Towards a complete census of active galactic nuclei in nearby galaxies: the incidence of growing black holes

Abstract

We investigate the local supermassive black hole (SMBH) density function and relative mass accretion rates of all active galactic nuclei (AGNs) identified in a volume‐limited sample of infrared (IR) bright galaxies (LIR > 3 × 109 L⊙) to D < 15 Mpc. A data base of accurate SMBH mass (MBH) estimates is compiled from literature sources using physically motivated AGN modelling techniques (reverberation mapping, maser mapping and gas kinematics) and well‐established indirect MBH estimation methods (the M–σ* and MBH–LK,bul relations). For the three sources without previously published MBH estimates, we use Two Micron All Sky Survey (2MASS) K‐band imaging and galfit to constrain the bulge luminosities, and hence SMBH masses. In general, we find the AGNs in the sample host SMBHs which are spread over a wide mass range [MBH≈ (0.1–30) × 107 M⊙], but with the majority in the poorly studied MBH≈ 106–107 M⊙ region. Using sensitive hard X‐ray (2–10 keV) and mid‐IR constraints we calculate the bolometric luminosities of the AGNs (LBol,AGN) and use them to estimate relative mass accretion rates. We use these data to calculate the volume‐averaged SMBH growth rate of galaxies in the local Universe and find that the AGNs hosting SMBHs in the mass range MBH≈ 106–107 M⊙ are dominated by optically unidentified AGNs. These relatively small SMBHs are acquiring a significant proportion of their mass in the present day, and are amongst the most rapidly growing in the local Universe (SMBH mass‐doubling times of ≈6 Gyr). Additionally, we find tentative evidence for an increasing volume‐weighted AGN fraction with decreasing SMBH mass in the MBH≈ 106–108 M⊙ range. Overall, we conclude that significant mass accretion on to small SMBHs may be missed in even the most sensitive optical surveys due to absent or weak optical AGN signatures.

Exploring the Connection Between Star Formation and Active Galactic Nucleus Activity in the Local Universe

“…Galaxies that are optically classified as composite systems are more akin to quiescent star-forming galaxies in terms of the mid-IR parameters that trace star formation: the smaller relative energetic importance of the AGN in composites does not seem to affect [Ne iii] and PAH luminosities. , it is potentially an unequivocal signature of AGN activity (Gilli et al 2010;Goulding et al 2010). We do not find a substantial fraction of optically identified star-forming galaxies with [NeV] detections as Goulding & Alexander (2009) do, but this is likely due to the different parameter space we are exploring (i.e., more distant and more massive galaxies).…”

“…These seeming disparities can be attributable to the different regime of parameter space we are exploring from Goulding & Alexander (2009). They focus on low mass galaxies within 15 Mpc, systems that SDSS is not optimized to identify since their velocity dispersions are below the SDSS spectral resolution (see Goulding et al 2010). The relative proximity of the Goulding & Alexander (2009) sample translates to a smaller physical aperture extraction area for IRS spectra, allowing for higher signal-to-noise detection of [NeV] which may become more insubstantial due to dilution effects when larger physical areas are probed, such as seen in our sample.…”

“…Ratios of IR fine structure lines also parameterize the ionization field hardness. For instance, [O iv] 26 μm and [Ne v] 14.32 μm are primarily ionized by AGNs DiamondStanic et al 2009;Armus et al 2007;Goulding et al 2010), whereas [Ne ii] 12.81 μm is excited by star formation (Ho & Keto 2007). The ratio of these lines can then indicate the relative importance of these two processes (Genzel et al 1998;Veilleux et al 2009;LaMassa et al 2010;Pereira-Santaella et al 2010).…”

“…Due to its high ionization potential (97.1 eV), [Ne v] has been claimed to cleanly trace SMBH accretion (Genzel et al 1998;Armus et al 2007;Pereira-Santaella et al 2010) and to be an unambiguous signature of AGN activity (Gilli et al 2010;Goulding et al 2010). Also, as it is formed in the narrow-line (Heckman et al 2005;Meléndez et al 2008;Rigby et al 2009;Diamond-Stanic et al 2009;LaMassa et al 2010).…”

The cosmic growth of the active black hole population at 1 <z <2 in zCOSMOS, VVDS and SDSS

“…Soria et al 2006;Ho 2008;Gallo et al 2010;Miller et al 2012). A different definition of an active BH will naturally result in a different active BHMF (Goulding et al 2010).…”