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Register a new userBarred Galaxies at z > 0.7: NICMOS HDFN Observations
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Previous optical studies found an unexpected deficit of bars at z > 0.7. To investigate the effects of bandshifting, we have studied the fraction of barred spirals in the NICMOS Deep Field North. At z > 0.7 we find at least four barred spirals, doubling the number previously detected. The number of barred galaxies is small because these (and previous) data lack adequate spatial resolution. A typical 5 kpc bar at z > 0.7 is only marginally detectable for WFPC2 at 0.8microns; the NICMOS data have even lower resolution and can only find the largest bars. The average size of the four bars seen at z > 0.7 is 12 kpc. The fraction of such large bars (4/95) is higher than that seen in nearby spirals (1/44); all known selection effects suggest that the observed fraction is a lower limit. However, important caveats such as small numbers and difficulties in defining comparable samples at high and low redshifts should be noted. We conclude that there is no significant evidence for a decrease in the fraction of barred spirals beyond z ~ 0.7.
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We perform a quantitative morphological comparison between the hosts of Active Galactic Nuclei (AGN) and quiescent galaxies at intermediate redshifts (z~0.7). The imaging data are taken from the large HST/ACS mosaics of the GEMS and STAGES surveys. Our main aim is to test whether nuclear activity at this cosmic epoch is triggered by major mergers. Using images of quiescent galaxies and stars, we create synthetic AGN images to investigate the impact of an optical nucleus on the morphological analysis of AGN hosts. Galaxy morphologies are parameterized using the asymmetry index A, concentration index C, Gini coefficient G and M20 index. A sample of ~200 synthetic AGN is matched to 21 real AGN in terms of redshift, host brightness and host-to-nucleus ratio to ensure a reliable comparison between active and quiescent galaxies. The optical nuclei strongly affect the morphological parameters of the underlying host galaxy. Taking these effects into account, we find that the morphologies of the AGN hosts are clearly distinct from galaxies undergoing violent gravitational interactions. In fact, the host galaxies distributions in morphological descriptor space are more similar to undisturbed galaxies than major mergers. Intermediate-luminosity (Lx < 10^44 erg/s) AGN hosts at z~0.7 show morphologies similar to the general population of massive galaxies with significant bulges at the same redshifts. If major mergers are the driver of nuclear activity at this epoch, the signatures of gravitational interactions fade rapidly before the optical AGN phase starts, making them undetectable on single-orbit HST images, at least with usual morphological descriptors. This could be investigated in future synthetic observations created from numerical simulations of galaxy-galaxy interactions.
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