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 the presence of a substantial population of very young massive stars, most of them classified as Wolf-Rayet (WR) galaxies. In this paper, the second of the series, we present the results of the analysis of long-slit intermediate-resolution spectroscopy of star-formation bursts for 16 galaxies of our sample. Methods. We study the spatial localization of the WR stars in each galaxy. We analyze the excitation mechanism and derive the reddening coefficient, physical conditions, and chemical abundances of the ionized gas. We study the kinematics of the ionized gas to check the rotation/turbulence pattern of each system. When possible, tentative estimates of the Keplerian mass of the galaxies are calculated. Results. Aperture effects and the exact positioning of the slit onto the WR-rich bursts seem to play a fundamental role in their detection. We check that the ages of the last star-forming burst estimated using optical spectra agree with those derived from Hα imagery. Our analysis shows that a substantial fraction of the galaxies exhibit evidence of perturbed kinematics. With respect to the results found in individual galaxies, we remark the detection of objects with different metallicity and decoupled kinematics in Haro 15 and Mkn 1199, the detection of tidal streams in IRAS 08208+2816, Tol 9, and perhaps in SBS 1319+579, and the development of a merging process in both SBS 0926+606 A and Tol 1457-262. Conclusions. All of these results -in combination with those obtained in Paper I -reinforce the hypothesis that interactions with or between dwarf objects is a very important mechanism in triggering massive-star formation in starburst galaxies, especially in dwarf ones. It must be highlighted that only deep and very detailed observations -as presented in this paper -can provide clear evidences that these subtle interaction processes are taking place.

Massive star formation in Wolf-Rayet galaxies

“…Cairós et al 2001a,b;Bergvall & Östlin 2002;Papaderos et al 2006;Amorín et al 2009). However, as we previously said (López-Sánchez et al 2004a), this seems not to be true in the particular case of members F of HCG 31, which clearly show no evidences of underlying old stellar populations. This was recently confirmed by deep Hubble Space Telescope imaging (Gallagher et al 2010), and hence these two objects are very likely experiencing their very first star-formation event.…”

“…Table 1 also lists some tidal dwarf galaxy (TDG) candidates (HCG 31 E, F1 and F2;Mkn 1087 #1 and #3, POX 4 Comp) and nearby external objects (Mkn 1087 N, Mkn 1199 NE, IRAS 08309+6517 C, Tol 1457-262 #15 and #16) surrounding a main galaxy. However, as we remarked in Paper I and in the analysis of the HCG 31 members (López-Sánchez et al 2004a), we must keep in mind that the B-magnitude of a starburst is increased by several magnitudes during the first 10 Myr with respect to its brightness in the quiescent phase, so we should expect that some of the objects with −18 ≥ M B ≥ −19.5 are indeed defined as dwarf objects during their quiescent phase.…”

“…(a) Colour excess, E(B − V), derived from our estimates of the reddening coefficient and assuming 2.868 c(Hβ). (d) All colours have been corrected for both extinction and emission of the gas, see López-Sánchez, Esteban (2008). (e) Age of the most recent star-forming burst (derived using the Hα equivalent width), in Myr.…”

“…In many cases, two or more slit positions were used to analyse the most interesting zones, knots or morphological structures belonging to each galaxy or even surrounding objects. Paper III (López-Sánchez & Esteban 2010) presented the analysis of the O and WR stellar populations within these galaxies. In this paper, the forth of the series, we globally compile and analyse the optical/NIR photometric data (Sect.…”

Extranuclear Hα-emitting complexes in low-z(U)LIRGs: precursors of tidal dwarf galaxies?

“…We tried to find these features in the spectra of the complexes, but could find no clear evidence of these bumps. This bump is much more difficult to detect than the other bump (such as the blue bump) characteristic of WRs because it is always weaker (e.g., Fernandes et al 2004;López-Sánchez & Esteban 2009). Sometimes it is not even detected again in Wolf-Rayet galaxies where it was previously seen (López-Sánchez & Esteban 2009).…”