3C 438


Basic Data
S178 Alpha FR Class ID Spectrum Best z mag. LAS lg P178 D
48.70.88IIRD Gal0.290R(c) = 17.90 23.2026.84 83.3


Size: 23.0 × 23.0 arcsec²
LUT: Linear
Beam: 0.29 arcsec
Frequency: 1534 MHz
Method: CLEAN/VTESS’ėˇ>0.29
Telescope: MERLIN/VLA
Credits: Leahy (1997)

3C 438 is the best example known of a DRAGN with well-defined strong-flavour jets in both lobes. The prominence of these jets is perhaps related to the surprising absence of hotspots; instead, the outer regions of the lobes on both sides show a high surface brightness with small-scale fluctuations at the level of tens of percent (notice that our image is displayed with a linear LUT).

The jets are very well collimated (they are best seen on our high-resolution supplementary image). The northern jet has three prominent knots, whose peaks are noticably off the jet axis. The southern jet brightens significantly from the bend about six arcsec from the core, and the jet seems to be brighter on the outside edge of the bend (this weak effect is at the limit of believability). If real, this suggests that the jet material is flowing around the bend in response to a pressure gradient, as opposed to ballistic models in which each part of the jet moves at constant velocity away from the core, so that bends indicate changes in the direction of ejection. Because of the strong depolarization, the magnetic field orientation is unknown.

The faint extension to the south from the north-west lobe is best seen in our C20 image.

The AGN powering 3C 438 is remarkably weak in the optical, with even narrow emission lines only just detected (Rawlings et al. 1989a). Surprisingly, 3C 438 is a relatively strong X-ray source (Feigelson & Berg 1983, Prieto 1996), but since the the host galaxy is in a cluster (Burbidge & Crowne 1979, de Koff et al. 1996) the X-rays may be from the ICM rather than the AGN. If so, this would be a massive cluster atmosphere, comperable to a rich Abell cluster. A very dense surrounding medium is also consistent with the strong depolarization. It may also explain why such a high-power DRAGN should have a relaxed double structure, which is normally found in objects with powers around the FR break. A very high pressure in the ambient medium will raise the lobe pressure, which in turn may lead to increased dissipation in the jets, reducing the energy flux available to power hotspots. At the same time the hotspot prominence is reduced anyway because high-pressure lobes will be relatively bright in the radio.

Weak fine-scale structure is undersampled in our supplementary full-resolution image and should not be relied on. The weak striping parallel to the jets is an artifacts, but there are features in the southern lobe that are not exactly parallel to the jet and are probably real.

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