EDP Sciences
Free access
Volume 405, Number 3, July III 2003
Page(s) 959 - 967
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20030678

A&A 405, 959-967 (2003)
DOI: 10.1051/0004-6361:20030678

I Zw 1 : Decomposition of the nearby QSO host

J. Scharwächter1, A. Eckart1, S. Pfalzner1, J. Moultaka1, C. Straubmeier1 and J. G. Staguhn2

1  I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
2  NASA/Goddard Space Flight Center, Building 21, Code 685, Greenbelt, MD 20771, USA

(Received 28 January 2003 / Accepted 28 April 2003 )

The paper presents an analysis of the nearby QSO host I Zw 1 based on new J-band imaging data, obtained with ISAAC at the Very Large Telescope (VLT) of the European Southern Observatory (ESO). As one of the best-studied QSOs with exceptional properties lying between those of QSOs, narrow-line Seyfert 1s, and ultraluminous infrared galaxies, I Zw 1 is a prime candidate for a detailed case study of a QSO host. With its high angular resolution and sensitivity, the new J-band image provides strong evidence for an interaction between I Zw 1 and the western companion galaxy. We present a procedure for subtracting the QSO nucleus from the I Zw 1 image to obtain an undisturbed view on the underlying host galaxy. Based on the derived surface brightness profile and the gas rotation curve, the host is classified as a high-surface-brightness Freeman type I galaxy with a central disk surface brightness significantly larger than that of normal spirals. The surface-brightness profile is decomposed into a Plummer bulge and a Kuzmin disk with similar luminosities and a bulge-to-disk scale length ratio of 0.29, comparable to that of nearby spiral hosts with active galactic nuclei (AGN). Different models for the decomposition of the gas rotation curve are discussed. The resulting J-band mass-to-light ratio ( M/L) of $(0.7\pm 0.4)\ M_{\odot}/L_{\odot}$ for the disk component suggests a mean solar-type stellar population with a tendency towards young stars. The bulge exhibits a lower M/L of $(0.4\pm 0.2)\ M_{\odot}/L_{\odot}$, which supports previous findings of enhanced starburst activity in the nuclear and circumnuclear region.

Key words: galaxies: individual: I Zw 1 -- galaxies: active -- galaxies: photometry -- galaxies: structure -- methods: observational

Offprint request: J. Scharwächter, scharw@ph1.uni-koeln.de

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© ESO 2003