Publications of the Astronomical Society of Australia (PASA) doi: 10.1017/pas.2021.xxx. Refining the mass estimate for the intermediate-mass black hole candidate in NGC 3319 Benjamin L. Davis1,2,∗ and Alister W. Graham1 1Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, VIC 3122, Australia 2Center for Astro, Particle, and Planetary Physics (CAP3), New York University Abu Dhabi Abstract Recent X-ray observations by Jiang et al. have identified an active galactic nucleus (AGN) in the bulgeless spiral galaxy NGC 3319, located just 14.3 ± 1.1 Mpc away, and suggest the presence of an 2 5 intermediate-mass black hole (IMBH; 10 ≤ M•/M ≤ 10 ) if the Eddington ratios are as high as 3 to 3 × 10−3. In an effort to refine the black hole mass for this (currently) rare class of object, we have explored multiple black hole mass scaling relations, such as those involving the (not previously used) velocity dispersion, logarithmic spiral-arm pitch angle, total galaxy stellar mass, nuclear star cluster mass, rotational velocity, and colour of NGC 3319, to obtain ten mass estimates, of differing accuracy. +7.02 4 5 We have calculated a mass of 3.14−2.20 × 10 M , with a confidence of 84% that it is ≤10 M , based on the combined probability density function from seven of these individual estimates. Our conservative approach excluded two black hole mass estimates (via the nuclear star cluster mass, and the fundamental plane of black hole activity — which only applies to black holes with low accretion rates) that were upper 5 5 limits of ∼10 M , and it did not use the M•–L2−10 keV relation’s prediction of ∼10 M . This target provides an exceptional opportunity to study an IMBH in AGN mode and advance our demographic knowledge of black holes. Furthermore, we introduce our novel method of meta-analysis as a beneficial technique for identifying new IMBH candidates by quantifying the probability that a galaxy possesses an IMBH. Keywords: black hole physics – galaxies: active – galaxies: evolution – galaxies: individual: NGC 3319 – galaxies: spiral – galaxies: structure Original unedited manuscript, accepted for publication by PASA, May 7, 2021. 1 INTRODUCTION Mezcua, 2017; Koliopanos, 2017; Inayoshi et al., 2020; Sahu et al., 2019a). There is a largely-missing population of intermediate- mass black holes (IMBHs) with masses higher than those As yet, there is no consensus as to how SMBHs formed by stable, single stars today (M• . 100 M ) and came to be. While the observed extent of quasar ac- arXiv:2105.04717v1 [astro-ph.GA] 11 May 2021 less massive than the supermassive black holes (SMBHs; tivity over the history of our Universe has revealed that 5 10 1 10 M ≤ M• . 10 M ) known to reside at the cen- the accretion of baryons fattened them up (e.g. Soltan, tres of massive galaxies. Not surprisingly, astronomers 1982; Shankar et al., 2004), we do not know what their around the world have been hotly pursuing the much- (potentially range of) birth masses were. Some theo- anticipated discovery of IMBHs for some time (e.g. Miller ries have speculated that their birth or ‘seed’ masses 5 & Colbert, 2004). In addition to providing a fundamen- were ≈10 M , thereby providing a kick-start to explain tal input to the cosmic inventory of our Universe, the the early-formation of the high-z, active galactic nuclei 9 abundance, or rarity, of IMBHs has implications for the (AGN) with sizeable black hole masses around ≈10 M formation of the Universe’s SMBHs (Graham, 2016b; (e.g. Mortlock et al., 2011; Yang et al., 2020; Mignoli et al., 2020). Theories have included primordial black ∗Author for correspondence: BLD, E-mail: [email protected] holes (e.g. Grobov et al., 2011), massive metal-free Pop- 1The massive central object in the quasar TON 618 is alleged 10 ulation III stars which subsequently collapse (or collide, to have the most massive black hole with a mass of 6.61×10 M , estimated from its Hβ emission line and a virial f-factor of 5.5 e.g. Alister Seguel et al., 2020) to form massive black (Shemmer et al., 2004; Onken et al., 2004). holes (e.g. Madau & Rees, 2001; Schneider et al., 2002), 1 2 Davis & Graham or the direct collapse of massive gas clouds, effectively ideas would place, at least some, IMBHs at the centres by-passing the stellar phase of evolution (e.g. Bromm & of galaxies, where established black hole mass scaling Loeb, 2003; Mayer et al., 2010). relations involving some property of the host galaxy can The suggestion of massive seeds arose from the no- be applied. tion that the ‘Eddington limit’ (Eddington, 1925) of gas Recent Chandra X-ray Observatory (CXO; Weisskopf accretion onto a black hole implied that stellar-mass et al., 2000) observations (Soria, 2016, see also Chilingar- black holes did not have sufficient time to grow into the ian et al. 2018 and Bi et al. 2020), have discovered IMBH SMBHs observed in the young, high-redshift AGN. How- candidates at the centres2 of several nearby, low-mass ever, the Eddington limit on the accretion rate applies galaxies. Long exposures have enabled the discovery of only to (unrealistic) spherical conditions (Nayakshin faint X-ray point-sources (consistent with low-mass black et al., 2012; Alexander & Natarajan, 2014) and can holes accreting with low Eddington ratios) in galaxies be significantly exceeded in real systems. For example, which have been predicted to host a central IMBH based super-critical (super-Eddington) accretion flows onto upon each galaxy’s velocity dispersion, luminosity, and massive black holes can occur when the accretion flow spiral-arm pitch angle (Koliopanos et al., 2017; Graham is mostly confined to the disk plane while most of the & Soria, 2019; Graham et al., 2019). The high-energy radiation emerges in outflows along the rotation axis X-ray photons, originating from the (not so) dead cen- (Abramowicz et al., 1980; Jiang et al., 2014; Pezzulli tres of the galaxies, are likely coming from the accretion et al., 2016). Hyper-Eddington accretion rates can exist disks around black holes because of their point-source in spherically-symmetric accretion flows when energy nature, where emission favours active black holes rather advection reduces radiative efficiency (Inayoshi et al., than spatially extended star formation. 2016). Thus, the practicality of super-critical accretion Several studies have identified IMBH candidates in has been invoked to explain the early existence of SMBHs galaxies based on single, or a few, black hole mass esti- at high redshifts (Volonteri & Rees, 2005; Volonteri, 2012; mates. In this work, we have selected a galaxy, NGC 3319, Volonteri & Bellovary, 2012; Volonteri et al., 2015). Be- where we can apply a wealth of independent black hole sides, most ultra-luminous X-ray sources are nowadays mass estimates. NGC 3319 is a gas-rich, bulgeless, late- explained as stellar-mass X-ray binaries accreting much type galaxy. It is a strongly-barred spiral galaxy classified faster than their Eddington limit (Feng & Soria, 2011; as SBcd(rs) (de Vaucouleurs et al., 1991) and has its bar Kaaret et al., 2017). Such accretion negates the need for aligned with the major axis (Randriamampandry et al., massive black hole seeds. 2015). Moreover, Jiang et al.(2018) identify it as pos- An additional motive for starting AGN with massive sessing a low-luminosity AGN with a high-accretion-rate seeds was that black holes with masses intermediate be- signalled by a nuclear X-ray point source and assume 2 5 tween that of stellar-mass black holes and SMBHs had a black hole mass between 3 × 10 M and 3 × 10 M not been directly observed, and therefore seemed not to based on a high Eddington ratio of 1 to 10−3, despite a exist. However, this may be a sample selection bias be- non-detection in the radio. Using the X-ray variability, 5±2 cause the sphere-of-gravitational-influence around such they report an estimate of ∼10 M , and using the IMBHs, where one would directly observe a Keplerian ‘fundamental plane of black hole activity’, they reported 5 rotation curve, is typically too small to resolve spatially. an upper limit of 10 M in the absence of radio data. Furthermore, there is now a rapidly rising number of NGC 3319 had previously been recognised as a possi- IMBH candidates based upon indirect estimates of the ble low-ionisation nuclear emission-line region (LINER) black hole mass (Farrell et al., 2009; Secrest et al., 2012; galaxy (Heckman et al., 1980; Pogge, 1989), or at least Baldassare et al., 2015; Graham et al., 2016; Kızıltan it possessed an uncertain H i i nucleus (Ho et al., 1997). et al., 2017; Nguyen et al., 2017; Chilingarian et al., Recently, Baldi et al.(2018) classified its nuclear type 2018; Mezcua et al., 2018; Jiang et al., 2018; Nguyen as a LINER based on BPT (Baldwin et al., 1981) di- et al., 2019; Graham & Soria, 2019; Graham et al., 2019; agram diagnostics. This classification is of significance Woo et al., 2019; Lin et al., 2020). In addition, there since AGN with black holes are suspected sources of are currently five IMBH candidates in the Milky Way stimulating LINER spectral emission (Heckman, 1980). (Takekawa et al., 2020). In this study, we endeavour to constrain better the There is no shortage of scenarios for how a bridging mass of the potential IMBH in the nucleus of NGC 3319 population of IMBHs may have arisen. Possible path- 2 ways include the runaway collapse of dense ‘nuclear star Some of the off-centre X-ray sources that were detected may also be IMBHs.