On the Origin of Exponential Disks at High Redshift

The major axis and ellipse-fit intensity profiles of spiral galaxies largerthan 0.3'' in the Hubble Space Telescope Ultra Deep Field (UDF) are generallyexponential, whereas the major axis profiles in irregular disk galaxies, calledclump-clusters in our previous studies, are clearly not. Here we show that thedeprojected positions of star-forming clumps in both galaxy types areexponential, as are the deprojected luminosity profiles of the total emissionsfrom these clumps. These exponentials are the same for both types whennormalized to the outer isophotal radii. The results imply that clumps form oraccrete in exponential radial distributions, and when they disperse they formsmooth exponential disks. The exponential scale lengths for UDF spirals average1.5 kpc for a standard cosmology. This length is smaller than the average forlocal spirals by a factor of 2. Selection effects that may account for thissize difference among spirals are discussed. Regardless of these effects, themere existence of small UDF galaxies with grand-design spiral arms differssignificantly from the situation in local fields, where equally small disks areusually dwarf Irregulars that rarely have spiral arms. Spiral arms require adisk mass comparable to the halo mass in the visible region -- something localspirals have but local dwarfs Irregulars do not. Our UDF result then impliesthat galaxy disks grow from the inside out, starting with a dense halo anddense disk that can form spiral arms, and then adding lower density halo anddisk material over time. Bars that form early in such small, dense, gas-richdisks should disperse more quickly than bars that form later in fully developeddisks.Comment: 15 pages, 7 figures, ApJ 634, Nov. 20, in pres