Used Hubble to study in detail the morphologies and colors of more than 100 radio-loud AGN. Found that host galaxies are luminous ellipticals with a very narrow range in intrinsic luminosity. Established that the host galaxy luminosity is independent of the nuclear power. Determined first constraints on evolution of host galaxies.
Key papers:
HST Observations of the Host Galaxies of BL~Lacertae Objects C. M. Urry, R. Falomo, R. Scarpa, J. E. Pesce, A. Treves, and M. Giavalisco, 1999, ApJ, 512, 88
The HST Survey of BL~Lacertae Objects. I. Surface Brightness Profiles, Magnitudes, and Radii of Host Galaxies R. Scarpa, C. Megan Urry, R. Falomo, J. E. Pesce, and A. Treves, 2000, ApJ, 532, 740-815
The HST Survey of BL~Lacertae Objects. II. Host Galaxies C. M. Urry, R. Scarpa, M. O’Dowd, R. Falomo, J. E. Pesce, and A. Treves, 2000, ApJ, 532, 816-829
The HST Survey of BL Lacertae Objects. III. Morphological Properties of Low-Redshift Host Galaxies R. Falomo, R. Scarpa, A. Treves, and C. M. Urry, 2000, ApJ, 542, 731-739
The HST Survey of BL Lacertae Objects. IV. Infrared Imaging of Host Galaxies R. Scarpa, C. M. Urry, P. Padovani, D. Calzetti, and M. O’Dowd, 2001, ApJ, 544, 258-268
Host Galaxy Evolution in Radio-Loud AGN M. O’Dowd and C. M. Urry, 2004, ApJ, 627, 97-124