Astronomy & Astrophysics

Free access article

Issue		A&A Volume 444, Number 3, December IV 2005
Page(s)		791 - 801
Section		Extragalactic astronomy
DOI		10.1051/0004-6361:20053863

A&A 444, 791-801 (2005)
DOI: 10.1051/0004-6361:20053863

Sub-arcsecond imaging of the radio continuum and neutral hydrogen in the Medusa merger

R. J. Beswick¹, S. Aalto², A. Pedlar¹ and S. Hüttemeister³

¹ The University of Manchester, Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, UK
e-mail: Robert.Beswick@manchester.ac.uk
² Onsala Rymdobservatorium, Chalmers Tekniska Högskola, 43992 Onsala, Sweden
e-mail: susanne@oso.chalmers.se
³ Astronomisches Institut Ruhr Universität Bochum, Universitätßtr. 150, 44780 Bochum, Germany

(Received 20 July 2005 / Accepted 24 August 2005 )

Abstract
We present sub-arcsecond, Multi-Element Radio Linked Interferometer (MERLIN) observations of the decimetre radio continuum structure and neutral hydrogen (HI) absorption from the nuclear region of the starburst galaxy NGC 4194 (the Medusa Merger). The continuum structure of the central kiloparsec of the Medusa has been imaged, revealing a pair of compact radio components surrounded by more diffuse, weak radio emission. Using the constraints provided by these observations and those within the literature we conclude that the majority of this radio emission is related to the ongoing star-formation in this merger system.

With these observations we also trace deep HI absorption across the detected radio continuum structure. The absorbing HI gas structure exhibits large variations in column densities. The largest column densities are found toward the south of the nuclear radio continuum, co-spatial with both a nuclear dust lane and peaks in ¹²CO ( $1\rightarrow 0$ ) emission. The dynamics of the HI absorption, which are consistent with lower resolution ¹²CO emission observations, trace a shallow north-south velocity gradient of ~320 km s^-1 kpc^-1. This gradient is interpreted as part of a rotating gas structure within the nuclear region. The HI and CO velocity structure, in conjunction with the observed gas column densities and distribution, is further discussed in the context of the fuelling and gas physics of the ongoing starburst within the centre of this merger.

Key words: galaxies: individual: NGC 4194 -- radio lines: galaxies -- ISM: kinematics and dynamics -- galaxies: ISM -- galaxies: starburst -- radio lines: ISM

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