Star-forming galaxies are the dominant contributor to the non-ionizing
UV radiation field in the universe. Are they a significant component of
the ionizing background as well? Simple arguments might suggest
otherwise. An H I column density of ~ 1
× 1018 cm-2 is sufficient to absorb
essentially all the ionizing radiation. Since the measured extinctions imply
column densities that are three or four orders of magnitude higher than
this, it might appear that essentially no ionizing radiation can
escape. However, the porosity of the ISM seen in the non-ionizing
continuum
( > 912 Å)
could very well extend below the Lyman edge and may dominate the shape
of the emergent spectrum.
The situation is sufficiently complex that the only
way to determine the escape fraction fesc of the
ionizing radiation is via a direct measurement.
Attempts to measure
fesc fall into two categories: observations of local
galaxies with a far-UV detector, or measurements using galaxies at
cosmological redshift, which are accessible from the ground with 8-m
class telescopes. Either technique has its advantages and
disadvantages. The "local" approach faces the obvious challenge of
extreme UV observations, whereas the "distant" measurement is affected
by the radiative transfer in the intergalactic medium. In addition, a
somewhat less direct method is to determine the Lyman continuum opacity
from a comparison of the
H and UV luminosity
functions in the local universe. Table 1 gives a
summary of recent results. Except for the last entry in this table, all
quoted studies find more or less stringent upper limits on
fesc both in the low- and
high-redshift universe. The ISM in the observed galaxies is highly
opaque, and very little stellar ionizing radiation leaks out.
Reference | Instrument | Objects | fesc |
Leitherer et al. (1995) | HUT | 4 galaxies;
z ![]() |
< 3% |
Hurwitz et al. (1997) | HUT | 4 galaxies;
z ![]() |
< 19% |
Deharveng et al. (1997) | H![]() |
local luminosity function | < 1% |
Deharveng et al. (2001) | FUSE | Mrk 54
z ![]() |
< 5% |
Giallongo et al. (2002) | FORS2 | z = 2.96, 3.32 | < 16% |
Fernández-Soto et al. (2003) | WFPC2 | HDF; 1.9 < z < 3.5 | < 4% |
Malkan et al. (2003) | STIS | 1.1 < z < 1.4 | < 1% |
Steidel et al. (2001) | Keck/LRIS | 29 galaxies;
z ![]() |
~ 100% |
Steidel et al. (2001)
detected significant Lyman-continuum flux in the composite spectrum of
29 Lyman-break galaxies with redshifts
z = 3.40 ± 0.09. If the inferred escaping Lyman-continuum
radiation is typical of galaxies at
z 3, then
these galaxies produce about 5 times more H-ionizing photons per unit
comoving volume than quasars at this redshift, with the obvious
cosmological implications.
Haehnelt et al. (2001)
fitted the composite spectrum by a standard stellar population and no
intrinsic H I opacity. Therefore,
fesc must be close to 100% for the observed
sample. Confirmation or rejection of this striking result will be a
major objective of observational cosmology.