Measuring the stellar velocity dispersion and BH mass - sigma relation of luminous QSOs
Direct measurements of MBH are restricted to the local Universe, but unobscured AGN allow to measure MBH with reverberation mapping applying the virial method to the motion of BLR clouds, MBH = f vBLR2 RBLR / G. Here, f is a virial factor that takes the unkown geometric and kinematical structure of the BLR into account and is calibrated such that the MBH - sigma relation for the reverberation-mapped AGN matches those of the local inactive galaxies. Currently, there is a big issue that the stellar velocity dispersion sigma are very difficult to measure in luminous unobscured QSO which host the more massive BH. Thus, the calibration of f for AGN is biassed towards lower BH masses. This is a challenge for understading the BH mass - host relation as a function redshift.
Optical 3D spectroscopy was obtained with VLT-MUSE IFU for the reverberation-mapped QSO PG1307+085 during commissioning. The data was analyzed and a careful spectral QSO-host deblending was performed with QDeblend3D. Although the QSO has a luminosity of Lbol>1046 erg/s and is 10x brighter than the host galaxy, we obtain a S/N~10 for the host galaxy spectrum integrated over the effective radius of the spheroid (re~1.4''). From spectral synthesis modeling we reliably measure a velocity dispersion of sigma=155+-15 km/s which places PG1307+085 on the local MBH-sigma relation. In addtion, a large ionized gas nebula is detected with several bright knots about 10-30kpc away from the nucleus and a faint bridge towards the massive 4L* galaxy at the center of a galaxy group. This suggest that PG1307+085 is currently interacting with its neighboring galaxies. More details on the analysis of the MUSE data can be found in associated article and the reduced data cube is publically available here.
Although PG1307+085 is just a single case, it shows the potential to study the host galaxies of luminous QSO with MUSE given its high sensitivy, spectral resolution, large field-of-view and excellent spatial resolution. Proposals to observe a larger sample of luminous QSOs with massive BHs is currently in review.