Ms Chandra Deep Field-North Research Articles

THE 2 Ms CHANDRA DEEP FIELD-NORTH SURVEY AND THE 250 Ks EXTENDED CHANDRA DEEP FIELD-SOUTH SURVEY: IMPROVED POINT-SOURCE CATALOGS

ABSTRACT We present improved point-source catalogs for the 2 Ms Chandra Deep Field-North (CDF-N) and the 250 ks Extended Chandra Deep Field-South (E-CDF-S) Surveys, implementing a number of recent improvements in Chandra source-cataloging methodology. For CDF-N/E-CDF-S, we provide a main catalog that contains 683/1003 X-ray sources detected with wavdetect at a false-positive probability threshold of 10−5 that also satisfy a binomial-probability source-selection criterion of /P < 0.002. Such an approach maximizes the number of reliable sources detected: a total of 196/275 main-catalog sources are new compared to the Alexander et al. CDF-N/Lehmer et al. E-CDF-S main catalogs. We also provide CDF-N/E-CDF-S supplementary catalogs that consist of 72/56 sources detected at the same wavdetect threshold and having P of 0.004–0.1/0.002–0.1 and mag counterparts. For all CDF-N and E-CDF-S sources, including the newly detected ones (these being generally fainter and more obscured), we determine X-ray source positions utilizing centroid and matched-filter techniques; we also provide multiwavelength identifications, apparent magnitudes of counterparts, spectroscopic and/or photometric redshifts, basic source classifications, and estimates of observed active galactic nucleus and galaxy source densities around respective field centers. Simulations show that both the CDF-N and E-CDF-S main catalogs are highly reliable and reasonably complete. Background and sensitivity analyses indicate that the on-axis mean flux limits reached represent a factor of –2.0 improvement over the previous CDF-N and E-CDF-S limits. We make our data products publicly available.

THE HOST GALAXIES OF X-RAY QUASARS ARE NOT STRONG STAR FORMERS

We use ultradeep SCUBA-2 850um observations (~0.37 mJy rms) of the 2 Ms Chandra Deep Field-North (CDF-N) and 4 Ms Chandra Deep Field-South (CDF-S) X-ray fields to examine the amount of dusty star formation taking place in the host galaxies of high-redshift X-ray AGNs. Supplementing with COSMOS, we measure the submillimeter fluxes of the 4-8 keV sources at z>1, finding little flux at the highest X-ray luminosities but significant flux at intermediate luminosities. We determine gray body and MIR luminosities by fitting spectral energy distributions to each X-ray source and to each radio source in an ultradeep Karl G. Jansky Very Large Array (VLA) 1.4 GHz (11.5uJy at 5-sigma) image of the CDF-N. We confirm the FIR-radio and MIR-radio correlations to z=4 using the non-X-ray detected radio sources. Both correlations are also obeyed by the X-ray less luminous AGNs but not by the X-ray quasars. We interpret the low FIR luminosities relative to the MIR for the X-ray quasars as being due to a lack of star formation, while the MIR stays high due to the AGN contribution. We find that the FIR luminosity distributions are highly skewed and the means are dominated by a small number of high-luminosity galaxies. Thus, stacking or averaging analyses will overestimate the level of star formation taking place in the bulk of the X-ray sample. We conclude that most of the host galaxies of X-ray quasars are not strong star formers, perhaps because their star formation is suppressed by AGN feedback.

IDENTIFICATIONS AND PHOTOMETRIC REDSHIFTS OF THE 2 Ms CHANDRA DEEP FIELD-SOUTH SOURCES

[Abridged] We present reliable multiwavelength identifications and high-quality photometric redshifts for the 462 X-ray sources in the ~2 Ms Chandra Deep Field-South. Source identifications are carried out using deep optical-to-radio multiwavelength catalogs, and are then combined to create lists of primary and secondary counterparts for the X-ray sources. We identified reliable counterparts for 446 (96.5%) of the X-ray sources, with an expected false-match probability of ~6.2%. A likelihood-ratio method is used for source matching, which effectively reduces the false-match probability at faint magnitudes compared to a simple error-circle matching method. We construct a master photometric catalog for the identified X-ray sources including up to 42 bands of UV-to-infrared data, and then calculate their photometric redshifts (photo-z's). The reliability of the photo-z's is evaluated using the subsample of 220 sources with secure spectroscopic redshifts. We achieve an accuracy of ~1% [|Delta z|/(1+z)] and an outlier fraction of ~1.4% for sources with spectroscopic redshifts. We performed blind tests to derive a more realistic estimate of the photo-z quality for sources without spectroscopic redshifts. We expect there are ~9% outliers for the relatively brighter sources (R<26), and the outlier fraction will increase to ~15-25% for the fainter sources (R>26). The typical photo-z accuracy is ~6-7%. The outlier fraction and photo-z accuracy do not appear to have a redshift dependence (for z~0-4). These photo-z's appear to be the best obtained so far for faint X-ray sources, and they have been significantly (>50%) improved compared to previous estimates of the photo-z's for the X-ray sources in the ~2 Ms Chandra Deep Field-North and ~1 Ms Chandra Deep Field-South.

CanChandraResolve the Remaining Cosmic X‐Ray Background?

The deepest extragalactic X-ray observation, the 2 Ms Chandra Deep Field North (CDF-N), resolves ~80% of the total extragalactic cosmic X-ray background (CXB) in the 1-2 keV band. Recent work has shown that 70% of the remaining CXB flux is associated with sources detected by the Hubble Space Telescope (HST). This paper uses the existing CDF-N data to constrain the X-ray flux distribution of these X-ray-undetected HST sources by comparing the number of 0.5-2 keV X-ray counts at the HST positions to those expected for model flux distributions. In the simple case where all the undetected HST X-ray sources have the same 0.5-2 keV flux, the data are best fit by 1.5-3 counts per source in 2 Ms, compared to a detection limit (at 10% completeness) of 9 counts. Assuming a more realistic power-law log N − log S distribution [N(> S) ∝ S−α], the data favor a relatively steep flux distribution, with α = 1.1−0.3+0.5 (limits are 99% confidence). This slope is very similar to that previously found for faint normal and starburst galaxies in the CDF-N. These results suggest deeper Chandra observations will detect a new population of faint X-ray sources, but extremely deep exposures are needed to resolve the remainder of the soft CXB. In the most optimistic scenario, when the HST sources have the flattest allowed flux distribution and all the sources without HST counterparts are detected, observations 5 times more sensitive than the existing ones would resolve at most ~60% of the remaining soft CXB.

Chandra Stacking Constraints on the Contribution of 24 μm Spitzer Sources to the Unresolved Cosmic X-Ray Background

We employ X-ray stacking techniques to examine the contribution from X-ray-undetected, mid-infrared-selected sources to the unresolved, hard (6-8 keV) cosmic X-ray background (CXB). We use the publicly available, 24 μm Spitzer Space Telescope MIPS catalogs from the Great Observatories Origins Deep Survey (GOODS)-North and South fields, which are centered on the 2 Ms Chandra Deep Field-North and 1 Ms Chandra Deep Field-South, to identify bright (S24 μm > 80 μJy) mid-infrared sources that may be powered by heavily obscured AGNs. We measure a significant stacked X-ray signal in all of the X-ray bands examined, including, for the first time, a significant (3.2 σ) 6-8 keV stacked X-ray signal from an individually undetected X-ray source population. We find that the X-ray-undetected MIPS sources make up ≈2% (or less) of the total CXB below 6 keV, but ≈6% in the 6-8 keV band. The 0.5-8 keV stacked spectrum is consistent with a hard power law (Γ = 1.44 ± 0.07), with the spectrum hardening at higher X-ray energies. Our findings show that these bright MIPS sources do contain obscured AGNs, but are not the primary source of the unresolved ~40% of 6-8 keV CXB. Our study rules out obscured, luminous quasars as a significant source of the remaining unresolved CXB and suggests that it most likely arises from a large population of obscured, high-redshift (z 1), Seyfert-luminosity AGNs.

X-ray selected starbursts in the Great Observatories Origins Deep Survey-North

We investigate claims that recent ultra-deep X-ray surveys are detecting starbursts at cosmologically interesting redshifts (z= 0–1). We combine X-ray data from the 2 Ms Chandra Deep Field-North and multiwavelength observations obtained as part of the Great Observatories Origins Deep Survey (GOODS)-North to build the Spectral Energy Distributions (UV, optical, infrared) of X-ray sources in this field. These are fit with model templates providing an estimate of the total infrared luminosity (3–1000 μm) of each source. We then exploit the tight correlation between infrared and X-ray luminosities for star-forming galaxies, established in the local Universe, to select sources that are dominated by star formation rather than supermassive black hole accretion. This approach is efficient in discriminating normal galaxies from active galactic nuclei (AGN) over a wide range of star formation rates, from quiescent systems to starbursts. The above methodology results in a sample of 45 X-ray selected star-forming systems at a median redshift z≈ 0.5, the majority of which (60 per cent) are either luminous or ultraluminous infrared galaxies. This sample is least affected by incompleteness and residual AGN contamination and is therefore well suited for cosmological studies. We quantify the X-ray evolution of these sources by constructing their differential X-ray counts, dN/dS, and comparing them with evolving luminosity function models. The results are consistent with luminosity evolution of the form (1 +z)p with p≈ 2.4. This is similar to the evolution rate of star-forming galaxies selected at other wavelengths, suggesting that the deep X-ray surveys, like the Chandra Deep Field-North, are indeed finding the starburst galaxy population that drives the rapid evolution of the global star formation rate density in the range z≈ 0–1. Our analysis also reveals a separate population of infrared-faint X-ray sources at moderate z. These include old galaxies but also systems that are X-ray luminous for their stellar mass compared to local ellipticals. We argue that these may be post-starbursts that will, over time, become fainter at X-ray wavelengths and will eventually evolve into early-type systems (i.e. E/S0).

The X‐Ray Spectral Properties of SCUBA Galaxies

Deep SCUBA surveys have uncovered a large population of massive submillimeter-emitting galaxies (SMGs; f850μm 4 mJy) at z 1. Although it is generally believed that these galaxies host intense star formation activity, there is growing evidence that a substantial fraction also harbor an active galactic nucleus (AGN; i.e., an accreting super-massive black hole [SMBH]). We present here possibly the strongest evidence for this viewpoint to date: the combination of ultradeep X-ray observations (the 2 Ms Chandra Deep Field-North) and deep Keck spectroscopic data of SMGs with radio counterparts. We find that the majority (≈75%) of these radio-selected spectroscopically identified SMGs host AGN activity; the other ≈25% have X-ray properties consistent with star formation (X-ray-derived star formation rates of ≈1300-2700 M☉ yr-1). The AGNs have properties generally consistent with those of nearby luminous AGNs (Γ ≈ 1.8 ± 0.5, NH ≈ 1020-1024 cm-2, and LX ≈ 1043-1044.5 ergs s-1), and the majority (≈80%) are heavily obscured (NH 1023 cm-2). We construct composite rest-frame 2-20 keV spectra for three different obscuration classes [NH 5 × 1023 cm-2], which reveal features not seen in the individual X-ray spectra. An ≈1 keV equivalent width Fe Kα emission line is seen in the composite X-ray spectrum of the most heavily obscured AGNs, suggesting Compton-thick or near Compton-thick absorption. Even taking into account the effects of absorption, we find that the average X-ray to far-IR luminosity ratio of the AGN-classified SMGs (LX/LFIR = 0.004) is approximately 1 order of magnitude below that found for typical quasars. This result suggests that intense star formation activity (of order ≈1000 M☉ yr-1) dominates the bolometric output of these SMGs. However, we also explore the possibility that the X-ray to far-IR luminosity ratio of the AGN components is intrinsically less than that found for typical quasars and postulate that some SMGs may be AGN dominated. We investigate the implications of our results for the growth of massive black holes, discuss the prospects for deeper X-ray observations, and explore the scientific potential offered by the next generation of X-ray observatories.

The spatial clustering of X-ray selected AGN and galaxies in the Chandra Deep Field South and North

We investigate the spatial clustering of X-ray selected sources in the two deepest X-ray fields to date, namely the 2 Ms Chandra Deep Field North (CDFN) and the 1 Ms Chandra Deep Field South (CDFS). The projected correlation function w(rp), measured on scales ∼0.2−10 h −1 Mpc for a sample of 240 sources with spectroscopic redshift in the CDFN and 124 sources in the CDFS at a median redshift of ¯ z ∼ 0.8 is used to constrain the amplitude and slope of the real space correlation function ξ(r) = (r/r0) −γ . The clustering signal is detected at high confidence (> 7σ) in both fields. The amplitude of the correlation is found to be significantly different in the two fields, the correlation length r0 being 8.6 ± 1.2 h −1 Mpc in the CDFS and 4.2 ± 0.4 h −1 Mpc in the CDFN, while the correlation slope γ is found to be flat in both fields: γ = 1.33 ± 0.11 in the CDFS and γ = 1.42 ± 0.07 in the CDFN (a flat Universe with Ωm = 0. 3a ndΩΛ = 0.7 is assumed; 1σ Poisson error estimates are considered). The correlation function also has been measured separately for sources classified as AGN or galaxies. In both fields AGN have a median redshift of ¯ ∼ 0. 9a nd am edian 0.5−10 keV luminosity of ¯ Lx ∼ 10 43 erg s −1 , i.e. they are generally in the Seyfert luminosity regime. As in the case of the total samples, we found a significant difference in the AGN clustering amplitude between the two fields, the best fit correlation parameters being r0 = 10.3 ± 1.7 h −1 Mpc, γ = 1.33 ± 0.14 in the CDFS, and r0 = 5.5 ± 0.6 h −1 Mpc, γ = 1.50 ± 0.12 in the CDFN. In the CDFN, where the statistics are sufficiently high, we were also able to measure the clustering of X-ray selected galaxies, finding r0 = 4.0 ± 0.7 h −1 Mpc and γ = 1.36 ± 0.15. Within each field no statistically significant difference is found between soft and hard X-ray selected sources or between type 1 and type 2 AGN. After having discussed and ruled out the possibility that the observed variance in the clustering amplitude is due to observational biases, we verified that the extra correlation signal in the CDFS is primarily due to the two prominent redshift spikes at z ∼ 0.7 reported by Gilli et al. (2003). The high (5−10 h −1 Mpc) correlation length measured for the X-ray selected AGN at z ∼ 1i n the twoChandra Ms fields is comparable to that of early type galaxies at the same redshift. This is consistent with the idea that, at z ∼ 1, AGN with Seyfert-like luminosities might be generally hosted by massive galaxies.

X-Ray Properties of Lyman Break Galaxies in the Great Observatories Origins Deep Survey

We constrain the X-ray emission properties of Lyman break galaxies (LBGs) at z ≈ 3–6 using the ≈2 Ms Chandra Deep Field North and ≈1 Ms Chandra Deep Field South. Large samples of LBGs were discovered using the Hubble Space Telescope as part of the Great Observatories Origins Deep Survey (GOODS). Deep optical and X-ray imaging over the GOODS fields have allowed us to place the most significant constraints on the X-ray properties of LBGs to date. Mean X-ray properties of 449, 1734, 629, and 247 LBGs with z ∼ 3, 4, 5, and 6, respectively, were determined using stacking techniques. When stacked, we detect X-ray emission from LBGs at z ∼ 3 (∼7 σ) and from an optically bright subset (brightest 25%) of LBGs at z ∼ 4 (∼3 σ); the latter is the highest redshift detection yet for "normal" galaxies in the X-ray band. The effective exposure times for these stacked observations are ≈0.7 and 0.5 Gs, respectively. The derived average rest-frame 2.0–8.0 keV luminosities are 1.5 × 1041 and 1.4 × 1041 ergs s-1, respectively. X-ray emission from these LBGs is likely due to high-mass X-ray binaries and Type II supernovae; the corresponding star formation rates are ≈10–30 M⊙ yr-1. The X-ray–to–B-band mean luminosity ratio (LX/LB) at z ∼ 3 is somewhat elevated with respect to that measured for starburst galaxies in the local universe (significance ∼3 σ). When stacking full samples of LBGs at z ∼ 4, 5, and 6, we do not obtain significant detections (<3 σ) and derive rest-frame 2.0–8.0 keV luminosity upper limits (3 σ) of 0.9, 2.8, and 7.1 × 1041 ergs s-1, respectively. These upper limits constrain any widespread active galactic nucleus (AGN) activity in these objects to be modest at best. Furthermore, we find that ∼0.5% of our LBGs from z ≈ 3 to z ≈ 6 are detected individually in the X-ray band. These LBGs have spectral shapes and luminosities characteristic of moderate-power AGNs (e.g., Seyfert galaxies and quasars).

The 172 ksChandraExposure of the LALA Bootes Field: X-Ray Source Catalog

We present an analysis of a deep, 172 ks Chandra observation of the Large Area Lyman Alpha survey (LALA) Bootes field, obtained with the Advanced CCD Imaging Spectrometer (ACIS-I) on board the Chandra X-Ray Observatory. This is one of the deepest Chandra images of the extragalactic sky; only the 2 Ms Chandra Deep Field North (CDF-N) and 1 Ms Chandra Deep Field South (CDF-S) are substantially deeper. In this paper we present the X-ray source catalog obtained from this image, along with an analysis of source counts and optical identifications. The X-ray image is composed of two individual observations obtained in 2002 and reaches 0.5–2.0 and 2.0–10.0 keV flux limits of 1.5 × 10-16 and 1.0 × 10-15 ergs cm-2 s-1, respectively, for point sources near the aim point. A total of 168 X-ray sources were detected: 160 in the 0.5–7.0 keV band, 132 in the 0.5–2.0 keV band, and 111 in the 2.0–7.0 keV band. The X-ray source counts were derived and compared with those from other Chandra deep surveys; the hard X-ray source density of the LALA Bootes field is 33% higher than that of CDF-S at the flux level of 2.0 × 10-15 ergs cm-2 s-1, confirming the field-to-field variances of the hard-band source counts reported by previous studies. The deep exposure resolves 72% of the 2.0–10.0 keV X-ray background. Our primary optical data are R-band imaging from the NOAO Deep Wide-Field Survey (NDWFS), with a limiting magnitude of R = 25.7 (Vega, 3 σ, and 4'' diameter aperture). We have found optical counterparts for 152 of the 168 Chandra sources (90%); 144 of these are detected in the R-band image, and eight have optical counterparts in other bands (either BW, V, I, or z'). Among the R-band nondetected sources, not more than 11 of them can possibly be at z > 5, based on the hardness ratios of their X-ray emission and nondetections in bluer bands (BW, V). The majority (~76%) of the X-ray sources are found to have log(fX/fR) within 0.0 ± 1; these are believed to be AGNs. Most of the X-ray–faint/optically bright sources [log(fX/fR) 1.0], which are harder in X-ray and are probably obscured AGNs.

X-ray spectroscopy and variability of AGN detected in the 2 Ms Chandra Deep Field-North Survey

We investigate the nature of the faint X-ray source population through X-ray spectroscopy and variability analyses of 136 active galactic nuclei (AGN) detected in the 2 Ms Chandra Deep Field-North Survey with >200 background-subtracted 0.5–8.0 keV counts [ F 0.5–8.0 keV = (1.4−200) × 10 −15 erg cm −2 s −1]. Our preliminary spectral analyses yield median spectral parameters of Γ = 1.61 and intrinsic N H = 6.2 × 10 21 cm −2 ( z = 1 assumed when no redshift available) when the AGN spectra are fitted with a simple absorbed power-law model. However, considerable spectral complexity is apparent (e.g., reflection, partial covering) and must be taken into account to model the data accurately. Moreover, the choice of spectral model (i.e., free vs. fixed photon index) has a pronounced effect on the derived JVH distribution and, to a lesser extent, the X-ray luminosity distribution. We also find that among the 136 AGN, 10 (≈7%) show significant Fe Kα emission-line features with equivalent widths in the range 0.1–1.3 keV. Two of these emission-line AGN could potentially be Compton thick (i.e., Γ < 1.0 and large Fe Kα equivalent width). Finally, we find that 81 (≈60%) of the 136 AGN show signs of variability, and that this fraction increases significantly (≈80–90%) when better photon statistics are available.

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 AGN source population in the Chandra Deep Field‐North Survey: constraints from X‐ray spectroscopy and variability

We briefly report here on our ongoing X-ray spectroscopy and variability analyses of 136 faint X-ray sources discovered in the 2 Ms Chandra Deep Field North observations spanning 1.4x10-15 to 2x10-13 ergs s-1 cm-2. Most sources are adequatley fitted with a simple intrinsic absorption plus power-law model, although the resulting photon index is much lower than the canonical average unabsorbed spectral slope found locally (suggesting additional spectral complexity). Among the 136 sources, 7% show significant Fe (Ka) emission features, with equivalent widths in the range 0.1-1.3 keV. Two of these emission-line sources appear to be Compton thick. For the subset of sources with redshifts, we also find that fewer than 10% are likely to be Compton thick. Finally, ~60% of the sample show signs of long-term variability. This fraction climbs to ~90% for the bright sources (>500 cnts) or BLAGN.

The Chandra Deep Field North Survey. XIV. X-Ray–Detected Obscured AGN[CLC]s[/CLC] and Starburst Galaxies in the Bright Submillimeter Source Population

We provide X-ray constraints and perform the first X-ray spectral analyses for bright (f_850>=5mJy; S/N>=4) SCUBA sources in an 8.4'x8.4' area of the 2 Ms Chandra Deep Field-North survey containing the Hubble Deep Field-North. X-ray emission is detected from 7 of the 10 bright submm sources in this region, corresponding to an X-ray detected submm source density of ~360 deg^-2 (>~36% of the bright submm source population). Two of the X-ray detected sources have nearby (within 3") X-ray companions, suggesting merging/interacting sources or gravitational lensing effects, and 3 lie within the approximate extent of a proto-cluster candidate. Five of the X-ray detected sources have flat X-ray spectral slopes, suggesting obscured AGN activity. X-ray spectral analyses suggest that one of these AGNs may be a Compton-thick source; of the other 4 AGNs, 3 appear to be Compton-thin sources and one has poor constraints. The rest-frame unabsorbed X-ray luminosities of these AGNs are more consistent with those of Seyfert galaxies than QSOs. Thus, the low X-ray detection rate of bright submm sources by moderately deep X-ray surveys appears to be due to the relatively low luminosities of the AGNs rather than Compton-thick absorption. A comparison of these sources to the well-studied heavily obscured AGN NGC6240 shows that the average AGN contribution is negligible at submm wavelengths. The X-ray properties of the other 2 X-ray detected sources are consistent with those expected from luminous star formation; however, we cannot rule out the possibility that low-luminosity AGNs are present. The 3 X-ray undetected sources appear to lie at high redshift (z>4) and could be either AGNs or starbust galaxies.

The Chandra Deep Field–North Survey. XI. X-Ray Emission from Luminous Infrared Starburst Galaxies

Using the 1 Ms Chandra Deep Field-North and 15 μm ISOCAM Hubble Deep Field-North surveys, we find a tight correlation between the population of strongly evolving starburst galaxiess discovered in faint 15 μm ISOCAM surveys and the apparently normal galaxy population detected in deep X-ray surveys. Up to 100% of the X-ray-detected emission-line galaxies (ELGs) have 15 μm counterparts, in contrast to 10%-20% of the X-ray-detected absorption-line galaxies and AGN-dominated sources. None of the X-ray-detected ELGs are detected in the hard band (2-8 keV), and their stacked-average X-ray spectral slope of Γ ≈ 2.0 suggests a low fraction of obscured AGN activity within the X-ray-detected ELG population. The characteristics of the z = 0.4-1.3 X-ray-detected ELGs are consistent with those expected for M82- and NGC 3256-type starburst galaxies; these X-ray-detected ELGs contribute ≈2% of the 0.5-8.0 keV extragalactic X-ray background. The only statistical difference between the X-ray-detected and X-ray-undetected 15 μm-selected ELGs is that a much larger fraction of the former have radio emission.

The Chandra Deep Field North Survey. IX. Extended X-Ray Sources

The ≈1 Ms Chandra Deep Field North observation is used to study the extended X-ray sources in the region surrounding the Hubble Deep Field North (HDF-N), yielding the most sensitive probe of extended X-ray emission at cosmological distances to date. A total of six such sources are detected, the majority of which align with small numbers of optically bright galaxies. Their angular sizes, band ratios, and X-ray luminosities—assuming they lie at the same distances as the galaxies coincident with the X-ray emission—are generally consistent with the properties found for nearby groups of galaxies. One source is notably different and is likely to be a poor-to-moderate X-ray cluster at high redshift (i.e., z 0.7). This source has a large angular extent, a double-peaked X-ray morphology, and an overdensity of unusual objects [very red objects, optically faint (I ≥ 24) radio and X-ray sources]. Another of the six sources is coincident with several z ≈ 1.01 galaxies located within the HDF-N itself, including the FR I radio galaxy VLA J123644+621133, and is likely to be a group or poor cluster of galaxies at that redshift. We are also able to place strong constraints on the optically detected cluster of galaxies ClG 1236+6215 at z = 0.85 and the wide-angle–tailed radio galaxy VLA J123725+621128 at z ~ 1–2; both sources are expected to have considerable associated diffuse X-ray emission, and yet they have rest-frame 0.5–2.0 keV X-ray luminosities of 3 × 1042 and (3–15) × 1042 ergs s-1, respectively. The environments of both sources are either likely to have a significant deficit of hot intracluster gas compared with local clusters of galaxies, or they are X-ray groups. We find the surface density of extended X-ray sources in this observation to be 167 deg-2 at a limiting soft-band flux of ≈3 × 10-16 ergs cm-2 s-1. No evolution in the X-ray luminosity function of clusters is needed to explain this value.

The [ITAL]Chandra[/ITAL] Deep Field–North Survey. VII. X-Ray Emission from Lyman Break Galaxies

We present results from stacking analyses, using the 1 Ms Chandra Deep Field-North data, that constrain the X-ray emission of Lyman break galaxies at z ≈ 2-4. Stacking the counts from 24 individually undetected Lyman break galaxies located within the Hubble Deep Field-North, we have obtained average detections of these objects in the resulting 0.5-8.0 and 0.5-2.0 keV images; these images have effective exposure times of 22.4 Ms (260 days). Monte Carlo testing empirically shows the detections to be highly significant. The average rest-frame 2-8 keV luminosity of a Lyman break galaxy is derived to be ≈3.2 × 1041 ergs s-1, comparable to that of the most X-ray luminous starbursts in the local universe. The observed ratio of X-ray to B-band luminosity is somewhat, but not greatly, higher than that seen from local starbursts. The X-ray emission probably arises from a combination of high-mass X-ray binaries, super-Eddington X-ray sources, and low-luminosity active galactic nuclei.