Astron. Astrophys. Suppl. Ser. 131, 317-318
H. Petit
Send offprint request: H. Petit
Observatoire de Marseille, 2 place Le Verrier, F-13248 Marseille Cedex 04, France
Received February 5; accepted March 19, 1998
A catalogue of HII regions in NGC 7331 is presented, with
positions relative to the center of the
Galaxy and relative H fluxes.
The catalogue can be found as an electronic file at the CDS
related to the companion
paper "Catalogue of HII region measured on 6 meter telescope
plates at Observatoire de Marseille".
Key words: galaxies: NGC 7331 -- galaxies: ISM -- HII regions -- catalogue
Several years ago, a program of H narrow-band photography of HII
regions in galaxies was undertaken with the f/1 focal reducer of Courtès
attached at the prime focus of the 6-m telescope of the soviet Special
Astrophysical
Observatory in Zelentchuk. In this
catalogue we present the results on the galaxy NGC 7331.
The photograph was obtained at the prime focus reducer of Courtès
(Courtès et al. 1987). It is a 120 m exposure on a 4415 Kodak film.
The 20'
circular field encompasses the full galaxy. The angular resolution is limited
by the seeing to about . The interference filter is centered at 6557 Å and
is 25 Å wide (FWHM).
A preliminary identification of the HII regions was conducted
by visual intercomparison of the H and continuum photograph. It
permitted
to reject foreground stars and gelatine defects. The H
photograph was
then digitized with the MAMA microdensitometer of the Observatoire de Paris
using a 10 microns square scanning slit and a 10 microns sampling step. The
data reduction was performed on the VAX 4000/500 computer of the Observatoire de
Marseille using MIDAS and special procedures designed within the framework of
MIDAS. The method is the same than that used for M 81
(Petit et al. 1988),
NGC 2403
(Sivan et al. 1990) and M 51
(Petit et al. 1996). The photographic
densities were converted into arbitrary intensities using a relative photometric
calibration of the Kodak emulsion.
We assigned to each HII region the positions of the photometric centre
provided by the corresponding isophotal system. The
coordinates, measured
on the MAMA reference frame, were then converted into a system centred on the
nucleus of NGC 7331,
with the X and Y axes in the E-W and N-S directions
respectively.
The flux for each HII region was computed by interactive integration of
the intensity contained within the outermost isophote down to the local
threshold and corrected for the background intensity; the aperture used for
the H flux integration of the individual HII regions corresponds to
20'' in diameter.
The diameters of the HII regions were measured following the method used in previous papers (Milliard & Marcelin 1981; Petit et al. 1988; Sivan et al. 1990; Courtès et al. 1993; Petit et al. 1996). These authors define the effective diameter of the HII region as the diameter of the region measured at half its total flux, corrected for instrumental broadening.
The correction is made by comparison with the observed half-flux diameter
of the field star images which have the same integrated flux. In other words,
the effective diameter is given by
where k is a scale factor, and A2 and A2* are the areas on the image containing
half the total flux, for the HII region and for a field star of the same total
flux, respectively. This method allowed us to determine the effective diameters
for 55.6% of the identified HII regions
(A2 > A2*). The half-flux effective
diameter method is well adapted to the HII regions of
relatively homogeneous
brightnesses, which is, for obvious reasons, the case for HII region diameters
close to the seeing disk (for distant galaxies this case is the most
frequent).
The table gives the following information:
Column 1: name of the galaxy.
Column 2: lists the reference number given to the individual HII regions.
Columns 3 and 4: give the
position (1950) of the centre of the
galaxy.
Columns 5 and 6: give the positions
of the photometric centre of the regions.
Column 7: lists the total H fluxes of the regions in
arbitrary units.
Column 8: gives the effective (half-flux) diameter D in arcsec.
Column 9: gives the geometrical diameter in arcsec.
Copyright The European Southern Observatory (ESO)