Extended Groth Strip Research Articles

Groups of galaxies in AEGIS: the 200-ksChandraextended X-ray source catalogue

We present the discovery of seven X-ray emitting groups of galaxies selected as extended X-ray sources in the 200 ksec Chandra coverage of the All-wavelength Extended Groth Strip International Survey (AEGIS). In addition, we report on AGN activity associated to these systems. Using the DEEP2 Galaxy Redshift Survey coverage, we identify optical counterparts and determine velocity dispersions. In particular, we find three massive high-redshift groups at z>0.7, one of which is at z=1.13, the first X-ray detections of spectroscopically selected DEEP2 groups. We also present a first look at the the L_X-T, L_X-sigma, and sigma-T scaling relations for high-redshift massive groups. We find that the properties of these X-ray selected systems agree well with the scaling relations of similar systems at low redshift, although there are X-ray undetected groups in the DEEP2 catalogue with similar velocity dispersions. The other three X-ray groups with identified redshifts are associated with lower mass groups at z~0.07 and together form part of a large structure or "supergroup" in the southern portion of the AEGIS field. All of the low-redshift systems are centred on massive elliptical galaxies, and all of the high-redshift groups have likely central galaxies or galaxy pairs. All of the central group galaxies host X-ray point sources, radio sources, and/or show optical AGN emission. Particularly interesting examples of central AGN activity include a bent-double radio source plus X-ray point source at the center of a group at z=0.74, extended radio and double X-ray point sources associated to the central galaxy in the lowest-redshift group at z=0.066, and a bright green valley galaxy (part of a pair) in the z=1.13 group which shows optical AGN emission lines.

The structures of distant galaxies — IV. A new empirical measurement of the time-scale for galaxy mergers — implications for the merger history

Abstract Understanding the role of mergers in galaxy formation is one of the most outstanding problems in extragalactic astronomy. While we now have an idea for how the merger fraction evolves at redshifts z < 3, converting this merger fraction into merger rates, and therefore how many mergers an average galaxy undergoes during its history, is still uncertain. The main reason for this is that the inferred number of mergers depends highly upon the time-scale observational methods are sensitive for finding ongoing or past mergers. While there are several theoretical and model-based estimates of merger times, there is currently no empirical measure of this time-scale. We present the first observationally based measurement of merger times utilizing the observed decline in the galaxy major merger fraction at z < 1.2 based on >20 000 galaxies in the Extended Groth Strip Survey and Cosmic Evolution Survey. Using a new methodology described in this Letter, we are able to determine how long a galaxy remains identifiable as a merging system within the CAS system. We find a maximum CAS major merger time-scale of 1.1 ± 0.3 Gyr at z < 1.2, and a most likely CAS merger time-scale of 0.6 ± 0.3 Gyr, in good agreement with results from N-body simulations. Utilizing this time-scale, we are able to measure the number of major mergers galaxies with masses M* > 1010 M⊙ undergo at z < 1.2, with a total number Nm= 0.90+0.44−0.23. We further show that this time-scale is inconsistent with a star formation origin for ultrahigh asymmetries, thereby providing further evidence that structural methods are able to locate mostly merging galaxies.

The deep optical imaging of the Extended Groth Strip

We present u′g′R optical images taken with the MMT/Megacam and the Subaru/Suprime telescopes of the Extended Groth Strip survey. The total survey covers an area of about ∼ 1 degree2, including four sub-fields and is optimized for the study of galaxies at z ∼ 3. Our methods for photometric calibration in AB magnitudes, the limiting magnitude and the galaxy number count are described. A sample of 1642 photometrically selected candidate Lyman–Break Galaxies (LBGs) to an apparent RAB magnitude limit of 25.0 is presented. The average sky surface density of our LBG sample is ∼1.0 arcmin−2, slightly higher than the previous finding.

The structures of distant galaxies - III. The merger history of over 20 000 massive galaxies atz< 1.2

Utilizing deep Hubble Space Telescope imaging from the two largest field galaxy surveys, the Extended Groth Strip (EGS) and the COSMOS survey, we examine the structural properties, and derive the merger history for 21,902 galaxies with M_*>10^{10} M_0 at z 10^{10} M_0, the merger fraction can be parameterised by f_m = f_0*(1+z)^m with the power-law slope m=2.3+/-0.4. By using the best available z = 0 prior the slope increases to m=3.8+/-0.2, showing how critical the measurement of local merger properties are for deriving the evolution of the merger fraction. We furthermore show that the merger fraction derived through structure is roughly a factor of 3-6 higher than pair fractions. Based on the latest cosmological simulations of mergers we show that this ratio is predicted, and that both methods are likely tracing the merger fraction and rate properly. We calculate, utilising merger time scales from simulations, and previously published merger fractions that the merger rate of galaxies with M_*>10^{10} M_0 increases linearly between z = 0.7 and z = 3, and that a typical M_*>10^{10} M_0 galaxy undergoes between 1-2 major mergers at z<1.2.

AEGIS-X: THE CHANDRA DEEP SURVEY OF THE EXTENDED GROTH STRIP

We present the AEGIS-X survey, a series of deep Chandra ACIS-I observations of the Extended Groth Strip. The survey comprises pointings at 8 separate positions, each with nominal exposure 200ks, covering a total area of approximately 0.67 deg2 in a strip of length 2 degrees. We describe in detail an updated version of our data reduction and point source detection algorithms used to analyze these data. A total of 1325 band-merged sources have been found to a Poisson probability limit of 4e-6, with limiting fluxes of 5.3e-17 erg/cm2/s in the soft (0.5-2 keV) band and 3.8e-16 erg/cm2/s in the hard (2-10 keV) band. We present simulations verifying the validity of our source detection procedure and showing a very small, <1.5%, contamination rate from spurious sources. Optical/NIR counterparts have been identified from the DEEP2, CFHTLS, and Spitzer/IRAC surveys of the same region. Using a likelihood ratio method, we find optical counterparts for 76% of our sources, complete to R(AB)=24.1, and, of the 66% of the sources that have IRAC coverage, 94% have a counterpart to a limit of 0.9 microJy at 3.6 microns (m(AB)=23.8). After accounting for (small) positional offsets in the 8 Chandra fields, the astrometric accuracy of the Chandra positions is found to be 0.8 arcsec RMS, however this number depends both on the off-axis angle and the number of detected counts for a given source. All the data products described in this paper are made available via a public website.

CHARACTERIZATION OF ACTIVE GALACTIC NUCLEI AND THEIR HOSTS IN THE EXTENDED GROTH STRIP: A MULTIWAVELENGTH ANALYSIS

We have employed a reliable technique of classification of active galactic nuclei (AGNs) based on the fit of well sampled spectral energy distributions (SEDs) with a complete set of AGN and starburst galaxy templates. We have compiled UV, optical, and IR data for a sample of 116 AGNs originally selected for their X-ray and mid-IR emissions (96 with single detections and 20 with double optical counterparts). This is the most complete compilation of multiwavelength data for such a large sample of AGN in the Extended Groth Strip. Through these SEDs, we are able to obtain highly reliable photometric redshifts and to distinguish between pure and host-dominated AGNs. For the objects with unique detection we find that they can be separated into five main groups, namely: Starburst-dominated AGNs (24% of the sample), Starburst-contaminated AGNs (7%), Type-1 AGNs (21%), Type-2 AGNs (24%), and Normal galaxy hosting AGN (24%). We find these groups concentrated at different redshifts: Type-2 AGNs and Normal galaxy hosting AGNs are concentrated at low redshifts, whereas Starburst-dominated AGNs and Type-1 AGNs show a larger span. Correlations between hard/soft X-ray and UV, optical and IR luminosities are reported for the first time for such a sample of AGNs spanning a wide range of redshifts. For the 20 objects with double detection, the percentage of Starburst-dominated AGNs increases up to 48%.

X-ray selected AGN in groups at redshiftsz≈ 1

We explore the role of the group environment in the evolution of AGN at the redshift interval 0.7<z<1.4, by combining deep Chandra observations with extensive optical spectroscopy from the All-wavelength Extended Groth strip International Survey (AEGIS). The sample consists of 3902 optical sources and 71 X-ray AGN. Compared to the overall optically selected galaxy population, X-ray AGN are more frequently found in groups at the 99% confidence level. This is partly because AGN are hosted by red luminous galaxies, which are known to reside, on average, in dense environments. Relative to these sources, the excess of X-ray AGN in groups is significant at the 91% level only. Restricting the sample to 0.7<z<0.9 and M_B<-20mag in order to control systematics we find that X-ray AGN represent (4.7\pm1.6) and (4.5\pm1.0)% of the optical galaxy population in groups and in the field respectively. These numbers are consistent with the AGN fraction in low redshift clusters, groups and the field. The results above, although affected by small number statistics, suggest that X-ray AGN are spread over a range of environments, from groups to the field, once the properties of their hosts (e.g. colour, luminosity) are accounted for. There is also tentative evidence, significant at the 98% level, that the field produces more X-ray luminous AGN compared to groups, extending similar results at low redshift to z~1. This trend may be because of either cold gas availability or the nature of the interactions occurring in the denser group environment (i.e. prolonged tidal encounters).

Exploring the Evolutionary Paths of the Most Massive Galaxies sincez∼ 2

We use Spitzer MIPS data from the FIDEL Legacy Project in the Extended Groth Strip to analyze the stellar mass assembly of massive (M>10^11 M_sun) galaxies at z 1, and more pronouncedly at z>1.3, the median specific SFRs of the disks and spheroids detected by MIPS are very similar, ranging from 0.1 to 1 Gyr^-1 (SFR=10-200 M_sun/yr). We estimate that massive spheroid-like galaxies may have doubled (at the most) their stellar mass from star-forming events at z<2: less than 20% mass increase at 1.7<z<2.0, up to 40% more at 1.1<z<1.7, and less than 20% additional increase at z<1. Disk-like galaxies may have tripled (at the most) their stellar mass at z<2 from star formation alone: up to 40% mass increase at 1.7<z<2.0, and less than 180% additional increase below z=1.7 occurred at a steady rate.

CATS: OPTICAL TO NEAR-INFRARED COLORS OF THE BULGE AND DISK OF TWOz= 0.7 GALAXIES USINGHUBBLE SPACE TELESCOPEAND KECK LASER ADAPTIVE OPTICS IMAGING

We have employed laser guide star (LGS) adaptive optics (AO) on the Keck II telescope to obtain near-infrared (NIR) images in the Extended Groth Strip deep galaxy survey field. This is a continuation of our Center for Adaptive Optics Treasury Survey program of targeting 0.5 < z < 1 galaxies where existing images with the Hubble Space Telescope (HST) are already in hand. Our AO field has already been imaged by the Advanced Camera for Surveys and the Near Infrared Camera and Multiobject Spectrograph (NICMOS). Our AO images at 2.2 ?m (K') are comparable in depth to those from the HST, have Strehl ratios up to 0.4, and full width at half-maximum resolutions superior to that from NICMOS. By sampling the field with the LGS at different positions, we obtain better quality AO images than with an immovable natural guide star. As examples of the power of adding LGS AO to HST data, we study the optical to NIR colors and color gradients of the bulge and disk of two galaxies in the field with z = 0.7.

Probing the 3.6 μm CIRB withSpitzerin Three DIRBE Dark Spots

We observed three regions of the sky with Spitzer in which the cosmic infrared background (CIRB) has been determined at 3.5 μm using the method of subtracting 2MASS stellar fluxes from zodiacal light-subtracted maps. For each of these regions we have obtained 270 s of integration time per pixel with IRAC on Spitzer over the central square degree. We present galaxy counts in each of these approximately 1 deg2 IRAC surveys. Along with deep galaxy counts in the extended Groth strip and GOODS North, we are able to compare the galactic contribution to the CIRB with the DIRBE minus 2MASS-determined L-band CIRB. Using the profile-fit photometry package GIM2D, we find a substantially larger flux contribution to the CIRB than that determined using aperture photometry. We have also made the first rigorous analysis of the uncertainties in determining the CIRB via galaxy counts in Spitzer images using a Monte Carlo Markov chain simulation of our data analysis. Using a simple broken power law model for galaxy counts as a function of magnitude we find a most probable contribution to the CIRB from galaxies at 3.6 μm of 10.8+2.1−1.1 kJy sr−1 (9.0+1.7−0.9 nW m−2 sr−1). Even with this restricted model, however, we find that galaxy counting does not strongly constrain the CIRB from above. We are able to find solutions in which the CIRB runs away to large intensities without the need for an additional diffuse source.

AEGIS: New Evidence Linking Active Galactic Nuclei to the Quenching of Star Formation

Using Chandra X-ray observations in the All-Wavelength Extended Groth Strip International Survey (AEGIS) we identify 241 X-ray-selected active galactic nuclei (AGNs; L2–10 > 10^42 ergs s^−1) and study the properties of their host galaxies in the range 0.4 < z < 1.4. By making use of infrared photometry from the Palomar Observatory and BRI imaging from the Canada-France-Hawaii Telescope, we estimate AGN host galaxy stellar masses and show that both stellar mass and photometric redshift estimates (where necessary) are robust to the possible contamination from AGNs in our X-ray-selected sample. Accounting for the photometric and X-ray sensitivity limits of the survey, we construct the stellar mass function of X-ray-selected AGN host galaxies and find that their abundance decreases by a factor of ~2 since z ~ 1 but remains roughly flat as a function of stellar mass. We compare the abundance of AGN hosts to the rate of star formation quenching observed in the total galaxy population. If the timescale for X-ray-detectable AGN activity is roughly 0.5-1 Gyr, as suggested by black hole demographics and recent simulations, then we deduce that the inferred AGN trigger rate matches the star formation quenching rate, suggesting a link between these phenomena. However, given the large range of nuclear accretion rates we infer for the most massive and red hosts, X-ray-selected AGNs may not be directly responsible for quenching star formation.

AEGIS: Radio and Mid‐Infrared Selection of Obscured AGN Candidates

The application of multiwavelength selection techniques is crucial for discovering a complete and unbiased set of active galactic nuclei (AGNs). Here, we select a sample of 72 AGN candidates in the extended Groth strip (EGS) using deep radio and mid-infrared (mid-IR) data from the Very Large Array (VLA) and the Spitzer Space Telescope, and analyze their properties across other wavelengths. Only 30% of these sources are detected in deep 200 ks Chandra X-Ray Observatory pointings. The X-ray-detected sources demonstrate moderate obscuration with column densities of NH 1022 cm−2. A stacked image of sources undetected by Chandra shows low levels of X-ray activity, suggesting they may be faint or obscured AGNs. Less than 40% of our sample are selected as AGNs with optical broad lines, mid-IR power laws, or X-ray detections. Thus, if our candidates are indeed AGNs, then the radio/mid-IR selection criteria we use provide a powerful tool for identifying sources missed by other surveys.

The Hα‐based Star Formation Rate Density of the Universe atz= 0.84

We present the results of an Hα near-infrared narrowband survey searching for star-forming galaxies at redshift z = 0.84. This work is an extension of our previous narrowband studies in the optical at lower redshifts. After removal of stars and redshift interlopers (using spectroscopic and photometric redshifts), we build a complete sample of 165 Hα emitters in the extended Groth strip and GOODS-N fields with L(Hα) > 10^41 ergs s^−1. We compute the Hα luminosity function at z = 0.84 after corrections for [N_π] flux contamination, extinction, systematic errors, and incompleteness. Our sources present an average dust extinction of A(H α) = 1.5 mag. Adopting Hα as a surrogate for the instantaneous SFR, we measure an extinction-corrected SFR density of 0.17^+0.03_−0.03 M_☉ yr^−1 Mpc^−3. Combining this result to our prior measurements at z = 0.02, 0.24, and 0.40, we derive an Hα-based evolution of the SFR density proportional to (1 + z)^β with β = 3.8 ± 0.5. This evolution is consistent with that derived by other authors using different SFR tracers.

The Environment on a Few Mpc Scales of Infrared Luminous Galaxies at Redshiftz∼1

We investigate the environment of infrared-luminous galaxies [ -->LIR(8–1000 μ m) > 1011 L☉]. We focus on the redshift range -->0.7 ≤ z≤ 1, where these galaxies dominate the star formation activity and play a significant role in galaxy evolution. We employ MIPS 24 μm data to identify infrared galaxies in the Extended Groth Strip (EGS). We use a local density indicator to probe the environment on a few Mpc scales and a group member catalog, both of which make use of the DEEP2 spectroscopic redshift catalog, to quantify the environment of these galaxies. We find that the local environment of LIRGs and ULIRGs is intermediate between that of blue and red galaxies. LIRGs and ULIRGs avoid underdense environments and inhabit local environments that are more dense on average than those of other DEEP2 galaxies at similar redshifts. However, when the comparison sample of the non-IR DEEP2 galaxies is restricted to have the same range of stellar mass, color, or luminosity as the IR galaxies, there is no longer any significant difference in environment: the IR galaxies follow the same trends in the color-environment and luminosity-environment relations observed at -->z ~ 1. We also find that about 30% of the LIRGs and ULIRGs belong to groups, associated with a minimum dark matter halo of -->6 × 1012 M☉ h−1. The group members constitute 20% of the sources responsible for the IR star formation rate density and comoving energy density at -->z ~ 1.

A Catalog of Mid‐Infrared Sources in the Extended Groth Strip

The Extended Groth Strip (EGS) is one of the premier fields for extragalactic deep surveys. Deep observations of the EGS with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope cover an area of 0.38 deg2 to a 50% completeness limit of 1.5 μJy at 3.6 μm. The catalog comprises 57,434 objects detected at 3.6 μm, with 84%, 28%, and 24% also detected at 4.5, 5.8, and 8.0 μm, respectively. Number counts are consistent with results from other Spitzer surveys. Color distributions show that the EGS IRAC sources comprise a mixture of populations: low-redshift star-forming galaxies, quiescent galaxies dominated by stellar emission at a range of redshifts, and high-redshift galaxies and AGNs.

The DEEP2 Galaxy Redshift Survey: the red sequence AGN fraction and its environment and redshift dependence

We measure the dependence of the AGN fraction on local environment at z~1, using spectroscopic data taken from the DEEP2 Galaxy Redshift Survey, and Chandra X-ray data from the All-Wavelength Extended Groth Strip International Survey (AEGIS). To provide a clean sample of AGN we restrict our analysis to the red sequence population; this also reduces additional colour-environment correlations. We find evidence that high redshift LINERs in DEEP2 tend to favour higher density environments relative to the red population from which they are drawn. In contrast, Seyferts and X-ray selected AGN at z~1 show little (or no) environmental dependencies within the same underlying population. We compare these results with a sample of local AGN drawn from the SDSS. Contrary to the high redshift behaviour, we find that both LINERs and Seyferts in the SDSS show a slowly declining red sequence AGN fraction towards high density environments. Interestingly, at z~1 red sequence Seyferts and LINERs are approximately equally abundant. By z~0, however, the red Seyfert population has declined relative to the LINER population by over a factor of 7. We speculate on possible interpretations of our results.

The properties of 70 μm-selected high-redshift galaxies in the Extended Groth Strip

We examine the infrared properties of 43 high-redshift (0.1 35, and for a large fraction (similar to 50 per cent) the SED turns over into the Rayleigh-Jeans regime at wavelengths longward of 90 mu m. For comparison, we also fit semi-empirical templates based on local galaxy data; however, these underestimate the far-infrared SED shape by a factor of at least 2 and in extreme cases up to 10 for the majority (similar to 70 per cent) of the sources. Further investigation of SED characteristics reveals that the mid-infrared (70/24 mu m) continuum slope is decoupled from various galaxy properties such as the total infrared luminosity and far-infrared peak, quantified by the L-160/L-70 ratio. In view of these results, we propose that these high-redshift galaxies have different properties to their local counterparts, in the sense that large amounts of dust cause heavy obscuration and are responsible for an additional cold emissive component, appearing as a far-infrared excess in their SEDs.

The Evolution of Galaxy Mergers and Morphology atz&lt; 1.2 in the Extended Groth Strip

We present the quantitative rest-frame B morphological evolution and galaxy merger fractions at 0.2 < z < 1.2 as observed by the All-wavelength Extended Groth Strip International Survey (AEGIS). We use the Gini coefficent and M_20 to identify major mergers and classify galaxy morphology for a volume-limited sample of 3009 galaxies brighter than 0.4 L_B^*, assuming pure luminosity evolution of 1.3 M_B per unit redshift. We find that the merger fraction remains roughly constant at 10 +/- 2% for 0.2 < z < 1.2. The fraction of E/S0/Sa increases from 21+/- 3% at z ~ 1.1 to 44 +/- 9% at z ~ 0.3, while the fraction of Sb-Ir decreases from 64 +/- 6% at z ~ 1.1 to 47 +/- 9% at z ~ 0.3. The majority of z < 1.2 Spitzer MIPS 24 micron sources with L(IR) > 10^11 L_sun are disk galaxies, and only ~ 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 ~ 1.1, while E/S0/Sa makeup over 90% of the red sequence at z ~ 0.3. Approximately 2% of our full sample are red mergers. We conclude (1) the galaxy 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 ~ 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 ~ 1.2.

The All-Wavelength Extended Groth Strip International Survey (AEGIS) Data Sets

In this the first of a series of Letters, we present a panchromatic data set in the Extended Groth Strip region of the sky. Our survey, the All-Wavelength Extended Groth Strip International Survey (AEGIS), aims to study the physical properties and evolutionary processes of galaxies at z ~ 1. It includes the following deep, wide-field imaging data sets: Chandra/ACIS X-ray, GALEX ultraviolet, CFHT/MegaCam Legacy Survey optical, CFHT/CFH12K optical, Hubble Space Telescope/ACS optical and NICMOS near-infrared, Palomar/WIRC near-infrared, Spitzer/IRAC mid-infrared, Spitzer/MIPS far-infrared, and VLA radio continuum. In addition, this region of the sky has been targeted for extensive spectroscopy using the Deep Imaging Multi-Object Spectrograph (DEIMOS) on the Keck II 10 m telescope. Our survey is compared to other large multiwavelength surveys in terms of depth and sky coverage.

AEGIS: The Color-Magnitude Relation for X-Ray-selected Active Galactic Nuclei

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 ~ 1 are luminous and red, with very few objects fainter than MB = -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.