The morphological mix of field galaxies to I=24.25 magnitudes (b=26 magnitudes) from a deep Hubble Space Telescope WFPC2 image

Simon P. Driver; Rogier A. Windhorst; Eric J. Ostrander; William C. Keel; Richard E. Griffiths; Kavan U. Ratnatunga

doi:10.1086/309637

paper

The morphological mix of field galaxies to I=24.25 magnitudes (b=26 magnitudes) from a deep Hubble Space Telescope WFPC2 image

arXiv:astro-ph/9511132 · doi:10.1086/309637

Abstract

We determine the morphological mix of field galaxies down to $m_{I} ≃ 24.25$ mag ( $m_{B} \sim 26.0$ mag) from a single ultradeep HST WFPC2 image in both the $V_{606}$ and $I_{814}$ filters. In total, we find 227 objects with $m_{I} \leq 24.5$ 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 to simple models in a standard flat cosmology. We find that both the elliptical and early-type spiral number counts are well described by {\it 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 {\it major} star-formation epoch in the giant galaxy populations to $z \geq 0.8$ . 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 either a modified {\it local} dwarf-rich LF, and/or strong evolution acting on the {\it local} LF. For the dwarf-rich case, a {\it steep} faint-end Schechter-slope ( $α ≃ - 1.8$ ) is required plus a five-fold increase in the dwarf normalization. For a purely evolving model based on a {\it flat} Loveday {\it et al.} (1992) LF ( $α ≃ - 1.0$ ), a ubiquitous starburst of $Δ I \sim$ 2.0 mag is needed at z $≃ 0.5$ for the {\it entire} late-type population. We argue for a combination of these possibilities, and show that for a steep Marzke {\it et al.} (1994) LF ( $α ≃ - 1.5$ ), a starburst of $\sim$ 1.3 mag is required

9 pages, 3 figures (2 colour). The figures are available at http://www.phys.unsw.edu.au/~spd/bib.html

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Are disappearing dwarfs just lying low ?

The morphological mix of field galaxies to I=24.25 magnitudes (b=26 magnitudes) from a deep Hubble Space Telescope WFPC2 image

Abstract

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