Massive star formation in Wolf-Rayet galaxies

Abstract

Aims. We have performed a comprehensive multiwavelength analysis of a sample of 20 starburst galaxies that show a substantial population of very young massive stars, most of them classified as Wolf-Rayet (WR) galaxies. In this paper, the forth of the series, we present the global analysis of the derived photometric and chemical properties. Methods. We compare optical/NIR colours and the physical properties (reddening coefficient, equivalent widths of the emission and underlying absorption lines, ionization degree, electron density, and electron temperature) and chemical properties (oxygen abundances and N/O, S/O, Ne/O, Ar/O, and Fe/O ratios) with previous observations and galaxy evolution models. We compile 41 independent star-forming regions -with oxygen abundances between 12 + log(O/H) = 7.58 and 8.75 -, of which 31 have a direct estimate of the electron temperature of the ionized gas.Results. According to their absolute B-magnitude, many of them are not dwarf galaxies, but they should be during their quiescent phase. We found that both c(Hβ) and W abs increase with increasing metallicity. The differences in the N/O ratio is explained assuming differences in the star formation histories. We detected a high N/O ratio in objects showing strong WR features (HCG 31 AC, UM 420, IRAS 0828+2816, III Zw 107, ESO 566-8 and NGC 5253). The ejecta of the WR stars may be the origin of the N enrichment in these galaxies. We compared the abundances provided by the direct method with those obtained through empirical calibrations, finding that (i) the Pilyugin method is the best suited empirical calibration for these star-forming galaxies; (ii) the relations provided by Pettini & Pagel (2004, MNRAS, 348, 59) give acceptable results for objects with 12 + log(O/H) > 8.0; and (iii) the results provided by empirical calibrations based on photoionization models are systematically 0.2-0.3 dex higher than the values derived from the direct method. The O and N abundances and the N/O ratios are clearly related to the optical/NIR luminosity; the dispersion of the data is a consequence of the differences in the star-formation histories. The L-Z relations tend to be tighter when using NIR luminosities, which facilitates distinguishing tidal dwarf galaxies candidates and pre-existing dwarf objects. Galaxies with redder colours tend to have higher oxygen and nitrogen abundances. Conclusions. Our detailed analysis is fundamental to understand the nature of galaxies that show strong starbursts, as well as to know their star formation history and the relationships with the environment. This study is complementary -but usually more powerful-to the less detailed analysis of large galaxy samples that are very common nowadays.

Massive star formation in Wolf-Rayet galaxies

“…HCG 31 F is the galaxy with highest M gas /M stars ratio (and lowest M stars /L B ratio), indicating its low content of evolved stars. This result agrees with our suggestion (López-Sánchez et al 2004a) that this TDG has created stars mainly using the neutral gas from the long arm-like H i structure (Verdes-Montenegro et al 2005) found between members AC and G of the HCG 31 galaxy system. Finally, we compare some mass-to-light ratios with the colors of the galaxies.…”

“…Indeed, HCG 31 F shows a much higher oxygen abundance than expected following the closed box model. The explanation of this behavior is that this object is a TDG that was very probably formed from the material stripped from HCG 31 AC during the fly-by encounter between member G and the A+C complex (López-Sánchez et al 2004a). The TDG has acreted a large fraction of the pre-enriched H i gas available in the arm-like structure and now hosts very intense star-formation.…”

“…It is well known that interactions in dwarf galaxies are usual not with nearby giant galaxies (Campos-Aguilar et al 1993;Telles & Terlevich 1995;Telles & Maddox 2000) but with low surface brightness galaxies (Wilcots et al 1996;Noeske et al 2001;Pustilnik et al 2001), or H i clouds (e.g., Taylor et al 1993Taylor et al , 1995Taylor et al , 1996Thuan et al 1999;van Zee et al 2001;Begum et al 2006;Ekta et al 2006;Hutchmeier et al 2008;López-Sánchez & Esteban 2008). Méndez & Esteban (2000) suggested, for the first time, that interactions with or between dwarf objects could be the main star formation triggering mechanism in dwarf galaxies.…”

“…In Papers I we described the optical/NIR broad-band and Hα photometry and in Paper II the intermediate-resolution optical spectroscopy analysis, of 16 up to 20 galaxies studied in this work. The analysis of the additional 4 systems were presented in previous papers: NGC 1741 (member AC within the HCG 31 group) in López-Sánchez et al (2004a), Mkn 1087 and their surrounding galaxies in López-Sánchez et al (2004b), the luminous blue compact galaxy IRAS 08339+6517 in , and NGC 5253 in López-Sánchez et al (2007). Paper III compiled the localization of the WR-rich star clusters within the galaxies and the analysis of their massive stellar populations.…”

“…Recent observations also suggest that interactions and mergers between dwarf galaxies also trigger the star-formation activity and play a fundamental role in the evolution of dwarf galaxies (i.e., Méndez & Esteban 2000;Östlin et al 2001Bergvall & Östlin 2002;Johnson et al 2004;Bravo-Alfaro et al 2004, 2006Cumming et al 2008;García-Lorenzo et al 2008;López-Sánchez & Esteban 2008, 2009James et al 2010). Many of these studies have been done on blue compact dwarf galaxies (BCDGs), which are low-luminosity, low-metallicity (∼10% solar) galaxies showing compact and irregular morphologies and undergoing an intense and short-lived episode of star-formation (i.e., Izotov & Thuan 1999;Cairós et al 2001a,b;Papaderos et al 2006), on top of an old underlying population with ages of several Gyrs (Noeske et al 2003(Noeske et al , 2005Amorín et al 2007Amorín et al , 2009.…”