The data set used in this study combines spectra of high-redshift galaxies
observed with the ESO VLT and spectra of local starburst
galaxies taken from the IUE archive. 51 of the high-redshift
spectra were selected (according to the criteria listed below) from
about 300 low-resolution spectra observed mainly during the
spectroscopic observing runs of the FDF program
(in 3 nights in Sep. and Oct. 2000 and 3.5 nights in Oct. and Dec. 2001).
For these
observations we used FORS1&2 at the VLT in MOS and MXU mode with a
slit width of 1
and the 150I grism. (For instrumental
details see the FORS Manual at the ESO web page www.eso.org).
A few additional spectra had already been obtained during the
commissioning phases of FORS1&2 in 1998, 1999 and 2000
using the same spectroscopical setup as described above.
All spectra cover a spectral range (in the observer's frame)
from about
3400 Å to about 10 000 Å with a spectral scale of 5 Å/pixel
and a spectral resolution of about 200.
Although spectra of galaxies as
faint as I = 26.0 mag have been observed successfully in the FDF, in the
present investigation only galaxies with I
mag were included.
Depending on the objects' magnitude and the seeing conditions
(
on average) the integration times ranged between 2 and 12 hours.
The data reduction (bias subtraction,
flatfielding, cosmic ray elimination, sky subtraction,
wavelength calibration, etc.), was performed using standard MIDAS
routines. A detailed
description of the FDF spectroscopic observing program
and the data reduction procedures will be presented by
Noll et al. (2002).
With respect to the photometric redshift catalogue of the FDF (see
Bender et al. 2001) our
spectroscopic sample is complete to about 85% for our limiting magnitude
(I mag) for photometric redshifts in the range
.
The distribution of the spectroscopic redshifts is in good agreement
with the photometric redshift
distribution of the FDF which has peaks at redshifts of around 2.4 and 3.4
(see Fig. 3 in Bender et al. 2001).
Exceptions are the redshifts in the range
where
a lack of
strong spectroscopic features in our observed wavelength range makes
a reliable spectroscopic redshift determination
rather difficult, resulting in an artificial low number
of objects in our spectroscopic sample.
![]() |
Figure 1:
Examples of the low-resolution galaxy spectra obtained with FORS.
The redshift of the objects is increasing from panel a) to
c). The dotted line indicates the noise level which (due to the
night sky spectrum and the wavelength dependent instrumental efficiency)
varies with wavelength. The S/N of these spectra is
about ![]() |
For the present investigation we selected those FDF galaxies
showing absorption line spectra with an adequate S/N
(>10 per resolution element) for a meaningful quantitative analysis of the C IV resonance doublet.
All these galaxies show typical
starburst characteristics in their spectra such as intense (rest frame)
UV continua and highly ionized metal absorption lines.
Three examples of FDF spectra are displayed in Fig. 1.
The 10 high-redshift galaxies which are dominated by their Ly emission were not included in our study.
This leads to a limitation of the sample to
galaxies with z < 4, since most of the few FDF galaxies with
larger redshifts (up to 5.0) observed so far show strong Ly
emission (while pure absorption line spectra dominate at lower redshifts; see Noll et al. 2002).
Also excluded from our study were 4 objects that show absorption lines with
clear emission components, forming P-Cygni profiles.
(The apparent emission peak redwards from the C IV absorption feature in Fig. 1c is not a P Cygni emission component.
The absorption line is unshifted and, as pointed out in the
caption, the apparent emission component is an artifact.)
In order to enlarge our sample somewhat we added 6 additional FORS spectra (matching the criteria listed above) which had been observed with the same setup during the FORS commissioning runs (and are now available from the VLT archive). Four of these additional spectra were selected among the gravitationally amplified galaxies behind the cluster 1E0657-558 (Mehlert et al. 2001). We further included two spectra from the HDF-S and AXAF Deep Field follow-up studies (Cristiani et al. 2000), which met our criteria.
The comparison sample of IUE low resolution spectra
from the IUE archive consists of
36 local (
)
starburst galaxies investigated by
Heckman et al. (1998)
. All these IUE spectra
were obtained with the Short Wavelength Prime Camera in the low
dispersion mode and, therefore,
cover a similar rest-frame spectral range with a slightly better
spectral resolution
as our FORS spectra of high redshift objects. The IUE spectra were reduced
using the pipeline provided by the archive and smoothed to attain the same
spectral resolution as the VLT spectra.
Copyright ESO 2002