Dynamics of " small galaxies" in the Hubble Deep Field

Wesley N. Colley, Oleg Y. Gnedin, Jeremiah P. Ostriker, James E. Rhoads

Research output: Contribution to journalArticle

14 Scopus citations

Abstract

We have previously found in the Hubble Deep Field (HDF) a significant angular correlation of faint, high-color redshift objects on scales below 1″, or several kiloparsecs in metric size. A correlation at this scale is most likely due to physical associations. We examine the correlation and nearest neighbor statistics to conclude that 38% of these objects in the HDF have a companion within 1″ (or ∼6 kpc), 3 times the number expected in a random distribution with the same number of objects; the total excess approaches 1.5 objects by separations of 10″. We next examine three possible dynamical scenarios for these object multiplets: (1) the objects are star-forming regions within normal galaxies, whose disks have been relatively dimmed by K-correction and surface brightness dimming; (2) they are fragments merging into large galaxies; (3) they are satellites accreting onto parent normal L* galaxies. We find that hypothesis 1 is most tenable. First, large galaxies in the process of a merger formation would have accumulated too much mass in their centers (5 × 1012 M inside 2 kpc) to correspond to any abundant category of present-day objects. Second, accretion by dynamical friction occurs with a predictable slope in density versus radius that is not seen among the faint HDF objects. Since the dynamical friction time is roughly 1 Gyr, a steady state should have been reached by redshift z ≲ 5. In the context of these two dynamical scenarios, we consider the possible effects of a gradient in mass-to-light ratio caused by induced star formation during infall. We note that star-forming regions within galaxies clearly present no dynamical problems, but also that large spirals would still appear as such in the HDF, which leads us to favor a scenario in which the faint compact sources in the HDF are giant star-forming regions within small normal galaxies, such as Magellanic irregulars. Last, we note that the "excess" number of correlated objects near a given faint source approaches 1.5, suggesting that the previous counts of objects have overestimated the number of galaxies by a factor of 2.5 while underestimating their individual luminosities by the same factor.

Original languageEnglish (US)
Pages (from-to)579-584
Number of pages6
JournalAstrophysical Journal
Volume488
Issue number2 PART I
DOIs
StatePublished - Jan 1 1997

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Keywords

  • Cosmology: observations
  • Galaxies: evolution
  • Galaxies: kinematics and dynamics
  • Galaxies: statistics

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Colley, W. N., Gnedin, O. Y., Ostriker, J. P., & Rhoads, J. E. (1997). Dynamics of " small galaxies" in the Hubble Deep Field. Astrophysical Journal, 488(2 PART I), 579-584. https://doi.org/10.1086/304730