Searcharxiv⌕ Search

arXiv subjects

R. C. Nichol

Publications and source records attributed to R. C. Nichol.

At least 235 records · Page 13Linked to original sources
arXiv1998

A Bayesian Inference Analysis of the X-ray Cluster Luminosity-Temperature Relation

We present a Bayesian inference analysis of the Markevitch (1998) and Allen & Fabian (1998) cooling flow corrected X-ray cluster temperature catalogs that constrains the slope and the evolution of the empirical X-ray cluster luminosity-temperature (L-T) relation. We find that for the luminosity range 10^44.5 erg s^-1 < L_bol < 10^46.5 erg s^-1 and the redshift range z < 0.5, L_bol is proportional to T^2.80(+0.15/-0.15)(1+z)^(0.91-1.12q_0)(+0.54/-1.22). We also determine the L-T relation that one should use when fitting the Press- Schechter mass function to X-ray cluster luminosity catalogs such as the Einstein Medium Sensitivity Survey (EMSS) and the Southern Serendipitous High- Redshift Archival ROSAT Catalog (Southern SHARC), for which cooling flow corrected luminosities are not determined and a universal X-ray cluster temperature of T = 6 keV is assumed. In this case, L_bol is proportional to T^2.65(+0.23/-0.20)(1+z)^(0.42-1.26q_0)(+0.75/-0.83) for the same luminosity and redshift ranges.

astro-ph↗
arXiv1998

The HST Medium Deep Survey Cluster Sample: Methodology and Data

We present a new, objectively selected, sample of galaxy overdensities detected in the Hubble Space Telescope Medium Deep Survey. These clusters/groups were found using an automated procedure which involved searching for statistically significant galaxy overdensities. The contrast of the clusters against the field galaxy population is increased when morphological data is used to search around bulge-dominated galaxies. In total, we present 92 overdensities above a probability threshold of 99.5%. We show, via extensive Monte Carlo simulations, that at least 60% of these overdensities are likely to be real clusters and groups and not random line-of-sight superpositions of galaxies. For each overdensity in the MDS cluster sample, we provide a richness and the average of the bulge-to-total ratio of galaxies within each system. This MDS cluster sample potentially contains some of the most distant clusters/groups ever detected, with about 25% of the overdensities having estimated redshifts z > 0.9. We have made this sample publicly available to facilitate spectroscopic confirmation of these clusters and help more detailed studies of cluster and galaxy evolution. We also report the serendipitous discovery of a new cluster close on the sky to the rich optical cluster CLl0016+16 at z=0.546. This new overdensity, HST 001831+16208, may be coincident with both an X-ray source and a radio source. HST 001831+16208 is the third cluster/group discovered near to CL0016+16 and appears to strengthen the claims of Connolly et al. (1996) of superclustering at high redshift.

astro-ph↗
arXiv1998

Deep probing of the Coma cluster

As a continuation of our study of the faint galaxy luminosity function in the Coma cluster of galaxies, we report here on the first spectroscopic observations of very faint galaxies (R $\le$ 21.5) in the direction of the core of this cluster. Out of these 34 galaxies, only one may have a redshift consistent with Coma, all others are background objects. The predicted number of Coma galaxies is 6.7 $\pm$ 6.0, according to Bernstein et al. (1995, B95). If we add the 17 galaxies observed by Secker (1997), we end up with 5 galaxies belonging to Coma, while the expected number is 16.0 $\pm$ 11.0 according to B95. Notice that these two independent surveys lead to the same results. Although the observations and predicted values agree within the error, such results raise into question the validity of statistical subtraction of background objects commonly used to derive cluster luminosity functions (e.g. B95). More spectroscopic observations in this cluster and others are therefore urgently needed. As a by-product, we report on the discovery of a physical structure at a redshift z$\simeq$0.5.

astro-ph↗
arXiv1997

The Southern SHARC Survey: the z = 0.3 - 0.7 Cluster XLF

We present the z = 0.3 - 0.7 cluster X-ray luminosity function (XLF) determined from the Southern SHARC (Serendipitous High-redshift Archival ROSAT Cluster) survey. Over the luminosity range L ~ (0.3 - 3) x 10^44 erg/s (0.5 - 2.0 keV) the XLF is in close agreement with that of the low redshift X-ray cluster population. This result greatly strengthens our previous claim of no evolution of the cluster population, at these luminosities, at a median redshift of z =0.44.

astro-ph↗
arXiv1997

X-ray Observations of Distant Optically Selected Cluster

We have measured fluxes or flux limits for 31 of the 79 cluster candidates in the Palomar Distant Cluster Survey (PDCS) using archival ROSAT/PSPC pointed observations. Our X-ray survey reaches a flux limit of $\simeq 3 \times 10^{-14}$ erg s$^{-1}$ cm$^{-2}$ (0.4 - 2.0 keV), which corresponds to luminosities of $L_x\simeq 5 \times 10^{43}$ erg s$^{-1}$ ($H_o$ = 50 km s$^{-1}$ Mpc$^{-1}$, $q_o$ = ${1/2}$), if we assume the PDCS estimated redshifts. Of the 31 cluster candidates, we detect six at a signal-to-noise greater than three. We estimate that $2.9^{+3.3}_{-1.4}$ (90% confidence limits) of these six detections are a result of X-ray emission from objects unrelated to the PDCS cluster candidates. The net surface density of X-ray emitting cluster candidates in our survey, $1.71^{+0.91}_{-2.19}$ clusters deg$^{-2}$, agrees with that of other, X-ray selected, surveys. It is possible, given the large error on our contamination rate, that we have not detected X-ray emission from any of our observed PDCS cluster candidates. We find no statistically significant difference between the X-ray luminosities of PDCS cluster candidates and those of Abell clusters of similar optical richness. This suggests that the PDCS contains objects at high redshift similar to the low redshift clusters in the Abell catalogs. We show that the PDCS cluster candidates are not bright X-ray sources, the average luminosity of the six detected candidates is only $\bar{L_x}=0.9\times10^{44}$ erg s$^{-1}$ (0.4-2.0 keV). This finding is in agreement with previous X-ray studies of high redshift, optically selected, rich clusters of galaxies.

astro-ph↗
arXiv1997

The Edinburgh-Durham Southern Galaxy Catalogue - VIII: The Cluster Galaxy Luminosity Function

We have re-examined the nature of the cluster galaxy luminosity function using the data from the Edinburgh-Durham Southern Galaxy Catalogue and the Edinburgh-Milano Redshift Survey. We derive a best fit luminosity function over the range M(bj)=-18 to -21, for a composite sample of 22 of the richer clusters that has M*=-20.16+/-0.02 and alpha=-1.22+/-0.04. The dominant error in these values results from the choice of background subtraction method. From extensive simulations we can show that when the LF is fitted over this narrow range, it is difficult to discriminate against bright values of M*in the single cluster fits, but that faint values provide a strong test of the universality of the luminosity function. We find that all the individual cluster data are well fit by a Schechter function with alpha fixed at -1.25, and that <=10% of these have fitted values of M* that disagree from the average at the 99% confidence level. We further show that fitting only a single parameter Schechter function to composite subsets of the data can give erroneous results for the derived M*. By considering two parameter fits, the results of Monte-Carlo simulations and direct two-sample chi-squared tests we conclude that there is only weak evidence for differences between the data when broken down into subsets based on physical properties (Bautz-Morgan class, richness, velocity dispersion): from our simulations, only the evidence for a difference between subsets based on velocity dispersion may in fact be significant. We find no evidence at all that a Schechter function is not a good model for the intrinsic cluster luminosity function over this absolute magnitude range. Models that invoke strong evolution of galaxy luminosity of all galaxies within clusters are inconsistent with our results.

astro-ph↗
arXiv1997

A Search For Large-Scale Structure at High Redshift

We present new and exciting results on our search for large-scale structure at high redshift. Specifically, we have just completed a detailed analysis of the area surrounding the cluster CL0016+16 (z=0.546) and have the most compelling evidence yet that this cluster resides in the middle of a supercluster. From the distribution of galaxies and clusters we find that the supercluster appears to be a sheet of galaxies, viewed almost edge-on, with a radial extent of 31 Mpc, transverse dimension of 12 Mpc, and a thickness of ~4 Mpc. The surface density and velocity dispersion of this coherent structure are consistent with the properties of the ``Great Wall'' in the CfA redshift survey.

astro-ph↗
arXiv1997

On The Evolution of X-ray Clusters at High Redshift

We report on the first results from a redshift survey of a flux-limited sample of X-ray clusters selected serendipitously from the ROSAT PSPC data archive. We spectroscopically confirm 15 clusters in the range 0.3 < z < 0.7, to a flux limit of ~ 3.9 x 10^-14 erg s^-1 cm^-2, over a survey area of 17.2 deg^2. The surface density of clusters in our survey is 2.0 (+0.4,-0.3) deg^-2, in good agreement with the number density of cluster candidates detected using algorithms designed to search for very extended sources. The number of clusters detected between 0.3 < z < 0.7 is consistent with a prediction based on a simple extrapolation of the local X-ray cluster luminosity function, which indicates that over this redshift range no significant evolution in the cluster population has taken place. These results are in conflict with recent claims that the number density of X-ray clusters found in deep ROSAT PSPC pointings evolves rapidly beyond z = 0.3.

astro-ph↗
arXiv1996

Evolution in the X-ray Cluster Luminosity Function Revisited

(Shortened Abstract) We present new X-ray data taken from the ROSAT PSPC pointing archive for 21 clusters in the Einstein Extended Medium Sensitivity Survey (EMSS). We have supplemented these data with new optical follow-up observations found in the literature and overall, 32 of the original 67 z>0.14 EMSS clusters now have new information. Using this improved EMSS sample, we have re-determined the EMSS X-ray Cluster Luminosity Function as a function of redshift. We have removed potential mis-classifications and included our new measurements of the clusters X-ray luminosities and redshifts. We find similar luminosity functions to those originally presented by Henry etal (1992); albeit with two important differences. First, we show that the original low redshift EMSS luminosity function is insufficiently constrained. Secondly, the power law shape of our new determination of the high redshift EMSS luminosity function (z=0.3 to 0.6) has a shallower slope than that seen by Henry etal. We have quantified the degree of evolution seen in the X-ray cluster luminosity using several statistical tests. The most restrictive analysis indicates that our low and high redshift EMSS luminosity functions are statistically different at only the 95% level. However, other tests indicate that these luminosity functions differ by as little as one sigma. These data are therefore, consistent with no evolution in the X-ray cluster luminosity function out to z=0.5.

astro-ph↗
arXiv1996

Optical/Near-Infrared Observations of GRO J1744-28

We present results from a series of optical (g and r-band) and near-infrared (K'-band) observations of the region of the sky including the entire XTE and ROSAT error circles for the ``Bursting Pulsar'' GRO J1744-28. These data were taken with the Astrophysical Research Consortium's 3.5-m telescope at Apache Point Observatory and with the 2.2-m telescope at the European Southern Observatory. We see no new object, nor any significant brightening of any known object, in these error circles, with the exception of an object detected in our 8 February 1996 image. This object has already been proposed as a near-infrared counterpart to GRO J1744-28. While it is seen in only two of our ten 8 February frames, there is no evidence that this is an instrumental artifact, suggesting the possibility of near-infrared flares from GRO J1744-28, similar to those that have been reported from the Rapid Burster. The distance to the ``Bursting Pulsar'' must be more than 2 kpc, and we suggest that it is more than 7 kpc.

astro-ph↗
arXiv1996

Superclustering at Redshift Z=0.54

We present strong evidence for the existence of a supercluster at a redshift of z=0.54 in the direction of Selected Area 68. From the distribution of galaxies with spectroscopic redshifts we find that there is a large over-density of galaxies (a factor of four over the number expected in an unclustered universe) within the redshift range 0.530 < z < 0.555. By considering the spatial distribution of galaxies within this redshift range (using spectroscopic and photometric redshifts) we show that the galaxies in SA68 form a linear structure passing from the South-West of the survey field through to the North-East (with a position angle of approximately 35 deg East of North). This position angle is coincident with the positions of the X-ray clusters CL0016+16, RX J0018.3+1618 and a new X-ray cluster, RX J0018.8+1602, centered near the radio source 54W084. All three of these sources are at a redshift of approximately z=0.54 and have position angles, derived from their X-ray photon distributions, consistent with that measured for the supercluster. Assuming a redshift of 0.54 for the distribution of galaxies and a FWHM dispersion in redshift of 0.020 this represents a coherent structure with a radial extent of 31 Mpc, transverse dimension of 12 Mpc, and a thickness of approximately 4 Mpc. The detection of this possible supercluster demonstrates the power of using X-ray observations, combined with multicolor observations, to map the large scale distribution of galaxies at intermediate redshifts.

astro-ph↗
arXiv1996

Low Surface Brightness Galaxies in the Core of the Coma Cluster

We present the results of a search for low surface brightness galaxies (hereafter LSBs) in the Coma cluster. Bernstein et al. report on deep CCD observations in R of a 7.5 by 7.5 arcminute region in the core of the Coma cluster, and we extend this work by finding and measuring 36 LSBs within this field. We report both R and Bj results. The average magnitude based on the best fit exponential to the images is 22.5 (R) and the typical exponential scale is 1.3 arcseconds. The range of exponential scales is 0.4 to 1.2 kpc (distance modulus 34.89), and the range of central surface brightnesses is 24 to 27.4 R magnitudes per square arcsecond. Many of these objects are similar in terms of scale length and central surface brightness to those found by others in nearby clusters such as Fornax (Bothun, Impey and Malin), as well as in the low luminosity end of the dwarfs cataloged in the review by Ferguson and Binggeli. We find no evidence for a dependence of color on central surface brightness or on distance from the D galaxies or the X-ray center of Coma. We also find that these LSBs make a small contribution to the overall mass of the cluster. We discuss these results in terms of possible scenarios of LSB formation and evolution.

astro-ph↗
arXiv1995

A search for distant X-ray galaxy clusters

We report on the progress of our search for serendipitous distant X-ray galaxy clusters in the ROSAT PSPC pointed observations archive. The initial aim of our work is to measure the X-ray luminosity function of clusters down to 4 x 10^-14 erg cm^-2 s^-1 and so test the claims of cluster evolution at moderate redshifts. We have R band images and spectroscopy for 45 extended sources detected in 50 of the 100 deepest southern fields. A preliminary analysis suggests we cover a range of distances, from nearby clusters to possible distant clusters, z \simeq 1.0.

astro-ph↗
arXiv1995

Galactic extinction and Abell clusters

In this paper, we present the results of comparing the angular distribution of Abell clusters with Galactic HI measurements. For most subsamples of clusters considered, their positions on the sky appear to be anti-correlated with respect to the distribution of HI column densities. The statistical significance of these observed anti-correlations is a function of both richness and distance class, with the more distant and/or richest systems having the highest significance (~3 sigma). The lower richness, nearby clusters appear to be randomly distributed compared to the observed Galactic HI column density.

astro-ph↗
arXiv1995

The Luminosity Function of the Coma Cluster Core for -25<M_R<-11

We determine the luminosity function (LF) of galaxies in the core of the Coma cluster for M_R<=-11.4 (assuming H_0=75 km/s/Mpc), a magnitude regime previously explored only in the Local Group. Objects are counted in a deep CCD image of Coma having RMS noise of 27.7 R mag~arcsec$^{-2}$. A correction for objects in the foreground or background of the Coma cluster---and the uncertainty in this correction---are determined from images of five other high-latitude fields, carefully matched to the Coma image in both resolution and noise level. Accurate counts of Coma cluster members are obtained as faint as R=25.5, or M_R=-9.4. The LF for galaxies is well fit by a power law dN/dL\propto L^α, with α=-1.42\pm0.05, over the range -19.4<M_R<-11.4; faintward of this range, the galaxies are unresolved and indistinguishable from globular clusters, but the data are consistent with an extrapolation of the power law. Surface brightness biases are minimized since galaxies are not subjected to morphological selection, and the limiting detection isophote is 27.6 R mag~arcsec$^{-2}$. We find the typical $M_R\approx-12$ Coma cluster galaxy to have an exponential scale length $\approx200$~pc, similar to Local Group galaxies of comparable magnitude. These extreme dwarf galaxies show a surface density increasing towards the giant elliptical NGC~4874 as $r^{-1.3}$, similar to the diffuse light and globular cluster distributions. The luminosity in the detected dwarf galaxies is at most a few percent of the total diffuse light of the giant galaxies in the cluster, and the contribution of the dwarfs to the mass of the cluster is likely negligible as well.

astro-ph↗
arXiv1995

The EDSGC - VII: The Edinburgh-Milano Cluster Redshift Survey

We present the redshifts of the galaxies and galaxy clusters which form the Edinburgh--Milano (EM) cluster redshift survey. A total of 777 galaxy redshifts have been measured in 94 clusters extracted from the digitised Edinburgh Durham Cluster Catalogue. We also present the radial velocity dispersions for 37 clusters. Observational and data reduction techniques are discussed in detail, together with the strategy adopted to determine the mean redshift of a cluster and to identify and discard plausible phantom clusters. Some 10% of our clusters show heavy contamination, indicating that projection is a serious problem for optically selected rich clusters. The median velocity dispersion estimated for a sub--sample of richness R \geq 1 clusters is 742 \pm63 km/sec From a simple comparison with Omega=1 Cold Dark Matter models of structure formation, these results favour a biasing parameter b=1.6-2.0 and are inconsistent with a bias outside of the range b=1.3-2.5.

astro-ph↗
arXiv1994

Cluster Alignments in the Edinburgh/Milano Cluster Redshift Survey

We present here the results of a statistical search for cluster alignments using the Edinburgh/Milano cluster redshift survey. This survey is a unique cluster database which has been objectively constructed to help minimise the systematic biases associated with previous optical cluster catalogues. We find some evidence for cluster alignments out to spatial separations of $<10\mpc$, however, it is not statistically significant. On larger scales, we find no evidence, statistically significant or not, for cluster alignments. These results are in most disagreement with the recent observations of West and Plionis; both of whom see significant cluster alignments out to $\simeq30\mpc$ and beyond in the Abell \& Lick catalogues of clusters. Our findings are consistent with other searches for cluster alignments that do not involve these catalogues.

astro-ph↗