Constraints on the Lyman continuum radiation from galaxies: First results with FUSE on Mrk 54

We present Far Ultraviolet Spectroscopic Explorer observations of the star-forming galaxy Mrk 54 at z=0.0448. The Lyman continuum radiation is not detected above the Hi absorption edge in our Galaxy. An upper limit is evaluated by comparison with the background measured in regions of the detector adjacent to the observed spectrum. A spectral window of 16 Angstrom, reasonably free of additional Hi Lyman series line absorption, is used. No correction is needed for molecular hydrogen absorption in our Galaxy but a foreground extinction of 0.29 mag is accounted for. An upper limit of 6.15 x 10(-16) erg cm(-2) s(-1) A(-1) is obtained for the flux at approximate to 900 Angstrom in the rest frame of Mrk 54. By comparison with the number of ionizing photons derived from the H alpha flux, this limit translates into an upper limit of f(esc) < 0.062 for the fraction of Lyman continuum photons that escape the galaxy without being absorbed by interstellar material. This limit compares with the limits obtained in three other nearby galaxies and is compatible with the escape fractions predicted by models. The upper limits obtained in nearby galaxies contrasts with the detection of Lyman continuum flux in the composite spectrum of Lyman-break galaxies at z <approximate to> 3.4. The difficulties and implications of a comparison are discussed.