Gravitational lens mass reconstruction

A highly desirable technique sought after by cosmology is one which enables the accu­rate mass measurement of rich galaxy clusters. From observations of their abundance and primarily their mass, clusters give strong constraints on the density parameter of the Universe, models of structure formation and normalisation of the power spectrum of density fluctuations. Gravitational lensing provides such a technique. Prevailing over X-ray temperature and virial velocity methods known to be problematic, lensing permits determination of cluster masses independent of dynamical state.This thesis concentrates mainly on the exploitation of the magnification properties of lenses rather than those of shear analysis which relies upon the quantification of galaxy image distortions. Magnification allows absolute mass measurements, breaking the slieet-mass degeneracy experienced by shear. To this extent, a theoretical analysis of the geometrical magnification of angular separations between galaxies lying behind a lensing cluster is performed. This sees application to the cluster Abell 1689 using V and I band observations to select background galaxies based on their V-I colour. The distribution of source number counts in the observed field of view results in the determination of a radial mass profile and a mass map for Abell 1689. This predicts a projected mass interior to 0.24/?,_1Mpc of M (< 0.24/i_1Mpc) = (0.50 ± 0.09) x 1O15/i_1M0.A new method of directly determining accurate, self-consistent lens mass and shear maps in the strong lensing regime from magnification is presented. The method relies upon pixellization of the surface mass density distribution which generates a simple, solvable set of equations. The concept of pixellization is also directed at shear analysis to give rise to a simplified method of application. Through use of cluster models,XI the method is verified before the magnification data from the colour-selected number counts is input to compute a self-consistent mass map of Abell 1689.The property of lens magnification to amplify observed background source fluxes is investigated. Using an independent set of observations in nine optimally chosen filters, photometric redshifts of objects lying in the field of Abell 1689 are calculated. In ad­dition to providing an unambiguous distinction between cluster members, foreground objects and background sources this also enables computation of the source luminos­ity function. Comparison of this with the distribution of luminosities in an observed offset field quantifies the lens-induced flux magnification to arrive at an independent mass profile measurement of Abell 1689. A projected mass interior to 0.25/?,-1 Mpc of M (< 0.25/i-1Mpc) = (0.48 ± 0.16) x 1015/»_1Mo is found

Evolution of the star formation histories of BLAST galaxies

We have measured star formation histories (SFHs) and stellar masses of galaxies detected by the Balloon-borne Large Aperture Sub-millimetre Telescope (BLAST) over approximately 9 square degrees centred on the Chandra Deep Field South. We have applied the recently developed SFH reconstruction method of Dye et al. to optical, near-infrared and mid-infrared photometry of 92 BLAST galaxies. We find significant differences between the SFHs of low mass (<10^11 M_sol) and high mass (>10^11 M_sol) galaxies. On average, low mass galaxies exhibit a dominant late burst of star formation which creates a large fraction of their stellar mass. Conversely, high mass systems tend to have a significant amount of stellar mass that formed much earlier. We also find that the high mass SFHs evolve more strongly than the low mass SFHs. These findings are consistent with the phenomenon of downsizing observed in optically selected samples of galaxies.Comment: Accepted by MNRAS Letters. 5 pages. 4 figure

Constraints on dark and visible mass in galaxies from strong gravitational lensing

We give a non-exhaustive review of the use of strong gravitational lensing in placing constraints on the quantity of dark and visible mass in galaxies. We discuss development of the methodology and summarise some recent results.Comment: To appear in proceedings of IAU Symposium 244, 'Dark Galaxies and Lost Baryons', 25th - 29th June 2007. Nine pages, five figures. Version 2 updates bibliograph

The stellar masses of 25000 galaxies at 0.2<z<1.0 estimated by the COMBO-17 survey

We present an analysis of stellar mass estimates for a sample of 25000 galaxies from the COMBO-17 survey over the interval 0.2<z<1.0. We have developed, implemented, and tested a new method of estimating stellar mass-to-light ratios, which relies on redshift and spectral energy distribution (SED) classification from 5 broadband and 12 medium band filters. We find that the majority (>60%) of massive galaxies with M_* > 10^{11} solar masses at all z<1 are non-star-forming; blue star-forming galaxies dominate at lower masses. We have used these mass estimates to explore the evolution of the stellar mass function since z=1. We find that the total stellar mass density of the universe has roughly doubled since z~1. Our measurements are consistent with other measurements of the growth of stellar mass with cosmic time and with estimates of the time evolution of the cosmic star formation rate. Intriguingly, the integrated stellar mass of blue galaxies with young stars has not significantly changed since z~1, even though these galaxies host the majority of the star formation: instead, the growth of the total stellar mass density is dominated by the growth of the total mass in the largely passive galaxies on the red sequence.Comment: Astronomy and Astrophysics in press. 15 pages, 12 figure

Nearly 5000 Distant Early-Type Galaxies in COMBO-17: a Red Sequence and its Evolution since z~1

We present the rest-frame colors and luminosities of ~25000 m_R<24 galaxies in the redshift range 0.2<z<1.1, drawn from 0.78 square degrees of the COMBO-17 survey. We find that the rest-frame color distribution of these galaxies is bimodal at all redshifts out to z~1. This bimodality permits a model-independent definition of red, early-type galaxies and blue, late-type galaxies at any given redshift. The colors of the blue peak become redder towards the present day, and the number density of blue luminous galaxies has dropped strongly since z~1. Focusing on the red galaxies, we find that they populate a color-magnitude relation. Such red sequences have been identified in galaxy cluster environments, but our data show that such a sequence exists over this redshift range even when averaging over all environments. The mean color of the red galaxy sequence evolves with redshift in a way that is consistent with the aging of an ancient stellar population. The rest-frame B-band luminosity density in red galaxies evolves only mildly with redshift in a Lambda-dominated cold dark matter universe. Accounting for the change in stellar mass-to-light ratio implied by the redshift evolution in red galaxy colors, the COMBO-17 data indicate an increase in stellar mass on the red sequence by a factor of two since z~1. The largest source of uncertainty is large-scale structure, implying that considerably larger surveys are necessary to further refine this result. We explore mechanisms that may drive this evolution in the red galaxy population, finding that both galaxy merging and truncation of star formation in some fraction of the blue, star-forming population are required to fully explain the properties of these galaxies.Comment: To appear in the Astrophysical Journal 20 June 2004. 16 pages, 6 embedded figures. Substantial revision of photometric redshifts and extensive minor changes to the paper throughout: conclusions unchange

Discovering strongly lensed quasar candidates with catalogue-based methods from DESI Legacy Surveys

The Hubble tension, revealed by a $\sim 5\sigma$ discrepancy between measurements of the Hubble-Lemaitre constant from early- and local-Universe observations, is one of the most significant problems in modern cosmology. In order to better understand the origin of this mismatch, independent techniques to measure $H_0$, such as strong lensing time delays, are required. Notably, the sample size of such systems is key to minimising statistical uncertainties and cosmic variance, which can be improved by exploring the datasets of large-scale sky surveys like DESI (Dark Energy Spectroscopic Instrument). We identify possible strong lensing time-delay systems within DESI by selecting candidate multiply imaged lensed quasars from a catalogue of 24,440,816 candidate QSOs contained in the 9th data release of the DESI Legacy Imaging Surveys (DESI-LS). Using a friend-of-friends-like algorithm on spatial co-ordinates, our method generates an initial list of compact quasar groups. This list is subsequently filtered using a measure of the similarity of colours of a group's members and the likelihood that they are quasars. A visual inspection finally selects candidate strong lensing systems based on the spatial configuration of the group members. We identify 620 new candidate multiply imaged lensed quasars (101 Grade-A, 214 Grade-B, 305 Grade-C). This number excludes 53 known spectroscopically confirmed systems and existing candidate systems identified in other similar catalogues. When available, these new candidates will be further checked by combining the spectroscopic and photometric data from DESI. The catalogues and images of the candidates in this work are available online (https://github.com/EigenHermit/lensed_qso_cand_catalogue_He-22/).Comment: Accepted by A&A. 14 pages, 11 figures. Comments are welcom

Auto-identification of unphysical source reconstructions in strong gravitational lens modelling

With the advent of next-generation surveys and the expectation of discovering huge numbers of strong gravitational lens systems, much effort is being invested into developing automated procedures for handling the data. The several orders of magnitude increase in the number of strong galaxy–galaxy lens systems is an insurmountable challenge for traditional modelling techniques. Whilst machine learning techniques have dramatically improved the efficiency of lens modelling, parametric modelling of the lens mass profile remains an important tool for dealing with complex lensing systems. In particular, source reconstruction methods are necessary to cope with the irregular structure of high-redshift sources. In this paper, we consider a convolutional neural network (CNN) that analyses the outputs of semi-analytic methods that parametrically model the lens mass and linearly reconstruct the source surface brightness distribution. We show the unphysical source reconstructions that arise as a result of incorrectly initialized lens models can be effectively caught by our CNN. Furthermore, the CNN predictions can be used to automatically reinitialize the parametric lens model, avoiding unphysical source reconstructions. The CNN, trained on reconstructions of lensed Sérsic sources, accurately classifies source reconstructions of the same type with a precision P > 0.99 and recall R > 0.99. The same CNN, without retraining, achieves P = 0.89 and R = 0.89 when classifying source reconstructions of more complex lensed Hubble Ultra-Deep Field (HUDF) sources. Using the CNN predictions to reinitialize the lens modelling procedure, we achieve a 69 per cent decrease in the occurrence of unphysical source reconstructions. This combined CNN and parametric modelling approach can greatly improve the automation of lens modelling