Astronomy & Astrophysics
using multiwavelength data
Rainbow over the Anglo-Australian Telescope in Siding Spring Observatory, NSW, Australia © Á.R.L-S.
Massive star formation in dwarf Wolf-Rayet galaxies
04.12.2006 - Universidad de La Laguna / Instituto de Astrofísica de Canarias, Spain
I present a detailed morphological, photometrical and spectroscopical analysis of a sample of 20 Wolf-Rayet (WR) galaxies. WR galaxies are a subtype of H II galaxies whose integrated spectra show broad emission lines attributed to WR stars, indicating the presence of an important population of massive stars and the youth of the starburst. The main aims are the study of the star formation and O and WR stellar populations in these galaxies and the role that interactions between low surface companion objects could have in the triggering mechanism of the bursts.
Deep and high spatial resolution imagery in optical and near-infrared (NIR) broad band and narrow-band (Hα and continuum) filters have been used to study the morphology of the stellar and ionized gas of each galaxy, its surroundings and possible faint companion objects. The ionizing stars content and the age of the recent star formation bursts and the old stellar underlying population are also analyzed. Long slit and echelle spectroscopy have been used to study the physical conditions (electron density and temperature, reddening, ionization nature), the chemical abundances and the kinematics of the ionized gas, as well as the massive star population content and its spatial location in each galaxy. This study led to disentangle the tidal/pre-existing nature of the companion objects surrounding the main galaxies.
We want to remark the quallity of the observations (some deep images obtained at 2.56m NOT have a seeing of around 0.6" and our deep echelle spectra of NGC 5253 obtained at 8.2m VLT are excellent) and the process of reduction and analysis of the data, that have led to perform a systematic and comprehensive analysis of the extiction in each region or galaxy.
Specially interesting are the cases of the groups of galaxies HCG 31 and Mkn 1087, where interactions involving more than two objects are needed to explain the tails, bridges, mergers and tidal dwarf galaxies observed in them. I also remark the detailed analysis of the star formation activity and stellar populations in the galaxy IRAS 08339+6517, where WR features are detected for the first time, and which is in interaction with a independent nearby dwarf object. However, one of the main goals of this thesis is the detection of the weak O II and C II recombination lines in our deep VLT spectra of the dwarf starburt galaxy NGC 5253, the first time reported in a starburst. Futhermore, we detect a localized nitrogen and helium enrichment, which agrees with that expected for the pollution by the ejecta of WR stars. The amount of enriched material needed to produce the observed overabundance is consistent with the mass lost by the number of WR stars estimated in the starbursts.
Our optical/NIR observations are completed with X-ray, far-infrared (FIR) and radio (H I and continuum) data extracted from the literature in order to achieve a global vision of the star formation activity and evolution of each galaxy. Finally, general results involving all the galaxy sample and considering all the multiwavelength data are presented. I conclude that the majority of the analyzed galaxies (16 up to 20, ~80% of the objects) show clear interaction features, confirming the hypothesis that interaction with or between dwarf objects trigger the star formation activity in Wolf-Rayet galaxies.
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