Narrow band imaging close to the nuclei of BL Lac objects

Abstract

A number of workers have discovered extended gaseous emission in the vicinity of AGNs of various types. Wilson (e.g., 1986) has investigated excited gas close to the cores of Seyfert galaxies, on length scales similar to those we might hope to see in BL Lacs. Stockton and MacKenty (e.g., 1987) have discovered extended structure around QSOs at correspondingly greater distances from the active nucleus.

The aim of our work is to investigate the relationship between sub-classes of AGN by comparing the incidence, morphology and ionization state of extended emission close to the nuclei of BL Lacs with that already found for Seyfert galaxies and quasars.

Using the Canada-France-Hawaii 3.6 meter telescope, the nuclear regions of 6 BL Lac objects were imaged using 70 Å wide interference filters whose bandpasses were centered on the redshifted wavelengths of Hα and [O III]. The objects were also observed through a 300 Å wide continuum filter. For comparison, similar observations were made of a Seyfert galaxy and a quasar, both known to exhibit extended gaseous emission.

Data reduction is not yet complete but our preliminary results indicate that one of the six BL Lacs we observed displayed extended ionized emission. The others, with one possible exception still under investigation, had extended emission whose morphology was the same in the Hα and [O III] lines as in the continuum, indicating its origin in the underlying galaxy.