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Publications (9)42.31 Total impact

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    ABSTRACT: Abridged. We use COSMOS to study in a self-consistent way the change in the number densities of quenched early-type galaxies (Q-ETGs) of a given size over the interval 0.2 < z < 1.0 to study the claimed size evolution of these galaxies. At 10^10.5<Mgalaxy<10^11 Msun, we see no change in the number density of compact Q-ETGs, while at >10^11 Msun we find a decrease by 30%. In both mass bins, the increase of the median sizes of Q-ETGs with time is primarily caused by the addition to the size function of larger and more diffuse Q-ETGs. At all masses, compact Q-ETGs become systematically redder towards later epochs, with a (U-V) difference consistent with passive evolution of their stellar populations, indicating that they are a population that does not appreciably evolve in size. At all epochs, the larger Q-ETGs (at least in the lower mass bin) have average rest-frame colors systematically bluer than those of the more compact Q-ETGs, suggesting that the former are younger than the latter. The idea that new, large, Q-ETGs are responsible for the observed growth in the median size of the population at a given mass is supported by the sizes and number of the star-forming galaxies that are expected to be progenitors of the new Q-ETGs over the same period. In the low mass bin, the new Q-ETG have 30% smaller sizes than their star-forming progenitors. This is likely due to the fading of their disks after they cease star-formation. Comparison with higher z shows that the median size of newly-quenched galaxies roughly scales, at constant mass, as (1+z)^-1. The dominant cause of the size evolution seen in the Q-ETG population is thus that the average sizes of individual Q-ETGs scale with the average density of the Universe at the time when they were quenched, with subsequent size changes in individual objects through eg merging of secondary importance, especially at masses <10^11 Msun.
    The Astrophysical Journal 02/2013; 773(2). · 6.73 Impact Factor
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    ABSTRACT: We present photometric measurements for the galaxies - and when possible their bulges and disks - in the 0.05<z<0.0585 groups of the Zurich Environmental Study (ZENS); these measurements include (B-I) colors, color gradients and maps, color dispersions, as well as stellar masses and star-formation rates. The ZENS galaxies are classified into quenched, moderately star-forming, and strongly star-forming using a combination of spectral features and FUV-to-optical colors; this approach optimally distinguishes quenched systems from dust-reddened star-forming galaxies. The latter contribute up to 50% to the (B-I) "red sequence" at ~10^10Msun. At fixed morphological or spectral type, we find that galaxy stellar masses are largely independent of environment, and especially of halo mass. As a first utilization of our photometric database, we study, at fixed stellar mass and Hubble type, how (B-I) colors, color gradients and color dispersion of disk satellites depend on group mass (M_GROUP), group-centric distance (R/R_200) and large-scale structure overdensity. The strongest environmental trend is found for disk-dominated satellites with M_GROUP and R/R_200. At M<10^10 Msun, disk-dominated satellites are redder in the inner regions of the groups than in the outer parts. At M>10^10 Msun, these satellites have shallower color gradients in higher mass groups and in the cores of groups compared with lower mass groups and the outskirts of groups. Stellar population analyses and semi-analytic models suggest that disk-dominated satellites undergo quenching of star formation in their outer disks, on timescales ~2 Gyr, as they progressively move inside the group potential.
    The Astrophysical Journal 06/2012; · 6.73 Impact Factor
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    ABSTRACT: We present structural measurements for the galaxies in the 0.05<z<0.0585 groups of the Zurich Environmental Study, aimed at establishing how galaxy properties depend on four environmental parameters: group halo mass M_GROUP, group-centric distance R/R_200, ranking into central or satellite, and large-scale structure density delta_LSS. Global galaxy structure is quantified both parametrically and non-parametrically. We correct all these measurements for observational biases due to PSF blurring and surface brightness effects as a function of galaxy size, magnitude, steepness of light profile and ellipticity. Structural parameters are derived also for bulges, disks and bars. We use the galaxy bulge-to-total ratios (B/T), together with the calibrated non-parametric structural estimators, to implement a quantitative morphological classification that maximizes purity in the resulting morphological samples. We investigate how the concentration C of satellite galaxies depends on galaxy mass for each Hubble type, and on M_GROUP, R/R_200 and delta_LSS. At galaxy masses M>10^10 M_sun, the concentration of disk satellites increases with increasing stellar mass, separately within each morphological bin of B/T. The known increase in concentration with stellar mass for disk satellites is thus due, at least in part, to an increase in galaxy central stellar density at constant B/T. The correlation between concentration and galaxy stellar mass becomes progressively steeper for later morphological types. The concentration of disk satellites shows a barely significant dependence on delta_LSS or R/R_200. The strongest environmental effect is found with group mass for M>10^10 M_sun disk-dominated satellites, which are ~10% more concentrated in high mass groups than in lower mass groups.
    The Astrophysical Journal 06/2012; 776(2). · 6.73 Impact Factor
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    ABSTRACT: The Zurich Environmental Study (ZENS) is based on a sample of ~1500 galaxy members of 141 groups in the mass range ~10^12.5-14.5 M_sun within the narrow redshift range 0.05<z<0.0585. ZENS adopts novel approaches, here described, to quantify four different galactic environments, namely: (1) the mass of the host group halo; (2) the projected halo-centric distance; (3) the rank of galaxies as central or satellites within their group halos; and (4) the filamentary large-scale structure (LSS) density. No self-consistent identification of a central galaxy is found in ~40% of <10^13.5 M_sun groups, from which we estimate that ~15% of groups at these masses are dynamically unrelaxed systems. Central galaxies in relaxed and unrelaxed groups have in general similar properties, suggesting that centrals are regulated by their mass and not by their environment. Centrals in relaxed groups have however ~30% larger sizes than in unrelaxed groups, possibly due accretion of small satellites in virialized group halos. At M>10^10 M_sun, satellite galaxies in relaxed and unrelaxed groups have similar size, color and (specific) star formation rate distributions; at lower galaxy masses, satellites are marginally redder in relaxed relative to unrelaxed groups, suggesting quenching of star formation in low-mass satellites by physical processes active in relaxed halos. Finally, relaxed and unrelated groups show similar stellar mass conversion efficiencies, peaking at halo masses around 10^12.5 M_sun. In the enclosed ZENS catalogue we publish all environmental diagnostics as well as the galaxy structural and photometric measurements described in companion ZENS papers II and III.
    The Astrophysical Journal 06/2012; 776(2). · 6.73 Impact Factor
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    ABSTRACT: ZENS is a survey of nearby (z 0.05) galaxy groups in the mass range 1012-1014 MSUN. From both spectroscopy and deep optical imaging, we have analyzed the structural, stellar population, and star-formation properties of the group galaxies. By comparing the galaxy populations, at fixed galactic stellar mass, across a wide range of environmental indicators - including group halo mass, group-centric radius, large scale structure density, and satellite vs. central galaxies - we are able to determine the dependence of galactic properties on each of these environmental measures. Our results indicate that the most significant environmental effects are seen for satellite galaxies as a function of the group-centric distance, where galaxies nearer the group centers are more likely to be quenched, be more bulge-dominated, and have redder colors (particularly in the disk component) than galaxies in the group outskirts. Group halo mass, LSS-density, and the central/satellite dichotomy tend to have smaller, although not always negligible, effects. Additionally, the group environment has a more pronounced affect on galaxies at lower stellar masses. We compare these results to those of several state-of-the art semi-analytic models of galaxy evolution. We find that the standard recipes tend to predict both an over-abundance of, and overly red colors for quenched galaxies. We instead find that a model in which the star-formation rate of galaxies is tied to the observed specific star formation evolution with redshift more accurately reproduced the numbers and colors of these quenched galaxies.
    05/2012;
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    Ewan Cameron
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    ABSTRACT: I present a critical review of techniques for estimating confidence intervals on binomial population proportions inferred from success counts in small-to-intermediate samples. Population proportions arise frequently as quantities of interest in astronomical research; for instance, in studies aiming to constrain the bar fraction, AGN fraction, SMBH fraction, merger fraction, or red sequence fraction from counts of galaxies exhibiting distinct morphological features or stellar populations. However, two of the most widely-used techniques for estimating binomial confidence intervals--the 'normal approximation' and the Clopper & Pearson approach--are liable to misrepresent the degree of statistical uncertainty present under sampling conditions routinely encountered in astronomical surveys, leading to an ineffective use of the experimental data (and, worse, an inefficient use of the resources expended in obtaining that data). Hence, I provide here an overview of the fundamentals of binomial statistics with two principal aims: (i) to reveal the ease with which (Bayesian) binomial confidence intervals with more satisfactory behaviour may be estimated from the quantiles of the beta distribution using modern mathematical software packages (e.g. R, matlab, mathematica, IDL, python); and (ii) to demonstrate convincingly the major flaws of both the 'normal approximation' and the Clopper & Pearson approach for error estimation.
    Publications of the Astronomical Society of Australia 12/2010; 28. · 3.12 Impact Factor
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    ABSTRACT: We use the high angular resolution in the near-infrared of the WFC3 on HST to determine YHVz color-color selection criteria to identify and characterize 1.5<z<3.5 galaxies in the HUDF09 and ERS (GOODS-South) fields. The WFC3 NIR images reveal galaxies at these redshifts that were undetected in the rest-frame UV HUDF/GOODS images, as well as true centers and regular disks in galaxies classified as highly irregular in rest-frame UV light. Across the 1.5<z<2.15 redshift range, regular disks are unveiled in the WFC3 images of ~25% of both intermediate and high mass galaxies, i.e., above 10^10 Msun. Meanwhile, galaxies maintaining diffuse and/or irregular morphologies in the rest-frame optical light---i.e., not yet dynamically settled---at these epochs are almost entirely restricted to masses below 10^11 Msun. In contrast at 2.25 < z < 3.5 these diffuse and/or irregular structures overwhelmingly dominate the morphological mix in both the intermediate and high mass regimes, while no regular disks, and only a small fraction (25%) of smooth spheroids, are evident above 10^11 Msun. Strikingly, by 1.5 < z < 2.25 roughly 2 out of every 3 galaxies at the highest masses are spheroids. In our small sample, the fraction of star-forming galaxies at these mass scales decreases concurrently from ~60% to ~5%. If confirmed, this indicates that z~2 is the epoch of both the morphological transformation and quenching of star-formation which assemble the first substantial population of massive ellipticals.
    The Astrophysical Journal 07/2010; 743. · 6.73 Impact Factor
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    ABSTRACT: We investigate the (large-scale) bar fraction in a mass-complete sample of M > 10^(10.5) M_⊙ disc galaxies at 0.2 < z < 0.6 in the Cosmological Evolution Survey (COSMOS) field. The fraction of barred discs strongly depends on mass, disc morphology and specific star formation rate (SSFR). At intermediate stellar mass (10^(10.5) < M < 10^(11) M_⊙) the bar fraction in early-type discs is much higher, at all redshifts, by a factor of ~2, than that in late-type discs. This trend is reversed at higher stellar mass (M > 10^(11) M_⊙), where the fraction of bars in early-type discs becomes significantly lower, at all redshifts, than that in late-type discs. The bar fractions for galaxies with low and high SSFRs closely follow those of the morphologically selected early- and late-type populations, respectively. This indicates a close correspondence between morphology and SSFR in disc galaxies at these earlier epochs. Interestingly, the total bar fraction in 10^(10.5) < M < 10^(11) M_⊙ discs is built up by a factor of ~2 over the redshift interval explored, while for M > 10^(11) M_⊙ discs it remains roughly constant. This indicates that, already by z ~ 0.6, spectral and morphological transformations in the most massive disc galaxies have largely converged to the familiar Hubble sequence that we observe in the local Universe, while for intermediate-mass discs this convergence is ongoing until at least z ~ 0.2. Moreover, these results highlight the importance of employing mass-limited samples for quantifying the evolution of barred galaxies. Finally, the evolution of the barred galaxy populations investigated does not depend on the large-scale environmental density (at least, on the scales which can be probed with the available photometric redshifts).
    Monthly Notices of the Royal Astronomical Society 01/2010; · 5.52 Impact Factor
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    ABSTRACT: We use the high angular resolution in the near-infrared of the WFC3 on HST to identify and characterize 1.5 < z < 3.5 galaxies in the HUDF09 and ERS fields. Specifically, (i) we construct H-band selected catalogs of galaxies complete down to AB=27(25) mag in the HUDF09(ERS) fields, and publish these source catalogs; (ii) present optimized color-selection criteria for identifying galaxies at 1.5 < z < 3.5 - i.e., a YHVz criterion, which offers a selection rate within the target redshift interval of 96%(92%) down to H < 27(25) mag in the HUDF09(ERS), with contamination from interlopers at lower or higher redshifts of only ~15%; and (iii) compare the WFC3/IR rest-frame optical morphologies of these galaxies with their ACS-based rest-frame UV morphologies. The WFC3 NIR images reveal galaxies at these redshifts that were undetected in the rest-frame UV HUDF/GOODS images, as well as true centers and regular disks in galaxies classified as highly irregular in rest-frame UV light. Across the entire 1.5 < z < 3.5 redshift range, galaxies in which regular disks are unveiled in the WFC3 images tend to be quite massive, i.e., >10^10.5 Msun. In contrast, less massive galaxies maintain an irregular morphology in the rest-frame optical light, indicating that, at these epochs, low-mass galaxies are not dynamically settled. At the highest masses, >10^11 Msun, galaxies at 2.25 < z < 3.5 show the whole variety of morphologies, from irregular to disk to spheroid, in roughly similar proportions. Strikingly, however, galaxies of similar high masses at 1.5 < z < 2.25 are virtually all elliptical-like spheroids. In our small sample, the fraction of star-forming galaxies at these mass scales decreases from ~60% to zero. If confirmed, this indicates that z ~ 2 is the epoch of both the morphological transformations and quenching of star-formation that results in the massive elliptical population.