Locating star-forming regions in quasar host galaxies

• Published in: Monthly notices of the Royal Astronomical Society 2014-02, Vol.438 (1), p.217-239
• Main Authors: Young, J. E., Eracleous, M., Shemmer, O., Netzer, H., Gronwall, C., Lutz, Dieter, Ciardullo, R., Sturm, Eckhard
• Format: Article
• Language: English
• Subjects: Astronomy; Astrophysics; Quasars; Star & galaxy formation; Starbursts
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stt2145

We present a study of the morphology and intensity of star formation in the host galaxies of eight Palomar-Green quasars using observations with the Hubble Space Telescope. Our observations are motivated by recent evidence for a close relationship between black hole growth and the stellar mass evolution in its host galaxy. We use narrow-band [O ii]λ3727, Hβ, [O iii]λ5007 and Paα images, taken with the Wide Field Planetary Camera 2 and NICMOS instruments, to map the morphology of line-emitting regions, and, after extinction corrections, diagnose the excitation mechanism and infer star-formation rates. Significant challenges in this type of work are the separation of the quasar light from the stellar continuum and the quasar-excited gas from the star-forming regions. To this end, we present a novel technique for image decomposition and subtraction of quasar light. Our primary result is the detection of extended line-emitting regions with sizes ranging from 0.5 to 5 kpc and distributed symmetrically around the nucleus, powered primarily by star formation. We determine star-formation rates of the order of a few tens of M yr−1. The host galaxies of our target quasars have stellar masses of the order of 1011 M and specific star-formation rates on a par with those of M82 and luminous infrared galaxies. As such they fall at the upper envelope or just above the star-formation mass sequence in the specific star formation versus stellar mass diagram. We see a clear trend of increasing star-formation rate with quasar luminosity, reinforcing the link between the growth of the stellar mass of the host and the black hole mass found by other authors.