arXiv subjects

F. E. Bauer

Publications and source records attributed to F. E. Bauer.

286 records · Page 16Linked to original sources

arXiv2002

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.

astro-ph↗

arXiv2002

On the nature of Extremely Red Objects: the 2 Ms Chandra Deep Field-North Survey results

Understanding the nature of Extremely Red Objects [EROs; (I-K)>=4] is one of the most challenging issues in observational cosmology. Here we report on the X-ray constraints provided by the 2 Ms Chandra Deep Field-North Survey (CDF-N). X-ray emission has been detected from 11 out of 36 EROs (approx 30%). Five of these have hard X-ray emission and appear to be obscured AGNs, while non-AGN emission (star formation or normal elliptical galaxy emission) is likely to be the dominant source of X-rays from the soft X-ray sources detected at the faintest X-ray flux levels.

astro-ph↗

arXiv2002

The Chandra Deep Field North Survey. XII. The Link Between Faint X-ray and Radio Source Populations

We investigate the relationship between faint X-ray and 1.4 GHz radio source populations detected within 3' of the Hubble Deep Field North using the 1 Ms Chandra and 40 uJy VLA surveys. Within this region, we find that ~42% of the 62 X-ray sources have radio counterparts and ~71% of the 28 radio sources have X-ray counterparts; thus a 40 uJy VLA survey at 1.4 GHz appears to be well-matched to a 1 Ms Chandra observation. Among the different source populations sampled, we find that the majority of the 18 X-ray detected emission-line galaxies (ELGs) have radio and mid-infrared ISOCAM counterparts and appear to be luminous star-forming galaxies at z=0.3-1.3. Importantly, the radio-detected ELGs make up ~35% of the X-ray source population at 0.5-8.0 keV X-ray fluxes between ~(1-5)x10e-16 erg/cm2/s and signal the emergence of the luminous, high-z starburst galaxy population in the X-ray band. We find that the locally-determined correlation between X-ray luminosities and 1.4 GHz radio luminosity densities of the late-type galaxies can easily be extended to include the luminous intermediate-redshift ELGs, suggesting that the X-ray and radio emission processes are generally associated in star-forming galaxies. This result implies that the X-ray emission can be used as an indicator of star formation rate for star-forming galaxies. Finally, we show that there appear to be two statistically distinct types of ISOCAM-detected star-forming galaxies: those with detectable radio and X-ray emission and those without. The latter type may have stronger mid-infrared emission-line features that increase their detectability at mid-infrared wavelengths.

astro-ph↗

arXiv2002

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 micron ISOCAM HDF-N surveys we find a tight correlation between the population of strongly evolving starbursts discovered in faint 15 micron 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 micron 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 is detected in the hard band (2-8 keV), and their stacked-average X-ray spectral slope of Gamma=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 starbursts; these X-ray detected ELGs contribute ~2% of the 0.5-8.0 keV X-ray background. The only statistical difference between the X-ray detected and X-ray undetected 15 micron selected ELGs is that a much larger fraction of the former have radio emission.

astro-ph↗

arXiv2002

The Chandra Deep Field-North Survey and the Cosmic X-ray Background

Chandra has performed a 1.4 Ms survey centred on the Hubble Deep Field-North (HDF-N), probing the X-ray Universe 55-550 times deeper than was possible with pre-Chandra missions. We describe the detected point and extended X-ray sources and discuss their overall multiwavelength (optical, infrared, submillimeter, and radio) properties. Special attention is paid to the HDF-N X-ray sources, luminous infrared starburst galaxies, optically faint X-ray sources, and high-to-extreme redshift AGN. We also describe how stacking analyses have been used to probe the average X-ray emission properties of normal and starburst galaxies at cosmologically interesting distances. Finally, we discuss plans to extend the survey and argue that a 5-10 Ms Chandra survey would lay key groundwork for future missions such as XEUS and Generation-X.

astro-ph↗

arXiv2002

Exotic sources detected in the 1 Ms Chandra Deep Field North Survey

We provide constraints on optically faint (I>=24) X-ray sources, z>6 AGN, Very Red Objects (VROs; I-K>=4), and optically faint radio sources using the 1 Ms Chandra Deep Field North survey. We argue that the majority of the optically faint X-ray sources are obscured AGN at z=1-3. These sources comprise up to 50% of the X-ray detected AGN. Approximately 30% of the optically faint X-ray sources have properties consistent with those expected for z>6 AGN; however we argue that the majority of these sources lie at lower redshifts. The constraint we place on the source density of z>6 AGN (<~0.09 arcmin^{-2}) is below that predicted by some models. Approximately 30% of the optically faint X-ray sources are also VROs. Within the K<=20.1 VRO population, ~15% of the sources are AGN dominated. Conversely, we present evidence suggesting that non-AGN dominated VROs are detected at the flux limit of this 1 Ms Chandra survey. Finally, we show that ~50% of the optically faint radio source population are detected with X-ray emission. The majority of these sources appear to be obscured AGN.

astro-ph↗

arXiv2001

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). 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-tail radio galaxy VLA J123725.7+621128 at z~1-2. With rest-frame 0.5--2.0 keV X-ray luminosities of <(3-15)e42 ergs s^{-1}, the environments of both sources are either likely to have a significant deficit of hot intra-cluster gas compared to 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 (+97,-67) deg^{-2} at a limiting soft-band flux of approximately 3e-16 ergs s^{-1} cm^{-2}. No evolution in the X-ray luminosity function of clusters is needed to explain this value. (Abridged)

astro-ph↗

arXiv2001

The Chandra Deep Field North Survey. X. X-ray Emission from Very Red Objects

The multi-wavelength properties of Very Red Objects (VROs; I-K>=4) are largely unknown as many of these sources are optically faint (I>=24) and undetected at most wavelengths. Here we provide constraints on the X-ray properties of VROs using the 1 Ms Chandra exposure of an 8.4'x8.4' region within the Hawaii Flanking-Field area containing the HDF-N. We find that VROs detected in the hard band (2-8 keV) have flat X-ray spectral slopes (Gamma ~ 0.9) and X-ray properties consistent with those expected from luminous obscured AGN. The fraction of such sources in the K<=20.1 VRO population is ~14%. Conversely, the average X-ray spectral slope of VROs detected in the soft band (0.5-2 keV) but not in the hard band is comparatively steep (Gamma>1.4), and the X-ray emission from these sources is consistent with that expected from less energetic processes (i.e., star formation, low-luminosity AGN activity, normal elliptical galaxy emission); star-formation and low-luminosity AGN activity scenarios are favored in those sources with irregular optical morphologies. Stacking analyses of the X-ray emission from VROs not individually detected at X-ray energies yield significant detections (>=99% confidence) in the soft band and the full band (0.5-8 keV). We find this X-ray emission is produced predominantly by the optically brightest VROs. The simplest explanation of this result is that we have detected the average X-ray emission from non-active VROs with low X-ray-to-optical flux ratios [log(FX/FI) ~ -2]; this is consistent with that expected if the majority of these VROs are ~ M* elliptical galaxies. A number of VROs are also detected with mid-IR (15um) and radio emission, and we provide constraints on the nature of this emission.

astro-ph↗

arXiv2001

The Chandra Deep Field North Survey. VIII. X-ray Constraints on Spiral Galaxies from 0.4 < z < 1.5

We present a statistical X-ray study of spiral galaxies in the Hubble Deep Field-North and its Flanking Fields using the Chandra Deep Field North 1 Ms dataset. We find that approximately L_star galaxies with 0.4 < z < 1.5 have ratios of X-ray to B-band luminosity similar to those in the local Universe, although the data indicate a likely increase in this ratio by a factor of approximately 2-3. We have also determined that typical spiral galaxies at 0.4 < z < 1.5 should be detected in the 0.5--2 keV band in the flux range 3-6 X 10^-18 erg cm^-2 s^-1.

astro-ph↗

arXiv2001

The Chandra Deep Field North Survey. VI. The Nature of the Optically Faint X-ray Source Population

We provide constraints on the nature of the optically faint (I>=24) X-ray source population from a 1 Ms Chandra exposure of a 8.4x8.4 arcmin region containing the Hubble Deep Field North. We detect 47 (2,400 deg^-2) optically faint X-ray sources. The fraction of optically faint X-ray sources is approximately constant (at approx 35%) for 0.5-8.0 keV fluxes from 3x10^-14 cgs down to the X-ray flux limit. A considerable fraction (approx 30%) of the sources are Very Red Objects (I-K>=4). Analysis of the optical and X-ray properties suggests a large number of optically faint X-ray sources are likely to host obscured AGN activity at z= 1-3. From these results we calculate that a significant fraction (approx 5-45%) could be obscured QSOs at z<=3. Given the number of X-ray sources without I-band counterparts, there are unlikely to be more than approx 15 sources at z>6. We provide evidence that the true number of z>6 sources is considerably lower. Nine optically faint X-ray sources have micro-Jy radio counterparts; approx 53% of the optically faint micro-Jy radio sources in this region. The most likely origin of the X-ray emission in these X-ray detected, optically faint micro-Jy radio sources is obscured AGN activity. However, two of these sources have been previously detected at sub-millimeter wavelengths and the X-ray emission from these sources could be due to luminous star formation activity. Assuming the SED of NGC 6240, we estimate the 175 micron flux of a typical optically faint X-ray source to be <10 mJy. As expected for sources with AGN activity, the two optically faint X-ray sources within the most sensitive area of the ISOCAM region have faint (approx 50 micro-Jy) 15 micron counterparts. (ABRIDGED).

astro-ph↗

arXiv2001

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

An extremely deep X-ray survey (about 1 Ms) of the Hubble Deep Field North and its environs (about 450 arcmin^2) 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 keV and 2-8 keV flux limits of 3 x 10^{-17} erg/cm^2/s and 2 x 10^{-16} erg/cm^2/s, 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 0.6-1.7 arcsec (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^{+1100}_{-940} deg^{-2} (at 4.2 x 10^{-17} erg/cm^2/s) and 4200^{+670}_{-580} deg^{-2} (at 3.8 x 10^{-16} erg/cm^2/s) are observed in the soft and hard bands, respectively.

astro-ph↗

arXiv2001

The Chandra 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 keV 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 x 10^{41} erg 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.

astro-ph↗

arXiv2001

A Chandra Study of the Circinus Galaxy Point-Source Population

We have used the Chandra X-ray Observatory to resolve spatially and spectrally the X-ray emission from the Circinus Galaxy. We report here on the nature of the X-ray emission from the off-nuclear point sources associated with the disk of Circinus. We find that many of the serendipitous X-ray sources are concentrated along the optical disk of the galaxy, but few have optical counterparts within 1" of their X-ray positions down to V=23-25. At 3.8 Mpc, their intrinsic 0.5-10 keV luminosities range from approx. 2E37 erg/s to 4E39 erg/s. One quarter of the sources are variable over the duration of the 67 ks observation, and spectral fitting of these off-nuclear sources shows a diverse range of spectral properties. The properties of the two strongest off-nuclear sources are remarkable, with average X-ray luminosities of 3.7E39 erg/s and 3.4E39 erg/s. The former displays large and periodic flux variations every 7.5 hr and is well fit by a multicolor blackbody accretion-disk model with T_in=1.35 keV, properties consistent with an eclipsing >50 M_sun black-hole binary. The latter appears to be a young supernova remnant, as it coincides with a non-thermal radio counterpart and an Hα-detected HII region. This source exhibits both long-term (approx. 4 yr) X-ray variability and a 6.67-6.97 keV iron emission-line blend with a 1.6 keV equivalent width. These two objects further support the notion that super-Eddington X-ray sources in nearby galaxies can be explained by a mixture of intermediate-mass black holes in X-ray binaries and young supernova remnants. (abridged)

astro-ph↗

arXiv2000

RBSC-NVSS Sample. I. Radio and Optical Identifications of a Complete Sample of 1500 Bright X-ray Sources

We cross-identified the ROSAT Bright Source Catalog (RBSC) and the NRAO VLA Sky Survey (NVSS) to construct the RBSC-NVSS sample of the brightest X-ray sources (>= 0.1 counts/s or ~1E-12 ergs/cm/cm/s in the 0.1-2.4 keV band) that are also radio sources (S >= 2.5 mJy at 1.4 GHz) in the 7.8 sr of extragalactic sky with |b| > 15 degrees. and delta > -40 degrees. The sky density of NVSS sources is low enough that they can be reliably identified with RBSC sources having average rms positional uncertainties = 10 arcsec. We used the more accurate radio positions to make reliable X-ray/radio/optical identifications down to the POSS plate limits. We obtained optical spectra for many of the bright identifications lacking published redshifts. The resulting X-ray/radio sample is unique in its size (N ~ 1500 objects), composition (a mixture of nearly normal galaxies, Seyfert galaxies, quasars, and clusters), and low average redshift ( ~ 0.1).

astro-ph↗

arXiv1999

High Resolution Radio Imaging of Distant Submillimeter Galaxies

Using a combination of radio and optical imaging at 0.1\arcs to 0.2\arcs resolution with the VLA/MERLIN and HST has led to a breakthrough in our understanding of radio emission from distant (0.1 4-6) and submillimeter sources with S > 2 mJy at 850 microns as measured with SCUBA on the JCMT. The far-infrared luminosities of these galaxies exceeds even the most intense starbursts found in the local universe (e.g., Arp 220), suggesting they are in the process of converting the bulk of their gass mass into stars. These galaxies, completely absent in optical surveys, constitute 50% - 90% of the star-formation density in the distant Universe. Given the poor sub-mm resolution (15\arcs) of the SCUBA/JCMT images, we use the 0.2\arcs radio imaging as a surrogate in order to understand the nature of the dominant emission mechanism driving the FIR luminosity (AGN vs. star-formation). Upcoming developments in radio instrumentation (the Expanded VLA Array and Square Kilometer Array) will soon increase sensitivity and resolution orders of magnitude, providing a natural complement to parallel developments in sub-mm facilities (e.g., ALMA). With dual radio continuum and sub-mm surveys of the distant Universe, a census of galaxy evolution to the earliest cosmic epochs (z = 5-30) will soon be possible.

astro-ph↗

arXiv1999

X-ray Properties of the Abell 644 Cluster of Galaxies

We use new ASCA observations and archival ROSAT Position Sensitive Proportional Counter (PSPC) data to determine the X-ray spectral properties of the intracluster gas in Abell 644. From the overall spectrum, we determine the average gas temperature to be 8.64 (+0.67,-0.56) keV, and an abundance of 0.32 (+/-0.04) $Z_{\odot}$. The global ASCA and ROSAT spectra imply a cooling rate of 214 (+100,-91) $M_{\odot}$ yr$^{-1}$. The PSPC X-ray surface brightness profile and the ASCA data suggest a somewhat higher cooling rate. We determine the gravitational mass and gas mass as a function of radius. The total gravitating mass within 1.2 Mpc is $6.2\times10^{14}$ $M_{\odot}$, of which 20% is in the form of hot gas. There is a region of elevated temperature 1.5-5 arcmin to the west of the cluster center. The south-southwest region of the cluster also shows excess emission in the ROSAT PSPC X-ray image, aligned with the major axis of the optical cD galaxy in the center of the cluster. We argue that the cluster is undergoing or has recently undergone a minor merger. The combination of a fairly strong cooling flow and evidence for a merger make this cluster an interesting case to test the disruption of cooling flow in mergers.

astro-ph↗