ICM/Radio-Galaxy Interaction

Figure: Left panel: Three-colour raw photon image of the central $24\times 24\arcsec^2$ regions of the 3C 401 system overlaid with contours of 20cm radio emission from MERLIN. The nucleus of 3C 401 lies at the center of this image and is coincident with the bright point X-ray source. Right panel: Adaptively smoothed total intensity image of this region. The anti-coincidence between the radio lobes and the X-ray emission is evident and results in the formation of a central X-ray bar.

In Fig. 2, we show both a raw 3-color X-ray image and an adaptively smoothed total intensity X-ray image of the central regions of the 3C 401 system. As before, the X-ray images have been overlaid with contours of 20cm radio emission from MERLIN. As is apparent in the raw image, there is a luminous point-like X-ray source at the location of the 3C 401 radio core (see Section 3.3 for more discussion of this source). Although the photon statistics are not good, the ICM emission is clearly not spherically-symmetric. In particular, there is a notable deficit of counts $3-5\arcsec$ north of the central source compared with the same distance east or west of the central source. This deficit is clearly shown in the smoothed total intensity map, which also reveals a similar deficit in the region 3-5arcsec south of the central source. In other words, there is a strong nuclear bar apparent, with a position angle orthogonal to the radio axis.

Furthermore, careful inspection of the raw image reveals hints of a cone-like structure bounding the northern radio-lobe. However, this structure does not appear to survive the adaptive smoothing process and so its reality cannot be confirmed.

Figure 3: (a) X-ray surface brightness profiles for a back-to-back pair of quadrants that include the radio lobes (dashed line) and the orthogonal pair of quadrants that avoid the radio lobes (solid line). (b) X-ray surface brightness profile analysis using eight 45degree sectors centered on 3C 401. The solid line shows the profile averaged over the four alternating sectors that avoid the spurs of the cross-like structure seen in Fig. 1. The dashed line shows the profile for the complementary sectors that are coincident with the spurs. The clear excess seen in this latter profile verifies that the cross-like structure is not an artifact of the adaptive smoothing procedure.

We further investigate these features by forming radial surface brightness profiles. Figure 3a shows the surface brightness profile within quadrants that are centered on the radio-axis of 3C 401 (the on-axis profile; red line), as well as the profile within the orthogonal quadrants (the off-axis profile; green line). The nuclear bar-like structure gives rise to a factor of two enhancement in the off-axis surface brightness at the smallest radii probed in our surface brightness analysis (7 pixels or 3.5arcsecs). There are other subtle differences between the two radial profiles (in the form of relative enhancements in the off-axis profile at 15arcsecs and 22arcsecs. These differences, while statistically significant with respect to Poisson noise, have no obvious corresponding features in either the raw or smoothed the X-ray image. Deeper X-ray images will be required to assess the nature of these particular deviations from circular symmetry.