There are two main types of radial velocity measurements in galaxies,
radio and optical ones. Radio observations give the velocity of neutral hydrogen,
mainly with the 21 cm line, meanwhile optical observations give the velocity of
ionized gas, mainly with H
and NII lines.
Both techniques are complementary since radio observations enable one to reach the
outermost parts of galaxies and optical observations provide a better spatial
resolution (a few arcseconds instead of typically 10 to 30 arcsec).
Most of the optical observations are based on slit spectroscopy, hence providing
information along a single line, whereas Fabry-Perot technique allows one to
obtain
velocity measurements all over the observed galaxy, hence providing a complete
2D velocity field. This enables one to derive the true rotation curve, whereas in the
case of slit spectroscopy it is often biased since the slit is oriented a
priori along the position of the major axis determined from photometry
observations, which is often off by several degrees from the kinematical
major axis.
Despite this great advantage of the Fabry-Perot over the slit spectroscopy,
most of the optical kinematical studies of galaxies are based on slit spectroscopy
and generally limited to the analysis of the rotation curve. A large number of
samples of rotation curves are available in the literature (e.g. Mathewson et al.
1992, with 965 H
rotation curves; Mathewson & Ford 1996, with 1051
H
rotation curves; Courteau 1997, with 304 galaxies; Dale et al. 1997,
1998 and 1999 with 522 rotation curves). However there is no large sample of
2D velocity fields at optical wavelengths, most of the Fabry-Perot observations
being concerned with individual peculiar objects or small samples, observed
at H
wavelength. The largest sample obtained up to now is that of
75 galaxies studied by Schommer et al. (1993) for measuring galaxy distances,
followed by the sample of 45 galaxies obtained by Amram et al. (1995) for studying
the behavior of galaxies in clusters. Let us mention also the sample of 7 galaxies
observed by Beauvais & Bothun (1999) for studying noncircular motions, which is
well suited for the Fabry-Perot since it is the only instrument capable of detecting
small scale structures in the velocity fields because of both spatial and spectral
resolution it offers.
By now, the total number of 2D velocity fields of galaxies available in the literature
must be around two hundred, observed with various telescopes and different
Fabry-Perot interferometers.
The main reason
is that the necessary exposure time is longer than with slit spectroscopy
(typically 2h instead of 10 mn), another reason being that the Fabry-Perot
technique is a bit more complicated for observing and for reducing data.
This lack of a large homogeneous sample of 2D velocity fields of galaxies at optical
wavelengths led us to initiate the GHASP project at the Observatoire de Haute-Provence.
GHASP (acronym for Gassendi H
survey of SPirals) is a survey of galaxies
led at Haute-Provence Observatory with a 1.93 m reflector equipped with a focal
reducer and a Fabry-Perot interferometer. The aim is to obtain high resolution
(both spatial and spectral) 2D velocity fields in the H
line of ionized
hydrogen for about 200 nearby spiral and irregular galaxies.
With this survey, we plan to complement the radio survey WHISP
(Westerbork survey of HI in SPiral galaxies) led at Westerbork
(Netherlands). High-resolution H
velocity fields are
complementary to HI velocity fields mapping the outer galactic
regions but suffering of beam-smearing and, most often, of a
lack of emission in the inner parts.
Began in 1993, WHISP is a survey of the neutral component in spiral and irregular
galaxies with the Westerbork Synthesis Radio Telescope (WSRT) at 21 cm wavelength.
Its aim is to obtain maps of the distribution and velocity structure of HI in 400 to 500 galaxies,
increasing the number of well-analyzed HI observations of galaxies by an order of
magnitude. The idea of its promoters is that such a uniform database will serve
as a basis for research in many areas,
for example: dark halos, effects of environment on the structure and growth of HI
disks, and galaxy distances.
More details can be found on the Web site http://www.astro.rug.nl/whisp/.
The GHASP survey will provide a homogeneous sample of 2D velocity fields which will cover a large range of luminosity and morphological types (except ellipticals) and will provide a complete reference sample at z = 0. This sample will be used for comparison with galaxies observed: in various environments (pairs, groups, clusters); at different stages of evolution (mergers, starbursts) and at higher redshifts. The 2D velocity fields will be used to constrain the mass distribution of spirals, as explained in Sect. 6. Complementary photometric observations are in progress at the Observatoire de Haute-Provence to derive the distribution of dark matter in the galaxies of our sample. Also, we are going to compare our velocity fields with models in order to derive a theoretical interpretation (e.g. Amram 2002, for more informations).
Copyright ESO 2002