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

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5. 21cm observations

The Seyfert galaxy was observed in H I with the 64-m radio telescope at the Parkes Observatory (ATNF) in July 1995 as part of program of measuring redshifts for obscured, low surface brightness galaxies behind the Milky Way (Kraan-Korteweg et al., in prep.). In this observing mode we used two IF's and offset 512 of the 1024 channel of each polarization by 22 MHz. This resulted in a velocity coverage of 0-10000 km s-1 and a velocity resolution of 12.6 km s-1 . The observations were carried out in total power mode. Six 10 minute ON-source observations were performed, preceded by an equal length OFF-source observation at the same Declination but 10.5 time minutes earlier in Right Ascension to cover the same patch of the sky for both the reference and the signal spectrum. The online control program automatically corrected for the zenith angle dependence of the telescope sensitivity. The telescope has a HPBW of [FORMULA] with a system temperature at 21 cm of typically Tsys = 39 Jy.

The data were reduced using the package "SLAP", or Spectral Line Analysis Program (Staveley-Smith 1985). The two polarisations of the 6 scans were averaged together. With a 5th-order polynomial baseline an r.m.s. of [FORMULA] = 3.3 mJy was obtained for this 60 minute integration. The reduced H I spectrum of the Seyfert galaxy is displayed in Fig. 5.

[FIGURE] Fig. 5. H I spectrum as obtained at the Parkes 64m radio telescope from a 60 minute integration. The velocity axis is in the radio convention.

A weak signal is seen at v[FORMULA] km s-1 , translating to a velocity in the optical convention of vopt = 5012 km s-1 . This is higher than the optical measurements ([FORMULA]v = 301 respectively 324 km s-1 for MEFOS and SAAO). A difference between the HI velocity and the optical measurement, as observed here for WKK 6092, has been seen before in Seyfert galaxies (Mirabel & Wilson 1984). It is caused by the net outflow of gas in the narrow emission-line regions (Mirabel & Wilson 1984). The observed velocity difference for WKK 6092, i.e., [FORMULA]300 km s-1 , is relatively large for Seyfert 1's.

The signal to noise ratio is only S/N = 2.8, however, the signal is consistent in flux and shape throughout all individual scans. The profile is narrow and Gaussian in shape - not unexpected considering that this galaxy is fairly face-on.

The linewidths measured at the 50% and 20% level are [FORMULA] = 88 km s-1 and [FORMULA] = 97 km s-1 . Even when corrected for inclination and a velocity dispersion in the z-direction (cf., Richter & Huchtmeier 1984) the linewidths (150 and 169 km s-1 ) seem relatively low for a luminous spiral galaxy. The integrated H I flux of the Seyfert is I = 0.93 Jy km s-1 .

Adopting a distance of 93 [FORMULA] Mpc (Kraan-Korteweg et al. 1996), global properties can be evaluated. The H I mass is:


The absolute blue magnitude, corrected for galactic foreground extinction, is [FORMULA]. The H I mass-to-blue light ratio then is:


Following Casertano & Shostak (1980), the total mass can be determined from the corrected linewidth, the distance R (in Mpc) and the extinction-corrected diameter Do (in arcmin):


Although the H I mass and H I mass-to-blue light ratio are quite low, they both lie well within the range typical of barred spiral galaxies of morphological type SBa to SBab (cf., Huchtmeier & Richter 1988, 1989 for a field and cluster sample). Considering furthermore the uncertainties due to the large foreground extinction corrections, the total mass is also in perfect agreement with the expectation for a barred early-type spiral.

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© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998