3.1. The nature of high-z peculiar galaxies
3.1.1. The evolving merger rate
Locally, most morphologically peculiar systems show dynamical evidence for tidal disruption, and it is tempting to assume that a large fraction of the diverse peculiar galaxy population seen on deep images are actually mergers in progress. But is this assumption justified? Figure 5 shows candidate z > 2 Lyman dropout systems in the Hubble Deep Field with IF814W < 25 mag. Clearly most are morphologically peculiar, but few resemble the classical appearance (ie. strongly bimodality, with prominent tidal tails) of ``canonical'' local merging systems. However, as discussed further below, the z > 2 regime accessible to Lyman limit searches probes rest wavelengths where the effects of bandshifting on morphology can be rather extreme. This is made clear by Hibbard & Vacca [46], who used HST FOC ultraviolet data of local merger-induced starburst galaxies to predict the appearance of the high-redshift counterparts. The usual signatures of mergers (tidal tails, distorted disks) are no longer visible at z > 2, and merging starbursts seem to provide at least qualitatively reasonable counterparts to many faint peculiar galaxies.
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Figure 5. Candidate lyman break (z > 2) systems in the Hubble Deep Field with I < 25 mag, taken from 84n den Bergh et al. (1996) [84]. |
Clearly the best way forward will be to incorporate dynamical information to determine directly which peculiar galaxies show distinct kinematical subcomponents. Unfortunately these observations are not currently feasible, although they may soon become possible with adaptive optics and the new generation of 8m-class telescopes. In the meantime, a promising approach to quantifying the fraction of mergers amongst the distant peculiar galaxy population may be to measure statistics which are relatively insensitive to image distortions resulting from bandshifting and surface-brightness biases, but which track probable merger activity. One such statistic is the ``Lee Ratio'', a measure of image bimodality. This statistic been applied to images of galaxies in the CFRS survey [56, 57] and to HDF galaxies, with the result that around ~ 40% of faint peculiar systems are significantly bimodal, with an ~ (1 + z)3 increase in the merger rate.