5. Consistency with the non-ionizing far UV radiation
As the H luminosity density of the local universe is a key input in our calculation, we have tried to evaluate how this quantity and the associated number of photons compare with other measured quantities tracing the star formation activity in the universe such as the far-UV luminosity density as determined from surveys of galaxies or the far-UV (non ionizing) background as measured by in-orbit experiments.
The former quantity is related to the luminosity density emitted at 900 Å by star formation activity through two parameters, the Lyman break of a pure star-forming population and the extinction at far-UV wavelengths due to dust mixed with the young stars in each galaxy. The far-UV spectral energy distribution (from longward of the Lyman break to 2000 Å) of a pure (without dust) and continuous star-forming population can be assumed to be flat in energy per frequency unit as shown by the models of Bruzual & Charlot (1993) and the observations of star-forming galaxies with little extinction by Calzetti et al. (1994). In the case of a continuous rate as we expect for the average star formation in the local universe, the Lyman break factor should be of the order of 4 according to Bruzual & Charlot (1993) and possibly between 6 and 20 depending on the initial mass function for massive stars according to Leitherer & Heckman (1995). With an average far-UV extinction of the order of 1 mag as discussed by Deharveng et al. (1994), we get a far-UV (observed) to 900 Å (emitted) luminosity density ratio between 1.6 and 8. For comparison, the 1600 Å (observed) to 900 Å (emitted) flux ratios in the four galaxies observed by Leitherer et al. (1995) are found to be 0.3, 1.5, 1.6 and 6. With the ratio above and our local luminosity density at the Lyman edge of erg s-1 Hz-1 Mpc-3 we get a far-UV luminosity density of 2 - 10 erg s-1 Hz-1 Mpc-3. This is in agreement with the local luminosity density of 3 W Hz-1 Mpc-3 (H0 =50 km s-1 Mpc-1) evaluated at 2800 Å by Lilly et al. (1996) and of 6 W Hz-1 Mpc-3 (H0 =50 km s-1 Mpc-1) evaluated at 2000 Å by Milliard et al. (1997).
Although the far-ultraviolet background is rich of several possible components, some of them of galactic origin, various arguments have established the accumulation of galaxy light along the line of sight as the dominant extragalactic contributor (e.g. Bowyer 1991, Jakobsen 1995, for a review) with an intensity in the range 50 - 150 photon cm-2 s-1 Å-1 sr-1 (or 0.7 - 2 10-21 erg cm-2 s-1 Hz-1 sr-1 at say 2000 Å). This background radiation can be converted back into a local luminosity density according to equation (2) which, with the same assumptions as above but without the neutral gas absorption term and the intergalactic opacity term, writes as
The integral upper bound is now 1.2, the value which shifts the Lyman break at 2000 Å. Assuming and in the range -1 to -3 (the case is observed for galaxies with little extinction and is not appropriate for the average spectral shape) the upper limit background of 2 10-21 erg cm-2 s-1 Hz-1 sr-1 gives a far-UV luminosity density in the range 2.6 - 6 W Hz-1 Mpc-3, again in satisfying agreement with the range of values that we have derived above.
© European Southern Observatory (ESO) 1997
Online publication: April 28, 1998