BH mass - host galaxy scaling relations

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posted by bhusemann on Sun, 08/30/2015 - 18:17

The origin of the tight correlations between the black hole masses and various properties of the galaxies in which they reside is still strongy debated. The existance of such tight correlations can be used as arguments for the self-regulated growth of galaxies and BH through AGN feedback or a simple co-evolution of both driven by the availability of gas. An alternative scenario is that sequences of galaxy mergers within the current cosmological framework will average the BH mass and host galaxy parameters from the initial distribution towards a mean relation just by statistics. Thus, it is important to study the evolution of the BH mass - host galaxy scaling relation with cosmic time, but in particular galaxies with AGN for which we know that the BH are currently in the growing phase.

 

Is the QSO HE 2158-0107 offset from the relation due to gas accretion from the environment?

As part of a large IFU survey of luminous QSOs, we observed the QSO HE 2158-0107 (z~0.2) with the PMAS integral field unit at the Calar Alto Observatory. This QSOs turned out to be unique in the sample as it displays a large tail-like ionized gas structure extended over several tens of kpc from the QSO. In addition, the line ratios are unusual for QSOs as the [NII] line ratios is undetected corresponding to a small [NII]/Ha line ratio which can be interpreted as a sub-solar metallicity gas. Combined with deep SDSS Stripe 82 optical and NTT-Sofi NIR photometry, we recover a compact (re~1.6kpc) bulge-dominated host galaxy with a significantly lower bulge luminosity, stellar mass and also a low velocity dispersion as probed by the line width of the [OIII] line in the QSO spectrum. It is possible that the origin of the offset from the relation is closely related to the current gas accretion event which is likely coming from the environment of the QSO, either smooth gas accretion or minor mergers. Given the very compact size of the galaxy, this galaxy might be a local analouge of typical galaxies at high redshift during an active phase.

More details can be found in this article...

Upper panel: PMAS IFU [OIII] narrow-band image of HE 2158-0107 and associated spectrum around Hb and Ha spectral regions. An ionized gas tail extents beyond the PMAS FoV which appears to have a very low metallicity given the undetection of [NII].

Lower panel: Various measurements of the BH mass - host properties of HE 2158-0107 in comparison to the local relations of inactive galaxies. Apparently, the host galaxy is offset towards higher BH mass.

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.