Astron. Astrophys. 356, 788-794 (2000)

2. Images

The longest ROSAT HRI pointing towards NGC 1275 was made in August 1994 with a total exposure time of about 52 ksec. The subsection of the HRI image, smoothed with a Gaussian, is shown in Fig. 1. The image is centered at NGC 1275. Two X-ray minima immediately to the north and south of NGC 1275 coincide (Böhringer et al. 1993) with bright lobes of radio emission at 332 MHz, mapped with the VLA by Pedlar et al. (1990). Another region of reduced brightness ( to the north-west from NGC 1275) was detected earlier in Einstein IPC and HRI images (Branduardi-Raymont et al. 1981, Fabian et al. 1981). It was suggested that reduced brightness in this region could be due to a foreground patch of a photoabsorbing material or pressure driven asymmetry in the thermally unstable cooling flow (Fabian et al. 1981). The complex shape of the X-ray surface brightness is much more clearly seen in Fig. 2 which shows the same image, adaptively smoothed using the procedure of Vikhlinin, Forman, Jones (1996). The "compressed" isophotes in the figure delineate a complex spiral-like structure. Comparison of Fig. 2 and Fig. 1 shows that the same structure is present in both images, i.e. it is not an artifact of the adaptive smoothing procedure.

Fig. 1. The subsection of the ROSAT HRI image convolved with a Gaussian. The image is centered at NGC 1275. Two X-ray minima immediately to the north and south of NGC 1275 coincide (Böhringer et al. 1993) with bright lobes of radio emission at 332 MHz. Another region of reduced brightness ( to the north-west from NGC 1275) was detected earlier in Einstein IPC and HRI images (Branduardi-Raymont et al. 1981, Fabian et al. 1981).

Fig. 2. The same image as in Fig. 1 adaptively smoothed using the wavelet-based procedure of Vikhlinin, Forman, Jones (1996). Contours are plotted with multiplicative increments of 1.05.

In order to estimate the amplitude of the substructure relative to the undisturbed ICM, we divided the original image (Fig. 1) by the azimuthally averaged radial surface brightness profile. The resulting image, convolved with the Gaussian is shown in Fig. 3. The regions having surface brightness higher than the azimuthally averaged value appear grey in this image and form a long spiral-like structure starting near the cluster center and ending from the center to the south-east. Of course the appearance of the excess emission as a "spiral" strongly depends on the choice of the "undisturbed" ICM model (which in the case of Fig. 3 is a symmetric distribution around NGC 1275). Other models would imply different shapes for the regions having excess emission. In particular a substantial part of the subtructure seen in Figs. 1 and 2 can be accounted for by a model consisting of a sequence of ellipses with varying centers and position angles (e.g. using the IRAF procedure ellipse due to Jedrzejewski, 1987). Nevertheless the image shown in Fig. 3 provides a convenient characterization of the deviations of the X-ray surface brightness relative to the azimuthally averaged value. Comparison of Fig. 3 and Figs. 1, 2 allows one to trace all features visible in Fig. 3 back to the original image.

Fig. 3. The subsection of the HRI image divided by the azimuthally averaged surface brightness profile at a given distance from NGC 1275 and convolved with a Gaussian. It characterizes the value of the surface brightness relative to the azimuthally averaged value. The darker the color, the larger is the value (white color corresponds to regions with surface brightness lower than azimuthally averaged value; dark grey corresponds to region which are 30% brighter than the azimuthally averaged value). Superposed onto the image are the contours of the radio flux at 1380 MHz (Pedlar et al. 1990). The radio data have a resolution of arcsec2 and the central region is not resolved. The compact feature to the west of NGC 1275, visible both in X-rays and radio is the radio galaxy NGC 1272.

Superposed onto the image shown in Fig. 3 are the contours of the radio image of 3C 84 at 1380 MHz (Pedlar et al. 1990). The radio image was obtained through DRAGN atlas (http://www.jb.man.ac.uk/atlas edited by J. P. Leahy, A. H. Bridle, and R. G. Strom). In this image, having a resolution of arcsec², the central region is not resolved (unlike the higher resolution image of the central area used in Böhringer et al. 1993) and it does not show features, corresponding to the gas-voids north and south of the nucleus. The compact feature to the west of NGC 1275, visible both in X-rays and radio, is the radio galaxy NGC 1272. Fig. 3 hints at possible relations between some prominent features in the radio and X-rays. In particular, the X-ray underluminous region to the North-West of NGC 1275 (Branduardi-Raymont et al. 1981, Fabian et al. 1981) seems to coincide with a "blob" in radio. A somewhat better correlation is seen if we compare our image with the radio map of Sijbring (1993), with its better angular resolution), but again the correlation is not one to one ¹. A similar partial correlation of X-rays and radio images also was found for another well studied object - M87 (Böhringer et al. 1995). For M87, the relatively compact radio halo surrounds the source and some morphological similarities of the X-ray and radio images are observed. Gull and Northover (1973) suggested that buoyancy plays an important role in the evolution of the radio lobes. Böhringer et al. (1993, 1995) pointed out that buoyant bubbles of cosmic rays may affect the X-ray surface brightness distribution in NGC 1275 and M87. Below we speculate on the hypothesis that this mechanism is operating in both sources and the disturbance of X-ray surface brightness is related, at least partly, with the activity of an AGN in the past.

© European Southern Observatory (ESO) 2000

Online publication: April 17, 2000
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