Browsing by Subject "ultraviolet: galaxies"

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CANDELS: The Contribution Of The Observed Galaxy Population To Cosmic Reionization
(2012-10) Finkelstein, Steven L.; Papovich, Casey; Ryan, Russell E.; Pawlik, Andreas H.; Dickinson, Mark; Ferguson, Henry C.; Finlator, Kristian; Koekemoer, Anton M.; Giavalisco, Mauro; Cooray, Asantha; Dunlop, James S.; Faber, Sandra M.; Grogin, Norman A.; Kocevski, Dale D.; Newman, Jeffrey A.; Finkelstein, Steven L.; Pawlik, Andreas H.
We present measurements of the specific ultraviolet luminosity density from a sample of 483 galaxies at 6 less than or similar to z less than or similar to 8. These galaxies were selected from new deep near-infrared Hubble Space Telescope imaging from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, Hubble UltraDeep Field 2009, and Wide Field Camera 3 Early Release Science programs. We investigate the contribution to reionization from galaxies that we observe directly, thus sidestepping the uncertainties inherent in complementary studies that have invoked assumptions regarding the intrinsic shape or the faint-end cutoff of the galaxy ultraviolet (UV) luminosity function. Due to our larger survey volume, wider wavelength coverage, and updated assumptions about the clumping of gas in the intergalactic medium (IGM), we find that the observable population of galaxies can sustain a fully reionized IGM at z = 6, if the average ionizing photon escape fraction (f(esc)) is similar to 30%. Our result contrasts with a number of previous studies that have measured UV luminosity densities at these redshifts that vary by a factor of five, with many concluding that galaxies could not complete reionization by z = 6 unless a large population of galaxies fainter than the detection limit were invoked, or extremely high values of f(esc) were present. The specific UV luminosity density from our observed galaxy samples at z = 7 and 8 is not sufficient to maintain a fully reionized IGM unless f(esc) > 50%. We examine the contribution from galaxies in different luminosity ranges and find that the sub-L* galaxies we detect are stronger contributors to the ionizing photon budget than the L > L* population, unless f(esc) is luminosity dependent. Combining our observations with constraints on the emission rate of ionizing photons from Ly alpha forest observations at z = 6, we find that we can constrain f(esc) < 34% (2 sigma) if the observed galaxies are the only contributors to reionization, or < 13% (2 sigma) if the luminosity function extends to a limiting magnitude of M-UV = -13. These escape fractions are sufficient to sustain an ionized IGM by z = 6. Current constraints on the high-redshift galaxy population imply that the volume ionized fraction of the IGM, while consistent with unity at z <= 6, appears to drop at redshifts not much higher than 7, consistent with a number of complementary reionization probes. If faint galaxies dominated the ionizing photon budget at z = 6-7, future extremely deep observations with the James Webb Space Telescope will probe deep enough to directly observe them, providing an indirect constraint on the global ionizing photon escape fraction.
CANDELS: The Evolution Of Galaxy Rest-Frame Ultraviolet Colors From Z=8 To 4
(2012-09) Finkelstein, Steven L.; Papovich, Casey; Salmon, Brett; Finlator, Kristian; Dickinson, Mark; Ferguson, Henry C.; Giavalisco, Mauro; Koekemoer, Anton M.; Reddy, Naveen A.; Bassett, Robert; Conselice, Christopher J.; Dunlop, James S.; Faber, S. M.; Grogin, Norman A.; Hathi, Nimish P.; Kocevski, Dale D.; Lai, Kamson; Lee, Kyoung-Soo; McLure, Ross J.; Mobasher, Bahram; Newman, Jeffrey A.; Finkelstein, Steven L.
We study the evolution of galaxy rest-frame ultraviolet (UV) colors in the epoch 4 less than or similar to z less than or similar to 8. We use new wide-field near-infrared data in the Great Observatories Origins Deep Survey-South field from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, Hubble Ultra Deep Field (HUDF) 2009, and Early Release Science programs to select galaxies via photometric redshift measurements. Our sample consists of 2812 candidate galaxies at z greater than or similar to 3.5, including 113 at z similar or equal to 7-8. We fit the observed spectral energy distribution to a suite of synthetic stellar population models and measure the value of the UV spectral slope (beta) from the best-fit model spectrum. We run simulations to show that this measurement technique results in a smaller scatter on beta than other methods, as well as a reduced number of galaxies with catastrophically incorrect beta measurements (i.e.,. Delta beta > 1). We find that the median value of beta evolves significantly from -1.82(-0.04)(+0.00) at z = 4 to -2.37(-0.06)(+0.26) at z = 7. Additionally, we find that faint galaxies at z = 7 have beta = -2.68(-0.24)(+0.39) (similar to-2.4 after correcting for observational bias); this is redder than previous claims in the literature and does not require "exotic" stellar populations (e. g., very low metallicities or top-heavy initial mass functions) to explain their colors. This evolution can be explained by an increase in dust extinction, from low amounts at z = 7 to A(V) similar to 0.5 mag at z = 4. The timescale for this increase is consistent with low-mass asymptotic giant branch stars forming the bulk of the dust. We find no significant (<2 sigma) correlation between beta and M-UV when measuring M-UV at a consistent rest-frame wavelength of 1500 angstrom. This is particularly true at bright magnitudes, though our results do show evidence for a weak correlation at faint magnitudes when galaxies in the HUDF are considered separately, hinting that dynamic range in sample luminosities may play a role. We do find a strong correlation between beta and the stellar mass at all redshifts, in that more massive galaxies exhibit redder colors. The most massive galaxies in our sample have similarly red colors at each redshift, implying that dust can build up quickly in massive galaxies and that feedback is likely removing dust from low-mass galaxies at z >= 7. Thus, the stellar-mass-metallicity relation, previously observed up to z similar to 3, may extend out to z = 7-8.
The Event Horizon Of M87
(2015-06) Broderick, Avery E.; Narayan, Ramesh; Kormendy, John; Perlman, Eric S.; Rieke, Marcia J.; Doeleman, Sheperd S.; Kormendy, John
The 6 x 10(9) M-circle dot supermassive black hole at the center of the giant elliptical galaxy M87 powers a relativistic jet. Observations at millimeter wavelengths with the Event Horizon Telescope have localized the emission from the base of this jet to angular scales comparable to the putative black hole horizon. The jet might be powered directly by an accretion disk or by electromagnetic extraction of the rotational energy of the black hole. However, even the latter mechanism requires a confining thick accretion disk to maintain the required magnetic flux near the black hole. Therefore, regardless of the jet mechanism, the observed jet power in M87 implies a certain minimum mass accretion rate. If the central compact object in M87 were not a black hole but had a surface, this accretion would result in considerable thermal near-infrared and optical emission from the surface. Current flux limits on the nucleus of M87 strongly constrain any such surface emission. This rules out the presence of a surface and thereby provides indirect evidence for an event horizon.
The Inhomogeneous Background Of H-2-Dissociating Radiation During Cosmic Reionization
(2009-04) Ahn, Kyungjin; Shapiro, Paul R.; Iliev, Illian T.; Mellema, Garrelt; Pen, Ue-Li; Shapiro, Paul R.
The first, self-consistent calculations are presented of the cosmological, H-2-dissociating UV background produced during the epoch of reionization by the sources of reionization. Large-scale radiative transfer simulations of reionization trace the impact of all the ionizing starlight on the intergalactic medium (IGM) from all the sources in our simulation volume down to dwarf galaxies of mass similar to 10(8) M-circle dot, identified by very high resolution N-body simulations, including the self-regulating effect of IGM photoheating on dwarf galaxy formation. The UV continuum emitted below 13.6 eV by each source is then transferred through the same IGM, attenuated by atomic H Lyman series resonance lines, to predict the evolution of the inhomogeneous radiation background in the Lyman-Werner (LW) bands of H-2 between 11 and 13.6 eV. On average, the intensity of this LW background is found to rise to the threshold level at which dissociation suppresses H-2 cooling and star formation inside minihalos, long before reionization is complete. Spatial variations in the LW background are found which result from the clustering of sources associated with large-scale structure formation, such that intensity fluctuations correlate with matter density fluctuations. As a result, the LW background rises to the threshold level for H-2 suppression earlier in the vicinity of the reionization sources and their H II regions.
The Next Generation Atlas of Quasar Spectral Energy Distributions from Radio To X-Rays
(2011-09) Shang, Zhaohui; Brotherton, Michael S.; Wills, Beverley J.; Wills, D.; Cales, Sabrina L.; Dale, Daniel A.; Green, Richard F.; Runnoe, Jessie C.; Nemmen, Rodrigo S.; Gallagher, Sarah C.; Ganguly, Rajib; Hines, Dean C.; Kelly, Benjamin J.; Kriss, Gerard A.; Li, Jun; Tang, Baitian; Xie, Yanxia; Wills, Beverley J.; Wills, D.
We have produced the next generation of quasar spectral energy distributions (SEDs), essentially updating the work of Elvis et al. by using high-quality data obtained with several space-and ground-based telescopes, including NASA's Great Observatories. We present an atlas of SEDs of 85 optically bright, non-blazar quasars over the electromagnetic spectrum from radio to X-rays. The heterogeneous sample includes 27 radio-quiet and 58 radio-loud quasars. Most objects have quasi-simultaneous ultraviolet-optical spectroscopic data, supplemented with some far-ultraviolet spectra, and more than half also have Spitzer mid-infrared Infrared Spectrograph spectra. The X-ray spectral parameters are collected from the literature where available. The radio, far-infrared, and near-infrared photometric data are also obtained from either the literature or new observations. We construct composite SEDs for radio-loud and radio-quiet objects and compare these to those of Elvis et al., finding that ours have similar overall shapes, but our improved spectral resolution reveals more detailed features, especially in the mid-and near-infrared.