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Register a new userHot Dust Obscured Galaxies with Excess Blue Light: Dual AGN or Single AGN Under Extreme Conditions?
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Hot Dust-Obscured Galaxies (Hot DOGs) are a population of hyper-luminous infrared galaxies identified by the WISE mission from their very red mid-IR colors, and characterized by hot dust temperatures ($T>60~rm K$). Several studies have shown clear evidence that the IR emission in these objects is powered by a highly dust-obscured AGN that shows close to Compton-thick absorption at X-ray wavelengths. Thanks to the high AGN obscuration, the host galaxy is easily observable, and has UV/optical colors usually consistent with those of a normal galaxy. Here we discuss a sub-population of 8 Hot DOGs that show enhanced rest-frame UV/optical emission. We discuss three scenarios that might explain the excess UV emission: (i) unobscured light leaked from the AGN by reflection over the dust or by partial coverage of the accretion disk; (ii) a second unobscured AGN in the system; or (iii) a luminous young starburst. X-ray observations can help discriminate between these scenarios. We study in detail the blue excess Hot DOG WISE J020446.13-050640.8, which was serendipitously observed by Chandra/ACIS-I for 174.5 ks. The X-ray spectrum is consistent with a single, hyper-luminous, highly absorbed AGN, and is strongly inconsistent with the presence of a secondary unobscured AGN. Based on this, we argue that the excess blue emission in this object is most likely either due to reflection or a co-eval starburst. We favor the reflection scenario as the unobscured star-formation rate needed to power the UV/optical emission would be $gtrsim 1000~rm M_{odot}~rm yr^{-1}$. Deep polarimetry observations could confirm the reflection hypothesis.
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Hot Dust-Obscured Galaxies (Hot DOGs) are among the most luminous galaxies in the Universe. Powered by highly obscured, possibly Compton-thick, active galactic nuclei (AGNs), Hot DOGs are characterized by SEDs that are very red in the mid-IR yet dominated by the host galaxy stellar emission in the UV and optical. An earlier study identified a sub-sample of Hot DOGs with significantly enhanced UV emission. One target, W0204-0506, was studied in detail and, based on Chandra observations, it was concluded that the enhanced emission was most likely due to either extreme unobscured star-formation (${rm SFR}>1000~M_{odot}~rm yr^{-1}$) or to light from the highly obscured AGN scattered by gas or dust into our line of sight. Here, we present a follow-up study of W0204-0506 as well as two more Hot DOGs with excess UV emission. For the two new objects we obtained Chandra/ACIS-S observations, and for all three targets we obtained HST/WFC3 F555W and F160W imaging. We conclude that the excess UV emission is primarily dominated by light from the central highly obscured, hyper-luminous AGN that has been scattered into our line of sight. We cannot rule out, however, that star-formation may significantly contribute to the UV excess of W0204-0506.
Recent models of super-massive black hole (SMBH) and host galaxy joint evolution predict the presence of a key phase where accretion, traced by obscured Active Galactic Nuclei (AGN) emission, is coupled with powerful star formation. Then feedback processes likely self-regulate the SMBH growth and quench the star-formation activity. AGN in this important evolutionary phase have been revealed in the last decade via surveys at different wavelengths. On the one hand, moderate-to-deep X-ray surveys have allowed a systematic search for heavily obscured AGN, up to very high redshifts (z~5). On the other hand, infrared/optical surveys have been invaluable in offering complementary methods to select obscured AGN also in cases where the nuclear X-ray emission below 10 keV is largely hidden to our view. In this review I will present my personal perspective of the field of obscured accretion from AGN surveys.
At low Eddington ratio (mdot), two effects make it harder to detect AGN given some selection criteria. First, even with fixed accretion physics, AGN are diluted/less luminous relative to their hosts; the magnitude of this depends on host properties and so on luminosity and redshift. Second, they may transition to a radiatively inefficient state, changing SED shape and dramatically decreasing in optical/IR luminosity. These effects lead to differences in observed AGN samples, even at fixed bolometric luminosity and after correction for obscuration. The true Eddington ratio distribution may depend strongly on luminosity, but this will be seen only in surveys robust to dilution and radiative inefficiency (X-ray or narrow-line samples); selection effects imply that AGN in optical samples will have uniformly high mdot. This also implies that different selection methods yield systems with different hosts: the clustering of faint optical/IR sources will be weaker than that of X-ray sources, and optical/IR Seyferts will reside in more disk-dominated galaxies while X-ray selected Seyferts will preferentially occupy early-type systems. If observed mdot distributions are correct, a large fraction of low-luminosity AGN currently classified as obscured are in fact diluted and/or radiatively inefficient, not obscured by gas or dust. This is equally true if X-ray hardness is used as a proxy for obscuration, since radiatively inefficient SEDs near mdot~0.01 are X-ray hard. These effects can explain most of the claimed luminosity/redshift dependence in the obscured AGN population, with the true obscured fraction as low as 20%.
We report the results of an investigation to determine the nature of the offset active galactic nucleus (AGN) found in the source CXO J101527.2+625911. Hubble Space Telescope and Chandra X-ray observatory data had suggested that the offset AGN, which has an angular separation of only 0farcs26 from the center of the host galaxy, is a recoiled Super Massive Black Hole (rSMBH). We carried out high angular resolution observations with both the VLBA (1.54 GHz) and the VLA (10.0 GHz & 33.0 GHz) and detected a single compact radio source in the center of the host galaxy, with no radio continuum emission associated with the offset AGN. The detected radio source has a high brightness temperature value of $T_b=7.2times10^{7}$ K, indicating that the radio emission is associated with an AGN. Furthermore, we present the decomposition of high-resolution KECK spectra of the [O III]5007AA line into two narrow emission line components, which is a characteristic sign of a dual black hole system. These new radio and optical wavelength results suggest that CXO J101527.2+625911 is the host of a dual black hole system rather than a rSMBH.
We present rest-frame optical spectroscopic observations of 24 Hot Dust-Obscured Galaxies (Hot DOGs) at redshifts 1.7-4.6 with KECK/NIRES. Our targets are selected based on their extreme red colors to be the highest luminosity sources from the WISE infrared survey. In 20 sources with well-detected emission we fit the key [O III], H$beta$, H$alpha$, [N II], and [S II] diagnostic lines to constrain physical conditions. Of the 17 targets with a clear detection of the [O III]$rm lambda$5007A emission line, 15 display broad blueshifted and asymmetric line profiles, with widths ranging from 1000 to 8000 $rm km s^{-1}$ and blueshifts up to 3000 $rm km s^{-1}$. These kinematics provide strong evidence for the presence of massive ionized outflows of up to $8000 rm M_odot yr^{-1}$, with a median of $150 rm M_odot yr^{-1}$. As many as eight sources show optical emission line ratios consistent with vigorous star formation. Balmer line star-formation rates, uncorrected for reddening, range from 30--1300 $rm M_odot yr^{-1}$, with a median of $50 rm M_odot yr^{-1}$. Estimates of the SFR from SED fitting of mid and far-infrared photometry suggest significantly higher values. We estimate the central black hole masses to be of order $10^{8-10}rm M_odot$, assuming the present-day $rm M_{BH}-sigma_*$ relation. The bolometric luminosities and the estimated masses of the central black holes of these galaxies suggest that many of the AGN-dominated Hot DOGs are accreting at or above their Eddington limit. The combination of ongoing star formation, massive outflows, and high Eddington ratios suggest Hot DOGs are a transitional phase in galaxy evolution.
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