Excess Of Galaxies Research Articles

A binary model for the UV-upturn of elliptical galaxies

The discovery of a flux excess in the far-ultraviolet (FUV) spectrum of elliptical galaxies was a major surprise in 1969. While it is now clear that this UV excess is caused by an old population of hot helium-burning stars without large hydrogen-rich envelopes, rather than young stars, their origin has remained a mystery. Here we show that these stars most likely lost their envelopes because of binary interactions, similar to the hot subdwarf population in our own Galaxy. We have developed an evolutionary population synthesis model for the FUV excess of elliptical galaxies based on the binary model developed by Han et al. for the formation of hot subdwarfs in our Galaxy. Despite its simplicity, it successfully reproduces most of the properties of elliptical galaxies with a UV excess: the range of observed UV excesses, both in (1550 - V) and (2000 - V), and their evolution with redshift. We also present colour-colour diagrams for use as diagnostic tools in the study of elliptical galaxies. The model has major implications for understanding the evolution of the UV excess and of elliptical galaxies in general. In particular, it implies that the UV excess is not a sign of age, as had been postulated previously, and predicts that it should not be strongly dependent on the metallicity of the population, but exists universally from dwarf ellipticals to giant ellipticals.

The red and blue galaxy luminosity function in the GOODS field: Evidence for an excess of red-dwarf galaxies

The red and blue galaxy luminosity function in the GOODS field: Evidence for an excess of red-dwarf galaxies

The cluster galaxy luminosity function at z= 0.3: a recent origin for the faint-end upturn?

We derive deep luminosity functions (LFs) (to M z = -15) for galaxies in Abell 1835 (z = 0.25) and AC 114 (z = 0.31), and compare these with the local z' LF for 69 clusters. The data show that the faint-end upturn, the excess of galaxies above a single Schechter function at M z < -17, does not exist in the higher redshift clusters. This suggests that the faint-end upturn galaxies have been created recently, by infall into clusters of star-forming field populations or via tidal disruption of brighter objects.

The first appearance of the red sequence of galaxies in proto-clusters at 2 lsimzlsim 3

We explore the evolved galaxy population in the proto-clusters around four high-z radio galaxies at 2 ≤ z ≤ 3 based on wide-field near-infrared (NIR) imaging. Three of the four fields are known proto-clusters as demonstrated by overdensities of line-emitting galaxies at the same redshifts as the radio galaxies found by narrow-band surveys and spectroscopic follow-up observations. We imaged the fields of three targets (PKS 1138-262, USS 0943-242 and MRC 0316-257) to a depth of K s ∼ 22 (Vega magnitude, 5σ) over a 4 x 7 arcmin 2 area centred on the radio galaxies with a new wide-field NIR camera, Multi-Object Infra-Red Camera and Spectrograph (MOIRCS), on the Subaru Telescope. Another target (USS 1558-003) was observed with Son of ISAAC on the New technology Telescope (NTT) to a depth of K s = 20.5 (5σ) over a 5 x 5 arcmin 2 area. We apply colour cuts in J-K s and/or JHK s in order to exclusively search for galaxies located at high redshifts: z > 2. To the 5σ limiting magnitudes, we see a significant excess of NIR-selected galaxies by a factor of 2 to 3 compared to those found in the field of GOODS-South. The spatial distribution of these NIR-selected galaxies is not uniform and traces structures similar to those of emission-line galaxies, although the samples of NIR-selected galaxies and emitters show little overlap, from which we conclude that the former tend to be an evolved population with much higher stellar mass than the latter, young and active emitters. We focus on the NIR colour-magnitude sequence of the evolved population and find that the bright-end (M stars > 10 11 M ⊙ ) of the red sequence is well populated by z ∼ 2 but much less so in the z ∼ 3 proto-clusters. This may imply that the bright-end of the colour-magnitude sequence first appeared between z = 3 and 2, an era coinciding with the appearance of sub-mm galaxies and the peak of the cosmic star formation rate. Our observations show that during the same epoch, massive galaxies are forming in high-density environments by vigorous star formation and assembly.

K-band imaging of strong Ca II-absorber host galaxies at z 1

We present K-band imaging of fields around 30 strong Ca ii absorption-line systems, at 0.7 < z < 1.2, three of which are confirmed damped Lyman α systems. A significant excess of galaxies is found within 6.0 arcsec (≃50 kpc) from the absorber line of sight. The excess galaxies are preferentially luminous compared to the population of field galaxies. A model in which field galaxies possess a luminosity-dependent cross-section for Ca ii absorption of the form (L/L*)0.7 reproduces the observations well. The luminosity-dependent cross-section for the Ca ii absorbers appears to be significantly stronger than the established (L/L*)0.4 dependence for Mg ii absorbers. The associated galaxies lie at large physical distances from the Ca ii-absorbing gas; we find a mean impact parameter of 24 kpc (H0= 70 km s−1 Mpc−1). Combined with the observed number density of Ca ii absorbers the large physical separations result in an inferred filling factor of only ∼10 per cent. The physical origin of the strong Ca ii absorption remains unclear, possible explanations vary from very extended discs of the luminous galaxies to associated dwarf galaxy neighbours, remnants of outflows from the luminous galaxies, or tidal debris from cannibalism of smaller galaxies.

The Environments of Ultrastrong MgiiAbsorbers

We present r'- or i'-band WIYN images of the fields of 15 Sloan Digital Sky Survey quasars that have spectra exhibiting intervening MgII absorption-line systems with rest equivalent widths 2.7A \le REW \le 6.0A and redshifts 0.42 < z_{abs} < 0.84. Such systems are rare and exhibit projected absorption velocity spreads in excess of \approx 300-650 km/s. Approximately 60% are expected to be damped Ly\alpha systems. In each of our fields we detect at least one galaxy that, if at the absorption redshift, would have impact parameter b \lesssim 40 kpc and luminosity L \gtrsim 0.3 L*. We measure a significant excess of galaxies at low-b to the sightlines over a large range of luminosity. Many of the sightlines are found to pass either through or close to the optically-luminous extent of a galaxy. Considering the very large velocity spreads seen in absorption, this suggests that these absorbing regions are more kinematically complex than local spirals such as the Milky Way. Our data indicate that interactions and galaxy pairs may be a contributing factor to the production of such large velocity spreads. Finally, we also find evidence that a population of galaxies with luminosities in the range 4L* \lesssim L \lesssim 13 L* may contribute to the presence of ultra-strong MgII absorption. Thus, some of the absorbing galaxies may represent a population intermediate to the very luminous high-redshift Lyman break galaxies and the fainter local starburst population.

Strong Dusty Bursts of Star Formation in Galaxies Falling into the Cluster RX J0152.7−1357

We have observed the cluster RX J0152.7-1357 (z ~ 0.83) at 24 μm with the Multiband Imaging Photometer for Spitzer (MIPS). We detected 22 sources associated with spectroscopically confirmed cluster members, while 10 more have photometric redshifts compatible with membership. Two of the 32 likely cluster members contain obvious active nuclei, while the others are associated with dusty star formation. The median IR-determined star formation rate among the remaining galaxies is estimated at 22 M☉ yr-1, significantly higher than in previous estimates from optical data. Most of the mid-infrared (MIR) emitting galaxies also have optical emission lines, but a few do not and hence have completely hidden bursts of star formation or AGN activity. An excess of MIR-emitting galaxies is seen in the cluster, compared to the field at the same redshift. The MIR cluster members are more associated with previously detected infalling late-type galaxies rather than triggered by the ongoing merging of bigger X-ray clumps. Rough estimates also show that ram pressure may not be capable of stripping the gas away from cluster outskirt galaxies, but it may disturb the gas enough to trigger the star formation activity. Harassment can also play a role if, for example, these galaxies belong to poor galaxy groups. Thus, bursts of star formation occur in the cluster environment and could also help consume the galaxy gas content, in addition to ram pressure, harassment, or galaxy-galaxy strong interactions.

Chandra point-source counts in distant galaxy clusters

With the superb angular resolution of the Chandra Observatory, it is now possible to detect X-ray point sources, either embedded in galaxy clusters or along the cluster line of sight, which could not be resolved by previous instruments. This now allows studies of source counts in distant cluster fields. A sample of 18 distant (0.25 < z < 1.01) galaxy clusters from the Chandra archive were used to analyze the inner region of clusters of galaxies to check for the presence of any overdensity of X-ray point sources embedded in the gas diffuse emission. We constructed the logN-logS, in both the soft and hard energy bands, for the X-ray point sources detected in the central cluster region (within 1 Mpc)to be compared with the counts of point sources detected in similarly deep fields without clusters. We find a ~ 2 sigma excess of cluster region sources at the bright end of the logN-logS. The radial distribution of the brightest X-ray point sources confirms this excess and indicates that it is confined to the inner 0.5 Mpc of the cluster region. The results suggest the possible existence of X-ray sources belonging to the cluster (most probably AGN, given their 0.5-10 keV luminosity ranging from 10^43 to 10^44 erg/s): on average one every three clusters. The X-ray source excess found here is much smaller than the excess of radio galaxies found recently in high-z X-ray selected clusters, possibly due to the higher sensitivity of the radio observations.

The properties of galaxies in voids

We present a comparison of the properties of galaxies in the most underdense regions of the Universe, where the galaxy number density is less than 10 per cent of the mean density, with galaxies from more typical regions. We have compiled a sample of galaxies in 46 large nearby voids that were identified using the Sloan Digital Sky Survey DR4, which provides the largest coverage of the sky. We study the u - r colour distribution, morphology, specific star formation rate (SFR) and radial number density profiles for a total of 495 galaxies fainter than M r = -20.4 + 5 log h located inside the voids and compare these properties with a control sample of field galaxies. We show that there is an excess of blue galaxies inside the voids. However, inspecting the properties of blue and red galaxies separately, we find that galaxy properties such as colour distribution, bulge-to-total ratios and concentrations are remarkably similar between the void and overall sample. The void galaxies also show the same specific SFR at fixed colour as the control galaxies. We compare our results with the predictions of cosmological simulations of galaxy formation using the Millennium Run semi-analytic galaxy catalogue. We show that the properties of the simulated galaxies in large voids are in reasonably good agreement with those found in similar environments in the real Universe. To summarize, in spite of the fact that galaxies in voids live in the least dense large-scale environment, this environment makes very little impact on the properties of galaxies.

Protoclusters with evolved populations around radio galaxies at z 2.5

We report the discovery of protocluster candidates around high-redshift radio galaxies at z ∼ 2.5 on the basis of clear statistical excess of colour-selected galaxies around them seen in the deep near-infrared imaging data obtained with CISCO on the Subaru Telescope. We have observed six targets, all at similar redshifts at z ∼ 2.5, and our data reach J = 23.5, H = 22.6 and K = 21.8 (5σ) and cover a 1.6 x 1.6 arcmin 2 field centred on each radio galaxy. We apply colour cuts in JHK bands in order to exclusively search for galaxies located at high redshifts, z > 2. Over the magnitude range of 19.5 2.3 by a factor of 2 around the combined radio galaxies fields compared to those found in the general field of the Great Observatories Origins Deep Survey-South (GOODS-S). The excess of galaxies around the radio galaxies fields becomes more than a factor of 3 around 19.5 < K < 20.5 when the two-colour cuts are applied with JHK bands. Such overdensity of the colour-selected galaxies suggests that those fields tend to host high-density regions at high redshifts, although there seems to be the variety of the density of the colour-selected galaxies in each field. In particular, two radio galaxies fields out of the six observed fields show very strong density excess and these are likely to be protoclusters associated with the radio galaxies which would evolve into rich clusters of galaxies dominated by old passively evolving galaxies.

Weak redshift discretisation in the Local Group of galaxies?

We discuss the distribution of radial velocities of galaxies belonging to the Local Group. Two independent samples of galaxies as well as several methods of reduction from the heliocentric to the galactocentric radial velocities are explored. We applied the power spectrum analysis using the Hann function as a weighting method, together with the jackknife error estimation. We performed a detailed analysis of this approach. The distribution of galaxy redshifts seems to be non-random. An excess of galaxies with radial velocities of $\sim 24 {km} \cdot {s}^{-1}$ and $\sim 36 {km} \cdot {s}^{-1}$ is detected, but the effect is statistically weak. Only one peak for radial velocities of $\sim 24 {km} \cdot {s}^{-1}$ seems to be confirmed at the confidence level of 95%.

Discovery of a Galaxy Cluster in the Foreground of the Wide‐Separation Quasar Pair UM 425

We report the discovery of a cluster of galaxies in the field of UM 425, a pair of quasars separated by 65. Based on this finding, we revisit the long-standing question of whether this quasar pair is a binary quasar or a wide-separation lens. Previous work has shown that both quasars are at z = 1.465 and show broad absorption lines. No evidence for a lensing galaxy has been found between the quasars, but there are two hints of a foreground cluster: diffuse X-ray emission observed with Chandra, and an excess of faint galaxies observed with the Hubble Space Telescope. Here we show, via VLT spectroscopy, that there is a spike in the redshift histogram of galaxies at z = 0.77. We estimate the chance of finding a random velocity structure of such significance to be about 5%, and thereby interpret the diffuse X-ray emission as originating from z = 0.77 rather than from the quasar redshift. The mass of the cluster, as estimated from either the velocity dispersion of the z = 0.77 galaxies or the X-ray luminosity of the diffuse emission, would be consistent with the theoretical mass required for gravitational lensing. The positional offset between the X-ray centroid and the expected location of the mass centroid is ~40 kpc, which is not too different from offsets observed in lower redshift clusters. However, UM 425 would be an unusual gravitational lens, by virtue of the absence of a bright primary lensing galaxy. Unless the mass-to-light ratio of the galaxy is at least 80 times larger than usual, the lensing hypothesis requires that the galaxy group or cluster plays a uniquely important role in producing the observed deflections.

Counts of Faint Blue Objects and the Anomalous Red Shift

The hypothesis that the spectra of young objects contain an anomalous red shift which decreases and disappears as they evolve is considered. The possibility of detecting features of this anomalous red shift in the spectra of postquasar objects is considered under the assumption that a large fraction of the quasars are local objects ejected from galactic nuclei. It is argued that the anomalous red shift has a significant effect on the form of the observed luminosity function for the local population of galaxies, depleting it at faint magnitudes. This approach is used to examine the problem of excess blue galaxies.

Detection of 20–30 h$\mathsf{^{-1}}$ Mpc-scale galaxy structures embedded in 100 h$\mathsf{^{-1}}$ Mpc-scale structures of quasars and MgII absorbers at $\textit{z}\simeq\mathsf{0.8}$ and $\textit{z} \simeq\mathsf{1.2}$

We report the finding of two large-scale structures of galaxies in a $40\times35~{\rm arcmin}^{2}$ field embedded in a $25\deg^2$ area where two 100$~h^{-1}\,{\rm Mpc}$-scale structures of quasars and $\ion{Mg}{ii}$ absorbers at $z\simeq0.8$ and $z\simeq1.2$ have been previously detected. Using deep optical ( V - and I -band) imaging, we are able to select high-redshift ($0.5\la z\la 1.3$) early-type galaxies as those redder than the cluster red sequence at $z=0.5$ and having $I red galaxies , corresponding to 563 galaxies across the field. The colour distribution of the excess galaxies shows a coherent peak at $2.7<V-I<3.1$ and the magnitude distribution is well described by a Schechter function, showing unambiguously that the galaxy excess is primarily due to a population of early-type galaxies at $z=0.83\pm0.08$. In follow-up NIR imaging of the four best cluster candidates, we identify three clusters at $z=0.8\pm0.1$, each having well defined red sequences of ~20 galaxies at $I-K_{\rm s}\simeq3.4$, forming a structure $20~h^{-1}\,{\rm Mpc}$ across. The fourth candidate, corresponding to a previously identified cluster at $z=1.2\pm0.1$, is confirmed with 28 $K_{\rm s}<20.5$ galaxies having $\textit{VIJK}_{\rm s}$ colours consistent with passively-evolving galaxies at $z\simeq1.2$ located within 1 arcmin of the cluster centre. In all four NIR fields we find numerous galaxies with the $\textit{VIJK}_{\rm s}$ colours of passively-evolving galaxies both at $z\simeq0.8$ and $z\simeq1.2$, including regions well away from the clusters. The results of this study support the presence of two sheet-like large-scale galaxy structures extending across the 20–3$0~h^{-1}\,{\rm Mpc}$ scales explored by the optical data, one “sheet” at $z\simeq0.8$ with three associated clusters, and a second “sheet” at $z\simeq1.2$ with one embedded cluster. Our results confirm that, at least inside the probed region, the two known 100$~h^{-1}\,{\rm Mpc}$-scale structures at $z\sim1$ of quasars and $\ion{Mg}{ii}$ absorbers also mark out volumes with an enhanced density of galaxies. The finding of two galaxy structures at $z\simeq 1$ on scales of 20–30$~h^{-1}\,{\rm Mpc}$ within much larger superstructures of quasars and $\ion{Mg}{ii}$ absorbers, firstly establishes the presence of galaxy sheet-like structures at $z\sim1$, and secondly, suggests that they may extend up to 100$~h^{-1}\,{\rm Mpc}$-scales.

Weak lensing of the high-redshift SNIa sample

The current sample of high-redshift Type Ia supernovae (SNeIa), which combines results from two teams, the High-z Supernova Search Team and the Supernova Cosmology Project, is analysed for the effects of weak lensing. After correcting supernovae (SN) magnitudes for cosmological distances, assuming recently published, homogeneous distance and error estimates, we find that brighter SNe are preferentially found behind regions (5–15 arcmin radius) that are overdense in foreground, z∼ 0.1 galaxies. This is consistent with the interpretation that SN fluxes are magnified by foreground galaxy excess and demagnified by foreground galaxy deficit, compared with a smooth universe case. The difference between most magnified and most demagnified SNe is approximately 0.3–0.4 mag. The effect is significant at the >99 per cent level. Simple modelling reveals that the slope of the relation between supernova magnitude and foreground galaxy density depends on the amount and distribution of matter along the line of sight to the sources, but does not depend on the specifics of the galaxy biasing scheme.

Lens or Binary?ChandraObservations of the Wide‐Separation Broad Absorption Line Quasar Pair UM 425

We have obtained a 110 ks Chandra ACIS-S exposure of UM 425, a pair of QSOs at z = 1.47 separated by 65, which show remarkably similar emission and broad absorption line (BAL) profiles in the optical/UV. Our 5000 count X-ray spectrum of UM 425A (the brighter component) is well fitted with a power law (photon spectral index Γ = 2.0) partially covered by a hydrogen column of 3.8 × 1022 cm-2. The underlying power-law slope for this object and for other recent samples of BALQSOs is typical of radio-quiet quasars, lending credence to the hypothesis that BALs exist in every quasar. Assuming the same Γ for the much fainter image of UM 425B, we detect an obscuring column 5 times larger. We search for evidence of an appropriately large lensing mass in our Chandra image and find weak diffuse emission near the quasar pair, with an X-ray flux typical of a group of galaxies at redshift z ~ 0.6. From our analysis of archival HST WFPC2 and NICMOS images, we find no evidence for a luminous lensing galaxy, but note a 3 σ excess of galaxies in the UM 425 field with plausible magnitudes for a z = 0.6 galaxy group. However, the associated X-ray emission does not imply sufficient mass to produce the observed image splitting. The lens scenario thus requires a dark (high M/L ratio) lens or a fortuitous configuration of masses along the line of sight. UM 425 may instead be a close binary pair of BALQSOs, which would boost arguments that interactions and mergers increase nuclear activity and outflows.

Clustering around radio galaxies at z∼1.5

The importance of studying old elliptical galaxies at redshift z∼1.5 is reviewed, considering both what can be learned by extending studies of the evolution of cluster galaxy scaling relations to earlier cosmic epochs, and the age-dating of old elliptical galaxies at high redshifts. Following this, the first results are provided of an on-going project to find such distant elliptical galaxies, through an investigation of the cluster environments of powerful radio sources with redshifts 1.44< z<1.7. These studies show a considerable excess of red galaxies in the radio source fields, with the magnitudes ( K≳17.5) and colours ( R− K>4) expected of old passively evolving galaxies at the radio source redshift. The red galaxy overdensities are found on two different scales around the radio sources; a pronounced small-scale peak at radial distances of ≲150 kpc, and a weaker large-scale excess extending out to 1–1.5 Mpc. The presence and richness of these red galaxy excesses varies considerably from source-to-source. An interpretation of these results is provided.

The evolution of active galaxies in clusters

The results of deep radio, sub-mm and X-ray observations of samples of high redshift (z∼1) clusters are presented. These reveal significant excesses of active galaxies associated with the clusters at all three wavelengths. The cluster radio source population shows evolution consistent with the (1+z)3 evolution typical of many AGN classes. A large fraction of the AGN are hosted by apparently passive early-type galaxies, often with a close companion. These results essentially constitute the detection of a counterpart of the Butcher-Oemler effect for both strongly star bursting galaxies and AGN.

Cluster of galaxies around seven radio-loud QSOs at $\mathsf 1&lt;z&lt;\mathsf{1.6}$

We have conducted a NIR study of the environments of seven radio-loud quasars at redshifts 1< z< 1:6. In present paper we describe deep K band images obtained for the fields of 6 6 arcmin around the quasars with 3 limiting magnitudes of K 20:5. These fields were previously studied using deep B and R band images (Sanchez & Gonzalez-Serrano 1999). Using optical and NIR data together, a significant excess of galaxies has been found whose optical-NIR colours, luminosity, spatial scale, and number of galaxies are compatible with clusters at the redshift of the quasar. We have selected a sample of cluster candidates analyzing the R K vs. K diagram. A25% of the candidates present red optical-NIR colours and an ultraviolet excess. This population has been also found in clusters around quasars at the same redshifts (Tanaka et al. 2000; Haines et al. 2001). These galaxies seem to follow a mixed evolution: a main passive evolution plus late star formation processes. The quasars do not inhabit the core of the clusters, being found in the outer regions. This result agrees with the hypothesis that the origin/feeding mechanism of the nuclear activity is merging processes. The quasars inhabit the region where a collision is most likely to produce a merger.

The lensing system towards the doubly imaged quasar SBS 1520+530

The gravitational potential responsible for the lensing effect in SBS 1520+530 is studied over length scales from a few arc-seconds to a few arc-minutes. For this purpose, we use sharply deconvolved Hubble Space Telescope images in the optical and near-IR, in combination with ground based optical data obtained over a wider field-of-view. In particular, we have carried out a multi-color analysis in order to identify groups or clusters of galaxies along the line of sight. Photometric redshifts are measured for 139 galaxies unveiling significant excesses of galaxies 1.0 arcmin, NW and 1.7 arcmin, SW of the main lensing galaxy. The photometric redshift inferred both for the main lensing galaxy and for the galaxy concentrations is z=0.9^{+0.10}_{-0.25}. This is in rough agreement with the measured spectroscopic redshift of the main lensing galaxy, z=0.71 (Burud et al. 2002), suggesting that it is part of a larger group or cluster. We investigate the impact of including the galaxy cluster, first on the modelling of the lensing system, and second on the expected time--delay between the two quasar images.