Spencer A. Stanford

University of Hertfordshire, Hatfield, England, United Kingdom

Are you Spencer A. Stanford?

Claim your profile

Publications (16)63.41 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present 4.5 {\mu}m luminosity functions for galaxies identified in 178 candidate galaxy clusters at 1.3 < z < 3.2. The clusters were identified as Spitzer/IRAC color-selected overdensities in the Clusters Around Radio-Loud AGN (CARLA) project, which imaged 421 powerful radio-loud AGN at z > 1.3. The luminosity functions are derived for different redshift and richness bins, and the IRAC imaging reaches depths of m*+2, allowing us to measure the faint end slopes of the luminosity functions. We find that {\alpha} = -1 describes the luminosity function very well in all redshifts bins and does not evolve significantly. This provides evidence that the rate at which the low mass galaxy population grows through star formation, gets quenched and is replenished by in-falling field galaxies does not have a major net effect on the shape of the luminosity function. Our measurements for m* are consistent with passive evolution models and high formation redshifts z_f ~ 3. We find a slight trend towards fainter m* for the richest clusters, implying that the most massive clusters in our sample could contain older stellar populations, yet another example of cosmic downsizing. Modelling shows that a contribution of a star-forming population of up to 40% cannot be ruled out. This value, found from our targeted survey, is significantly lower than the values found for slightly lower redshift, z ~ 1, clusters found in wide-field surveys. The results are consistent with cosmic downsizing, as the clusters studied here were all found in the vicinity of radio-loud AGNs -- which have proven to be preferentially located in massive dark matter halos in the richest environments at high redshift -- they may therefore be older and more evolved systems than the general protocluster population.
    The Astrophysical Journal 03/2014; 786(1). · 6.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: By combining data from the NASA Wide-field Infrared Survey Explorer (WISE) mission with optical spectroscopy from the W. M. Keck telescope, we discover a mid-IR color criterion that yields a 78% success rate in identifying rare, typically radio-quiet, 1.6 z 4.6 dusty Lyα emitters (LAEs). Of these, at least 37% have emission extended on scales of 30-100 kpc and are considered Lyα "blobs" (LABs). The objects have a surface density of only ~0.1 deg–2, making them rare enough that they have been largely missed in deep, small area surveys. We measured spectroscopic redshifts for 92 of these galaxies, and find that the LAEs (LABs) have a median redshift of 2.3 (2.5). The WISE photometry coupled with data from Herschel (Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA) reveals that these galaxies are in the Hyper Luminous IR galaxy regime (L IR 1013-1014 L ☉) and have warm colors. They are typically more luminous and warmer than other dusty, z ~ 2 populations such as submillimeter-selected galaxies and dust-obscured galaxies. These traits are commonly associated with the dust being illuminated by intense active galactic nucleus activity. We hypothesize that the combination of spatially extended Lyα, large amounts of warm IR-luminous dust, and rarity (implying a short-lived phase) can be explained if the galaxies are undergoing brief, intense "feedback" transforming them from an extreme dusty starburst/QSO into a mature galaxy.
    The Astrophysical Journal 05/2013; 769(2):91. · 6.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report the first results from the Clusters Around Radio-Loud AGN program, a Cycle 7 and 8 Spitzer Space Telescope snapshot program to investigate the environments of a large sample of obscured and unobscured luminous radio-loud active galactic nuclei (AGNs) at 1.2 < z < 3.2. These data, obtained for 387 fields, reach 3.6 and 4.5 μm depths of [3.6]AB = 22.6 and [4.5]AB = 22.9 at the 95% completeness level, which is two to three times fainter than L* in this redshift range. By using the color cut [3.6] – [4.5] > –0.1 (AB), which efficiently selects high-redshift (z > 1.3) galaxies of all types, we identify galaxy cluster member candidates in the fields of the radio-loud AGN. The local density of these Infrared Array Camera (IRAC)-selected sources is compared to the density of similarly selected sources in blank fields. We find that 92% of the radio-loud AGN reside in environments richer than average. The majority (55%) of the radio-loud AGN fields are found to be overdense at a ≥2σ level; 10% are overdense at a ≥5σ level. A clear rise in surface density of IRAC-selected sources toward the position of the radio-loud AGN strongly supports an association of the majority of the IRAC-selected sources with the radio-loud AGN. Our results provide solid statistical evidence that radio-loud AGN are likely beacons for finding high-redshift galaxy (proto-)clusters. We investigate how environment depends on AGN type (unobscured radio-loud quasars versus obscured radio galaxies), radio luminosity and redshift, finding no correlation with either AGN type or radio luminosity. We find a decrease in density with redshift, consistent with galaxy evolution for this uniform, flux-limited survey. These results are consistent with expectations from the orientation-driven AGN unification model, at least for the high radio luminosity regimes considered in this sample.
    The Astrophysical Journal 05/2013; 769(1):79. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We measure the faint end slope of the galaxy luminosity function (LF) for cluster galaxies at 1<z<1.5 using Spitzer IRAC data. We investigate whether this slope, alpha, differs from that of the field LF at these redshifts, and with the cluster LF at low redshifts. The latter is of particular interest as low-luminosity galaxies are expected to undergo significant evolution. We use seven high-redshift spectroscopically confirmed galaxy clusters drawn from the IRAC Shallow Cluster Survey to measure the cluster galaxy LF down to depths of M* + 3 (3.6 microns) and M* + 2.5 (4.5 microns). The summed LF at our median cluster redshift (z=1.35) is well fit by a Schechter distribution with alpha[3.6] = -0.97 +/- 0.14 and alpha[4.5] = -0.91 +/- 0.28, consistent with a flat faint end slope and is in agreement with measurements of the field LF in similar bands at these redshifts. A comparison to alpha in low-redshift clusters finds no statistically significant evidence of evolution. Combined with past studies which show that M* is passively evolving out to z~1.3, this means that the shape of the cluster LF is largely in place by z~1.3. This suggests that the processes that govern the build up of the mass of low-mass cluster galaxies have no net effect on the faint end slope of the cluster LF at z<1.3.
    The Astrophysical Journal 11/2012; 761(2). · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present Spitzer 3.6 and 4.5 μm photometry and positions for a
    The Astronomical Journal 11/2012; 144:148. · 4.97 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on the results of an optical spectroscopic survey at high Galactic latitude (b\geq30{\deg}) of a sample of WISE-selected targets, grouped by WISE W1 ({\lambda}_eff = 3.4 \mum) flux, which we use to characterize the sources WISE detected. We observed 762 targets in 10 disjoint fields centered on ultra-luminous infrared galaxy (ULIRG) candidates using the DEIMOS spectrograph on Keck II. We find 0.30 \pm 0.02 galaxies arcmin^-2 with a median redshift of z=0.33 \pm 0.01 for the sample with W1 \geq 120 \muJy. The foreground stellar densities in our survey range from 0.23 \pm 0.07 arcmin^(-2) to 1.1 \pm 0.1 arcmin^(-2) for the same sample. We obtained spectra that produced science grade redshifts for \geq 90% of our targets for sources with W1 flux \geq 120 \muJy that also had i-band flux \geq 18 \muJy. We used for targeting very preliminary data reductions available to the team in August of 2010. Our results therefore present a conservative estimate of what is possible to achieve using WISE's Preliminary Data Release for the study of field galaxies.
    The Astronomical Journal 11/2011; 143(1). · 4.97 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present results from a systematic study of star formation in local galaxy clusters using 22 micron data from the Wide-field Infrared Survey Explorer (WISE). The 69 systems in our sample are drawn from the Cluster Infall Regions Survey (CIRS), and all have robust mass determinations. The all-sky WISE data enables us to quantify the amount of star formation, as traced by 22 micron, as a function of radius well beyond R200, and investigate the dependence of total star formation rate upon cluster mass. We find that the fraction of star-forming galaxies increases with cluster radius but remains below the field value even at 3 R200. We also find that there is no strong correlation between the mass-normalized total specific star formation rate and cluster mass, indicating that the mass of the host cluster does not strongly influence the total star formation rate of cluster members.
    The Astrophysical Journal 08/2011; 743. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present the discovery of only the second radio-selected z ~ 6 quasar. We identified SDSS J222843.54+011032.2 (z = 5.95) by matching the optical detections of the deep Sloan Digital Sky Survey Stripe 82 with their radio counterparts in the Stripe 82 Very Large Array Survey. We also matched the Canadian-France-Hawaiian Telescope Legacy Survey Wide with the Faint Images of the Radio Sky at Twenty cm survey but have yet to find any z ~ 6 quasars in this survey area. The discovered quasar is optically faint, z = 22.3 and M 1450 ~ –24.5, but radio bright, with a flux density of f 1.4 GHz, peak = 0.31 mJy and a radio loudness of R ~ 1100 (where R ≡ f 5 GHz/f 2500). The i – z color of the discovered quasar places it outside the color selection criteria for existing optical surveys. We conclude by discussing the need for deeper wide-area radio surveys in the context of high-redshift quasars.
    The Astrophysical Journal 07/2011; 736(1):57. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report two new low-metallicity blue compact dwarf galaxies (BCDs), WISEP J080103.93+264053.9 (hereafter W0801+26) and WISEP J170233.53+180306.4 (hereafter W1702+18), discovered using the Wide-field Infrared Survey Explorer (WISE). We identified these two BCDs from their extremely red colors at mid-infrared wavelengths and obtained follow-up optical spectroscopy using the Low Resolution Imaging Spectrometer on Keck I. The mid-infrared properties of these two sources are similar to the well-studied, extremely low metallicity galaxy SBS 0335-052E. We determine metallicities of 12 + log (O/H) = 7.75 and 7.63 for W0801+26 and W1702+18, respectively, placing them among a very small group of very metal deficient galaxies (Z ≤ 1/10 Z ☉). Their >300 Å Hβ equivalent widths, similar to SBS 0335-052E, imply the existence of young (<5 Myr) star-forming regions. We measure star formation rates of 2.6 and 10.9 M ☉ yr–1 for W0801+26 and W1702+18, respectively. These BCDs, showing recent star formation activity in extremely low metallicity environments, provide new laboratories for studying star formation in extreme conditions and are low-redshift analogs of the first generation of galaxies to form in the universe. Using the all-sky WISE survey, we discuss a new method to identify similar star-forming, low-metallicity BCDs.
    The Astrophysical Journal Letters 06/2011; 736(1):L22. · 6.35 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present composite 3.6 and 4.5 μm luminosity functions (LFs) for cluster galaxies measured from the Spitzer Deep, Wide-Field Survey for 0.3 < z < 2. We compare the evolution of m* for these LFs to models for passively evolving stellar populations to constrain the primary epoch of star formation in massive cluster galaxies. At low redshifts (z 1.3), our results agree well with models with no mass assembly and passively evolving stellar populations with a luminosity-weighted mean formation redshift zf = 2.4 assuming a Kroupa initial mass function (IMF). We conduct a thorough investigation of systematic biases that might influence our results, and estimate systematic uncertainties of Δzf = +0.16 –0.18 (model normalization), Δzf = +0.40 –0.05 (α), and Δzf = +0.30 –0.45 (choice of stellar population model). For a Salpeter-type IMF, the typical formation epoch is thus strongly constrained to be z ~ 2-3. Higher formation redshifts can only be made consistent with the data if one permits an evolving IMF that is bottom-light at high redshift, as suggested by van Dokkum. At high redshifts (z 1.3), we also witness a statistically significant (>5σ) disagreement between the measured LF and the continuation of the passive evolution model from lower redshifts. After considering potential systematic biases that might influence our highest redshift data points, we interpret the observed deviation as potential evidence for ongoing mass assembly at this epoch.
    The Astrophysical Journal 08/2010; 720(1):284. · 6.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We request 4 nights to obtain SOAR/OSIRIS near-IR photometry and spectroscopy follow-up observations of a sample of the extreme ultra- luminous infrared galaxy (ULIRG) candidates at z > 1 selected using infrared data from the Wide-Field Infrared Survey Explorer (WISE). One of the two primary science goals of WISE, a NASA MIDEX mission currently surveying the entire sky at 3.4, 4.6, 12 and 22 micron, is to identify such extreme ULIRGs (the other is to identify cool brown dwarfs). The candidates are selected to have mid-IR colors indicating starburst- dominated ULIRGs at z=1.2 - 3, but 100 times more luminous than local ULIRGs with L_IR > 10^14L_&sun;. In combination with the mid-infrared photometry, the near-IR photometric and spectroscopic observations will allow us to distinguish high-z targets from local red populations, determine the luminosity, and further study the star formation activity from hydrogen recombination lines, extinction toward the star formation regions, and SED modeling on the stellar population of these galaxies.
    NOAO Proposal. 08/2010;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We propose to image a single 12 square degree area with IRAC. The area is unique in that it is being imaged by three mm-wave experiments - SPT, ACT, and APEX - as they conduct Sunyaev-Zel'dovich Effect surveys for galaxy clusters. We have also begun to survey with XMM this same area to provide an X-ray selected galaxy cluster catalog. We have already obtained griz images of this area in an NOAO optical imaging survey to provide crucial photometric redshifts at z < 1. The proposed IRAC data will allow us to determine phot-z for the SZE cluster candidates up to at least z = 1.5, to uniformly select galaxies and AGN for carrying out clustering analyses, to estimate galaxy masses, and to cross check the SZE and X-ray cluster selection functions within the same area. Compared to other similar total areas surveyed by IRAC previously, our survey area has the distinct advantages of 1) being targetted by the new generation of mm-wave surveys, and 2) being the target of moderately deep XMM imaging.
    Spitzer Proposal. 05/2007;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Spitzer is uniquely able to provide the data we need to make an unbiased estimate of the total amount of current star formation in galaxies in protoclusters. Such information is crucial to understanding the relative formation timescales of the stellar populations, their galaxies, and even the intracluster medium of the host protoclusters. We propose to obtain deep MIPS 24 micron observations of the protocluster MRC 1138-262 at z=2.1. The rest frame wavelengths sampled by the MIPS bandpass will contain the strong PAH feature at 7.7 microns, so the data will be sensitive to current star formation. The proposed depth would detect an L^* luminous IR galaxy with L(IR) = 10^{11} L_sun at the 5 sigma level.
    Spitzer Proposal. 06/2005;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We propose to complete the mapping of the Lynx supercluster at z=1.27 in the mid-infrared, to cover a large range of environments that will include newly discovered galaxy groups. The proposed observations would obtain IRAC data on the 7 recently discovered groups, and MIPS 24 micron imaging of all the groups and the two central clusters. The two short wavelength IRAC bands sample the rest-frame near-IR, where the SED of old stellar populations peaks, so these data are useful for constraining estimates of stellar masses. Knowledge of the way that the stellar mass function varies with local galaxy density at z > 1 would be valuable for comparisons with the results for the low z universe derived from SDSS. We will combine the four IRAC bands with our optical and NIR data to measure accurate spectral energy distributions, to which evolutionary spectral synthesis models can be fit in order to estimate the stellar population ages and masses, as well as refined photometric redshifts. We will use the MIPS 24 micron observations to measure star-formation activity as a function of local environment in the supercluster to investigate the location of star formation during the transitions of field galaxies through groups and into clusters.
    Spitzer Proposal. 06/2005;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We propose to obtain deep IRAC images in all four bands of a z~3 proto galaxy cluster. We have spectroscopically identified 31 star forming galaxies at the redshift of a z=3.16 radio galaxy, MRC0316-257. The IRAC data will sample the rest frame near-IR in the member galaxies, allowing an estimate to be made of the stellar masses. Combined with our existing optical and near-IR photometry, the IRAC data will allow us to search for non-star-forming galaxies in the protocluster, which should be easily detectable if their stellar populations formed at z ~ 5 or greater. Our target lies in the redshift gap between the z ~ 2 and z ~ 4 protoclusters targetted by GTO programs in Cycle 1. So by combining our data with those in the archive, we will be able to determine the formation history of massive galaxies in clusters as they virialize from z ~ 4 to 2, and test the predictions of the hierarchical vs monolithic collapse scenarios.
    Spitzer Proposal. 09/2004;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We propose to conduct an IRAC survey of a contiguous 100 deg2 area within the deepest South Pole Telescope survey field. The resulting IRAC dataset will be used in combination with Sunyaev-Zel'dovich information from the SPT to study the complete baryon budget in dense environments at all redshifts, to measure halo masses at 1 < z< 2 via lensing of the CMB, and to cross calibrate SZ, X-ray, and optical-IR methods of constructing galaxy cluster catalogs and estimating galaxy cluster masses. The target field will contain a publicly-available suite of X-ray, optical, NIR, FIR, and millimeter data, making it an ideal location to conduct a wide-field IRAC extragalactic legacy survey.

Publication Stats

24 Citations
63.41 Total Impact Points

Institutions

  • 2013
    • University of Hertfordshire
      • Centre for Astrophysics Research (CAR)
      Hatfield, England, United Kingdom
  • 2010–2013
    • University of California, Davis
      • Department of Physics
      Davis, California, United States