Morphology and evolution of emission-line galaxies in the Hubble ultra deep field

We investigate the properties and evolution of a sample of galaxies selected to have prominent emission lines in low-resolution grism spectra of the Hubble Ultra Deep Field (HUDF). These objects, eGRAPES, are late-type blue galaxies characterized by small proper sizes (R₅₀ ≤ 2 kpc) in the 4350 ̊ rest frame, low masses (5 × 10⁹ M _⊙), and a wide range of luminosities and surface brightnesses. The masses, sizes, and volume densities of these objects appear to change very little up to a redshift of z = 1.5. On the other hand, their surface brightness decreases significantly from z = 1.5 to 0, while their mass-to-light ratio increases twofold. This could be a sign that most low-redshift eGRAPES have a stellar population older than high-redshift eGRAPES and hence that most eGRAPES formed at higher redshifts. The average volume density of eGRAPES is (1.8 ± 0.3) × 10^-3 h₇₀³ Mpc ^-3 at 0.3 < z ≤ 1.5. Many eGRAPES would have been formally classified as luminous compact blue galaxies (LCBGs) if these had been selected based on small physical size, blue intrinsic color, and high surface brightness, while the remainder of the sample discussed in this paper forms an extension of LCBGs toward fainter luminosities.