Extra-planar gas in the spiral galaxy NGC 4559

¹ INAF – Osservatorio Astronomico, via Ranzani 1, 40127 Bologna, Italy e-mail: claudia.barbieri@bo.astro.it
² Università degli Studi di Milano, Dipartimento di Fisica, via Celoria 16, 20133 Milano, Italy
³ Theoretical Physics, University of Oxford, 1 Keble Road, Oxford OX1 3NP, UK
⁴ ASTRON, PO Box 2, 7990 AA, Dwingeloo, The Netherlands
⁵ Kapteyn Astronomical Institute, University of Groningen, The Netherlands

Received: 19 November 2004
Accepted: 15 April 2005

Abstract

We present 21-cm line observations of the spiral galaxy NGC 4559, made with the Westerbork Synthesis Radio Telescope. We have used them to study the H I distribution and kinematics, the relative amount and distribution of luminous and dark matter in this galaxy and, in particular, the presence of extra-planar gas. Our data do reveal the presence of such a component, in the form of a thick disk, with a mass of (one tenth of the total H I mass) and a mean rotation velocity 25-50 km s^-1 lower than that of the thin disk. The extra-planar gas may be the result of galactic fountains but accretion from the IGM cannot be ruled out. With this study we confirm that lagging, thick H I layers are likely to be common in spiral galaxies.