3.2. The rest-frame optical spectroscopy
We have also obtained near-IR spectroscopy for some Lyman-break galaxies
in our
sample, which allowed us to study the optical rest-frame spectrum of
these systems (Pettini
et al. 1998, in prep.). These observations have been carried out at the
UKIRT 3.8-m
telescope with the CGS4 spectrograph and had, therefore, to be limited
to the brightest
galaxies of our sample. Even in this case, we could essentially only
study the nebular emission lines of [OII],
H
and [OIII] of the
galaxies and only barely detect their
continua (see Figure 5). Nevertheless, these
data have provided very useful complementary
information on the amount of extinction and the kinematics of
Lyman-break galaxies.
|
Figure 5. Two examples of near-IR spectra
(K band) of Lyman-break galaxies obtained with
the UKIRT telescope and the CSG4 spectrograph (Pettini et al. 1998, in
preparation). The
observed nebular emission lines are labeled. The vertical dashed lines
mark the position of
major night sky lines. In each case, the continuum emission is just
barely detected. The
emission lines are resolved and their velocity widths correspond to
velocity dispersions in the
range of 50 |
There is a broad agreement between the star-formation rates derived from
the H
fluxes and the ones obtained from the far-UV continuum luminosity
density, once
moderate corrections for dust obscuration are included. A typical net
correction to the
observed star-formation rates would be again in the range of 3x to 7x,
depending on
the adopted obscuration law. This seems to exclude, at least in the
galaxies for which
near-IR spectroscopy is available, large amounts of dust obscuration, as
have been suggested by some (e.g.,
Meurer et al. 1997).
Interestingly, using the
Calzetti's law (1997)
to compute H fluxes
from the UV continuum results in values that are in line with the
observed ones only if continuous star formation is assumed, while
significantly higher
fluxes are found in the case of a burst (Pettini et al. 1998, in preparation).
As discussed above, the near-IR spectra have also shown differences of
up to ~ 400
km s-1 between the redshifts of the UV absorption lines and
those of the optical nebular
emission lines, with the latter having higher redshift than the former
and being more
likely to be closer to the systemic velocities of the galaxies than the
UV absorption lines.
The lines are all resolved and their widths, if interpreted in terms of
velocity dispersion, yield values in the range 50
160 km
s-1. If the kinematics of these lines is a
good tracer of the gravitational motions within the galaxies, then
combining the velocity
dispersion with the sizes derived from the HST images gives
dynamical masses in the
range 1010-1011
M
, comparable to the
mass of the Milky Way bulge
(Dwek et al. 1995)
and to the mass of the innermost few kpc of an L* elliptical
galaxy. While the extent
to which the optical nebular lines can be used as dynamical indicators
is not known, we
note that the total masses involved are very likely to be substantially
greater, given that
the present IR observations sample only the innermost cores of the
galaxies, where the star formation rates are highest.