Leipski, C. and Haas, M. and Siebenmorgen, R. and Meusinger, H. and Albrecht, M. and Cesarsky, C. and Chini, R. and Cutri, R. and Drass, H. and Huchra, J. P. and Ott, S. and Wilkes, B. J. (2007) The reddest ISO-2MASS quasar. Astronomy and Astrophysics, 473 (1). pp. 121-128. ISSN 0004-6361 http://resolver.caltech.edu/CaltechAUTHORS:20100113-100848475
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Context. While there is growing consensus on the existence of numerous dust-enshrouded red quasars, their discovery and detailed exploration is still an observational challenge. In the course of the near-mid-infrared AGN search combining the 6.7 μm ISOCAM Parallel Survey and 2MASS we have discovered 24 type-1 quasars about a third of which are too red to be discriminated by optical/UV search techniques. Aims. Here we report on a detailed case study of the reddest type-1 quasar of our sample, 2MASS J23410667−0914327 (for short J2341) at redshift z = 0.236 with M_K = −25.8 and J − K_s = 1.95. This source has a very red optical appearance but lacks the far-infrared emission typically seen in known dust enshrouded AGN. Therefore we here explore its enigmatic nature. Methods. We performed spectroscopy in the optical with VLT/FORS1 and in the mid-infrared (5–38 μm) with the Spitzer Space Telescope as well as near-infrared (NIR) imaging with ISPI at the CTIO 4m-telescope. To explain the red optical continuum of the quasar we examine nuclear dust reddening of an intrinsically blue quasar continuum in combination with dilution by stellar light of the host galaxy. Results. The optical and NIR observations reveal a star forming emission-line galaxy at the same redshift as the quasar with a projected linear separation of 1".8 (6.7 kpc). The quasar and its companion are embedded in diffuse extended continuum emission. Compared with its companion the quasar exhibits redder optical-NIR colours, which we attribute to hot nuclear dust. The mid-infrared spectrum shows only few emission lines superimposed on a power-law spectral energy distribution typically seen in type-1 AGN. However, the lack of strong far-infrared emission suggests that our potentially interacting object contains much less gas and dust and is in a stage different from dust reddened ULIRG-AGN like Mrk 231. The optical spectrum shows signatures for reddening in the emission-lines and no post-starburst stellar population is detected in the host galaxy of the quasar. The optical continuum emission of the active nucleus appears absorbed and diluted. Conclusions. Even the combination of absorption and host dilution is not able to match J2341 with standard quasar templates.While the BLR shows only a rather moderate absorption of E_(B−V) = 0.3, the continuum shorter than 4500 Å requires strong obscuration with E_(B−V) = 0.7, clearly exceeding the constraints from the low upper limit on the silicate 9.7 μm absorption. This leads us to conclude that the continuum of the quasar J2341 is intrinsically redder than that of typical quasars.
|Additional Information:||© ESO 2007. Received 30 August 2006. Accepted 11 July 2007. Part of this work was supported by Sonderforschungsbereich SFB 591 “Universelles Verhalten gleich-gewichtsferner Plasmen” der Deutschen Forschungsgemeinschaft, and by Nordrhein-Westfälische Akademie der Wissenschaften. We thank Dan Weedman for kindly providing the Spitzer spectrum of Mrk 231. We also thank Andy Robinson for his helpful comments on the manuscript. Based on observations with the Infrared Space Observatory ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and UK) and with the participation of ISAS and NASA. The Two Micron All Sky Survey is a joint project of the University of Massachusetts and IPAC/Caltech, funded by the National Aeronautics and Space Administration and the National Science Foundation.|
|Subject Keywords:||galaxies : active; quasars : general; infrared : galaxies|
|Official Citation:||The reddest ISO-2MASS quasar C. Leipski, M. Haas, R. Siebenmorgen, H. Meusinger, M. Albrecht, C. Cesarsky, R. Chini, R. Cutri, H. Drass, J. P. Huchra, S. Ott and B. J. Wilkes A&A 473 (1) 121-128 (2007) DOI: 10.1051/0004-6361:20066323|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Tony Diaz|
|Deposited On:||25 Jan 2010 18:59|
|Last Modified:||26 Dec 2012 11:41|
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