Amy J. Barger

The Star formation histories of galaxies in distant clusters

We present a detailed analysis of the spectroscopic catalog of galaxies in 10 distant clusters from Dressler et al. We investigate the nature of the different spectral classes defined by Dressler et al., including star-forming, poststarburst, and passive galaxy populations, and reproduce their basic properties using our spectral synthesis model. We attempt to identify the evolutionary pathways between the various spectral classes in order to search for the progenitors of the numerous poststarburst galaxies. The comparison of the spectra of the distant galaxy populations with samples drawn from the local universe leads us to identify a significant population of dust-enshrouded starburst galaxies, showing both strong Balmer absorption and relatively modest [O II] emission, that we believe are the most likely progenitors of the poststarburst population. We present the differences between the field and cluster galaxies at z=0.4-0.5. We then compare the spectral and morphological properties of the distant cluster galaxies, exploring the connection between the quenching of star formation inferred from the spectra and the strong evolution of the S0 population discussed by Dressler et al. We conclude that either two different timescales and/or two different physical processes are responsible for the spectral and morphological transformation.

Submillimetre-wavelength detection of dusty star-forming galaxies at high redshift

Optical surveys of the global star-formation rate in high-redshift galaxies show a strong peak in activity at a redshift of z ≈ 1.5, which implies that most of the star formation has already been seen. High-redshift galaxies may, however, emit most of their energy at submillimetre wavelengths, if they contain substantial amounts of dust that absorbs the starlight and reradiates it as far-infrared light. Here we report a deep survey of a blank region of sky, performed at submillimetre wavelengths (450 and 850 μm). We detect luminous sources in the 850-μm band which, if they have similar spectra to low-redshift ultraluminous infrared galaxies and are primarily powered by star formation, must each be converting more than 100 solar masses of gas per year into stars: this is larger than themaximum star-formation rates inferred for most optically selected galaxies. The total amount of star formation at high redshifts is essentially fixed by the level of background light, but where the peak activity occurs at submillimetre wavelengths is not yet well established. However, the background light inferred from the sources that we have detected is already comparable to that from the optically selected sources. Establishing the main epoch of star formation will therefore require a combination of optical and submillimetre studies.

Resolving the extragalactic hard X-ray background

The origin of the hard (2–10 keV) X-ray background has been a mystery for over 35 years. Most of the soft X-ray background has been resolved into individual sources (mainly quasars), but these sources do not have the spectral energy distribution required to match the spectrum of the X-ray background as a whole. Here we report the results of a deep survey, using the Chandra satellite, in which the detected hard X-ray sources account for at least 75 per cent of the hard X-ray background. The mean X-ray spectral energy distribution of these sources is in good agreement with that of the background. Moreover, most of those hard X-ray sources are associated unambiguously with either the nuclei of otherwise normal bright galaxies, or with optically faint sources. The latter could be active nuclei in dust-enshrouded galaxies or a population of quasars at extremely high redshift.

The Chandra Deep Survey of the Hubble Deep Field-North Area. II. Results from the Caltech Faint Field Galaxy Redshift Survey Area*

A deep X-ray survey of the Hubble Deep Field-North (HDF-N) and its environs is performed using data collected by the Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-Ray Observatory. Currently a 221.9 ks exposure is available, the deepest ever presented, and here we give results on X-ray sources located in the 86 × 87 area covered by the Caltech Faint Field Galaxy Redshift Survey (the Caltech area). This area has (1) deep photometric coverage in several optical and near-infrared bands; (2) extensive coverage at radio, submillimeter, and mid-infrared wavelengths; and (3) some of the deepest and most complete spectroscopic coverage ever obtained. It is also where the X-ray data have the greatest sensitivity; the minimum detectable fluxes in the 0.5-2 keV (soft) and 2-8 keV (hard) bands are ≈1.3 × 10-16 and ≈6.5 × 10-16 ergs cm-2 s-1, respectively. More than ≈80% of the extragalactic X-ray background in the hard band is resolved. The 82 Chandra sources detected in the Caltech area are correlated with more than 25 multiwavelength source catalogs, and the results of these correlations as well as spectroscopic follow-up results obtained with the Keck and Hobby-Eberly Telescopes are presented. All but nine of the Chandra sources are detected optically with R 26.5. Redshifts are available for 39% of the Chandra sources, including 96% of the sources with R 5.0) objects. A total of 16 of the 67 1.4 GHz μJy sources in the Caltech area are detected in the X-ray band, and the detection rates for starburst-type and AGN-candidate μJy sources are comparable. Only two of the 17 red, optically faint (I > 25) μJy sources are detected in X-rays. While many of the starburst-type μJy sources appear to contain obscured active galactic nuclei (AGNs), the Chandra data are consistent with the majority of the μJy radio sources being powered by star formation. A total of 11 of the ≈100 ISO mid-infrared sources found in and near the HDF-N are detected in X-rays. In the HDF-N itself, where both the infrared coverage and the X-ray coverage are deepest, it is notable that six of the eight Chandra sources are detected by ISO; most of these are known to be AGNs where the X-ray and infrared detections reveal both the direct and indirect accretion power being generated. The high X-ray-to-infrared matching rate bodes well for future sensitive infrared observations of faint X-ray sources. Four of the 33 very red objects that have been identified in the Caltech area are detected in X-rays; these four are among our hardest Chandra sources, and we argue that they contain moderately luminous obscured AGNs. Overall, however, the small Chandra detection fraction suggests a relatively small AGN content in the optically selected very red object population. A stacking analysis of the very red objects not detected individually by Chandra yields a soft-band detection with an average soft-band X-ray flux of ≈1.9 × 10-17 ergs cm-2 s-1; the observed emission may be associated with the hot interstellar media of moderate-redshift elliptical galaxies. Constraints on AGN candidates, extended X-ray sources, and Galactic objects in the Caltech area are also presented.

The Chandra Deep Field North Survey. V. 1 Ms Source Catalogs

An extremely deep X-ray survey (≈1 Ms) of the Hubble Deep Field North (HDF-N) and its environs (≈450 arcmin2) has been performed with the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory. This is one of the two deepest X-ray surveys ever performed; for point sources near the aim point, it reaches 0.5–2.0 and 2–8 keV flux limits of ≈3 × 10-17 and ≈2 × 10-16 ergs cm-2 s-1, respectively. Here we provide source catalogs, along with details of the observations, data reduction, and technical analysis. Observing conditions, such as background, were excellent for almost all of the exposure. We have detected 370 distinct point sources: 360 in the 0.5–8.0 keV band, 325 in the 0.5–2.0 keV band, 265 in the 2–8 keV band, and 145 in the 4–8 keV band. Two new Chandra sources in the HDF-N itself are reported and discussed. Source positions are accurate to within 06–17 (at ≈90% confidence), depending mainly on the off-axis angle. We also detect two highly significant extended X-ray sources and several other likely extended X-ray sources. We present basic number count results for sources located near the center of the field. Source densities of 7100 deg-2 (at 4.2 × 10-17 ergs cm-2 s-1) and 4200 deg-2 (at 3.8 × 10-16 ergs cm-2 s-1) are observed in the soft and hard bands, respectively.

Optical and Infrared Properties of the 2 Ms Chandra Deep Field North X-Ray Sources

We present an optical and near-infrared catalog for the X-ray sources in the ?2 Ms Chandra observation of the Hubble Deep Field North region. We have high-quality multicolor imaging data for all 503 X-ray point sources in the X-ray?selected catalog and reliable spectroscopic redshifts for 284. We spectroscopically identify six high-redshift (z > 1) type II quasars (L2?8keV > 1044 ergs s-1) in our sample. Our spectroscopic completeness for the R ? 24 sources is 87%. The spectroscopic redshift distribution shows two broad redshift spikes that have clearly grown over those originally seen in the ?1 Ms exposure. The spectroscopically identified extragalactic sources already comprise 75% of the measured 2?8 keV light. Redshift slices versus 2?8 keV flux show that an impressive 54% of the measured 2?8 keV light arises from sources at z 5.7 that would classify them as extremely red objects (EROs). The photometric redshifts of these EROs are all between z ~ 1.5 and z ~ 2.5. We use our wide wavelength coverage to determine rest-frame colors for the X-ray sources with spectroscopic or photometric redshifts. We find that many of the X-ray sources have the rest-frame colors of evolved red galaxies and that there is very little evolution in these colors with redshift. We also determine absolute magnitudes and find that many of the non?broad-line sources are more luminous than M, even at high redshifts. We therefore infer that deep X-ray observations may provide an effective way of locating M* galaxies with colors similar to present-day early-type galaxies to high redshifts.

The diversity of SCUBA-selected galaxies

We present extensive observations of a sample of distant, submillimetre (submm) galaxies detected in the field of the massive cluster lens, Abell1835, using the Submm Common-User Bolometer Array (SCUBA). Taken in conjunction with earlier observations of other submm-selected sources (Ivison et al. 1998; Smail et al. 1999; Soucail et al. 1999) we now have detailed, multi-wavelength observations of seven examples of the submm population, having exploited the combination of achromatic amplification by cluster lenses and lavish archival datasets. These sources, all clearly at z > 1, illustrate the wide range in the radio and optical properties of distant submm-selected galaxies. We include detailed observations of the first candidate ‘pure’ starburst submm galaxy at high redshift, a z = 2.56 interacting galaxy which shows no obvious sign of hosting an AGN. The remaining sources have varying degrees of inferred AGN activity (three from seven of the most luminous show some evidence of the presence of an AGN) although even when an AGN is obviously present it is still not apparent if reprocessed radiation from this source dominates the submm emission. In contrast with the variation in the spectral properties, we see relatively homogeneous morphologies for the population, with a large fraction of merging or interacting systems. Our study shows that virtually identical spectral energy distributions are seen for galaxies which exhibit strikingly different optical/UV spectral-line characteristics. We conclude that standard optical/UV spectral classifications are misleading when applied to distant, highly obscured galaxies and that we must seek other means of determining the various contributions to the overall energy budget of submm galaxies and hence to the far-infrared extragalactic background.

Mapping the Evolution of High-Redshift Dusty Galaxies with Submillimeter Observations of a Radio-selected Sample

Direct submillimeter imaging has recently revealed the 850 μm background to be mostly composed of a population of distant ultraluminous infrared galaxies, but identifying the optical/near-infrared (NIR) counterparts to these sources has proved difficult because of the poor submillimeter spatial resolution. However, the proportionality of both centimeter and submillimeter data to the star formation rate suggests that high-resolution radio continuum maps with subarcsecond positional accuracy can be exploited to locate submillimeter sources. In this paper we present results from a targeted SCUBA survey of microjansky radio sources in the flanking fields of the Hubble Deep Field. The sources were selected from the uniform (8 μJy at 1 σ) 1.4 GHz VLA image of Richards. Even with relatively shallow SCUBA observations (a 3 σ detection limit of 6 mJy at 850 μm), we were successful at making submillimeter detections of optical/NIR-faint (I 24 and K 21–22) radio sources, and our counts closely match the bright counts from submillimeter surveys. An important corollary is that a large fraction of the bright (>6 mJy) submillimeter sources in untargeted submillimeter surveys have extremely faint optical/NIR counterparts and hence are inaccessible to optical imaging and spectroscopy. However, redshift estimates can be made from the ratio of the submillimeter flux to the radio flux across the 100 GHz break in the spectral energy distribution. This procedure, which we refer to as millimetric redshift estimation, places the bright submillimeter population at z = 1–3, where it forms the high-redshift tail of the faint radio population. The star formation rate density (SFRD) due to ultraluminous infrared galaxies increases by more than 2 orders of magnitude from z ~ 0 to z ~ 1–3. The SFRD at high-redshift inferred from our >6 mJy submillimeter observations is comparable with that observed in the ultraviolet/optical.

A Highly Complete Spectroscopic Survey of the GOODS-N Field

We present a table of redshifts for 2907 galaxies and stars in the 145 arcmin2 HST ACS GOODS-North, making this the most spectroscopically complete redshift sample obtained to date in a field of this size. We also include the redshifts, where available, in a table containing just under 7000 galaxies from the ACS area with -->Ks, AB Ks image obtained with WIRCam on the CFHT, as well as in a table containing 1016 sources with NUV -->AB AB F 435WAB = 24.5, -->F 850LPAB = 23.3, and -->Ks, AB = 21.5, and to 24 μm fluxes of 250 μJy. An extensive analysis of these data appear in a companion paper, but here we test the efficiency of color-selection techniques to identify populations of high-redshift galaxies and active galactic nuclei. We also examine the feasibility of doing tomography of the intergalactic medium with a 30 m telescope.

Resolving the Submillimeter Background: The 850 Micron Galaxy Counts

Recent deep blank-field submillimeter surveys have revealed a population of luminous high-redshift galaxies that emit most of their energy in the submillimeter. The results suggest that much of the star formation at high redshift may be hidden to optical observations. In this Letter we present wide-area 850 ?m Submillimeter Common-User Bolometer Array data on the Hawaii Survey Fields SSA13, SSA17, and SSA22. Combining these new data with our previous deep-field data, we establish the 850 ?m galaxy counts from 2 to 10 mJy with a greater than 3 ? detection limit. The area coverage is 104 arcmin2 to 8 mJy and 7.7 arcmin2 to 2.3 mJy. The differential 850 ?m counts are well described by the function n(S)=N0/(a+S3.2), where S is the flux in mJy, N0 = 3.0 ? 104 deg-2 mJy-1, and a=0.4-1.0 is chosen to match the 850 ?m extragalactic background light. Between 20% and 30% of the 850 ?m background resides in sources brighter than 2 mJy. Using an empirical fit to our data above 2 mJy constrained by the extragalactic background light at lower fluxes, we argue that the bulk of the 850 ?m extragalactic background light resides in sources with fluxes near 1 mJy. The submillimeter sources are plausible progenitors of the present-day spheroidal population.