Number Of Galaxies Research Articles

Analysis of the properties of clusters of galaxies in the region of the Ursa Major supercluster

We analyze the properties of the clusters of galaxies in the region of the Ursa Major (UMa) supercluster using observational data from SDSS and 2MASS catalogs. The region studied includes a supercluster (with a galaxy and cluster overdensity of 3 and 15, respectively) and field clusters inside the 150-Mpc diameter surrounding region. The total dynamical mass of 10 clusters of galaxies in UMa is equal to 2.25 × 1015 M ⊙, and the mass of 11 clusters of galaxies in the UMa neighborhood is equal to 1.70 × 1015 M ⊙. The fraction of early-type galaxies brighter than M * K + 1 in the virialized regions of clusters is, on the average, equal to 70%, and it is virtually independent on the mass of the cluster. The fraction of these galaxies and their average photometric parameters are almost the same both for UMa clusters and for the clusters located in its surroundings. Parameters of the clusters of galaxies, such as infrared luminosities up to a fixed magnitude, the mass-to-luminosity ratio, and the number of galaxies have almost the same correlations with the cluster mass as in other samples of galaxies clusters. However, the scatter of these parameters for UMa member clusters is twice smaller than the corresponding scatter for field clusters, possibly, due to the common origin of UMa clusters and synchronized dynamical evolution of clusters in the supercluster.

AGN Environments in the Sloan Digital Sky Survey. I. Dependence on Type, Redshift, and Luminosity

We explore how the local environment is related to the redshift, type, and luminosity of active galactic nuclei (AGNs). Recent simulations and observations are converging on the view that the extreme luminosity of quasars is fueled in major mergers of gas-rich galaxies. In such a picture, quasars are expected to be located in regions with a higher density of galaxies on small scales where mergers are more likely to take place. However, in this picture, the activity observed in low-luminosity AGNs is due to secular processes that are less dependent on the local galaxy density. To test this hypothesis, we compare the local photometric galaxy density on kiloparsec scales around spectroscopic type I and type II quasars to the local density around lower luminosity spectroscopic type I and type II AGNs. To minimize projection effects and evolution in the photometric galaxy sample we use to characterize AGN environments, we place our random control sample at the same redshift as our AGNs and impose a narrow redshift window around both the AGNs and control targets. We find that higher luminosity AGNs have more overdense environments compared to lower luminosity AGNs on all scales out to our 2 h70−1 Mpc limit. In addition, in the range 0.3 ≤ z ≤ 0.6, type II quasars have similarly overdense environments to those of bright type I quasars on all scales out to our 2 h70−1 Mpc limit, while the environment of dimmer type I quasars appears to be less overdense than the environment of type II quasars. We see increased overdensity for type II AGNs compared to type I AGNs on scales out to our limit of 2 h70−1 Mpc in overlapping redshift ranges. We also detect marginal evidence for evolution in the number of galaxies within 2 h70−1 Mpc of a quasar with redshift.

Cosmic Evolution of Stellar Disk Truncations: Fromz ∼ 1 to the Local Universe

We have conducted the largest systematic search so far for stellar disk truncations in disk-like galaxies at intermediate redshift (z<1.1), using the Great Observatories Origins Deep Survey South (GOODS-S) data from the Hubble Space Telescope - ACS. Focusing on Type II galaxies (i.e. downbending profiles) we explore whether the position of the break in the rest-frame B-band radial surface brightness profile (a direct estimator of the extent of the disk where most of the massive star formation is taking place), evolves with time. The number of galaxies under analysis (238 of a total of 505) is an order of magnitude larger than in previous studies. For the first time, we probe the evolution of the break radius for a given stellar mass (a parameter well suited to address evolutionary studies). Our results suggest that, for a given stellar mass, the radial position of the break has increased with cosmic time by a factor 1.3+/-0.1 between z~1 and z~0. This is in agreement with a moderate inside-out growth of the disk galaxies in the last ~8 Gyr. In the same period of time, the surface brightness level in the rest-frame B-band at which the break takes place has increased by 3.3+/-0.2 mag/arcsec^2 (a decrease in brightness by a factor of 20.9+/-4.2). We have explored the distribution of the scale lengths of the disks in the region inside the break, and how this parameter relates to the break radius. We also present results of the statistical analysis of profiles of artificial galaxies, to assess the reliability of our results.

Clustering properties of galaxies with different galaxy morphologies

From the volume-limited main galaxy sample of the Sloan Digital Sky Survey Data Release 6 (SDSS DR6), we construct an early-type sample and a late-type sample, and perform comparative studies of clustering properties between them. It is found that the distribution of late-type galaxies is more filamentary. In addition, in the late-type sample, the fractions of single, close double and multiple galaxies are higher than the ones of the early-type sample, but the fraction of grouped galaxies is lower. We also randomly extract a subsample from the late-type sample, which has the same galaxy number and number density as the early-type sample, compare clustering properties of this subsample with ones of the early-type sample, and reach the same conclusions.

Calibrating Redshift Distributions beyond Spectroscopic Limits with Cross‐Correlations

We describe a new method that can measure the true redshift distribution of any set of objects that are studied only photometrically. Measuring the angular cross-correlation between objects in the photometric sample with objects in some spectroscopic sample as a function of the spectroscopic -->z, along with other, standard correlation measurements, provides sufficient information to reconstruct the redshift distribution of the photometric sample. The spectroscopic sample need not resemble the photometric sample in galaxy properties, but must fall within its sky coverage. We test this hybrid, photometric-spectroscopic cross-correlation technique with Monte Carlo simulations based on realistic error estimates (including sample variance). The rms errors in recovering both the mean redshift and -->σ of the redshift distribution for a single photometric redshift bin with true distribution given by a Gaussian are -->1.4 × 10−3(σz/0.1)(Σp/10)−0.3(dNs/dz/25,000)−1/2, where -->σz is the true Gaussian -->σ , -->Σp is the surface density of the photometric sample in galaxies arcmin −2, and -->dNs/dz is the number of galaxies with a spectroscopic redshift per unit -->z. We test the impact of non-Gaussian redshift outliers and of systematic errors due to unaccounted-for bias evolution, errors in measuring autocorrelations, photometric zero-point variations, or mistaken cosmological assumptions, and find that none will dominate measurement uncertainties in reasonable scenarios. The true redshift distributions of even arbitrarily faint photometric samples may be determined to the precision required by proposed dark energy experiments ( -->Δz 3 × 10−3 at -->z ~ 1) with this method.

The magnetic fields and angular sizes of radio sources displaying synchrotron self-absorption

A method for estimating the magnetic-field strengths and angular sizes of radio sources displaying synchrotron self-absorption based on their observed radio spectra is considered. The method is used to derive the angular sizes of compact radio sources (components) and the magnetic fields in these regions, as well as the energy and number of relativistic electrons and the radiative power of a number of quasars and radio galaxies.

Luminosity Function of the Cluster of Galaxies Abell 566

We investigate the Luminosity Function (LF) of the cluster of galaxies Abell 566. The photometric data of 15 intermediate-bands are obtained from the Beijing-Arizona-Taiwan-Connecticut (BATC) photometric sky survey. For each of the 15 wavebands, the LF of cluster galaxies is well modelled by the Schechter function, with characteristic luminosities from –18.0 to –21.9 magnitude, from the a- to the p-band. Morphological dependence of the LF is investigated by separating the cluster members into `red' and `blue' subsamples. It is clear that late type galaxies have a steeper shape of LF than the early type galaxies. We also divided the sample galaxies by their local environment. It was found that galaxies in the sparser region have steeper shape of LF than galaxies in the denser region. Combining the results of morphological and environmental dependence of LFs, we show that Abell 566 is a well relaxed cluster with positive evidence of galaxy interaction and merger, and excess number of bright early type galaxies located in its denser region.

Redshift distribution of the submillimeter extragalactic background light

The submillimeter (submm) extragalactic background light (EBL) traces the integrated star formation history throughout the cosmic time. Deep blank-field 850 μm and 1.4 GHz surveys and optical follow-up have been only able to determine the redshift of ∼20% of the submm EBL. The majority (80%) of the submm EBL is still below the confusion and sensitivity limits of current submm and radio instruments. We break through these limits with stacking analyses on our deep 850 μm image in the GOODS-N and find that the submm EBL mostly comes from galaxies at redshifts around 1.0. This redshift is much lower than the redshift of z=2–3 previously implied from radio identified submm sources. This result significantly decreases the number of high redshift galaxies that may be seen by ALMA.

Photometric Redshifts and Signal‐to‐Noise Ratios

We investigate the impact of photometric signal-to-noise ratio (S/N) on the precision of photometric redshifts in multiband imaging surveys using both simulations and real data. We simulate the optical four-band (BVRz) Deep Lens Survey (DLS) and use the publicly available Bayesian Photometric Redshift code BPZ. The simulations include a realistic range of magnitudes and colors and vary from infinite S/N to S/N = 5. The real data are from DLS photometry and two spectroscopic surveys and explore a range of S/Ns by adding noise to initially very high S/N photometry. Precision degrades steadily as S/N drops, both because of direct S/N effects and because lower S/N is linked to fainter galaxies with a weaker magnitude prior. If a simple S/N cut were used, S/N ≥ 17 in R (corresponding, in the DLS, to lower S/N in other bands) would be required to keep the scatter in Δ z ≡ (zspec − zphot)/(1 + zspec) to less than 0.1. However, cutting on ODDS (a measure of the peakiness of the probability density function provided by BPZ) greater than 0.4 provides roughly double the number of usable galaxies with the same σΔ z. Ellipticals form the tightest zspec − zphot relation, and cutting on elliptical type provides better precision than the ODDS > 0.9 cut, but this eliminates the vast majority of galaxies in a deep survey. In addition to being more efficient than a type cut, ODDS also has the advantages of working with all types of galaxies (although ellipticals are overrepresented) and of being a continuous parameter for which the severity of the cut can be adjusted as desired.

Dynamical evolution of galaxy clusters in the framework of the N-body problem. The formation of supermassive cD galaxies

We have carried out numerical simulations of the dynamical evolution of galaxy clusters taking into account merging when the relative velocities of the colliding galaxies are low. In particular, we study the evolution of the structure, mass spectrum, and velocity spectrum of a cluster of a thousand galaxies, as well as the growth of the central supermassive cD galaxy. The initial velocity dispersion of the galaxies and the rotation of the cluster were taken into account. The observed logarithmic spectrum dN ∼ % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfKttLearuqr1ngBPrgarmWu51MyVXgatC% vAUfeBSjuyZL2yd9gzLbvyNv2CaeHbuLwBLnhiov2DGi1BTfMBaeHb% d9wDYLwzYbItLDharqqtubsr4rNCHbGeaGqiVu0Je9sqqrpepC0xbb% L8F4rqqrFfpeea0xe9Lq-Jc9vqaqpepm0xbba9pwe9Q8fs0-yqaqpe% pae9pg0FirpepeKkFr0xfr-xfr-xb9adbaqaaeGaciGaaiaadeWaaq% aadaqbaaGcbaWaaSqaaSqaaiabdsgaKjabd2eanbqaaiabd2eanbaa% aaa!3E82! $$\tfrac{{dM}}{M}$$ was adopted as the initial mass spectrum. The dynamical evolution of galaxy clusters, allowing for the possible merging of colliding galaxies, results in the emergence of a central supermassive galaxy, whose mass continuously increases due to mergers. This occurs only if the mass of the central galaxy becomes greater than ∼0.1 of the total mass of the cluster. The observation of cD galaxies with relative masses of ∼0.01 suggests that they initially formed in the cluster core, merged with nearby galaxies, and accreted intergalactic gas. The model indicates that a logarithmic galaxy mass spectrum is preserved during the cluster evolution, despite the substantial decrease in the number of galaxies in the cluster with time. The model can also reproduce the observed mass distribution with distance from the cluster center, M r ∼ r 1.7.

THE STATISTICAL PROPERTIES OF GALAXY MORPHOLOGICAL TYPES IN COMPACT GROUPS OF MAIN GALAXIES FROM THE SDSS DATA RELEASE 4

In order to explore the statistical properties of galaxy morphological types in compact groups (CGs), we construct a random group sample which has the same distributions of redshift and number of member galaxies as those of the CG sample. It turns out that the proportion of early-type galaxies in different redshift bins for the CG sample is statistically higher than that for random group sample, and with growing redshift z this kind of difference becomes more significant. This may be due to the existence of interactions and mergers within a significant fraction of SDSS CGs. We also compare statistical results of CGs with those of more compact groups and pairs, but do not observe as large statistical difference as Hickson's [Astrophys. J.255 (1982) 382] results.

The mass distribution in early-type disc galaxies: declining rotation curves and correlations with optical properties

We present rotation curves for 19 early-type disc galaxies (S0-Sab). The galaxies span a B-band absolute magnitude range from -17.5 to -22, but the majority have a high luminosity with M-B <-20. Rotation velocities are measured from a combination of H I velocity fields and long-slit optical emission line spectra along the major axis; the resulting rotation curves probe the gravitational potential on scales ranging from 100 pc to 100 kpc. We find that the rotation curves generally rise rapidly in the central regions and often reach rotation velocities of 200-300 km s(-1) within a few hundred parsecs of the centre. The detailed shape of the central rotation curves shows a clear dependence on the concentration of the stellar light distribution and the bulge-to-disc luminosity ratio: galaxies with highly concentrated stellar light distributions reach the maximum in their rotation curves at relatively smaller radii than galaxies with small bulges and a relatively diffuse light distribution. We interpret this as a strong indication that the dynamics in the central regions are dominated by the stellar mass. At intermediate radii, many rotation curves decline, with the asymptotic rotation velocity typically 10-20 per cent lower than the maximum. The strength of the decline is correlated with the total luminosity of the galaxies, more luminous galaxies having on average more strongly declining rotation curves. At large radii, however, all declining rotation curves flatten out, indicating that substantial amounts of dark matter must be present in these galaxies too. A comparison of our rotation curves with the Universal Rotation Curve from Persic et al. reveals large discrepancies between the observed and predicted rotation curves; we argue that rotation curves form a multiparameter family which is too complex to describe with a simple formula depending on total luminosity only. In a number of galaxies from our sample, there is evidence for the presence of rapidly rotating gas in the inner few hundred parsecs from the centres. The inferred central masses and mass densities are too high to be explained by the observed stellar components and suggest the presence of supermassive black holes in these galaxies.

Peculiar motions in the region of the Ursa Major supercluster of galaxies

We have investigated the peculiar motions of clusters of galaxies in the Ursa Major (UMa) supercluster and its neighborhood. Based on SDSS (Sloan Digital Sky Survey) data, we have compiled a sample of early-type galaxies and used their fundamental plane to determine the cluster distances and peculiar velocities. The samples of early-type galaxies in the central regions (within R 200) of 12 UMa clusters of galaxies, in three main subsystems of the supercluster—the filamentary structures connecting the clusters, and in nine clusters from the nearest UMa neighborhood have similar parameters. The fairly high overdensity (3 by the galaxy number and 15 by the cluster number) suggests that the supercluster as a whole is gravitationally bound, while no significant peculiar motions have been found: the peculiar velocities do not exceed the measurement errors by more than a factor of 1.5–2. The mean random peculiar velocities of clusters and the systematic deviations from the overall Hubble expansion in the supercluster are consistent with theoretical estimates. For the possible approach of the three UMa subsystems to be confirmed, the measurement accuracy must be increased by a factor of 2–3.

Anisotropic Galactic Outflows and Enrichment of the Intergalactic Medium. I. Monte Carlo Simulations

We combine an analytic model for anisotropic outflows and galaxy formation with numerical simulations of large-scale structure and halo formation to study the impact of galactic outflows on the evolution of the IGM. We have simulated the evolution of a comoving volume (15 Mpc)^3 in the LCDM universe. We follow the formation of 20000-60000 galaxies and simulate the galactic outflows produced by these galaxies, for five outflow opening angles, alpha=60, 90, 120, 150, and 180 degrees (isotropic outflows). Anisotropic outflows follow the path of least resistance and thus travel preferentially into low-density regions, away from cosmological structures where galaxies form. These anisotropic outflows are less likely to overlap with one another, or to hit pre-galactic collapsing halos and strip them of their gas, preventing a galaxy from forming. Going from 180 deg to 60 deg, the number of galaxies that actually form doubles, producing twice as many outflows, and these outflows overlap to a lesser extent. As a result, the metal volume filling factor of the IGM goes from 8% for isotropic outflows up to 28% for anisotropic ones. High density regions are more efficiently enriched than low density ones (~80% compared to ~20% by volume), even though most enriched regions are low densities. Increasing the anisotropy of outflows increases the extent of enrichment at all densities, low and high. This is in part because anisotropic outflows are more numerous. When this effect is factored-out, we find that the probability a galaxy will enrich systems at densities up to 10 rho_mean is higher for increasingly anisotropic outflows. This is an effect of the dynamical evolution of the IGM. Anisotropic outflows expand preferentially into underdense gas, but that gas can later accrete onto overdense structures.

The color properties of compact groups of main galaxies from the SDSS Data Release 4

In order to compare color statistical properties of member galaxies in compact groups with those of field galaxies, we use a compact group (CG) sample which contains 4217 CGs extracted from the MAIN galaxy sample of the SDSS Data Release 4 (SDSS4), and construct a random group sample which has the same distributions of redshift and number of member galaxies as those of the CG sample. It turns out that the mean colors of galaxies in CGs are redder than those of galaxies in random groups. Additionally, we also find that member galaxies in real CGs have a smaller dispersion of colors than those in random groups-at least for g-r, r-i, and i-z colors.

RASS-SDSS galaxy cluster survey

We explore the mass-to-light ratio in galaxy clusters and its relation to the cluster mass. We study the relations among the optical luminosity ($L_{op}$), the cluster mass ($M_{200}$) and the number of cluster galaxies within $r_{200}$ ($N_{gal}$) in a sample of 217 galaxy clusters with confirmed 3D overdensity. We correct for projection effects, by determining the galaxy surface number density profile in our cluster sample. This is best fitted by a cored King profile in low and intermediate mass systems. The core radius decreases with cluster mass, and, for the highest mass clusters, the profile is better represented by a generalized King profile or a cuspy Navarro, Frenk & White profile. We find a very tight proportionality between $L_{op}$ and $N_{gal}$, which, in turn, links the cluster mass-to-light ratio to the Halo Occupation Distribution $N_{gal}$ vs. $M_{200}$. After correcting for projection effects, the slope of the $L_{op}-M_{200}$ and $N_{gal}-M_{200}$ relations is found to be $0.92\pm0.03$, close, but still significantly less than unity. We show that the non-linearity of these relations cannot be explained by variations of the galaxy luminosity distributions and of the galaxy M/L with the cluster mass. We suggest that the nonlinear relation between number of galaxies and cluster mass reflects an underlying nonlinear relation between number of subhaloes and halo mass.

Dwarf galaxies in voids: suppressing star formation with photoheating

We study the structure formation in cosmological void regions using high-resolution hydrody-namical simulations. Despite being significantly underdense, voids are populated abundantly with small dark matter haloes which should appear as dwarf galaxies if their star formation is not suppressed significantly. We here investigate to which extent the cosmological ultraviolet (UV) background reduces the baryon content of dwarf galaxies, and thereby limits their cooling and star formation rates. Assuming a Haardt & Madau UV background with reionization at redshift z = 6, our samples of simulated galaxies show that haloes with masses below a characteristic mass of M c (z = 0) = 6.5 x 10 9 h -1 M ⊙ are baryon-poor, but in general not completely empty, because baryons that are in the condensed cold phase or are already locked up in stars resist evaporation. In haloes with mass M? M c , we find that photoheating suppresses further cooling of gas. The redshift- and UV-background-dependent characteristic mass M c (z) can be understood from the equilibrium temperature between heating and cooling at a characteristic overdensity of δ ≃ 1000. If a halo is massive enough to compress gas to this density despite the presence of UV-background radiation, gas is free to 'enter' the condensed phase and cooling continues in the halo, otherwise it stalls. By analysing the mass accretion histories of dwarf galaxies in voids, we show that they can build up a significant amount of condensed mass at early times before the epoch of reionization. Later on, the amount of mass in this phase remains roughly constant, but the masses of the dark matter haloes continue to increase. Consequently, photoheating leads to a reduced baryon fraction in void dwarf galaxies, endows them with a rather old stellar population, but still allows late star formation to some extent. We estimate the resulting stellar mass function for void galaxies. While the number of galaxies at the faint end is significantly reduced due to photoheating, additional physical feedback processes may be required to explain the apparent paucity of dwarfs in observations of voids.

Distribution and evolution of high-z galaxies in the Subaru/XMM-Newton Deep Survey Field (SXDF)

AbstractWe present results from our on-going study of the distribution and evolution of high-z galaxies. We exploited the large area coverage (~1.3 deg2) of the Subaru / XMM-Newton Deep Survey field (SXDF) to search for Lyman-break galaxies (LBGs) and Lyα Emitters (LAEs) at 3 &lt;z&lt; 7. We have found filamentary large scale structures, which are made of LAEs, with ~10 to 40 Mpc (in commoving units) scale voids as early as at z = 5.7. Galaxies at this redshift show clustering of ~1 Mpc in diameter (in physical unit). The inferred star formation rate density of these clustering is ~130 times the mean of the whole area, indicates burst of star formation activities. Combined with the UKIDSS/UDS photometry, the optical+near-IR SED fitting provides the estimate of masses for some of these high-z galaxies. Preliminary results indicate a number of galaxies at z &gt;5 have already grown up to ~1011 solar mass.

The Faint-End of the Galaxy Luminosity Function in the Hydra I Cluster

AbstractWe examine the galaxy luminosity functions (LFs) in the Hydra I cluster (Abell 1060) at z = 0.0126, for which very faint galaxies (down to M ~ −10) have not been surveyed yet. We conclude that the total LF has a slightly steep slope (α ~ −1.6) at −20 &lt; M &lt; −10 in the B- and RC-bands. The numbers of galaxies at the faint end (M ≳ −14) differ in between the cluster center and the outskirts slightly. We divide the Hydra member galaxies into red/blue galaxies and find that the shape of LFs in the faint magnitude range is determined by red dwarf galaxies.

X‐Ray‐selected Intermediate‐Redshift Groups of Galaxies

We present spectroscopic confirmation of nine moderate-redshift galaxy groups and poor clusters selected from the ROSAT Deep Cluster Survey. The groups span the redshift range z ~ 0.23-0.59 and have between 4 and 20 confirmed members. The velocity dispersions of these groups range from ~125 to 650 km s-1. Similar to X-ray groups at low redshift, these systems contain a significant number of early-type galaxies. Therefore, the trend for X-ray-luminous groups to have high early-type fractions is already in place by at least z ~ 0.5. In four of the nine groups, the X-ray emission is clearly peaked on the most luminous early-type galaxy in the group. However, in several cases the central galaxy is composed of multiple luminous nuclei, suggesting that the brightest group galaxy may still be undergoing major mergers. In at least three (and possibly five) of the groups in our sample, a dominant early-type galaxy is not found at the center of the group potential. This suggests that many of our groups are not dynamically evolved despite their high X-ray luminosities. While similar systems have been identified at low redshift, the X-ray luminosities of the intermediate-redshift examples are 1-3 orders of magnitude higher than those of their low-redshift counterparts. We suggest that this may be evidence for group downsizing: while massive groups are still in the process of collapsing and virializing at intermediate redshifts, only low-mass groups are in the process of forming at the present day.