Multiwavelength Observations of the Gas‐rich Host Galaxy of PDS 456: A New Challenge for the ULIRG‐to‐QSO Transition Scenario

We report new K-band, radio continuum, and CO (1-0) imaging observations and850 micron photometric observations of PDS 456, the most luminous QSO in thelocal universe (z<0.3). The 0.6'' resolution K-band image obtained using theKeck telescope shows three compact K~16.5 (M(K)~ -21) sources at a projecteddistance of ~10 kpc to the southwest, and the host galaxy of PDS 456 may beinteracting or merging with one or more companions. The observations using theOVRO millimeter array has revealed a narrow CO (1-0) line (FWHM = 181 km/s)centered at z=0.1849, and 9 x 10^9 solar mass of molecular gas mass isinferred. Radio continuum luminosity is nearly an order of magnitude largerthan expected from its FIR luminosity, and the radio source, unresolved by the2" beam of the VLA, is dominated by the AGN activity. Our 850 micronphotometric observations suggest that the cold dust content of the host galaxyis less than one half of the amount in Arp 220. Its SED has both a QSO-like anda ULIRG-like nature, and the observed IR, X-ray, and gas properties suggestthat the AGN activity dominates its luminosity. ULIRGs and IR QSOs form a broadcontinuous track in the ``star formation efficiency'' plot in the mannerconsistent with the ULIRG-QSO transition scenario, relating the evolution inthe dust processed luminosity with the available fuel (gas and dust) supply.However, the location of PDS 456 is clearly offset from the apparent tracktraced by the ULIRGs and IR QSOs on this plot, and PDS 456 appears to be arare, exceptional object, and the duration of the physical process governingits present properties must be short compared with the length of the luminousQSO phase.Comment: 17 pages including 8 figures, Latex, uses emulateapj5.sty and apjfonts.sty, to appear in the Astrophysical Journal (2004