A kinematic study of the irregular dwarf galaxy NGC 4861 using H I and Hα observations

Abstract

Context. Outflows powered by the injection of kinetic energy from massive stars can strongly affect the chemical evolution of galaxies, in particular of dwarf galaxies, as their lower gravitational potentials enhance the chance of a galactic wind. Aims. We therefore performed a detailed kinematic analysis of the neutral and ionised gas components in the nearby star-forming irregular dwarf galaxy NGC 4861. Similar to a recently published study of NGC 2366, we want to make predictions about the fate of the gas and to discuss some general issues about this galaxy. Methods. Fabry-Perot interferometric data centred on the Hα line were obtained with the 1.93 m telescope at the Observatoire de Haute-Provence. They were complemented by H i synthesis data from the VLA. We performed a Gaussian decomposition of both the Hα and the H i emission lines in order to search for multiple components indicating outflowing gas. The expansion velocities of the detected outflows were compared to the escape velocity of NGC 4861, which was modelled with a pseudo-isothermal halo.Results. Both in Hα and H i the galaxy shows several outflows, three directly connected to the disc and probably forming the edges of a supergiant shell, and one at kpc-distance from the disc. We measured velocity offsets of 20 to 30 km s −1 , which are low in comparison to the escape velocity of the galaxy and therefore minimise the chance of a galactic wind.

Massive star formation in Wolf-Rayet galaxies

“…This trend seems to be a consequence of the metallicity-dependence of nitrogen production in both massive and intermediate-mass stars (e.g., Pilyugin et al 2003), the N/O ratio increases at higher metallicities. Hence, nitrogen is essentially a secondary element in this metallicity regime (Torres-Peimbert 1989;Vila-Costas & Edmunds 1993;Henry et al 2000;van Zee & Haynes 2006). Besides the uncertainties (that are higher than those estimated in other objects because the T e error is higher at higher metallicities) our data agree with the tendency found in other galaxy samples, as that compiled by Pilyugin et al (2003).…”

“…Besides the uncertainties (that are higher than those estimated in other objects because the T e error is higher at higher metallicities) our data agree with the tendency found in other galaxy samples, as that compiled by Pilyugin et al (2003). However, notice that the galaxy sample compiled by van Zee et al (1998) does not agree with our data, as her data have a systematically lower N/O ratio. In some cases, differences higher than 0.5 dex in the N/O ratio are found for a particular oxygen abundance.…”

“…(Right) N/O ratio vs. oxygen abundance for all galaxies analysed in this work (also included objects which chemical abundances are derived using empirical calibrations). For comparison, we have included other galaxy samples from the literature whose O/H ratio has been determined using empirical calibrations: Izotov et al ( , 2006, van Zee et al (1998), IT99 (Izotov & Thuan 1999) and PTV03 (Pilyugin et al 2003).…”

“…We compare our data with the two galaxy samples previously indicated (Izotov & Thuan 1999;) and with other galaxy samples whose data have been obtained using empirical calibrations: Izotov et al Izotov et al 2006, whose data were extracted from the SDSS, and van Zee et al (1998), who study data from H ii regions within spiral galaxies with chemical abundances computed via the direct method or using the McGaugh 1994 empirical calibration. In some sense, the N/O ratio of a galaxy is an indicator of the time that has elapsed since the bulk of star formation occurred, or of the nominal age of the galaxy as suggested by Edmunds & Pagel 1978.…”

“…However, the observational evidence for low-mass galaxies with galactic winds is still unclear (i.e. Marlowe et al 1995;Bomans et al 2007;Dubois & Teyssier 2008;van Eymeren et al 2008van Eymeren et al , 2009van Eymeren et al , 2010 Consequently, the chemical properties of these galaxies would vary very fast (few tens of Myr) during the starburst phase (Kobulnicky & Skillman 1998). The delayedrelease hypothesis also predicts that BCDGs with high N/O ratios are experiencing their first burst of massive star formation after a relatively long quiescent interval (oxygen has still not been completely delayed by massive stars and mixed with the surrounding ISM), while those objects with low N/O ratios have had little or no quiescent interval.…”