The above discussion has focussed on identifying young galaxies at the highest accessible redshifts. The observed optical emission then necessarily samples the rest-frame UV. However, galaxy formation is an ongoing process; locally we see several galaxies with metallicities close to primordial (Kunth et al. 1994; Thuan & Izotov 1997). These nearby young systems sample a different segment of the galaxy luminosity function from the high-redshift systems discussed above - the expectation is that high-redshift galaxies are the progenitors of present-day massive galaxies (L* systems and larger) while the local low-metallicity systems are all very low-mass dwarf galaxies. Nevertheless, high signal-to-noise (S/N) UV observations of the local protogalaxy population provide a very useful laboratory for studying and understanding the high-redshift population.
As mentioned earlier, models predict strong (50-200 Å equivalent
width) Ly
emission from
young, dust-free galaxies forming their first generation of stars (e.g.,
Charlot & Fall 1991).
However, International Ultraviolet Explorer (IUE)
observations of local star-forming galaxies revealed
Ly strengths
considerably weaker than predicted by case B recombination: the
Ly /
H
intensity
ratio was always found to be
10, as opposed to
the theoretical value of 33. Furthermore, some galaxies showed
Ly absorption rather
than emission. Small amounts of dust intermixed with the extended
neutral gas was the assumed culprit (e.g.,
Hartmann, Huchra, &
Geller 1984).
Hartmann et al. (1988)
showed evidence for an anticorrelation of
Ly strength with
metallicity, which conformed to a simplistic scheme of chemical
enhancement and motivated searches for strong
Ly emission in distant
galaxies with anticipated primordial abundances. More recently,
HST observations of the two most metal-deficient galaxies known,
I Zw 18 (Z =
Z
/ 51;
Kunth et al. 1994)
and SBS 0335-052 (Z =
Z / 40;
Thuan & Izotov 1997)
show Ly in
absorption rather than emission, at odds with the results of
Hartmann et al. (1988).
Kunth et al. (1998a)
present HST UV spectra of eight H II
galaxies covering a wide range of metallicity. The observations were
designed to cover both the
Ly region and the region
around O I
1302 and Si
II
1304. The former
region allows study of the
Ly emission and
absorption properties and an estimate of the H I
column, while the latter allows a crude estimate of the chemical
composition of the gas and a measure of the velocity of the gas with
respect to the systemic velocity of the system as measured from optical
emission lines. Surprisingly, they find that the primary indicator of
Ly strength is
kinematics, not metallicity. The four systems with metallic lines static
with respect to the ionized gas show damped
Ly absorption, while the
four systems with
Ly emission show the
metallic lines blueshifted by
200 km s-1
with respect to the ionized gas. The implications are that even nearly
primordial clouds undergoing star formation have sufficient dust columns
to suppress
Ly emission provided the
kinematics of the neutral gas allows resonant scattering of the
Ly emission. In all
cases reporting
Ly emission in the
Kunth et al. (1998a)
sample, an asymmetric profile with a sharp blue cutoff is observed.
In addition to the local star-forming galaxies with (1) broad, damped
Ly absorption centered
at the wavelength corresponding to the redshift of the H II gas and (2) galaxies with
Ly emission marked by
blueshifted absorption features,
Kunth et al. (1998b)
notes a third morphology of
Ly line that is
occasionally observed in the local universe: (3) galaxies showing "pure"
Ly emission, i.e.,
galaxies which show no
Ly absorption whatsoever
and have symmetric
Ly emission profiles.
Terlevich et al. (1993)
presents IUE spectra of two examples: C0840+1201 and T1247-232,
both of which are extremely low-metallicity H II
galaxies.
Thuan, Izotov, &
Lipovetsky (1997)
present a high S/N HST spectrum of the latter galaxy, noting that
with Z =
Z / 23,
it is the lowest metallicity local star-forming galaxy showing
Ly in emission. At
higher S/N and higher dispersion than the IUE spectrum, the line
shows multiple absorption features near the redshift of the emission,
bringing into question the "pure" designation.
Tenorio-Tagle et
al. (1999)
have proposed a scenario to explain the variety of
Ly profiles based on the
hydrodynamical evolution of superbubbles powered by massive starbursts.
This scenario and observations of local star-forming galaxies have two
important implications for studies of high-redshift
protogalaxies. First, they provide a natural explanation for asymmetric
profiles which seem to characterize high-redshift
Ly (e.g., see
Fig. 9), but also imply that although the
asymmetric profile may be a sufficient condition for identification of a
strong line with
Ly, it is not a
necessary condition. This point is particularly important for judging
the identification of serendipitous and narrowband survey emission
sources whose spectra are dominated by a solitary, high equivalent width
emission line. Second, if
Ly emission is primarily
a function of kinematics and perhaps evolutionary phase of a starburst,
attempts to derive the comoving star formation rate at high redshifts
from
Ly emission will require
substantial and uncertain assumptions regarding the relation of observed
Ly properties to the
intrinsic star formation rate; use of the UV continuum
(
1500 Å) may be
preferred for measuring star formation rates.
 |
Figure 9. Co-averaged spectrum of the
serendipitously discovered galaxy 0140+326 RD1 at z = 5.34
(I = 26.1;
Dey et al. 1998).
The total exposure time is 36.2 ks. Note the strong
Ly |