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

4. Photometry

4.1. B_J and R_C CCD surface photometry

In April 1996, we obtained and CCD images of WKK 6092 with the 1-m telescope of the SAAO. The TEK #8 CCD was used, giving a field of view of and a pixel size of 0.347 arcsec/pixel.

WKK 6092 was observed under good seeing conditions ( and for and respectively) with exposure times of 1200 seconds in and 2400 seconds in . The R-band image is displayed in the upper panel of Fig. 1. This image clearly illustrates the inherent difficulties in identifying galaxies at these low galactic latitudes. Two of the more serious complications are:

1. The overwhelming number of foreground stars; applying DAOPHOT a thousand stars were, for instance, detected 3 above the background on the R-band image.

2. The uncertain and non-uniform galactic foreground extinction at these low galactic latitudes () and its effect on the observed properties of the regarded galaxies.

In the lower panel of Fig. 1, the star-substracted CCD image is displayed. This image demonstrates the effectiveness of the star substraction routine (important for the determination of the magnitude) and reveals further detailed structure of the Seyfert. The galaxy has a very blue nucleus. The bar of the galaxy is quite distinct and the disk very smooth with a clear superimposed ring; it is in fact a SBa(r) galaxy. The features of the Seyfert and the nearby companion, a dS0, do not reveal any indication that this galaxy pair is in gravitational interaction, despite their close position on the sky and in velocity space.

To determine the radial surface brightness profile and total apparent magnitudes one must first detect and then mask all the foreground stars in the image. This was done with DAOPHOT as implemented in IRAF. Once all the stars are identified, stellar light above a certain isophotal treshold is masked, this is approximately 23 mag/ in and 24 mag/ in .

The surface brightness profile is determined using Jedrzejewski's method (Jedrzejewski 1987) in IRAF. Both the ellipticity and position angle are kept fixed whilst determining the radial profile. This way the average counts per pixel will be based on the unmasked pixels. This method only works if less than 50% of the light is masked within the ellipse. A more detailed description of the surface photometry will be given in a separate paper, where we will map the galactic extinction from and photometry combined with the Mg₂ index of elliptical galaxies (Woudt et al., in prep.).

The upper panel of Fig. 4 shows the radial surface brightness profile of the Seyfert galaxy. The dotted line at the B = 24.5 mag/ isophotal level corresponds to the isophotal level of our diameter estimate on the IIIaJ film copy (Kraan-Korteweg et al. 1995). Our 'eye' estimate of 56" x 47" from the IIIaJ SRC film copy agrees very well with the CCD data = 58" x 52".

Fig. 4. Radial surface brightness profile (upper panel), Radial colour profile (middle panel) and the integrated magnitudes (lower panel).

With an observed axial ratio of d/D = 0.89 and an intrinsic flattening of r_o = 0.2, the inclination of the Seyfert according to the formalism given by Holmberg (1946), cos²i = (r² - )/(1 - ) is i = .

The middle panel of Fig. 4 shows the ( - ) colour. The inner 2.5 arcsec reveal a strong gradient: within the inner area the ( - ) colour changes by 0.5 - 0.6 mag. Not surprisingly, the nucleus of the Seyfert is very blue.

The lower panel shows the integrated magnitude as a function of radius. Within each ellipse the sum of the masked and unmasked pixels is multiplied by the average counts per pixel and this is then integrated over the entire galaxy. The total magnitude at the asymptotic value was found to be = mag, respectively = mag.

4.2. Near-infrared observations

The 0.75-m telescope of SAAO was used to obtain single aperture (9") JHKL (1.25 - 3.4 µm) broadband photometry of WKK 6092. The Seyfert was observed twice in May 1996. The observations were made in the same photometric system as reported by Glass & Moorwood (1985) and further details on observation and reduction procedures are described there.

The resulting J, H, K and L magnitudes of WKK 6092 are 12.91, 11.91, 11.51 and 10.60 (cf., Table 2), and the respective near infrared colours are (J - H) = 0.93, (H - K) = 0.47, (K - L) = 0.91. The typical errors in J, H and K are 0.03 mag, whereas the error in L is somewhat larger (0.15 mag).

Table 2. Observational parameters of WKK 6092

Assuming = 1.6 mag (cf., Sects. 3.2 and 6) and the relative extinction values in the near infrared by Cardelli et al. (1989), the extinction-corrected colours are (J - H)₀ = 0.82, (H - K)₀ = 0.38, (K - L)₀ = 0.84. These colours compare well with known Seyfert 1's (e.g. NGC 1566), although there is evidence for a significant contribution of the underlying galaxy - even with the 9" aperture which primarily contains the nucleus.

© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998
helpdesk.link@springer.de