The (Black Hole)-Bulge Mass Scaling Relation at Low Masses
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The Astrophysical Journal, 798:54 (12pp), 2015 January 1 doi:10.1088/0004-637X/798/1/54 C 2015. The American Astronomical Society. All rights reserved. THE (BLACK HOLE)-BULGE MASS SCALING RELATION AT LOW MASSES Alister W. Graham1 and Nicholas Scott2,3 1 Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, VIC 3122, Australia 2 Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia 3 ARC Centre of Excellence for All-sky Astrophysics, 44 Rosehill Street, Redfern, NSW 2016, Australia Received 2014 August 21; accepted 2014 October 27; published 2014 December 19 ABSTRACT Several recent papers have reported on the occurrence of active galactic nuclei (AGNs) containing undermassive black holes relative to a linear scaling relation between black hole mass (Mbh) and host spheroid stellar mass (Msph,∗). However, dramatic revisions to the Mbh–Msph,∗ and Mbh–Lsph relations, based on samples containing 8 predominantly inactive galaxies, have recently identified a new steeper relation at Mbh (2–10)×10 M, roughly 11 corresponding to Msph,∗ (0.3–1)×10 M. We show that this steeper, quadratic-like Mbh–Msph,∗ relation defined by the Sersic´ galaxies, i.e., galaxies without partially depleted cores, roughly tracks the apparent offset of the AGN 5 8 having 10 Mbh/M 0.5 × 10 . That is, these AGNs are not randomly offset with low black hole masses, but also follow a steeper (nonlinear) relation. As noted by Busch et al., confirmation or rejection of a possible AGN offset from the steeper Mbh–Msph,∗ relation defined by the Sersic´ galaxies will benefit from improved stellar mass-to-light ratios for the spheroids hosting these AGNs. Several implications for formation theories are noted. Furthermore, reasons for possible under- and overmassive black holes, the potential existence of intermediate 5 ∝ 2.7±0.7 mass black holes (<10 M), and the new steep (black hole)–(nuclear star cluster) relation, Mbh Mnc ,are also discussed. Key words: black hole physics – galaxies: bulges – galaxies: fundamental parameters – galaxies: nuclei Supporting material: machine-readable table 1. INTRODUCTION Hohl 1975; Hohl & Zang 1979; Combes & Sanders 1981; Kormendy 1982, 1993; Kormendy & Kennicutt 2004). This 4 Several years ago, Graham (2007a, 2008a, 2008b) and Hu would agree with one of the scenarios presented by Hu (2008) and Graham (2008a), and subsequently Kormendy & (2008) reported on galaxies whose black hole masses, Mbh, appeared undermassive relative to expectations based on their Bender (2011), but if correct would present a contradiction stellar velocity dispersion, σ. This apparent sub-structure in with the picture presented in the preceding paragraph. However, pseudobulges are particularly difficult to reliably identify (Wyse the Mbh–σ diagram (Ferrarese & Merritt 2000; Gebhardt et al. 2000) was due to barred galaxies located, on average, 0.3 dex et al. 1997) because they can possess the same physical properties as low-mass, merger-built bulges, including Sersic´ in the log Mbh direction below the Mbh–σ relation defined by barless galaxies. Graham & Li (2009) subsequently revealed (1968) index, rotation, the presence of embedded disks, and a that galaxies with active galactic nuclei (AGNs) also display systematic departure from the bright end of any scaling relation that has used “effective” radii or “effective” surface brightnesses this same general separation in the Mbh–σ diagram, supporting the earlier introduction of barred-, barless-, and elliptical-galaxy (e.g., Dominguez-Tenreiro et al. 1998; Aguerri et al. 2001; Bekki 2010; Saha et al. 2012; Querejeta et al. 2014; Graham 2014b, Mbh–σ relations (see also Gultekin¨ et al. 2009; Greene et al. 2010). It was noted from the start that either the black hole 2013, and references therein). masses could be low in the barred galaxies or that an elevated To address the above contradiction and bypass the issue of velocity dispersion may account for their apparent offset in the pseudobulges, we start by noting that the near-linear scaling relations between M and host spheroid luminosity, L , and Mbh–σ diagram. Hartmann et al. (2014; see also Brown et al. bh sph 2013; Debattista et al. 2013; Monari et al. 2014) have recently also host spheroid stellar mass, Msph,∗ (Dressler 1989;Yee1992; used simulations to demonstrate that the observed offset is an Kormendy & Richstone 1995; Magorrian et al. 1998; Marconi 5 expected result from bar dynamics, which inflate the measured & Hunt 2003;Haring¨ & Rix 2004) have recently been shown velocity dispersion by exactly the amount observed (Graham by Graham (2012a) to provide an incomplete description of the ∝ 5 et al. 2011). Given that this can fully account for the offsets in (black hole)–spheroid relationship. In essence, the Mbh σ (Ferrarese & Merritt 2000; Merritt & Ferrarese 2001a; Graham the Mbh–σ diagram, it implies that the barred galaxies do not possess undermassive black holes, and thus should not be offset et al. 2011; McConnell et al. 2011; Graham & Scott 2013) and L ∝ σ 2 (Davies et al. 1983;Heldetal.1992; Matkovic´ in the black hole mass–spheroid mass (Mbh–Msph) diagram. sph A few papers (e.g., Jiang et al. 2011a; Jiang et al. 2013; & Guzman´ 2005; de Rijcke et al. 2005; Balcells et al. 2007b; Mathur et al. 2012; Reines et al. 2013), however, have shown Chilingarian et al. 2008; Forbes et al. 2008; Cody et al. 2009; Tortora et al. 2009; Kourkchi et al. 2012) scaling relations that there is an offset at the low-mass end of the Mbh–Msph diagram, such that the black hole mass is lower than predicted 4 by the near-linear Mbh–Msph relation established using galaxies Pseudobulges could be offset low in the Mbh–Msph diagram if secular having predominantly higher-mass black holes. These offset evolution disproportionately increases the central bulge mass relative to the growth of the black hole. galaxies have been labeled by some to contain pseudobulges— 5 The linear MQSO–Mgalaxy relationship proposed by Yee (1992) pertains to spheroidal components thought to be produced by the secular the limit in massive spheroids, and it is effectively the relationship for which evolution of a disk and associated with bars (Bardeen 1975; Magorrian et al. (1998) and Laor (2001) later provided the zero-point. 1 The Astrophysical Journal, 798:54 (12pp), 2015 January 1 Graham & Scott for “Sersic´ spheroids” necessitates a nonlinear Mbh–Lsph and additive, dry major merger events that created the near-linear Mbh–Msph,∗ relation. Sersic´ spheroids are elliptical galaxies and (one-to-one) Mbh-Msph,∗ relation. The process of “mechanical” the bulges of disk galaxies that do not have partially depleted or “radio mode” AGN feedback may therefore subsequently 6 8 cores ; they typically have B-band absolute magnitudes MB maintain, rather than establish, this linear relation. −20.5 ± 1 mag, and Sersic´ indices n 3–4. Graham (2012a) Here we investigate if galaxies with AGNs hosting low- pointed out that in these spheroids, one expects to find that mass black holes that have been reported in the literature to 2.5 Mbh ∝ L and that the relationship between Mbh and Msph,∗ be offset from the near-linear Mbh-Msph,∗ relation (defined by should be better described by a near-quadratic relation than a predominantly massive spheroids) might simply be following linear relation, as was further shown in Graham & Scott (2013) the steeper relation of the Sersic´ galaxies. If so, then they may and Scott et al. (2013). As noted in these works, it is only at high not be discrepant galaxies with unusually low Mbh/Msph,∗ mass 8 masses (Mbh 10 M) that a near-linear Mbh–Msph,∗ relation ratios, but rather abide by the main relation defined by the is evident, giving rise to this “broken” scaling relation. Due to majority of galaxies today. This will have dramatic implications the scatter in the Mbh–Msph diagram, coupled with the location for cosmological hydrodynamical simulations, such as Illustris of the brighter Sersic´ galaxies at the high-mass end of the near- (Sijacki et al. 2014), which are tied to the near-linear Mbh–Msph,∗ quadratic Mbh–Msph,∗ relation, surveys that have not sufficiently relation. Our study has been performed using data from many 7 probed below Mbh ≈ 10 M can readily miss the bend in the authors, thereby avoiding possible biases in any one study Mbh–Msph relation (e.g., Sani et al. 2011; Beifiori et al. 2012; and deriving spheroid stellar masses when not done in the Vika et al. 2012; van den Bosch et al. 2012; McConnell & Ma original papers. In Section 2, we introduce the galaxy/black 2013; Sanghvi et al. 2014; Feng et al. 2014). A steeper than hole samples used, and in Section 3, we present their location linear, although not bent, relation was, however, detected early in the Mbh–Msph,∗ diagram. Section 4 provides a discussion on (Laor 1998, 2001; Wandel 1999) and a number of recent of related topics such as formation theories, expectations for theoretical works have now revealed a steepening relationship intermediate mass black holes, coexistence with nuclear star at lower masses (Dubois et al. 2012; Khandai et al. 2012; Bonoli clusters, and potential evolutionary pathways for possible under- et al. 2014; Bellovary et al. 2014), although Khandai et al. (2014, and overmassive black holes. their Figure 22) does not. This would appear to be bringing things more in line with the prediction by Haehnelt et al. (1998) 2. SAMPLE AND DATA that M αM5/3 . bh halo 2.1. Reference Sample These scaling relationships are important for several reasons. Given the broken, or rather bent, Mbh–Msph,∗ relation, it implies Our initial reference sample consists of 75 galaxies with that within the Sersic´ galaxies, the supermassive black holes directly measured supermassive black hole masses and spheroid grow more rapidly than the stellar spheroids (Graham 2012a).