Search

(Abridged) We have performed a comprehensive multiwavelength analysis of a sample of 20 starburst galaxies that show the presence of a substantial population of massive stars. The main aims are the study of the massive star formation and stellar populations in these galaxies, and the role that interactions with or between dwarf galaxies and/or low surface companion objects have in triggering the bursts. We completed new deep optical and \NIR\ broad-band images, as well as the new continuum-subtracted H$\alpha$ maps, of our sample of Wolf-Rayet galaxies. We analyze the morphology of each system and its surroundings and quantify the photometric properties of all important objects. All data were corrected for both extinction and nebular emission using our spectroscopic data. The age of the most recent star-formation burst is estimated and compared with the age of the underlying older low-luminosity population. The \Ha-based star-formation rate, number of O7V equivalent stars, mass of ionized gas, and mass of the ionizing star cluster are also derived. We found interaction features in many (15 up to 20) of the analyzed objects, which were extremely evident in the majority. We checked that the correction for nebular emission to the broad-band filter fluxes is important in compact objects and/or with intense nebular emission to obtain realistic colors and compare with the predictions of evolutionary synthesis models. The estimate of the age of the most recent star-formation burst is derived consistently. With respect to the results found in individual objects, we remark the strong \Ha\ emission found in IRAS 08208+2816, UM 420, and SBS 0948+532, the detection of a double-nucleus in SBS 0926+606A, a possible galactic wind in Tol 9, and one (two?) nearby dwarf star-forming galaxies surrounding Tol 1457-437., Comment: Published in A&A, 491, 131 (2008). 39 Figures have NOT been included here. Full version in http://www.atnf.csiro.au/people/Angel.Lopez-Sanchez/papers/9409WRG1.pdf False color pictures of the galaxies can be found in http://www.atnf.csiro.au/people/Angel.Lopez-Sanchez/pictures.html

We present the main results of the PhD Thesis carried out by L\'opez-S\'anchez (2006), in which a detailed morphological, photometrical and spectroscopical analysis of a sample of 20 Wolf-Rayet (WR) galaxies was realized. 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 have in the triggering of the bursts. We analyze the morphology, stellar populations, physical conditions, chemical abundances and kinematics of the ionized gas, as well as the star-formation activity of each system., Comment: 16 pages, 15 figures

We present the first results of a project aimed at the combined study of massive stars and their surrounding nebulae by means of a detailed study of Galactic HII regions ionized by only one massive star. With this, we intend to check the validity of the new generation of massive star model atmosphere codes in terms of ionizing flux distribution. We take into account the effect of the nebular density distribution in our analyses. Various types of stellar and nebular observations have been collected for this purpose., Comment: 2 pages, 2 figures. To appear in the proceedings of the workshop "Massive Stars: Fundamental Parameters and Circumstellar Interactions"

We present echelle spectrophotometry of the blue compact dwarf galaxy (BCDG) NGC 5253. The data have been taken with the Very Large Telescope UVES echelle spectrograph in the 3100 to 10400 angstroms range. We have measured the intensities of a large number of permitted and forbidden emission lines in four zones of the central part of the galaxy. In particular, we detect faint C II and O II recombination lines (RLs). This is the first time that these lines are unambiguously detected in a dwarf starburst galaxy. The physical conditions of the ionized gas have been derived using a large number of different line intensity ratios. Chemical abundances of He, N, O, Ne, S, Cl, Ar, and Fe have been determined following the standard methods. In addition, C^{++} and O^{++} abundances have been derived from pure RLs. These abundances are larger than those obtained from collisionally excited lines, (CELs) (from 0.30 to 0.40 dex for C^{++} and from 0.19 to 0.28 dex for O^{++}). This result is consistent with a temperature fluctuations parameter (t^2) between 0.050 and 0.072. We confirm previous results that indicate the presence of a localized N enrichment in certain zones of the center of the galaxy. Moreover, our results also indicate a possible slight He overabundance in the same zones. The enrichment pattern agrees with that expected for the pollution by the ejecta of massive stars in the Wolf-Rayet (WR) phase. 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. Finally, we discuss the possible origin of the difference between abundances derived from RLs and CELs in H II regions, finding that a recent hypothesis based on the delayed enrichment by SNe ejecta inclusions seems not to explain the observed features., Comment: 28 pages, 13 tables, 10 figures. Accepted for publication in ApJ

Based on O II recombination lines we present a new calibration (called O II[RL]) of Pagel's O23 indicator to determine the O/H abundance ratio in extragalactic H II regions and emission line galaxies. The O II(RL) calibration produces O/H abundances about a factor of two higher than those derived from the T(4363) method with t2 = 0.00. The O II(RL) calibration has implications for the study of different properties of emission line galaxies such as their metallicity, star formation rate, and initial mass function. The O II(RL) calibration also affects the abundance determinations based on other O/H indicators, that include collisionally excited lines, like those known as O3N2, N2, S23, Ar3O3, and S3O3. We argue that the controversy between the T(4363) method and the photoionization models method to derive O/H values is mainly due to temperature variations inside the observed H II regions., Comment: 8 pages, 3 figures. From the Congference "First Light Science with the Gran Telescopio Canarias" (2006). To appear in RevMexAA, SC

IRAS 08339+6517 is a luminous infrared and Ly$\alpha$-emitting starburst galaxy that possesses a dwarf companion object at a projected distance of 56 kpc. An \ion{H}{i} tidal tail has recently been detected between both galaxies, suggesting that about 70% of the neutral gas has been ejected from them.We present deep broad-band optical images, together with narrow band H$\alpha$ CCD images, and optical intermediate-resolution spectroscopy of both galaxies. The images reveal interaction features between both systems and strong H$\alpha$ emission in the inner part of IRAS 08339+6517. The chemical composition of the ionized gas of the galaxies is rather similar. The analysis of their kinematics also indicates interaction features and reveals an object that could be a candidate tidal dwarf galaxy or a remnant of an earlier merger. Our data suggest that the \ion{H}{i} tail has been mainly formed from material stripped from the main galaxy. We find weak spectral features that could be attributed to the presence of Wolf--Rayet stars in this starburst galaxy and estimate an age of the most recent burst of around 4 -- 6 Myr. A more evolved underlying stellar population, with a minimal age between 100 -- 200 Myr, is also detected and fits an exponential intensity profile. A model which combines 85% young and 15% old populations can explain both the spectral energy distribution and the \ion{H}{i} Balmer and \ion{He}{i} absorption lines presented in our spectrum. The star formation rate of the galaxy is consistently derived using several calibrations, giving a value of $\sim$9.5 \Mo yr$^{-1}$. IRAS 08339+6517 does satisfy the criteria of a luminous compact blue galaxy, rare objects in the local universe but common at high redshifts, being a very interesting target for detailed studies of galaxy evolution and formation., Comment: 23 pages, 8 tables, 18 figures. Accepted for publication in Astronomy & Astrophysics

We present new broad-band optical and near-infrared CCD imaging together with deep optical intermediate-resolution spectroscopy of Mkn 1087 and its surrounding objects. We analyze the morphology and colors of the stellar populations of the brightest objects, some of them star-formation areas, as well as the kinematics, physical conditions and chemical composition of the ionized gas associated with them. Mkn 1087 does not host an Active Galactic Nucleus, but it could be a Luminous Compact Blue Galaxy. Although it was classified as a suspected Wolf-Rayet galaxy, we do not detect the spectral features of these sort of massive stars. Mkn 1087 shows morphological and kinematical features that can be explained assuming that it is in interaction with two nearby galaxies: the bright KPG 103a and a dwarf ($M_B\sim-18$) star-forming companion. We argue that this dwarf companion is not a tidal object but an external galaxy because of its low metallicity [12+log(O/H) = 8.24] with respect to the one derived for Mkn 1087 [12+log(O/H) = 8.57] and its kinematics. Some of the non-stellar objects surrounding Mkn 1087 are connected by bridges of matter with the main body, host star-formation events and show similar abundances despite their different angular distances. These facts, together their kinematics, suggest that they are tidal dwarf galaxies formed from material stripped from Mkn 1087. A bright star-forming region at the south of Mkn 1087 (knot #7) does not show indications of being a tidal galaxy or the product of a merging process as suggested in previous works. We argue that Mkn 1087 and its surroundings should be considered a group of galaxies., Comment: Accepted by A&A, 21 pages, 13 figures, 8 tables

We present new observations of IRAS 04000+5052, a Galactic HII region associated with a young stellar cluster and possibly located at the Perseus arm. Contrary to previous claims, we have found that this object is not a compact metal-poor HII region. The electron density and chemical composition of the nebula are similar to those of normal HII regions of the Galactic disk. The radial velocity of the ionized gas coincides with that obtained from CO observations, indicating that the nebula is associated with a molecular cloud. Probably, the ionizing source of the HII region is a Herbig Be star of B0.5 spectral type., Comment: 15 pages, 2 tables, 5 figures. Accepted for publication in PASP

We present new broad-band optical and near-infrared CCD imaging together with deep optical intermediate-resolution spectroscopy of the Hickson Compact Group 31. We analyze the morphology and colors of the stellar populations of the galaxies, as well as the kinematics, physical conditions and chemical composition of the ionized gas in order to get a more complete view on the origin and evolution of the system. We estimate the ages of the most recent star formation bursts of the system, finding an excellent consistency among the values obtained with different indicators and starburst models. We find that member F hosts the youngest starburst of the group, showing a substantial population of Wolf-Rayet stars. The chemical abundances are fairly similar in all the members of the group despite their very different absolute magnitudes. We argue that the use of traditional metallicity-luminosity relations based on the absolute $B$-magnitude is not appropriate for dwarf starburst galaxies, because their luminosity is dominated by the transient contribution of the starburst to the blue luminosity. We think that members E and F of the group are candidate tidal dwarf galaxies because of their high metallicity, their kinematics, and the absence of underlying old stellar populations. Finally, we propose that HCG~31 is suffering several almost simultaneous interaction processes. The most relevant of these processes are: (a) the merging of members A and C, that would have produced two optical tidal tails; and (b) a fly-by encounter between G and the A+C complex, that would have produced an \ion{H}{1} tidal tail from the stripping of the external gas of A+C, from which members F and E have originated., Comment: Accepted for publication in ApJS, 41 pages, 15 figures, 9 tables