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 and 850 μm photometric observations of PDS 456, the most luminous QSO in the local universe (z < 0.3). The 0″.6 resolution K-band image obtained using the Keck Telescope shows three compact m_K ∼ 16.5 (M _K ∼ -21) sources at a projected distance of ∼10 kpc to the southwest, and the host galaxy of PDS 456 may be interacting or merging with one or more companions. Observations using the Owens Valley Radio Observatory millimeter array have revealed a narrow CO (1-0) line (FWHM = 181 km s ^-1) centered at z = 0.1849, and 9 × 10⁹ M _⊙ of molecular gas mass is inferred. The radio continuum luminosity is nearly an order of magnitude larger than expected from its FIR luminosity, and the radio source, unresolved by the 2″ beam of the Very Large Array, is dominated by active galactic nucleus (AGN) activity. Our 850 μm photometric observations suggest that the cold dust content of the host galaxy is less than one-half the amount in Arp 220. The analysis of the spectral energy distribution reveals both a QSO-like and a ULIRG-like nature, and the observed IR, X-ray, and gas properties suggest that the AGN activity dominates its luminosity. PDS 456 displays many characteristics expected of an object undergoing a transition from an ultraluminous infrared galaxy (ULIRG) to a classical QSO phase as proposed by Sanders et al., including an optical spectrum dominated by broad emission lines, large X-ray and IR luminosity, a large cold gas/dust content, and an extremely large L_FIR/M _H2 ratio (≳100 L_⊙/M_⊙). ULIRGs and IR QSOs form a broad continuous track in the "star formation efficiency" plot in a manner consistent with the ULIRG-QSO transition scenario, relating the evolution in the dust-processed luminosity to the available fuel (gas and dust) supply. However, the location of PDS 456 is clearly offset from the apparent track traced by the ULIRGs and IR QSOs on this plot. Therefore, PDS 456 appears to be a rare, exceptional object, and the duration of the physical process governing its present properties must be short compared with the length of the luminous QSO phase.