V. Desai

Paris Diderot University, Lutetia Parisorum, Île-de-France, France

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Publications (97)349 Total impact

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    ABSTRACT: We present a 48 orbit cycle-21 program to image three of the four Hubble Frontier Fields at near-ultraviolet wavelengths. Each lensing cluster will be observed for eight orbits in both the F275W and F336W filters. The primary science goals are to measure the luminosity functions of faint star-forming galaxies at 1.5<z<3, measure the Lyman continuum escape fractions, and study the sizes of star forming regions in galaxies at 1. Observations will likely begin in October. We will present the first images and compare to the existing optical Hubble images. In addition, we will show previous results from a deep near-UV imaging program in Abell 1689 as a demonstration of what can be done with these observations.
    01/2014;
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    ABSTRACT: Ultra-faint star-forming galaxies produce a significant fraction of global star formation rate density at high redshifts. The magnification provided by strong gravitational lensing from massive clusters enables us to detect the faint background galaxies that are beyond our current detection limits. Using the massive lensing cluster Abell 1689 along with deep HST/WFC3 ultraviolet imaging (30 orbits in the F275W filter), we find that the UV luminosity function is steep down to very faint magnitudes (MUV = -13 AB mag) and shows no turnover. Our new HST program images Abell 1689 for 10 and 14 orbits in F225W and F336W bands, respectively. We again use the Lyman break technique to select star-forming galaxies as F225W and F336W “dropouts” at z=1.5 and z=2.7, respectively. Finally, we end up with a large sample of ultra-faint star-forming galaxies at the peak epoch of star formation, 1 < z < 3. We study the evolution of the faint-end slope of the UV luminosity function as well as a variety of properties of faint star-forming galaxies in this sample. We also measure the Lyman continuum escape fraction in these feeble sources, as they play an important role in making up the ionizing background radiation at both intermediate redshifts (1<z<3) and the epoch of reionization (z>6).
    01/2014;
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    ABSTRACT: We explore the relationship between gas, dust and star formation in a sample of 12 ultra-luminous infrared galaxies (ULIRGs) at high redshift compared to a similar sample of local galaxies. We present new CO observations and/or Spitzer mid-IR spectroscopy for 6 70 micron selected galaxies at z~1 in order to quantify the properties of the molecular gas reservoir, the contribution of an active galactic nuclei (AGN) to the mid-IR luminosity and the star formation efficiency (SFE=LIR/L'CO). The mid-IR spectra show strong polycyclic aromatic hydrocarbon (PAH) emission and our spectral decomposition suggests that the AGN makes a minimal contribution (<25%) to the mid-IR luminosity. The 70 micron selected ULIRGs which we find to be spectroscopic close pairs, are observed to have high SFE, similar to local ULIRGs and high redshift submillimeter galaxies, consistent with enhanced IR luminosity due to an ongoing major merger. Combined with existing observations of local and high redshift ULIRGs, we further compare the PAH, IR and CO luminosities. We show that the ratio LPAH6.2/LIR decreases with increasing IR luminosity for both local and high redshift galaxies but the trend for high redshift galaxies is shifted to higher IR luminosities; the average LPAH6.2/LIR ratio at a given LIR is ~3 times higher at high redshift. When we normalize by the molecular gas, we find this trend to be uniform for galaxies at all redshifts and that the molecular gas is correlated with the PAH dust emission.The similar trends seen in the [CII] to molecular gas ratios in other studies suggests that PAH emission, like [CII], continues to be a good tracer of photodissociation regions even at high redshift. Together the CO, PAH and far-IR fine structure lines should be useful for constraining the interstellar medium conditions in high redshift galaxies.
    The Astrophysical Journal 06/2013; 772(2). · 6.73 Impact Factor
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    ABSTRACT: We use the Spitzer Space Telescope Enhanced Imaging Products (SEIP) and the Spitzer Archival Far-InfraRed Extragalactic Survey (SAFIRES) to study the spectral energy distributions of spectroscopically confirmed type 1 quasars selected from the Sloan Digital Sky Survey (SDSS). By combining the Spitzer and SDSS data with the 2-Micron All Sky Survey (2MASS) we are able to construct a statistically robust rest-frame 0.1-100 micron type 1 quasar template. We find the quasar population is well-described by a single power-law SED at wavelengths less than 20 microns, in good agreement with previous work. However, at longer wavelengths we find a significant excess in infrared luminosity above an extrapolated power-law, along with signifiant object-to-object dispersion in the SED. The mean excess reaches a maximum of 0.8 dex at rest-frame wavelengths near 100 microns.
    The Astrophysical Journal 03/2013; 768(1). · 6.73 Impact Factor
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    ABSTRACT: We have obtained a deep (30-orbit) Hubble Space Telescope WFC3/UVIS F275W image of Abell 1689, a massive cluster at z=0.183, and one of the best studied HST targets for lensing surveys. We use the new UV data, in conjunction with existing high resolution optical and IR data to constrain the residual star-formation in the cluster early-type galaxy population, determine the ages of young stellar populations, providing insight into the buildup of the cluster red sequence.
    01/2013;
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    ABSTRACT: The NASA/IPAC Infrared Science Archive (IRSA) curates both data and analysis tools from NASA's infrared missions. As part of our primary goal, we provide long term access to mission-specific software from projects such as IRAS and Spitzer. We will review the efforts by IRSA (and within the greater IPAC before that) to keep the IRAS and Spitzer software tools current and available. Data analysis tools are a vital part of the Spitzer Heritage Archive. The IRAS tools HIRES and SCANPI have been in continual use since the 1980's. Scanpi offers a factor of 2 to 5 gain in sensitivity over the IRAS Point Source Catalog by performing 1D scan averaging of raw survey data at specified arbitrary position. In 2007 SCANPI was completely modernized, with major code revisions. HIRES returns IRAS survey images with higher resolution than the IRAS Sky Survey Atlas (ISSA). We are currently undertaking a modest revision to the tool to ensure continued reliability. In the next two years, the US Planck Data Center plans to adapt both tools for use with Planck data, and deliver them to IRSA for long term curation.
    09/2012;
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    ABSTRACT: We present observations of CO J=2-1 line emission in infrared-luminous cluster galaxies at z~1 using the IRAM Plateau de Bure Interferometer. Our two primary targets are optically faint, dust-obscured galaxies (DOGs) found to lie within 2 Mpc of the centers of two massive (>10^14 Msun) galaxy clusters. CO line emission is not detected in either DOG. We calculate 3-sigma upper limits to the CO J=2-1 line luminosities, L'_CO < 6.08x10^9 and < 6.63x10^9 K km/s pc^2. Assuming a CO-to-H_2 conversion factor derived for ultraluminous infrared galaxies in the local Universe, this translates to limits on the cold molecular gas mass of M_H_2 < 4.86x10^9 Msun and M_H_2 < 5.30x10^9 Msun. Both DOGs exhibit mid-infrared continuum emission that follows a power-law, suggesting that an AGN contributes to the dust heating. As such, estimates of the star formation efficiencies in these DOGs are uncertain. A third cluster member with an infrared luminosity, L_IR < 7.4x10^11 Lsun, is serendipitously detected in CO J=2-1 line emission in the field of one of the DOGs located roughly two virial radii away from the cluster center. The optical spectrum of this object suggests that it is likely an obscured AGN, and the measured CO line luminosity is L'_CO = (1.94 +/- 0.35)x10^10 K km/s pc^2, which leads to an estimated cold molecular gas mass M_H_2 = (1.55+/-0.28)x10^10 Msun. A significant reservoir of molecular gas in a z~1 galaxy located away from the cluster center demonstrates that the fuel can exist to drive an increase in star-formation and AGN activity at the outskirts of high-redshift clusters.
    The Astrophysical Journal 04/2012; 752(2). · 6.73 Impact Factor
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    ABSTRACT: Dust-obscured galaxies (DOGs) are a subset of high-redshift (z \approx 2) optically-faint ultra-luminous infrared galaxies (ULIRGs, e.g. L_{IR} > 10^{12} Lsun). We present new far-infrared photometry, at 250, 350, and 500 um (observed-frame), from the Herschel Space Telescope for a large sample of 113 DOGs with spectroscopically measured redshifts. Approximately 60% of the sample are detected in the far-IR, confirming their high IR luminosities, which range from 10^{11.6} Lsun < L_{IR} (8-1000 um) <10^{13.6} Lsun. 90% of the Herschel detected DOGs in this sample are ULIRGs and 30% have L_{IR} > 10^{13} Lsun. The rest-frame near-IR (1 - 3 um) SEDs of the Herschel detected DOGs are predictors of their SEDs at longer wavelengths. DOGs with "power-law" SEDs in the rest-frame near-IR show observed-frame 250/24 um flux density ratios similar to the QSO-like local ULIRG, Mrk 231. DOGs with a stellar "bump" in their rest-frame near-IR show observed-frame 250/24 um flux density ratios similar to local star-bursting ULIRGs like NGC 6240. For the Herschel detected DOGs, accurate estimates (within \approx 25%) of total IR luminosity can be predicted from their rest-frame mid-IR data alone (e.g. from Spitzer observed-frame 24 um luminosities). Herschel detected DOGs tend to have a high ratio of infrared luminosity to rest-frame 8 um luminosity (the IR8= L_{IR}(8-1000 um)/v L_{v}(8 um) parameter of Elbaz et al. 2011). Instead of lying on the z=1-2 "infrared main-sequence" of star forming galaxies (like typical LIRGs and ULIRGs at those epochs) the DOGs, especially large fractions of the bump sources, tend to lie in the starburst sequence. While, Herschel detected DOGs are similar to scaled up versions of local ULIRGs in terms of 250/24 um flux density ratio, and IR8, they tend to have cooler far-IR dust temperatures (20-40 K for DOGs vs. 40-50 K for local ULIRGs). Abridged.
    The Astronomical Journal 03/2012; 143(5). · 4.97 Impact Factor
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    ABSTRACT: The Spitzer Science Center and NASA Infrared Science Archive (IRSA) will produce and release a set of Enhanced Imaging Products from the Spitzer Heritage Archive. We anticipate a preliminary release of a subset of the data in time for the January 2012 AAS. A release of the full set of products for the Spitzer cryogenic mission will fall in mid-to-late 2012. These products will include enhanced mosaics created using data from multiple programs where appropriate and a source list (SL) of photometry for compact sources. The primary requirement on the SL is very high reliability -- with areal coverage, completeness and limiting depth being secondary considerations. The enhanced imaging products will include data from the four channels of IRAC (3-8 microns) and the 24 micron channel of MIPS. The products will be generated for Spitzer observations of about 1500 square degrees and include around 30 million sources.
    01/2012;
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    ABSTRACT: The primary goal of the Local Cluster Survey is to measure the variations in the spatial extent of cold disk gas relative to the stellar disk for approximately 400 low-redshift group and cluster galaxies (z < 0.037) in order to quantify the relative importance of the physical mechanisms that cause galaxies to evolve from blue, actively star-forming galaxies to red, passive galaxies. The sample consists of 9 groups and clusters that span a range of X-ray luminosities, and all have optical photometry and spectroscopy from the SDSS, infrared 24-micron imaging from the Spitzer Space Telescope, and radio data from the ALFALFA survey. The wide areal coverage of these data allows us to track the evolution of disk gas from the dense cluster core to the surrounding field. Here we present a preliminary analysis of the stellar and gas radial profiles for the group and cluster galaxies. We compare with those of the surrounding field galaxies to look for signatures of environmentally-driven gas depletion.
    01/2012;
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    ABSTRACT: The Spitzer Space Telescope has identified a population of ultraluminous infrared galaxies (ULIRGs) at z ~ 2 that may play an important role in the evolution of massive galaxies. We measure the stellar masses (M *) of two populations of Spitzer-selected ULIRGs that have extremely red R – [24] colors (dust-obscured galaxies, or DOGs) and compare our results with submillimeter-selected galaxies (SMGs). One set of 39 DOGs has a local maximum in their mid-infrared (mid-IR) spectral energy distribution (SED) at rest frame 1.6 μm associated with stellar emission ("bump DOGs"), while the other set of 51 DOGs have power-law mid-IR SEDs that are typical of obscured active galactic nuclei ("power-law DOGs"). We measure M * by applying Charlot & Bruzual stellar population synthesis models to broadband photometry in the rest-frame ultraviolet, optical, and near-infrared of each of these populations. Assuming a simple stellar population and a Chabrier initial mass function, we find that power-law DOGs and bump DOGs are on average a factor of 2 and 1.5 more massive than SMGs, respectively (median and inter-quartile M * values for SMGs, bump DOGs, and power-law DOGs are log(M */M ☉) = 10.42+0.42 – 0.36, 10.62+0.36 – 0.32, and 10.71+0.40 – 0.34, respectively). More realistic star formation histories drawn from two competing theories for the nature of ULIRGs at z ~ 2 (major merger versus smooth accretion) can increase these mass estimates by up to 0.5 dex. A comparison of our stellar masses with the instantaneous star formation rate (SFR) in these z ~ 2 ULIRGs provides a preliminary indication supporting high SFRs for a given M *, a situation that arises more naturally in major mergers than in smooth accretion-powered systems.
    The Astrophysical Journal 12/2011; 744(2):150. · 6.73 Impact Factor
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    ABSTRACT: The Spitzer Space Telescope has identified a population of ultra-luminous infrared galaxies (ULIRGs) at z ∼ 2 that may play an important role in the evolution of massive galaxies. We measure the stellar masses (M *) of two populations of Spitzer-selected ULIRGs that have extremely red R − [24] colors (dust-obscured galaxies, or DOGs) and compare our results with sub-millimeter selected galaxies (SMGs). One set of 39 DOGs have a local maximum in their mid-infrared (mid-IR) spectral energy distribution (SED) at rest-frame 1.6µm associated with stellar emission ("bump DOGs"), while the other set of 51 DOGs have power-law mid-IR SEDs that are typical of obscured AGN ("power-law DOGs"). We measure M * by applying Charlot & Bruzual stellar population synthesis models to broad-band photometry in the rest-frame ultra-violet, optical, and near-infrared of each of these populations. Assuming a simple stellar population and a Chabrier initial mass function (IMF), we find that power-law DOGs and bump DOGs are on average a factor of 2 and 1.5 larger than SMGs, respectively (median and inter-quartile M * values for SMGs, bump DOGs and power-law DOGs are log(M * /M ⊙) = 10.42 +0.42 −0.36 , 10.62 +0.36 −0.32 , and 10.71 +0.40 −0.34 , respectively). More realistic star-formation histories drawn from two competing theories for the nature of ULIRGs at z ∼ 2 (major merger vs. smooth accretion) can increase these mass estimates by up to 0.5 dex. A comparison of our stellar masses with the instantaneous star-formation rate (SFR) in these z ∼ 2 ULIRGs provides a preliminary indication supporting high SFRs for a given M * , a situation that arises more naturally in major mergers than in smooth accretion powered systems.
    12/2011;
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    ABSTRACT: Exploiting the capabilities of four different surveys --- the Padova-Millennium Galaxy and Group Catalogue (PM2GC), the WIde-field Nearby Galaxy-cluster Survey (WINGS), the IMACS Cluster Building Survey (ICBS) and the ESO Distant Cluster Survey (EDisCS) --- we analyze the galaxy stellar mass distribution as a function of local density in mass-limited samples, in the field and in clusters from low (z>0.04) to high (z<0.8) redshift. We find that at all redshifts and in all environments, local density plays a role in shaping the mass distribution. In the field, it regulates the shape of the mass function at any mass above the mass limits. In clusters, it seems to be important only at low masses (log M_ast/M_sun <10.1 in WINGS and log M_ast/M_sun < 10.4 in EDisCS), otherwise it seems not to influence the mass distribution. Putting together our results with those of Calvi et al. and Vulcani et al. for the global environment, we argue that at least at $z\leq 0.8$ local density is more important than global environment in determining the galaxy stellar mass distribution, suggesting that galaxy properties are not much dependent of halo mass, but do depend on local scale processes.
    Monthly Notices of the Royal Astronomical Society 11/2011; · 5.52 Impact Factor
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    ABSTRACT: Detailed analysis of the substructure of Lya nebulae can put important constraints on the physical mechanisms at work and the properties of galaxies forming within them. Using high resolution HST imaging of a Lya nebula at z~2.656, we have taken a census of the compact galaxies in the vicinity, used optical/near-infrared colors to select system members, and put constraints on the morphology of the spatially-extended emission. The system is characterized by (a) a population of compact, low luminosity (~0.1 L*) sources --- 17 primarily young, small (Re~1-2 kpc), disky galaxies including an obscured AGN --- that are all substantially offset (>20 kpc) from the line-emitting nebula; (b) the lack of a central galaxy at or near the peak of the Lya emission; and (c) several nearly coincident, spatially extended emission components --- Lya, HeII, and UV continuum --- that are extremely smooth. These morphological findings are difficult to reconcile with theoretical models that invoke outflows, cold flows, or resonant scattering, suggesting that while all of these physical phenomena may be occurring, they are not sufficient to explain the powering and large extent of Lya nebulae. In addition, although the compact galaxies within the system are irrelevant as power sources, the region is significantly overdense relative to the field galaxy population (by at least a factor of 4). These observations provide the first estimate of the luminosity function of galaxies within an individual Lya nebula system, and suggest that large Lya nebulae may be the seeds of galaxy groups or low-mass clusters.
    The Astrophysical Journal 11/2011; 752(2). · 6.73 Impact Factor
  • 07/2011;
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    ABSTRACT: We present 3.6 and 4.5 micron Spitzer IRAC imaging over 0.77 square degrees at the Virgo cluster core for the purpose of understanding the formation mechanisms of the low surface brightness intracluster light features. Instrumental and astrophysical backgrounds that are hundreds of times higher than the signal were carefully characterized and removed. We examine both intracluster light plumes as well as the outer halo of the giant elliptical M87. For two intracluster light plumes, we use optical colors to constrain their ages to be greater than 3 & 5 Gyr, respectively. Upper limits on the IRAC fluxes constrain the upper limits to the masses, and optical detections constrain the lower limits to the masses. In this first measurement of mass of intracluster light plumes we find masses in the range of 5.5 x 10^8 - 4.5 x 10^9 and 2.1 x 10^8 - 1.5 x 10^9 solar masses for the two plumes for which we have coverage. Given their expected short lifetimes, and a constant production rate for these types of streams, integrated over Virgo's lifetime, they can account for the total ICL content of the cluster implying that we do not need to invoke ICL formation mechanisms other than gravitational mechanisms leading to bright plumes. We also examined the outer halo of the giant elliptical M87. The color profile from the inner to outer halo of M87 (160 Kpc) is consistent with either a flat or optically blue gradient, where a blue gradient could be due to younger or lower metallicity stars at larger radii. The similarity of the age predicted by both the infrared and optical colors (> few Gyr) indicates that the optical measurements are not strongly affected by dust extinction.
    The Astrophysical Journal 04/2011; 735(2). · 6.73 Impact Factor
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    The Astronomical Journal 04/2011; 141:141. · 4.97 Impact Factor
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    ABSTRACT: The sample of galaxies analyzed in this paper consists of spectroscopic early-type objects. We considered the flux-calibrated spectra reduced in Halliday et al. (2004, Cat. J/A+A/427/397) and Milvang-Jensen et al. (2008, Cat. J/A+A/482/419) of galaxies with early spectral type (1 or 2). (3 data files).
    VizieR Online Data Catalog. 04/2011;
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    ABSTRACT: We present Hubble Space Telescope (HST) imaging of 22 ultra-luminous infrared galaxies (ULIRGs) at z~2 with extremely red R-[24] colors (called dust-obscured galaxies, or DOGs) which have a local maximum in their spectral energy distribution (SED) at rest-frame 1.6um associated with stellar emission. These sources, which we call "bump DOGs", have star-formation rates of 400-4000 Msun/yr and have redshifts derived from mid-IR spectra which show strong polycyclic aromatic hydrocarbon emission --- a sign of vigorous on-going star-formation. Using a uniform morphological analysis, we look for quantifiable differences between bump DOGs, power-law DOGs (Spitzer-selected ULIRGs with mid-IR SEDs dominated by a power-law and spectral features that are more typical of obscured active galactic nuclei than starbursts), sub-millimeter selected galaxies (SMGs), and other less-reddened ULIRGs from the Spitzer extragalactic First Look Survey (XFLS). Bump DOGs are larger than power-law DOGs (median Petrosian radius of 8.4 +/- 2.7 kpc vs. 5.5 +/- 2.3 kpc) and exhibit more diffuse and irregular morphologies (median M_20 of -1.08 +/- 0.05 vs. -1.48 +/- 0.05). These trends are qualitatively consistent with expectations from simulations of major mergers in which merging systems during the peak star-formation rate period evolve from M_20 = -1.0 to M_20 = -1.7. Less obscured ULIRGs (i.e., non-DOGs) tend to have more regular, centrally peaked, single-object morphologies rather than diffuse and irregular morphologies. This distinction in morphologies may imply that less obscured ULIRGs sample the merger near the end of the peak star-formation rate period. Alternatively, it may indicate that the intense star-formation in these less-obscured ULIRGs is not the result of a recent major merger.
    03/2011;
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    ABSTRACT: Using data from the Spitzer Space Telescope, we analyze the mid-infrared (3-70 μm) spectral energy distributions of dry merger candidates in the Boötes field of the NOAO Deep Wide-Field Survey. These candidates were selected by previous authors to be luminous, red, early-type galaxies with morphological evidence of recent tidal interactions. We find that a significant fraction of these candidates exhibit 8 and 24 μm excesses compared to expectations for old stellar populations. We estimate that a quarter of dry merger candidates have mid-infrared-derived star formation rates greater than ~1 M ☉ yr–1. This represents a "frosting" on top of a large old stellar population, and has been seen in previous studies of elliptical galaxies. Further, the dry merger candidates include a higher fraction of star-forming galaxies relative to a control sample without tidal features. We therefore conclude that the star formation in these massive ellipticals is likely triggered by merger activity. Our data suggest that the mergers responsible for the observed tidal features were not completely dry, and may be minor mergers involving a gas-rich dwarf galaxy.
    The Astrophysical Journal 03/2011; 730(2):130. · 6.73 Impact Factor

Publication Stats

1k Citations
349.00 Total Impact Points

Institutions

  • 2013
    • Paris Diderot University
      Lutetia Parisorum, Île-de-France, France
  • 1925–2013
    • California Institute of Technology
      • Spitzer Science Center
      Pasadena, California, United States
  • 2009
    • University of Kansas
      • Department of Physics and Astronomy
      Lawrence, Kansas, United States
    • University of Sussex
      • Astronomy Centre
      Brighton, England, United Kingdom
  • 2006–2009
    • University of Washington Seattle
      • Department of Astronomy
      Seattle, WA, United States
    • Cornell University
      • Department of Astronomy
      Ithaca, New York, United States
  • 2005
    • University of Crete
      • Department of Physics
      Retimo, Crete, Greece