1995/08/10 by Simon P. Driver, Rogier A. Windhorst, E. J. Ostrander +6 · 7 citations
Physics and Astronomy · #Advanced Camera for Surveys #Astronomy and Astrophysical Research #Astrophysics #Cosmology #Elliptical galaxy #Galaxies: Formation, Evolution, Phenomena #Galaxy #Hubble space telescope #Physics #Spiral galaxy #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/309637
published as The Astrophysical Journal 449 (1995) L23-L27 · 9 pages, 3 figures (2 colour). The figures are available at http://www.phys.unsw.edu.au/~spd/bib.html
openalex publication_date 1995/08/10 · arxiv created 1995/11/28 · openalex created_date 2016/06/24 · arxiv updated 2016/08/30 · openalex updated_date 2026/08/05
We determine the morphological mix of field galaxies down to m I ≃ 24.25 mag ( m B ~ 26.0 mag) from a single ultradeep Hubble Space Telescope wide field planetary camera (WFPC2) image in both the V 606 and the I 814 filters. In total, we find 227 objects with m I ≤ 24.5 mag and classify these into three types: ellipticals (16%), early-type spirals (37%), and late-type spirals/irregulars (47%). The differential number counts for each type are compared with simple models in a standard flat cosmology. We find that both the elliptical and the early-type spiral number counts are well described by little-or-no-evolution models, but only when normalized at b J = 18.0 mag. Given the uncertainties in the luminosity function (LF) normalization, both populations are consistent with a mild evolutionary scenario based on a normal/low rate of star formation. This constrains the end of the last major star formation epoch in the giant galaxy populations to z ≥ 0.8. Conversely, the density of the observed late-type/irregular population is found to be a factor of 10 in excess of the conventional no-evolution model. This large population might be explained by a modified local dwarf-rich LF and/or strong evolution acting on the local LF. For the dwarf-rich case, a steep faint-end Schechter slope (α ≃ -1.8) is required, plus a fivefold increase in the dwarf normalization. For a purely evolving model based on a flat Loveday et al. LF (α ≃ -1.0), a ubiquitous starburst of Δ I ~ 2.0 mag is needed at z ≃ 0.5 for the entire late-type population. We argue for a combination of these possibilities, and show that for a steep Marzke et al. LF (α ≃ -1.5) a starburst of ~1.3 mag is required at z ≃ 0.5 in the entire late-type population, or ~2.0 mag in ~20% of the population.