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 det ected above the Hi absorption edge in our Galaxy. An upper limit is evaluat ed by comparison with the background measured in regions of the detector ad jacent to the observed spectrum. A spectral window of 16 Angstrom, reasonab ly free of additional Hi Lyman series line absorption, is used. No correcti on is needed for molecular hydrogen absorption in our Galaxy but a foregrou nd 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 Ang strom in the rest frame of Mrk 54. By comparison with the number of ionizin g photons derived from the H alpha flux, this limit translates into an uppe r limit of f(esc) < 0.062 for the fraction of Lyman continuum photons that escape the galaxy without being absorbed by interstellar material. This lim it 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 t o> 3.4. The difficulties and implications of a comparison are discussed.