PKS1413 + 135: a BL Lac object in a disc galaxy

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Abstract
As part of a programme to study the host galaxies of BL Lac objects, we have recently obtained deep B-, V-, R-, I- and K-band images of PKS1413 + 135. The K-band image is dominated by the unresolved BL Lac core, although the surrounding galaxy is visible. The optical images are completely dominated by the host galaxy. The radial surface brightness distributions in R and I are best fitted by a combination of a large exponential disc and a small central bulge. The disc/bulge ratio is ~ 4 – 7 and the colours of the host galaxy are quite red so we tentatively classify it as an S0b–c galaxy (using the DDO system). This is the first BL Lac object for which there is conclusive evidence for an underlying disc galaxy and only the second object for which a disc-type host has been reported at all. In the ‘unified schemes’ for active galaxies, BL Lac objects are generally thought to be double radio sources seen end-on. Since no powerful double radio sources have yet been associated with disc-dominated galaxies, our observations do not fit in easily with the currently accepted unified schemes.

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