TY - JOUR
T1 - Are hubble deep field galaxy counts whole numbers?
AU - Colley, Wesley N.
AU - Rhoads, James E.
AU - Ostriker, Jeremiah P.
AU - Spergel, David N.
N1 - Funding Information:
W. N. C. is most grateful for the continued support of the Fannie and John Hertz Foundation and for partial support from NSF grant AST-9529120. The work of J. E. R. and D. N. S. has been supported by NSF grants AST 91-17388 and NASA ADP grant 5-2567, and the NSF traineeship of J. E. R. has been supported by DGE-9354937. J. E. R. also thanks IPAC for its hospitality. The work of J. P. O. has been partially supported by NSF grant AST-9424416. We would also like to thank Robert H. Lupton, J. Richard Gott III, Warrick Couch, Sangeeta Malhotra, and anonymous referees for their very useful communications. We thank Jill Knapp for her kind support and encouragement. Finally, we thank the HDF team for their hard work and generosity in preparing the data for public release.
PY - 1996
Y1 - 1996
N2 - The Hubble Deep Field (HDF) offers the best view to date of the optical sky at faint magnitudes and small angular scales. Early reports suggested that faint source counts continue to rise to the completeness limit of the data, which implies a very large number of galaxies. In this Letter, we use the two-point angular correlation function and number-magnitude relation of sources within the HDF in order to assess their nature. We find that the correlation peaks between 0″.25 and 0″.4 with amplitude of 2 or greater and is much higher for the smallest objects. This angular scale corresponds to physical scales of order 1 kpc for redshifts z ≳ 1. The correlation must therefore derive from objects with subgalactic separations. At faint magnitudes, the counts satisfy the relation number α1/flux, which is expected for images that are subdivisions of larger ones. Several explanations for these observed correlations are possible, but a conservative explanation can suffice to produce our results. Since high-redshift space (z ≳ 0.5) dominates the volume of the sample, observational redshift effects are important. Rest-frame ultraviolet radiation appears in the HDF's visible and near-UV bands, and surface brightness dimming enhances the relative brightness of unresolved objects versus resolved objects. Both work to increase the prominence of compact star-forming regions over diffuse stellar populations. Thus, a "normal" gas-rich galaxy at high redshift can appear clumpy and asymmetric in the visible bands. For sufficiently faint and distant objects, the compact star-forming regions in such galaxies peak above undetectable diffuse stellar backgrounds. Our results do not exclude asymmetric formation or fragmentation scenarios.
AB - The Hubble Deep Field (HDF) offers the best view to date of the optical sky at faint magnitudes and small angular scales. Early reports suggested that faint source counts continue to rise to the completeness limit of the data, which implies a very large number of galaxies. In this Letter, we use the two-point angular correlation function and number-magnitude relation of sources within the HDF in order to assess their nature. We find that the correlation peaks between 0″.25 and 0″.4 with amplitude of 2 or greater and is much higher for the smallest objects. This angular scale corresponds to physical scales of order 1 kpc for redshifts z ≳ 1. The correlation must therefore derive from objects with subgalactic separations. At faint magnitudes, the counts satisfy the relation number α1/flux, which is expected for images that are subdivisions of larger ones. Several explanations for these observed correlations are possible, but a conservative explanation can suffice to produce our results. Since high-redshift space (z ≳ 0.5) dominates the volume of the sample, observational redshift effects are important. Rest-frame ultraviolet radiation appears in the HDF's visible and near-UV bands, and surface brightness dimming enhances the relative brightness of unresolved objects versus resolved objects. Both work to increase the prominence of compact star-forming regions over diffuse stellar populations. Thus, a "normal" gas-rich galaxy at high redshift can appear clumpy and asymmetric in the visible bands. For sufficiently faint and distant objects, the compact star-forming regions in such galaxies peak above undetectable diffuse stellar backgrounds. Our results do not exclude asymmetric formation or fragmentation scenarios.
KW - Cosmology: observations
KW - Galaxies: structure
KW - Techniques: image processing
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U2 - 10.1086/310394
DO - 10.1086/310394
M3 - Article
AN - SCOPUS:21444442718
SN - 0004-637X
VL - 473
SP - L63-L66
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 2 PART II
ER -