3C 296


Basic Data
S178 Alpha FR Class ID Spectrum Best z mag. LAS lg P178 D
14.20.67IBTJ Gal0.0237R = 11.43 437.0024.08 192.6


Size: 382.5 × 384.0 arcsec²
LUT: Logarithmic
Beam: 4.9 arcsec
Frequency: 1477 MHz
Method: CLEAN/VTESS’ėˇ>4.9
Telescope: VLA B+C
Credits: Leahy & Perley (1991)

Study of 3C 296 has been rather accident prone. Its host galaxy, the large elliptical NGC 5532, has often been incorrectly called IC 5532. Further, it is sometimes claimed to be a member of galaxy cluster Abell 1890, which in fact is more than twice as distant. Worse, in the definitive 3CR catalogue (Bennett 1962) the declination is misprinted as 9° instead of 11°; although this was noticed by many workers, it was overlooked when the 3CRR sample was first compiled, so 3C 296 was left out as below the 10° declination limit. It was only restored to the sample in about 1980, and as a result this beautiful example of a bridged twin-jet is much less well-known than it should be.

A radio-optical overlay and another version of this image are given in Alan Bridle's gallery.

Unlike most FR Is, 3C 296 is in a relatively low-density environment. Although NGC 5532 is surrounded by much smaller galaxies (which may however be in the background), and it has a comperably-sized neighbour, NGC 5531, 140 kpc to the north-west, there is not even a poor cluster, and the surrounding hot gas is too faint to show up in X-rays. Here is another source of confusion: 3C 296 is an X-ray source and this emission has often been discussed as if due to hot gas, but the ROSAT high-resolution image shows that it comes from within 2 kpc of the nucleus (Hardcastle & Worrall 1999) and it is very likely due to the AGN.

Nevertheless there is some gas around 3C 296, visible via its Faraday rotation of the radio polarization at long wavelengths (Garrington et al. 1996). From this we can tell that the the southern lobe is the more distant one, behind the gas. As the brighter jet is in the north, 3C 296 is an example of the Laing-Garrington effect.

The HST image of NGC 5532 (Martel et al. 1999) reveals a nearly edge-on dust disk roughly perpendicular to the radio jets. The disk is tilted enough to show that its south-western edge is closer to us, so if the jets are coming out along the disk axis, as jets usually do, this again confirms that the north-eastern jet is the closer.

High-resolution images of the inner jets are given by Hardcastle et al. (1997a), which clearly show the larger brightness asymmetry in the inner jets, as expected if the jets are slowing from initially relativistic speeds.

The bright point to the west of the north lobe is a background source; another background source is partially visible in the bottom left corner of the image. Other "lumpy" structure of the faint regions of our image is just due to noise, as we don't have quite enough sensitivity to see all the structure at this resolution. The C20 image on the Other Images page shows that we are not missing much.

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Page created: 2009 Apr 2 14:16:43
J. P. Leahy