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

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

Research output: Contribution to journalArticle

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Abstract

We determine the morphological mix of field galaxies down to mI ≃ 24.25 mag (mB ∼ 26.0 mag) from a single ultradeep Hubble Space Telescope wide field planetary camera (WFPC2) image in both the V606 and the I814 filters. In total, we find 227 objects with m1 ≤ 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 bJ = 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 ΔI ∼ 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.

Original languageEnglish (US)
JournalAstrophysical Journal
Volume449
Issue number1 PART 2
StatePublished - Aug 10 1995

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Hubble Space Telescope
galaxies
population type
luminosity
star formation
cosmology
filter
cameras
time measurement
slopes
filters
normalisation

Keywords

  • Galaxies: elliptical and lenticular, cD
  • Galaxies: evolution
  • Galaxies: irregular
  • Galaxies: luminosity function, mass function
  • Galaxies: spiral

ASJC Scopus subject areas

  • Space and Planetary Science

Cite this

The morphological mix of field galaxies to mI = 24.25 magnitudes (bJ ∼ 26 magnitudes) from a deep Hubble Space Telescope WFPC2 image. / Driver, Simon P.; Windhorst, Rogier; Ostrander, Eric J.; Keel, William C.; Griffiths, Richard E.; Ratnatunga, Kavan U.

In: Astrophysical Journal, Vol. 449, No. 1 PART 2, 10.08.1995.

Research output: Contribution to journalArticle

Driver, Simon P. ; Windhorst, Rogier ; Ostrander, Eric J. ; Keel, William C. ; Griffiths, Richard E. ; Ratnatunga, Kavan U. / The morphological mix of field galaxies to mI = 24.25 magnitudes (bJ ∼ 26 magnitudes) from a deep Hubble Space Telescope WFPC2 image. In: Astrophysical Journal. 1995 ; Vol. 449, No. 1 PART 2.
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abstract = "We determine the morphological mix of field galaxies down to mI ≃ 24.25 mag (mB ∼ 26.0 mag) from a single ultradeep Hubble Space Telescope wide field planetary camera (WFPC2) image in both the V606 and the I814 filters. In total, we find 227 objects with m1 ≤ 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 bJ = 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 ΔI ∼ 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.",
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T1 - The morphological mix of field galaxies to mI = 24.25 magnitudes (bJ ∼ 26 magnitudes) from a deep Hubble Space Telescope WFPC2 image

AU - Driver, Simon P.

AU - Windhorst, Rogier

AU - Ostrander, Eric J.

AU - Keel, William C.

AU - Griffiths, Richard E.

AU - Ratnatunga, Kavan U.

PY - 1995/8/10

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N2 - We determine the morphological mix of field galaxies down to mI ≃ 24.25 mag (mB ∼ 26.0 mag) from a single ultradeep Hubble Space Telescope wide field planetary camera (WFPC2) image in both the V606 and the I814 filters. In total, we find 227 objects with m1 ≤ 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 bJ = 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 ΔI ∼ 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.

AB - We determine the morphological mix of field galaxies down to mI ≃ 24.25 mag (mB ∼ 26.0 mag) from a single ultradeep Hubble Space Telescope wide field planetary camera (WFPC2) image in both the V606 and the I814 filters. In total, we find 227 objects with m1 ≤ 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 bJ = 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 ΔI ∼ 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.

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KW - Galaxies: evolution

KW - Galaxies: irregular

KW - Galaxies: luminosity function, mass function

KW - Galaxies: spiral

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