Volume 539, March 2012
|Number of page(s)||9|
|Published online||23 February 2012|
I. Major and minor mergers
1 Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina
2 ICATE, CONICET, CC 49, 5400 San Juan, Argentina
3 IATE, CONICET, OAC, Universidad Nacional de Córdoba, Laprida 854, X5000BGR, Córdoba, Argentina
4 Departamento de Astronomia, Instituto de Astronomia, Geofísica e Ciências Atmosféricas da USP, Rua do Matão 1226, Cidade Universitária, 05508-090, São Paulo, Brazil
Received: 17 August 2011
Accepted: 29 November 2011
Aims. We study galaxy pair samples selected from the Sloan Digital Sky Survey (SDSS-DR7) and we perform an analysis of minor and major mergers with the aim of investigating the dependence of galaxy properties on interactions.
Methods. We build a galaxy pair catalog requiring rp < 25 kpc h-1 and ΔV < 350 km s-1 within redshift z < 0.1. By visual inspection of SDSS images we remove false identifications and we classify the interactions into three categories: pairs undergoing merging, M; pairs with evident tidal features, T; and non disturbed, N. We also divide the pair sample into minor and major interactions according to the luminosity ratio of the galaxy members. We study star formation activity through colors, the 4000 Å break, and star formation rates.
Results. We find that ~10% of the pairs are classified as M. These systems show an excess of young stellar populations as inferred from the Dn(4000) spectral index, colors, and star formation rates of the member galaxies, an effect which we argue is directly related to the ongoing merging process. We find ~30% of the pairs exhibiting tidal features (T pairs) with member galaxies showing evidence of old stellar populations. This can be associated either to the disruptive effect of some tidal interactions, or to the longer time-scale of morphological disturbance with respect to the bursts of the tidal induced star formation. Regardless of the color distribution, we find a prominent blue peak in the strongest mergers, while pairs with tidal signs under a minor merger show a strong red peak. Therefore, our results show that galaxy interactions are important in driving the evolution of galaxy bimodality. By adding stellar masses and star formation rates of the two members of the pairs, we explore the global efficiency of star formation of the pairs as a whole. We find that, at a given total stellar mass, major mergers are significantly more efficient (a factor ≈ 2) in forming new stars, with respect to both minor mergers or a control sample of non-interacting galaxies.
We conclude that the characteristics of the interactions and the ratio of luminosity galaxy pair members involved in a merger are important parameters in setting galaxy properties.
Key words: galaxies: evolution / galaxies: interactions / galaxies: formation / galaxies: statistics / galaxies: starburst / galaxies: star formation
© ESO, 2012
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