at Paranal), and at other sites also considered as alternatives to the reference sites: Aklim, Macon, and Izana (Observatorio del Teide; OT). In this article, we present a statistical analysis of the ground meteorological properties recorded at these sites, making use of automatic weather stations (AWSs) equipped with standard meteorological sensors providing the air temperature, relative humidity, barometric pressure, wind speed, and wind direction, using standard procedures across all sites. Meteorology offers but one discriminant in the complicated question of where to site such a major facility as the E-ELT (other factors being seeing, local geology, the economics of the logistics, etc.), both for determining the feasibility of telescope and instrumentation design and construction and for determining the useful observing time. However, the final decision of where to locate a major telescope depends in part on all these—and other—considerations and not on any one criterion alone. In summary, for 90% of the nighttime, the wind speed is lower than 18 m s-1, the telescope operational limit at all the sites except Macon. For this reason, Macon was discarded in the final site selection as, for 25% of the time, the wind speed is greater than 17 m s-1. The smallest nighttime temperature gradient is at ORM, whereas the lowest mean relative humidity value is reached at the Ventarrones site. Izana was discarded in the site selection study from the very beginning due to lack of funding to install further site-testing equipement (e.g., Multi-Aperture Scintillation Sensor-Differential Image Motion Monitor (MASS-DIMM)). We investigate the statistical distributions of annual and monthly meteorological conditions for day and night separately. This is the third paper in a series discussing the E-ELT FP6 site characterization project.

We present the study of nineteen low X-ray luminosity galaxy clusters (L$_X \sim$ 0.5--45 $\times$ $10^{43}$ erg s$^{-1}$), selected from the ROSAT Position Sensitive Proportional Counters (PSPC) Pointed Observations (Vikhlinin et al. 1998) and the revised version of Mullis et al. (2003) in the redshift range of 0.16 to 0.7. This is the introductory paper of a series presenting the sample selection, photometric and spectroscopic observations and data reduction. Photometric data in different passbands were taken for eight galaxy clusters at Las Campanas Observatory; three clusters at Cerro Tololo Interamerican Observatory; and eight clusters at the Gemini Observatory. Spectroscopic data were collected for only four galaxy clusters using Gemini telescopes. With the photometry, the galaxies were defined based on the star-galaxy separation taking into account photometric parameters. For each galaxy cluster, the catalogues contain the PSF and aperture magnitudes of galaxies within the 90\% completeness limit. They are used together with structural parameters to study the galaxy morphology and to estimate photometric redshifts. With the spectroscopy, the derived galaxy velocity dispersion of our clusters ranged from 507 km~s$^{-1}$ for [VMF98]022 to 775 km~s$^{-1}$ for [VMF98]097 with signs of substructure. Cluster membership has been extensively discussed taking into account spectroscopic and photometric redshift estimates. In this sense, members are the galaxies within a projected radius of 0.75 Mpc from the X-ray mission peak and with cluster centric velocities smaller than the cluster velocity dispersion or 6000 km~s$^{-1}$, respectively. These results will be used in forthcoming papers to study, among the main topics, the red cluster sequence, blue cloud and green populations; the galaxy luminosity function and cluster dynamics., Comment: 13 pages, 6 tables, 9 figures. Uses emulateapj. Accepted for publication in The Astronomical Journal. Some formatting errors fixed

We analyse the dependence of clustering properties of galaxies as a function of their large-scale environment. In order to characterize the environment on large scales, we use the catalogue of future virialized superstructures (FVS) by Luparello et al. and separate samples of luminous galaxies according to whether or not they belong to FVS. In order to avoid biases in the selection of galaxies, we have constructed different subsamples so that the distributions of luminosities and masses are comparable outside and within FVS. As expected, at large scales, there is a strong difference between the clustering of galaxies inside and outside FVS. However, this behaviour changes at scales r ≤ 1 h−1 Mpc, where the correlations have similar amplitudes. The amplitude of the two-halo term of the correlation function for objects inside FVS does not depend on their mass, but rather on that of the FVS. This is confirmed by comparing this amplitude with that expected from extended Press–Schechter fits. In order to compare these observational results with current models for structure formation, we have performed a similar analysis using a semi-analytic implementation in a Λcold dark matter (ΛCDM) cosmological model. We find that the cross-correlation functions from the mock catalogue depend on the large-scale structures in a similar way to the observations. From our analysis, we conclude that the clustering of galaxies within the typical virialized regions of groups mainly depends on the halo mass, irrespective of the large-scale environment.

We study the relative fraction of low and high surface brightness galaxies (LSBGs and HSBGs) at void walls in the SDSS DR7. We focus on galaxies in equal local density environments. We assume that the host dark-matter halo mass (for which we use SDSS group masses) is a good indicator of local density. This analysis allows to examine the behavior of the abundance of LSBG and HSBG galaxies at a fixed local density and distinguish the large-scale environment defined by the void geometry. We compare galaxies in the field, and in the void walls; the latter are defined as the volume of void shells of radius equal to that of the void. We find a significant decrement, a factor $\sim 4$, of the relative fraction of blue, active star-forming LSBGs in equal mass groups at the void walls and the field. This decrement is consistent with an increase of the fraction of blue, active star-forming HSBGs. By contrast, red LSBGs and HSBGs show negligible changes. We argue that these results are consistent with a scenario where LSBGs with blue colors and strong star formation activity at the void walls are fueled by gas from the expanding void regions. This process could lead to LSBG to HSBG transformations.

We present a new catalogue of galaxy triplets derived from the Sloan Digital Sky Survey (SDSS) Data Release 7. The identification of systems was performed considering galaxies brighter than Mr =− 20.5 and imposing constraints over the projected distances, radial velocity differences of neighbouring galaxies and isolation. To improve the identification of triplets, we employed a data pixelization scheme, which allows us to handle large amounts of data as in the SDSS photometric survey. Using spectroscopic and photometric data in the redshift range 0.01 ≤ z ≤ 0.40, we obtain 5901 triplet candidates. We have used a mock catalogue to analyse the completeness and contamination of our methods. The results show a high level of completeness (∼80 per cent) and low contamination (∼5 per cent). By using photometric and spectroscopic data, we have also addressed the effects of fibre collisions in the spectroscopic sample. We have defined an isolation criterion considering the distance of the triplet brightest galaxy to the closest neighbour cluster, to describe a global environment, as well as the galaxies within a fixed aperture, around the triplet brightest galaxy, to measure the local environment. The final catalogue comprises 1092 isolated triplets of galaxies in the redshift range 0.01 ≤ z ≤ 0.40. Our results show that photometric redshifts provide very useful information, allowing us to complete the sample of nearby systems whose detection is affected by fibre collisions, as well as extending the detection of triplets to large distances, where spectroscopic redshifts are not available.

The site for the future European Extremely Large Telescope (E-ELT) is already known to be Armazones, near Paranal (Chile). The selection was based on a variety of considerations, with an important one being the quality of the atmosphere for the astronomy planned for the ELT. We present an overview of the characterization of the atmospheric parameters of candidate sites, making use of standard procedures and instruments as carried out within the Framework Programme VI (FP6) of the European Union. We have achieved full characterization of the selected sites for the parameters considered. Further details on adaptive optics results and climatology will be the subject of two forthcoming articles. A summary of the results of the FP6 site-testing campaigns at the different sites is provided.

We construct catalogues of present superstructures that, according to a LCDM scenario, will evolve into isolated, virialized structures in the future. We use a smoothed luminosity density map derived from galaxies in SDSS-DR7 data and separate high luminosity density peaks. The luminosity density map is obtained from a volume-limited sample of galaxies in the spectroscopic galaxy catalogue, within the SDSS-DR7 footprint area and in the redshift range 0.04 < z < 0.12. Other two samples are constructed for calibration and testing purposes, up to z = 0.10 and z = 0.15. The luminosity of each galaxy is spread using an Epanechnikov kernel of 8Mpc/h radius, and the map is constructed on a 1 Mpc/h cubic cells grid. Future virialized structures (FVS) are identified as regions with overdensity above a given threshold, calibrated using a LCDM numerical simulation, and the criteria presented by D\"unner et al. (2006). We assume a constant mass-to-luminosity ratio and impose the further condition of a minimum luminosity of 10^{12}Lsol. According to our calibrations with a numerical simulation, these criteria lead to a negligible contamination by less overdense (non FVS) superstructures.We present a catalogue of superstructures in the SDSS-DR7 area within redshift 0.04 < z < 0.12 and test the reliability of our method by studying different subsamples as well as a mock catalogue.We compute the luminosity and volume distributions of the superstructures finding that about 10% of the luminosity (mass) will end up in future virialized structures. The fraction of groups and X-ray clusters in these superstructures is higher for groups/clusters of higher mass, suggesting that future cluster mergers will involve the most massive systems. We also analyse known structures in the present Universe and compare with our catalogue of FVS.

We have measured redshifts for 243 z ~3 quasars in nine VLT VIMOS LBG redshift survey areas, each of which is centred on a known bright quasar. Using spectra of these quasars, we measure the cross-correlation between neutral hydrogen gas causing the Lya forest and 1020 Lyman-break galaxies at z ~3. We find an increase in neutral hydrogen absorption within 5 h^-1 Mpc of a galaxy in agreement with the results of Adelberger et al. (2003, 2005). The Lya-LBG cross-correlation can be described by a power-law on scales larger than 3 h^-1 Mpc. When galaxy velocity dispersions are taken into account our results at smaller scales (

We present the initial imaging and spectroscopic data acquired as part of the VLT VIMOS Lyman-break galaxy Survey. UBR (or UBVI) imaging covers five 36'x36' fields centred on bright z>3 QSOs, allowing ~21,000 2 2 LBGs and 10 z>2 QSOs from a total of 19 VIMOS pointings. From the galaxy spectra, we observe a 625+-510 km/s velocity offset between the ISM absorption and Ly-alpha emission line redshifts. Using the photometric and spectroscopic catalogues, we have analysed the galaxy clustering at z~3. In the photometric case, the angular correlation function, w(theta), is well fit by a double power-law with clustering scale-length, r_0 = 3.19+0.32-0.54 Mpc/h for r ^1/2 ~ 700 km/s, higher than the ~400 km/s found by previous authors. We also measure a value for the gravitational growth rate parameter of beta(z=3) = 0.48+-0.17, implying a low value for the bias of b = 2.06+1.1-0.5. This value is consistent with the galaxy clustering amplitude which gives b = 2.22+-0.16, assuming the standard cosmology, implying that the evolution of the gravitational growth rate is also consistent with Einstein gravity. We have compared our LBG clustering amplitudes with lower redshift measurements and find that the clustering strength is not inconsistent with that of low-redshift L* spirals for simple 'long-lived' galaxy models.

We present a catalogue of galaxy photometric redshifts and k-corrections for the Sloan Digital Sky Survey Seven Data Release (SDSS-DR7), available on the World Wide Web. The photometric redshifts were estimated with an artificial neural network using five ugriz bands, concentration indices and Petrosian radii in the g and r bands. We have explored our redshift estimates with different training set concluding that the best choice to improve redshift accuracy comprises the Main Galaxies Sample (MGS), the Luminous Red Galaxies, and galaxies of active galactic nuclei covering the redshift range 0

We perform a statistical analysis of the peculiar velocity field around dark matter haloes in numerical simulations. We examine different properties of the infall of material onto haloes and its relation to central halo shapes and the shape of the large scale surrounding regions (LSSR). We find that the amplitude of the infall velocity field along the halo shape minor axis is larger than that along the major axis. This is consistent for general triaxial haloes, and for both prolate and oblate systems. We also report a strong anisotropy of the velocity field along the principal axes of the LSSR. The infall velocity field around dark matter haloes reaches a maximum value along the direction of the minor axis of the LSSR, whereas along the direction of its major axis, it exhibits the smallest velocities. We also analyse the dependence of the matter velocity field on the local environment. The amplitude of the infall velocity at high local density regions is larger than at low local density regions. The velocity field tends to be more laminar along the direction towards the minor axis of the LSSR, where the mean ratio between flow velocity and velocity dispersion is of order unity and nearly constant up to scales of 15 Mpc/h. We also detect anisotropies in the outflowing component of the velocity field, showing a maximum amplitude along the surrounding LSSR major axis.

We study the properties of Sloan Digital Sky Survey (SDSS) galaxies with and without active galactic nucleus (AGN) detection as a function of the local and global environment measured via the local density, mass of the galaxy host group (parametrized by the group luminosity) and distance to massive clusters. Our results can be divided into two main subjects: the environments of galaxies and their relation to the assembly of their host haloes, and the environments of AGNs. (i) For the full SDSS sample, we find indications that the local galaxy density is the most efficient parameter to separate galaxy populations, but we also find that galaxies at a fixed local density show some remaining variation in their properties as a function of the distance to the nearest cluster of galaxies (in a range of 0-10 cluster virial radii). These differences seem to become less significant, if the galaxy samples are additionally constrained to be hosted by groups of similar total luminosity. If instead of fixing the local density, the mass of the host group is held fixed at a given value, the fraction of red galaxies also increases as the distance to clusters diminishes, indicating that neither the local density nor the host halo mass contains all the information on the environment. (ii) In AGN host galaxies, the morphology-density relation is much less notable when compared to the behaviour of the full SDSS sample, indicating a lack of sensitivity to the host group mass during the AGN phase, probably due to the higher typical luminosities of the AGN hosts. In order to interpret this result, we analyse control samples constructed using galaxies with no detected AGN activity with matching distributions of redshifts, stellar masses, r-band luminosities, g - r colours, concentrations, local densities, host group luminosities and fractions of central and satellite galaxies; the aim in using the control sample is to detect any correlations between the AGN detection and other AGN host properties that are unrelated to the AGN selection. The control samples also show a similar small dependence on the local density, indicating an influence from the AGN selection, but their colours are slightly bluer compared to the AGN hosts, regardless of the local density. Furthermore, even when the local density is held fixed at intermediate or high values, and the distance to the closest cluster of galaxies is allowed to vary, AGN control galaxies away from clusters tend to be bluer than the AGN hosts. However, AGN in bright, low-concentration hosts (i.e. discy morphologies) are bluer than galaxies in the control sample, connecting the presence of discs to AGN activity even under a controlled comparison between active and inactive galaxies.

We analyze optical and radio properties of radiogalaxies detected in the Sloan Digital Sky Survey (SDSS). The sample of radio sources are selected from the catalogue of Kimball & Ivezi\'c (2008) with flux densities at 325, 1400 and 4850 MHz, using WENSS, NVSS and GB6 radio surveys and from flux measurements at 74 MHz taken from VLA Low-frequency Sky Survey \citep{cohen}. We study radiogalaxy spectral properties using radio colour-colour diagrams and find that our sample follows a single power law from 74 to 4850 MHz. The spectral index vs. spectroscopic redshift relation ($\alpha-z$) is not significant for our sample of radio sources. We analyze a subsample of radio sources associated with clusters of galaxies identified from the maxBCG catalogue and find that about 40% of radio sources with ultra steep spectra (USS, $\alpha

We explore properties of close galaxy pairs and merging systems selected from the Sloan Digital Sky Survey Data Release 4 in different environments with the aim to assess the relative importance of the role of interactions over global environmental processes. For this purpose, we perform a comparative study of galaxies with and without close companions as a function of local density and host halo mass, carefully removing sources of possible biases. We find that at low- and high-local-density environments, colours and concentration indices of close galaxy pairs are very similar to those of isolated galaxies. At intermediate densities, we detect significant differences, indicating that close pairs could have experienced a more rapid transition on to the red sequence than isolated galaxies. The presence of a correlation between concentration index and colours indicates that the physical mechanism responsible for the colour transformation also operates in the transformation of the luminous matter distribution. At fixed local densities, we find a dependence of the red galaxy fraction on dark matter halo mass for galaxies with or without a close companion. This suggests the action of host halo mass related effects. Regardless of dark matter halo mass, we show that the percentage of red galaxies in close pairs and in the control sample are comparable at low- and high-local-density environments. However, at intermediate local densities, the gap in the red fraction between close pairs and the control galaxies increases from ∼10 per cent in low-mass haloes up to ∼50 per cent in the most massive ones. Interestingly, we also detect that 50 per cent of merging systems populate the intermediate local environments, with a large fraction of them being extremely red and bulge dominated. Our findings suggest that in intermediate-density environments galaxies are efficiently pre-processed by close encounters and mergers before entering higher local density regions., Facultad de Ciencias Astronómicas y Geofísicas

We analyse the relation between active galactic nuclei (AGN) host properties and large-scale environment for a representative red and blue AGN host galaxy sample selected from the Data Release 4 Sloan Digital Sky Survey. A comparison is made with two carefully constructed control samples of non-active galaxies, covering the same redshift range and colour baseline. The cross-correlation functions show that the density distribution of neighbours is almost identical for blue galaxies, either active or non-active. Although active red galaxies inhabit environments less dense compared to non-active red galaxies, both reside in environments considerably denser than those of blue hosts. Moreover, the radial density profile of AGN relative to galaxy group centres is less concentrated than galaxies. This is particularly evident when comparing red AGN and non-active galaxies. The properties of the neighbouring galaxies of blue and red AGN and non active galaxies reflect this effect. While the neighbourhood of the blue samples is indistinguishable, the red AGN environs show an excess of blue-star-forming galaxies with respect to their non-active counterpart. On the other hand, the active and non-active blue systems have similar environments but markedly different morphological distributions, showing an excess of blue early-type AGN, which are argued to be late-stage mergers. This comparison reveals that the observable differences between active red and blue host galaxy properties including star formation history and AGN activity depends on the environment within which the galaxies form and evolve. Fil: Coldwell Lloveras, Georgina Vanesa. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Geofísica y Astronomía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Garcia Lambas, Diego Rodolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Astronomía Teórica y Experimental. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba. Instituto de Astronomía Teórica y Experimental; Argentina Fil: Söchting, Ilona K.. University of Oxford; Reino Unido Fil: Gurovich, Sebastian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Astronomía Teórica y Experimental. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba. Instituto de Astronomía Teórica y Experimental; Argentina

We perform a statistical study of the characteristics of galaxies in voids and void walls in the SDSS and 2dFGRS catalogues. We investigate dependencies of the distribution of galaxy spectral types and colours as a function of the relative position to the void centres for different luminosity and local density ranges. We find a trend towards bluer, star forming galaxies in void walls beyond the local density dependence. These results indicate that luminosity and local density do not entirely determine the distribution of galaxy properties such as colours and spectral types, and point towards a large scale modulation of star formation. We argue that this effect is due to the lower accretion and merger history of galaxies arriving at void walls from the emptier inner void regions., 5 pages, 6 figures, submitted to MRNAS Letters

We use photometric redshifts to analyse the effect of local environment on galaxy colours at redshifts z < 0.63 in the SDSS data release 6. We construct mock SDSS-DR6 catalogues using semi-analytic galaxies to study possible systematic effects on the characterisation of environment and colour statistics due to the uncertainty in the determination of redshifts. We use the projected galaxy density derived from the distance to the nearest neighbours with a suitable radial velocity threshold to take into account the uncertainties in the photometric redshift estimates. Our findings indicate that the use of photometric redshifts greatly improves estimates of projected local galaxy density when galaxy spectra are not available. We find a tight relationship between spectroscopic and photometric derived densities, both in the SDSS-DR6 data (up to z=0.3) and mock catalogues (up to z=0.63). At z=0, faint galaxies show a clear increase of the red galaxy fraction as the local density increases. Bright galaxies, on the other hand, show a constant red galaxy fraction. We are able to track the evolution of this fraction to z=0.55 for galaxies brighter than M_r=-21.5 and find that the fraction of blue galaxies with respect to the total population progressively becomes higher as the redshift increases, at a rate of 15% Gyr. Also, at any given redshift, bright galaxies show a larger red population, indicating that the star-formation activity shifts towards smaller objects as the redshift decreases.

We have conducted an optical and infrared imaging in the neighbourhoods of 4 triplets of quasars. R, z', J and Ks images were obtained with MOSAIC II and ISPI at Cerro Tololo Interamerican Observatory. Accurate relative photometry and astrometry were obtained from these images for subsequent use in deriving photometric redshifts. We analyzed the homogeneity and depth of the photometric catalog by comparing with results coming from the literature. The good agreement shows that our magnitudes are reliable to study large scale structure reaching limiting magnitudes of R = 24.5, z' = 22.5, J = 20.5 and Ks = 19.0. With this catalog we can study the neighbourhoods of the triplets of quasars searching for galaxy overdensities such as groups and galaxy clusters., The paper contains 12 figures and 3 tables

We perform a statistical analysis of AGN host characteristics and nuclear activity for AGNs in pairs and without companions. Our study concerns a sample of AGNs derived from the SDSS-DR4 data by Kauffmann et al (2003) and pair galaxies obtained from the same data set by Alonso et al. (2006). An eye-ball classification of images of 1607 close pairs (rp < 25 kpc h −1 , ∆V < 350 km s −1 ) according to the evidence of interaction through distorted morphologies and tidal features provides us with a more confident assessment of galaxy interactions from this sample. We notice that, at a given luminosity or stellar mass content, the fraction of AGNs is larger for pair galaxies exhibiting evidence for strong interaction and tidal features which also show sings of strong star formation activity. Nevertheless, this process accounts only for a ∼ 10% increase of the fraction of AGNs. As in previous works, we find AGN hosts to be redder and with a larger concentration morphological index than non-AGN galaxies. This effect does not depend whether AGN hosts are in pairs or in isolation. The OIII luminosity of AGNs with strong interaction features is found to be significantly larger than that of other AGNs, either in pairs or in isolation. Estimations of the accretion rate, L[OIII]/MBH , show that AGNs in merging pairs are actively feeding their black holes, regardless of their stellar masses. We also find that the luminosity of the companion galaxy seems to be a key parameter in the determination of the black hole activity. At a given host luminosity, both the OIII luminosity and the L[OIII]/MBH are significantly larger in AGNs with a bright companion (Mr < −20) than otherwise. Fil: Alonso, M. Sol. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Complejo Astronómico "el Leoncito". Universidad Nacional de Córdoba. Complejo Astronómico "el Leoncito". Universidad Nacional de la Plata. Complejo Astronómico "el Leoncito". Universidad Nacional de San Juan. Complejo Astronómico "el Leoncito"; Argentina Fil: Garcia Lambas, Diego Rodolfo. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Tissera, Patricia Beatriz. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina Fil: Coldwell, Georgina. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba; Argentina

We developed a Monte Carlo code to generate long-duration gamma-ray burst (LGRB) events within cosmological hydrodynamical simulations consistent with the concordance Λ cold dark matter model. As structure is assembled, LGRBs are generated in the substructure that formed galaxies today. We adopted the collapsar model so that LGRBs are produced by single, massive stars at the final stage of their evolution. We found that the observed properties of the LGRB host galaxies (HGs) are reproduced if LGRBs are also required to be generated by low-metallicity stars. The low-metallicity condition imposed on the progenitor stars of LGRBs selects a sample of HGs with mean gas abundances of 12 + log O/H ~ 8.6. For z < 1 the simulated HGs of low-metallicity LGRB progenitors tend to be faint, slow rotators with high star formation efficiency, compared with the general galaxy population, in agreement with observations. At higher redshift, our results suggest that larger systems with high star formation activity could also contribute to the generation of LGRBs from low-metallicity progenitors since the fraction of low-metallicity gas available for star formation increases for all systems with look-back time. Under the hypothesis of our LGRB model, our results support the claim that LGRBs could be unbiased tracers of star formation at high redshifts. Fil: Nuza, Sebastian Ernesto. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina Fil: Tissera, Patricia Beatriz. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina Fil: Pellizza González, Leonardo Javier. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina Fil: Garcia Lambas, Diego Rodolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Astronomia Teórica y Experimental. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba. Instituto de Astronomia Teórica y Experimental; Argentina Fil: Scannapieco, Cecilia. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina Fil: de Rossi, Maria Emilia. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

We measure the effect of the environment on the intrinsic shapes of spiral and elliptical galaxies by finding the 3D shape distribution and dust extinction that fits better the projected shape of galaxies in different environment. We find that spiral galaxies in groups are very similar to field spirals with similar intrinsic properties (magnitudes, sizes and colours). But for spirals in groups, those in denser environments or closer to the centre of the group tend to have a more circular disc than similar galaxies in less dense environments or far from the group centres. Also we find that central spiral galaxies in their groups tend to be thinner than other similar spirals. For ellipticals, we do not find any important dependence of their shape on their position in a group or on the local density. However, we find that elliptical galaxies in groups tend to be more spherical than field ellipticals with similar intrinsic properties. We find that, once in groups, the shape of member galaxies do not depend on group mass, regardless of their morphological type., Comment: 8 pages, 5 figures, accepted for publication in MNRAS

Inspired on the well known dynamical dichotomy predicted in voids, where some underdense regions expand whereas others collapse due to overdense surrounding regions, we explored the interplay between the void inner dynamics and its large scale environment. The environment is classified depending on its density as in previous works. We analyse the dynamical properties of void-centered spherical shells at different void-centric distances depending on this classification. The above dynamical properties are given by the angular distribution of the radial velocity field, its smoothness, the field dependence on the tracer density and shape, and the field departures from linear theory. We found that the velocity field in expanding voids follows more closely the linear prediction, with a more smooth velocity field. However when using velocity tracers with large densities such deviations increase. Voids with sizes around $18\,h^{-1}\,Mpc$ are in a transition regime between regions with expansion overpredicted and underpredicted from linear theory. We also found that velocity smoothness increases as the void radius, indicating the laminar flow dominates the expansion of larger voids (more than $18\,h^{-1}\,Mpc$). The correlations observed suggest that nonlinear dynamics of the inner regions of voids could be dependent on the evolution of the surrounding structures. These also indicate possible scale couplings between the void inner expansion and the large scale regions where voids are embedded. These results shed some light to the origin of nonlinearities in voids, going beyond the fact that voids just quickly becomes nonlinear as they become emptier., Comment: 12 pages, 8 Figures, accepted for publication in MNRAS

We perform a statistical study on the distribution and dynamics of voids in the 2dFGRS. Our statistics are tested and calibrated using mock 2dFGRS catalogues. We analyse the dynamics around voids in mock and real 2dFGRS surveys. The void-galaxy cross-correlation redshift-space distortions show evidence of the continuing growth of voids, confirming another prediction of the hierarchical clustering scenario. A non-linear outflow model can be used to provide quantitative estimates of the outflow velocities around 2dFGRS voids. These results are consistent with maximum outflows of 110km/s, 210km/s and 270km/s for voids of =7.5, 12.5 and 17.5h^-1Mpc, assuming a galaxy bias of b=1. As an application for future surveys, our study of the mock catalogues shows that direct measurements of the expansion of voids can be obtained using peculiar velocity data. We find that it would also be possible to detect differences in the velocity dispersion of galaxies in the directions parallel and perpendicular to the void walls., Comment: 13 pages, 13 figures, published in MNRAS

We analyse $u-r$ colour distributions for several samples of galaxies in groups drawn from the Fourth Data Release of the Sloan Digital Sky Survey. For all luminosity ranges and environments considered the colour distributions are well described by the sum of two Gaussian functions. We find that the fraction of galaxies in the red sequence is an increasing function of group virial mass. We also study the evolution of the galaxy colour distributions at low redshift, $z\le0.18$ in the field and in groups for galaxies brighter than $M_r-5\log(h)=-20$, finding significant evidence of recent evolution in the population of galaxies in groups. The fraction of red galaxies monotonically increases with decreasing redshift, this effect implies a much stronger evolution of galaxies in groups than in the field.

We perform a statistical analysis of galaxies in pairs with a Λ-CDM scenario by using the chemical GADGET-2 of Scannapieco et al. (2005, MNRAS, 364, 552) in order to study the effects of galaxy interactions on colours and metallicities. We find that galaxy-galaxy interactions can produce a bimodal colour distribution where galaxies with significant recent star formation activity contribute mainly to blue colours. In the simulations, the colours and the fractions of recently formed stars of galaxies in pairs depend on their environments more strongly than those of galaxies without close companions, suggesting that interactions play an important role in galaxy evolution. If the metallicity of the stellar populations is used as the chemical indicator, we find that the simulated galaxies determine luminosity-metallicity and stellar mass-metallicity relations that do not depend on the presence of a close companion. However, in the case of the luminosity-metallicity relation, at a given level of enrichment, we detect a systematic displacement of the relation to brighter magnitudes for active star forming systems. Regardless of relative distance and current level of star formation (SF) activity, galaxies in pairs have stellar populations with higher level of enrichment than galaxies without a close companion. In the case of the gas component, this is no longer valid for galaxies in pairs with passive star formation that only show an excess of metals for very close pair members, consequence of an important recent past star-formation activity. In agreement with observations, the signature of gas inflows driven by interactions can be also detected in the lower mean O/H abundances measured in the central regions of galaxies in pairs. Our results suggest that interactions play a significant role in the determination of colour and chemical properties of galaxies in hierarchical clustering scenarios, although SN energy feedback is needed to achieve a full agreement., Facultad de Ciencias Astronómicas y Geofísicas

We analyse the environment of low redshift, z < 0.2, SDSS quasars using the spectral and photometric information of galaxies from the Sloan Digital Sky Survey Third Data Release (SDSS-DR3). We compare quasar neighbourhoods with field and high density environments through an analysis on samples of typical galaxies and groups. We compute the surrounding surface number density of galaxies finding that quasar environments systematically avoid high density regions. Their mean environments correspond to galaxy density enhancements similar to those of typical galaxies. We have also explored several galaxy properties in these environments, such as spectral types, specific star formation rates, concentration indexes, colours and active nuclei activity. We conclude that low redshift quasar neighbourhoods (r_p < 1 Mpc h^-1, Delta V < 500 km/s) are populated by bluer and more intense star forming galaxies of disk-type morphology than galaxies in groups and in the field. Although star formation activity is thought to be significantly triggered by interactions, we find that quasar fueling may not require the presence of a close companion galaxy (r_p < 100 kpc h^-1, Delta V< 350 km/s). As a test of the unified AGN model, we have performed a similar analysis to the neighbours of a sample of active galaxies. The results indicate that these neighbourhoods are comparable to those of quasars giving further support to this unified scenario.

We present measurements of the clustering properties of galaxies in the field of redshiftrange 0.5 . z . 1.5 Ultra Steep Spectrum (USS) radio sources selected from SydneyUniversity Molonglo Sky Survey and NRAO VLA Sky Survey. Galaxies in these USSfields were identified in deep near-IR observations, complete down to K s = 20, usingIRIS2instrument at the AAT telescope. We used the redshift distribution of K s < 20galaxies taken from Cimatti et al. (2002) to constrain the correlation length r 0 . Wefind a strong correlation signal of galaxies with K s < 20 around our USS sample. Acomoving correlation length r 0 = 14.0±2.8 h −1 Mpc and γ = 1.98±0.15 are derivedin a flat cosmological model Universe.We compare our findings with those obtained in a cosmological N–body simu-lation populated with GALFORM semi-analytic galaxies. We find that clusters ofgalaxies with masses in the range M = 10 13 .4−14 2 h −1 M⊙ have a cluster–galaxycross–correlation amplitude comparable to those found between USS hosts and galax-ies. These results suggest that distant radio galaxies are excellent tracers of galaxyoverdensities and pinpoint the progenitors of present day rich clusters of galaxies.Key words: cosmology: large-scale structure of Universe–galaxies: galaxies: high-redshift.

We analyse a high redshift sample ($0.4, Comment: The paper contain 6 pages, 5 figures and 1 table Acepted MNRAS 2006 March 15

We analyze photometric properties of 1384 cluster galaxies as a function of the normalized distance to cluster center. These galaxies were selected in the central region ($r/r_{200} \leq$ 0.8) of 14 southern Abell clusters chosen from the Southern Abell Cluster Redshifts Survey (SARS). For 507 of these galaxies we also obtained their luminosity profiles. We have studied the morphology-clustercentric distance relation on the basis of the shape parameter $n$ of the S\'ersic's law. We also have analyzed the presence of a possible segregation in magnitude for both, the galaxy total luminosity and that of their components (i.e. the bulge and the disk). Results show a marginal ($2\sigma$ level) decrease of the total luminosity as a function of normalized radius. However, when bulges are analyzed separately, a significant luminosity segregation is found ($3\sigma$ and $2\sigma$ for galaxies in projection and member galaxies respectively). The fraction of bulges brighter than $M_B \leq -22$ is three times larger in the core of clusters than in the outer region. Our analysis of the disk component suggests that disks are, on average, less luminous in the cluster core than at $r/r_{200} \sim 0.8$. In addition, we found that the magnitude-size relation as a function of $r/r_{200}$ indicates (at $2\sigma$ level) that disks are smaller and centrally brighter in the core of clusters. However, the Kormendy relation (the bulge magnitude-size relation) appears to be independent of environment., Comment: To appear in the AJ

We have analyzed galaxy properties in the environment of a sample of 70 Ultra Steep Spectrum (USS) radio sources selected from the Sydney University Molonglo sky Survey and the NRAO VLA Sky Survey catalogues, using near-IR data complete down to Ks=20. We have quantified galaxy excess around USS targets using an Abell-type measurement N0.5 (Hill & Lilly 1991). We find that most of the USS fields studied are compatible with being Abell class 0 richness clusters. A statistical analysis of the distribution of companion galaxies around USS radio sources show a pronounced tendency for such objects to be found in the direction defined by the radio axis, suggesting that they may be related to the presence of the radio sources. We have also measure the central concentration of light of the USS sample and compare these to the values obtained for field galaxies and galaxies selected through other methods. By using Spearman statistics to disentangle richnesses and concentration indices dependences, we detect a weak, but significant, positive correlation. We find that at z > 2 USS radio sources are more concentrated than field galaxies at similar redshifts, indicating that these objects trace the most massive systems at high redshift.

We compute the galaxy luminosity function in spectroscopically selected nearby groups and clusters. Our sample comprises 728 systems extracted from the third release of the Sloan Digital Sky Survey in the redshift range $0.03 < z < 0.06$ with virial mass range $10^{11}M_\odot < M_{vir} < 2\times 10^{14}M_\odot$. In order to compute the galaxy luminosity function, we apply a statistical background subtraction method following usually adopted techniques. In the $r$ band, the composite galaxy luminosity function shows a slope $\alpha=-1.3$ in the bright--end, and an upturn of the slope in the faint--end, $M_r\ga -18+5log(h)$, to slopes $-1.9, Comment: 8 pages, 10 figures, Submitted to Astronomy and Astrophysics

We compute the real-space cluster-galaxy cross-correlation xi_cg(r) using the ROSAT-ESO Flux Limited X-ray (REFLEX) cluster survey, a group catalogue constructed from the final version of the 2dFGRS, and galaxies extracted from 2MASS and APM surveys. This first detailed calculation of the cross-correlation for X-ray clusters and groups, is consistent with previous works and shows that xi_cg(r) can not be described by a single power law. We analyse the clustering dependence on the cluster X-ray luminosity L_X and virial mass M_vir thresholds as well as on the galaxy limiting magnitude. We also make a comparison of our results with those obtained for the halo-mass cross-correlation function in a LambdaCDM N-body simulation to infer the scale dependence of galaxy bias around clusters. Our results indicate that the distribution of galaxies shows a significant anti-bias at highly non-linear small cluster-centric distances (b_cg(r) ~ 0.7), irrespective of the group/cluster virial mass or X-ray luminosity and galaxy characteristics, which show that a generic process controls the efficiency of galaxy formation and evolution in high density regions. On larger scales b_cg(r) rises to a nearly constant value of order unity, the transition occuring at approximately 2 Mpch^(-1) for 2dF groups and 5 Mpch^(-1) for REFLEX clusters.

We have analyzed CCD images of 14 Abell clusters in the R filter of the Southern Abell Clusters Redshifts Survey (SARS) with cz < 40,000 km s-1. We have obtained the luminosity profiles for 507 galaxies, of which 232 (46%) have known redshifts. In order to fit the luminosity profiles, we used the de Vaucouleurs law for bulge systems and an exponential profile for disk systems, and we also fitted the Sersic's law (rn) to all galaxy profiles. We have found that 162 galaxies (32%) in the sample have pure r1/4 profiles, 168 (33%) have pure exponential profiles, and 93 (18%) have luminosity profiles that are well fitted by a combination of bulge and disk profiles. On the other hand, we could not fit the classical bulge + disk profile to the remaining 84 galaxies (17%) of the sample. For such cases we have used only the Sersic law. We have also analyzed how seeing and sky cleaning affect the structural and photometric parameters obtained through profile fitting. In addition, we have studied several relations between these parameters. We have found that bulges and disks show consistency with a unique relation in the μe– log re plane. We also found that bulges and disks obey a magnitude-size relation in the sense that large bulges and disks (large re values) have high luminosities. On the other hand, Sersic-law fitting parameters n, rs, and μs show a strong correlation, in agreement with the findings of other authors, suggesting that not only elliptical galaxies but all galaxies are likely to be understood as a one-parameter family.

We analyse the projected radial distribution of satellites around bright primary galaxies in the 2dFGRS. We have considered several primary-satellite subsamples to search for dependences of the satellite number density profile, \rho(r_p), on properties of satellites and primaries. We find significant differences of the behaviour of \rho(r_p) depending on primary characteristics. In star-forming primaries, the satellite number density profile is consistent with power laws within projected distance 20100 kpc), the density profiles of all primaries is well described by power laws, although we notice that for red, early spectral type primaries, the outer slope obtained is steeper than that corresponding to blue, late spectral type ones. We have tested our results by control samples of galaxies identical to the samples of satellites in apparent magnitude and projected distance to the primary, but with a large relative velocity. This sample of unphysical primary-galaxy pairs shows a flat radial density beyond r_p=20 kpc indicating that our results are not biased toward a decrease of the true number of objects due to catalogue selection effects. Our results can be understood in terms of dynamical friction and tidal stripping on satellites in the primary haloes. These processes can effectively transfer energy to the dark matter, flattening the central steep profiles of the satellite distribution in evolved systems., Comment: 8 pages, 6 figures. Accepted for publication in MNRAS

We have analysed a sample of satellite and primary galaxies in the 2dF galaxy redshift survey. In our study we find a strong statistical evidence of the Holmberg effect (that is a tendency for satellites to avoid regions along the line defined by the primary plane) within 500 kpc of projected distance to the primary. This effect is present only when we restrict to objects with radial velocity relative to the primary $|\Delta v

We present the preliminary results of the astronomical site testing, which the group IATE of the OAC is developing in northwest of Argentine in collaboration with ESO and the Department of Astronomy of Cornell University.

We study galaxy pairs in the field selected from the 100 K public release of the 2dF galaxy redshift survey. Our analysis provides a well defined sample of 1258 galaxy pairs, a large database suitable for statistical studies of galaxy interactions in the local universe, $z \le 0.1$. Galaxy pairs where selected by radial velocity ($\Delta V$) and projected separation ($r_{\rm p}$) criteria determined by analyzing the star formation activity within neighbours (abridged). The ratio between the fractions of star forming galaxies in pairs and in isolation is a useful tools to unveil the effects of having a close companion. We found that about fifty percent of galaxy pairs do not show signs of important star formation activity (independently of their luminosities) supporting the hypothesis that the internal properties of the galaxies play a crucial role in the triggering of star formation by interactions.