The nature of the HE0450−2958 system

Magain et al. argued that the host galaxy of the quasar in HE0450−2958 is substantially underluminous given the quasar brightness. Using kinematical information from the spectra of the quasar and the companion galaxy, an ultraluminous infrared galaxy, we test the hypothesis that the black hole powering the quasar was ejected from the companion galaxy during a merger. We argue that the ejection model is unlikely, since the kick velocity required to remove the black hole from the galaxy is 500 km s −1 , inconsistent with the presence of narrow emission-line gas at the same redshift as the quasar nucleus. We also show that the quasar in HE0450−2958 has the spectral characteristics of a narrow-line Seyfert 1 galaxy and calculate a mass for its black hole that is roughly an order of magnitude smaller than that estimated by Magain et al. The predicted luminosity of the host galaxy is then consistent with the upper limits inferred by those authors.