We present the results of a 400 ks Chandra survey of 29 extended Lyα emitting nebulae (Lyα Blobs,... more We present the results of a 400 ks Chandra survey of 29 extended Lyα emitting nebulae (Lyα Blobs, LABs) in the z = 3.09 proto-cluster in the SS A22 field. We detect luminous X-ray counterparts in five LABs, implying a large fraction of active galactic nuclei (AGN) in LABs, f AGN = 17 +12 −7 % down to L 2−32keV ∼ 10 44 erg s −1 . All of the AGN appear to be heavily obscured, with spectral indices implying obscuring column densities of N H > 10 23 cm −2 . The AGN fraction should be considered a lower limit, since several more LABs not detected with Chandra show AGN signatures in their mid-infrared emission. We show that the UV luminosities of the AGN are easily capable of powering the extended Lyα emission via photo-ionization alone. When combined with the UV flux from a starburst component, and energy deposited by mechanical feedback, we demonstrate that 'heating' by a central source, rather than gravitational cooling is the most likely power source of LABs. We argue that all LABs could be powered in this manner, but that the luminous host galaxies are often just below the sensitivity limits of current instrumentation, or are heavily obscured. No individual LABs show evidence for extended X-ray emission, and a stack equivalent to a > ∼ 9 Ms exposure of an average LAB also yields no statistical detection of a diffuse X-ray component. The resulting diffuse X-ray/Lyα luminosity limit implies there is no hot (T > ∼ 10 7 K) gas component in these halos, and also rules out inverse Compton scattering of cosmic microwave background photons, or local far-infrared photons, as a viable power source for LABs. Subject headings: galaxies: active -galaxies: high-redshift -galaxies: evolution Recently there has been great interest in the highly extended (∼30-200 kpc in projected linear extent) Lyα lineemitting nebulae (L Lyα ∼ 10 43−44 erg s −1 ) identified in highredshift narrowband surveys: 'Lyα Blobs' (LABs) (Fynbo et
We present early results from the deepest X-ray stacking of <5000 high redshift galaxies from ... more We present early results from the deepest X-ray stacking of <5000 high redshift galaxies from z=1 to 8 using the recently acquired Chandra Deep Field South (CDF-S) 4Ms data. The galaxy samples were selected using the Lyman break technique based on recent HST ACS and WFC3 observations. Based on the X-ray stacking analyses for the 1<zɜ LBGs, we find that the mean LX/SFR ratio remains roughly constant, consistent with X-ray binary population synthesis models. We do not obtain significant X-ray detections from stacking LBGs at z 6, 7, and 8 and obtain upper limits that are inconsistent with recently published results, constraining the SMBH accretion history of the Universe.
We report the detection of extended X-ray emission around two powerful high-z radio galaxies (HzR... more We report the detection of extended X-ray emission around two powerful high-z radio galaxies (HzRGs) at z~3.6 (4C03.24 & 4C19.71) and use these to investigate the origin of extended, Inverse Compton (IC) powered X-ray halos at high z. The halos have X-ray luminosities of Lx~3e44 erg/s and sizes of ~60kpc. Their morphologies are broadly similar to the ~60-kpc long radio lobes around these galaxies suggesting they are formed from IC scattering by relativistic electrons in the radio lobes, of either CMB or FIR photons from the dust-obscured starbursts in these galaxies. These observations double the number of z>3 HzRGs with X-ray detected IC halos. We compare the IC X-ray to radio luminosity ratios for these new detections to the two previously detected z~3.8 HzRGs. Given the similar redshifts, we would expect comparable X-ray IC luminosities if CMB mm photons are the seed field for the IC emission. Instead the two z~3.6 HzRGs, which are ~4x fainter in the FIR, also have ~4x fainter X-ray IC emission. Including a further six z>2 radio sources with IC X-ray halos from the literature, we suggest that in the more compact (lobe sizes <100-200kpc), majority of radio sources, the bulk of the IC emission may be driven by scattering of locally produced FIR photons from luminous, dust-obscured starbursts within these galaxies, rather than CMB photons. The resulting X-ray emission can ionise the gas on ~100-200-kpc scales around these systems and thus form their extended Ly-alpha emission line halos. The starburst and AGN activity in these galaxies are thus combining to produce an effective and wide-spread "feedback" process, acting on the long-term gas reservoir for the galaxy. If episodic radio activity and co-eval starbursts are common in massive, high-z galaxies, then this IC-feedback mechanism may affect the star-formation histories of massive galaxies. [Abridged]
We present partial-correlation analyses that examine the strengths of the relationships between l... more We present partial-correlation analyses that examine the strengths of the relationships between l2500 ˚ A, l2 keV, �OX, and redshift for optically-selected AGNs. We extend the work of Strateva et al. (2005), that analyzed optically-selected AGNs from the Sloan Digital Sky Survey (SDSS), by including 52 moderate-luminosity, optically-selected AGNs from the COMBO-17 survey with corresponding deep (≈ 250 ks to
Bulletin of the American Astronomical Society, Dec 1, 2006
Using the 2 Ms Chandra Deep Field North, the deepest X-ray observation to date, we employ X-ray s... more Using the 2 Ms Chandra Deep Field North, the deepest X-ray observation to date, we employ X-ray stacking techniques to constrain the maximum 6-8 keV flux that can be contributed by the X-ray undetected Great Observatories Origins Deep Survey (GOODS) sources from both Hubble's Advanced Camera for Surveys (ACS) and Spitzer's Infrared Array Camera (IRAC) source catalogs. We compare the combined X-ray flux of these sources with recent models of the Cosmic X-ray Background (CXB) and show that the "missing" 6-8 keV CXB cannot be completely recovered from the integrated X-ray emission from the known optical and infrared sources in the field.
We present the results of a 400 ks Chandra survey of 29 extended Lyα emitting nebulae (Lyα Blobs,... more We present the results of a 400 ks Chandra survey of 29 extended Lyα emitting nebulae (Lyα Blobs, LABs) in the z = 3.09 protocluster in the SS A22 field. We detect luminous X-ray counterparts in five LABs, implying a large fraction of active galactic nuclei (AGN) in LABs, f AGN = 17+12 -7% down to L 2-32 keV ~ 1044 erg s-1. All of the AGN appear to be heavily obscured, with spectral indices implying obscuring column densities of N H > 1023 cm-2. The AGN fraction should be considered a lower limit, since several more LABs not detected with Chandra show AGN signatures in their mid-infrared (mid-IR) emission. We show that the UV luminosities of the AGN are easily capable of powering the extended Lyα emission via photoionization alone. When combined with the UV flux from a starburst component, and energy deposited by mechanical feedback, we demonstrate that "heating" by a central source, rather than gravitational cooling is the most likely power source of LABs. We argue that all LABs could be powered in this manner, but that the luminous host galaxies are often just below the sensitivity limits of current instrumentation, or are heavily obscured. No individual LABs show evidence for extended X-ray emission, and a stack equivalent to a gsim9 Ms exposure of an average LAB also yields no statistical detection of a diffuse X-ray component. The resulting diffuse X-ray/Lyα luminosity limit implies there is no hot (T gsim 107 K) gas component in these halos, and also rules out inverse Compton scattering of cosmic microwave background photons, or local far-IR photons, as a viable power source for LABs.
We investigate the local supermassive black hole (SMBH) density function and relative mass accret... more We investigate the local supermassive black hole (SMBH) density function and relative mass accretion rates of all active galactic nuclei (AGNs) identified in a volume-limited sample of infrared (IR) bright galaxies (L_IR > 3 x 10^9 L_sun) to D<15 Mpc (Goulding & Alexander 2009). A database of accurate SMBH mass (M_BH) estimates is compiled from literature sources using physically motivated AGN modeling techniques (reverberation mapping, maser mapping and gas kinematics) and well-established indirect M_BH estimation methods (the M-sigma and M_BH-L_(K,bul) relations). For the three sources without previously published M_BH estimates, we use 2MASS K-band imaging and GALFIT to constrain the bulge luminosities, and hence SMBH masses. In general, we find the AGNs in the sample host SMBHs which are spread over a wide mass range (M_BH ~ (0.1-30) x 10^7 M_sun), but with the majority in the poorly studied M_BH ~ 10^6-10^7 M_sun region. Using sensitive hard X-ray (2-10 keV) and mid-IR constraints we calculate the bolometric luminosities of the AGNs (L_(Bol,AGN)) and use them to estimate relative mass accretion rates. We use these data to calculate the volume-average SMBH growth rate of galaxies in the local Universe and find that the AGNs hosting SMBHs in the mass range M_BH ~ 10^6-10^7 M_sun are dominated by optically unidentified AGNs. These relatively small SMBHs are acquiring a significant proportion of their mass in the present-day, and are amongst the most rapidly growing in the local Universe (SMBH mass doubling times of ~6 Gyrs). Additionally, we find tentative evidence for an increasing volume-weighted AGN fraction with decreasing SMBH mass in the M_BH ~ 10^6-10^8 M_sun range. Overall, we conclude that significant mass accretion onto small SMBHs may be missed in even the most sensitive optical surveys due to absent or weak optical AGN signatures.
We investigate the local supermassive black hole (SMBH) density function and relative mass accret... more We investigate the local supermassive black hole (SMBH) density function and relative mass accretion rates of all active galactic nuclei (AGNs) identified in a volume-limited sample of infrared (IR) bright galaxies (L IR&amp;amp;amp;gt; 3× 10 9 L⊙) to D&amp;amp;amp;lt; 15 Mpc. A data base of accurate ...
The entire ~400ks Chandra exposure consisted of four separate Chandra observations of ~70-120ks t... more The entire ~400ks Chandra exposure consisted of four separate Chandra observations of ~70-120ks taken between 2007 October 1 and 2007 December 30. (2 data files).
We employ X-ray stacking techniques to examine the contribution from X-ray undetected, mid-infrar... more We employ X-ray stacking techniques to examine the contribution from X-ray undetected, mid-infraredselected sources to the unresolved, hard (6 − 8 keV) cosmic X-ray background (CXB). We use the publicly available, 24µm 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 the 1 Ms Chandra Deep Field-South, to identify bright (S 24µm > 80µJy) 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 (3.2σ) 6 − 8 keV stacked X-ray signal from an X-ray undetected source population. We find that the X-ray-undetected MIPS sources make up about 2% (or less) of the total CXB below 6 keV, but about 6% in the 6 − 8 keV band. The 0.5 − 8 keV stacked X-ray 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 50% of 6 − 8 keV CXB. Our study rules out obscured, luminous QSOs 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.
Designed to study gamma ray bursts, the Swift satellite has also ultraviolet and optical capabili... more Designed to study gamma ray bursts, the Swift satellite has also ultraviolet and optical capabilities. In this study, we use the UVOT instrument aboard Swift to discover 1<z<2 Lyman break galaxies (LBGs). Using UVOT near-ultraviolet color selection, we select UV-dropouts in the Chandra Deep Field South (CDFS). We match the selected sources with available multi-wavelength data from ACS GOODS-South, MUSYC, and COMBO-17 to characterize spectral energy distributions and determine stellar masses, star formation rates (SFRs) and dust attenuation in these galaxies. Additionally, working with these multi-wavelength catalogs, we explore how the Swift UVOT selection of LBGs (and possibly the UV-dropout selection in general) might bias the final LBG sample. In this talk, I will present how the properties of the selected sample compare with other samples: z 3 LBGs, z<1 UV luminous galaxies, and other z=1-2 CDFS galaxies, and I will present the potential for using Swift to search for Ly...
The entire Chandra observation program consisted of nine separate Chandra observations taken betw... more The entire Chandra observation program consisted of nine separate Chandra observations taken between 2004 February 29 and November 20. (3 data files).
We present X-ray point-source catalogs for a deep 400 ks Chandra ACIS-I exposure of the SSA22 fie... more We present X-ray point-source catalogs for a deep 400 ks Chandra ACIS-I exposure of the SSA22 field. The observations are centred on a z = 3.09 protocluster, which is populated by Lyman break galaxies (LBGs), Lyalpha emitters (LAEs), and extended Lyalpha-emitting blobs (LABs). The survey reaches ultimate (3 count) sensitivity limits of 5.7 X 10^-17 and 3.0 X 10^-16 ergs cm^-2 s^-1 for the 0.5-2 keV and 2-8 keV bands, respectively. These limits make SSA22 the fourth deepest extragalactic Chandra survey yet conducted, and the only one focused on a known high redshift structure. In total, we detect 297 X-ray point sources and identify one obvious bright extended X-ray source over a ~330 arcmin^2 region. In addition to our X-ray catalogs, we provide all available optical spectroscopic redshifts and near-infrared and mid-infrared photometry available for our sources. The basic X-ray and infrared properties of our Chandra sources indicate a variety of source types, although absorbed active galactic nuclei (AGNs) appear to dominate. In total, we have identified 12 X-ray sources (either via optical spectroscopic redshifts or LAE selection) at z =3.06-3.12 that are likely to be associated with the SSA22 protocluster. These sources have X-ray and multiwavelength properties that suggest they are powered by AGN with 0.5-8 keV luminosities in the range of 10^43-10^45 ergs s^-1 We have analysed the AGN fraction of sources in the protocluster as a function of local LAE source density and find suggestive evidence for a correlation between AGN fraction and local LAE source density (at the 96 per cent confidence level), implying that supermassive black hole growth at z = 3 is strongest in the highest density regions.
We investigate the evolution over the last 6.3 Gyr of cosmic time (i.e., since z ~ 0.7) of the av... more We investigate the evolution over the last 6.3 Gyr of cosmic time (i.e., since z ~ 0.7) of the average X-ray properties of early-type galaxies within the Extended Chandra Deep Field-South (E-CDF-S). Our early-type galaxy sample includes 539 objects with red-sequence colors and Sersic indices larger than n = 2.5, which were selected jointly from the COMBO-17 (Classifying Objects by Medium-Band Observations in 17 Filters) and GEMS (Galaxy Evolution from Morphologies and SEDs) surveys. We utilize the deep Chandra observations over the E-CDF-S and X-ray stacking analyses to constrain primarily the average X-ray emission from "normal" early-type galaxies (i.e., those that are not dominated by luminous active galactic nuclei [AGNs]). In our analyses, we study separately optically luminous (L_B ~ 10^[10-11] L_B,sol) and faint (L_B ~ 10^[9.3-10] L_B,sol) galaxy samples, which we expect to have soft (0.5-2.0 keV) X-ray emission dominated by hot (~1 keV) interstellar gas and low mass X-ray binary (LMXB) populations, respectively. We detect individually 49 (~9%) of our galaxies in the X-ray band, and classify these sources as either normal early-type galaxies (17 galaxies) or AGN candidates (32 galaxies). The AGN fraction of our optically luminous samples evolves with redshift in a manner consistent with the (1+z)^3 evolution observed in other investigations of X-ray-selected AGNs. After removing potential AGNs from our samples, we find that the X-ray-to-B-band mean luminosity ratio (L_X/L_B) for optically luminous early-type galaxies does not evolve significantly over the redshift range z~0.0-0.7. This lack of X-ray evolution implies a general balance between the heating and cooling of the hot interstellar gas [Abridged].
We analyze a population of intermediate-redshift (z = 0.05-0.3) off-nuclear X-ray sources located... more We analyze a population of intermediate-redshift (z = 0.05-0.3) off-nuclear X-ray sources located within the optical extent of optically-bright galaxies in the Great Observatories Origins Deep Survey (GOODS) and Galaxy Evolution from Morphology and SEDs (GEMS) fields. A total of 19 off-nuclear sources are classified using deep Chandra exposures from the Chandra Deep Field-North, Chandra Deep Field-South, and Extended Chandra Deep Field-South; ten of these sources are newly identified. These sources have average X-ray spectral shapes and optical environments similar to those of ultraluminous X-ray sources (ULXs) in the local univese. This sample improves the available source statistics for intermediate-redshift off-nuclear sources with 0.5-2.0 keV luminosities L{0.5-2.0 keV} > 1039.5 erg s-1, and places significant new constraints on the redshift evolution of the off-nuclear source frequency in field galaxies. We find that the fractions of intermediate-redshift field galaxies containing an off-nuclear source are elevated by a factor of two with respect to those observed for ULXs in the local universe for 0.5-2.0 keV luminosities in the range of 1039-40.5 erg s-1; the rise in these fractions is consistent with that expected from the observed increase in global star-formation density with redshift.
The CDF-S consists of 23 separate observations: The ~1 Ms catalogs for the first 11 observations ... more The CDF-S consists of 23 separate observations: The ~1 Ms catalogs for the first 11 observations taken between 1999 October 14 and 2000 December 23 were presented in Giacconi et al., 2002 (Cat. J/ApJS/139/369) and Alexander et al., 2003 (Cat. J/AJ/126/539).The second ~1 Ms exposure consisted of 12 observations taken between 2007 September 20 and 2007 November 4. (3 data files).
We find evidence that the high-mass X-ray binary (HMXB) populations in relatively metal- and dust... more We find evidence that the high-mass X-ray binary (HMXB) populations in relatively metal- and dust- poor UV-selected galaxies differs from those in other normal, local, star-forming galaxies, yielding higher 2-10 keV X-ray luminosities per star formation rate (SFR). Our metal and dust poor UV-selected galaxies are drawn from populations that span the majority of cosmic history -- ranging from Lyman break galaxies (LBGs) in the early Universe (z=1.5-4) to Lyman break analogs (LBAs) in the present-day Universe ( 0.1). Using X-ray stacking analyses of the 4 Ms Chandra Deep Field South (CDF-S) data with a sample of ~4000 z=1.5-4 LBGs, we find that the 2-10 keV X-ray luminosity per SFR (L_X/SFR) evolves mildly with redshift, consistent with predictions from X-ray binary population synthesis models. Additional Chandra observations of 6 relatively nearby ( 0.1) LBAs yield individual X-ray detections with elevated L_X/SFR ratios compared to local galaxies, but similar to the z>2 LBGs. In ...
We present the results of a 400 ks Chandra survey of 29 extended Lyα emitting nebulae (Lyα Blobs,... more We present the results of a 400 ks Chandra survey of 29 extended Lyα emitting nebulae (Lyα Blobs, LABs) in the z = 3.09 proto-cluster in the SS A22 field. We detect luminous X-ray counterparts in five LABs, implying a large fraction of active galactic nuclei (AGN) in LABs, f AGN = 17 +12 −7 % down to L 2−32keV ∼ 10 44 erg s −1 . All of the AGN appear to be heavily obscured, with spectral indices implying obscuring column densities of N H > 10 23 cm −2 . The AGN fraction should be considered a lower limit, since several more LABs not detected with Chandra show AGN signatures in their mid-infrared emission. We show that the UV luminosities of the AGN are easily capable of powering the extended Lyα emission via photo-ionization alone. When combined with the UV flux from a starburst component, and energy deposited by mechanical feedback, we demonstrate that 'heating' by a central source, rather than gravitational cooling is the most likely power source of LABs. We argue that all LABs could be powered in this manner, but that the luminous host galaxies are often just below the sensitivity limits of current instrumentation, or are heavily obscured. No individual LABs show evidence for extended X-ray emission, and a stack equivalent to a > ∼ 9 Ms exposure of an average LAB also yields no statistical detection of a diffuse X-ray component. The resulting diffuse X-ray/Lyα luminosity limit implies there is no hot (T > ∼ 10 7 K) gas component in these halos, and also rules out inverse Compton scattering of cosmic microwave background photons, or local far-infrared photons, as a viable power source for LABs. Subject headings: galaxies: active -galaxies: high-redshift -galaxies: evolution Recently there has been great interest in the highly extended (∼30-200 kpc in projected linear extent) Lyα lineemitting nebulae (L Lyα ∼ 10 43−44 erg s −1 ) identified in highredshift narrowband surveys: 'Lyα Blobs' (LABs) (Fynbo et
We present early results from the deepest X-ray stacking of <5000 high redshift galaxies from ... more We present early results from the deepest X-ray stacking of <5000 high redshift galaxies from z=1 to 8 using the recently acquired Chandra Deep Field South (CDF-S) 4Ms data. The galaxy samples were selected using the Lyman break technique based on recent HST ACS and WFC3 observations. Based on the X-ray stacking analyses for the 1<zɜ LBGs, we find that the mean LX/SFR ratio remains roughly constant, consistent with X-ray binary population synthesis models. We do not obtain significant X-ray detections from stacking LBGs at z 6, 7, and 8 and obtain upper limits that are inconsistent with recently published results, constraining the SMBH accretion history of the Universe.
We report the detection of extended X-ray emission around two powerful high-z radio galaxies (HzR... more We report the detection of extended X-ray emission around two powerful high-z radio galaxies (HzRGs) at z~3.6 (4C03.24 & 4C19.71) and use these to investigate the origin of extended, Inverse Compton (IC) powered X-ray halos at high z. The halos have X-ray luminosities of Lx~3e44 erg/s and sizes of ~60kpc. Their morphologies are broadly similar to the ~60-kpc long radio lobes around these galaxies suggesting they are formed from IC scattering by relativistic electrons in the radio lobes, of either CMB or FIR photons from the dust-obscured starbursts in these galaxies. These observations double the number of z>3 HzRGs with X-ray detected IC halos. We compare the IC X-ray to radio luminosity ratios for these new detections to the two previously detected z~3.8 HzRGs. Given the similar redshifts, we would expect comparable X-ray IC luminosities if CMB mm photons are the seed field for the IC emission. Instead the two z~3.6 HzRGs, which are ~4x fainter in the FIR, also have ~4x fainter X-ray IC emission. Including a further six z>2 radio sources with IC X-ray halos from the literature, we suggest that in the more compact (lobe sizes <100-200kpc), majority of radio sources, the bulk of the IC emission may be driven by scattering of locally produced FIR photons from luminous, dust-obscured starbursts within these galaxies, rather than CMB photons. The resulting X-ray emission can ionise the gas on ~100-200-kpc scales around these systems and thus form their extended Ly-alpha emission line halos. The starburst and AGN activity in these galaxies are thus combining to produce an effective and wide-spread "feedback" process, acting on the long-term gas reservoir for the galaxy. If episodic radio activity and co-eval starbursts are common in massive, high-z galaxies, then this IC-feedback mechanism may affect the star-formation histories of massive galaxies. [Abridged]
We present partial-correlation analyses that examine the strengths of the relationships between l... more We present partial-correlation analyses that examine the strengths of the relationships between l2500 ˚ A, l2 keV, �OX, and redshift for optically-selected AGNs. We extend the work of Strateva et al. (2005), that analyzed optically-selected AGNs from the Sloan Digital Sky Survey (SDSS), by including 52 moderate-luminosity, optically-selected AGNs from the COMBO-17 survey with corresponding deep (≈ 250 ks to
Bulletin of the American Astronomical Society, Dec 1, 2006
Using the 2 Ms Chandra Deep Field North, the deepest X-ray observation to date, we employ X-ray s... more Using the 2 Ms Chandra Deep Field North, the deepest X-ray observation to date, we employ X-ray stacking techniques to constrain the maximum 6-8 keV flux that can be contributed by the X-ray undetected Great Observatories Origins Deep Survey (GOODS) sources from both Hubble's Advanced Camera for Surveys (ACS) and Spitzer's Infrared Array Camera (IRAC) source catalogs. We compare the combined X-ray flux of these sources with recent models of the Cosmic X-ray Background (CXB) and show that the "missing" 6-8 keV CXB cannot be completely recovered from the integrated X-ray emission from the known optical and infrared sources in the field.
We present the results of a 400 ks Chandra survey of 29 extended Lyα emitting nebulae (Lyα Blobs,... more We present the results of a 400 ks Chandra survey of 29 extended Lyα emitting nebulae (Lyα Blobs, LABs) in the z = 3.09 protocluster in the SS A22 field. We detect luminous X-ray counterparts in five LABs, implying a large fraction of active galactic nuclei (AGN) in LABs, f AGN = 17+12 -7% down to L 2-32 keV ~ 1044 erg s-1. All of the AGN appear to be heavily obscured, with spectral indices implying obscuring column densities of N H > 1023 cm-2. The AGN fraction should be considered a lower limit, since several more LABs not detected with Chandra show AGN signatures in their mid-infrared (mid-IR) emission. We show that the UV luminosities of the AGN are easily capable of powering the extended Lyα emission via photoionization alone. When combined with the UV flux from a starburst component, and energy deposited by mechanical feedback, we demonstrate that "heating" by a central source, rather than gravitational cooling is the most likely power source of LABs. We argue that all LABs could be powered in this manner, but that the luminous host galaxies are often just below the sensitivity limits of current instrumentation, or are heavily obscured. No individual LABs show evidence for extended X-ray emission, and a stack equivalent to a gsim9 Ms exposure of an average LAB also yields no statistical detection of a diffuse X-ray component. The resulting diffuse X-ray/Lyα luminosity limit implies there is no hot (T gsim 107 K) gas component in these halos, and also rules out inverse Compton scattering of cosmic microwave background photons, or local far-IR photons, as a viable power source for LABs.
We investigate the local supermassive black hole (SMBH) density function and relative mass accret... more We investigate the local supermassive black hole (SMBH) density function and relative mass accretion rates of all active galactic nuclei (AGNs) identified in a volume-limited sample of infrared (IR) bright galaxies (L_IR > 3 x 10^9 L_sun) to D<15 Mpc (Goulding & Alexander 2009). A database of accurate SMBH mass (M_BH) estimates is compiled from literature sources using physically motivated AGN modeling techniques (reverberation mapping, maser mapping and gas kinematics) and well-established indirect M_BH estimation methods (the M-sigma and M_BH-L_(K,bul) relations). For the three sources without previously published M_BH estimates, we use 2MASS K-band imaging and GALFIT to constrain the bulge luminosities, and hence SMBH masses. In general, we find the AGNs in the sample host SMBHs which are spread over a wide mass range (M_BH ~ (0.1-30) x 10^7 M_sun), but with the majority in the poorly studied M_BH ~ 10^6-10^7 M_sun region. Using sensitive hard X-ray (2-10 keV) and mid-IR constraints we calculate the bolometric luminosities of the AGNs (L_(Bol,AGN)) and use them to estimate relative mass accretion rates. We use these data to calculate the volume-average SMBH growth rate of galaxies in the local Universe and find that the AGNs hosting SMBHs in the mass range M_BH ~ 10^6-10^7 M_sun are dominated by optically unidentified AGNs. These relatively small SMBHs are acquiring a significant proportion of their mass in the present-day, and are amongst the most rapidly growing in the local Universe (SMBH mass doubling times of ~6 Gyrs). Additionally, we find tentative evidence for an increasing volume-weighted AGN fraction with decreasing SMBH mass in the M_BH ~ 10^6-10^8 M_sun range. Overall, we conclude that significant mass accretion onto small SMBHs may be missed in even the most sensitive optical surveys due to absent or weak optical AGN signatures.
We investigate the local supermassive black hole (SMBH) density function and relative mass accret... more We investigate the local supermassive black hole (SMBH) density function and relative mass accretion rates of all active galactic nuclei (AGNs) identified in a volume-limited sample of infrared (IR) bright galaxies (L IR&amp;amp;amp;gt; 3× 10 9 L⊙) to D&amp;amp;amp;lt; 15 Mpc. A data base of accurate ...
The entire ~400ks Chandra exposure consisted of four separate Chandra observations of ~70-120ks t... more The entire ~400ks Chandra exposure consisted of four separate Chandra observations of ~70-120ks taken between 2007 October 1 and 2007 December 30. (2 data files).
We employ X-ray stacking techniques to examine the contribution from X-ray undetected, mid-infrar... more We employ X-ray stacking techniques to examine the contribution from X-ray undetected, mid-infraredselected sources to the unresolved, hard (6 − 8 keV) cosmic X-ray background (CXB). We use the publicly available, 24µm 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 the 1 Ms Chandra Deep Field-South, to identify bright (S 24µm > 80µJy) 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 (3.2σ) 6 − 8 keV stacked X-ray signal from an X-ray undetected source population. We find that the X-ray-undetected MIPS sources make up about 2% (or less) of the total CXB below 6 keV, but about 6% in the 6 − 8 keV band. The 0.5 − 8 keV stacked X-ray 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 50% of 6 − 8 keV CXB. Our study rules out obscured, luminous QSOs 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.
Designed to study gamma ray bursts, the Swift satellite has also ultraviolet and optical capabili... more Designed to study gamma ray bursts, the Swift satellite has also ultraviolet and optical capabilities. In this study, we use the UVOT instrument aboard Swift to discover 1<z<2 Lyman break galaxies (LBGs). Using UVOT near-ultraviolet color selection, we select UV-dropouts in the Chandra Deep Field South (CDFS). We match the selected sources with available multi-wavelength data from ACS GOODS-South, MUSYC, and COMBO-17 to characterize spectral energy distributions and determine stellar masses, star formation rates (SFRs) and dust attenuation in these galaxies. Additionally, working with these multi-wavelength catalogs, we explore how the Swift UVOT selection of LBGs (and possibly the UV-dropout selection in general) might bias the final LBG sample. In this talk, I will present how the properties of the selected sample compare with other samples: z 3 LBGs, z<1 UV luminous galaxies, and other z=1-2 CDFS galaxies, and I will present the potential for using Swift to search for Ly...
The entire Chandra observation program consisted of nine separate Chandra observations taken betw... more The entire Chandra observation program consisted of nine separate Chandra observations taken between 2004 February 29 and November 20. (3 data files).
We present X-ray point-source catalogs for a deep 400 ks Chandra ACIS-I exposure of the SSA22 fie... more We present X-ray point-source catalogs for a deep 400 ks Chandra ACIS-I exposure of the SSA22 field. The observations are centred on a z = 3.09 protocluster, which is populated by Lyman break galaxies (LBGs), Lyalpha emitters (LAEs), and extended Lyalpha-emitting blobs (LABs). The survey reaches ultimate (3 count) sensitivity limits of 5.7 X 10^-17 and 3.0 X 10^-16 ergs cm^-2 s^-1 for the 0.5-2 keV and 2-8 keV bands, respectively. These limits make SSA22 the fourth deepest extragalactic Chandra survey yet conducted, and the only one focused on a known high redshift structure. In total, we detect 297 X-ray point sources and identify one obvious bright extended X-ray source over a ~330 arcmin^2 region. In addition to our X-ray catalogs, we provide all available optical spectroscopic redshifts and near-infrared and mid-infrared photometry available for our sources. The basic X-ray and infrared properties of our Chandra sources indicate a variety of source types, although absorbed active galactic nuclei (AGNs) appear to dominate. In total, we have identified 12 X-ray sources (either via optical spectroscopic redshifts or LAE selection) at z =3.06-3.12 that are likely to be associated with the SSA22 protocluster. These sources have X-ray and multiwavelength properties that suggest they are powered by AGN with 0.5-8 keV luminosities in the range of 10^43-10^45 ergs s^-1 We have analysed the AGN fraction of sources in the protocluster as a function of local LAE source density and find suggestive evidence for a correlation between AGN fraction and local LAE source density (at the 96 per cent confidence level), implying that supermassive black hole growth at z = 3 is strongest in the highest density regions.
We investigate the evolution over the last 6.3 Gyr of cosmic time (i.e., since z ~ 0.7) of the av... more We investigate the evolution over the last 6.3 Gyr of cosmic time (i.e., since z ~ 0.7) of the average X-ray properties of early-type galaxies within the Extended Chandra Deep Field-South (E-CDF-S). Our early-type galaxy sample includes 539 objects with red-sequence colors and Sersic indices larger than n = 2.5, which were selected jointly from the COMBO-17 (Classifying Objects by Medium-Band Observations in 17 Filters) and GEMS (Galaxy Evolution from Morphologies and SEDs) surveys. We utilize the deep Chandra observations over the E-CDF-S and X-ray stacking analyses to constrain primarily the average X-ray emission from "normal" early-type galaxies (i.e., those that are not dominated by luminous active galactic nuclei [AGNs]). In our analyses, we study separately optically luminous (L_B ~ 10^[10-11] L_B,sol) and faint (L_B ~ 10^[9.3-10] L_B,sol) galaxy samples, which we expect to have soft (0.5-2.0 keV) X-ray emission dominated by hot (~1 keV) interstellar gas and low mass X-ray binary (LMXB) populations, respectively. We detect individually 49 (~9%) of our galaxies in the X-ray band, and classify these sources as either normal early-type galaxies (17 galaxies) or AGN candidates (32 galaxies). The AGN fraction of our optically luminous samples evolves with redshift in a manner consistent with the (1+z)^3 evolution observed in other investigations of X-ray-selected AGNs. After removing potential AGNs from our samples, we find that the X-ray-to-B-band mean luminosity ratio (L_X/L_B) for optically luminous early-type galaxies does not evolve significantly over the redshift range z~0.0-0.7. This lack of X-ray evolution implies a general balance between the heating and cooling of the hot interstellar gas [Abridged].
We analyze a population of intermediate-redshift (z = 0.05-0.3) off-nuclear X-ray sources located... more We analyze a population of intermediate-redshift (z = 0.05-0.3) off-nuclear X-ray sources located within the optical extent of optically-bright galaxies in the Great Observatories Origins Deep Survey (GOODS) and Galaxy Evolution from Morphology and SEDs (GEMS) fields. A total of 19 off-nuclear sources are classified using deep Chandra exposures from the Chandra Deep Field-North, Chandra Deep Field-South, and Extended Chandra Deep Field-South; ten of these sources are newly identified. These sources have average X-ray spectral shapes and optical environments similar to those of ultraluminous X-ray sources (ULXs) in the local univese. This sample improves the available source statistics for intermediate-redshift off-nuclear sources with 0.5-2.0 keV luminosities L{0.5-2.0 keV} > 1039.5 erg s-1, and places significant new constraints on the redshift evolution of the off-nuclear source frequency in field galaxies. We find that the fractions of intermediate-redshift field galaxies containing an off-nuclear source are elevated by a factor of two with respect to those observed for ULXs in the local universe for 0.5-2.0 keV luminosities in the range of 1039-40.5 erg s-1; the rise in these fractions is consistent with that expected from the observed increase in global star-formation density with redshift.
The CDF-S consists of 23 separate observations: The ~1 Ms catalogs for the first 11 observations ... more The CDF-S consists of 23 separate observations: The ~1 Ms catalogs for the first 11 observations taken between 1999 October 14 and 2000 December 23 were presented in Giacconi et al., 2002 (Cat. J/ApJS/139/369) and Alexander et al., 2003 (Cat. J/AJ/126/539).The second ~1 Ms exposure consisted of 12 observations taken between 2007 September 20 and 2007 November 4. (3 data files).
We find evidence that the high-mass X-ray binary (HMXB) populations in relatively metal- and dust... more We find evidence that the high-mass X-ray binary (HMXB) populations in relatively metal- and dust- poor UV-selected galaxies differs from those in other normal, local, star-forming galaxies, yielding higher 2-10 keV X-ray luminosities per star formation rate (SFR). Our metal and dust poor UV-selected galaxies are drawn from populations that span the majority of cosmic history -- ranging from Lyman break galaxies (LBGs) in the early Universe (z=1.5-4) to Lyman break analogs (LBAs) in the present-day Universe ( 0.1). Using X-ray stacking analyses of the 4 Ms Chandra Deep Field South (CDF-S) data with a sample of ~4000 z=1.5-4 LBGs, we find that the 2-10 keV X-ray luminosity per SFR (L_X/SFR) evolves mildly with redshift, consistent with predictions from X-ray binary population synthesis models. Additional Chandra observations of 6 relatively nearby ( 0.1) LBAs yield individual X-ray detections with elevated L_X/SFR ratios compared to local galaxies, but similar to the z>2 LBGs. In ...
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Papers by Bret Lehmer