Metallicity of high stellar mass galaxies with signs of merger events

¹ Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina e-mail: salonso@icate-conicet.gob.ar, [leo;dgl]@mail.oac.uncor.edu
² Instituto de Ciencias Astronómicas, de la Tierra y del Espacio, San Juan, Argentina
³ IATE, CONICET, OAC, Universidad Nacional de Córdoba, Laprida 854, X5000BGR, Córdoba, Argentina
⁴ Centre de Recherche Astrophysique de Lyon, Université de Lyon, Université Lyon 1, Observatoire de Lyon, École Normale Supérieure de Lyon, CNRS, UMR 5574, 9 avenue Charles André, 69230 Saint-Genis Laval, France

Received: 2 July 2009
Accepted: 12 January 2010

Abstract

Aims. We focus on an analysis of galaxies of high stellar mass and low metallicity. Evidence of recent merger events in their optical images allow us to classify galaxies into either disturbed or undisturbed, and to study galaxy properties such as morphology, colours, stellar populations, and global environment for the different metallicity ranges and disturbance classes.

Methods. We cross-correlated the Millenium Galaxy Catalogue (MGC) and the Sloan Digital Sky Survey (SDSS) galaxy catalogue to provide a sample of MGC objects with high resolution imaging and both spectroscopic and photometric information available in the SDSS database. For each galaxy in our sample, we conducted a systematic morphological analysis by visual inspection of MGC images using their luminosity contours. The galaxies are classified as either disturbed or undisturbed objects. We divide the sample into three metallicity regions, within wich we compare the properties of disturbed and undisturbed objects.

Results. We find that the fraction of galaxies that are strongly disturbed, indicative of being merger remnants, is higher when lower metallicity objects are considered. The three bins analysed consist of approximatively 15%, 20%, and 50% disturbed galaxies (for high, medium, and low metallicity, respectively). Moreover, the ratio of the disturbed to undisturbed relative distributions of the population age indicator, D(4000), in the low metallicity bin, indicates that the disturbed objects have substantially younger stellar populations than their undisturbed counterparts. In addition, we find that an analysis of colour distributions provides similar results, showing that low metallicity galaxies with a disturbed morphology are bluer than those that are undisturbed. The bluer colours and younger populations of the low metallicity, morphologically disturbed objects suggest that they have experienced a recent merger with an associated enhanced star formation rate.