The field of AGN host galaxies has seen great progress over the last few years, and is the most promising approach to understand which phenomenon triggers nuclear activity. Type-1 QSOs have typically been studied with HST, because any bright seeing-enlarged central light source swamps the light of the host galaxy, making the delicate subtraction of the overpowering AGN source impossible. Adaptive optics may become an alternative ground-based technique, but suffer currently still from significant and variable wings in their PSF. Type-2 AGN appear like pure host galaxies in the optical and NIR and are hence trivial to examine.
Kukula et al. (2001) and Dunlop et al. (2003) targetted type-1 QSOs of luminosity MV < - 23.5 with NICMOS at z = [0.9, 1.9] and with WFPC2 at z < 0.2, respectively. These monochromatic images showed host galaxies to be normal giant ellipticals obeying the normal Kormendy relation expected at their redshifts.
Kauffmann et al. (2003) examined type-2 AGN at z < 0.3 in the SDSS spectroscopic sample and found normal-looking early-type (spheroidal) galaxies as well as some disks and disturbed systems. At lowest AGN luminosities these exhibit the usual red colours expected in early-type galaxies, but at higher luminosity they show a mild excess of blue star light compared to non-AGN early-type objects. Detailed diagnostics using the H absorption line suggest that most of them experienced a starburst within the previous Gyr.
Most recently, two studies on low-luminosity type-1 AGN (MV = [- 24, - 20]) measured not only shapes but also colours of type-1 hosts. Jahnke et al. investigated a sample at z < 0.2 and Sanchez et al. used a z = [0.5, 1.1] sample from the GEMS (Galaxy Evolution from Morphologies and SEDs) survey. They both found mostly spheroidal morphologies but also some disks. Again, many early-types showed a moderate excess of blue star light compared to non-AGN.
These studies appear to show consistently, that AGN live mostly (but not exclusively) in 'young bulges', i.e. mostly large ellipticals with a moderate excess of blue stellar light compared to non-AGN ellipticals. This suggests that the AGN phenomenon is accompanied by some amount of star formation, which is either still ongoing or has predated the AGN phase as a recent starburst. This picture is not surprising, given that a powerful AGN requires both a massive black hole and abundant fuel supply to operate. While only massive early-type galaxies would contain such black holes, only galaxies with significant recent or ongoing star formation provide the fuel supply.