2007A&A...473..121L

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. 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. We performed spectroscopy in the optical with VLT/FORS1 and in the mid-infrared (5-38µm) 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. 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" (6.7kpc). 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 Mrk231. 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. 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 4500Å requires strong obscuration with E_B–V=0.7, clearly exceeding the constraints from the low upper limit on the silicate 9.7µm absorption. This leads us to conclude that the continuum of the quasar J2341 is intrinsically redder than that of typical quasars.