The Θ-z relation for HST¹ bulges and disks out to z ≃ 0.8

We present HST scale lengths and ground-based redshifts for 63 faint field galaxies down to 1 ≲ 21.5 mag from the Medium-Deep Survey. These have measured redshifts z ≲ 0.8 and half-light radii 0″.1 ≲ r_e, r_s ≲ 5″. We present the Θ-z relation for r^1/4-bulges and exponential disks separately for world models with q₀ = 0.0-1.0. We show that selection against low surface brightness galaxies in the HST images and ground-based spectra is comparable to that in local surveys. We compare our HST disk-dominated galaxies to a magnitude-limited subsample of the ESO-Uppsala local spirals. Extrapolating the best-fit local exponential disk scale length (r_s = 3.5 kpc for H₀ = 75) out to z = 0.8, we find a rather symmetrical distribution of HST disks around this value. This implies that galaxy disks have been stable since z ≃ 0.8. We compare our HST bulge-dominated galaxies with r^1/4-profiles to a magnitude-limited subsample of local Seven Samurai ellipticals. The latter have a local best fit r_e = 5.7 kpc. Our HST bulge sample shows a similar distribution for z ≲ 0.8. Elliptical galaxy scale lengths have thus also remained rather constant since z ≲ 0.8. We set limits to their possible evolution.