The highly obscured region around PKS 1343 – 601 - I. Galactic interstellar extinctions using DENIS galaxy colours

The highly obscured radio-bright galaxy PKS 1343 – 601 at Galactic coordinates of $(l,b) = (309\fdg7, +1\fdg8$) has been suspected to mark the centre of a hitherto unknown cluster in the wider Great Attractor region. As such it presents an ideal region for a search of galaxies in the near-infrared (NIR) and an in-depth study of their colours as a function of extinction. A visual search of a ~30 square-degree area centred on this radio galaxy on images of the NIR DENIS survey ($I J K$) revealed 83 galaxies (including two AGNs) and 39 possible candidates. Of these, 49 are also listed in the 2MASS Extended Source Catalog 2MASX. Taking the IRAS/DIRBE extinction values (Schlegel et al. 1998, ApJ, 500, 525) at face value, the absorption in the optical ( A B ) ranges from ~ 2 m to over 100 m across the Galactic Plane. Comparing the detections with other systematic surveys, we conclude that this search is highly complete up to the detection limits of the DENIS survey and certainly surpasses any automatic galaxy finding algorithm applied to crowded areas. The NIR galaxy colours from the $7\arcsec$ aperture were used as a probe to measure total Galactic extinction. A comparison with the IRAS/DIRBE Galactic reddening maps suggests that the IRAS/DIRBE values result in a slight overestimate of the true extinction at such low Galactic latitudes, the inferred extinction from the galaxy colours corresponds to about 87% of the IRAS/DIRBE extinctions. Although this determination still shows quite some scatter, it proves the usefulness of NIR surveys for calibrating the IRAS/DIRBE maps in the extinction range of about $2^{\rm{m}} \la A_B \la 12^{\rm{m}}$.