Browsing by Author "Lehmer, B. D."

Now showing 1 - 11 of 11
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    A magnetar-powered X-ray transient as the aftermath of a binary neutron-star merger
    (2019) Xue, Yongquan; Zheng, X. C.; Li, Y.; Brandt, W. N.; Zhang, Bryna; Luo, Bruce; Zhang, Boce; Bauer, Franz Erik; Sun, H.; Lehmer, B. D.; Wu, Xuefeng; Yang, Guang; Kong, Xu; Li, J. Y.; Sun, M. Y.; Wang, Junxian; Vito, Fabio
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    An extragalactic spectroscopic survey of the SSA22 field
    (2015) Saez, C.; Lehmer, B. D.; Bauer, Franz Erik; Stern, D.; Gonzales, A.; Rreza, I.; Alexander, D. M.; Matsuda, Y.; Geach, J. E.; Harrison, F. A.
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    Chandra reveals a luminous Compton-thick QSO powering a Lyα blob in a z=4 starbursting protocluster
    (2020) Vito, F.; Brandt, W. N.; Lehmer, B. D.; Vignali, C.; Zou, F.; Bauer, F. E.; Bremer, M.; Gilli, R.; Ivison, R. J.; Spingola, C.
    Context. Galaxy clusters in the local universe descend from high-redshift overdense regions known as protoclusters. The large gas reservoirs and high rate of galaxy interaction in protoclusters are expected to enhance star-formation activity and trigger luminous supermassive black-hole accretion in the nuclear regions of the host galaxies.Aims. We investigated the active galactic nucleus (AGN) content of a gas-rich and starbursting protocluster at z=4.002, known as the Distant Red Core (DRC). In particular, we search for luminous and possibly obscured AGN in 13 identified members of the structure, and compare the results with protoclusters at lower redshifts. We also test whether a hidden AGN can power the Ly alpha blob (LAB) detected with VLT/MUSE in the DRC.Methods. We observed all of the identified members of the structure with 139 ks of Chandra ACIS-S imaging. Being less affected by absorption than optical and IR bands, even in the presence of large column densities of obscuring material, X-ray observations are the best tools to detect ongoing nuclear activity in the DRC galaxies.Results. We detect obscured X-ray emission from the two most gas-rich members of the DRC, named DRC-1 and DRC-2. Both of them are resolved into multiple interacting clumps in high-resolution Atacama Large Millimeter Array and Hubble Space Telescope observations. In particular, DRC-2 is found to host a luminous (L2-10 keV approximate to 3x10(45) erg s(-1) ) Compton-thick (N-H greater than or similar to 10(24) cm(-2)) quasar (QSO) candidate, comparable to the most luminous QSOs known at all cosmic times. The AGN fraction among DRC members is consistent with results found for lower redshift protoclusters. However, X-ray stacking analysis reveals that supermassive black hole (SMBH) accretion is likely also taking place in other DRC galaxies that are not detected individually by Chandra.Conclusions. The luminous AGN detected in the most gas-rich galaxies in the DRC and the widespread SMBH accretion in the other members, which is suggested by stacking analysis, point toward the presence of a strong link between large gas reservoirs, galaxy interactions, and luminous and obscured nuclear activity in protocluster members. The powerful and obscured QSO detected in DRC-2 is likely powering the nearby LAB detected with VLT/MUSE, possibly through photoionization; however, we propose that the diffuse Ly alpha emission may be due to gas shocked by a massive outflow launched by DRC-2 over a approximate to 10 kpc scale.
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    REVEALING A POPULATION OF HEAVILY OBSCURED ACTIVE GALACTIC NUCLEI AT z ≈ 0.5-1 IN THE CHANDRA DEEP FIELD-SOUTH
    (2011) Luo, B.; Brandt, W. N.; Xue, Y. Q.; Alexander, D. M.; Brusa, M.; Bauer, F. E.; Comastri, A.; Fabian, A. C.; Gilli, R.; Lehmer, B. D.; Rafferty, D. A.; Schneider, D. P.; Vignali, C.
    Heavily obscured (NH greater than or similar to 3 x 10(23) cm(-2)) active galactic nuclei (AGNs) not detected even in the deepest X-ray surveys are often considered to be comparably numerous to the unobscured and moderately obscured AGNs. Such sources are required to fit the cosmic X-ray background (XRB) emission in the 10-30 keV band. We identify a numerically significant population of heavily obscured AGNs at z approximate to 0.5-1 in the Chandra Deep Field-South (CDF-S) and Extended Chandra Deep Field-South by selecting 242 X-ray undetected objects with infrared-based star-formation rates (SFRs) substantially higher (a factor of 3.2 or more) than their SFRs determined from the UV after correcting for dust extinction. An X-ray stacking analysis of 23 candidates in the central CDF-S region using the 4 Ms Chandra data reveals a hard X-ray signal with an effective power-law photon index of Gamma = 0.6(-0.4)(+ 0.3), indicating a significant contribution from obscured AGNs. Based on Monte Carlo simulations, we conclude that 74% +/- 25% of the selected galaxies host obscured AGNs, within which approximate to 95% are heavily obscured and approximate to 80% are Compton-thick (CT; N-H > 1.5 x 10(24) cm(-2)). The heavily obscured objects in our sample are of moderate intrinsic X-ray luminosity (approximate to(0.9-4) x 10(42) erg s(-1) in the 2-10 keV band). The space density of the CT AGNs is (1.6 +/- 0.5) x 10(-4) Mpc(-3). The z approximate to 0.5-1 CT objects studied here are expected to contribute approximate to 1% of the total XRB flux in the 10-30 keV band, and they account for approximate to 5%-15% of the emission in this energy band expected from all CT AGNs according to population-synthesis models. In the 6-8 keV band, the stacked signal of the 23 heavily obscured candidates accounts for <5% of the unresolved XRB flux, while the unresolved approximate to 25% of the XRB in this band can probably be explained by a stacking analysis of the X-ray undetected optical galaxies in the CDF-S (a 2.5 sigma stacked signal). We discuss prospects to identify such heavily obscured objects using future hard X-ray observatories.
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    SUPERMASSIVE BLACK HOLE GROWTH IN STARBURST GALAXIES OVER COSMIC TIME: CONSTRAINTS FROM THE DEEPEST CHANDRA FIELDS
    (2011) Rafferty, D. A.; Brandt, W. N.; Alexander, D. M.; Xue, Y. Q.; Bauer, F. E.; Lehmer, B. D.; Luo, B.; Papovich, C.
    We present an analysis of deep multiwavelength data for z approximate to 0.3-3 starburst galaxies selected by their 70 mu m emission in the Extended-Chandra Deep Field-South and Extended Groth Strip. We identify active galactic nuclei (AGNs) in these infrared sources through their X-ray emission and quantify the fraction that host an AGN. We find that the fraction depends strongly on both the mid-infrared color and rest-frame mid-infrared luminosity of the source, rising to similar to 50%-70% at the warmest colors (F-24 (mu m)/F-70 (mu m) less than or similar to 0.2) and highest mid-infrared luminosities (corresponding to ultraluminous infrared galaxies), similar to the trends found locally. Additionally, we find that the AGN fraction depends strongly on the star formation rate (SFR) of the host galaxy (inferred from the observed-frame 70 mu m luminosity after subtracting the estimated AGN contribution), particularly for more luminous AGNs (L0.5-8.0keV greater than or similar to 10(43) erg s(-1)). At the highest SFRs (similar to 1000 M-circle dot yr(-1)), the fraction of galaxies with an X-ray detected AGN rises to approximate to 30%, roughly consistent with that found in high-redshift submillimeter galaxies. Assuming that the AGN fraction is driven by the SFR (rather than stellar mass or redshift, for which our sample is largely degenerate), this result implies that the duty cycle of luminous AGN activity increases with the SFR of the host galaxy: specifically, we find that luminous X-ray detected AGNs are at least similar to 5-10 times more common in systems with high SFRs (greater than or similar to 300 M-circle dot yr(-1)) than in systems with lower SFRs (less than or similar to 30 M-circle dot yr(-1)). Lastly, we investigate the ratio between the supermassive black hole accretion rate (inferred from the AGN X-ray luminosity) and the bulge growth rate of the host galaxy (approximated as the SFR) and find that, for sources with detected AGNs and star formation (and neglecting systems with low star formation rates to which our data are insensitive), this ratio in distant starbursts agrees well with that expected from the local scaling relation assuming the black holes and bulges grew at the same epoch. These results imply that black holes and bulges grow together during periods of vigorous star formation and AGN activity.
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    The 2 Ms Chandra Deep Field-North Survey and the 250 ks Extended Chandra Deep Field-South Survey : Improved Point-Source Catalogs
    (2016) Xue, Y. Q.; Luo, B.; Brandt, W. N.; Alexander, D. M.; Bauer, Franz Erik; Lehmer, B. D.; Yang, G.
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    THE 4 Ms CHANDRA DEEP FIELD-SOUTH NUMBER COUNTS APPORTIONED BY SOURCE CLASS: PERVASIVE ACTIVE GALACTIC NUCLEI AND THE ASCENT OF NORMAL GALAXIES
    (2012) Lehmer, B. D.; Xue, Y. Q.; Brandt, W. N.; Alexander, D. M.; Bauer, F. E.; Brusa, M.; Comastri, A.; Gilli, R.; Hornschemeier, A. E.; Luo, B.; Paolillo, M.; Ptak, A.; Shemmer, O.; Schneider, D. P.; Tozzi, P.; Vignali, C.
    We present 0.5-2 keV, 2-8 keV, 4-8 keV, and 0.5-8 keV (hereafter soft, hard, ultra-hard, and full bands, respectively) cumulative and differential number-count (log N-log S) measurements for the recently completed approximate to 4 Ms Chandra Deep Field-South (CDF-S) survey, the deepest X-ray survey to date. We implement a new Bayesian approach, which allows reliable calculation of number counts down to flux limits that are factors of approximate to 1.9-4.3 times fainter than the previously deepest number-count investigations. In the soft band (SB), the most sensitive bandpass in our analysis, the approximate to 4 Ms CDF-S reaches a maximum source density of approximate to 27,800 deg(-2). By virtue of the exquisite X-ray and multiwavelength data available in the CDF-S, we are able to measure the number counts from a variety of source populations (active galactic nuclei (AGNs), normal galaxies, and Galactic stars) and subpopulations (as a function of redshift, AGN absorption, luminosity, and galaxy morphology) and test models that describe their evolution. We find that AGNs still dominate the X-ray number counts down to the faintest flux levels for all bands and reach a limiting SB source density of approximate to 14,900 deg(-2), the highest reliable AGN source density measured at any wavelength. We find that the normal-galaxy counts rise rapidly near the flux limits and, at the limiting SB flux, reach source densities of approximate to 12,700 deg(-2) and make up 46% +/- 5% of the total number counts. The rapid rise of the galaxy counts toward faint fluxes, as well as significant normal-galaxy contributions to the overall number counts, indicates that normal galaxies will overtake AGNs just below the approximate to 4 Ms SB flux limit and will provide a numerically significant new X-ray source population in future surveys that reach below the approximate to 4 Ms sensitivity limit. We show that a future approximate to 10 Ms CDF-S would allow for a significant increase in X-ray-detected sources, with many of the new sources being cosmologically distant (z greater than or similar to 0.6) normal galaxies.
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    The deepest X-ray view of high-redshift galaxies : constraints on low-rate black hole accretion
    (2016) Vito, F.; Gilli, R.; Vignali, C.; Brandt, W. N.; Comastri, A.; Yang, G.; Lehmer, B. D.; Luo, B.; Basu-Zych, A.; Bauer, Franz Erik
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    THE EVOLUTION OF NORMAL GALAXY X-RAY EMISSION THROUGH COSMIC HISTORY: CONSTRAINTS FROM THE 6 MS CHANDRA DEEP FIELD-SOUTH
    (2016) Lehmer, B. D.; Basu-Zych, A. R.; Mineo, S.; Brandt, W. N.; Eufrasio, R. T.; Fragos, T.; Hornschemeier, A. E.; Luo, B.; Xue, Y. Q.; Bauer, Franz Erik
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    The XMM-SERVS survey: new XMM-Newton point-source catalogue for the XMM-LSS field
    (2018) Chen, C-T J.; Brandt, W. N.; Luo, B.; Ranalli, P.; Yang, G.; Alexander, D. M.; Bauer, Franz Erik; Kelson, D. D.; Lacy, M.; Nyland, K.; Tozzi, P.; Vito, F.; Cirasuolo, M.; Gilli, R.; Jarvis, M. J.; Lehmer, B. D.; Paolillo, M.; Schneider, D. P.|