Physics
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997hst..prop.7260m&link_type=abstract
HST Proposal ID #7260
Physics
Hst Proposal Id #7260 Agn/Quasars
Scientific paper
In deep ground based CCD observations we discovered that the BL Lacertae object, PKS1413+135 apparently lies in a disc host galaxy. In cycle 2 HST observations we showed that the `host' was a type Sa spiral galaxy. In the conventional `unified schemes' for active galaxies this result is extremely surprising as the parent population of BL Lac objects are thought to be low power radio galaxies which almost all lie in large elliptical galaxies. No similar radio source has yet been found in a spiral galaxy. One way around this problem is to suppose that the BL Lac does not lie in the `host' galaxy, but is a background quasar which is being gravitationally lensed by the apparent host. If the BL Lac does live in the host galaxy then we expect it to be exactly centered. However, a background lensed object will typically lie a fraction of an arcsecond from the center of the lensing galaxy. The BL Lac core has a very steep spectrum and dominates the galaxy in ground based K-band images but is undetectable in the cycle 2 V band image {or ground based R and I band images}. We ask, therefore, for a deep H band image which will detect both the galaxy and core well, to determine the precise location of the BL Lac object relative to the underlying galaxy. If we find the BL Lac to be exactly centred on the underlying galaxy, the conventional `unified schemes' will have to be re-thought.
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