Hubble Space Telescope observations of the BL Lac object PKS Formula: the host galaxy revealed

Abstract

In an earlier paper, we showed that the BL Lac object PKS |$1413+135$| appears to lie in a disc-dominated galaxy with colours closer to those of an elliptical than a spiral galaxy. Subsequently, a large neutral hydrogen absorption feature was found at the optical redshift of the proposed host galaxy together with a substantial absorbing column in the Einstein X-ray spectrum, and so it was concluded that the galaxy is a spiral. If the BL Lac object does lie in a spiral galaxy, then there are severe difficulties for the standard unified schemes of active galaxies, as the putative parent population of BL Lacs – low-luminosity radio galaxies – never lie in spirals. In this paper, we present a Hubble Space Telescope (HST) observation which shows that the galaxy contains a previously unresolved dust lane and is, indeed, very likely to be an early-type spiral galaxy viewed edge-on. This observation explains both the large absorbing column and the rather unusual optical colours. The unified schemes may be saved if the BL Lac is actually a background object, gravitationally lensed by the apparent host galaxy. However, from the HST image we can derive the projected light distribution and, for all reasonable mass-to-light ratios, this matter distribution would produce multiple images of a background BL Lac object. As these extra images are not seen in either the optical or radio data, we conclude that PKS |$1413+135$| probably does lie in the centre of a spiral galaxy. We discuss the possible implications of this result.