A 7 deg(2) survey for galaxy-scale gravitational lenses with the HST imaging archive(star)

We present the results of a visual search for galaxy-scale gravitational lenses in similar to 7 deg(2) of Hubble Space Telescope (HST) images. The data set comprises the whole imaging data ever taken with the Advanced Camera for Surveys (ACS) in the filter F814W (I-band) up to 2011 August 31, i.e. 6.03 deg(2) excluding the field of the Cosmic Evolution Survey which has been the subject of a separate visual search. In addition, we have searched for lenses in the whole Wide Field Camera 3 (WFC3) near-IR imaging data set in all filters (1.01 deg(2)) up to the same date. Our primary goal is to provide a sample of lenses with a broad range of different morphologies and lens-source brightness contrast in order to estimate a lower limit to the number of galaxy-scale strong lenses in the future Euclid survey in its VIS band. Our criteria to select lenses are purely morphological as we do not use any colour or redshift information. The final candidate selection is very conservative hence leading to a nearly pure but incomplete sample. We find 49 new lens candidates: 40 in the ACS images and 9 in the WFC3 images. Out of these, 16 candidates are secure lenses owing to their striking morphology, 21 more are very good candidates and 12 more have morphologies compatible with gravitational lensing but also compatible with other astrophysical objects such as ring and chain galaxies or mergers. Interestingly, some lens galaxies include low surface brightness galaxies, compact groups and mergers. The imaging data set is heterogeneous in depth and spans a broad range of galactic latitudes. It is therefore insensitive to cosmic variance and allows us to estimate the number of galaxy-scale strong lenses on the sky for a putative survey depth, which is the main result of this work. Because of the incompleteness of the sample, the estimated lensing rates should be taken as lower limits. Using these, we anticipate that a 15 000 deg(2) space survey such as Euclid will find at least 60 000 galaxy-scale strong lenses down to a limiting AB magnitude of I = 24.5 (10 Sigma) or I = 25.8 (3 Sigma).

THE Ly alpha REFERENCE SAMPLE. I. SURVEY OUTLINE AND FIRST RESULTS FOR MARKARIAN 259

Hakim Atek

The Ly alpha Reference Sample (LARS) is a substantial program with the Hubble Space Telescope (HST) that provides a sample of local universe laboratory galaxies in which to study the detailed astrophysics of the visibility and strength of the Ly alpha line of neutral hydrogen. Ly alpha is the dominant spectral line in use for characterizing high-redshift (z) galaxies. This paper presents an overview of the survey, its selection function, and HST imaging observations. The sample was selected from the combined GALEX+Sloan Digital Sky Survey catalog at z = 0.028-0.19, in order to allow Ly alpha to be captured with combinations of long-pass filters in the Solar Blind Channel (SBC) of the Advanced Camera for Surveys (ACS) onboard HST. In addition, LARS utilizes H alpha and H beta narrowband and u, b, i broadband imaging with ACS and the Wide Field Camera 3 (WFC3). In order to study galaxies in which large numbers of Ly alpha photons are produced (whether or not they escape), we demanded an H alpha equivalent width W(H alpha) >= 100 angstrom. The final sample of 14 galaxies covers far-UV (FUV, lambda similar to 1500 angstrom) luminosities that overlap with those of high-z Ly alpha emitters (LAEs) and Lyman break galaxies (LBGs), making LARS a valid comparison sample. We present the reduction steps used to obtain the Ly alpha images, including our LARS eXtraction software (LaXs), which utilizes pixel-by-pixel spectral synthesis fitting of the energy distribution to determine and subtract the continuum at Ly alpha. We demonstrate that the use of SBC long-pass-filter combinations increase the signal-to-noise ratio by an order of magnitude compared to the nominal Ly alpha filter available in SBC. To exemplify the science potential of LARS, we also present some first results for a single galaxy, Mrk 259 (LARS #1). This irregular galaxy shows bright and extended (indicative of resonance scattering) but strongly asymmetric Ly alpha emission. Spectroscopy from the Cosmic Origins Spectrograph on board HST centered on the brightest UV knot shows a moderate outflow in the neutral interstellar medium (probed by low ionization stage absorption features) and Ly alpha emission with an asymmetric profile. Radiative transfer modeling is able to reproduce the essential features of the Ly alpha line profile and confirms the presence of an outflow. From the integrated photometry we measure an Ly alpha luminosity of L-Ly alpha= 1.3x10(42) erg s(-1) an equivalent width W(Ly alpha) = 45 angstrom and an FUV absolute magnitude M-FUV = -19.2 (AB). Mrk 259 would hence be detectable in high-z Ly alpha and LBG surveys. The total Ly alpha escape fraction is 12%. This number is higher than the low-z average, but similar to that at z > 4, demonstrating that LARS provides a valid comparison sample for high-z galaxy studies.

American Astronomical Society2014

Cosmic Evolution Of Virial And Stellar Mass In Massive Early-Type Galaxies

We measure the average mass properties of a sample of 41 strong gravitational lenses at moderate redshift (z similar to 0.4-0.9) and present the lens redshift for six of these galaxies for the first time. Using the techniques of strong and weak gravitational lensing on archival data obtained from the Hubble Space Telescope, we determine that the average mass overdensity profile of the lenses can be fit with a power-law profile (Delta Sigma proportional to R-0.86 +/- 0.16) that is within 1 sigma of an isothermal profile (Delta Sigma proportional to R-1) with velocity dispersion sigma(upsilon) = 260 +/- 20 km s(-1). Additionally, we use a two-component de Vaucouleurs + Navarro-Frenk-White (NFW) model to disentangle the total mass profile into separate luminous and dark matter components and determine the relative fraction of each component. We measure the average rest frame V-band stellar mass-to-light ratio (gamma(V) = 4.0 +/- 0.6hM(circle dot)/L-circle dot) and virial mass-to-light ratio (tau(V) = 300 +/- 90h M-circle dot/L-circle dot) for our sample, resulting in a virial-to-stellar mass ratio of M-vir/M-* = 75 +/- 25. Relaxing the NFW assumption, we estimate that changing the inner slope of the dark matter profile by similar to 20% yields a similar to 30% change in stellar mass-to-light ratio. Finally, we compare our results to a previous study using low-redshift lenses to understand how galaxy mass profiles evolve over time. We investigate the evolution of M-vir/M-(z) = alpha(1 + z)(beta), and find best-fit parameters of alpha = 51 +/- 36 and beta = 0.9 +/- 1.8, constraining the growth of virial-to-stellar mass ratio over the last similar to 7 Gyr. We note that, by using a sample of strong lenses, we are able to constrain the growth of M-vir/M-(z) without making any assumptions about the initial mass function of the stellar population.

2010

THE Ly alpha REFERENCE SAMPLE. I. SURVEY OUTLINE AND FIRST RESULTS FOR MARKARIAN 259

Hakim Atek

American Astronomical Society2014