Low-Mass Active Galactic Nuclei With Rapid X-Ray Variability

Abstract

We present a detailed study of the optical spectroscopic properties of 12 active galactic nuclei (AGNs) with candidate low-mass black holes (BHs) selected by Kamizasa et al. through rapid X-ray variability. The highquality, echellette Magellan spectra reveal broad Hα emission in all the sources, allowing us to estimate robust viral BH masses and Eddington ratios for this unique sample. We confirm that the sample contains low-mass BHs accreting at high rates: the median M BH = 1.2 × 10 6 M ⊙ and median L bol /L Edd = 0.44. The sample follows the M BH − σ * relation, within the considerable scatter typical of pseudobulges, the probable hosts of these low-mass AGNs. Various lines of evidence suggest that ongoing star formation is prevalent in these systems. We propose a new strategy to estimate star formation rates in AGNs hosted by low-mass, low-metallicity galaxies, based on modification of an existing method using the strength of [O II] λ3727, [O III] λ5007, and X-rays.

Supermassive Black Holes With High Accretion Rates in Active Galactic Nuclei. V. A New Size–luminosity Scaling Relation for the Broad-Line Region

“…1 is the minimum velocity width of Hβ (see Equation (15) in Paper IV). We note that the minimum observed FWHM value of Hα (which is comparable to Hβ) is~-10 km s 3 1 among low-mass AGNs (Greene & Ho 2007;Ho & Kim 2016). Indeed, this limit is consistent with the saturation trend of b r H (Figure 6(a)).…”

Observational evidence for intermediate-mass black holes

“…of ∼ 10 3 M ) BHs. The virial BH masses of the 10 low-mass AGN from Ho & Kim (2016) are found to follow the M BH − σ * relation (if the considerably large scatter in the pseudobulge regime is considered, e.g., Ho & Kim 2016; grey squares in Fig. 10), as found for other samples of low-mass AGN (also included in Table 3 and Fig.…”

“…However, this is not the case for the M BH −M bulge relation: the BH mass of IMBHs/low-mass AGN tends to be lower at a given bulge mass than expected from an extrapolation of the relation found for classical bulges (e.g., Greene et al 2008;Jiang et al 2011;Graham & Scott 2013Baldassare et al 2015Baldassare et al , 2017Nguyen et al 2017). SMBHs in inactive galaxies with pseudobulges (Hu 2008;Kormendy & Ho 2013) and spiral galaxies with megamaser BH mass measurements (Greene et al 2010;Läsker et al 2016) are also found to fall below the M BH − M bulge relation of early-type galaxies (Kormendy & Ho 2013), which suggests that most IMBHs/low-mass AGN are hosted by late-type galaxies (Greene & Ho 2007a;Ho & Kim 2016). Finally, low-mass late-type galaxies tend to lie as well below the M BH − M * relation compared to bulge-dominated and elliptical (early-type) galaxies (Reines & Volonteri 2015), which could be explained by IMBH quenching of the star formation during an early gas-rich phase in the life of the dwarf galaxy (Silk 2017).…”

“…Based on rapid X-ray variability, Kamizasa et al (2012) presented an additional sample of 15 low-mass AGN candidates, 7 of which have M BH < 2 × 10 6 M and are thus candidates to IMBHs. Optical spectroscopy of 12 of these low-mass BHs revealed the presence of broad Hα emission lines, from which BH masses in the range 10 5 − 10 6 M were estimated (Ho & Kim 2016). …”

“…The uncertainty on the BH masses of POX 52 and the Greene & Ho (2007a) and Ho & Kim (2016) sources is taken as 0.5 dex (e.g., Ho & Kim 2014). a BH mass measured using reverberation mapping.…”