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Issue A&A
Volume 439, Number 3, September I 2005
Page(s) 947 - 956
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20042395



A&A 439, 947-956 (2005)
DOI: 10.1051/0004-6361:20042395

Extra-planar gas in the spiral galaxy NGC 4559

C. V. Barbieri1, 2, F. Fraternali3, T. Oosterloo4, G. Bertin2, R. Boomsma5 and R. Sancisi1, 5

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 $5.9 \times 10^{8}~M_{\odot}$ (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.


Key words: galaxies: halos -- galaxies: individual: NGC 4559 -- galaxies: ISM -- galaxies: kinematics and dynamics

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© ESO 2005


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