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2. The sample

2.1. Binary galaxies

The ideal sites to study galaxy encounters are pairs and groups of galaxies. The pioneers in the studies of pairs of galaxies are Holmberg (1937, 1958) and Page (1952, 1961) and more recently Karachentsev (1972). Major effort has been focused into assembly complete samples of isolated pairs of galaxies for the Southern Hemisphere (Reduzzi & Rampazzo 1995; Soares et al. 1995; see Sulentic 1992 for a review on the subject). Several studies have analyzed the effect of interaction in binary galaxies, e.g. Rampazzo & Sulentic (1992); Rampazzo et al. (1995); de Mello et al. (1996); Marquez & Moles 1996; Reduzzi & Rampazzo (1996), and de Souza et al. (1997).

One simple approach in studying tidal effects is to analyze galaxy morphology in binary galaxies. A standard morphological classification of binaries is based on their components morphology. For instance, early-type binaries, usually called EE, are formed by two early-type galaxies and late-type binaries (SS) by two spiral galaxies. Mixed morphology binaries, ES, are formed by an early and a late-type galaxy. The observed fraction of the three types is 0.60 for SS, 0.30 for for ES and 0.10 for EE (Sulentic 1992). The existence of many isolated mixed binaries raises problems for models of galaxy formation that envision local environment as a major determinant of morphology. However, there are a few evidences, like morphological distortion in the late-type galaxy and young stellar population in the early-type showing that mixed binaries are physically interacting (de Mello et al. 1996; Rampazzo & Sulentic 1992). Therefore, if mergers of spiral galaxies are the mechanism for the formation of ellipticals then binaries containing an elliptical would be the product of interaction between at least 3 galaxies. To understand whether galaxy transformation is a common phenomenon in galaxy evolution more studies of galaxy morphologies in interacting systems are needed. In this work we present the results of such analysis for a sample of binary galaxies.

2.2. Selection criteria

One of the most important goals of this study is to build a photometric and spectroscopic database in order to study interacting galaxy evolution. For this, the bright and faint pairs should constitute a carefully matched sample with projected separations such that the objects would be expected to merge in much less than a Hubble time.

The faint (19<mR<22) sample was selected from automatic compiled catalogs of existing (de Mello et al. 1997a,b) CTIO 4 m images at the equator; the bright sample (16<mV<18) was selected from the catalog of isolated binary galaxies (CPG) compiled by Karachentsev (1972), which is the best available catalog that meets the local control sample requirements.

Pairs of faint galaxies (separation tex2html_wrap_inline1644) were chosen such that tex2html_wrap_inline1646. Here tex2html_wrap_inline1648 is the minimum separation at which pairs can be reliably separated (nominally 2''); tex2html_wrap_inline1652 corresponds to some physical separation, tex2html_wrap_inline1654, chosen so that: (i) physical pairs are doomed to merge in < 109 yr (on the basis of both empirical studies and conventional dynamics); and (ii) most pairs in the faint sample are physically associated. These conditions are satisfied by tex2html_wrap_inline1658 kpc, which corresponds to tex2html_wrap_inline1660 at z = 0.35.

What fraction of the faint pairs is expected to be physical pairs and merge? Infante et al. (1996) have calculated the angular correlation function, tex2html_wrap_inline1664 for pairs at mR < 21.5 and tex2html_wrap_inline1668. It follows that tex2html_wrap_inline1670 of faint pairs will be physically associated. Furthermore, a cut in tex2html_wrap_inline1672 (say tex2html_wrap_inline1674 km s-1) should isolate most of the physical pairs. Following Carlberg et al. (1994) the fraction of galaxies that will merge is tex2html_wrap_inline1678. Thus, it is expected that tex2html_wrap_inline1680 of galaxies in pairs will eventually merge.

The two samples should be affected by identical selection effects. The CPG contains 603 binaries brighter than tex2html_wrap_inline1682, is the most complete catalog of binaries and has complete redshift information. In order to match the properties of z=0.35 pairs we looked at all Karachentsev pairs with characteristics similar to what we expected for pairs at high z. We have selected all pairs with projected linear separation tex2html_wrap_inline1688 kpc tex2html_wrap_inline1690 and radial velocity difference tex2html_wrap_inline1692.

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