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Astron. Astrophys. 353, 92-96 (2000)

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5. Discussion and conclusions

On the basis of analysis of the HDF-N and HDF-S images we selected 25 galaxies with probable tidal tails with z=0.5-1.5. Integral photometric characteristics of the suspected tails are close to that for local interacting galaxies. Considering the subsample of tailed galaxies with z=0.5-1.0 (14 objects), we estimated that co-moving volume density of such galaxies changes approximately as (1+z)4. (Hence the volume density of tailed galaxies at [FORMULA] is [FORMULA] for [FORMULA].) Inclusion in the sample of the galaxies with tidal bridges does not noticeably change the value of the exponent (Paper I). Therefore, we estimated the change of the rate of close encounters leading to the formation of extended tails. If this rate reflects the merger rate, we have obtained evidence of a steeply increasing merger rate at [FORMULA]. (Our result is related to field galaxies. The evolution in clusters might even be stronger than in the field. For instance, van Dokkum et al. 1999 found [FORMULA] for the merger fraction in rich clusters of galaxies.)

How does our estimation of m agree with values obtained by other methods? The recent surveys of the evolution of galaxy pairs with z are consistent with [FORMULA] (see references in Abraham 1998). Evolution of the rate of interactions according to our data is characterized by close (within quoted errors) value of m. Direct analysis of the morphology of distant galaxies at [FORMULA] suggests a significant increase of the fraction of irregular and peculiar systems with redshift (Fig. 4). If interactions and mergers are responsible for the observed asymmetries of galaxies (e.g. Conselice & Bershady 1998), this increase can reflect the increase of the interaction rate with z. As one can see in Fig. 4, relative fraction of Irr/Pec galaxies changes in accordance with [FORMULA]. Naim et al.'s (1997) result (35%[FORMULA]15% of peculiar galaxies down to [FORMULA]) agrees with [FORMULA] also. Many other observational surveys and numerical works indicate a large ([FORMULA]3) exponent m (Sect. 1). Comparison with predictions of analytical and numerical works shows that current observational estimates of the merger rate favor a zero curvature ([FORMULA]) universe (e.g. Carlberg 1991, Governato et al. 1997).

[FIGURE] Fig. 4. The dependence of the relative fraction of irregular and peculiar galaxies on redshift. The data for z=0.4 and 0.8 are from Brinchmann et al. (1998); for z=0.54 are from Roche et al. (1998); the local value (z=0) is estimated from Marzke et al. (1998) LF. Dashed line - (1+z)4 relation.

Our results indicate that further detailed statistics of galaxies with tidal structures will be a powerful tool to quantify the interaction and merging rates evolution.

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© European Southern Observatory (ESO) 2000

Online publication: December 8, 1999