R. G. Abraham

University of Toronto, Toronto, Ontario, Canada

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Publications (171)652.73 Total impact

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    ABSTRACT: We recently identified a population of low surface brightness objects in the field of the z=0.023 Coma cluster, using the Dragonfly Telephoto Array. Here we present Keck spectroscopy of one of the largest of these "ultra-diffuse galaxies" (UDGs), confirming that it is a member of the cluster. The galaxy has prominent absorption features, including the Ca II H+K lines and the G-band, and no detected emission lines. Its radial velocity of cz=6280 +- 120 km/s is within the 1 sigma velocity dispersion of the Coma cluster. The galaxy has an effective radius of 4.3 +- 0.3 kpc and a Sersic index of 0.89 +- 0.06, as measured from Keck imaging. We find no indications of tidal tails or other distortions, at least out to a radius of ~2 r_e. We show that UDGs are located in a previously sparsely populated region of the size - magnitude plane of quiescent stellar systems, as they are ~6 magnitudes fainter than normal early-type galaxies of the same size. It appears that the luminosity distribution of large quiescent galaxies is not continuous, although this could largely be due to selection effects. Dynamical measurements are needed to determine whether the dark matter halos of UDGs are similar to those of galaxies with the same luminosity or to those of galaxies with the same size.
    04/2015; 804(1). DOI:10.1088/2041-8205/804/1/L26
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    ABSTRACT: Most massive passive galaxies are compact at high redshifts, but similarly compact massive galaxies are rare in the local universe. The most common interpretation of this phenomenon is that massive galaxies have grown in size by a factor of about five since redshift z=2. An alternative explanation is that recently quenched massive galaxies are larger (a "progenitor bias"). In this paper we explore the importance of progenitor bias by looking for systematic differences in the stellar populations of compact early-type galaxies in the DEEP2 survey as a function of size. Our analysis is based on applying the statistical technique of bootstrap resampling to constrain differences in the median ages of our samples and to begin to characterize the distribution of stellar populations in our co-added spectra. The light-weighted ages of compact early-type galaxies at redshifts 0.5 < z < 1.4 are compared to those of a control sample of larger galaxies at similar redshifts. We find that massive compact early-type galaxies selected on the basis of red color and high bulge-to-total ratio are younger than similarly selected larger galaxies, suggesting that size growth in these objects is not driven mainly by progenitor bias, and that individual galaxies grow as their stellar populations age. However, compact early-type galaxies selected on the basis of image smoothness and high bulge-to-total ratio are older than a control sample of larger galaxies. Progenitor bias will play a significant role in defining the apparent size changes of early-type galaxies if they are selected on the basis of the smoothness of their light distributions.
    The Astrophysical Journal 11/2014; 798(1). DOI:10.1088/0004-637X/798/1/26 · 6.28 Impact Factor
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    ABSTRACT: We report the discovery of 47 low surface brightness objects in deep images of a 3 x 3 degree field centered on the Coma cluster, obtained with the Dragonfly Telephoto Array. The objects have central surface brightness mu(g,0) ranging from 24 - 26 mag/arcsec^2 and effective radii r_e = 3"-10", as measured from archival Canada France Hawaii Telescope images. From their spatial distribution we infer that most or all of the objects are galaxies in the Coma cluster. This relatively large distance is surprising as it implies that the galaxies are very large: with r_e = 1.5 - 4.6 kpc their sizes are similar to those of L* galaxies even though their median stellar mass is only ~6 x 10^7 Solar masses. The galaxies are relatively red and round, with = 0.8 and <b/a> = 0.74. One of the 47 galaxies is fortuitously covered by a deep Hubble Space Telescope ACS observation. The ACS imaging shows a large spheroidal object with a central surface brightness mu(g,0) = 25.8 mag/arcsec^2, a Sersic index n=0.6, and an effective radius of 7", corresponding to 3.4 kpc at the distance of Coma. The galaxy is unresolved, as expected for a Coma cluster object. To our knowledge such "ultra-diffuse galaxies" have not been predicted in any modern galaxy formation model. We speculate that UDGs may have lost their gas supply at early times, possibly resulting in very high dark matter fractions.
    10/2014; 798(2). DOI:10.1088/2041-8205/798/2/L45
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    ABSTRACT: We study the spatially resolved stellar kinematics of two star-forming galaxies at z similar to 0.1 from the larger DYnamics of Newly Assembled Massive Objects (DYNAMO) sample. These galaxies, which have been characterized by high levels of star formation and large ionized-gas velocity dispersions, are considered possible analogues to high-redshift clumpy discs. They were observed using the Gemini Multi-Object Spectrograph instrument in integral field spectroscopy (IFS) mode at the Gemini Observatory with high spectral resolution (R similar or equal to 5400, equivalent to sigma similar or equal to 24 km s(-1) at the observed wavelengths) and similar to 6 h exposure times in order to measure the resolved stellar kinematics via absorption lines. We also obtain higher quality emission-line kinematics than previous observations. The spatial resolution (1.2 kpc) is sufficient to show that the ionized gas in these galaxies (as traced by H beta emission) is morphologically irregular, forming multiple giant clumps while stellar continuum light is smooth and well described by an exponential profile. Clumpy gas morphologies observed in IFS data are confirmed by complementary narrow-band H alpha imaging from the Hubble Space Telescope. Morphological differences between the stars and ionized gas are not reflected dynamically as stellar kinematics are found to be closely coupled to the kinematics of the ionized gas: both components are smoothly rotating with large velocity dispersions (similar to 40 km s(-1)) suggesting that the high gas dispersions are not primarily driven by star formation feedback. In addition, the stellar population ages of these galaxies are estimated to be quite young (60-500 Myr). The large velocity dispersions measured for these young stars suggest that we are seeing the formation of thick discs and/or stellar bulges in support of recent models which produce these from clumpy galaxies at high redshift.
    Monthly Notices of the Royal Astronomical Society 06/2014; 442(4):3206-3221. DOI:10.1093/mnras/stu1029 · 5.23 Impact Factor
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    ABSTRACT: In this letter we report the discovery of CO fluxes, suggesting very high gas fractions in three disk galaxies seen in the nearby Universe (z ~ 0.1). These galaxies were investigated as part of the DYnamics of Newly Assembled Massive Objects (DYNAMO) survey. High-resolution Hubble Space Telescope imaging of these objects reveals the presence of large star forming clumps in the bodies of the galaxies, while spatially resolved spectroscopy of redshifted Halpha reveals the presence of high dispersion rotating disks. The internal dynamical state of these galaxies resembles that of disk systems seen at much higher redshifts (1 < z < 3). Using CO(1-0) observations made with the Plateau de Bure Interferometer, we find gas fractions of 20-30% and depletion times of tdep ~ 0.5 Gyr (assuming a Milky Way-like CO conversion factor). These properties are unlike those expected for low- redshift galaxies of comparable specific star formation rate, but they are normal for their high-z counterparts. DYNAMO galaxies break the degeneracy between gas fraction and redshift, and we show that the depletion time per specific star formation rate for galaxies is closely tied to gas fraction, independent of redshift. We also show that the gas dynamics of two of our local targets corresponds to those expected from unstable disks, again resembling the dynamics of high-z disks. These results provide evidence that DYNAMO galaxies are local analogues to the clumpy, turbulent disks, which are often found at high redshift.
    The Astrophysical Journal Letters 05/2014; 790(2). DOI:10.1088/2041-8205/790/2/L30 · 5.60 Impact Factor
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    ABSTRACT: We study the spatially resolved stellar kinematics of two star-forming galaxies at z = 0.1 from the larger DYnamics of Newly Assembled Massive Objects (DYNAMO) sample. These galaxies, which have been characterized by high levels of star formation and large ionized gas velocity dispersions, are considered possible analogs to high-redshift clumpy disks. They were observed using the GMOS instrument in integral field spectroscopy (IFS) mode at the Gemini Observatory with high spectral resolution (R=5400, equivalent to 24 km/s at the observed wavelengths) and 6 hour exposure times in order to measure the resolved stellar kinematics via absorption lines. We also obtain higher-quality emission line kinematics than previous observations. The spatial resolution (1.2 kpc) is sufficient to show that the ionized gas in these galaxies (as traced by H-beta emission) is morphologically irregular, forming multiple giant clumps while stellar continuum light is smooth and well described by an exponential profile. Clumpy gas morphologies observed in IFS data are confirmed by complementary narrow band H-alpha imaging from the Hubble Space Telescope. Morphological differences between the stars and ionized gas are not reflected dynamically as stellar kinematics are found the be closely coupled to the kinematics of the ionized gas: both components are smoothly rotating with large velocity dispersions (~40 km/s) suggesting that the high gas dispersions are not primarily driven by star-formation feedback. In addition, the stellar population ages of these galaxies are estimated to be quite young (60-500 Myr). The large velocity dispersions measured for these young stars suggest that we are seeing the formation of thick disks and/or stellar bulges in support of recent models which produce these from clumpy galaxies at high redshift.
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    ABSTRACT: Dwarf satellite galaxies are a key probe of dark matter and of galaxy formation on small scales and of the dark matter halo masses of their central galaxies. They have very low surface brightness, which makes it difficult to identify and study them outside of the Local Group. We used a low surface brightness-optimized telescope, the Dragonfly Telephoto Array, to search for dwarf galaxies in the field of the massive spiral galaxy M101. We identify seven large, low surface brightness objects in this field, with effective radii of 10-30 arcseconds and central surface brightnesses of μg ~ 25.5-27.5 mag arcsec–2. Given their large apparent sizes and low surface brightnesses, these objects would likely be missed by standard galaxy searches in deep fields. Assuming the galaxies are dwarf satellites of M101, their absolute magnitudes are in the range –11.6 MV –9.3 and their effective radii are 350 pc-1.3 kpc. Their radial surface brightness profiles are well fit by Sersic profiles with a very low Sersic index (n ~ 0.3-0.7). The properties of the sample are similar to those of well-studied dwarf galaxies in the Local Group, such as Sextans I and Phoenix. Distance measurements are required to determine whether these galaxies are in fact associated with M101 or are in its foreground or background.
    The Astrophysical Journal Letters 05/2014; 787(2):L37. DOI:10.1088/2041-8205/787/2/L37 · 5.60 Impact Factor
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    ABSTRACT: We use a new telescope concept, the Dragonfly Telephoto Array, to study the low surface brightness outskirts of the spiral galaxy M101. The radial surface brightness profile is measured down to mu_g ~ 32 mag/arcsec^2, a depth that approaches the sensitivity of star count studies in the Local Group. We convert surface brightness to surface mass density using the radial g-r color profile. The mass density profile shows no significant upturn at large radius and is well-approximated by a simple bulge + disk model out to R = 70 kpc, corresponding to 18 disk scale lengths. Fitting a bulge + disk + halo model we find that the best-fitting halo mass M_halo ~ 1.7 x 10^8 M_sun. The total stellar mass of M101 is M_tot,* ~ 5.3 x 10^10 Msun, and we infer that the halo mass fraction f_halo = M_halo / M_tot,* ~ 0.003. This mass fraction is lower than that of the Milky Way (f_halo ~ 0.02) and M31 (f_halo ~ 0.04). All three galaxies fall below the f_halo - M_tot,* relation predicted by recent cosmological simulations that trace the light of disrupted satellites, with M101's halo mass a factor of ~10 below the median expectation. However, the predicted scatter in this relation is large, and more galaxies are needed to better quantify this possible tension with galaxy formation models. Dragonfly is well suited for this project: as integrated-light surface brightness is independent of distance, large numbers of galaxies can be studied in a uniform way.
    The Astrophysical Journal Letters 01/2014; 782(2). DOI:10.1088/2041-8205/782/2/L24 · 5.60 Impact Factor
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    Roberto G. Abraham, Pieter G. van Dokkum
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    ABSTRACT: We describe the Dragonfly Telephoto Array, a robotic imaging system optimized for the detection of extended ultra low surface brightness structures. The array consists of eight Canon 400mm f/2.8 telephoto lenses coupled to eight science-grade commercial CCD cameras. The lenses are mounted on a common framework and are co-aligned to image simultaneously the same position on the sky. The system provides an imaging capability equivalent to a 0.4m aperture f/1.0 refractor with a 2.6 deg X 1.9 deg field of view. The system has no obstructions in the light path, optimized baffling, and internal optical surfaces coated with a new generation of anti-reflection coatings based on sub-wavelength nanostructures. As a result, the array's point spread function has a factor of ~10 less scattered light at large radii than well-baffled reflecting telescopes. The Dragonfly Telephoto Array is capable of imaging extended structures to surface brightness levels below 30 mag/arcsec^2 in 10h integrations (without binning or foreground star removal). This is considerably deeper than the surface brightness limit of any existing wide-field telescope. At present no systematic errors limiting the usefulness of much longer integration times has been identified. With longer integrations (50-100h), foreground star removal and modest binning the Dragonfly Telephoto Array is capable of probing structures with surface brightnesses below 32 mag/arcsec^2. Detection of structures at these surface brightness levels may hold the key to solving the "missing substructure" and "missing satellite" problems of conventional hierarchical galaxy formation models. The Dragonfly Telephoto Array is therefore executing a fully-automated multi-year imaging survey of a complete sample of nearby galaxies in order to undertake the first census of ultra-faint substructures in the nearby Universe.
    Publications of the Astronomical Society of the Pacific 01/2014; 126(935). DOI:10.1086/674875 · 3.23 Impact Factor
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    ABSTRACT: (abridged) DYNAMO is a multi-wavelength, spatially-resolved survey of local ($z \sim 0.1$) star-forming galaxies designed to study evolution through comparison with samples at z~2. Half of the sample has integrated H-alpha luminosities of >$10^{42}$ erg/s, the typical lower limit for resolved spectroscopy at z~2. The sample covers a range in stellar mass ($10^9$-$10^{11}$ Msun) and star-formation rate (0.2-100 Msun/yr). In this first paper of a series, we present integral-field spectroscopy of H-alpha emission for the sample of 67 galaxies. We infer gas fractions in our sample as high as ~0.8, higher than typical for local galaxies. Gas fraction correlates with stellar mass in galaxies with star-formation rates below 10 Msun/yr, as found by COLDGASS, but galaxies with higher star-formation rates have higher than expected gas fractions. There is only a weak correlation, if any, between gas fraction and gas velocity dispersion. Galaxies in the sample visually classified as disc-like are offset from the local stellar-mass Tully-Fisher relation to higher circular velocities, but this offset vanishes when both gas and stars are included in the baryonic Tully-Fisher relation. The mean gas velocity dispersion of the sample is ~50 km/s, and V/sigma ranges from 2 to 10 for most of the discs, similar to 'turbulent' galaxies at high redshift. Half of our sample show disc-like rotation, while ~20 percent show no signs of rotation. The division between rotating and non-rotating is approximately...
    Monthly Notices of the Royal Astronomical Society 10/2013; 437(2). DOI:10.1093/mnras/stt1882 · 5.23 Impact Factor
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    ABSTRACT: The Near Infrared Imager and Slitless Spectrograph (NIRISS) aboard the James Webb Space Telescope (JWST) will offer wide-field slitless spectroscopy (WFSS) with a resolving power R = 150 over the wavelength range 0.8 to 2.25 microns. In this band, NIRISS will be sensitive to Lyman-alpha emission from galaxies with redshifts 6 < z < 17. To explore its ability to observe such high-redshift galaxies, we have modeled a NIRISS observation of the massive galaxy cluster MACS J0647+7015. Using published images, photometry, and redshifts from the CLASH survey, we constructed a series of simulated direct and dispersed images in the six filters used for WFSS with NIRISS. To each image were added 180 high-redshift galaxies distributed uniformly in space, redshift, and magnitude. Using Source Extractor, we identified 7200 galaxies in the F200W direct image, including 165 of the high-redshift sources (the remainder were lost to bright foreground objects). From this catalog, we selected 1000 objects, including all 165 of the high-redshift sources. We performed photometry of these 1000 sources in each direct image and extracted their spectra (using the aXe software package) from each dispersed image. A subset of our team was given these data and asked to identify the high-redshift galaxies. We will present the results of this analysis and discuss their implications for the ability of NIRISS to detect and parameterize high-redshift galaxies in crowded fields. NIRISS is provided to the JWST project by the Canadian Space Agency under the leadership of René Doyon of the Université de Montréal. The prime contractor is COM DEV Canada.
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    ABSTRACT: We present an analysis of the detectability of faint tidal features in galaxies from the wide-field component of the Canada-France-Hawaii Telescope Legacy Survey. Our sample consists of 1781 luminous M_r<-19.3 mag) galaxies in the magnitude range 15.5<r<17 mag and in the redshift range 0.04<z<0.2. Although we have classified tidal features according to their morphology (e.g. streams, shells and tails), we do not attempt to interpret them in terms of their physical origin (e.g. major versus minor merger debris). Instead, we provide a catalog that is intended to provide raw material for future investigations which probe the nature of low surface brightness substructure around galaxies. We find that around 12% of the galaxies in our sample show clear tidal features at the highest confidence level. This fraction rises to about 18% if we include systems with convincing albeit weaker tidal features, and to 26% if we include systems with more marginal features that may or may not be tidal in origin. These proportions are a strong function of rest-frame colour and of stellar mass. Linear features, shells, and fans are much more likely to occur in massive galaxies with stellar masses >10^10.5 M_sun, and red galaxies are twice as likely to show tidal features than are blue galaxies.
    The Astrophysical Journal 01/2013; 765(1). DOI:10.1088/0004-637X/765/1/28 · 6.28 Impact Factor
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    Richard C. Y. Chou, Carrie R. Bridge, Roberto G. Abraham
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    ABSTRACT: We have studied the kinematics of ~2800 candidate close pair galaxies at 0.1<z<1.2 identified from the Canada-France-Hawaii Telescope Legacy Survey fields. Spectra of these systems were obtained using spectrometers on the 6.5m Magellan and 5m Hale telescopes. These data allow us to constrain the rate of dry mergers at intermediate redshifts and to test the `hot halo' model for quenching of star formation. Using virial radii estimated from the correlation between dynamical and stellar masses published by Leauthaud et al. (2011), we find that around 1/5 of our candidate pairs are likely to share a common dark matter halo (our metric for close physical association). These pairs are divided into red-red, blue-red and blue-blue systems using the rest-frame colors classification method introduced in Chou et al. (2011). Galaxies classified as red in our sample have very low star-formation rates, but they need not be totally quiescent, and hence we refer to them as `damp', rather than `dry', systems. After correcting for known selection effects, the fraction of blue-blue pairs is significantly greater than that of red-red and blue-red pairs. Red-red pairs are almost entirely absent from our sample, suggesting that damp mergers are rare at z~0.5. Our data supports models with a short merging timescale (<0.5 Gyr) in which star-formation is enhanced in the early phase of mergers, but quenched in the late phase. Hot halo models may explain this behaviour, but only if virial shocks that heat gas are inefficient until major mergers are nearly complete.
    The Astrophysical Journal 10/2012; 760(2). DOI:10.1088/0004-637X/760/2/113 · 6.28 Impact Factor
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    ABSTRACT: The Fine Guidance Sensor (FGS) is one of the four science instruments on board the James Webb Space Telescope (JWST). FGS features two modules: an infrared camera dedicated to fine guiding of the observatory and a science camera module, the Near-Infrared Imager and Slitless Spectrograph (NIRISS) covering the wavelength range between 0.7 and 5.0 μm with a field of view of 2.2' X 2.2'. NIRISS has four observing modes: 1) broadband imaging featuring seven of the eight NIRCam broadband filters, 2) wide-field slitless spectroscopy at a resolving power of rv150 between 1 and 2.5 μm, 3) single-object cross-dispersed slitless spectroscopy enabling simultaneous wavelength coverage between 0. 7 and 2.5 μm at Rrv660, a mode optimized for transit spectroscopy of relatively bright (J > 7) stars and, 4) sparse aperture interferometric imaging between 3.8 and 4.8 μm enabling high­ contrast ("' 10-4) imaging of M < 8 point sources at angular separations between 70 and 500 milliarcsec. This paper presents an overview of the FGS/NIRISS design with a focus on the scientific capabilities and performance offered by NIRISS.
    Proceedings of SPIE - The International Society for Optical Engineering 09/2012; DOI:10.1117/12.926578 · 0.20 Impact Factor
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    ABSTRACT: We present observational evidence for the inhibition of bar formation in dispersion-dominated (dynamically hot) galaxies by studying the relationship between galactic structure and host galaxy kinematics in a sample of 257 galaxies between 0.1 $<$ z $\leq$ 0.84 from the All-Wavelength Extended Groth Strip International Survey (AEGIS) and the Deep Extragalactic Evolutionary Probe 2 (DEEP2) survey. We find that bars are preferentially found in galaxies that are massive and dynamically cold (rotation-dominated) and on the stellar Tully-Fisher relationship, as is the case for barred spirals in the local Universe. The data provide at least one explanation for the steep ($\times$3) decline in the overall bar fraction from z=0 to z=0.84 in L$^*$ and brighter disks seen in previous studies. The decline in the bar fraction at high redshift is almost exclusively in the lower mass (10 $<$ log M$_{*}$(\Msun)$<$ 11), later-type and bluer galaxies. A proposed explanation for this "downsizing" of the bar formation / stellar structure formation is that the lower mass galaxies may not form bars because they could be dynamically hotter than more massive systems from the increased turbulence of accreting gas, elevated star formation, and/or increased interaction/merger rate at higher redshifts. The evidence presented here provides observational support for this hypothesis. However, the data also show that not every disk galaxy that is massive and cold has a stellar bar, suggesting that mass and dynamic coldness of a disk are necessary but not sufficient conditions for bar formation -- a secondary process, perhaps the interaction history between the dark matter halo and the baryonic matter, may play an important role in bar formation.
    The Astrophysical Journal 08/2012; 758(2). DOI:10.1088/0004-637X/758/2/136 · 6.28 Impact Factor
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    ABSTRACT: Recent deep Hubble Space Telescope WFC3 imaging suggests that a majority of compact quiescent massive galaxies at z~2 may contain disks. To investigate this claim, we have compared the ellipticity distribution of 31 carefully selected high-redshift massive quiescent compact galaxies to a set of mass-selected ellipticity and Sersic index distributions obtained from 2D structural fits to ~40,000$ nearby galaxies from the Sloan Digital Sky Survey. A Kolmogorov-Smirnov test shows that the distribution of ellipticities for the high-redshift galaxies is consistent with the ellipticity distribution of a similarly chosen sample of massive early-type galaxies. However the distribution of Sersic indices for the high-redshift sample is inconsistent with that of local early-type galaxies, and instead resembles that of local disk-dominated populations. The mismatch between the properties of high-redshift compact galaxies and those of both local early-type and disk-dominated systems leads us to conclude that the basic structures of high-redshift compact galaxies probably do not closely resemble those of any single local galaxy population. Any galaxy population analog to the high-redshift compact galaxies that exists at the current epoch is either a mix of different types of galaxies, or possibly a unique class of objects on their own.
    The Astrophysical Journal Letters 06/2012; 754(2). DOI:10.1088/2041-8205/754/2/L24 · 5.60 Impact Factor
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    ABSTRACT: We present the first observational evidence for the inhibition of bar formation in dispersion dominated (dynamically hot) galaxies. We compare the presence of galactic structures (bars) and the host galaxy kinematics in a sample of 200 disk galaxies from the All-Wavelength Extended Groth Strip International Survey (AEGIS) and the Deep Extragalactic Evolutionary Probe 2 (DEEP2) survey. We find that bars are preferentially found in galaxies that lie on the Tully-Fisher relationship and are rotation-dominated, whereas few bars are found in galaxies that are dispersion dominated. The data provide at least one explanation for the steep (x3) decline in the bar fraction from z=0 to z=0.84 previously observed in L* disk galaxies in the COSMOS field. In the COSMOS data, the decline in bars is primarily found in the low mass, late-type systems. A proposed explanation for the trend was that at higher redshifts, lower mass systems were more dispersion dominated because they were more easily harassed by the increased interaction and merger rate. The data presented here provides observational support for this hypothesis.
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    ABSTRACT: We present an analysis of the size growth seen in early-type galaxies over 10 Gyr of cosmic time. Our analysis is based on a homogeneous synthesis of published data from 17 spectroscopic surveys observed at similar spatial resolution, augmented by new measurements for galaxies in the Gemini Deep Deep Survey. In total, our sample contains structural data for 465 galaxies (mainly early-type) in the redshift range 0.2<z<2.7. The size evolution of passively-evolving galaxies over this redshift range is gradual and continuous, with no evidence for an end or change to the process around z~1, as has been hinted at by some surveys which analyze subsets of the data in isolation. The size growth appears to be independent of stellar mass, with the mass-normalized half-light radius scaling with redshift as R_e (1+z)^(-1.62 +/- 0.34). Surprisingly, this power law seems to be in good agreement with the recently reported continuous size evolution of UV-bright galaxies in the redshift range z~0.5-3.5. It is also in accordance with the predictions from recent theoretical models.
    The Astrophysical Journal Letters 08/2011; 739(2). DOI:10.1088/2041-8205/739/2/L44 · 5.60 Impact Factor
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    ABSTRACT: In this paper we present the coordinates of 67 55' x 55' patches of sky which have the rare combination of both high stellar surface density (>0.5 arcmin^{-2} with 13<R<16.5 mag) and low extinction (E(B-V)<0.1). These fields are ideal for adaptive-optics based follow-up of extragalactic targets. One region of sky, situated near Baade's Window, contains most of the patches we have identified. Our optimal field, centered at RA: 7h24m3s, Dec: -1deg27'15", has an additional advantage of being accessible from both hemispheres. We propose a figure of merit for quantifying real-world adaptive optics performance, and use this to analyze the performance of multi-conjugate adaptive optics in these fields. We also compare our results to those that would be obtained in existing deep fields. In some cases adaptive optics observations undertaken in the fields given in this paper would be orders of magnitude more efficient than equivalent observations undertaken in existing deep fields.
    Publications of the Astronomical Society of the Pacific 01/2011; 123(901). DOI:10.1086/658931 · 3.23 Impact Factor
  • Roberto G. Abraham
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    ABSTRACT: In keeping with the spirit of a meeting on ‘masks,’ this talk presents two short stories on the theme of dust. In the first, dust plays the familiar role of the evil obscurer, the enemy to bedefeated by the cunning observer in order to allow a key future technology (adaptive optics) to be exploited fully by heroic astronomers. In the second story, dust itself emerges as the improbable hero, in the form of a circumstellar debris disks. I will present evidence of a puzzling near-infrared excess in the continuum of high-redshift galaxies and will argue that the seemingly improbable origin of this IR excess is a population of young circumstellar disks formed around high-mass stars in distant galaxies. Assuming circumstellar disks extend down to lower masses,as they do in our own Galaxy, the excess emission presents us with an exciting opportunity to measure the formation rate of planetary systems in distant galaxies at cosmic epochs before our own solar system formed.
    12/2010: pages 409-424;

Publication Stats

5k Citations
652.73 Total Impact Points

Institutions

  • 1994–2014
    • University of Toronto
      • Department of Astronomy and Astrophysics
      Toronto, Ontario, Canada
  • 2008
    • San Francisco State University
      • Department of Physics and Astronomy
      San Francisco, California, United States
  • 2007
    • Carnegie Institution for Science
      Washington, West Virginia, United States
  • 2003–2004
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, MD, United States
  • 1992–2002
    • University of Cambridge
      • Institute of Astronomy
      Cambridge, ENG, United Kingdom
  • 1998
    • National Research Council Canada
      • Dominion Radio Astrophysical Observatory
      Ottawa, Ontario, Canada
    • University of Colorado at Boulder
      • Center for Astrophysics and Space Astronomy
      Boulder, Colorado, United States
  • 1997
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 1993
    • Durham University
      Durham, England, United Kingdom
    • Government of British Columbia, Canada
      Vancouver, British Columbia, Canada
  • 1991
    • University of Southampton
      • Department of Physics and Astronomy
      Southampton, England, United Kingdom