EDP Sciences
Free access
Volume 398, Number 3, February II 2003
Page(s) 983 - 991
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20021725

A&A 398, 983-991 (2003)
DOI: 10.1051/0004-6361:20021725

Interstellar H $\mathsf{_2}$ in M 33 detected with FUSE

H. Bluhm1, K. S. de Boer1, O. Marggraf1, P. Richter2 and B. P. Wakker3

1  Sternwarte, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
2  Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
3  University of Wisconsin-Madison, 475 N. Charter Street, Madison, WI 53706, USA

(Received 30 October 2002 / Accepted 18 November 2002)

FUSE spectra of the four brightest $\ion{H}{ii}$ regions in M 33 show absorption by interstellar gas in the Galaxy and in M 33 . On three lines of sight molecular hydrogen in M 33 is detected. This is the first measurement of diffuse H 2 in absorption in a Local Group galaxy other than the Magellanic Clouds. A quantitative analysis is difficult because of the low signal to noise ratio and the systematic effects produced by having multiple objects in the FUSE aperture. We use the M 33 FUSE data to demonstrate in a more general manner the complexity of interpreting interstellar absorption line spectra towards multi-object background sources. We derive H 2 column densities of $\approx$ 1016 to 1017 cm -2 along 3 sight lines ( NGC 588 , NGC 592 , NGC 595 ). Because of the systematic effects, these values most likely represent upper limits and the non-detection of H 2 towards NGC 604 does not exclude the existence of significant amounts of molecular gas along this sight line.

Key words: galaxies: individual: M 33 -- ISM: abundances -- ISM: molecules -- ultraviolet: ISM

Offprint request: H. Bluhm, hbluhm@astro.uni-bonn.de

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