Gas Content and Efficiency of AGN Feedback in Low-redshift Quasars

The interstellar medium content is crucial to understand the physicsof active galaxies and the coevolution between supermassive black holes andtheir host galaxies.  In a popular scenario, major mergers of gas-rich galaxiesfuel vigorous star formation and obscured (type 2) quasar activity until energy feedback from the quasar clears away the gas and dust to reveal an unobscured(type 1) quasar.  Under this scenario, the precursor type 2 quasars should bemore gas-rich than their type 1 counterparts, and both types of quasars areexpected to be gas-deficient relative to normal, star-forming galaxies ofsimilar stellar mass.  However, direct gas measurements are time-consuming, andnot without their own limitations and uncertainties.  Dust provides an efficientand effective indirect probe of the total (atomic and molecular) gas content ofgalaxies, active or not.  We apply this technique to a large sample of quasars,whose total gas content would be prohibitively expensive, if not presentlyimpossible, to measure.  I will introduce a comprehensive study of the full (1to 500 micron) infrared spectral energy distributions of low-redshift (z<0.5)type 1 and type 2 matched quasar samples, using photometric measurementsobtained from 2MASS, WISE, and Herschel, combined with Spitzer mid-infrared (5to 40 micron) spectra. With a newly developed Bayesian Markov chain Monte Carlofitting method, we quantitatively decompose various overlapping contributions tothe integrated spectral energy distribution, including starlight, hot and warmdust from the central engine and torus, and cooler dust emanating from largerscales in the host galaxy.  This procedure yields a robust total dust mass,which we use to infer the total gas mass, using a gas-to-dust ratio constrainedby the host galaxy stellar mass.  Contrary to expectations, the gas content ofthe host galaxies of type 2 quasars is nearly indistinguishable from that oftype 1 quasar hosts, and neither type exhibits the predicted deficit in gasrelative to normal galaxies.  The gas mass fraction of quasar hosts appearsunaffected by the bolometric luminosity, although their interstellar radiationfield is preferentially higher than that of normal galaxies, potentiallyimplicating the heating of the large-scale galactic dust from the central engine. 

Speaker: 
Shangguan Jinyi
Location: 
1st Seminar Room, KIAA-PKU
Time: 
Friday, March 23, 2018 - 5:00pm to 6:00pm