Lensed galaxies in Abell 370 I. Modeling the number counts and redshift distribution of background sources

We present new observations of the cluster-lens Abell 370: a deep HST/WFPC2 F675W image and ESO 3.6m spectroscopy of faint galaxies. These observations shade new lights on the statistical properties of faint lensed galaxies. In particular, we spectroscopically confirm the multiple image nature of the B2--B3 gravitational pair (Kneib et al. 1993), and determine a redshift of z=0.806 which is in very good agreement with earlier predictions. A refined mass model of the cluster core (that includes cluster galaxy halos) is presented, based on a number of newly identified multiple images. Following Bezecourt et al. (1998a), we combine the new cluster mass model with a spectrophotometric prescription for galaxy evolution to predict the arclets number counts and redshift distribution in the HST image. In particular, the ellipticity distribution of background sources is taken into account, in order to properly estimate the statistical number and redshift distribution of arclets. We show that the redshift distribution of arclets, and particularly its high redshift tail can be used as a strong constraint to disentangle different galaxy evolution scenario. A hierarchical model which includes a number density evolution is favored by our analysis. Finally, we compute the depletion curves in the faint galaxies number counts and discuss its wavelength dependence.Comment: 10 pages, Astronomy and Astrophysics in pres

A ring galaxy at z=1 lensed by the cluster Abell 370

We present a study of a very peculiar object found in the field of the cluster-lens Abell 370. This object displays, in HST imaging, a spectacular morphology comparable to nearby ring-galaxies. From spectroscopic observations at the CFHT, we measured a redshift of $z=1.062$ based on the identification of [O ii] 3727 \AA and [Ne v] 3426 \AA emission lines. These emission lines are typical of starburst galaxies hosting a central active nucleus and are in good agreement with the assumption that this object is a ring-galaxy. This object is also detected with ISO in the LW2 and LW3 filters, and the mid Infra-Red (MIR) flux ratio favors a Seyfert 1 type. The shape of the ring is gravitationally distorted by the cluster-lens, and most particularly by a nearby cluster elliptical galaxy. Using the cluster mass model, we can compute its intrinsic shape. Requiring that the outer ring follows an ellipse we put constraints on the M/L ratio of the nearby galaxy and derive a magnification factor of 2.5 $\pm$ 0.2. The absolute luminosities of the source are then $L_B = 1.3 \ 10^{12} L_{B \odot}$and$\nu$L$_\nu \simeq 4. 10^{10}$L$_\odot$ in the mid-IR.Comment: 5 pages, 5 figures, uses aa.cls, accepted to A&A Letters. Minor changes, Figure 1 revisited and typos adde

Mass Distributions of HST Galaxy Clusters from Gravitational Arcs

Although N-body simulations of cosmic structure formation suggest that dark matter halos have density profiles shallower than isothermal at small radii and steeper at large radii, whether observed galaxy clusters follow this profile is still ambiguous. We use one such density profile, the asymmetric NFW profile, to model the mass distributions of 11 galaxy clusters with gravitational arcs observed by HST. We characterize the galaxy lenses in each cluster as NFW ellipsoids, each defined by an unknown scale convergence, scale radius, ellipticity, and position angle. For a given set of values of these parameters, we compute the arcs that would be produced by such a lens system. To define the goodness of fit to the observed arc system, we define a chi^2 function encompassing the overlap between the observed and reproduced arcs as well as the agreement between the predicted arc sources and the observational constraints on the source system. We minimize this chi^2 to find the values of the lens parameters that best reproduce the observed arc system in a given cluster. Here we report our best-fit lens parameters and corresponding mass estimates for each of the 11 lensing clusters. We find that cluster mass models based on lensing galaxies defined as NFW ellipsoids can accurately reproduce the observed arcs, and that the best-fit parameters to such a model fall within the reasonable ranges defined by simulations. These results assert NFW profiles as an effective model for the mass distributions of observed clusters.Comment: Submitted to ApJ, 14 figures include

Magellan Spectroscopy of the Galaxy Cluster RX J1347.5-1145: Redshift Estimates for the Gravitationally Lensed Arcs

We present imaging and spectroscopic observations of the gravitationally lensed arcs in the field of RX J1347.5-1145, the most X-ray luminous galaxy cluster known. Based on the detection of the [OII] 3727 emission line, we confirm that the redshift of one of the arcs is z = 0.806. Its color and [OII] line strength are consistent with those of distant, actively star forming galaxies. In a second arc, we tentatively identify a pair of absorption lines superposed on a red continuum; the lines are consistent with Ca II H & K at z = 0.785. We detected a faint blue continuum in two additional arcs, but no spectral line features could be measured. We establish lower limits to their redshifts based on the absence of [OII] emission, which we argue should be present and detectable in these objects. Redshifts are also given for a number of galaxies in the field of the cluster.Comment: To appear in The Astrophysical Journal (September 2002). 6 page

Source Ellipticity and the Statistics of Lensed Arcs

The statistics of gravitationally lensed arcs, which can be used for a variety of cosmological tests, are sensitive to the intrinsic shapes of the source galaxies. I present an analytic formalism that makes it simple to include elliptical sources in analytic calculations of lens statistics. For cuspy lens models, sources with an axis ratio of 2:1 enhance the total number of arcs longer than 10:1 by a factor of order two, while modestly decreasing the number ratio of radial arcs to tangential arcs. Source ellipticity is therefore an important systematic effect in detailed quantitative studies, but it should not hinder cosmological applications such as attempts to constrain cluster dark matter profiles with arc statistics.Comment: 4 pages, emulateapj5; accepted in Ap

Resolving the Stellar Populations in a z=4 Lensed Galaxy

We present deep near-infrared Keck/NIRC imaging of a recently-discovered z=4.04 galaxy (Frye & Broadhurst 1998). This is lensed by the rich foreground cluster Abell~2390 (z~0.23) into highly-magnified arcs 3-5arcsec in length. Our H- and K'-band NIRC imaging allows us to map the Balmer+4000Ang break amplitude. In combination with high-quality archival HST/WFPC2 data, we can spatially resolve stellar populations along the arcs. The WFPC2 images clearly reveal several bright knots, which correspond to sites of active star formation. However, there are considerable portions of the arcs are significantly redder, consistent with being observed >100Myr after star formation has ceased. Keck/LRIS long-slit spectroscopy along the arcs reveals that the Ly-alpha emission is spatially offset by ~1arcsec from the rest-UV continuum regions. We show that this line emission is most probably powered by star formation in neighboring HII regions, and that the z=4 system is unlikely to be an AGN.Comment: Accepted for publication in the Astrophysical Journal. Uses emulateapj.sty and graphics.sty (included). 34 pages - has 5 tables and 21 encapsulated postscript figures, 4 in colour mail (B&W versions also provided