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

¹ Jodrell Bank Centre for Astrophysics, School of Physics & Astronomy, The University of Manchester, Alan Turing Building, Oxford Road, Manchester, M13 9PL, UK e-mail: Janine.VanEymeren@manchester.ac.uk
² Astronomisches Institut der Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum, Germany
³ Australia Telescope National Facility, CSIRO, PO Box 76, Epping, NSW 1710, Australia
⁴ Laboratoire d'Astrophysique de Marseille, OAMP, Université Aix-Marseille & CNRS, 38 rue Frédéric Joliot-Curie, 13013 Marseille, France

Received: 2 February 2009
Accepted: 20 July 2009

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.