arXiv subjects

L. M. Lubin

Publications and source records attributed to L. M. Lubin.

At least 37 records · Page 2Linked to original sources

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.

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arXiv2006

The Gravitational Lens -- Galaxy Group Connection. II. Groups Associated with B2319+051 and B1600+434

We report on the results of a spectroscopic survey of the environments of the gravitational lens systems CLASS B1600+434 (z_l = 0.41, z_s = 1.59) and CLASS B2319+051 (z_l = 0.62). The B1600+434 system has a time delay measured for it, and we find the system to lie in a group with a velocity dispersion of 100 km/s and at least six members. B2319+051 has a large group in its immediate foreground with at least 10 members and a velocity dispersion of 460 km/s and another in the background of the lens with a velocity dispersion of 190 km/s. There are several other small groups in the fields of these lens systems, and we describe the properties of these moderate redshift groups. Furthermore, we quantify the effects of these group structures on the gravitational lenses and find a ~5% correction to the derived value of H_0 for B1600+434.

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arXiv2006

Mass Along the Line of Sight to the Gravitational Lens B1608+656: Galaxy Groups and Implications for H_0

We report the discovery of four groups of galaxies along the line of sight to the B1608+656 gravitational lens system. One group is at the redshift of the primary lensing galaxy (z = 0.631) and appears to have a low mass, with eight spectroscopically-confirmed members and an estimated velocity dispersion of 150+/-60 km/s. The three other groups are in the foreground of the lens. These groups contain ~10 confirmed members each, and are located at redshifts of 0.265, 0.426, and 0.520. Two of the three additional groups are centered roughly on the lens system, while the third is centered ~1 arcminute south of the lens. We find that each group provides an external convergence (kappa_ext) of ~0.005--0.060, depending on the assumptions made in calculating the convergence. For lens systems where no additional observables that can break the mass-sheet degeneracy exist, the determination of H_0 will be biased high by a factor of (1-kappa_ext)^{-1} if the external convergence is not properly included in the model. For the B1608+656 system, the stellar velocity dispersion of the lensing galaxy has been measured, thus breaking the mass-sheet degeneracy due to the group that is physically associated with the lens. The effect of the other groups along the line of sight can be folded into the overall uncertainties due to large-scale structure (LSS) along the line of sight. Because the B1608+656 system appears to lie along an overdense line of sight, the LSS will cause the measurement of H_0 to be biased high for this system. The systematic bias introduced by LSS could be 5% or greater. (Abridged)

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arXiv2005

Pushing the Boundaries of the Cl 1604 Supercluster at z~0.9

The Cl 1604 supercluster at z~0.9 is known to contain at least four distinct member clusters, separated in both projection and redshift. In this paper we present deep, multicolor wide-field imaging of a region spanning ~45' on a side, corresponding to 21 h^{-1}_{70} Mpc (physical) at the supercluster redshift. We select galaxies whose colors correspond to those of spectroscopically confirmed cluster members in the r' vs. (r'-i') color-magnitude diagram. Using an adaptive kernel, we generate a map of the projected red galaxy density and identify numerous new candidate clusters which are likely supercluster members. Assuming that all of the density peaks are associated with the supercluster, its transverse size is ~10 h^{-1}_{70} Mpc, which is still significantly smaller than the nearly 93 h^{-1}_{70} Mpc depth in redshift space.

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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.

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arXiv2004

Spectroscopic Confirmation of the Cl 1604 Supercluster at z~0.9

We present spectroscopic confirmation of the Cl 1604 supercluster at z~0.9. Originally detected as two individual clusters, Cl 1604+4304 at z = 0.90 and Cl 1604+4321 at z = 0.92, which are closely separated in both redshift and sky position, subsequent imaging revealed a complex of red galaxies bridging the two clusters, suggesting that the region contained a large scale structure. We have carried out extensive multi-object spectroscopy, which, combined with previous measurements, provides ~600 redshifts in this area, including 230 confirmed supercluster members. We detect two additional clusters that are part of this structure, Cl 1604+4314 at z = 0.87 and Cl 1604+4316 at z = 0.94. All four have properties typical of local clusters, with line-of-sight velocity dispersions between 489 and 962 km/s. The structure is significantly extended in redshift space, which, if interpreted as a true elongation in real space, implies a depth of 93 Mpc. We examine the spatial and redshift distribution of the supercluster members.

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arXiv2004

Keck spectroscopy of CLASS gravitational lenses

We present the optical spectra of four newly discovered gravitational lenses from the Cosmic Lens All-Sky Survey (CLASS). These observations were carried out using the Low Resolution Imaging Spectrograph on the W. M. Keck-I Telescope as part of a program to study galaxy-scale gravitational lenses. From our spectra we found the redshift of the background source in CLASS B0128+437 (z_s=3.1240+-0.0042) and the lensing galaxy redshifts in CLASS B0445+123 (z_l=0.5583+-0.0003) and CLASS B0850+054 (z_l=0.5883+-0.0006). Intriguingly, we also discovered that CLASS B0631+519 may have two lensing galaxies (z_l,1=0.0896+-0.0001, z_l,2=0.6196+-0.0004). We also found a single unidentified emission line from the lensing galaxy in CLASS B0128+437 and the lensed source in CLASS B0850+054. We find the lensing galaxies in CLASS B0445+123 and CLASS B0631+519 (l,2) to be early-type galaxies with Einstein Radii of 2.8-3.0 h^{-1} kpc. The deflector in CLASS B0850+054 is a late-type galaxy with an Einstein Radius of 1.6 h^{-1} kpc.

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arXiv2003

The Age of the Oldest Stars in the Local Galactic Disk From Hipparcos Parallaxes of G and K Subgiants

We review the history of the discovery of field subgiant stars and their importance in the age dating of the Galactic disk. We use the cataloged data from the Hipparcos satellite in this latter capacity. Based on Hipparcos parallaxes accurate to 10% or better, the absolute magnitude of the lower envelope of the nearly horizontal subgiant sequence for field stars in the H-R diagram for B-V colors between 0.85 and 1.05 is measured to be M_V = 4.03 +/- 0.06. The age of the field stars in the solar neighborhood is found to be 7.9 +/- 0.7 Gyr by fitting the theoretical isochrones for [Fe/H] = +0.37 to the lower envelope of the Hipparcos subgiants. The same grid of isochrones yields ages, in turn, of 4.0 +/- 0.2 Gyr, 6.2 +/- 0.5 Gyr, and 7.5 to 10 Gyr for the old Galactic clusters M67, NGC188, and NGC6791. The ages of both the Galactic disk in the solar neighborhood and of NGC6791 are, nevertheless, likely between 3 and 5 Gyr younger than the oldest halo globular clusters, which have ages of 13.5 Gyr. The most significant results are (1) the supermetallicity of the oldest local disk stars, and (2) the large age difference between the most metal-poor component of the halo and the thick and thin disk in the solar neighborhood. These facts are undoubtedly related and pose again the problem of the proper scenario for the timing of events in the formation of the halo and the Galactic disk in the solar neighborhood. [Abstract Abridged]

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arXiv2001

The Gravitational Lens -- Galaxy Group Connection: I. Discovery of a Group Coincident with CLASS B0712+472

Previous observations of the environments of the lensing galaxies in gravitational lens systems suggest that many of the lensing galaxies are associated with small groups of galaxies. As a result, we have begun a coordinated program to study the local environments of all known gravitational lens systems. In this paper, we present results on the gravitational lens system CLASS B0712+472, which has previously measured source and lens redshifts of (z_l,z_s) = (0.4060,1.339). Although we have not found a galaxy group associated with the primary lensing galaxy, we have found a foreground group which is spatially coincident with the lens system. Based on multi-object spectroscopy taken at the Keck 10-m telescope, we have confirmed ten group members with a mean redshift of = 0.2909. The resulting velocity dispersion and estimated virial mass are 306(+110/-58) km/s and 3.0(+2.2/-1.2) x 10^13 h^-1 M_sun, respectively, for (Omega_m,Omega_Lambda) = (0.2,0.0). The dynamical properties of this moderate-redshift group are completely consistent with the range of values found in nearby groups of galaxies. Five of the group members are red, elliptical-like galaxies, while the remaining five are active, star-forming galaxies. Based on the spectroscopic results and the publically-available HST imaging of nine group members, we find that the early-type fraction is 40%. We estimate that the effect of this foreground group on the gravitational lensing potential of B0712+472 is small, producing an external shear which is only a few percent, although the shear could be larger if the group centroid is significantly closer to the lens system than it appears to be.

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arXiv2000

A Probabilistic Quantification of Galaxy Cluster Membership

Clusters of galaxies are important laboratories for understanding both galaxy evolution and constraining cosmological quantities. Any analysis of clusters, however, is best done when one can reliably determine which galaxies are members of the cluster. While this would ideally be done spectroscopically, the difficulty in acquiring a complete sample of spectroscopic redshifts becomes rather daunting, especially at high redshift where the background contamination becomes increasingly larger. Traditionally, an alternative approach of applying a statistical background correction has been utilized, which, while useful in a global sense, does not provide information for specific galaxies. In this paper, we develop a more robust technique which uses photometrically estimated redshifts to determine cluster membership. This technique can either be used as an improvement over the commonly used statistical correction method or it can be used to determine cluster candidates on an individual galaxy basis. By tuning the parameters of our algorithm, we can selectively maximize our completeness or, alternatively, minimize our contamination. Furthermore, our technique provides a statistical quantification of both our resulting completeness and contamination from foreground and background galaxies.

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arXiv2000

CFHT Optical PDCS Survey II: Evolution in the Space Density of Clusters of Galaxies

We present the first dynamical study of the optically selected Palomar Distant Cluster Survey (PDCS). We show that eleven out of twelve PDCS clusters we observed are real over-densities of galaxies. Most clusters have velocity dispersions appropriate for clusters of galaxies. However, we find a fraction (1/3) of objects in the PDCS which have velocity dispersions in the range of groups of galaxies (200 +/- 100 km/s) but have richnesses appropriate for clusters of galaxies. We measure the richness function, X-ray luminosity function (using both the detections and upper limits), and the mass function derived from our velocity dispersions. We confirm that the space density, as a function of richness, of clusters of galaxies in the PDCS is ~5 times that of the Abell catalog. Excluding the above fraction of 1/3 of objects with low velocity dispersions, we measure a space density ~3 times that of the Abell catalog for equivalent mass clusters of galaxies. Our space density estimates are in agreement with other low-redshift, optically-selected cluster surveys such as the EDCC, APM and EDCC2, which raises the possibility that the Abell catalog is incomplete. Our X-ray luminosity function agrees with other measurements based on both X-ray and optically selected samples, so we find that the PDCS does not miss clusters of galaxies that would be found in an X-ray selected survey. Our resulting mass function, centered around 10e14 Msun/h, agrees with the expectations from such surveys as the Canadian Network for Observational Cosmology cluster survey, though errors on our mass measurements are too large to constrain cosmological parameters.

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arXiv2000

A Definitive Optical Detection of a Supercluster at z = 0.91

We present the results from a multi-band optical imaging program which has definitively confirmed the existence of a supercluster at z = 0.91. Two massive clusters of galaxies, CL1604+4304 at z = 0.897 and CL1604+4321 at z = 0.924, were originally observed in the high-redshift cluster survey of Oke, Postman & Lubin (1998). They are separated by 4300 km/s in radial velocity and 17 arcminutes on the plane of the sky. Their physical and redshift proximity suggested a promising supercluster candidate. Deep BRi imaging of the region between the two clusters indicates a large population of red galaxies. This population forms a tight, red sequence in the color--magnitude diagram at (R-i) = 1.4. The characteristic color is identical to that of the spectroscopically-confirmed early-type galaxies in the two member clusters. The red galaxies are spread throughout the 5 Mpc region between CL1604+4304 and CL1604+4321. Their spatial distribution delineates the entire large scale structure with high concentrations at the cluster centers. In addition, we detect a significant overdensity of red galaxies directly between CL1604+4304 and CL1604+4321 which is the signature of a third, rich cluster associated with this system. The strong sequence of red galaxies and their spatial distribution clearly indicate that we have discovered a supercluster at z = 0.91.

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arXiv1999

A Keck Survey of Gravitational Lens Systems: I. Spectroscopy of SBS 0909+532, HST 1411+5211, and CLASS B2319+051

We present new results from a continuing Keck program to study gravitational lens systems. We have obtained redshifts for three lens systems, SBS 0909+532, HST 1411+5211, and CLASS B2319+051. For all of these systems, either the source or lens redshift (or both) has been previously unidentified. We find (z_l, z_s) = (0.830, 1.377) for SBS 0909+532; (z_l, z_s) = (0.465, 2.811) for HST 1411+5211, although the source redshift is still tentative; and (z_l1, z_l2) = (0.624, 0.588) for the two lensing galaxies in CLASS B2319+051. The background radio source in B2319+051 has not been detected optically; its redshift is, therefore, still unknown. We find that the spectral features of the central lensing galaxy in all three systems are typical of an early-type galaxy. The observed image splittings in SBS 0909+532 and HST 1411+5211 imply that the masses within the Einstein ring radii of the lensing galaxies are 1.4 x 10^{11} and 2.0 x 10^{11} h^{-1} M_sun, respectively. The resulting B band mass-to-light ratio for HST 1411+5211 is 41.3 +/- 1.2 h (M/L)_sun, a factor of 5 times higher than the average early-type lensing galaxy. This large mass-to-light is almost certainly the result of the additional mass contribution from the cluster CL 3C295 at z = 0.46. For the lensing galaxy in SBS 0909+532, we measure (M/L)_B = 4^{+11}_{-3} h (M/L)_sun where the large errors are the result of significant uncertainty in the galaxy luminosity. While we cannot measure directly the mass-to-light ratio of the lensing galaxy in B2319+051, we estimate that (M/L)_B is between 3-7 h (M/L)_sun.

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arXiv1999

Spectroscopic Observations of Optically Selected Clusters of Galaxies from the Palomar Distant Cluster Survey

We have conducted a redshift survey of sixteen cluster candidates from the Palomar Distant Cluster Survey (PDCS) to determine both the density of PDCS clusters and the accuracy of the estimated redshifts presented in the PDCS catalog (Postman et. al. 1996). We find that the matched-filter redshift estimate presented in the PDCS has an error sigma_z = 0.06 in the redshift range 0.1 < z < 0.35 based on eight cluster candidates with three or more concordant galaxy redshifts. We measure the low redshift (0.1 < z < 0.35) space density of PDCS clusters to be 31.3^{+30.5}_{-17.1} * E-06 h^3 Mpc^-3 (68% confidence limits for a Poisson distribution) for Richness Class 1 systems. We find a tentative space density of 10.4^{+23.4}_{-8.4}* E-06 h^3 Mpc^-3 for Richness Class 2 clusters. These densities compare favorably with those found for the whole of the PDCS and support the finding that the space density of clusters in the PDCS is a factor of ~5 above that of clusters in the Abell catalog (Abell 1958; Abell, Corwin, and Olowin 1989). These new space density measurements were derived as independently as possible from the original PDCS analysis and therefore, demonstrate the robustness of the original work. Based on our survey, we conclude that the PDCS matched-filter algorithm is successful in detecting real clusters and in estimating their true redshifts in the redshift range we surveyed.

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arXiv1999

Photometric Redshifts: A New Tool for Studying High-Redshift Clusters

We present the first results of our application of photometric redshifts to the study of galaxy populations in high-redshift clusters. For this survey, we are examining a sample of galaxy clusters at z > 0.6 which have already been well-studied in the optical and infrared wavelengths (Oke, Postman & Lubin 1998). Our main goal is to use photometric redshifts to delineate accurately between field and cluster galaxies. Once we isolate the cluster galaxies, we can directly study the properties of the galaxy population in each high-redshift cluster. Specifically, we are studying the cluster morphological fractions, the morphology-density relation, and the large scale structure distribution. Although we have encountered some operational problems with our photometric redshift technique, early results suggest that this procedure will become a significant tool for studying high-redshift clusters.

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arXiv1999

The afterglow, the redshift, and the extreme energetics of the gamma-ray burst 990123

Afterglow, or long-lived emission, has now been detected from about a dozen well-positioned gamma-ray bursts. Distance determinations made by measuring optical emission lines from the host galaxy, or absorption lines in the afterglow spectrum, place the burst sources at significant cosmological distances, with redshifts ranging from ~1--3. The energy required to produce the bright gamma-ray flashes is enormous: up to ~10^{53} erg or 10 percent of the rest mass energy of a neutron star, if the emission is isotropic. Here we present the discovery of the optical afterglow and the redshift of GRB 990123, the brightest well-localized GRB to date. With our measured redshift of >1.6, the inferred isotropic energy release exceeds the rest mass of a neutron star thereby challenging current theoretical models for the origin of GRBs. We argue that the optical and IR afterglow measurements reported here may provide the first observational evidence of beaming in a GRB, thereby reducing the required energetics to a level where stellar death models are still tenable.

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arXiv1998

A Group-Group Merger at a Redshift of z = 0.84?

We present a dynamical study of the CL0023+0423 system at a redshift of z = 0.84. This system consists of two components separated in velocity space by 2900 km/s and on the plane of the sky by 0.23 h^{-1} Mpc. A kinematic analysis indicates that the two components are a poor cluster with a velocity dispersion of 415 km/s and a less massive group with a velocity dispersion of 158 km/s (Postman, Lubin & Oke 1998). An analytic two-body calculation and N-body simulations indicate that the system is most likely not bound but simply a chance projection on the sky; however, within the observational uncertainties, there do exist bound solutions where the two galaxy groups are currently moving toward each other and will eventually merge. A statistical analysis of 1000 N-body simulations indicates that there is an 20% chance that the two groups will merge. If the CL0023+0423 system does merge, it will appear as a cluster on the sky, as well as in velocity space, within 1-2 Gyrs. The cluster will evolve dynamically for more than 3 Gyrs, appearing during this time more similar to an open, irregular cluster. The final merged system has a velocity dispersion which is consistent with a local Abell richness class 1 cluster. The morphological analysis of the galaxy populations of CL0023+0423 suggests that both groups are largely dominated by spiral galaxies with early-type fractions of 33% or less (Lubin et al. 1998). These modest early-type fractions may have implications for both cluster formation and group evolution. A comparison with nearby and intermediate-redshift groups and clusters of galaxies suggest that some fraction of early-type galaxies are formed at redshifts of z < 1.

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arXiv1998

A Study of Nine High-Redshift Clusters of Galaxies : III. HST Morphology of Clusters 0023+0423 and 1604+4304

We present a detailed morphological analysis of the galaxy populations in the first two clusters to be completed in an extensive observational study of nine high-redshift clusters of galaxies (Oke, Postman & Lubin 1998). These two clusters, CL0023+0423 and CL1604+4304, are at redshifts of z = 0.84 and z = 0.90, respectively. The morphological studies are based on high-angular resolution imagery taken with WFPC2 aboard the Hubble Space Telescope. These data are combined with deep, ground-based BVRI photometry and spectra taken with the Keck 10-meter telescopes. Based on these observations, CL0023+0423 has a galaxy population which is more similar to groups of galaxies and the field. This system is almost completely dominated by spiral galaxies. CL1604+4304, however, has a morphological composition which is more typical of a normal, present-day cluster; early-type galaxies comprise approximately 76% of all galaxies brighter than M_V = -19.0 + 5 log h in the central 0.5 h^{-1} Mpc. The ratio of S0 galaxies to ellipticals in this cluster is 1.7 +/- 0.9, consistent with local cluster populations. The morphological results support the conclusions of the dynamical analysis presented in the second paper of this series (Postman, Lubin & Oke 1998). CL0023+0423 consists of two galaxy groups which are separated by approximately 2900 km/s in radial velocity. On the other hand, CL1604+4304 has a morphological composition, velocity dispersion, and implied mass which are consistent with an Abell richness class 2 or 3 cluster.

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