Life of the Most Massive Black Holes

Abstract

Supermassive black holes (BHs) are mostly found in the center of galaxy bulges, growing in mass during the active galactic nucleus (AGN) phase. The most massive limit is identified as present day ten billion solar mass BHs, in massive quiescent galaxies. We search for observational signatures to investigate the physical evolution of extremely massive BHs (EMBHs, >10^9.5M⊙). We aim to constrain the massive limit of AGNs, probe the massive end evolution of BH mass, search for evolution in the AGN dusty structure, and establish the mid-infrared fundamental scaling relation of massive BH host galaxies.

First, we examine the reliability of the BH mass for the most massive AGNs by estimating their mass using optical mass estimators, and comparing them to the previous rest-frame UV mass estimations of ~10^10M⊙ for 27 AGNs at 0.7<z<2.5. We find that the Balmer BH masses are overall consistent with the MgII masses up to ~10^10M⊙, although a fraction of the >10^10M⊙ AGN spectra suffer from disk emitter features on top of the broad Hα emission, which are weakly noticed near the UV broad line emission. On the other hand, the CIV-based BH masses show a large scatter with respect to the Hα based, with 4 out of 8 quasars with CIV BH mass of >10^10M⊙ having Hα BH mass less than 10^10M⊙. These results suggest that previously reported rest-UV BH mass of >10^10M⊙ should be carefully interpreted. We test for the biases in the BH mass from systematic uncertainty in the BH mass estimator, but find that our Hα BH mass measurements will likely stay at ~10^10M⊙.

Second, we study the rest-frame optical spectra of 155 luminous quasars at 3.3<z<6.4 taken with the AKARI space telescope. We extend the scaling relation between 5100 angstrom and Hα luminosities of AGNs to the high luminosity, high redshift regime. Remarkably, a single linear relation can be applied to the AGN luminosities suggesting that the physical mechanism governing the relation does not evolve with redshift (z=0-6), over five decades in luminosity (10^43<L_bol<10^48ergs/s). Similar scaling relations are found between the optical and the UV continuum luminosities and line widths. Applying the scaling relations to the Hβ BH mass estimator of local AGNs, we derive the BH mass estimators based on Hα, MgII, and CIV lines finding that the UV-line based masses are overall consistent with the Balmer-line based, but with a large intrinsic scatter for the CIV estimates. Our 43 BH mass estimates from Hα confirm the existence of BHs as massive as ~10^10 and 10^9M⊙ out to z~5 and 6, respectively, and provide a secure footing for previous UV-line based studies that a rapid BH mass growth has occurred in the early universe.

Next, as a possible sign of AGN structure formation and evolution on the way of becoming EMBHs, we identify and characterize a population of luminous, dust-poor quasars at z<5 that is photometrically similar to dust-poor quasars found at z>6 previously. We fit the rest-frame UV-to-IR spectral energy distribution (SED) of 41,000 optically selected type 1 quasars with L_bol>10^45.7ergs/s, to find 0.6% of the sample to be hot dust-poor, with rest-frame 2.3μm-to-0.51μm flux density ratios of -0.5dex or less. The dust-poor SEDs are blue in the UV-optical and weak in the mid-IR, such that their accretion disks are relatively unobscured and the hot dust emission traces that of warm dust down to the dust-poor regime. At a given bolometric luminosity, dust-poor quasars are lower in black hole mass, higher in Eddington ratio, and higher in redshift than general luminous quasars, suggesting that they are in a rapidly growing evolutionary state in which the dust-poor phase appears as a short or rare phenomenon.

Lastly, we study the local fundamental plane (FP) relation of early-type galaxies in the mid-infrared (MIR), the final evolutionary stage of massive BH hosts. The scaling relation between size, surface brightness, and velocity dispersion of early-type galaxies has been found to be tilted against the simple virial expectation, prompting debates on its origin. In order to investigate the contribution of systematic stellar population variation to the FP tilt, we used a sample of 56 early-type galaxies for which visible, near-infrared, and MIR (Spitzer IRAC) data are available. The derived slope of the FP in the MIR suggests that the stellar population effect can explain more than half of the FP suggesting that the MIR light better represents mass than the shorter wavelengths.

Through the study of the rest-frame optical spectra of high redshift AGNs, we find that the governing physics in AGNs appear to be unchanging at z=0-6, and provide further support that the EMBHs in AGNs appear at z=5 with a vigorous growth happening at z=6 involving a dust-poor phase. When EMBHs settle in at quiescent, massive galaxies today, the host galaxies possess the infrared fundamental plane relation only slightly off from a simple virial expectation when using the MIR luminosity as a proxy of mass.