Browsing by Author "Ho, L. C."
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- ItemA hard X-ray view of luminous and ultra-luminous infrared galaxies in GOALS - I. AGN obscuration along the merger sequence(2021) Ricci, C.; Privon, G. C.; Pfeifle, R. W.; Armus, L.; Iwasawa, K.; Torres-Albà, N.; Satyapal, S.; Bauer, F. E.; Treister, E.; Ho, L. C.; Aalto, S.; Arévalo, P.; Barcos-Muñoz, L.; Charmandaris, V.; Diaz-Santos, T.; Evans, A. S.; Gao, T.; Inami, H.; Koss, M. J.; Lansbury, G.; Linden, S. T.; Medling, A.; Sanders, D. B.; Song, Y.; Stern, D.; U, V.; Ueda, Y.; Yamada, S.The merger of two or more galaxies can enhance the inflow of material from galactic scales into the close environments of active galactic nuclei (AGNs), obscuring and feeding the supermassive black hole (SMBH). Both recent simulations and observations of AGN in mergers have confirmed that mergers are related to strong nuclear obscuration. However, it is still unclear how AGN obscuration evolves in the last phases of the merger process. We study a sample of 60 luminous and ultra-luminous IR galaxies (U/LIRGs) from the GOALS sample observed by NuSTAR. We find that the fraction of AGNs that are Compton thick (CT;N-H >= 10(24)cm(-2) ) peaks at at a late merger stage, prior to coalescence, when the nuclei have projected separations (d(sep)) of 0.4-6 kpc. A similar peak is also observed in the median N-H [[(1.6 +/- 0.5) x 10(24) cm(-2)].]. The vast majority (85(-9)(+7) per cent)) of the AGNs in the final merger stages (d(sep) less than or similar to 10 kpc) are heavily obscured (N-H = 10(23) cm(-2)), and the median N-H of the accreting SMBHs in our sample is systematically higher than that of local hard X-ray-selected AGN, regardless of the merger stage. This implies that these objects have very obscured nuclear environments, with the gas almost completely covering the AGN in late mergers. CT AGNs tend to have systematically higher absorption-corrected X-ray luminosities than less obscured sources. This could either be due to an evolutionary effect, with more obscured sources accreting more rapidly because they have more gas available in their surroundings, or to a selection bias. The latter scenario would imply that we are still missing a large fraction of heavily obscured, lower luminosity (L2-10 less than or similar to 10(43) erg s(-1)) AGNs in U/LIRGs.
- ItemBASS XXXVII: The Role of Radiative Feedback in the Growth and Obscuration Properties of Nearby Supermassive Black Holes(2022) Ricci, C.; Ananna, T. T.; Temple, M. J.; Urry, C. M.; Koss, M. J.; Trakhtenbrot, B.; Ueda, Y.; Stern, D.; Bauer, F. E.; Treister, E.; Privon, G. C.; Oh, K.; Paltani, S.; Stalevski, M.; Ho, L. C.; Fabian, A. C.; Mushotzky, R.; Chang, C. S.; Ricci, F.; Kakkad, D.; Sartori, L.; Baer, R.; Caglar, T.; Powell, M.; Harrison, F.We study the relation between obscuration and supermassive black hole (SMBH) accretion using a large sample of hard X-ray selected active galactic nuclei (AGNs). We find a strong decrease in the fraction of obscured sources above the Eddington limit for dusty gas (log lambda(Edd) >= -2) confirming earlier results, and consistent with the radiation-regulated unification model. This also explains the difference in the Eddington ratio distribution functions (ERDFs) of type 1 and type 2 AGNs obtained by a recent study. The break in the ERDF of nearby AGNs is at log lambda*(Edd) = -1.34 +/- 0.07. This corresponds to the lambda(Edd) where AGNs transition from having most of their sky covered by obscuring material to being mostly devoid of absorbing material. A similar trend is observed for the luminosity function, which implies that most of the SMBH growth in the local universe happens when the AGN is covered by a large reservoir of gas and dust. These results could be explained with a radiation-regulated growth model, in which AGNs move in the N-H-lambda(Edd) plane during their life cycle. The growth episode starts with the AGN mostly unobscured and accreting at low lambda(Edd). As the SMBH is further fueled, lambda(Edd), N-H and the covering factor increase, leading the AGN to be preferentially observed as obscured. Once lambda(Edd) reaches the Eddington limit for dusty gas, the covering factor and N-H rapidly decrease, leading the AGN to be typically observed as unobscured. As the remaining fuel is depleted, the SMBH goes back into a quiescent phase.
- ItemBASS. XLII. The Relation between the Covering Factor of Dusty Gas and the Eddington Ratio in Nearby Active Galactic NucleiRicci, C.; Ichikawa, K.; Stalevski, M.; Kawamuro, T.; Yamada, S.; Ueda, Y.; Mushotzky, R.; Privon, G. C.; Koss, M. J.; Trakhtenbrot, B.; Fabian, A. C.; Ho, L. C.; Asmus, D.; Bauer, Franz Erik; Chang, C. S.; Gupta, K. K.; Oh, K.; Powell, M.; Pfeifle, R. W.; Rojas, A.; Ricci, F.; Temple, M. J.; Toba, Y.; Tortosa, A.; Treister, Ezequiel; Harrison, F.; Stern, D.; Urry, C. M.Accreting supermassive black holes (SMBHs) located at the centers of galaxies are typically surrounded by large quantities of gas and dust. The structure and evolution of this circumnuclear material can be studied at different wavelengths, from the submillimeter to the X-ray. Recent X-ray studies have shown that the covering factor of the obscuring material tends to decrease with increasing Eddington ratio, likely due to radiative feedback on dusty gas. Here we study a sample of 549 nearby (z less than or similar to 0.1) hard X-ray (14-195 keV) selected nonblazar active galactic nuclei (AGN) and use the ratio between the AGN infrared and bolometric luminosity as a proxy of the covering factor. We find that, in agreement with what has been found by X-ray studies of the same sample, the covering factor decreases with increasing Eddington ratio. We also confirm previous findings that showed that obscured AGN typically have larger covering factors than unobscured sources. Finally, we find that the median covering factors of AGN located in different regions of the column density-Eddington ratio diagram are in good agreement with what would be expected from a radiation-regulated growth of SMBHs.
- ItemBAT AGN Spectroscopic Survey - XII. The relation between coronal properties of active galactic nuclei and the Eddington ratio.(2018) Ricci, Claudio; Bauer, Franz Erik; Treister, Ezequiel; Ho, L. C.; Fabian, AC.; Trakhtenbrot, B.; Koss, M.; Ueda, Y.; Lohfink, A.; Shimizu, T.; Mushotzky, R.
- ItemGrowing supermassive black holes in the late stages of galaxy mergers are heavily obscured.(2017) Ricci, Claudio; Bauer, Franz Erik; Treister, Ezequiel; Privon, G. C.; Schawinski, K.; Blecha, L.; Arevalo, P.; Armus, Lee; Harrison, F.; Ho, L. C.; Iwasawa, K.; Sanders, David B.; Stern, Daniel
- ItemJoint ALMA/X-ray monitoring of the radio-quiet type 1 active galactic nucleus IC 4329A(2024) Shablovinskaya, E.; Ricci, C.; Chang, C. -s.; Tortosa, A.; del Palacio, S.; Kawamuro, T.; Aalto, S.; Arzoumanian, Z.; Balokovic, M.; Bauer, F. E.; Gendreau, K. C.; Ho, L. C.; Kakkad, D.; Kara, E.; Koss, M. J.; Liu, T.; Loewenstein, M.; Mushotzky, R.; Paltani, S.; Privon, G. C.; Smith, K.; Tombesi, F.; Trakhtenbrot, B.The origin of a compact millimeter (mm, 100-250 GHz) emission in radio-quiet active galactic nuclei (RQ AGN) remains debated. Recent studies propose a connection with self-absorbed synchrotron emission from the accretion disk X-ray corona. We present the first joint ALMA (similar to 100 GHz) and X-ray (NICER/XMM-Newton/Swift; 2-10 keV) observations of the unobscured RQ AGN, IC 4329A (z = 0.016). The time-averaged mm-to-X-ray flux ratio aligns with recently established trends for larger samples, but with a tighter scatter (similar to 0.1 dex) compared to previous studies. However, there is no significant correlation on timescales of less than 20 days. The compact mm emission exhibits a spectral index of -0.23 +/- 0.18, remains unresolved with a 13 pc upper limit, and shows no jet signatures. Notably, the mm flux density varies significantly (by factor of 3) within four days, exceeding the contemporaneous X-ray variability and showing the largest mm variations ever detected in RQ AGN over daily timescales. The high amplitude variability rules out scenarios of heated dust and thermal free-free emission, pointing toward a synchrotron origin for the mm radiation in a source of similar to 1 light day (similar to 120 gravitational radii) size. While the exact source is not yet certain, an X-ray corona scenario emerges as the most plausible compared to a scaled-down jet or outflow-driven shocks.
- ItemThe QUEST-La Silla AGN Variability Survey: Selection of AGN Candidates through Optical Variability(2019) Sanchez-Saez, P.; Lira, P.; Cartier, R.; Miranda, N.; Ho, L. C.; Arevalo, P.; Bauer, F. E.; Coppi, P.; Yovaniniz, C.We used data from the QUEST-La Silla Active Galactic Nucleus (AGN) variability survey to construct light curves for 208,583 sources over similar to 70 deg(2), with a limiting magnitude r similar to 21. Each light curve has at least 40 epochs and a length of >= 200 days. We implemented a random forest algorithm to classify our objects as either AGN or non-AGN according to their variability features and optical colors, excluding morphology cuts. We tested three classifiers, one that only includes variability features (RF1), one that includes variability features and also r - i and i - z colors (RF2), and one that includes variability features and also g - r, r - i, and i - z colors (RF3). We obtained a sample of high-probability candidates (hp-AGN) for each classifier, with 5941 candidates for RF1, 5252 candidates for RF2, and 4482 candidates for RF3. We divided each sample according to their g - r colors, defining blue (g - r <= 0.6) and red subsamples (g - r > 0.6). We find that most of the candidates known from the literature belong to the blue subsample, which is not necessarily surprising given that, unlike many literature studies, we do not cut our sample to point-like objects. This means that we can select AGNs that have a significant contribution from redshifted starlight in their host galaxies. In order to test the efficiency of our technique, we performed spectroscopic follow-up, confirming the AGN nature of 44 among 54 observed sources (81.5% efficiency). From the campaign, we concluded that RF2 provides the purest sample of AGN candidates.