arXiv subjects

L. L. Cowie

Publications and source records attributed to L. L. Cowie.

At least 91 records · Page 5Linked to original sources

arXiv2002

2-8 keV X-ray number counts determined from Chandra blank field observations

We present the 2-8 keV number counts from the 1Ms Chandra observation of the Chandra Deep Field North (CDF-N). We combine these with the number counts from a 78ks exposure of the Hawaii Survey Field SSA22 and with the number counts obtained in independent analyses of the CDF-S and the Hawaii Survey Field SSA13 to determine the number counts from 2.10^(-16) to 10^(-13) ergs/cm2/s. Over this flux range the contribution to the X-ray background is 1.1 10^(-11) ergs/cm2/s/square degree. When the contributions above 10^(-13) ergs/cm2/s from BeppoSAX or ASCA observations are included, the total rises to 1.4 10^(-11) ergs/cm2/s/square degree. However, there appears to be substantial field-to-field variation in the counts in excess of the statistical uncertainties. When the statistical and flux calibration uncertainties (in both the background and source measurements) are taken into account, as much as 0.5 10^(-11) ergs/cm2/s/square degree could still be present in an unresolved component.

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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.

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arXiv2001

Resolving the Structure of Ionized Helium in the Intergalactic Medium with the Far Ultraviolet Spectroscopic Explorer

The neutral hydrogen and the ionized helium absorption in the spectra of quasars are unique probes of structure in the early universe. We present Far-Ultraviolet Spectroscopic Explorer observations of the line of sight to the quasar HE2347-4342 in the 1000-1187 A band at a resolving power of 15,000. We resolve the He II Ly alpha absorption as a discrete forest of absorption lines in the redshift range 2.3 to 2.7. About 50 percent of these features have H I counterparts with column densities log N(HI) > 12.3 that account for most of the observed opacity in He II Ly alpha. The He II to H I column density ratio ranges from 1 to >1000 with an average of ~80. Ratios of <100 are consistent with photoionization of the absorbing gas by a hard ionizing spectrum resulting from the integrated light of quasars, but ratios of >100 in many locations indicate additional contributions from starburst galaxies or heavily filtered quasar radiation. The presence of He II Ly alpha absorbers with no H I counterparts indicates that structure is present even in low-density regions, consistent with theoretical predictions of structure formation through gravitational instability.

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arXiv2001

The Submillimeter Properties of the 1 Ms Chandra Deep Field North X-ray Sample

We present submillimeter observations for 136 of the 370 X-ray sources detected in the 1 Ms exposure of the Chandra Deep Field North. Ten of the X-ray sources are significantly detected in the submillimeter. The average X-ray source in the sample has a significant 850 micron flux of 1.69+/-0.27 mJy. This value shows little dependence on the 2-8 keV flux from 5e-16 erg/cm^2/s to 1e-14 erg/cm^2/s. The ensemble of X-ray sources contribute about 10% of the extragalactic background light at 850 microns. The submillimeter excess is found to be strongest in the optically faint X-ray sources that are also seen at 20 cm, which is consistent with these X-ray sources being obscured and at high redshift (z>1).

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arXiv2001

Supermassive Black Hole Accretion History Inferred from a Large Sample of Chandra Hard X-ray Sources

We describe the optical, near-infrared, and radio properties of a sample of hard (2-7 keV) X-ray sources detected in a deep Chandra observation of the field surrounding the Abell 370 cluster. We combine these data with similar observations of the Chandra Deep Field-North and the Hawaii Survey Field SSA13 to obtain a sample of 69 hard X-ray sources (45 are spectroscopically identified) with extremely deep 20 cm observations. We find that about 4% of the >Lstar galaxy population is X-ray luminous at any time and hence that black hole accretion has a duration of about half a Gyr. We find that about 30% of the summed 2-7 keV flux from our total sample is from sources at z<1. We estimate the bolometric luminosities of accretion onto supermassive black holes and the mass inflow rates. The time history of the accretion rate density is evaluated, and its integrated value is reasonably consistent with the value inferred from the local black hole mass to bulge mass ratio.

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arXiv2001

Multi-wavelength observations of serendipitous Chandra sources

We describe progress in a programme to study a sample of sources typical of those which contribute a large fraction of the hard X-ray background. The sources are selected from the fields of approximately 10 ks Chandra cluster observations with follow-up in the near-infrared and optical. The X-ray data indicate that many of these are powerful, obscured sources. Two hard objects which are lensed by the cluster are good candidates for X-ray type-2 quasars, with luminosities of the order 10^{44}-10^{45} erg s^{-1} and obscuring column densities of the order 10^{23} cm^{-2}. We find that the sources are bright in the infra-red (K typically 17-18). From Keck optical spectra and photometric redshifts, we find that the host galaxies are consistent with being early-type, massive hosts at redshifts ranging from 0.2 to 3, with median redshift approximately 1.5. The two obscured quasars also have mid-infrared detections and 850-micron upper-limits implying that the surrounding dust is at warm-to-hot temperatures (T of the order of 1000 K).

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arXiv2001

Multi-wavelength observations of serendipitous Chandra X-ray sources in the field of A2390

We present optical spectra and near-infrared imaging of a sample of serendipitous X-ray sources detected in the field of Chandra observations of the A2390 cluster of galaxies. The sources have 0.5-7 keV fluxes of 0.6-8 x 10^{-14} erg cm^{-2} s^{-1} and lie around the break in the 2-10 keV source counts. They therefore are typical of sources dominating the X-ray Background in that band. Twelve of the fifteen targets for which we have optical spectra show emission lines, most of which have soft X-ray spectra. Including photometric redshifts and published spectra we have redshifts for seventeen of the sources, ranging from z~0.2 up to z~3 with a peak between z=1-2. Ten of our sources have hard X-ray spectra indicating a spectral slope flatter than that of a typical unabsorbed quasar. Two hard sources that are gravitationally lensed by the foreground cluster are obscured quasars, with intrinsic 2-10 keV luminosities of 0.2-3 x 10^{45} erg/s and absorbing columns of N_H>10^{23} cm^{-2}. Both were detected by ISO, showing that the absorbed flux emerges in the far-infrared.

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arXiv2001

Detecting high redshift evolved galaxies as the hosts of optically faint hard X-ray sources

We combine deep Subaru near-infrared images of the massive lensing clusters A2390 and A370 with Keck optical data to map the spectral energy distributions (SEDs) of Chandra X-ray sources lying behind the clusters. The three sources behind A2390 are found to have extremely red colors with SEDs consistent with evolved galaxies at redshifts z>1.4. One source has extremely anomalous colors, which we interpret as evidence for a type Sa SED at a redshift around 2.5. The photometric redshift of another source has been confirmed at z=1.467 from near-infrared spectroscopy using the CISCO spectrograph on Subaru. Mapping of optically faint hard X-ray sources may prove to be an extremely efficient way to locate luminous evolved galaxies at high redshifts.

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arXiv2001

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

We present results from a 221.9 ks Chandra exposure of the HDF-N and its vicinity, concentrating on the 8.6' X 8.7' area covered by the Caltech Faint Field Galaxy Redshift Survey (the `Caltech area'). The minimum detectable fluxes in the 0.5-2 keV and 2-8 keV bands are 1.3e-16 cgs and 6.5e-16 cgs, respectively and a total of 82 sources are detected. More than 80% of the extragalactic X-ray background in the 2-8 keV band is resolved. Redshifts are available for 96% of the sources with R<23; the redshift range is 0.1-3.5 with most sources having z < 1.5. Eight of the X-ray sources are located in the HDF-N itself, including two not previously reported. A population of X-ray faint, optically bright, nearby galaxies emerges at soft-band fluxes of ~< 3e-16 cgs. We set the tightest constraints to date on the X-ray emission properties of microJy radio sources, mid-infrared sources detected by ISO, and very red (R-K_s > 5.0) objects. Where both the infrared and the X-ray coverage are deepest, 75% of the X-ray sources are detected by ISO; 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 by Hogg et al. (2000) are detected in X-rays; these four are among our hardest Chandra sources, and we argue that they contain moderately luminous obscured AGN. Overall, however, the small Chandra detection fraction suggests a relatively small AGN content in the optically selected very red object population. (Abridged)

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arXiv2000

The Nature of the Hard X-ray Background Sources: Optical, Near-infrared, Submillimeter, and Radio Properties

With recent Chandra observations, at least 60% of the 2-10 keV background is now resolved into discrete sources. Here we present deep optical, NIR, submm, and 20 cm (radio) images, as well as high-quality optical spectra, of a complete sample of 20 hard X-ray sources in a deep Chandra observation of the SSA13 field. The thirteen I<23.5 galaxies have redshifts in the range 0.1 to 2.6. Two are quasars, five show AGN signatures, and six are z<1.5 luminous bulge-dominated galaxies whose spectra show no obvious optical AGN signatures. The seven spectroscopically unidentified sources have colors that are consistent with evolved early galaxies at z=1.5-3. Only one hard X-ray source is significantly detected in an ultradeep submm map; its millimetric redshift is in the range z=1.2-2.4. None of the remaining 19 sources is detected in the submm. These results probably reflect the fact that the 850-micron flux limits obtainable with SCUBA are quite close to the expected fluxes from obscured AGN. The hard X-ray sources have an average L(FIR)/L(2-10 keV)~60, similar to that of local obscured AGN. The same ratio for a sample of submm selected sources is in excess of 1100, suggesting that their FIR light is primarily produced by star formation. Our data show that luminous hard X-ray sources are common in bulge-dominated optically luminous galaxies. We use our measured bolometric corrections with the 2-10 keV EBL to infer the growth of supermassive black holes. Even with a high radiative efficiency of accretion (e=0.1), the black hole mass density required to account for the observed light is comparable to the local black hole mass density. (Abridged)

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arXiv2000

ISO deep far-infrared survey in the "Lockman Hole" II. Power spectrum analysis: evidence of a strong evolution in number counts

We investigate the characteristics of FIR brightness fluctuations at 90 micron and 170 micron in the Lockman Hole, which were surveyed with ISOPHOT aboard the Infrared Space Observatory(ISO). We first calculated the angular correlation function of each field and then its Fourier transform(the angular Power Spectral Density: PSD) over the spatial frequency range of f=0.05 - 1 arcmin^-1. The PSDs are found to be rather flat at low spatial frequencies (f <= 0.1 arcmin^-1), slowly decreasing toward higher frequencies. These spectra are unlike the power-law ones seen in the IR cirrus fluctuations, and are well explained by randomly distributed point sources. Furthermore, point-to-point comparison between 90 micron and 170 micron brightness shows a linear correlation between them, and the slope of the linear fit is much shallower than that expected from the IR cirrus color, and is consistent with the color of galaxies at low or moderate redshift(z < 1). We conclude that the brightness fluctuations in the Lockman Hole are not caused by the IR cirrus, but are most likely due to faint star-forming galaxies. We also give the constraints on the galaxy number counts down to 35 mJy at 90 micron and 60 mJy at 170 micron, which indicate the existence of a strong evolution down to these fluxes in the counts. The galaxies responsible for the fluctuations also significantly contribute to the cosmic infrared background radiation.

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arXiv2000

FUSE Observations of the Galactic and Intergalactic Medium Towards H1821+643

We have obtained moderate resolution spectra of the bright QSO H1821+643 (z=0.297) in the wavelength interval 990-1185A with FUSE. Strong, complex OVI 1032 and FeII 1145 absorption arising in gas above several spiral arms and the outer warp of our Galaxy is detected. We have identified absorption by a high negative-velocity (-215 km/s) component of OVI, which corresponds to the limiting velocity of v_LSR = 190+/-20 km/s for Milky Way gas along this line of sight, assuming corotation of disk and halo gas and a flat rotation curve for the outer galaxy. We report the detection of four absorbers in the intergalactic medium, through detections of Lyman beta at z=0.2454 and z=0.1213, Lyman delta, epsilon, zeta in a system at z=0.2249, CIII 977 at z=0.1705, and OVI 1032 at z=0.12137. The FUSE data show that the Lyman absorbers at z=0.225 are composed of 2 components separated by 70 km/s. Finally, we have observed associated absorption from OIII 832, OIV 787, and possibly SV 786 at the redshift of the QSO (z= 0.297). NeVIII 770,780 is not detected in this associated system. When combined with the previous detection of associated OVI 1032 (Savage et al, 1998), we conclude that this system is a multiphase absorber, and we discuss the origin of the absorption in this context.

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arXiv2000

Resolving the extragalactic hard X-ray background

The origin of the hard (2-10 keV) X-ray background has remained mysterious for over 35 years. Most of the soft (0.5-2 keV) X-ray background has been resolved into discrete sources, which are primarily quasars; however, these sources do not have the flat spectral shape required to match the X-ray background spectrum. Here we report the results of an X-ray survey 30 times more sensitive than previous studies in the hard band and four times more sensitive in the soft band. The sources detected in our survey account for at least 75 per cent of the hard X-ray background. The mean X-ray spectrum of these sources is in good agreement with that of the background. The X-ray emission from the majority of the detected sources is unambiguously associated with either the nuclei of otherwise normal bright galaxies or optically faint sources, which could either be active nuclei of dust enshrouded galaxies or the first quasars at very high redshifts.

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arXiv2000

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

Direct submm imaging has recently revealed the 850-micron background to be mostly composed of a population of distant ultraluminous infrared galaxies, but identifying the optical/NIR counterparts to these sources has proved difficult due to the poor submm spatial resolution. However, the proportionality of both cm and submm data to the star formation rate suggests that high resolution radio continuum maps with subarcsecond positional accuracy can be exploited to locate submm sources. In this paper we present results from a targeted SCUBA survey of micro-Jansky radio sources in the flanking fields of the Hubble Deep Field. Even with relatively shallow 850-micron SCUBA observations (>6 mJy at 3-sigma), we were successful at making submm detections of optical/NIR-faint (I>24 and K~21-22) radio sources, and our counts closely match the bright counts from submm surveys. Redshift estimates can be made from the ratio of the submm flux to the radio flux across the 100 GHz break in the spectral energy distribution. This millimetric redshift estimation places the bright submm 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 two orders of magnitude from z~0 to z~1-3. The SFRD at high redshift inferred from our >6 mJy submm observations is comparable to that observed in the UV/optical. (Abridged)

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arXiv1999

Optically Faint Microjansky Radio Sources

We report on the identifications of radio sources from our survey of the Hubble Deep Field and the SSA13 fields, both of which comprise the deepest radio surveys to date at 1.4 GHz and 8.5 GHz respectively. About 80% of the microjansky radio sources are associated with moderate redshift starburst galaxies or AGNs within the I magnitude range of 17 to 24 with a median of I = 22 mag. Thirty-one (20%) of the radio sources are: 1) fainter than $I>$25 mag, with two objects in the HDF $I_{AB}>$28.5, 2) often identified with very red objects $I-K>$4, and 3) not significantly different in radio properties than the brighter objects. We suggest that most of these objects are associated with heavily obscured starburst galaxies with redshifts between 1 and 3. However, other mechanisms are discussed and cannot be ruled out with the present observations.

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arXiv1999

The Discovery of ERO Counterparts to Faint Submm Galaxies

We use deep ground-based imaging in the near-IR to search for counterparts to the luminous submm sources in the catalog of Smail et al (1998). For the majority of the submm sources the near-IR imaging supports the counterparts originally selected from deep optical images. However, in two cases (10% of the sample) we find a relatively bright near-IR source close to the submm position, sources that were unidentified in the deep HST and ground-based R-band images used in Smail et al (1998). We place limits on colours of these sources from deep high-resolution Keck II imaging and find they have 2-sigma limits of (I-K)>6.8 and (I-K)>6.0 respectively. Both sources thus class as EROs. Using the spectral properties of the submm source in the radio and submm we argue that these EROs are probably the source of the submm emission, rather than the bright spiral galaxies previously identified by Smail et al. (1998). From the surface density of these submm-bright EROs we suggest that this class accounts for the majority of the reddest members of the ERO population, in good agreement with the preliminary conclusions of pointed submm observations of individual EROs. We conclude that the most extreme EROs represent a population of dusty, ultraluminous galaxies at high redshifts; further study of these will provide insights into the nature of star formation in obscured galaxies in the early Universe. The identification of similar counterparts in blank field submm surveys will be extremely difficult owing to their faintness (K~20.5, I>26.5). Finally, we discuss the radio and submm properties of the two submm-bright EROs discovered here and suggest that both galaxies lie at z>2.

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arXiv1999

The view of galaxy formation from Hawaii: Seeing the dark side of the universe

The strength of the submillimeter background light shows directly that much of the energy radiated by star formation and AGN is moved to far infrared wavelengths. However, it is only as this background at 850 microns has been resolved with direct submillimeter imaging that we have seen that it is created by a population of ultraluminous (or near ultraluminous) infrared galaxies (ULIGs) which appear to lie at relatively high redshifts (z>1). Mapping the redshift evolution of this major portion of the universal star formation has been difficult because of the poor submillimeter spatial resolution, but this difficulty can be overcome by using extremely deep cm continuum radio observations to obtain precise astrometric information since the bulk of the brighter submillimeter sources have detectable radio counterparts. However, with this precise position information available, we find that most of the submillimeter sources are extremely faint in the optical and near infrared (I>>24 and K=21-22) and inaccessible to optical spectroscopy. Rough photometric redshift estimates can be made from combined radio and submillimeter energy distributions. We shall refer to this procedure as millimetric redshift estimation to distinguish it from photometric estimators in the optical and near IR. These estimators place the bulk of the submillimeter population at z=1-3, where it corresponds to the high redshift tail of the faint cm radio population. While still preliminary, the results suggest that the submillimeter population appears to dominate the star formation in this redshift range by almost an order of magnitude over the mostly distinct populations selected in the optical-ultraviolet.

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arXiv1999

The Submillimeter Search for Very High Redshift Galaxies

Identifying the optical/near-infrared (NIR) counterparts to the distant submillimeter (submm) source population has proved difficult due to poor submm spatial resolution. However, the proportionality of both centimeter and submm data to the star formation rate suggests that high resolution radio continuum maps with subarcsecond positional accuracy could be exploited to locate submm sources. We targeted with SCUBA a sample of micro-Jy radio sources in the flanking fields of the Hubble Deep Field selected from the uniform (8 micro-Jy at 1-sigma) 1.4 GHz VLA image of Richards (1999). We find that the majority of bright (>6 mJy) submm sources have detectable radio counterparts. With the precise positions from the radio, we also find that these submm sources are extremely faint in the optical and NIR (I>>24 and K=21-22) and are therefore inaccessible to optical spectroscopy. Redshift estimates can, however, be made from the shape of the spectral energy distribution in the radio and submm. This procedure, which we refer to as millimetric redshift estimation, places the bright submm population at z=1-3, where it forms the high redshift tail of the faint radio population.

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