arXiv subjects

V. E. Margoniner

Publications and source records attributed to V. E. Margoniner.

15 recordsLinked to original sources

arXiv2009

Shear-Selected Clusters From the Deep Lens Survey III: Masses from Weak Lensing

We present weak lensing mass estimates of seven shear-selected galaxy cluster candidates from the Deep Lens Survey. The clusters were previously identified as mass peaks in convergence maps of 8.6 sq. deg of R band imaging, and followed up with X-ray and spectroscopic confirmation, spanning a redshift range 0.19 - 0.68. Most clusters contained multiple X-ray peaks, yielding 17 total mass concentrations. In this paper, we constrain the masses of these X-ray sources with weak lensing, using photometric redshifts from the full set of BVRz' imaging to properly weight background galaxies according to their lensing distance ratios. We fit both NFW and singular isothermal sphere profiles, and find that the results are insensitive to the assumed profile. We also show that the results do not depend significantly on the assumed prior on the position of the mass peak, but that this may become an issue in future larger samples. The inferred velocity dispersions for the extended X-ray sources range from 250-800 km/s, with the exception of one source for which no lensing signal was found. This work further establishes shear selection as a viable technique for finding clusters, but also highlights some unresolved issues such as determination of the mass profile center without biasing the mass estimate, and fully accounting for line-of-sight projections. A follow-up paper will examine the mass-X-ray scaling relations of these clusters.

astro-ph.CO↗

arXiv2007

Photometric Redshifts and Photometry Errors

We examine the impact of non-Gaussian photometry errors on photometric redshift performance. We find that they greatly increase the scatter, but this can be mitigated to some extent by incorporating the correct noise model into the photometric redshift estimation process. However, the remaining scatter is still equivalent to that of a much shallower survey with Gaussian photometry errors. We also estimate the impact of non-Gaussian errors on the spectroscopic sample size required to verify the photometric redshift rms scatter to a given precision. Even with Gaussian {\it photometry} errors, photometric redshift errors are sufficiently non-Gaussian to require an order of magnitude larger sample than simple Gaussian statistics would indicate. The requirements increase from this baseline if non-Gaussian photometry errors are included. Again the impact can be mitigated by incorporating the correct noise model, but only to the equivalent of a survey with much larger Gaussian photometry errors. However, these requirements may well be overestimates because they are based on a need to know the rms, which is particularly sensitive to tails. Other parametrizations of the distribution may require smaller samples.

astro-ph↗

arXiv2007

Photometric Redshifts and Signal-to-Noise

We investigate the impact of photometric signal-to-noise (S/N) on the precision of photometric redshifts in multi-band imaging surveys, using both simulations and real data. We simulate the optical 4-band (BVRz) Deep Lens Survey (DLS, Wittman etal 2002), and use the publicly available Bayesian Photometric Redshift code BPZ by Benitez (2000). 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/N 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 Deltaz = (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 scatter. Ellipticals form the tightest zspec-zphot relation, and cutting on type=elliptical 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.

astro-ph↗

arXiv2006

First Results On Shear-Selected Clusters From the Deep Lens Survey: Optical Imaging, Spectroscopy, and X-ray Followup

We present the first sample of galaxy clusters selected on the basis of their weak gravitational lensing shear. The shear induced by a cluster is a function of its mass profile and its redshift relative to the background galaxies being sheared; in contrast to more traditional methods of selecting clusters, shear selection does not depend on the cluster's star formation history, baryon content, or dynamical state. Because mass is the property of clusters which provides constraints on cosmological parameters, the dependence on these other parameters could induce potentially important biases in traditionally-selected samples. Comparison of a shear-selected sample with optically and X-ray selected samples is therefore of great importance. Here we present the first step toward a new shear-selected sample: the selection of cluster candidates from the first 8.6 deg$^2$ of the 20 deg$^2$ Deep Lens Survey (DLS), and tabulation of their basic properties such as redshifts and optical and X-ray counterparts.

astro-ph↗

arXiv2006

Three Gravitational Lenses for the Price of One: Enhanced Strong Lensing through Galaxy Clustering

We report the serendipitous discovery of two strong gravitational lens candidates (ACS J160919+6532 and ACS J160910+6532) in deep images obtained with the Advanced Camera for Surveys on the Hubble Space Telescope, each less than 40 arcsec from the previously known gravitational lens system CLASS B1608+656. The redshifts of both lens galaxies have been measured with Keck and Gemini: one is a member of a small galaxy group at z~0.63, which also includes the lensing galaxy in the B1608+656 system, and the second is a member of a foreground group at z~0.43. By measuring the effective radii and surface brightnesses of the two lens galaxies, we infer their velocity dispersions based on the passively evolving Fundamental Plane (FP) relation. Elliptical isothermal lens mass models are able to explain their image configurations within the lens hypothesis, with a velocity dispersion compatible with that estimated from the FP for a reasonable source-redshift range. Based on the large number of massive early-type galaxies in the field and the number-density of faint blue galaxies, the presence of two additional lens systems around CLASS B1608+656 is not unlikely in hindsight. Gravitational lens galaxies are predominantly early-type galaxies, which are clustered, and the lensed quasar host galaxies are also clustered. Therefore, obtaining deep high-resolution images of the fields around known strong lens systems is an excellent method of enhancing the probability of finding additional strong gravitational lens systems.

astro-ph↗

arXiv2004

Weak Lensing Detection of Cl 1604+4304 at z = 0.90

We present a weak lensing analysis of the high-redshift cluster Cl 1604+4304. At z=0.90, this is the highest-redshift cluster yet detected with weak lensing. It is also one of a sample of high-redshift, optically-selected clusters whose X-ray temperatures are lower than expected based on their velocity dispersions. Both the gas temperature and galaxy velocity dispersion are proxies for its mass, which can be determined more directly by a lensing analysis. Modeling the cluster as a singular isothermal sphere, we find that the mass contained within projected radius R is 3.69+-1.47 * (R/500 kpc) 10^14 M_odot. This corresponds to an inferred velocity dispersion of 1004+-199 km/s, which agrees well with the measured velocity dispersion of 989+98-76 km/s (Gal & Lubin 2004). These numbers are higher than the 575+110-85 km/s inferred from Cl 1604+4304 X-ray temperature, however all three velocity dispersion estimates are consistent within ~ 1.9 sigma.

astro-ph↗

arXiv2004

The Deep Lens Survey Transient Search I : Short Timescale and Astrometric Variability

We report on the methodology and first results from the Deep Lens Survey transient search. We utilize image subtraction on survey data to yield all sources of optical variability down to 24th magnitude. Images are analyzed immediately after acquisition, at the telescope and in near-real time, to allow for followup in the case of time-critical events. All classes of transients are posted to the web upon detection. Our observing strategy allows sensitivity to variability over several decades in timescale. The DLS is the first survey to classify and report all types of photometric and astrometric variability detected, including solar system objects, variable stars, supernovae, and short timescale phenomena. Three unusual optical transient events were detected, flaring on thousand-second timescales. All three events were seen in the B passband, suggesting blue color indices for the phenomena. One event (OT 20020115) is determined to be from a flaring Galactic dwarf star of spectral type dM4. From the remaining two events, we find an overall rate of η= 1.4 events deg-2 day-1 on thousand-second timescales, with a 95% confidence limit of η< 4.3. One of these events (OT 20010326) originated from a compact precursor in the field of galaxy cluster Abell 1836, and its nature is uncertain. For the second (OT 20030305) we find strong evidence for an extended extragalactic host. A dearth of such events in the R passband yields an upper 95% confidence limit on short timescale astronomical variability between 19.5 < R < 23.4 of η_R < 5.2. We report also on our ensemble of astrometrically variable objects, as well as an example of photometric variability with an undetected precursor.

astro-ph↗

arXiv2003

Weak Lensing Discovery and Tomography of a Cluster at z=0.68

We report the weak lensing discovery, spectroscopic confirmation, and weak lensing tomography of a massive cluster of galaxies at $z=0.68$, demonstrating that shear selection of clusters works at redshifts high enough to be cosmologically interesting. The mass estimate from weak lensing, $11.1 +- 2.8 x 10^{14} (r/Mpc)$ solar masses within projected radius r, agrees with that derived from the spectroscopy ($σ_v = 980 km s^{-1}$), and with the position of an arc which is likely to be a strongly lensed background galaxy. The redshift estimate from weak lensing tomography is consistent with the spectroscopy, demonstrating the feasibility of baryon-unbiased mass surveys. This tomographic technique will be able to roughly identify the redshifts of any dark clusters which may appear in shear-selected samples, up to z ~ 1.

astro-ph↗

arXiv2003

Shear-selected clusters from the Deep Lens Survey

Weak gravitational lensing has the potential to select clusters independently of their baryon content, dynamical state, and star formation history. We present steps toward the first shear-selected sample of clusters, from the Deep Lens Survey (DLS), a deep BVRz' imaging survey of 28 square degrees. Cluster redshifts are determined from photometric redshifts of members and from lensing tomography, and in some cases have been confirmed spectroscopically. DLS imaging data are also used to derive mass-to-light ratios, and upcoming Chandra and XMM time will yield X-ray luminosities and temperatures for a subsample of 12 clusters. Thus we can begin to address any baryon or luminous-matter bias which may be present in current optical and X-ray samples. When the DLS is complete, we expect to have a sample of ~ 100 shear-selected clusters from z ~ 0.2-1.

astro-ph↗

arXiv2002

The Deep Lens Survey

The Deep Lens Survey (DLS) is a deep BVRz' imaging survey of seven 2x2 degree fields, with all data to be made public. The primary scientific driver is weak gravitational lensing, but the survey is also designed to enable a wide array of other astrophysical investigations. A unique feature of this survey is the search for transient phenomena. We subtract multiple exposures of a field, detect differences, classify, and release transients on the Web within about an hour of observation. Here we summarize the scientific goals of the DLS, field and filter selection, observing techniques and current status, data reduction, data products and release, and transient detections. Finally, we discuss some lessons which might apply to future large surveys such as LSST.

astro-ph↗

arXiv2002

Ground-Based Optical Deep Pencil Beam Surveys

The initial results of the Deep Lens Survey (http://dls.bell-labs.com) to identify possible brown dwarfs and extremely metal poor red halo subdwarfs near the hydrogen burning limit are presented. Individual deep CCD high galactic latitude survey fields appear to offer a low probability of discovering field BD's, but taken collectively offer an opportunity to begin addressing questions regarding the scale height and distribution of these objects. In all likelihood, the very depth of such surveys will greatly increase our knowledge of the coolest extreme halo objects, which currently are known in far fewer numbers than T dwarfs. Ultimately, the large volume surveyed by the Large Synoptic Survey Telescope will identify vast numbers of such objects, providing a more complete picture of their spatial distribution.

astro-ph↗

arXiv2001

Discovery of a Galaxy Cluster via Weak Lensing

We report the discovery of a cluster of galaxies via its weak gravitational lensing effect on background galaxies, the first spectroscopically confirmed cluster to be discovered through its gravitational effects rather than by its electromagnetic radiation. This fundamentally different selection mechanism promises to yield mass-selected, rather than baryon or photon-selected, samples of these important cosmological probes. We have confirmed this cluster with spectroscopic redshifts of fifteen members at z=0.276, with a velocity dispersion of 615 km/s. We use the tangential shear as a function of source photometric redshift to estimate the lens redshift independently and find z_l = 0.30 +- 0.08. The good agreement with the spectroscopy indicates that the redshift evolution of the mass function may be measurable from the imaging data alone in shear-selected surveys.

astro-ph↗

arXiv2000

The Butcher-Oemler Effect in 295 Clusters: Strong Redshift Evolution and Cluster Richness Dependence

We examine the Butcher-Oemler effect and its cluster richness dependence in the largest sample studied to date: 295 Abell clusters. We find a strong correlation between cluster richness and the fraction of blue galaxies, f_B, at every redshift. The slope of the f_B(z) relation is similar for all richnesses, but at a given redshift, f_B is systematically higher for poor clusters. This is the chief cause of scatter in the f_B vs. z diagram: the spread caused by the richness dependence is comparable to the trend in f_B over a typical redshift baseline, so that conclusions drawn from smaller samples have varied widely. The two parameters, z, and a consistently defined projected galaxy number density, N, together account for all of the observed variation in f_B within the measurement errors. The redshift evolution of f_B is real, and occurs at approximately the same rate for clusters of all richness classes.

astro-ph↗

arXiv2000

Photometric Properties of 47 Clusters of Galaxies: I. The Butcher-Oemler Effect

We present gri CCD photometry of 44 Abell clusters and 4 cluster candidates. Twenty one clusters in our sample have spectroscopic redshifts. Fitting a relation between mean g, r and i magnitudes, and redshift for this subsample, we have calculated photometric redshifts for the remainder with an estimated accuracy of 0.03. The resulting redshift range for the sample is 0.03<z<0.38. Color-magnitude diagrams are presented for the complete sample and used to study evolution of the galaxy population in the cluster environment. Our observations show a strong Butcher-Oemler effect (Butcher & Oemler 1978, 1984), with an increase in the fraction of blue galaxies (f_B) with redshift that seems more consistent with the steeper relation estimated by Rakos and Schombert (1995) than with the original one by Butcher & Oemler (1984). However, in the redshift range between ~ 0.08 and 0.2, where most of our clusters lie, there is a wide range of f_B values, consistent with no redshift evolution of the cluster galaxy population. A large range of f_B values is also seen between ~ 0.2 and 0.3, when Smail at al. (1998) x-ray clusters are added to our sample. The discrepancies between samples underscore the need for an unbiased sample to understand how much of the Butcher-Oemler effect is due to evolution, and how much to selection effects. We also tested the idea proposed by Garilli et al. (1996) that there is a population of unusually red galaxies which could be associated either with the field or clusters, but we find that these objects are all near the limiting magnitude of the images (20.5<r<22) and have colors that are consistent with those expected for stars or field galaxies at z ~ 0.7.

astro-ph↗

arXiv1999

The Northern Sky Optical Cluster Survey I: Detection of Galaxy Clusters in DPOSS

The Northern Sky Optical Cluster Survey is a project to create an objective catalog of galaxy clusters over the entire high-galactic-latitude Northern sky, with well understood selection criteria. We use the object catalogs generated from the Digitized Second Palomar Sky Survey (DPOSS, Djorgovski et al. 1999) as the basis for this survey. We apply a color criterion to select against field galaxies, and use a simple adaptive kernel technique to create galaxy density maps, combined with the bootstrap technique to make significance maps, from which density peaks are selected. This survey attempts to eliminate some of the subjective criteria and assumptions of past surveys, including detection by eye (Abell 1958, ACO 1989) and assumed luminosity functions and cluster profiles (PDCS, Postman et al. 1995). We also utilize more information (especially colors) than the most similar recent survey, the APM (Dalton et al. 1992). This paper presents the details of our cluster detection technique, as well as some initial results for two small areas totaling ~60 square degrees. We find a mean surface density of ~1.5 clusters per square degree, consistent with the detection of richness class 0 and higher clusters to z~0.3. In addition, we demonstrate an effective photometric redshift estimator for our clusters.

astro-ph↗