The Astrophysical Journal, 827:81 (16pp), 2016 August 10 doi:10.3847/0004-637X/827/1/81 © 2016. The American Astronomical Society. All rights reserved. DO CIRCUMNUCLEAR DENSE GAS DISKS DRIVE MASS ACCRETION ONTO SUPERMASSIVE BLACK HOLES? Takuma Izumi1, Nozomu Kawakatu2, and Kotaro Kohno1,3 1 Institute of Astronomy, School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan; [email protected] 2 Faculty of Natural Sciences, National Institute of Technology, Kure College, 2-2-11 Agaminami, Kure, Hiroshima 737-8506, Japan 3 Research Center for the Early Universe, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan Received 2015 November 12; revised 2016 May 16; accepted 2016 June 8; published 2016 August 10 ABSTRACT We present a positive correlation between the mass of dense molecular gas (Mdense) of ∼100 pc scale circumnuclear disks (CNDs) and the black hole mass accretion rate (M˙ BH) in atotal of10 Seyfert galaxies, based on data 7–8 compiled from the literature and an archive (median aperture θmed=220 pc). A typical Mdense of CNDs is 10 M, estimated from the luminosity of the dense gas tracer, the HCN(1–0) emission line. Because dense molecular gas is the site of star formation, this correlation is virtually equivalent to the one between thenuclear star-formation rate and M˙ BH revealed previously. Moreover, the MMdense– ˙ BH correlation was tighter for CND-scale gas than for the gas on kiloparsec or larger scales. This indicates that CNDs likely play an important role in fueling black holes, whereas greater than kiloparesecscale gas does not. To demonstrate a possible approach for studying the CND- scale accretion process with the Atacama Large Millimeter/submillimeter Array, we used a mass accretion model where angular momentum loss due to supernova explosions is vital. Based on the model prediction, we suggest that only the partial fraction of the mass accreted from the CND (M˙acc) is consumed as M˙ BH. However, M˙acc agrees well with the total nuclear mass flow rate (i.e., M˙ BH + outflow rate). Although these results are still tentative with large uncertainties, they support the view that star formation in CNDs can drive mass accretion onto supermassive black holes in Seyfert galaxies. Key words: galaxies: active – galaxies: evolution – galaxies: Seyfert – ISM: molecules 1. INTRODUCTION would be sufficient to explain lower-luminosity activity, such as that observed in Seyfert galaxies (e.g., Hopkins & Hern- The mass accretion onto a supermassive black hole (SMBH, quist 2009;Treisteretal.2012). The lack of enhanced signatures with a mass of M 106 M ) is commonly believed to BH of major mergers or strong interactions in local Seyfert galaxies produce the enormous amount of energy observed as an active ( ) ( supports this view Gabor et al. 2009; Cisternas et al. 2011 . galactic nucleus AGN; Antonucci 1993; Urry & Pado- fi ) Although large-scale structures, like a bar, would ef ciently vani 1995 . However, the physics of the angular momentum ( ( transport gases toward the central region e.g., Sakamoto transfer of the accreting gas remains unclear Alexander & ) ) et al. 1999; Sheth et al. 2005 , there is little or no clear Hickox 2012 for a review . SMBHs have been claimed to difference in that scale morphologies between AGN hosts and reside at the centers of galaxies with spheroidal components, inactive galaxies (e.g., Mulchaey & Regan 1997; Martini and there is a correlation between MBH and the properties of the ( ) et al. 2003; Simões Lopes et al. 2007; Cheung et al. 2015; host galaxies, such as bulge mass Mbulge and stellar velocity Cisternas et al. 2015). Even for smaller-scale morphologies, dispersion (s ). This, a so-called co-evolutionary relationship ( ( * such as nuclear bars and nuclear spirals e.g., Shlosman Magorrian et al. 1998; Ferrarese & Merritt 2000; Tremaine et al. 1990; Hopkins & Quataert 2010), this trend holds, at least et al. 2002; Marconi & Hunt 2003; Gültekin et al. 2009; ( ) in late-type galaxies typical hosts of Seyfert nuclei, e.g., Kormendy & Ho 2013 and references therein , indicates that Martini et al. 2003; Hunt & Malkan 2004). Regarding early- bulges and SMBHs evolved together, by regulating each other. type galaxies, on the other hand, Simões Lopes et al. (2007) Thus, to understand the mechanism of the angular momentum found that Seyfert nuclei preferentially accompany dusty transfer of the accreting gas at various spatial scales (from a ( ) structures. Thus, the presence of a nuclear dusty equivalently host galaxy to an accretion disk , which is directly connected to gaseous) structure is necessary, but not sufficient, to trigger the mass accumulation of an SMBH, is of great importance to AGNs. To summarize, there is apparently no unique mech- unveil the currently unknown co-evolutionary mechanism. anism in Seyfert galaxies at 100 pc from the center, and direct Recent numerical simulations have shown that the radial triggers of AGN activity should exist at the innermost 100 pc fi streaming of gas caused by major mergers can ef ciently feed region. the central SMBH, which triggers a powerful AGN as well as At that spatial scale, many studies have proposed that the ( ) starbursts e.g., Hopkins et al. 2008; Hopkins & Quataert 2010 . inflowing gas would form a circumnuclear disk (CND)4 due to While such a violent mechanism would be vital for fueling the remaining angular momentum (e.g., Thompson et al. 2005; luminous quasars, rather secular processes induced by, for Ballantyne 2008; Kawakatu & Wada 2008; Vollmer et al. − example, barred gravitational potential, galaxy galaxy interac- 2008). Indeed, such dense molecular gas disks have been found tions, or even joint effects of star formation and large-scale observationally around Seyfert nuclei (e.g., Krips et al. 2007; dynamics (e.g., Shlosman et al. 1990; Barnes & Hernquist 1992; Hicks et al. 2009, 2013; Davies et al. 2012; Sani et al. 2012; Kormendy & Kennicutt 2004; Hopkins & Hernquist 2006; ) 4 Jogee 2006; Rocca-Volmerange et al. 2015 , or minor mergers A CND refers to a massive gaseous disk with sizes of ∼1–100 parsec [pc] in (e.g., Mihos & Hernquist 1994;Taniguchi1999;Kaviraj2014), this work. 1 The Astrophysical Journal, 827:81 (16pp), 2016 August 10 Izumi, Kawakatu, & Kohno Izumi et al. 2013, 2015; García-Burillo et al. 2014), though few the case for the SFR–M˙ BH correlations. Whether there is any of them have been resolved spatially. Recently, Hicks et al. dependence of the correlation strength on the probed spatial (2013) reported on systematic differences at the CND scale scale has not yet been investigated. Here, we note that, if we between active and inactive galaxies: Seyfert galaxies showed use dense gas tracers for our investigation rather than the more centrally concentrated profiles of both the stellar conventional CO(1–0) line that traces total molecular gas, / continuum and H2 1–0S(1) line emission with enhanced H2 including diffuse and or foreground ones, we can expect less luminosity. Therefore, molecular surface brightness is clearly contamination, at least from the foreground component (e.g., elevated in CNDs of Seyfert galaxies. galactic disks). Moreover, dense gas is indeed the source of Because a CND would be a massive reservoir of molecular massive star formation. Thus, to provide further insights on the gas, we can reasonably expect active star formation there. AGN−starburst connection more directly and to understand the Observationally, prominent star formation has been found as a underlying mass accretion processes at the CND scale, it would (circum-)nuclear starburst (e.g., Heckman et al. 1995; Cid be desirable to establish a correlation between AGN activity Fernandes et al. 2004; Imanishi & Wada 2004; Davies and some molecular properties based on emission lines that et al. 2007; Diamond-Stanic & Rieke 2012; Alonso-Herrero faithfully trace star-forming regions, to check the variation in et al. 2013; Esquej et al. 2014). Interestingly, there are the spatial scale probed, and to explore the origin of the correlations between the star-formation rate (SFR) and the correlations. black hole accretion rate (M˙ BH) in Seyfert galaxies, which are tighter when the SFR is measured in closer vicinity to the 1.1. This Work ( ) AGN, whereas it is weaker for larger-scale kpc SFRs In this work, motivated by the idea above, we explored the (Diamond-Stanic & Rieke 2012; Esquej et al. 2014). From a possible correlation between M˙ BH and the mass of the CND- theoretical viewpoint, such a correlation would indeed be scale molecular gas as a natural extension of the previous ( predicted to exist due to various mechanisms e.g., Kawakatu galactic-scale measurements (Yamada 1994; Monje ) & Wada 2008; Hopkins & Quataert 2010 . et al. 2011). The mass of dense molecular gas (M ) was ˙ dense On the origin of the SFR–MBH correlation, we should investigated preferentially for the reasons above. Thus, we ( ) consider the results of Davies et al. 2007 , who showed a time compiled currently available data for a typical dense gas tracer – delay of 50 200 Myr between the onset of star formation and at the millimeter band, specifically, the HCN(1–0) emission the peak epoch of AGN activity (see also Schawinski line (Section 2.1). This allows us to straightforwardly compare et al. 2009; Wild et al. 2010). That delay might be compelling results with the SFR–M˙ BH correlations, because Mdense is evidence of a causal connection between these activities: that directly convertible to SFR (e.g., Gao & Solomon 2004b). The star formation provides the fuel for the SMBH (Section 5).Asa detailed spatial distribution (or gas surface density) and candidate mechanism to make such a link, Hopkins & Quataert kinematics of dense molecular gas, however, cannot be (2010) suggested, for example, the importance of a series of discussed in this paper as most CNDs have not been gravitational instability and the resulting stellar gravitational spatiallyresolved at this point.