Abundance determination of multiple star-forming regions in the H ii galaxy SDSS J165712.75+321141.4

Guillermo F. H agele 0 3 4 Rub en Garca-Benito 1 4 Enrique P erez-Montero 6 A ngeles I. Daz 4 M onica V. Cardaci 3 4 Ver onica Firpo 3 Elena Terlevich 2 5 Roberto Terlevich 5 7 0 Member of the Carrera de Investigador Cient fico, Consejo Nacional de Investigaciones Cient ficas y Te cnicas (CONICET) 1 Kavli Institute of Astronomy and Astrophysics, Peking University , 100871 Beijing , China 2 Visiting Astronomer at the Institute of Astronomy, University of Cambridge , Madingley Road, Cambridge CB3 0HA 3 Facultad de Cs. Astrono micas y Geof sicas, Universidad Nacional de La Plata , Paseo del Bosque s/n, 1900 La Plata , Argentina 4 Departamento de F sica Teo rica, Mo dulo 15, Universidad Auto noma de Madrid , 28049 Madrid , Spain 5 Instituto Nacional de Astrof sica , O ptica y Electro nica, Tonantzintla, Apdo. Postal 51, 72000 Puebla, Me xico 6 Instituto de Astrof sica de Andaluc a, CSIC, Apdo. 3004, 18080 Granada , Spain 7 Research Affiliate, Institute of Astronomy, University of Cambridge , Madingley Road, Cambridge CB3 0HA A B S T R A C T We analyse high signal-to-noise ratio spectrophotometric observations acquired simultaneously with TWIN, a double-arm spectrograph, from 3400 to 10 400 of three star-forming regions in the H II galaxy SDSS J165712.75+321141.4. We have measured four line temperatures - T e([O III]), T e([S III]), T e([O II]) and T e([S II]) - with high-precision, rms errors of the order of 2, 5, 6 and 6 per cent, respectively, for the brightest region, and slightly worse for the other two. The temperature measurements allowed the direct derivation of ionic abundances of oxygen, sulphur, nitrogen, neon and argon. We have computed CLOUDY tailor-made models which reproduce the O2+-measured thermal and ionic structures within the errors in the three knots, with deviations of only 0.1 dex in the case of O+ and S2+ ionic abundances. In the case of the electron temperature and the ionic abundances of S+/H+, we find major discrepancies which could be the consequence of the presence of colder diffuse gas. The star formation history derived using STARLIGHT shows a similar age distribution of the ionizing population among the three star-forming regions. This fact suggests a similar evolutionary history which is probably related to the process of interaction with a companion galaxy that triggered the star formation in the different regions almost at the same time. The hardness of the radiation field mapped through the use of the softness parameter is the same within the observational errors for all three regions, implying that the equivalent effective temperatures of the radiation fields are very similar for all the studied regions of the galaxy, in spite of some small differences in the ionization state of different elements. Regarding the kinematics of the galaxy, the gas rotation curve shows a deviation from the circular motion probably due either to an interaction process or to an expanding bubble or shell of the ionized gas approaching us. A dynamical mass of 2.5 1010 M is derived from the rotation curve. - younger Universe. The origin of the term starburst (coined as starburst nuclei by Weedman et al. 1981) dates back to the early observations of dust-obscured star-forming regions in the centres of nearby galaxies at the end of the 1970s and beginning of the 1980s, but the basic concept extends further back (e.g. Hodge 1969; Searle, Sargent & Bagnuolo 1973). The level of intensity of a starburst is highly variable. According to Terlevich (1997), in a starburst galaxy, the energy output of the starburst (LSB) is much larger than the one coming from the rest of the galaxy (LG); a galaxy with LSB LG is a galaxy with starbursts and in a normal galaxy LSB LG. This classification shows the variety of environments of the bursts. It is clear that the visibility of the burst depends not only on its intensity but also on its environment. Terlevich (1997) also proposed a division in phases of the starburst. The first one, the nebular phase, is characterized by the presence of strong emission lines from the gas photoionized by young massive stars, with an age of less than 10 Myr. The early continuum phase goes from 10 to 100 Myr, when some Balmer lines appear in absorption and others in emission. Finally, the late continuum phase is when only some weak emission lines appear in the spectrum. The H II galaxies are typical examples of the first phase. H II galaxies are gas-rich dwarf galaxies experiencing a violent star formation period which dominates the optical spectrum of the host galaxy. They have one of the highest intensity levels of the starforming activity. In general, these galaxies have a central region which contains one or more star-forming knots, with a diameter of several hundred parsecs with a high surface brightness, and a low-luminosity underlying galaxy (MV 17). The activity of the star formation episodes cannot be sustained continuously for long periods of time, since the central region cannot have enough gas to fuel these processes for longer than 109 yr and to match the gas content and metallicity with theoretical considerations (Thuan, Hibbard & Levrier 2004). Spectroscopically, H II galaxies are essentially identical to the giant H II regions found in nearby irregular and late-type galaxies. The correlation among structural parameters (H luminosity, velocity dispersion, linewidths) and between these parameters and the chemical composition (Terlevich & Melnick 1981) favours the interpretation of H II galaxies as giant H II regions in distant dwarf irregular galaxies similar to the ones found nearby (Melnick, Terlevich & Eggleton 1985). Other important characteristic of H II galaxies is their low metallicity (Z /50 Z Z /3; Kunth & Sargent 1983). The fact that H II galaxies are metal poor and very blue objects seems to suggest that they are young. Nevertheless, there is evidence which indicates the presence of populations older than the ones in the starburst. This is seen in the behaviour of the surface brightness profile which is exponential in the external zones, or in the colour index, which turns redder in V R and V I (Telles & Terlevich 1997). IZw18 in particular was considered as the best candidate for a truly young galaxy. Early studies of the stellar population of IZw18 did not reveal any old population (Hunter & Thronson 1995). This contradicted some models which predict that during a starburst the heavy elements produced by the massive stars are ejected with high velocities into a hot phase, leaving the starburst region without immediate contribution to the enrichment of the interstellar medium (TenorioTagle 1996). In this scenario, the metals observed now would have their origin in a previous star formation event and an underlying old stellar population would be expected. In fact, Garnett et al. (1997) attributed the high carbon abundance that they found in HST spectroscopy of IZw 18 as evidence for the presence of an old ste (...truncated)