A quantitative study of peculiarities in galaxy morphology

I have developed a refined version of the asymmetry parameter first presented by Abraham et al. (1996). Coupled with a simple concentration index, this pair of indices, AW-CW, is compared to Abraham et al.'s log(AA)-log(C A) algorithm. These indices are then applied to a large sample of galaxies in the Hubble Deep Field with photometric redshifts from Connolly (1999). This allows investigation of trends at redshifts which have been largely inaccessible until now. The final sample consists of ∼350 objects detected by the AW-CW algorithms with first-moment radii, r1 ≥ 0″.2 (corresponding to F814WAB ∼ 26).; The distribution of objects in the AW-CW plane is found to change with redshift. The region of the AW -CW plane populated by objects out to z ∼ 1.2 is roughly constant. Accounting for bandpass shifting effects, this region resembles that populated by local bright galaxies (Frei et al. 1996) cosmologically simulated at z = 0.8.; Beyond z ∼ 1.2, the distribution of galaxies undergoes a significant change. Highly concentrated objects disappear from the AW-C W plane, while high asymmetry objects appear. The high concentration objects below z ∼ 1.2 all have early-type morphologies. Very low resolution SEDs of these galaxies created using colors from Williams et al. (1996) are used to calculate the expected appearance of these objects at higher redshifts (z = 1.5 and 2.3). Assuming their luminosities and SEDs have not changed, one-third of the z ∼ 0.8 objects would be undetected at z = 1.5, and 97% would disappear by z = 2.3. Any true drop in the number density of these objects would be completely masked by such strong selection effects.; An increase in the number and degree of asymmetric objects is also seen beyond z ∼ 1.2. Many of these objects exhibit multiple condensations, consistent with merging. Though the shifting of the ultraviolet galaxy spectrum into the F814W-band may be partially responsible, the majority of the increase in both number and degree of asymmetry appears to reflect a genuinely higher number density of asymmetric objects at earlier epochs.; AW and CW are shown to be useful tools for exploring galaxy morphology over a wide range of redshifts. Future refinements combined with better SEDs and evolutionary models will undoubtedly reveal valuable insight into galaxy evolution.