Hot Dust-Obscured Galaxies with Excess Blue Light
R.J. Assef, M. Brightman, D.J. Walton, D.Stern, F.E. Bauer, A.W. Blain, T. Diaz-Santos, P.R.M. Eisenhardt, R.C. Hickox, H.D. Jun, A. Psychogyios, C.-W. Tsai, J.W. Wu
aa r X i v : . [ a s t r o - ph . GA ] M a y Draft version May 14, 2019
Preprint typeset using L A TEX style emulateapj v. 12/16/11
HOT DUST-OBSCURED GALAXIES WITH EXCESS BLUE LIGHT
R.J. Assef , M. Brightman , D.J. Walton , D. Stern , F.E. Bauer , A.W. Blain , T. D´ıaz-Santos ,P.R.M. Eisenhardt , R.C. Hickox , H.D. Jun , A. Psychogyios , C.-W. Tsai , J.W. Wu Draft version May 14, 2019
ABSTRACTHot 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 arecharacterized by SEDs that are very red in the mid-IR yet dominated by the host galaxy stellar emis-sion in the UV and optical. An earlier study identified a sub-sample of Hot DOGs with significantlyenhanced UV emission. One target, W0204–0506, was studied in detail and, based on
Chandra obser-vations, it was concluded that the enhanced emission was most likely due to either extreme unobscuredstar-formation (SFR > M ⊙ yr − ) or to light from the highly obscured AGN scattered by gas ordust into our line of sight. Here, we present a follow-up study of W0204–0506 as well as two more HotDOGs 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 theexcess UV emission is primarily dominated by light from the central highly obscured, hyper-luminousAGN that has been scattered into our line of sight. We cannot rule out, however, that star-formationmay significantly contribute to the UV excess of W0204–0506.
Keywords: galaxies: active — galaxies: evolution — galaxies: high-redshift — quasars: general —infrared: galaxies INTRODUCTION
Hot Dust-Obscured Galaxies (Hot DOGs;Eisenhardt et al. 2012; Wu et al. 2012) are some ofthe most luminous galaxies in the Universe, withbolometric luminosities L bol > L ⊙ and a signif-icant fraction with L bol > L ⊙ (Wu et al. 2012;Tsai et al. 2015). Discovered by the Wide-field InfraredSurvey Explorer (WISE; Wright et al. 2010), HotDOGs are characterized by very red mid-IR colorsand spectral energy distributions (SEDs) that peakat rest-frame ∼ µ m. This implies that Hot DOGsare powered by highly obscured, hyper-luminous AGNthat dominate the SED from the mid- to the far-IR(Eisenhardt et al. 2012; Wu et al. 2012, 2014; Fan et al. N´ucleo de Astronom´ıa de la Facultad de Ingenier´ıa y Cien-cias, Universidad Diego Portales, Av. Ej´ercito Libertador 441,Santiago, Chile. Cahill Center for Astrophysics, California Institute of Tech-nology, 1216 East California Boulevard, Pasadena, CA 91125,USA Institute of Astronomy, University of Cambridge, MadingleyRoad, Cambridge CB3 0HA, UK Jet Propulsion Laboratory, California Institute of Technol-ogy, 4800 Oak Grove Drive, Pasadena, CA 91109, USA Instituto de Astrof´ısica, Facultad de F´ısica, Pontificia Uni-versidad Cat´olica de Chile, 306, Santiago 22, Chile Millennium Institute of Astrophysics (MAS), NuncioMonse˜nor S´otero Sanz 100, Providencia, Santiago, Chile Space Science Institute, 4750 Walnut Street, Suite 205, Boul-der, Colorado 80301 Physics & Astronomy, University of Leicester, 1 UniversityRoad, Leicester LE1 7RH, UK Department of Physics and Astronomy, Dartmouth College,6127 Wilder Laboratory, Hanover, NH 03755, USA School of Physics, Korea Institute for Advanced Study, 85Hoegiro, Dongdaemun-gu, Seoul 02455, Republic of Korea Department of Physics, University of Crete, 71003, Herak-lion, Greece National Astronomical Observatories, Chinese Academy ofSciences, 20A Datun Road, Chaoyang District, Beijing, 100012,People’s Republic of China ± . Yet their numberdensity is comparable to that of similarly luminousunobscured quasars (Assef et al. 2015) and of heavilyreddened type 1 quasars (Banerji et al. 2015).X-ray studies have shown that the obscuration of thecentral engine in Hot DOGs is very high, with col-umn densities ranging from somewhat below to abovethe Compton-thick limit (i.e., N H > . × cm − Stern et al. 2014; Piconcelli et al. 2015; Assef et al. 2016;Ricci et al. 2017; Vito et al. 2018). As the AGN emissionis highly obscured, the host galaxy is observable at rest-frame UV, optical and near-IR wavelengths. A study oftheir SEDs by Assef et al. (2015) showed that their stel-lar masses, as derived from their rest-frame near-IR lumi-nosities, imply that either the super-massive black holes(SMBHs) are accreting well above the Eddington limit,or that their SMBH masses ( M BH ) are well above the lo-cal relations between M BH and the mass of the spheroidalcomponent of the host galaxy (see, e.g., Magorrian et al.1998; Bennert et al. 2011). Indeed, recent results byWu et al. (2018) and Tsai et al. (2018) suggest that HotDOGs are radiating at or above the Eddington limit,which in turn suggests that Hot DOGs are likely experi-encing strong AGN feedback that could easily affect thewhole host galaxy and its immediate environment. In-deed, D´ıaz-Santos et al. (2016) presented a study of the[C ii ] 157.7 µ m emission line in the highest luminosity HotDOG, and possibly the most luminous galaxy known,WISEA J224607.56–052634.9 (W2246–0526; Tsai et al.2015), and determined based on the emission-line kine-matics that the central gas of the host galaxy is likelyundergoing an isotropic outflow event. Further ionizedgas outflow signatures have been observed in the opticalnarrow emission lines of some other Hot DOGs (Wu et al.2018; Jun et al. in prep.), supporting the presence ofstrong AGN feedback in the ISM of these targets.Assef et al. (2015, also see Eisenhardt et al. 2012;Tsai et al. 2015, 2018) showed that the UV through mid-IR SED of the majority of Hot DOGs (specifically “W12–drops” with z >
1) can be well modeled as a combina-tion of a star-forming galaxy that dominates the opti-cal/UV emission, and a luminous, obscured AGN thatdominates the mid-IR SED and the bolometric lumi-nosity of the system. However, this is not the case forall Hot DOGs. In a later work, Assef et al. (2016, A16hereafter) presented a small sample of eight Hot DOGswhose optical/UV emission is not well modeled by a star-forming galaxy, but instead needs a second, unobscuredAGN component that is only ∼
1% as luminous as theobscured component. A16 posited that the SED couldbe explained by three different scenarios: i) that theUV/optical emission is dominated by leaked or scatteredlight from the hyper-luminous, highly obscured AGN;ii) that the system is a dual quasar, with a more lumi-nous, highly obscured quasar and a less luminous, un-obscured one; and iii) that the system is undergoing anextreme star-formation event with little dust obscurationsuch that the broad-band UV/optical SED is similar tothat of an AGN.One of these objects, WISEA J020446.13–050640.8(W0204–0506 hereafter), was serendipitously observedby the
Chandra X-ray Observatory as part of theLarge-Area Lyman Alpha survey (LALA; Rhodes et al.2000). A16 studied this object in detail usingthese observations along with broad-band SED andoptical spectroscopic observations. A16 determinedthat the X-ray spectrum of W0204–0506 is consis-tent with a single, hyper-luminous, highly absorbedAGN (log L −
10 keV / erg s − = 44 . +0 . − . , N H =6 . +8 . − . × cm − ), and highly inconsistent with asecondary, unobscured AGN with the luminosity nec-essary to explain the optical/UV emission. Instead,A16 found that the UV/optical continuum was bet-ter explained by a starburst with a star-formation rate & M ⊙ yr − , or by scattered light from the hyper-luminous, highly obscured central engine. While star-formation rates (SFRs) & M ⊙ yr − are routinelyfound through far-IR/sub-mm observations of highly ob-scured systems such as SMGs and ULIRGs, rates above ∼ M ⊙ yr − have never been observed throughUV/optical wavelengths in Lyman break galaxies, whichhave the strongest UV/optical star-formations measured(Barger et al. 2014). Due to the large SFR needed to ex-plain the optical/UV SED of this object as a starburst,A16 favored the scattered AGN-light scenario.In this paper we present Hubble Space Telescope ( HST ) observations of W0204–0506 to further exploreits nature, and we explore in detail two more Blue-Excess Hot DOGs (BHDs), WISE J022052.12+013711.6(W0220+0137 hereafter) and WISE J011601.41-050504.0(W0116–0505 hereafter), using
HST and
Chandra obser-vations as well as optical spectroscopy and broad-bandUV through mid-IR SEDs. In § § Chandra
X-ray observations. In § §
5. Throughout the article all magnitudesare presented in their natural system unless otherwisestated, namely AB for ugriz and Vega for all the rest.We assume a concordance flat ΛCDM cosmology with H = 70 km s − Mpc − , Ω Λ = 0 . M = 0 .
3. Forall quantities derived from X-ray spectra, we quote 90%confidence interval, while for all other quantities we quote68.3% confidence intervals instead. SAMPLE AND OBSERVATIONS
Blue-Excess Hot DOGs
A16 identified 8 BHDs from a sample of 36 HotDOGs with W4 < z > ugriz modelMag photometry in the SDSS DR12database with S/N > < E ( B − V ), assuming R V = 3 . ugriz SDSS DR12 modelMag photometry,
Spitzer /IRAC [3.6] and [4.5] pho-tometry from Griffith et al. (2012), and the WISE W3and W4 photometry from the WISE All-Sky Data Re-lease (Cutri et al. 2012). Additionally, whenever possi-ble, A16 used the J, Ks and deeper r -band imaging pre-sented by Assef et al. (2015). For the three sources con-sidered in this article, the deeper r -band imaging was ob-tained using the 4.1m Southern Astrophysical ResearchTelescope (SOAR) with the SOAR Optical Imager (SOI).For W0116–0505, images were obtained with an exposuretime of 3 ×
600 s on the night of UT 2013 August 28. Forthe other two sources, the images were obtained on UT2011 November 20, with exposure times of 3 ×
500 s forW0204–0506 and of 2 ×
500 s for W0220+0137. In allcases the images were reduced following standard pro-cedures, and the photometric calibration was performedby comparing bright stars in each field with their respec-tive SDSS magnitudes. The details of the NIR imagingcan be found in Assef et al. (2015). All magnitudes areshown in Table 1. Table 1
Photometric DataWISE ID J011601.41–050504.0 J020446.13–050640.8 J022052.12+013711.6SDSS u ± ± ± g ± ± ± ± ± ± r ± ± ± r ± ± ± i ± ± ± z ± ± ± J · · · ± ± ± ± ± Ks · · · · · · ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± A16 found that the approach described above was notable to accurately model the UV/optical emission for afraction of their sample, which were significantly bluerthan allowed by the SED templates. They identifiedeight objects for which an additional, secondary AGNcomponent with independent normalization and redden-ing provided a significant improvement in χ to the best-fit SED model. A16 presented a detailed study of theproperties of one of these targets: W0204–0506. Herewe extend this analysis to an additional one of theseeight targets, W0220+0137, as well as to another verysimilar target, W0116–0505. The W1=17.13 ± > χ by the secondary AGN component is spurious forthis source. A16 argued that these probabilities are likelyoverestimated and hence conservative, as the F-test usedto estimate them does not take into account the con-straints provided by the non-negative requirement of thelinear combination of templates for the best-fit model.The broad band SEDs as well as best-fit SED modelsof the three targets are shown in Figure 1. We note thatthe SED of W0204–0506 differs slightly from that pre-sented by A16 as, for consistency with the other twosources, the SED presented here only uses the SDSSDR12 bands in the UV/optical instead of the deeperimaging of Finkelstein et al. (2007). Table 2 shows, foreach target, the best-fit E ( B − V ) to both the primaryand secondary AGN components. The table also showsthe reddening-corrected monochromatic luminosities at6 µ m, L µ m , calculated from the template fit to each AGNcomponent. In all three targets the secondary AGN hasa much lower ( . µ m.The uncertainties for the parameters shown in Table 2have been estimated using a Monte-Carlo method follow-ing a similar prescription to that used by A16. For eachobject we first apply a scaling factor to the photomet-ric uncertainties such that the best-fit SED model has areduced χ ( χ ν ) of 1. We then create 1,000 realizationsof the observed SED of each object by re-sampling itsphotometry according to the aforementioned scaled un-certainties and assuming a Gaussian distribution. We fit Figure 1.
UV through mid-IR SEDs of the three BHDs discussedin this study. The green solid points show the observed flux den-sities in the photometric bands discussed in § P Ran that the improvement in χ gained from adding thesecondary AGN component is spurious. each of the 1,000 simulated SEDs and compile the dis-tribution of each parameter. We assign the uncertaintiesto the 68.3% intervals of these distributions around thevalues of the best-fit model. Optical Spectra
We obtained optical spectra of W0116–0505 andW0220+0137 using the Multiple Mirror Telescope
Table 2
Best-fit SED ParametersPrimary AGN Secondary AGNObject Redshift log L µ m E ( B − V ) log L µ m E ( B − V ) P ran (erg s − ) (mag) (erg s − ) (mag) (10 − )W0116 − +0 . − . +2 . − . +0 . − . +0 . − . − +0 . − . +1 . − . +0 . − . +0 . − . +0 . − . +2 . − . +0 . − . +0 . − . (MMT) spectrograph on the night of UT 2010 December4. We used the blue channel with the 1200 lines/mmgrating, and obtained 3 ×
600 s exposures on each targetthrough a longslit with a width of 1.5 ′′ . The optical spec-trum of W0204–0506 was obtained using the GMOS-Sspectrograph on the Gemini South telescope on UT 2011November 27 using a longslit with a width of 1.5 ′′ aswell. These observations have been previously presentedby A16, and we refer the reader to that study for furtherdetails on these observations. All three spectra were re-duced using standard tools.The optical spectra of W0116–0505, W0204–0506 andW0220+0137 are presented in Figures 2, 3 and 4, respec-tively. In addition to the MMT spectra, we also showlower S/N spectra obtained by SDSS for W0116–0505and W0220+0137 on UT 2013 October 3 and UT 2015September 13 respectively. The difference between theequivalent widths of the emission lines suggests eitherthat there is a small amount of variability in the contin-uum and/or the emission lines, or that the emission linescome from a region that has a different spatial exten-sion than the continuum such that differences in extrac-tion apertures can account for the discrepancy. Unlikethe MMT spectra, the SDSS observations were obtainedthrough a much larger 3 ′′ fiber. The spectra of W0116–0505 and W0220+0137 show clear broad, high ioniza-tion emission lines that are typically observed in type 1quasars. Single Gaussian fits to the C iv emission line,following the prescription of Assef et al. (2011, and ref-erences therein) to fit the continuum and define the spec-tral region on which to fit the emission line, have FWHMof approximately 2800 km s − and 3500 km s − respec-tively for W0116–0505 and W0220+0137. Based on theseemission lines we measure a redshift of z = 3 . ± . z = 3 . ± .
002 for W0220+0137.In particular both spectra show blended Ly β and O vi emission features. W0204–0506 is at a significantly lowerredshift of z = 2 . ± . HST Observations
A joint program between
Chandra and
HST was ap-proved during
Chandra
Cycle 17 (PID 17700696) to ob-tain
HST imaging in two bands of all three targetsand obtain
Chandra /ACIS-S observations of W0116–0505 and W0220+0137. These targets were selectedfor having some of the clearest blue excess emission interms of the χ improvement, and for having some ofthe highest expected count rates in ACIS-S. The archival Chandra /ACIS-I observations for W0204–0506 presentedby A16 are sufficient to accomplish our science goals, sono further observations were requested. In this section
Figure 2.
Optical spectrum of W0116–0505, obtained with theMMT spectrograph (black) and by SDSS (gray).
Figure 3.
Optical spectrum of W0204–0506, obtained with theGMOS-S instrument at the Gemini South Observatory.
Figure 4.
Optical spectrum of W0220+0137, obtained with theMMT spectrograph (black) and by SDSS (gray). we focus on the
HST observations, while the
Chandra observations are described in the next section.Imaging observations were obtained using the WFC3camera onboard
HST of all three BHD targets in both theF555W and the F160W bands. Each target was observedduring one orbit, with two exposures in the F555W bandfollowed by three exposures in the F160W band. Theexposure times in the F555W band were 738 s and 626 sfor each image for W0116–0505 and W0204–0506, and735 s and 625 s each for W0220+0137. All exposure timesin the F160W band were 353 s. For the F160W band weuse the reduced images provided by the
HST archive.Cutouts of 5 ′′ × ′′ centered on the F160W coordinatesof the target, are shown in the middle panels of Figure5. For the F555W band we do not use the archive pro-vided reductions, as the pipeline cosmic ray rejection issignificantly compromised by the acquisition of only twoimages. Instead, we took the fully-reduced single framesprovided by the archive, including the charge transfer ef-ficiency correction, and used the LACOSMIC algorithm(van Dokkum 2001) to remove cosmic rays. We thenused those cosmic-ray corrected images to continue withthe pipeline processing and combine the frames. Wealigned the F555W image to the F160W image usingstars detected in both bands. The final images are shownin the left panels of Figure 5. Table 1 presents the 4 ′′ di-ameter aperture magnitudes measured in each band foreach object.The right panel of Figure 5 show an RGB compositeof the images created using the Lupton et al. (2004) al-gorithm as implemented through the astropy v2.0.1 function make lupton rgb . We assigned the F555W im-age to the blue channel and the F160W image to the redchannel, while leaving the green channel empty. Beforeproducing the RGB composite, we convolve the F555Wimage with a Gaussian kernel to match its PSF to thatof the F160W image. We assume that the PSFs of bothimages are well modeled by Gaussian PSFs with the re-spective FWHM as provided by the WFC3 documenta-tion , namely 0.067 ′′ for the F555W channel, and 0.148 ′′ for the F160W channel. Hence, the Gaussian kernel usedon the F555W image corresponds to a Gaussian functionwith FWHM = FWHM − FWHM .Figure 6 shows the radial profiles of each source com-pared to that of a fiducial point source with a GaussianPSF. The emission of the three objects is clearly resolvedin both bands. For W0116–0505 and W0220+0137 themorphologies seem to be broadly undisturbed in bothbands, with the F160W emission having a larger extentand a higher luminosity. The emission peaks in bothbands are spatially co-located. W0204–0506 is, on theother hand, quite clearly disturbed, with the F160Wmorphology (rest-frame 5200˚A) suggestive of a recent in-teraction. The F555W emission (rest-frame 1800˚A) ispatchy, reminiscent of a starburst. We discuss the impli-cations of this UV morphology further in § , Lotz et al. (2004) to measure the Gini, M and A co-efficients (Lotz et al. 2004, and references therein). TheGini coefficient (Abraham et al. 2003) measures how uni-formly distributed is the light among the pixels of agalaxy in an image, such that Gini is 0 if all pixels havea uniform brightness and is 1 if all brightness is concen-trated in a single pixel. The M coefficient measures thesecond order moment of the brightest 20% of the flux ofthe galaxy as compared to the total second order mo-ment, M tot . The moments are computed around a cen-ter chosen to minimize M tot . The A coefficient measuresthe rotational asymmetry of a galaxy by subtracting animage of the galaxy rotated by 180 degrees. The rota-tional center is chosen to minimize A . For further detailson these coefficients, we refer the reader to Lotz et al.(2004) and Conselice (2014).We start by subtracting the background using SExtractor ( v2.19.5 , Bertin & Arnouts 1996) as wellas obtaining the centroid of each object in each band.We then compute the Petrosian radius (Petrosian 1976)and generate the segmentation map following Lotz et al.(2004), and finally proceed to measure the coefficientsdiscussed above. The values and uncertainties of theGini, M and A coefficients for each object in each bandare shown in Table 3. We estimate the uncertainties ineach parameter through a Monte Carlo approach. For agiven object in a given band, we use the uncertainty ineach pixel to generate 1,000 resampled images assumingGaussian statistics. We then repeat the measurementin each resampled image following the procedure out-lined above. We assign the measurement error to be thedispersion of the coefficient measurements in the 1,000resampled images.Recently, Farrah et al. (2017) measured these coeffi-cients for 12 Hot DOGs using HST /WFC3 images in theF160W. Using the boundaries proposed by Lotz et al.(2004) in the Gini– A plane and by Lotz et al. (2008)in the Gini– M plane, Farrah et al. (2017) determinedthat while Hot DOGs have a high merger fraction ( ∼ z ∼
2, leading them to conclude thatHot DOGs are not preferentially associated with merg-ers. These results are generally consistent with those ofFan et al. (2016b) who also found a high merger fraction(62 ± ∼ µ m imaging of atriple major merger in the the most luminous Hot DOG,W2246–0526. If we adopt the same boundaries used byFarrah et al. (2017) to classify our sources according totheir F160W morphologies, and noting that all caveatsidentified by Farrah et al. (2017) also apply here, we findthat the host galaxies of W0116–0505 and W0220+0137are not consistent with mergers but instead are classifiedas undisturbed early-type galaxies. For W0204–0506, onthe other hand, we find that its host galaxy morphologyis best classified as an on-going merger. These results areconsistent with our visual characterization of the hostgalaxies. Chandra Observations
We have obtained
Chandra /ACIS-S observations oftwo of our targets: W0116–0505 and W0220+0137 (pro-posal ID 17700696). Each object was observed with a
W0116-0505 F555W
N E
F160W
PSF FWHM
W0204-0506 F555W
N E
F160W
PSF FWHM
W0220+0137 F555W
N E
F160W
PSF FWHM
Figure 5.
HST /WFC3 images of W0116–0505 (top row), W0204–0506 (middle row) and W0220+0137 (bottom row) in the F555W (leftpanels) and F160W (middle panels) bands. The right panels show a color-composite where the F160W band has been mapped to red andthe F555W band has been mapped to blue, and we have matched the PSF of the F555W band to that of F160W. Each panel shows a5 ′′ × ′′ region centered on the F160W centroid of each target. The magenta circle in the F555W image of W0204–0506 shows the brightestUV clump. Table 3
Morphology of Blue Excess Hot DOGsSource Band Gini M A W0116–0505 F555W 0.499 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Figure 6.
Radial profile of the flux surface density, Σ, in theF555W (left panel) and F160W (right panel) bands of all threesources, namely W0116–0505 (solid red lines), W0204–0506 (solidblue lines) and W0220+0137 (solid green lines). The dashed graylines show the radial profile of a Gaussian PSF with the appropriateFWHM for each band. All profiles are shown from a minimumradius of 1 pixel, and all profiles have been normalized to Σ = 1 atthat pixel. total exposure time of 70 ks. W0116–0505 was observedcontinuously, while the observations of W0220+0137were split into one 30 ks and two 20 ks visits spreadthroughout seven days. It is worth noting that these ob-servations have previously been presented by Vito et al.(2018) in the context of a larger sample of Hot DOGs ob-served in X-rays. They find both sources are heavily ab-sorbed at those wavelengths. Goulding et al. (2018) an-alyzed the observations for W0220+0137 as well, but inthe context of a large sample of Extremely Red Quasars(ERQs), and also found the source to be heavily absorbedat X-ray energies, qualitatively consistent with the restof the ERQ population analyzed. Here we analyze thedata following the approach of A16, who analyzed thearchival
Chandra /ACIS-I observations of W0204–0506.We use ciao v4.7 to analyze these data. The spec-tral data products, including the source and backgroundspectra, and the response files were created using the specextract tool. Source events were extracted fromcircular regions with 2 ′′ radii centered on the source,while background events were extracted from annuliwith inner and outer radii of 3 and 6 ′′ , respectively.For W0220+0137, the spectral products from the threeobservations were combined into one using the tool combine spectra . We use the heasoft tool grppha togroup the spectra with a minimum of one count per bin.After subtracting the background, 74 counts are de-tected for W0116–0505, and only 18 for W0220+0137.Figures 7 and 8 show their respective unfolded spec-tra. For reference, we also show the ACIS-I spectrumof W0204–0506 in Figure 9, which had a significantlylonger exposure time of 160 ks. The shape of all threespectra differ significantly from that of an unabsorbedpower-law, suggesting the emission is dominated by ahighly obscured AGN, as expected from the SED model-ing presented in § X-RAY DATA MODELING
The X-ray spectra of W0116–0505 and W0220+0137are clearly hard, implying the emission is most likely
Figure 7. (Top panel)
X-ray spectrum of W0116–0505 obtainedusing
Chandra /ACIS-S (see § §
3. The dashed-gray line shows the emission expectedfor a second, unobscured AGN in the system powering the ob-served UV/optical emission. (Bottom panel)
The data points showthe ratio between the observed spectrum and the best-fit model.
Figure 8.
Same as Fig. 7 but for the
Chandra /ACIS-S spectrumof W0220+0137.
Figure 9.
Same as Fig. 7 but for the
Chandra /ACIS-I spectrumof W0204–0506. Adapted from Fig. 4 of A16. dominated by a highly obscured AGN. To better con-strain the properties of the obscured AGN, we fitthe emission of both objects using the models ofBrightman & Nandra (2011), following the same ap-proach as in A16. These models predict the X-ray spec-trum as observed through an optically thick mediumwith a toroidal geometry, as posited by the AGN uni-fied scheme. The models employ Monte-Carlo techniquesto simulate the transfer of X-ray photons through theoptically-thick neutral medium, self-consistently includ-ing the effects of photoelectric absorption, Compton scat-tering and fluorescence from Fe K, amongst other ele-ments. Treating these effects self consistently rather thanseparately has the advantage of reducing the number offree parameters and of gaining constraints on the spec-tral parameters. It is therefore particularly useful forlow count spectra such as those we are fitting here. Wetherefore carry out the parameter estimation by mini-mizing the Cash statistic (Cash 1979), modified throughthe W-statistic provided by XSPEC to account for thesubtracted background. Also following the approach ofA16, we require the photon index Γ to be ≥ .
6, as it ispoorly constrained by our data and lower values are onlyappropriate for low Eddington ratios. In practice, thefitting procedure we used allows values of Γ in the range1.6–3.0, and values of N H in the range 10 − cm − .Figures 7 and 8 show the best-fit models to thespectra of W0116–0505 and W0220+0137, respectively.The best-fit absorbed AGN model to W0116–0505has an absorption column density of neutral hydro-gen of N H = 1 . +1 . − . × cm − , a photon-index ofΓ = 1 . +0 . − . and an absorption-corrected luminosity oflog L −
10 keV / erg s − = 45 . +0 . − . . The best-fit model https://heasarc.gsfc.nasa.gov/xanadu/xspec/manual/XSappendixStatistics.html has a Cash statistic of C = 56 . ν = 65 degrees offreedom. For W0220+0137, the best-fit model has N H =3 . × cm − , Γ = 2 . L −
10 keV / erg s − =45 . +0 . − . , with C = 8 . ν = 15. Note that be-cause of the low number of counts for W0220+0137, the90% confidence uncertainties in the best-fit Γ and N H are determined by the boundaries of the model. Thesame is true for the lower-bound of the best-fit Γ inW0116–0505. The best-fit values of N H and L −
10 keV are consistent within the uncertainties with those foundby Vito et al. (2018) for both sources. The best-fit valuesfor W0220+0137 are also consistent within the (large)error bars with those found by Goulding et al. (2018).For W0204–0506, A16 found that the best-fit absorbedAGN has N H = 0 . +0 . − . × cm − , Γ = 1 . +0 . − . andlog L −
10 keV / erg s − = 44 . +0 . − . , with C = 66 .
08 and ν = 77.The spectra of all three objects are likely dominatedby a luminous AGN with very high absorption. Inthe case of W0116–0505 and W0220+0137, the ab-sorption is consistent with the objects being Compton-thick (i.e., N rmH > . × cm − ). This isin qualitative agreement with the SED modeling pre-sented in § µ m, L µ m ,which has been shown to be well correlated with the L −
10 keV
X-ray luminosity by a number of authors(Fiore et al. 2009; Gandhi et al. 2009; Bauer et al. 2010;Mateos et al. 2015; Stern 2015; Chen et al. 2017). Weuse the best-fit relation of Stern (2015) between L µ m and L −
10 keV , which accurately traces this relation upto very high L µ m and is hence most appropriate forour targets. From the L µ m of the most luminous andobscured AGN component of W0116–0505, this relationpredicts log L P redicted −
10 keV / erg s − = 45 . ± .
62, which isin excellent agreement with the luminosity of the best-fit model to the X-ray data of log L −
10 keV / erg s − =45 . +0 . − . . For W0220+0137 we also find excellentagreement, with log L P redicted −
10 keV / erg s − = 45 . ± . L −
10 keV / erg s − = 45 . +0 . − . . A16 nominallyfound a good agreement as well for W0204–0506, as theyestimated log L P redicted −
10 keV / erg s − = 45 . ± .
37 and foundlog L −
10 keV / erg s − = 44 . +0 . − . from the best-fit X-raymodel. However, when jointly considering this with thebest-fit and expected absorption, their Figure 5 suggestsW0204–0506 may be somewhat X-ray weak.From the SED modeling we also have an estimateof the amount of dust that is obscuring the luminousAGN that dominates in both the mid-IR and the X-rays. Comparing to the column densities of neutral hy-drogen constrained by the modeling of X-ray spectra,we find dust-to-gas ratios of E ( B − V ) /N H = 3 . ± . × − cm mag for W0116–0505, where the un-certainty corresponds to the 68.3% confidence intervaland has been derived, for simplicity, assuming Gaussianstatistics. For W0220+0137 we find E ( B − V ) /N H =2 . × − cm mag. As N H is not constrained atthe 90% level within the model boundaries, we cannotderive a meaningful confidence interval. For W0204–0506, A16 found a larger ratio of E ( B − V ) /N H =1 . ± . × − cm mag. For comparison, the me-dian dust-to-gas ratio in AGN found by Maiolino et al.(2001) is 1 . × − cm mag. This value is compa-rable to that found in W0204–0506, while those foundin W0116–0505 and W0220+0137 are lower. Unfortu-nately the large uncertainties in this quantity make thisresult difficult to interpret, but it is worth noting that re-cently Yan et al. (2019) identified a very low dust-to-gasratio of ≈ × − cm for a heavily obscured nearbyquasar at z = 0 .
218 with N H ≈ × cm − , witharound N H ≈ cm − coming from the ISM, whichcould be a better analog to our objects. If the dust-to-gas ratio is indeed significantly lower in W0116–0505and W0220+0137 than in W0204–0506, it could eitherimply a low metallicity for the former systems such thatthere is a deficit of dust overall in the host galaxy, orthat a higher than typical fraction of absorbing gas ex-ists within the dust sublimation radius of the accretiondisk. We speculate the latter could be consistent with therecent results of Wu et al. (2018) that show Hot DOGsare accreting close to the Eddington limit, perhaps as aresult of higher gas densities in the vicinity of the SMBH.Taken together, these results could imply that W0116–0505 and W0220+0137 represent a different class of ob-ject than W0204–0506, as the former are either dust-pooror gas-rich in the nuclear regions, but have normal X-rayluminosities, while the latter has a normal amount ofdust but might be somewhat X-ray weak. The morphol-ogy of the HST imaging strongly differs between theseobjects, as discussed in § SOURCE OF THE EXCESS BLUE EMISSION
Dual AGN
One of the possible scenarios proposed by A16 is thatBHDs could be powered by two AGNs instead of one,where a primary luminous, highly obscured AGN dom-inates the mid-IR emission, and a secondary fainter,unobscured or lightly obscured AGN dominates theUV/optical emission. As discussed above, the formerwould be expected to dominate the hard X-ray emissionof these sources, and that is exactly what is observed.However, the less luminous component would contributesignificant soft X-ray emission, that can be constrainedby the
Chandra observations. In Table 2 we list the expected intrinsic 6 µ m luminosity of the primary andsecondary best-fit AGN components for both W0116–0505 and W0220+0137. It is important to note that forthe secondary AGN components we have no useful con-straints in the IR, as the rest-frame near-IR is dominatedby the host galaxy and the mid-IR is dominated by theprimary AGN component. Its 6 µ m luminosity comes in-stead indirectly from the template fit to the rest-frameUV/optical SED. As we did in §
3, we can estimate theexpected 2–10 keV luminosity using the relation of Stern(2015). Hence, if the secondary component is a real sec-ond AGN in the system, for W0116–0505 we expect itto have an X-ray luminosity of log L P redicted −
10 keV / erg s − =44 . ± .
37, and for W0220+0137 we expect it to havelog L P redicted −
10 keV / erg s − = 44 . ± .
62. The gray-dashedcurves in Figures 7 and 8 show the expected X-rayspectrum of these secondary components for W0116–0505 and W0220+0137 respectively. We assume power-law spectra with Γ = 1 . L −
10 keV / erg s − < .
95 in W0116–0505, and oflog L −
10 keV / erg s − < .
93 in W0220+0137. Theselimits are marginally consistent with the 2–10 keV lumi-nosities expected given the optical/UV luminosities ob-served. For W0204–0506 on the other hand, A16 wasable to rule out this scenario with high confidence. Un-like the analysis presented here, A16 reached this con-clusion by comparing the change in Cash statistic of theX-ray spectra modeling obtained by requiring or not thepresence of the secondary AGN emission with the ex-pected luminosity. Specifically, A16 found that includ-ing the secondary component resulted in an increase inthe Cash statistic ∆ C = 128 .
38, which allowed to ruleout the dual AGN scenario with > .
9% confidence.We do not replicate this analysis for W0116–0505 andW0220+0137, as the interpretation of the change in the C statistic (∆ C = 17 . C = 18 .
7, respectively)is complicated by the lower number of counts detected,particularly in the case of W0220+0137.Hence, the X-ray spectra of all three objects are betterdescribed by the single, highly absorbed AGN model,suggesting that BHDs are not dual AGN. The case isstrongest for W0204–0506, while for W0116–0505 andW0220+0137 we cannot completely reject the dual AGNscenario with high confidence using the current data sets.
Extreme Star-formation
Another possibility discussed by A16 is that theUV/optical SED of BHDs is powered by unobscured ex-treme star-formation rather than by unobscured AGNemission. This would account for a very blue UV/opticalSED without the X-ray contribution expected for a sec-ondary AGN.This scenario was studied in detail by A16 for W0204–0506. Modeling the UV/optical SED of this object usingthe Starburst99 v7.0.0 code (Leitherer et al. 1999, 2010,2014; V´azquez & Leitherer 2005) in combination withthe EzGal package of Mancone & Gonzalez (2012), they0determined that the SED could be consistent with beingpowered by a young starburst but only if the SFR wasvery high. Specifically, they assumed the latest Genevamodels available for the used version of Starburst 99(see Leitherer et al. 2014, for details), a constant SFR,and a solar metallicity, and determined that the SEDcould only be powered by a starburst of age . & M ⊙ yr − with 90% confidence. Alower metallicity somewhat eases these constraints, withthe lowest metallicity available for the Geneva models inStarburst99 of Z = 0 .
001 implying age .
100 Myr andSFR & M ⊙ yr − . However, A16 considered that sucha low metallicity was unlikely given the large amount ofdust available in the inner regions of the system thatgive rise to the high specific luminosities in the mid-IR.Furthermore, due to the large, unobscured SFR impliedby the solar metallicity models, A16 considered that theUV/optical SED was unlikely powered by a starburst.However, the morphology of the UV emission in the HST imaging we have obtained (see § ∼ ∼ § HST imaging available for W0204–0506 then imply that if itsUV/optical SED is solely powered by a starburst, thenthe system must be in a very uncommon state. On onehand, it could be that the system has a very large metal-licity gradient, such that in the outskirts, where star-formation dominates, the metallicity is close to primor-dial and SFR is only & M ⊙ yr − , yet near the SMBHthe metallicity is high enough to allow for the substantialamount of dust needed to obscure the hyper-luminousAGN. The other possibility would be that W0204–0506does not have a substantial metallicity gradient but isinstead powered by the strongest unobscured starburstknown with SFR & ⊙ yr − .A third and more likely option is that while a moder-ate starburst is ongoing in the system, the UV/opticalemission is still dominated by light leaking from the cen-tral highly obscured AGN. As shown in Table 3 (alsosee discussion in § M coefficient than the other twoBHDs studied. While the large M is consistent withthe observed patchiness of the system, the high Gini co-efficient implies that the light is strongly concentrated inthe brightest regions. In the left panel of Figure 5 it canbe appreciated that the NW UV clump (marked by themagenta circle, 0.2 ′′ diameter) is brighter than the rest,containing approximately 10% of the total F555W fluxmeasured in the 4 ′′ radius aperture. This region is closeto the geometrical center of the F160W light distribution,and could correspond to the position of the buried AGN.That the optical spectrum of this source (Fig. 3) showsa mixture of narrow and broad emission lines is also con- sistent with this picture, as A16 reported a FWHM of1630 ±
220 km s − for C iv but of only 550 ±
100 km s − for C iii].For W0116–0505 and W0220+0137 the situation issomewhat different. The optical spectra, shown in Fig-ures 2 and 4, show clear broad, high-ionization featurescharacteristic of type 1 AGNs. The UV emission, whilespatially extended, is strongly concentrated in both ob-jects (see discussion in § Z = 0 .
001 discussedabove. As we do not include nebular emission we only usethe bands that are redward of the Ly α emission line andexclude the F160W band, which can be strongly contami-nated by [O ii ] emission. Indeed, if we include the F160Wband we find best-fit χ values a factor ∼ ∼ χ values ofthe best fits are quite large when it is considered that weare fitting three different parameters. This, coupled withUV/optical spectral features (i.e., the presence of broademission lines) and the morphology in the HST imagingsuggest that the UV/optical emission in these objects isunlikely dominated by unobscured starbursts.
Leaked AGN Light
The third possibility to explain the nature of BHDsis that the blue excess emission found in these objectscorresponds to light coming from the highly obscuredprimary AGN that is leaking into our line of sight. Asdiscussed by A16, this could happen either due to dust orgas scattering of the AGN emission into our line of sight,or due to a small gap in the dust that allows for a par-tial view towards the accretion disk and the broad-lineregion. However, the latter is unlikely, as discussed byA16, as the UV/optical SED is consistent with the emis-sion of an unobscured accretion disk. As radiation atprogressively shorter wavelengths is emitted in progres-sively inner regions of the accretion disk, a gap that onlyallows ∼
1% of the emitted light through but does notdistort the accretion disk spectrum would need to cover99% of the effective disk size at each wavelength. Whilenot impossible, the shape of such a gap would be exceed-ingly contrived, making this unlikely. Furthermore, wewould not expect the UV/optical emission to be spatially1
Figure 10. (Upper panel)
The solid black line shows the best-fitStarburst99 SED model to the UV/optical broad-band photome-try of W0116–0505, assuming solar metallicity (see text for details).The gray shaded area shows all SED shapes within the 90% confi-dence interval. (Bottom panel)
Same as in the top panel but for ametallicity of Z = 0 . Figure 11.
Same as Fig. 10 but for W0220+0137. extended, as found in § Figure 12.
The contours show a χ map of the best-fit Star-burts99 models as discussed in the text. The contours for W0116–0505 and W0220+0137 are shown in the top and bottom panels,respectively. Dark contours assume solar metallicity, while graycontours assume Z = 0 . sorbed by dust, while 1% will be scattered into our lineof sight by either dust or gas, or both. Reflection nebu-lae are known to make the reflected SED bluer than theemitted SED in the UV ( λ . Chandra observa-tions (Draine 2003a,b). Hence, the non-detection of aclear unabsorbed component in the X-ray spectra in § SUMMARY
We have investigated the source of the blue excessemission in three BHDs, two of which were identi-fied as such by A16, and a third one which has anSED consistent with that of a Hot DOG although itdoes not meet the formal selection criteria due to be-ing slightly too bright in the W1 band. While allHot DOGs are characterized by mid-IR emission thatis most naturally explained by a highly obscured hyper-luminous AGN (Eisenhardt et al. 2012; Assef et al. 2015;Tsai et al. 2015), BHDs have a UV/optical SED thatis significantly bluer than expected based on templatefitting results. Using a similar approach to that of2Assef et al. (2015), we find that the SEDs of BHDs arebest modeled using two AGN components: a primaryhyper-luminous, highly obscured AGN that dominatesthe mid-IR emission, and a secondary lower luminos-ity but unobscured AGN that dominates the UV/opticalemission. The bolometric luminosity of the secondaryAGN SED is ∼
1% of that of the primary component.A16 identified three possible scenarios to produce the ex-cess blue emission, namely: (i) a secondary, less luminousbut unobscured AGN in the system, (ii) an extreme star-burst, or (iii) leaked UV/optical light from the primary,highly luminous, highly obscured AGN that dominatesthe mid-IR.For one of the sources (W0204–0506), A16 ruled outa secondary AGN as the source of the blue excess emis-sion, and instead concluded that the excess was causedby either unobscured star formation with an SFR & M ⊙ yr − , or by UV/optical light from the cen-tral engine leaking into our line of sight due to scatter-ing or through a partially obscured sight-line, with thescattered AGN light hypothesis deemed more likely. Inthis paper, we have presented HST /WFC3 imaging ofW0204–0506 showing a morphology consistent with anon-going merger and evidence of an on-going widespreadstarburst. Considering, however, the very high SFRneeded to explain the UV emission by star-formationalone, we conclude it is more likely that the UV emissionof W0204–0505 arises from a combination of scatteredAGN light and star-formation.We also studied in detail two other BHDs, W0116–0505 and W0220+0137. We present observations ob-tained with
Chandra /ACIS-S and interpret them us-ing the Brightman & Nandra (2011) models. We findthat the X-ray spectra are consistent with single lumi-nous, highly absorbed AGNs dominating the X-ray emis-sion. We find that the L −
10 keV luminosities of theseAGNs are consistent with those expected for the pri-mary AGNs based on their estimated L µ m according tothe L µ m − L −
10 keV relation of Stern (2015). We alsofind that the gas-to-dust ratios of the AGNs in thesesystems are somewhat below the median value foundin AGNs by Maiolino et al. (2001) and lower than thatfound in W0204–0506, suggestive of a lower metallicityor of a higher fraction of absorbing gas within the dust-sublimation radius of the AGN. Based on the UV throughmid-IR SED models of these sources, we estimate the ex-pected X-ray luminosity of the putative secondary AGNcomponents assuming it is a second, independent AGNin the system. We found that the X-ray observations areonly marginally consistent with the presence of a secondAGN component in both W0116–0505 and W0220+0137,suggesting the dual AGN scenario is unlikely.We followed A16 and modeled the UV emission ofW0116–0505 and W0220+0137 assuming a pure star-burst scenario, and found that while the best-fit SFRsare generally high, comparable to those found by A16for W0204–0506, they are not well constrained due tothe large degeneracies between SFR, age and metallicity.We found, however, that the χ values of the best-fit star-burst models are large ( ∼
12 for W0116–0506 and ∼ HST /WFC3 F555W and F160W images of W0116–0505 and W0220+0137 and found them to be undis-turbed with the UV emission being centrally concen-trated. An analysis based on the Gini, M and A coef-ficients showed that these systems are best characterizedas undisturbed early type galaxies, consistent with theleaked AGN scenario. Considering all of this, we con-clude that the source of the UV emission in W0116–0505and W0220+0137 is scattered light from the hyperlu-minous, highly obscured AGN that powers the mid-IRSED. Given the detail of our data and SED modeling,we cannot determine whether the scattering material isprimarily gas, dust, or a mixture of both.That all three BHDs we have investigated are due toscattered light from the highly obscured, hyperluminousAGN highlights how powerful the central engine is in HotDOGs: with only 1% of the emission of the accretiondisk scattered into our line of sight, it is still more lumi-nous than the entire stellar emission of the host galaxy inthe UV. This is in general agreement with recent resultswhich show that the SMBHs in Hot DOGs are accretingabove the Eddington limit (Wu et al. 2018; Tsai et al.2018) and are injecting large amounts of energy into theISM of their host galaxies (D´ıaz-Santos et al. 2016), andhence are experiencing strong events of AGN feedback.We thank J. Comerford and B. Weiner for carry-ing out observations presented in this article. RJAwas supported by FONDECYT grants number 1151408and 1191124. DJW acknowledges financial supportfrom STFC Ernest Rutherford fellowships. HDJwas supported by Basic Science Research Programthrough the National Research Foundation of Korea(NRF) funded by the Ministry of Education (NRF-2017R1A6A3A04005158). FEB acknowledges supportfrom CONICYT-Chile (Basal AFB-170002) and the Min-istry of Economy, Development and Tourism’s MilleniumScience Initiative through grant IC120009, awarded toThe Millenium Institute of Astrophysics, MAS. JW issupported by the NSFC Grant 11690024 and SPRP CASgrant XDB23000000. Based on observations made withthe NASA/ESA Hubble Space Telescope , obtained at theSpace Telescope Science Institute, which is operated bythe Association of Universities for Research in Astron-omy, Inc., under NASA contract NAS 5-26555. Theseobservations are associated with program
Chandra X-ray Observatory and observations madeby the
Chandra X-ray Observatory and published previ-ously in cited articles. Support for this work was pro-vided by the National Aeronautics and Space Adminis-tration through Chandra Award Number 17700696 is-sued by the Chandra X-ray Center, which is operatedby the Smithsonian Astrophysical Observatory for and3on behalf of the National Aeronautics Space Adminis-tration under contract NAS8-03060. This publicationmakes use of data products from the
Wide-field InfraredSurvey Explorer , which is a joint project of the Universityof California, Los Angeles, and the Jet Propulsion Labo-ratory/California Institute of Technology, funded by theNational Aeronautics and Space Administration. Thiswork is based in part on observations made with the
Spitzer Space Telescope , which is operated by the JetPropulsion Laboratory, California Institute of Technol-ogy under a contract with NASA. The Pan-STARRS1Surveys (PS1) have been made possible through con-tributions of the Institute for Astronomy, the Univer-sity of Hawaii, the Pan-STARRS Project Office, theMax-Planck Society and its participating institutes, theMax Planck Institute for Astronomy, Heidelberg andthe Max Planck Institute for Extraterrestrial Physics,Garching, The Johns Hopkins University, Durham Uni-versity, the University of Edinburgh, Queen’s Univer-sity Belfast, the Harvard-Smithsonian Center for Astro-physics, the Las Cumbres Observatory Global TelescopeNetwork Incorporated, the National Central Universityof Taiwan, the Space Telescope Science Institute, theNational Aeronautics and Space Administration underGrant No. NNX08AR22G issued through the PlanetaryScience Division of the NASA Science Mission Direc-torate, the National Science Foundation under Grant No.AST-1238877, the University of Maryland, and EotvosLorand University (ELTE) and the Los Alamos NationalLaboratory. Funding for SDSS-III has been provided bythe Alfred P. Sloan Foundation, the Participating Insti-tutions, the National Science Foundation, and the U.S.Department of Energy Office of Science. The SDSS-III web site is . SDSS-III ismanaged by the Astrophysical Research Consortium forthe Participating Institutions of the SDSS-III Collabo-ration including the University of Arizona, the BrazilianParticipation Group, Brookhaven National Laboratory,Carnegie Mellon University, University of Florida, theFrench Participation Group, the German ParticipationGroup, Harvard University, the Instituto de Astrofisicade Canarias, the Michigan State/Notre Dame/JINA Par-ticipation Group, Johns Hopkins University, LawrenceBerkeley National Laboratory, Max Planck Institute forAstrophysics, Max Planck Institute for ExtraterrestrialPhysics, New Mexico State University, New York Uni-versity, Ohio State University, Pennsylvania State Uni-versity, University of Portsmouth, Princeton University,the Spanish Participation Group, University of Tokyo,University of Utah, Vanderbilt University, University ofVirginia, University of Washington, and Yale University.Some of the observations reported here were obtained atthe MMT Observatory, a joint facility of the SmithsonianInstitution and the University of Arizona.REFERENCES
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