Abstract
We present new near-infrared VLTI/GRAVITY interferometric spectra that spatially resolve the broad Brγ emission line in the nucleus of the active galaxy IRAS 09149−6206. We use these data to measure the size of the broad line region (BLR) and estimate the mass of the central black hole. Using an improved phase calibration method that reduces the differential phase uncertainty to 0.05° per baseline across the spectrum, we detect a differential phase signal that reaches a maximum of ∼0.5° between the line and continuum. This represents an offset of ∼120 μas (0.14 pc) between the BLR and the centroid of the hot dust distribution traced by the 2.3 μm continuum. The offset is well within the dust sublimation region, which matches the measured ∼0.6 mas (0.7 pc) diameter of the continuum. A clear velocity gradient, almost perpendicular to the offset, is traced by the reconstructed photocentres of the spectral channels of the Brγ line. We infer the radius of the BLR to be ∼65 μas (0.075 pc), which is consistent with the radius–luminosity relation of nearby active galactic nuclei derived based on the time lag of the Hβ line from reverberation mapping campaigns. Our dynamical modelling indicates the black hole mass is ∼1 × 108 M⊙, which is a little below, but consistent with, the standard MBH–σ* relation.
Highlights
Massive black holes in the centres of galaxies are a key component of galaxy evolution because of the role that accreting black holes have in the feedback that regulates star formation and galaxy growth (Booth & Schaye 2009; Fabian 2012; Somerville & Davé 2015; Dubois et al 2016)
In this paper we present an analysis of new GRAVITY observations for IRAS 09149−6206 (α = 09:16:09.39, δ = −62:19: 29.9)
We found a substantial improvement in residual phase noise by retaining all data independent of FT signal-to-noise ratio (S/N) or the estimated visibility loss2 (GC18; GRAVITY Collaboration 2020a, hereafter GC20a)
Summary
Massive black holes in the centres of galaxies are a key component of galaxy evolution because of the role that accreting black holes have in the feedback that regulates star formation and galaxy growth (Booth & Schaye 2009; Fabian 2012; Somerville & Davé 2015; Dubois et al 2016). In GRAVITY Collaboration (2018, hereafter, GC18), we reported the first robust measurements of BLR size and kinematics for 3C 273 by combining differential phase spectra with the Paα emission line profile to model the BLR as a thick rotating disk under the gravitational influence of a black hole of ∼3 × 108 M. Parametric models of the BLR geometry and dynamics have been successfully applied to fewer than two dozen AGNs that have both high S/N spectra and high cadence monitoring (Pancoast et al 2014a; Grier et al 2017a; Williams et al 2018; Li & Wang 2018) These enable one to fit for radial (inflow or outflow) motion of the clouds in addition to rotation, and to derive the virial factors for individual objects. 1 pc subtends 0.87 mas on sky and 1 μas corresponds to 1.37 light day at the redshift of IRAS 09149−6206
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