Spectroastrometry of rotating gas disks for the detection of supermassive black holes in galactic nuclei

Abstract

We measure the black hole mass in the nearby active galaxy Centaurus A (NGC 5128) using a new method based on spectroastrometry of a rotating gas disk. The spectroastrometric approach consists in measuring the photocenter position of emission lines for different velocity channels. In a previous paper we focused on the basic methodology and the advantages of the spectroastrometric approach with a detailed set of simulations demonstrating the possibilities for black hole mass measurements going below the conventional spatial resolution. In this paper we apply the spectroastrometric method to multiple longslit and integral field near infrared spectroscopic observations of Centaurus A. We find that the application of the spectroastrometric method provides results perfectly consistent with the more complex classical method based on rotation curves: the measured BH mass is nearly independent of the observational setup and spatial resolution and the spectroastrometric method allows the gas dynamics to be probed down to spatial scales of ∼0.02 , i.e. 1/10 of the spatial resolution and ∼1/50 of BH sphere of influence radius. The best estimate for the BH mass based on kinematics of the ionised gas is then log(M BH sin i 2 /M ) 7.5 ± 0.1 which corresponds to M BH = 9.6 +2.5 −1.8 × 10 7 M for an assumed disk inclination of i = 35• . The complementarity of this method with the classic rotation curve method will allow us to put constraints on the disk inclination which cannot be otherwise derived from spectroastrometry. With the application to Centaurus A, we have shown that spectroastrometry opens up the possibility of probing spatial scales smaller than the spatial resolution, extending the measured M BH range to new domains which are currently not accessible: smaller BHs in the local universe and similar BHs in more distant galaxies.

Supermassive Black Holes with High Accretion Rates in Active Galactic Nuclei. VIII. Structure of the Broad-line Region and Mass of the Central Black Hole in Mrk 142

“…Unfortunately, the objects with both RM monitoring and the other independent measurements are still extremely few (Peterson 2014). On the other hand, new techniques such as spectro-astrometry (Gnerucci et al 2010;Stern et al 2015) and spectro-interferometry (Kraus 2012;Petrov et al 2012) are in the process of development with the purpose of spatially resolving gas dynamics surrounding the central BHs using the current ground-based 10m class telescopes (Gnerucci et al 2011(Gnerucci et al , 2013Rakshit et al 2015). Hopefully, in the near future, there will be sufficient data sample with independent mass measurements that allow us to explore the systematic errors of our dynamical models.…”

X-ray spectra of hot accretion flows

“…To reduce the above limitations, we gathered from literature the values of the intrinsic X-ray slopes from detailed models of broad-band data sets (including Integral, I, and/or Suzaku, S, data) of several well studied, nearby objects: NGC 4258 (S, Yamada et Brenneman et al 2012). Crucially for precision of λ 2−10 , the first three have M estimated directly from gas or maser kinematics; for Circinus we use M = (1.7 ± 0.3) × 10 6 M ⊙ (Greenhill et al 2003), for Cen A, M = 9.6 +2.5 −1.8 × 10 7 M ⊙ (Gnerucci et al 2011), and for NGC 4258, M = 3.6 × 10 7 M ⊙ (Miyoshi et al 1995;uncertainty not reported). For the remaining two, M is found from reverberation mapping, M = 6.7(±2.6) × 10 7 M ⊙ for NGC 5548 (Peterson et al 2004) and M = 4.6(±0.6) × 10 7 M ⊙ for NGC 4151 (Bentz et al 2006); the estimated errors do not take into account much larger systematic uncertainties.…”

Past, Present, and Future of the Scaling Relations of Galaxies and Active Galactic Nuclei

“…Spectro-astrometry is another promising tool: the approach is based on the different photocenter positions of emission lines at different velocities (Gnerucci et al, 2011). Although a relatively modest spectral resolution ( ∼ 10 km s −1 ) is sufficient, sub-arcsec spatial resolution is required, obviously the higher the better, achievable only from space or from ground using active optics (Abuter et al, 2021).…”