36 research outputs found
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 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
0.2. The absolute luminosities of the source are then $L_B = 1.3 \
10^{12} L_{B \odot}\nu_\nu \simeq 4. 10^{10}_\odot$ 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