Up: Dynamics of blue compact fields
Eight galaxies were selected (see Sect. 1) for observations on the three dark nights
allocated at the ESO 3.6 m telescope on La Silla,
from August 30 to September 2 1995.
Observing conditions were photometric all nights. The seeing (measured at the telescope)
was slightly above one arcsecond.
The exposure times ranged between 24 minutes (1 minute per channel) and 160 minutes
(almost
7 minutes per channel), plus calibration exposures. The used instrument was
CIGALE, attached to the Cassegrain focus of the telescope.
CIGALE is basically composed of a focal reducer (bringing the original
f/8 focal ratio of the Cassegrain focus to f/3), a scanning
Fabry-Perot interferometer, an interference filter (to isolate the emission line and
suppress the sky brightness) and an IPCS detector (2-D photon-counting system).
The basic principles of this instrument were described in
Amram et al. (1991).
The pixel size, projected on the sky, is 0.91
with a resulting field of view
of
. The IPCS,
with a time resolution of 1/50 seconds and zero readout noise makes it
possible to scan the interferometer rapidly, avoiding problems with varying sky
transparency, airmass and seeing during long exposures; and thus has several
advantages over a CCD for this application. Narrow band interference filters were
used to isolate the region around the redshifted H
line. Different
interference filters were used for observing galaxies with different radial
velocities, see
Table 2. Only in two cases (ES0 350-IG38
and Tololo 0341-407) the [N II]
line might
be partly transmitted. This line is however observed to be very weak as compared to
H
in the two galaxies
(Bergvall & Östlin 1999;
Terlevich et al. 1991)
so there is no risk that this affect our
results.
In order to save observing time and to increase the
the spectral resolution,
with respect to the small velocity range of the observed galaxies, we
decided to use a high order Fabry-Perot interferometer
(hereafter simply referred
as to FP2 for Fabry-Perot interferometer number 2, see Table 1)
giving at H
a mean finesse of 10, a free spectral range (FSR) of 117
km s-1, and a scanning step of 4.8 km s-1. The FSR is the separation between
two consecutive interference orders and is therefore the maximum wavelength (velocity)
range that can be observed before wavelength overlap occurs. The finesse is
effectively the ratio between the FSR and the width of the Airy function. The Airy
function (or apparatus function) is the instrumental line profile. Thus by scanning
the FSR in a number of steps that equals at least twice the finesse, adequate
spectral sampling is obtained. The spectral resolution, R, given in
Table 1 is calculated from the width of the apparatus function, but for high
signal to noise (S/N) the effective R will be higher.
One galaxy was also observed using a lower order Fabry-Perot
interferometer (hereafter simply referred to as
FP1, see Table 1), which gave us the possibility to check the consistency
of our results. Calibrations were obtained by scanning a narrow neon line before and
after the observations of each galaxy. Table 1 lists some characteristics of
the set-up used for the observations. Table 2 gives the target names and
some parameters for the observed
galaxies.
Table 1:
Journal of observations
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Table 2:
List of observed galaxies. R.A. and Decl. give the right ascension (h m s)
and declination (
) of the targets for epoch 1950. mV is
the apparent magnitude in V.
is the velocity used for selecting
narrow band filter for the observations. "Filter" is the central wavelengths of the
used interference filters as measured in laboratory and corrected to the observing
conditions: a temperature of
C and a focal ratio of f/8. FWHM is the
full width half maximum of the used narrow band filter. The transmission curve for the
filters have a quite squared shape, thus the full width at (practically) zero
transmission is not much larger than the FWHM. Unless noted otherwise, all
observations were made with FP2
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Up: Dynamics of blue compact fields
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