Interstellar H₂ in M 33 detected with FUSE ^*

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

Corresponding author: H. Bluhm, hbluhm@astro.uni-bonn.de

Received: 30 October 2002
Accepted: 18 November 2002

Abstract

Fuse spectra of the four brightest 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 H₂ 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 H₂ column densities of ≈ 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 H₂ towards NGC 604 does not exclude the existence of significant amounts of molecular gas along this sight line.