3. Comparison of the mid-infrared and optical images
The LW3 flux (12-17 µm) is shown in Fig. 1 in red contours superimposed on the HST WFPC2 combined images in the V (5252 Å) and I (8269 Å) filters by Whitmore et al. (1997). The infrared emission appears to be associated with the nuclei of both galaxies, the star forming ring in the northern galaxy NGC 4038, and with the relatively obscured overlap region of the two disks, which is 5x3 kpc in extent. The emission is very clumpy and bright knots dominate the mid-infrared emission, which as discussed below, comes from gas and dust heated by massive stars.
The two optical nuclei are detected as bright knots in the mid-infrared, but the brightest knot in the 15 µm map is 2.3 kpc east of the nucleus of the southern galaxy NGC 4039. This knot is toward the southern corner of the region of overlap of the two disks. In this region, several optical red objects were found (Whitmore & Schweizer, 1995) associated with discrete intensity peaks in the centimeter continuum maps (Hummel & van der Hulst, 1986) and millimeter CO maps (Stanford et al. 1990). The 5x3 kpc overlap region contributes about half of the 2.16 Jy flux radiated from the whole system at 15 µm. An average extinction of 70 mag in the overlap region has been derived by Kunze et al. (1996) using SWS ISO observations. A mean absorption of 73 mag was independently inferred by Stanford et al. (1990) from CO interferometric observations.
At 15 µm the brightest area of the overlap region, located east of the nucleus of NGC 4039, is resolved into two peaks separated by . The fainter one has a bright WFPC2 visual counterpart. However, the brighter eastern peak in the 15 µm map has only a faint I band counterpart of 48 pc in effective radius (Whitmore & Schweizer, 1995), but no conspicuous V band counterpart is seen in the HST image of Fig. 1. The brighter eastern knot alone contributes 15% of the total luminosity at 15 µm and at this wavelength it is almost 100 times brighter than the nearby nucleus of NGC 4039 (Vigroux et al. 1996). From J, H, and K band images (Duc & Mirabel, 1997), we measure an excess of 7 mag in the visual absorption in front of the eastern knot relative to the western knot.
The most luminous knots at 15 µm do not coincide with the most prominent dark lanes of the HST image. An even more striking displacement between the mid-infrared and optical dust lanes has been found in Centaurus A (Mirabel et al. 1998). These displacements could be due to different spatial distributions of the warm dust that radiates at 15 µm, and the cool dust that causes most of the optical obscuration. The dark lanes at optical wavelengths result from dust in the foreground of the optically emitting material, and projection effects also may play a role.
© European Southern Observatory (ESO) 1998
Online publication: April 15, 1998