Hot-Dust-Poor Type 1 Active Galactic Nuclei in the COSMOS Survey
Heng Hao, Martin Elvis, Francesca Civano, Giorgio Lanzuisi, Marcella Brusa, Elisabeta Lusso, Gianni Zamorani, Andrea Comastri, Angela Bongiorno, Chris D. Impey, Anton M. Koekemoer, Emeric Le Floc'h, Mara Salvato, David Sanders, Jonathan R. Trump, Cristian Vignali
aa r X i v : . [ a s t r o - ph . C O ] N ov Received to ApJ 2010 July 8; accepted 2010 October 13; published 2010 October 29
Preprint typeset using L A TEX style emulateapj v. 03/07/07
HOT-DUST-POOR TYPE 1 ACTIVE GALACTIC NUCLEI IN THE COSMOS SURVEY
Heng Hao , Martin Elvis , Francesca Civano , Giorgio Lanzuisi , Marcella Brusa , Elisabeta Lusso , GianniZamorani , Andrea Comastri , Angela Bongiorno , Chris D. Impey , Anton M. Koekemoer , Emeric Le Floc’h ,Mara Salvato , David Sanders , Jonathan R. Trump , and Cristian Vignali Received to ApJ 2010 July 8; accepted 2010 October 13; published 2010 October 29
ABSTRACTWe report a sizable class of type 1 active galactic nuclei (AGNs) with unusually weak near-infrared(1–3 µ m) emission in the XMM-COSMOS type 1 AGN sample. The fraction of these “hot-dust-poor”AGNs increases with redshift from 6% at low redshift ( z <
2) to 20% at moderate high redshift(2 < z < . µ m emission relative to the 1 µ memission is a factor of 2–4 smaller than the typical Elvis et al. AGN spectral energy distribution(SED), which indicates a ‘torus’ covering factor of 2%–29%, a factor of 3–40 smaller than requiredby unified models. The weak hot dust emission seems to expose an extension of the accretion diskcontinuum in some of the source SEDs. We estimate the outer edge of their accretion disks to lie at(0.3–2.0) × Schwarzschild radii, ∼ Subject headings: galaxies: evolution – quasars: general INTRODUCTION
Characteristically, active galactic nuclei (AGNs) havehotter dust than starburst galaxies, which has long beenemployed to select AGNs in near-infrared (NIR) surveys(e.g., Miley et al. 1985, Lacy et al. 2004, 2007, Stern etal. 2005, Donley et al. 2008). In AGNs, dust reachesmaximum temperature ( ∼ z ∼ Electronic address: [email protected], [email protected] Harvard-Smithsonian Center for Astrophysics, 60 GardenStreet, Cambridge, MA 02138, USA INAF-IASF Roma, Via del Fosso del Cavaliere 100, 00133Roma, Italy Max Planck Institut f¨ur extraterrestrische Physik Giessen-bachstrasse 1, D–85748 Garching, Germany INAF-Osservatorio Astronomico di Bologna, via Ranzani 1,I-40127 Bologna, Italy Steward Observatory, University of Arizona, 933 North CherryAvenue, Tucson, AZ 85721, USA Space Telescope Science Institute, 3700 San Martin Drive,Baltimore, MD 21218, USA Institute for Astronomy, University of Hawaii, 2680 WoodlawnDrive, Honolulu, HI 96822, USA IPP-Max-Planck-Institute for Plasma Physics, Boltz-mannstrasse 2, D-85748, Garching, Germany Dipartimento di Astronomia, Universit`a degli Studi diBologna, via Ranzani 1, I-40127 Bologna, Italy
Emmering et al. (1992) proposed a BLR/torus struc-ture in the disk–wind scenario. The two structurescorrespond to different regions of a clumpy wind com-ing off the accretion disk rotating around the blackhole (BH). The disk–wind scenario predicts that thetorus disappears at luminosities lower than ∼ erg s − because the accretion onto the central BHcan no longer sustain the required cloud outflow rate(Elitzur & Shlosman 2006). In a sample of nearbyAGNs, even the BLR disappears at luminosities lowerthan 5 × ( M/ M J ) / erg s − (Elitzur & Ho2009).The AGN structure is reflected in the shape of thespectral energy distribution (SED). The maximum dusttemperature leads to a characteristic drop in the NIRemission at 1 µ m (Elvis et al. 1994, E94 hereafter; Glik-man et al. 2006). AGNs with weak or no ‘torus’ have anSED with weak or no IR bump, or no 1 µ m inflection.We have found a substantial ( ∼ > − ; Elvis et al. 2010) AGNs. All these X-ray sourceshave secure optical and infrared identifications (Brusa etal. 2010) and at least one optical spectrum, from eitherthe Magellan (Trump et al. 2009a), Sloan Digital SkySurvey (SDSS; Schneider et al. 2007), or Very LargeTelescope (VLT; Lilly et al. 2007, 2009) surveys. SELECTION OF HDP OBJECTS
The HDP AGNs are selected based on the NIR andoptical SED shapes. We plot NIR versus optical slopeson either side of 1 µ m (rest frame, Figure 1; for details seeHao et al. 2010). The 1 µ m point is not chosen arbitrar-ily. It is where the blackbody emission of the hot dustat the maximum sublimation temperature (e.g., 1500 K;Barvainis 1987) normally begins to outshine the emissionof the accretion disk.The plot is equivalent to a color–color plot, but utilizes Hao et al.more than four photometric bands. Briefly, we fit powerlaws ( νF ν ∝ ν α ) on either side of the 1 µ m inflectionpoint of the rest-frame SED: 1 µ m–3000˚A to derive anoptical slope ( α OP T ) and 3 µ m - 1 µ m to derive a NIRslope ( α NIR ). In normal type 1 AGNs, these slopes are α OP T ∼ α NIR ∼ − . satisfy thesecriteria. The errors of the slopes are the standard errorsof the linear fit.The α OP T − α NIR plot can be conveniently used to sep-arate the nuclear from the host galaxy emission. Figure 1shows the α OP T and α NIR for the E94 radio-quiet AGNtemplate (red cross) and three galaxy templates (a spiral— Spi4, a 5 Gyr old elliptical — Ell5, and a starburst —NGC 6090; big blue triangles) from the “SWIRE Tem-plate Library” (Polletta et al. 2007). The black curvesshow the slopes of the SED templates obtained by mix-ing the AGN and galaxy with different fraction (0%–100%) after normalizing the E94 and galaxy templatesat 1 µ m. The mixing curves of NGC 6090 and Spi4 definethe boundaries of the possible slopes obtained by mixingthe E94 with all the 16 available galaxy templates in theSWIRE library (small blue triangles). The magenta lineshows the OPT and NIR slopes of E94 template whenreddening ( E ( B − V ) = 0 − . Red-dening primarily affects α OP T .The majority of the XMM-COSMOS AGNs ( ∼ α OP T − α NIR plot, boundedby the reddening and mixing curves. However, ∼ σ beyond the AGN–host–reddening triangle.These sources have an optical slope consistent with typ-ical AGNs, but a relatively weak IR bump. We namethem “hot-dust-poor” (HDP) quasars. HDP SEDS
There is a range of SED shapes for the 41 HDP AGNs.We further divide these AGNs into three classes, accord-ing to their relative positions to the equal slope (red) linein the α OP T − α NIR plot. Figure 2 shows examples ofSEDs for each class. All wavelengths in this section referto the rest frame.Class I. Twenty-four sources lie below the equal slopeline (blue symbols in Figure 1). These sources have anormal big blue bump (BBB), but a factor of ∼ µ m. They all have a ratherflat infrared SED shape ( α NIR ∼
0) extending to at least10 µ m (rest frame; Figure 2, left). Severn have high lumi-nosities ( > L ∗ , ranging from 5.3 L ∗ to 24 L ∗ , where L ∗ isthe break point of the galaxy luminosity function; Cira-suolo et al. 2007) at 1 µ m and hence should have weakhost galaxy contribution, while the rest have luminosities Three out of the other four (XID=2119, 5320, 5617) are low-redshift AGNs with only K band and one IRAC band in the rest-frame NIR SED. The remaining source (XID=54439) is the highestredshift AGN with only two IRAC bands in the rest-frame NIRSED. We applied the IDL dereddening routine ‘‘FM UNRED.PRO’’ ,with the SMC extinction curve (Gordon et al. 2003).
Fig. 1.—
Slope–slope plot of the XMM-COSMOS type 1 AGNs.Red cross and red circle show the E94 mean SED and the 1 σ disper-sion of the E94 sources. The blue triangles show different SWIREgalaxy templates. The black lines connecting the E94 and thegalaxy templates are mixing curves. The purple line shows thereddening vector of E94. The straight red solid line shows the α OPT = α NIR line. Different colors of the points show differentclass of the HDP AGNs (I, II, and III, see the text for details).The black dots show all the other XMM-COSMOS type 1 AGNs. . L ∗ –4 L ∗ .Class II. Six sources are consistent with lying on theequal slope line (green symbols in Figure 1). The infraredemission could be the exposed continuation of the BBBto longer wavelengths ( ∼ µ m). Two out of six, havehigh luminosities ( > L ∗ , 5 . L ∗ –13 L ∗ ) at 1 µ m, while therest have luminosities 0 . L ∗ –2 . L ∗ .Class III. Eleven sources lie above the equal slope line(red symbols in Figure 1). These sources have flatterBBB than normal, possibly due to reddening. Someof them show quite strong ∼ µ m continuum emission(Figure 2, right). The class III sources have luminositiesof 0 . L ∗ –5 . L ∗ .We calculated the mean SEDs of the HDP AGNs ineach class and compared them with the mean SEDs ofthe other type 1 AGNs in the XMM-COSMOS samplehaving the same range of optical slopes for that class,i.e., normal type 1 AGNs with same BBB shape (Figure2, upper). The HDP AGNs show relatively weaker 1–3 µ m emission, by a factor of 1.6 for class I, 3 for class II,and 2.5 for class III. SOURCE PROPERTIES
Correlations
We compared a number of observed properties of theHDP AGNs with the whole XMM-COSMOS sample:redshift ( z ), bolometric luminosity ( L bol , 24 µ m–912 ˚A),BH mass ( M BH ; Trump et al. 2009b; Merloni et al.2010), Eddington ratio ( λ E ; Lusso et al. 2010), the op-ot-Dust-Poor Quasars 3 Fig. 2.—
Mean SED and examples of SEDs for each HDP classes: Upper row: mean SED of the three classes of HDP AGNs (black solidline) compared with E94 RQ mean SED (dotted red line) and the mean SED of the normal XMM-COSMOS type 1 AGNs with similarBBB (solid red line). Lower left: class I SED example (XID=2105, redshift z = 1 . z = 2 . z = 2 . α OPT and α NIR . The host galaxy contribution isnegligible, for XID=2105 and XID=96 at high luminosity (4 L ∗ and 13 L ∗ at 1 µ m). The green dashed line shows the blackbody fitting toouter edge of the accretion disk. tical to X-ray spectral index ( α ox ) , the X-ray lumi-nosity at rest frame 2keV ( L keV ), the X-ray hardnessratio (HR) , and radio loudness ( q ) . All the val-ues of these parameters were reported in Lusso et al.(2010), Brusa et al. (2010), and Hao et al. (2010). Inthe HDP sample, only one source is radio-loud (Elviset al. 2010).We used the Kolmogorov–Smirnov (K-S) test to com-pare the distributions of each parameter for the HDPAGNs with the whole sample (HDP AGNs excluded).The K-S probabilities are reported in Table 1. Only z shows a significant correlation ( p K − S = 0 . M BH , L keV , log λ E , and L bol ) are indistinguishable for the two samples. We willstudy the X-ray properties of the HDP AGNs in a follow-ing paper. We can also see some of these results in the α ox = 0 . × log (cid:20) F ν (2 keV ) F ν (2500 ˚ A ) (cid:21) , where α ox <
0, and larger α ox means X-ray louder (Tananbaum et al. 1979). HR= ( H − S ) / ( H + S ) where H are the XMM counts in the2–10 keV band and S those in the 0.5–2 keV band (Brusa et al.2007, 2010). q =log( f µm /f . GHz ) < XID=167, class III.
TABLE 1K-S Probability
Parameter N (HDP)/ N (total − HDP) p K − S z λ E α OX L keV M BH L bol The number of sources that have the detectionof the physical parameter in the format of (HDPAGN)/(whole type 1 AGN sample exclude the HDPAGN). fraction plot in Figure 3, where we divided two param-eters into nine bins and plotted the fraction of HDPAGNs in each bin.The fraction of the HDP AGNs clearly increases with z . This fraction is 5 . ± .
2% at z < . ± .
1% at 2 < z < .
5. Beyond z = 3 .
5, the sam-ple is too small (one source in two bins) to give a useful One ( z ) with the lowest p K − S and one ( L bol ) with the highest p K − S . Hao et al.
Fig. 3.—
Distribution in the redshift space (top) and fractionof the HDP in the whole sample as a function of redshift (middle)and L bol (bottom). The black (number divided by 5) and bluehistograms show the distribution of all the AGNs and the HDP,respectively. constraint. A linear regression fit of the fractions gives f HDP = (0 . ± . z ) (1 . ± . . The photometryused in the SED comes from a point source correction tothe 3 ′′ aperture photometry. Low-redshift SEDs there-fore include less host compared to the high redshift ones.This effect will only move the source along the mixingcurve, so it cannot explain the observed HDP fractionincrease versus redshift. SED Fitting and Physical Parameters
Studies of NIR spectrum of quasars showed that ablackbody spectrum is needed to fit the observed spec-trum (Glikman et al. 2006). The BBB is mainly causedby the emission of the accretion disk (e.g., Elvis et al.1994) and could be reddened by the dust in the host(e.g., Hopkins et al. 2004). At ∼ µ m, host galaxy hasthe maximum contribution to the observed type 1 AGNSED (e.g., Polletta et al. 2007). Considering all theseeffects, we fit all the HDP AGNs observed NIR–opticalSEDs with three components: (1) accretion disk emis-sion: we use the standard α -disk model in Schwarzschildgeometry with electron scattering and Comptonizationof soft photons in the disk atmosphere (Siemiginowskaet al. 1995), with measured BH mass and accretion ratewhere available (Hao et al. 2010) and E ( B − V ) =0.1–0.2in eight cases to get good chi-square in the UV. (2)hotdust emission: we use a single temperature blackbodyspectrum. (3)host galaxy emission: we use a 5 Gyr oldelliptical galaxy (Polletta et al. 2007). Examples of thefit results are shown in the lower panel of Figure 2. Covering Factor
The NIR SEDs are fitted with a blackbody to get themaximum dust temperature ( T d ). The sublimation tem-perature for graphite and silicate grains are ∼ ∼ T d ranges from 800 K to 1900 K.Most (19 out of 34) HDP AGNs with dust componenthave T d > r d = 1 . L / uv, T − . pc (Barvainis1987), where L uv, is the total ultraviolet (1 µ m–912˚A)luminosity in units of 10 erg s − and T is the max-imum dust temperature in units of 1500 K. We find r d ranges from 0.2 pc to 3.9 pc for the HDP sources. Theemission area ( A d ) is given from the blackbody fitting,and A d ranges from 0.1 pc to 10.68 pc . The coveringfactor of the dust component is then f c = A d / (4 πr d ).For example, the results for source XID=2105 (Figure 2,left) are T d =1500 K and A d =1.76 pc . The evaporationradius is 0.83 pc, corresponding to f c = 20%. f c rangesfrom 1.9% to 29%, which are a factor of 2–40 smallerthan the 75% expected from the unified models to givethe observed type 2 to type 1 ratio (e.g., Krolik & Begel-man 1988). Even if considering the dependence of type2 to type 1 ratio on X-ray luminosity (e.g., Gilli et al.2007), the f c of HDP AGN is still smaller than the 50%expected for the X-ray bright AGNs.The above innermost radius estimation is compara-ble to those reported in Kishimoto et al. (2007, 2009).Kishimoto et al. (2007) also showed that the innermostradii measured by near-IR reverberation are systemati-cally smaller by a factor of ∼ Disk Outer Radius
For 11 HDP AGNs with little host galaxy contamina-tion (defined as <
50% at 1 µ m), and available M BH and λ E (Lusso et al. 2010; Trump et al. 2009b; Merloni etal. 2010), the lack of hot dust emission allows us to seewhat appears to be the accretion disk emission extendingfurther out well beyond 1 µ m.We fitted the outer edge of the accretion disk compo-nent (red dashed line in Figure 2) with a single temper-ature ( T c ) blackbody spectrum (green dashed line). Theouter radius of the accretion disk (Frank et al. 2002): R out = 1 . × T − c α − η − M λ E f pc, where M = M/ (10 M J ) and f = h − (cid:0) GMR c (cid:1) i . Weassume α = 0 . η = 0 . M = 3 . λ E = 0 . R out =0 .
47 pc, ∼ . × Schwarzschild radius ( r s ), 13.6 timesthe gravitational stability radius of the accretion disk(Goodman 2003, i.e., the radius beyond which the diskis unstable to self-gravity and should break up). For the11 HDP AGNs, the T c range from 2200 K to 4500 K.The R out range from 0.09 pc to 0.99 pc, correspondingto (0 . − × r s . These R out are ∼ DISCUSSION AND CONCLUSIONS ot-Dust-Poor Quasars 5In a sample of 404 XMM-selected type 1 AGNs (ex-cluding four with incomplete NIR photometry), we foundthat 10 . ± .
7% have weak NIR emission, indicating arelative paucity of hot dust emission. We call these HDPAGNs.HDP AGNs have not been reported from SDSS or ear-lier samples. We have made an initial check of wherethe Richards et al. (2006) and E94 samples lie on the α OP T − α NIR plane. We find similar fraction of HDPAGNs and will report on this study in detail in a laterpaper. The α OP T − α NIR plane is a useful tool for lo-cating outliers.The fraction of HDP AGNs clearly increases to red-shift z = 3 . f HDP = (0 . ± . z ) (1 . ± . ).No trends of the fraction of HDP AGNs with other pa-rameters, notably L bol , are observed.We divided these HDP AGNs into three classes accord-ing to their α OP T and α NIR . We fitted the HDP SEDswith three components: the accretion disk, a blackbodyto represent the hot dust, and the host galaxy. We foundthe dust covering factors are 1.9%–29%, well below thetypical 75% required by unified models (e.g., Krolik &Begelman 1988). For the 11 HDP AGNs with little hostcontribution ( <
50% at 1 µ m) and available M BH and λ E , we estimated the lower limit of the accretion diskouter radius to be (0.29–2) × r s (0.09–0.99 pc), corre-sponding to 10–23 times the gravitational stability radiusof an α disk. How these disks stabilized is an open ques-tion, or the long wavelength turndown is not R out as weassumed. These results agree with the NIR disk spec-trum uncovered using polarized light in Kishimoto et al.(2008). We ignore further discussion of the sources withstrong host galaxy contribution as the system is morecomplicated to include the also unknown galaxy part.There are several possibilities to explain the lack ofNIR emission in HDP sources. First, most XMM–COSMOS HDPs are at 1 . < z <
3, when the universeis 2.1–4.2 Gyr old, and they have 1–3 Gyr from reioniza-tion to form a torus. The Jiang et al. (2010) proposedexplanation thus seems unlikely for the XMM–COSMOSHDP AGNs. Second, the luminosity and Eddington ratiodistributions of the HDP AGNs and the normal XMM–COSMOS type 1 AGNs are indistinguishable. This rulesout the possibility that HDP AGNs are low-luminosity or low-accretion rate sources, unable to support a dustytorus (Elitzur & Ho 2009). Third, evolutionary scenarios(e.g., Hopkins et al. 2008) predict short-lived Eddingtonlimited outbursts after a merger, which could destroy theinnermost dust. The HDP AGNs might then be quasarsthat had insufficient time to reform dust in the inner re-gion. Fourth, alternatively, when two galaxies merge ,the chaotic dynamics might destroy the “torus”. The‘torus’ then reforms in a timescale that might allow 10%of quasars to be HDP. The dust formation timescale de-pends strongly on pressure and temperature (Whittet2003; Kr¨ugel 2003) making estimates unreliable as bothquantities are poorly known. Last, Guedes et al. (2010)suggested that a BH ejected by gravitational wave recoiland carrying along its accretion disk and broad emissionline region, but not the hot dust “torus”, would appearas an HDP AGN.The unusual SEDs of HDP AGNs can affect estimatesof galaxy bulge and BH masses derived from SED fittingassuming the BH SED to be E94 mean SED. Merloni etal (2010) recently used this method to show that z ∼ . M − σ relationship (H¨aring& Rix 2004). Three of the Merloni et al. (2010) quasarsbelong to our HDP sample. For these sources, the SEDfitting using E94 template gives a small AGN contribu-tion. Allowing an HDP SED to fit would increase theBH mass and decrease the stellar mass, making themdeviate even more from the local M − σ relationship.New AGN templates with variable dust bump strengthsare needed to derive accurate galaxy and BH masses inthese objects.The properties of the HDP AGNs still need to be in-vestigated to understand their formation scenario andput them in the context of galaxy and SMBH evolution.In particular, we will check the existing Hubble imagesand optical and X-ray spectra of HDP AGNs in futurepapers. ACKNOWLEDGMENTS
H.H. thanks Sumin Tang for useful discussions. Thiswork was supported in part by NASA Chandra grantnumber GO7-8136A (H.H., F.C., M.E.). In Italy thiswork is supported by ASI/INAF grants I/023/05.
REFERENCESAntonucci, R. 1993, ARA&A, 31, 473Appleton, P. N., et al. 2004, ApJS, 154, 147Barvainis, R. 1987, ApJ, 320, 537Brusa, M., et al. 2007, ApJS, 172, 353Brusa, M., et al. 2010, ApJ, 716, 348.Capak, P., et al. 2007, ApJS, 172, 99Cappelluti, N., et al. 2009, A&A, 497, 635Cirasuolo, M., et al. 2007, MNRAS, 380, 585Donley, J. L., Rieke, G. H., P´erez-Gonz´alez, P. G. & Barro, G.2008, ApJ, 687, 111Elitzur, M. & Ho, L. C. 2009, ApJ, 701, L91Elitzur, M. & Shlosman, I. 2006, ApJ, 648, L101Elvis, M. et al., 1994, ApJS, 95, 1Elvis, M., et al., 2010, ApJ in preparationEmmering, R. T., Blandford, R. D. & Shlosman, I. 1992, ApJ, 385,460Frank, J., King, A., & Raine, D. 2002, Accretion Power inAstrophysics (Cambridge University Press)Gilli, R., Comastri, A. & Hasinger, G., 2007, A&A, 463, 79Glikman, E., Helfand, D. J. & White, R. L. 2006, ApJ, 640, 579Goodman, J., 2003, MNRAS, 339, 937 Gordon, K. D., Clayton, G. C., Misselt, K. A., Landolt, A. U. &Wolff, M. J. 2003, ApJ, 594, 279Guedes, J., Madau, P., Mayer, L. & Callegari, S. 2010, ApJ,submitted, astro-ph 1008.2032Hao, H., et al., ApJ, in preperationH¨aring, N. & Rix, H.-W., 2004, ApJ, 604, L89Hasinger, G., et al., 2007, ApJS, 172, 29Hopkins, P. F., et al., 2004, AJ, 128, 1112Hopkins, P. F., Hernquist, L.,Cox, T. J. & Kere, D. 2008, ApJS,175, 356Jiang, L. et al., 2010, Nature, 464, 380Kishimoto, M., H¨onig, S. F., Beckert, T., & Weigelt, G. 2007, A&A,476, 713Kishimoto, M., Antonucci, R., Blaes, O., Lawrence, A., Boisson,C., Albrecht, M., & Leipski, C. 2008, Nature, 454, 492Kishimoto, M., H¨onig, S. F., Antonucci, R., Kotani, T., Barvainis,R., Tristram, K. R. W. & Weigelt, G. 2009, A&A, 507, L57Krolik, J. H. & Begelman, M. C., 1988, ApJ, 329, 702Kr¨ugel, E., 2003, The Physics of Interstellar Dust (Bristol andPhiladelphia: Institute of Physics)Lacy, M., et al., 2004, ApJS, 154, 166