Browsing by Author "Vito, Fabio"

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    A Large Population of Obscured AGN in Disguise as Low-luminosity AGN in Chandra Deep Field South
    (2020) Lambrides, Erini L.; Chiaberge, Marco; Heckman, Timothy; Gilli, Roberto; Vito, Fabio; Norman, Colin
    Population synthesis models of actively accreting supermassive black holes (or active galactic nuclei (AGN)) predict a large fraction that must grow behind dense, obscuring screens of gas and dust. Deep X-ray surveys are thought to have provided the most complete and unbiased samples of AGN, but there is strong observational evidence that a portion of the population of obscured AGN is being missed. In this paper, we use a sample of AGN derived from the deepest X-ray survey to date, the Chandra 7 Ms GOODS-South Survey, to investigate the nature of low-flux X-ray sources. We make full use of the extensive multiwavelength coverage of the GOODS-South field and cross-match our objects with wavelengths from the radio to the IR. We find that the low X-ray flux AGN in our sample have X-ray luminosities that indicate low-luminosity AGN classification, while their radio, IR, and optical counterparts indicate moderate to powerful AGN classification. We find that the predicted column densities are, on average, an order of magnitude higher than the calculated column densities via X-ray detections for X-ray-faint sources. We interpret our results as evidence of obscured AGN disguised as low-luminosity AGN via their X-ray luminosities. When we compare the estimation of the obscured AGN space density with and without these objects, we find a difference of 40% in the lowest X-ray luminosity regime probed by our sample.
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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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    Identification and Characterization of a Large Sample of Distant Active Dwarf Galaxies in XMM-SERVS
    (2023) Zou, Fan; Brandt, W. N.; Ni, Qingling; Zhu, Shifu; Alexander, David M.; Bauer, Franz E.; Chen, Chien-Ting J.; Luo, Bin; Sun, Mouyuan; Vignali, Cristian; Vito, Fabio; Xue, Yongquan; Yan, Wei
    Active dwarf galaxies are important because they contribute to the evolution of dwarf galaxies and can reveal their hosted massive black holes. However, the sample size of such sources beyond the local universe is still highly limited. In this work, we search for active dwarf galaxies in the recently completed XMM-Spitzer Extragalactic Representative Volume Survey (XMM-SERVS). XMM-SERVS is currently the largest medium-depth X-ray survey covering 13 deg(2) in three extragalactic fields, which all have well-characterized multiwavelength information. After considering several factors that may lead to misidentifications, we identify 73 active dwarf galaxies at z < 1, which constitutes the currently largest X-ray-selected sample beyond the local universe. Our sources are generally less obscured than predictions based on the massive-AGN (active galactic nucleus) X-ray luminosity function and have a low radio-excess fraction. We find that our sources reside in environments similar to those of inactive dwarf galaxies. We further quantify the accretion distribution of the dwarf-galaxy population after considering various selection effects and find that it decreases with X-ray luminosity, but redshift evolution cannot be statistically confirmed. Depending on how we define an AGN, the active fraction may or may not show a strong dependence on stellar mass. Their Eddington ratios and X-ray bolometric corrections significantly deviate from the expected relation, which is likely caused by several large underlying systematic biases when estimating the relevant parameters for dwarf galaxies. Throughout this work, we also highlight problems in reliably measuring photometric redshifts and overcoming strong selection effects for distant active dwarf galaxies.
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    Piercing through Highly Obscured and Compton-thick AGNs in the Chandra Deep Fields. I. X-Ray Spectral and Long-term Variability Analyses
    (2019) Li, Junyao; Xue, Yongquan; Sun, Mouyuan; Liu, Teng; Vito, Fabio; Brandt, William N.; Hughes, Thomas M.; Yang, Guang; Tozzi, Paolo; Zhu, Shifu; Zheng, Xuechen; Luo, Bin; Chen, Chien-Ting; Vignali, Cristian; Gilli, Roberto; Shu, Xinwen
    We present a detailed X-ray spectral analysis of 1152 active galactic nuclei (AGNs) selected in the Chandra Deep Fields (CDFs), in order to identify highly obscured AGNs (N-H > 10(23) cm(-2)). By fitting spectra with physical models, 436 (38%) sources with L-X > 10(42) erg s(-1) are confirmed to be highly obscured, including 102 Compton-thick (CT) candidates. We propose a new hardness ratio measure of the obscuration level that can be used to select highly obscured AGN candidates. The completeness and accuracy of applying this method to our AGNs are 88% and 80%, respectively. The observed log N-log S relation favors cosmic X-ray background models that predict moderate (i.e., between optimistic and pessimistic) CT number counts. Nineteen percent (6/31) of our highly obscured AGNs that have optical classifications are labeled as broad-line AGNs, suggesting that, at least for part of the AGN population, the heavy X-ray obscuration is largely a line-of-sight effect, i.e., some high column density clouds on various scales (but not necessarily a dust-enshrouded torus) along our sight line may obscure the compact X-ray emitter. After correcting for several observational biases, we obtain the intrinsic N-H distribution and its evolution. The CT/highly obscured fraction is roughly 52% and is consistent with no evident redshift evolution. We also perform long-term (approximate to 7 yr in the observed frame) variability analyses for 31 sources with the largest number of counts available. Among them, 17 sources show flux variabilities: 31% (5/17) are caused by the change of N-H, 53% (9/17) are caused by the intrinsic luminosity variability, 6% (1/17) are driven by both effects, and 2 are not classified owing to large spectral fitting errors.
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    Piercing through Highly Obscured and Compton-thick AGNs in the Chandra Deep Fields. II. Are Highly Obscured AGNs the Missing Link in the Merger-triggered AGN-Galaxy Coevolution Models?
    (2020) Li, Junyao; Xue, Yongquan; Sun, Mouyuan; Brandt, William N.; Yang, Guang; Vito, Fabio; Tozzi, Paolo; Vignali, Cristian; Comastri, Andrea; Shu, Xinwen; Fang, Guanwen; Fan, Lulu; Luo, Bin; Chen, Chien-Ting; Zheng, Xuechen
    By using a large, highly obscured ( N-H > 10(23) cm(-2)) active galactic nucleus (AGN) sample (294 sources at z similar to 0-5) selected from detailed X-ray spectral analyses in the deepest Chandra.surveys, we explore distributions of these X-ray sources in various optical/infrared/X-ray color-color diagrams and their host-galaxy properties, aiming at characterizing the nuclear obscuration environment and the triggering mechanism of highly obscured AGNs. We find that the refined Infrared Array Camera (IRAC) color-color diagram fails to identify the majority of X-ray-selected, highly obscured AGNs, even for the most luminous sources with log L-X(erg s(-1)) > 44. Over 80% of our sources will not be selected as heavily obscured candidates using the flux ratio of f(24 mu m)/f(R) > 1000 and R - K > 4.5 criteria, implying complex origins and conditions for the obscuring materials that are responsible for the heavy X-ray obscuration. The average star formation rate (SFR) of highly obscured AGNs is similar to that of stellar mass-(M*-) and z-controlled normal galaxies, while a lack of quiescent hosts is observed for the former. Partial correlation analyses imply that highly obscured AGN activity (traced by L-X) appears to be more fundamentally related to M*, and no dependence of N-H on either M* or SFR is detected. Morphology analyses reveal that 61% of our sources have a significant disk component, while only similar to 27% of them exhibit irregular morphological signatures. These findings together point toward a scenario where secular processes (e.g., galacticdisk instabilities), instead of mergers, are most probable to be the leading mechanism that triggers accretion activities of X-ray-selected, highly obscured AGNs.
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    The Most Obscured AGNs in the XMM-SERVS Fields
    (2023) Yan, Wei; Brandt, W. N.; Zou, Fan; Zhu, Shifu; Chen, Chien-Ting J.; Hickox, Ryan C.; Luo, Bin; Ni, Qingling; Alexander, David M.; Bauer, Franz E.; Vignali, Cristian; Vito, Fabio
    We perform X-ray spectral analyses to derive the characteristics (e.g., column density, X-ray luminosity) of & AP;10,200 active galactic nuclei (AGNs) in the XMM-Spitzer Extragalactic Representative Volume Survey, which was designed to investigate the growth of supermassive black holes across a wide dynamic range of cosmic environments. Using physical torus models (e.g., Borus02) and a Bayesian approach, we uncover 22 representative Compton-thick (CT; N (H) > 1.5 x 10(24) cm(-2)) AGN candidates with good signal-to-noise ratios as well as a large sample of 136 heavily obscured AGNs. We also find an increasing CT fraction (f (CT)) from low (z < 0.75) to high (z > 0.75) redshift. Our CT candidates tend to show hard X-ray spectral shapes and dust extinction in their spectral energy distribution fits, which may shed light on the connection between AGN obscuration and host-galaxy evolution.
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    The XMM-SERVS Survey: XMM-Newton Point-source Catalogs for the W-CDF-S and ELAIS-S1 Fields
    (2021) Ni, Qingling; Brandt, W. N.; Chen, Chien-Ting; Luo, Bin; Nyland, Kristina; Yang, Guang; Zou, Fan; Aird, James; Alexander, David M.; Bauer, Franz Erik; Lacy, Mark; Lehmer, Bret D.; Mallick, Labani; Salvato, Mara; Schneider, Donald P.; Tozzi, Paolo; Traulsen, Iris; Vaccari, Mattia; Vignali, Cristian; Vito, Fabio; Xue, Yongquan; Banerji, Manda; Chow, Kate; Comastri, Andrea; Del Moro, Agnese; Gilli, Roberto; Mullaney, James; Paolillo, Maurizio; Schwope, Axel; Shemmer, Ohad; Sun, Mouyuan; Timlin III, John D.; Trump, Jonathan R.
    We present the X-ray point-source catalogs in two of the XMM-Spitzer Extragalactic Representative Volume Survey (XMM-SERVS) fields, W-CDF-S (4.6 deg(2)) and ELAIS-S1 (3.2 deg(2)), aiming to fill the gap between deep pencil-beam X-ray surveys and shallow X-ray surveys over large areas. The W-CDF-S and ELAIS-S1 regions were targeted with 2.3 and 1.0 Ms of XMM-Newton observations, respectively; 1.8 and 0.9 Ms exposures remain after flare filtering. The survey in W-CDF-S has a flux limit of 1.0 x 10(-14) erg cm(-2) s(-1) over 90% of its area in the 0.5-10 keV band; 4053 sources are detected in total. The survey in ELAIS-S1 has a flux limit of 1.3 x 10(-14) erg cm(-2) s(-1) over 90% of its area in the 0.5-10 keV band; 2630 sources are detected in total. Reliable optical-to-IR multiwavelength counterpart candidates are identified for approximate to 89% of the sources in W-CDF-S and approximate to 87% of the sources in ELAIS-S1. A total of 3129 sources in W-CDF-S and 1957 sources in ELAIS-S1 are classified as active galactic nuclei (AGNs). We also provide photometric redshifts for X-ray sources; approximate to 84% of the 3319/2001 sources in W-CDF-S/ELAIS-S1 with optical-to-near-IR forced photometry available have either spectroscopic redshifts or high-quality photometric redshifts. The completion of the XMM-Newton observations in the W-CDF-S and ELAIS-S1 fields marks the end of the XMM-SERVS survey data gathering. The approximate to 12,000 pointlike X-ray sources detected in the whole approximate to 13 deg(2) XMM-SERVS survey will benefit future large-sample AGN studies.
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    X-ray properties of dust-obscured galaxies with broad optical/UV emission lines
    (2020) Zou, Fan; Brandt, William N.; Vito, Fabio; Chen, Chien-Ting; Garmire, Gordon P.; Stern, Daniel; Ayubinia, Ashraf
    Dust-obscured galaxies (DOGs) with extreme infrared luminosities may represent a key phase in the co-evolution of galaxies and supermassive black holes. We select 12 DOGs at 0.3 less than or similar to z less than or similar to 1.0 with broad Mg II or H beta emission lines and investigate their X-ray properties utilizing snapshot observations (similar to 3 ks per source) with Chandra. By assuming that the broad lines are broadened due to virial motions of broad-line regions, we find that our sources generally have high Eddington ratios (lambda(Edd)). Our sources generally have moderate intrinsic X-ray luminosities (L-X less than or similar to 10(45) erg s(-1)), which are similar to those of other DOGs, but are more obscured. They also present moderate outflows and intense starbursts. Based on these findings, we conclude that high-lambda(Edd) DOGs are closer to the peaks of both host-galaxy and black hole growth compared to other DOGs, and that (active galactic nucleus) AGN feedback has not swept away their reservoirs of gas. However, we cannot fully rule out the possibility that the broad lines are broadened by outflows, at least for some sources. We investigate the relations among L-X, AGN rest-frame 6 mu m monochromatic luminosity, and AGN bolometric luminosity, and find the relations are consistent with the expected ones.