Astron. Astrophys. 338, 8-14 (1998)

3. Spectroscopy

The spectroscopic observations described here are part of a larger program to gauge the 3-dimensional distribution of galaxies behind the southern Milky Way. The reader is referred to Kraan-Korteweg et al. (1994) for details and preliminary results on the individual approaches.

3.1. MEFOS spectroscopy

The MEFOS (M eudon E SO F ibre O bject S pectrograph) observation of WKK 6092 was made in February 1994. MEFOS is mounted at the prime focus of the 3.6-m telescope of the European Southern Observatory (ESO), La Silla, resulting in a 1 degree field (Felenbok et al. 1997). The CCD Tek #32 was used together with grating #15, yielding a dispersion of 170Å/mm and a resolution of about 11Å. The exposure time was 2 x 30 minutes.

The MEFOS spectrum of WKK 6092 (cf., Fig. 2, upper panel) revealed broad Balmer emission lines, indicative of it being a Seyfert 1 galaxy. The emission lines yield an observed radial velocity of km s^-1 . Further analysis is not possible from this low resolution, narrow wavelength range (3850Å - 6100Å) spectrum, and the galaxy was subsequently reobserved at the SAAO (cf., Fig. 3, next section). The neighbouring galaxy WKK 6103, visible left of the Seyfert in Fig. 1, has been observed during the same run on another MEFOS field. It has a radial velocity of km s^-1 (determined from standard cross-correlation techniques following Tonry & Davies 1979), is hence at a similar redshift as the Seyfert and might be a companion of WKK 6092.

Fig. 2. The MEFOS spectrum of the Seyfert WKK 6092 (upper panel) and the companion WKK 6103 (lower panel).

Both spectra are displayed in Fig. 2. The upper panel shows the Seyfert and the lower panel the neighbouring galaxy.

3.2. S.A.A.O. spectroscopy

We reobserved WKK 6092 for a total of 2500 seconds in June 1995 at the SAAO using the 1.9-m Radcliffe reflector with "Unit" spectrograph and a reticon photon-counting detector. This results in a wider spectral range (3500Å - 7500Å) including therewith also the H emission line. More importantly, we observed a spectro-photometric standard star (LTT 7379) allowing the determination of the relative instrumental response and its correction.

The spectrum is displayed in Fig. 3. Its features identify it as a Seyfert 1 galaxy: broadened Balmer lines with a full width at zero intensity of 7500 km s^-1 , but with significant narrow emission superposed, and strong narrow forbidden lines (cf., labels in Fig. 3).

Fig. 3. Flux calibrated SAAO spectrum of the Seyfert 1 WKK 6092. The spectrum was calibrated using the spectrophotometric standard star LTT 7379. The emission lines are labelled.

The line strengths could be measured to an accuracy of approximately 10%. For most narrow emission lines, the uncertainty is somewhat higher due to the the difficulty in separating the broad from the narrow emission lines, particularly in H where the peak of the broad component is heavily masked by the [NII] lines on either side of the narrow H emission line. The line strengths are listed in Table 1. The header line lists the identified emission lines with their respective wavelengths in Ångstrom. The fluxes are expressed relative to a total H-line intensity of 100.

Table 1. Line strengths determined from the flux calibrated SAAO spectrum.

The broad emission lines show a Balmer decrement (H/H) of 4.24, the narrow lines indicate a general Balmer decrement of 4.47. Given the difficulty in separating the broad from the narrow component in the SAAO spectrum, these values are in good agreement within the observational errors.

The steepening in the Balmer decrement of the narrow component - the recombination value is 2.85 (Aller 1984) - is caused by reddening alone (Osterbrock 1989) and corresponds to a galactic extinction of mag. This is in good agreement with values derived from the galactic H I column density which - at the position of the Seyfert - is atoms cm^-2 (Kerr et al. 1986). Assuming a constant gas-to-dust ratio, the formalism given by Burstein & Heiles (1982) predicts an absorption in the blue of =1.5 mag. This value is furthermore supported by extinction measurements from fits to the ROSAT PSPC X-ray spectrum (cf., Sect. 6).

The ratio of the [SII] lines 6716/6731 indicates an electron density (N_e) of approximately 600 cm^-3. Generally high excitation is shown by the presence of weak [OIII] 4363 and [NeIII] 3869. The strength of the 4363 line suggests a temperature above 15000 K. The continuum can be described by a power law.

© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998
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