We present new observational determinations of the evolution of the 2–10 keV X-ray luminosity function (XLF) of active galactic nuclei (AGN). We utilize data from a number of surveys including both ...the 2 Ms Chandra Deep Fields and the AEGIS-X 200 ks survey, enabling accurate measurements of the evolution of the faint end of the XLF. We combine direct, hard X-ray selection and spectroscopic follow-up or photometric redshift estimates at z < 1.2 with a rest-frame UV colour pre-selection approach at higher redshifts to avoid biases associated with catastrophic failure of the photometric redshifts. Only robust optical counterparts to X-ray sources are considered using a likelihood ratio matching technique. A Bayesian methodology is developed that considers redshift probability distributions, incorporates selection functions for our high-redshift samples and allows robust comparison of different evolutionary models. We statistically account for X-ray sources without optical counterparts to correct for incompleteness in our samples. We also account for Poissonian effects on the X-ray flux estimates and sensitivities and thus correct for the Eddington bias. We find that the XLF retains the same shape at all redshifts, but undergoes strong luminosity evolution out to z∼ 1, and an overall negative density evolution with increasing redshift, which thus dominates the evolution at earlier times. We do not find evidence that a luminosity-dependent density evolution, and the associated flattening of the faint-end slope, is required to describe the evolution of the XLF. We find significantly higher space densities of low-luminosity, high-redshift AGN than in prior studies, and a smaller shift in the peak of the number density to lower redshifts with decreasing luminosity. The total luminosity density of AGN peaks at z= 1.2 ± 0.1, but there is a mild decline to higher redshifts. We find that >50 per cent of black hole growth takes place at z > 1, with around half in LX < 1044 erg s−1 AGN.
We present infrared observations of 66 starburst galaxies over the full range of oxygen abundances observed in local star-forming galaxies, from image to 8.9. The data include imaging and ...spectroscopy from the Spitzer Space Telescope, supplemented by ground-based near-infrared imaging. We confirm a strong correlation of aromatic emission with metallicity, with a threshold at image. We show that the far-infrared color temperature of the large dust grains increases toward lower metallicity, peaking at a metallicity of 8 before turning over. We compute dust masses and compare them to H i masses from the literature to derive the ratio of atomic gas to dust, which increases by nearly 3 orders of magnitude between solar metallicity and a metallicity of 8, below which it flattens out. The abrupt change in aromatic emission at mid-infrared wavelengths thus appears to be reflected in the far-infrared properties, indicating that metallicity changes affect the composition of the full range of dust grain sizes that dominate the infrared emission. Although the great majority of galaxies show similar patterns of behavior as described above, there are three exceptions, SBS 0335-052E, Haro 11, and SHOC 391. Their infrared SEDs are dominated energetically by the mid- IR near 24 mum rather than by the 60-200 mum region. In addition, they have very weak near-infrared outputs and their SEDs are dominated by emission by dust at wavelengths as short as 1.8 mum. The latter behavior indicates that the dominant star-forming episodes in them are extremely young. The component of the ISM responsible for the usual far-infrared emission appears to be either missing or inefficiently heated in these three galaxies.
We present the quantitative rest-frame B morphological evolution and galaxy merger fraction at 0.2 < z < 1.2 as observed by the All-Wavelength Extended Groth Strip International Survey (AEGIS). We ...use the Gini coefficient and M sub(20) to identify major mergers and classify galaxy morphology for a volume-limited sample of 3009 galaxies brighter than 0.4L super(*) sub(B), assuming pure luminosity evolution. We find that the merger fraction remains roughly constant at 10% plus or minus 2% for 0.2 < z < 1.2. The fraction of E/S0/Sa galaxies increases from 21% plus or minus 3% at z similar to 1.1 to 44% plus or minus 9% at z similar to 0.3, while the fraction of Sb-Ir galaxies decreases from 64% plus or minus 6% at z similar to 1.1 to 47% similar to 9% at z similar to 0.3. The majority of z < 1.2 Spitzer MIPS 24 mu m sources with L(IR) > 10 super(11) L unk are disk galaxies, and only similar to 15% are classified as major merger candidates. Edge-on and dusty disk galaxies (Sb-Ir) are almost a third of the red sequence at z similar to 1.1, while E/S0/Sa make up over 90% of the red sequence at z similar to 0.3. Approximately 2% of our full sample are red mergers. We conclude (1) the merger rate does not evolve strongly between 0.2 < z < 1.2; (2) the decrease in the volume-averaged star formation rate density since z similar to 1 is a result of declining star formation in disk galaxies rather than a disappearing population of major mergers; (3) the build-up of the red sequence at z < 1 can be explained by a doubling in the number of spheroidal galaxies since z similar to 1.2.
We characterize the mass-dependent evolution of more than 8000 galaxies using spectroscopic redshifts from the DEEP2 Galaxy Redshift Survey in the range 0.4 < z < 1.4 and stellar masses calculated ...from K-band photometry obtained at Palomar Observatory. This sample spans more than 1.5 deg super(2) in four independent fields. Using rest-frame U - B color and O II equivalent widths, we distinguish star-forming from passive populations in order to explore the nature of "downsizing"--a pattern in which the sites of active star formation shift from high-mass galaxies at early times to lower mass systems at later epochs. We identify a mass limit, M sub(Q), above which star formation appears to be quenched and show that the physical mechanisms responsible for downsizing can thus be empirically quantified by charting the evolution in this threshold mass. We find that M sub(Q) decreases with time by a factor of 63 across our redshift range according to M sub(Q) 8(1 + z) super(3.5). To further constrain possible quenching mechanisms, we investigate how downsizing depends on local galaxy environment using the projected third-nearest-neighbor statistic D sub(p,3). For the majority of galaxies near the median density, there is no significant correlation between downsizing and environment. However, a trend is observed in the comparison between environments that are more than 3 times overdense or underdense relative to the median. Here, downsizing appears accelerated in overdense regions that host higher numbers of massive, early-type galaxies as compared to the underdense regions. Our results significantly constrain recent suggestions for the origin of downsizing and indicate that the process for quenching star formation must, primarily, be internally driven.
We discuss the relationship between rest-frame color and optical luminosity for X-ray sources in the range 0.6< z <1.4 selected from the Chandra survey of the Extended Groth Strip. These objects are ...almost exclusively active galactic nuclei (AGNs). While there are a few luminous QSOs, most are relatively weak or obscured AGNs whose optical colors should be dominated by host galaxy light The vast majority of AGN hosts at z 6 1 are luminous and red, with very few objects fainter than M sub(B) = -20.5 or bluer than U-B = 0.6. This places the AGNs in a distinct region of color-magnitude space, on the "red sequence" or at the top of the "blue cloud," with many in between these two modes in galaxy color. A key stage in the evolution of massive galaxies is when star formation is quenched, resulting in a migration from the blue cloud to the red sequence. Our results are consistent with scenarios in which AGNs either cause or maintain this quenching. The large number of red-sequence AGNs implies that strong, ongoing star formation is not a necessary ingredient for AGN activity, as black hole accretion appears often to persist after star formation has been terminated.
The DEEP2 and COMBO-17 surveys are compared to study luminosity functions of red and blue galaxies to z similar to 1. The two surveys have different methods and sensitivities, but nevertheless ...results agree. After z similar to 1, M* sub(B) has dimmed by 1.2-1.3 mag for all colors of galaxies, phi * for blue galaxies has hardly changed, and phi * for red galaxies has at least doubled (our formal value is similar to 0.5 dex). Luminosity density J sub(B) has fallen by 0.6 dex for blue galaxies but has remained nearly constant for red galaxies. These results imply that the number and total stellar mass of blue galaxies have been substantially constant since z similar to 1, whereas those of red galaxies (near L*) have been significantly rising. To explain the new red galaxies, a "mixed" scenario is proposed in which star formation in blue cloud galaxies is quenched, causing them to migrate to the red sequence, where they merge further in a small number of stellar mergers. This mixed scenario matches the local boxy-disky transition for nearby ellipticals, as well as red sequence stellar population scaling laws such as the color-magnitude and Mg- sigma relations (which are explained as fossil relics from blue progenitors). Blue galaxies enter the red sequence via different quenching modes, each of which peaks at a different characteristic mass and time. The red sequence therefore likely builds up in different ways at different times and masses, and the concept of a single process that is "downsizing" (or upsizlng) probably does not apply. Our claim in this paper of a rise in the number of red galaxies applies to galaxies near L*. Accurate counts of brighter galaxies on the steep part of the Schechter function require more accurate photometry than is currently available.
We measure the clustering of DEEP2 galaxies at z = 1 as a function of luminosity on scales 0.1-20 h super(-1) Mpc. Drawing from a parent catalog of 25,000 galaxies at 0.7 < z < 1.3 in the full DEEP2 ...survey, we create volume-limited samples having upper luminosity limits between M sub(B) = -19 and -20.5, roughly 0.2-1L* at z = 1. We find that brighter galaxies are more strongly clustered than fainter galaxies and that the slope of the correlation function does not depend on luminosity for L < L*. The brightest galaxies, with L > L,* have a steeper slope. The clustering scale length, r sub(0), varies from 3.69 c 0.14 for the faintest sample to 4.43 c 0.14 for the brightest sample. The relative bias of galaxies as a function of L/L* is steeper than the relation found locally for SDSS galaxies by Zehavi et al. in 2005 over the luminosity range that we sample. The absolute bias of galaxies at z 61 is scale dependent on scales r sub(p) < 1 h super(-1) Mpc, and rises most significantly on small scales for the brightest samples. For a concordance cosmology, the large-scale bias varies from 1.26 c 0.04 to 1.54 c 0.05 as a function of luminosity and implies that DEEP2 galaxies reside in dark matter halos with a minimum mass of 6 super((1-3)) x 10 super(12) h super(-1) M sub( ).
We present Spitzer IRS mid-infrared spectra for 15 gravitationally lensed, 24 mum-selected galaxies, and combine the results with four additional very faint galaxies with IRS spectra in the ...literature. The median intrinsic 24 mum flux density of the sample is 130 muJy, enabling a systematic survey of the spectral properties of the very faint 24 mum sources that dominate the number counts of Spitzer cosmological surveys. Six of the 19 galaxy spectra (32%) show the strong mid-IR continuua expected of AGNs; X-ray detections confirm the presence of AGNs in three of these cases, and reveal AGNs in two other galaxies. These results suggest that nuclear accretion may contribute more flux to faint 24 mum-selected samples than previously assumed. Almost all the spectra show some aromatic (PAH) emission features; the measured aromatic flux ratios do not show evolution from image. In particular, the high signal-to-noise mid-IR spectrum of SMM J163554.2+661225 agrees remarkably well with low-redshift, lower luminosity templates. We compare the rest-frame 8 mum and total infrared luminosities of star-forming galaxies, and find that the behavior of this ratio with total IR luminosity has evolved modestly from image to image. Since the high aromatic-to-continuum flux ratios in these galaxies rule out a dominant contribution by AGNs, this finding implies systematic evolution in the structure and/or metallicity of infrared sources with redshift. It also has implications for the estimates of star-forming rates inferred from 24 mum measurements, in the sense that at image, a given observed frame 24 mum luminosity corresponds to a lower bolometric luminosity than would be inferred from low-redshift templates of similar luminosity at the corresponding rest wavelength.
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