The QSO HE 0450-2958: Scantily Dressed or Heavily Robed? A Normal Quasar as Part of an Unusual ULIRG

Abstract

The luminous z = 0.286 quasar HE 0450-2958 is interacting with a companion galaxy at 6.5 kpc distance and the whole system radiates in the infrared (IR) at the level of an ultraluminous infrared galaxy (ULIRG). A so far undetected host galaxy triggered the hypothesis of a mostly "naked" black hole (BH) ejected from the companion by three-body interaction. We present new Hubble Space Telescope (HST)/NICMOS 1.6 μm imaging data at 0farcs1 resolution and VLT/VISIR 11.3 μm images at 0farcs35 resolution that are for the first time resolving the system in the near- and mid-infrared. We combine these data with existing optical HST and CO maps. (1) At 1.6 μm we find an extension NE of the quasar nucleus that is likely a part of the host galaxy, though not its main body. If true, a combination with upper limits on a main body co-centered with the quasar brackets the host-galaxy luminosity to within a factor of ~4 and places HE 0450-2958 directly onto the M _BH - M _bulge relation for nearby galaxies. (2) A dust-free line of sight to the quasar suggests a low dust obscuration of the host galaxy, but the formal upper limit for star formation (SF) lies at 60 M _sun yr^-1. HE 0450-2958 is consistent with lying at the high-luminosity end of Narrow-Line Seyfert 1 Galaxies, and more exotic explanations like a "naked quasar" are unlikely. (3) All 11.3 μm radiation in the system is emitted by the quasar nucleus. It has warm ULIRG-strength IR emission powered by BH accretion and is radiating at super-Eddington rate, L/L _Edd = 6.2^+3.8 _-1.8, or 12 M _sun yr^-1. (4) The companion galaxy is covered in optically thick dust and is not a collisional ring galaxy. It emits in the far-infrared at ULIRG strength, powered by Arp220-like SF (strong starburst-like). An M82-like SED is ruled out. (5) With its BH accretion rate, HE 0450-2958 produces not enough new stars to maintain its position on the M _BH - M _bulge relation, and SF and BH accretion are spatially disjoint. This relation can either only be maintained averaging over a longer timescale (lsim500 Myr) and/or the bulge has to grow by redistribution of pre-existing stars. (6) Systems similar to HE 0450-2958 with spatially disjoint ULIRG-strength star formation and quasar activity might be common at high redshifts but at z < 0.43 we only find <4% (3/77) candidates for a similar configuration.