Cosmic voids are becoming key players in testing the physics of our Universe. Here we concentrate on the abundances and the dynamics of voids as these are among the best candidates to provide information on cosmological parameters. Cai, Padilla & Li
(2014) use the abundance of voids to tell apart Hu & Sawicki $f(R)$ models from General Relativity. An interesting result is that even though, as expected, voids in the dark matter field are emptier in $f(R)$ gravity due to the fifth force expelling away from the void centres, this result is reversed when haloes are used to find voids. The abundance of voids in this case becomes even lower in $f(R)$ compared to GR for large voids. Still, the differences are significant and this provides a way to tell apart these models. The velocity field differences between $f(R)$ and GR, on the other hand, are the same for halo voids and for dark matter voids. Paz et al. (2013), concentrate on the velocity profiles around voids. First they show the necessity of four parameters to describe the density profiles around voids given two distinct void populations, voids-in-voids and voids-in-clouds. This profile is used to predict peculiar velocities around voids, and the combination of the latter with void density profiles allows the construction of model void-galaxy cross-correlation functions with redshift space distortions. When these models are tuned to fit the measured correlation functions for voids and galaxies in the Sloan Digital Sky Survey, small voids are found to be of the void-in-cloud type, whereas larger ones are consistent with being void-in-void. This is a novel result that is obtained directly from redshift space data around voids. These profiles can be used to remove systematics on void-galaxy Alcock-Pacinsky tests coming from redshift-space distortions.
The angular momentum of galactic discs in semi-analytic models of galaxy formation is usually updated in time as material is accreted to the disc by adopting a constant dimensionless spin parameter and little attention is paid to the effects of accre
tion with misaligned angular momenta. These effects are the subject of this paper, where we adopt a Monte-Carlo simulation for the changes in the direction of the angular momentum of a galaxy disc as it accretes matter based on accurate measurements from dark-matter haloes in the Millennium II simulation. In our semi-analytic model implementation, the flips seen the dark matter haloes are assumed to be the same for the cold baryons; however, we also assume that in the latter the flip also entails a difficulty for the disc to increase its angular momentum which causes the disc to become smaller relative to a no-flip case. This makes star formation to occur faster, specially in low mass galaxies at all redshifts allowing galaxies to reach higher stellar masses faster. We adopt a new condition for the triggering of starbursts during mergers. As these produce the largest flips it is natural to adopt the disc instability criterion to evaluate the triggering of bursts in mergers instead of one based on mass ratios as in the original model. The new implementation reduces the average lifetimes of discs by a factor of 2, while still allowing old ages for the present-day discs of large spiral galaxies. It also provides a faster decline of star formation in massive galaxies and a better fit to the bright end of the luminosity function at z = 0.
Aims: We present a new method that uses luminosity or stellar mass functions combined with clustering measurements to select samples of galaxies at different redshifts likely to follow a progenitor-to-descendant relationship. As the method uses clust
ering information, we refer to galaxy samples selected this way as clustering-selected samples. We apply this method to infer the number of mergers during the evolution of MUSYC early-type galaxies (ETGs) from z~1 to the present-day. Methods: The method consists in using clustering information to infer the typical dark-matter halo mass of the hosts of the selected progenitor galaxies. Using LambdaCDM predictions, it is then possible to follow these haloes to a later time where the sample of descendants will be that with the clustering of these descendant haloes. Results: This technique shows that ETGs at a given redshift evolve into brighter galaxies at lower redshifts (considering rest-frame, passively evolved optical luminosities). This indicates that the stellar mass of these galaxies increases with time and that, in principle, a stellar mass selection at different redshifts does not provide samples of galaxies in a progenitor-descendant relationship. Conclusions: The comparison between high redshift ETGs and their likely descendants at z=0 points to a higher number density for the progenitors by a factor 5.5+-4.0, implying the need for mergers to decrease their number density by today. Because the luminosity densities of progenitors and descendants are consistent, our results show no need for significant star-formation in ETGs since z=1, which indicates that the needed mergers are dry, i.e. gas free.
We measure the evolution of galaxy clustering out to a redshift of z~1.5 using data from two MUSYC fields, the Extended Hubble Deep Field South (EHDF-S) and the Extended Chandra Deep Field South (ECDF-S). We use photometric redshift information to ca
lculate the projected-angular correlation function, omega(sigma), from which we infer the projected correlation function Xi(sigma). We demonstrate that this technique delivers accurate measurements of clustering even when large redshift measurement errors affect the data. To this aim we use two mock MUSYC fields extracted from a LambdaCDM simulation populated with GALFORM semi-analytic galaxies which allow us to assess the degree of accuracy of our estimates of Xi(sigma) and to identify and correct for systematic effects in our measurements. We study the evolution of clustering for volume limited subsamples of galaxies selected using their photometric redshifts and rest-frame r-band absolute magnitudes. We find that the real-space correlation length r_0 of bright galaxies, M_r<-21 (rest-frame) can be accurately recovered out to z~1.5, particularly for ECDF-S given its near-infrared photometric coverage. There is mild evidence for a luminosity dependent clustering in both fields at the low redshift samples (up to <z>=0.57), where the correlation length is higher for brighter galaxies by up to 1Mpc/h between median rest-frame r-band absolute magnitudes of -18 to -21.5. As a result of the photometric redshift measurement, each galaxy is assigned a best-fit template; we restrict to E and E+20%Sbc types to construct subsamples of early type galaxies (ETGs). Our ETG samples show a strong increase in r_0 as the redshift increases, making it unlikely (95% level) that ETGs at median redshift z_med=1.15 are the direct progenitors of ETGs at z_med=0.37 with equivalent passively evolved luminosities. (ABRIDGED)
We study the properties of SDSS galaxies with and without AGN detection as a function of the local and global environment measured via the local density, the mass of the galaxy host group (parameterised by the group luminosity) and distance to massiv
e clusters. Our results can be divided in two main subjects, the environments of galaxies and their relation to the assembly of their host haloes, and the environments of AGN. (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 fixed local density show some remaining variation of their properties as a function of the distance to the nearest cluster of galaxies (in a range of 0 to 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. (ii) In AGN host galaxies, the morphology-density relation is much less noticeable when compared to the behaviour of the full SDSS sample. 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 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 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. (ABRIDGED)
Based on recent results on the frequency of MgII absorption line systems in the QSO behind RCS clusters survey (QbC), we analyse the effects of the cluster environment on the sizes of baryonic haloes around galaxies. We use two independent models, i)
an empirical halo occupation model which fits current measurements of the clustering and luminosity function of galaxies at low and high redshifts, and ii) the GALFORM semi-analytic model of galaxy formation, which follows the evolution of the galaxy population from first principles, adjusted to match the statistics of low and high redshift galaxies. In both models we constrain the MgII halo sizes of field and cluster galaxies using observational results on the observed MgII statistics. Our results for the field are in good agreement with previous works, indicating a typical mgii halo size of $r_MgII ~ 50h_71^-1kpc in the semi-analytic model, and slightly lower in the halo occupation number approach. For the cluster environment, we find that both models require a median MgII halo size of r_MgII< 10h_71^-1kpc in order to reproduce the observed statistics on absorption line systems in clusters of galaxies. Based on the Chen & Tinker (2008) result that stronger systems occur closer to the MgII halo centre, we find that strong absorption systems in clusters of galaxies occur at roughly a fixed fraction of the cold-warm halo size out to 1h_71^-1Mpc from the cluster centres. In contrast, weaker absorption systems appear to occur at progressively shorter relative fractions of this halo as the distance to the cluster centre decreases.
In this paper, we study the variations of group galaxy properties according to the assembly history in SDSS-DR6 selected groups. Using mock SDSS group catalogues, we find two suitable indicators of group formation time: i) the isolation of the group,
defined as the distance to the nearest neighbor in terms of its virial radius, and ii) the concentration, measured as the group inner density calculated using the fifth nearest bright galaxy to the group centre. Groups within narrow ranges of mass in the mock catalogue show increasing group age with isolation and concentration. However, in the observational data the stellar age, as indicated by the spectral type, only shows a correlation with concentration. We study groups of similar mass and different assembly history, finding important differences in their galaxy population. Particularly, in high mass SDSS groups, the number of members, mass-to-light ratios, red galaxy fractions and the magnitude difference between the brightest and second brightest group galaxies, show different trends as a function of isolation and concentration, even when it is expected that the latter two quantities correlate with group age. Conversely, low mass SDSS groups appear to be less sensitive to their assembly history. The correlations detected in the SDSS are not consistent with the trends measured in the mock catalogues. However, discrepancies can be explained in terms of the disagreement found in the age-isolation trends, suggesting that the model might be overestimating the effects of environment.We discuss how the modeling of the cold gas in satellite galaxies could be responsible for this problem. These results can be used to improve our understanding of the evolution of galaxies in high-density environments.
We study the alignments between the angular momentum of individual objects and the large-scale structure in cosmological numerical simulations and real data from the Sloan Digital Sky Survey, Data Release 6. To this end we measure anisotropies in the
two point cross-correlation function around simulated halos and observed galaxies, studying separately the 1- and 2-halo regimes. The alignment of the angular momentum of dark-matter haloes in LCDM simulations is found to be dependent on scale and halo mass. At large distances (2-halo regime), the spins of high mass haloes are preferentially oriented in the direction perpendicular to the distribution of matter; lower mass systems show a weaker trend that may even reverse to show an angular momentum in the plane of the matter distribution. In the 1-halo term regime, the angular momentum is aligned in the direction perpendicular to the matter distribution; the effect is stronger than for the 1-halo term and increases for higher mass systems. On the observational side, we focus our study on galaxies in the Sloan Digital Sky Survey, Data Release 6 (SDSS-DR6) with elongated apparent shapes, and study alignments with respect to the major semi-axis. We find an excess of structure in the direction of the major semi-axis for all samples; the red sample shows the highest alignment (2.7+-0.08%) and indicates that the angular momentum of flattened spheroidals tends to be perpendicular to the large-scale structure. (Abridged)
We determine the underlying shapes of spiral and elliptical galaxies in the Sloan Digital Sky Survey Data Release 6 from the observed distribution of projected galaxy shapes, taking into account the effects of dust extinction and reddening. We assume
that the underlying shapes of spirals and ellipticals are well approximated by triaxial ellipsoids. The elliptical galaxy data are consistent with oblate spheroids, with a correlation between luminosity and ellipticity: the mean values of minor to middle axis ratios are 0.41+-0.03 for Mr ~ -18 ellipticals, and 0.76+-0.04 for Mr ~-22.5 ellipticals. Ellipticals show almost no dependence of axial ratio on galaxy colour, implying a negligible dust optical depth. There is a strong variation of spiral galaxy shapes with colour indicating the presence of dust. The intrinsic shapes of spiral galaxies in the SDSS-DR6 are consistent with flat disks with a mean and dispersion of thickness to diameter ratio of (21+-2)%, and a face-on ellipticity, e, of ln(e)=-2.33+-0.79. Not including the effects of dust in the model leads to disks that are systematically rounder by up to 60%. More luminous spiral galaxies tend to have thicker and rounder disks than lower-luminosity spirals. Both elliptical and spiral galaxies tend to be rounder for larger galaxies. The marginalised value of the edge-on r-band dust extinction E_0 in spiral galaxies is E_0 ~ 0.45 magnitudes for galaxies of median colours, increasing to E_0=1 magnitudes for g-r>0.9 and E_0=1.9 for the luminous and most compact galaxies, with half-light radii <2kpc/h.
