Matthew Ashby

Publications (2)0 Total impact

• Source

  Available from: Nimish P. Hathi

  Article: Luminous and High Stellar Mass Candidate Galaxies at z ~ 8 Discovered in CANDELS

  Haojing Yan, Steven L. Finkelstein, Kuang-Han Huang, Russell E. Ryan, Henry C. Ferguson, Anton M. Koekemoer, Norman A. Grogin, Mark Dickinson, Jeffrey A. Newman, Rachel S. Somerville, [......], Nimish P. Hathi, Giovanni G. Fazio, Matthew Ashby, Benjamin J. Weiner, Ray A. Lucas, Avishai Dekel, Laura Pentericci, Christopher J. Conselice, Dale D. Kocevski, Kamson Lai
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  ABSTRACT: One key goal of the Hubble Space Telescope Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey is to track galaxy evolution back to z ~ 8. Its two-tiered "wide and deep" strategy bridges significant gaps in existing near-infrared surveys. Here we report on z ~ 8 galaxy candidates selected as F105W-band dropouts in one of its deep fields, which covers 50.1 square arcmin to 4 ks depth in each of three near-infrared bands in the Great Observatories Origins Deep Survey southern field. Two of our candidates have J<26.2 mag, and are > 1 mag brighter than any previously known F105W-dropouts. We derive constraints on the bright-end of the rest-frame ultraviolet luminosity function of galaxies at z ~ 8, and show that the number density of such very bright objects is higher than expected from the previous Schechter luminosity function estimates at this redshift. Another two candidates are securely detected in Spitzer Infrared Array Camera images, which are the first such individual detections at z ~ 8. Their derived stellar masses are on the order of a few x 10^9 M_sun, from which we obtain the first measurement of the high-mass end of the galaxy stellar mass function at z ~ 8. The high number density of very luminous and very massive galaxies at z ~ 8, if real, could imply a large stellar-to-halo mass ratio and an efficient conversion of baryons to stars at such an early time.
  12/2011;
• Source

  Available from: Alaina Henry

  Article: Expanding the search for galaxies at z ~7-10 with new NICMOS Parallel Fields

  Alaina Henry, Brian Siana, Matthew Malkan, Matthew Ashby, Carrie Bridge, Ranga-Ram Chary, James Colbert, Mauro Giavalisco, Harry Teplitz, Patrick McCarthy
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  ABSTRACT: We have carried out a search for galaxies at z ~ 7-10 in ~14.4 sq. arcmin of new NICMOS parallel imaging taken in the Great Observatories Origins Deep Survey (GOODS, 5.9 sq. arcmin), the Cosmic Origins Survey (COSMOS, 7.2 sq. arcmin), and SSA22 (1.3 sq. arcmin). These images reach 5 sigma sensitivities of J110 = 26.0-27.5 (AB), and combined they increase the amount of deep near-infrared data by more than 60% in fields where the investment in deep optical data has already been made. We find no z>7 candidates in our survey area, consistent with the Bouwens et al. (2008) measurements at z~7 and 9 (over 23 sq. arcmin), which predict 0.7 galaxies at z~7 and <0.03 galaxies at z~9. We estimate that 10-20% of z>7 galaxies are missed by this survey, due to incompleteness from foreground contamination by faint sources. For the case of luminosity evolution, assuming a Schecter parameterization with a typical phi* = 10^-3 Mpc^-3, we find M* > -20.0 for z~7 and M* > -20.7 for z~9 (68% confidence). This suggests that the downward luminosity evolution of LBGs continues to z~7, although our result is marginally consistent with the z~6 LF of Bouwens et al.(2006, 2007). In addition we present newly-acquired deep MMT/Megacam imaging of the z~9 candidate JD2325+1433, first presented in Henry et al. (2008). The resulting weak but significant detection at i' indicates that this galaxy is most likely an interloper at z~2.7.
  03/2009;
• Article: Charting Cluster Mass Build-up using Luminous IR Galaxies

  Min Yun, Itziar Aretxaga, Matthew Ashby, Jason Austermann, Giovanni Fazio, Jiasheng Huang, David Hughes, James Lowenthal, Thushara Perera, Kimberly Scott, Grant Wilson, Josh Younger
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  ABSTRACT: An important legacy of Spitzer telescope is that it has established the increasing importance of IR luminous (LIRG) and ultraluminous IR galaxies (ULIRGs) in the cosmic star formation and mass assembly history to z>1. Hierarchical models of galaxy formation and evolution and the popular notion of ``downsizing'' suggest that so-called 'submillimeter galaxies' (SMGs) with L(IR)=10^{12-13}L_odot likely play an important role in the mass assembly history at z>1-2, particularly in high density environments. Using the AzTEC instrument on James Clerk Maxwell Telescope (JCMT) and Atacama Submillimeter Telescope Experiment (ASTE) telescope in Chile, we have constructed a new data base that includes 1000 SMGs (>2 times larger than all previous SMG surveys combined) covering both ``blank'' and biased/overdensity fields. The main aim of this Spitzer proposal is to map the process by which clusters build up their mass using luminous IR galaxies identified using MIPS 24 micron and AzTEC 1100 micron surveys and to put the SMG phenomenon in the broader context of galaxy and large scale structure evolution.
• Article: The Spitzer/MIPS Star Formation Reference Survey

  Giovanni Fazio, Matthew Ashby, Pauline Barmby, Sukanya Chakrabarti, Eduardo Gonzalez-Alfonso, Jia-Sheng Huang, Suzanne Madden, Kai Noeske, Michael Pahre, Casey Papovich, Thomas Robitaille, Howard Smith
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  ABSTRACT: We propose a statistically robust study of 380 nearby, bright star-forming galaxies of all types to better understand the nature of star formation. The goal of this MIPS reference survey will be to better understand global star formation via mid-IR continuum emission, with an emphasis on quantitative comparisons of multiple global star formation indicators including ultraviolet emission, H-alpha, and radio continuum measurements. The sample is selected to be fully representative of the entire ranges of infrared luminosity, dust temperature, and stellar mass exhibited by star-forming galaxies in the local universe: the sample galaxies exhibit all existing combinations of these properties with the minimum overall number, selected in a manner that allows results to be applied to the entire local galaxy population. Here we propose Spitzer/MIPS 24 micron photometry for the 267 out of 380 objects which lack suitable observations in the Spitzer archive. All sample galaxies already have extensive complementary data available including global ugrizJHK photometry plus radio continuum intensities. Most also have GALEX imaging; in addition we have already begun a ground-based campaign to acquire global H-alpha imaging for the complete sample. We are submitting this proposal in the context of a larger campaign that includes a GTO proposal to complete the four-band IRAC micron imaging, and a GO proposal to acquire the IRS low-resolution spectroscopy. Although these companion proposals will significantly increase the scientific return of our survey program, the success of this proposal is not contingent in any way on any other Spitzer proposal. Our international team is dedicated, experienced, and has adequate manpower and institutional resources, with expertise in all the relevant disciplines to ensure the success of this undertaking.