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Astron. Astrophys. 362, 119-132 (2000)

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2. Observations and data reduction

The dataset consists of spectra of the normal late-type spirals NGC 1963, IC 2531, NGC 3044, NGC 4302, NGC 4402, NGC 4634, and of the irregular galaxy NGC 2188. All objects in the sample are seen nearly edge-on (see Table 1). Slit positions were determined from narrow band [FORMULA] images on which diffuse HII gas at high galactic latitudes was detected (Rossa & Dettmar 2000).


[TABLE]

Table 1. Sample overview.
Notes:
a) for [FORMULA], b) Bottinelli et al. 1984, c) Teerikorpi et al. 1992
d) The Surface Photometry Catalogue of the ESO-Uppsala Galaxies; Lauberts A. and Valentijn E.A., 1989
e) NASA Extragalactic Database
f) Third Reference Catalogue of Bright Galaxies; de Vaucouleurs et al., 1991
g) Rossa & Dettmar 2000


The spectroscopic data were obtained during January/February 1995 with the ESO 1.52m telescope in combination with the Boller & Chivens spectrograph at La Silla (Chile). A Ford Aerospace CCD (2048 [FORMULA] 2048 pixel) with a pixel size of 15 [FORMULA] 15 µm and a spatial resolution of 0.82" pix-1 was used during the observations. Grating [FORMULA] 23 provided a dispersion of 126 Å [FORMULA] which resulted in a coverage of a wavelength range from 3500 Å - 7450 Å. We determined the spectral resolution to 4.6 Å by measuring the FWHM of an unbroadened emission line in a HeAr-calibration spectrum. The total integration time per slit position was 90 minutes.

Data reduction was performed using the IRAF and MIDAS software packages. All data were bias subtracted and dome flats were used to reduce small scale sensitivity variations. An extinction correction was performed by adopting the atmospheric extinction curve for La Silla (Tüg 1977). Spectroscopy of the standard star Hiltner 600 yielded the wavelength dependent response function. We divided all object frames by this function to correct for wavelength dependent sensitivity variations. Subsequently a flux calibration of our spectra was applied.

The slit length of [FORMULA] allowed a precise night sky subtraction. This reduction process did not influence the emission lines of HeI [FORMULA]5876 and [OI ][FORMULA]6300 since they are sufficiently redshifted from the atmospheric lines for our objects. Due to the large wavelength range we could not apply a single continuum fit to the whole spectrum. Therefore the continuum level was estimated and subtracted row by row from the total emission of each galaxy using linear fits to the continuum on each side of prominent emission lines. The data for NGC 2188 are already presented by Domgörgen & Dettmar (1997) and will not be further discussed here. As a consistency check these spectra were reanalysed and the previous results were fully reproduced.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 19100
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