Astronomy Object of the Month: 2024, December

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4C70.19 - a very twisted radio galaxy

Radio galaxies are typically associated with an active galactic nucleus (AGN) in the centre of an elliptical or sometimes spiral galaxy, that emits a powerful jet of particles extending over tens to hundreds of kiloparsecs (kpc) or even up to several megaparsecs (Mpc). Interacting with the surrounding intergalactic medium (IGM) these jets form a variety of shapes, lobes or plumes, depending on the initial energy and the properties of the medium through which they travel. Sometimes, the produced shapes make it difficult to associate the radio galaxy to a known class of objects. 4C70.19 is a great example.

4C70.19 is a radio galaxy with radio jets that show a very distorted morphology, including an unusual, 'hook-like' structure on one side (Lara et al. 2001, A&A, 370, 409). The other side looks relatively more typical with a kind of 'hot spot', that could be associated with an FRII type (Fanaroff & Riley 1974, MNRAS, 167, 31) of radio galaxies. Recent observations at low radio frequencies by the LOFAR telescope (van Haarlem et al. 2013, A&A, 556, 2) confirmed this morphology with high resolution and sensitivity (Figure 1). This would however mean that a single source combines properties of different classes of radio galaxies. To further investigate this interesting radio galaxy, we used available multiwavelength data, including dedicated, sensitive polarimetric radio observations with the Effelsberg single-dish telescope. The results of the latter are presented in Figure 2. The contours of the high-frequency map at 8.35GHz are shown along with the lines marking the projected orientation of the magnetic field lines. The high sensitivity to diffuse emission of the Effelsberg telescope revealed the large-scale emission around the source. Interestingly, the magnetic field seems to be perpendicular to the jet axis except for the region of the northern bend, where it becomes parallel with the propagation of the jet.

To study the propagation path of both jets of 4C70.19 in more detail, we used the high-resolution LOFAR map and applied the Sobel filtering that enhances areas with the highest gradients of the emission. The processed map is presented in Figure 3, where the colour lines are drawn along the propagation path with more distinct changes in the direction marked with crosses and labels. Now, the lines clearly suggest that not only the northern jet bends significantly. Also in the southern jet (red line in Figure 3) a sudden turn (outwards) in the area of the highest radio brightness is visible and the jet seems to follow in the western direction. This would mean that the observed asymmetry comes from the fact, that we see the northern jet (and its bend) roughly in the sky plane, while the southern jet bends in a similar way but the bend occurs outwards, along the line of sight. In such case, an otherwise highly asymmetric radio source becomes fairly symmetric. The distance measured along the jet propagation paths is the same within 25 kpc, reaching 300kpc in each direction from the AGN.




Also, the behaviour of the magnetic field in 4C70.19 was investigated closely to see if its observed properties could explain the complex morphology of the source. In Figure 4 we present the combined VLA+Effelsberg map of 4C70.19 with lines of the magnetic field, whose lengths are proportional to the degree of polarisation. This kind of presentation allows to look for areas with enhanced magnetic fields. Three such areas can be recognised: at the bend of the northern jet, at its end, as well as in the southern 'radio lobe'.

It is worth to note here, that any enhancement of the magnetic field can be a result of either compression or the shearing of the magnetic field. While the compression of the magnetic field needs to be accompanied by a local increase in radio intensity (proportional to the strength of the magnetic field), the shearing effects might work without any significant brightenings. The latter case seems to be observed in both northern and southern jet bends. This can be explained with a weakening of the jet, which cannot overcome the external pressure of the surrounding medium and follows the 'easier' path, likely enforced by the motion of the host galaxy. The high resolution of this map also confirmed the findings from a more sensitive map presented in Figure 2, that the orientation of the magnetic field changes at the bend from perpendicular to parallel to the jet axis. At the tip of the northern jet, however, a different situation is observed. Here, a slight compression takes place, which marks the termination shock of the jet, also visible as a brightening in Figure 3 (end of the black line).

The results of our observations presented above suggest that the peculiar look of 4C70.19 cannot be explained exclusively with changes in the density of the surrounding intergalactic medium. The full explanation likely needs to include the influence of the galaxy group in which NGC6048, the host galaxy of 4C70.19, resides. Especially, that two companion galaxies seem to be present in the direct vicinity of NGC6048 (Figure 5). One of them, PGC214442 (bottom left in both panels of Figures 5), is tidally distorted along the direction towards NGC6048. Also the infrared halo around NGC6048 (right panel of Figure 5) is elongated in this direction. Furthermore, if we compare the extent of this halo with the shape of the northern radio jet of 4C70.19 (Figure 6), we might conclude that the termination of this jet likely occurs in the outer parts of the halo of its host galaxy NGC6048.

Last, but not least, the above findings suggest that the large-scale radio emission visible in the sensitive Effelsberg map at 8.35GHz is not originating from a radio halo around 4C70.19, but corresponds to the 'patchy' emission visible in the LOFAR map (Figure 1). This emission, in turn, can be easily explained with radio plumes that form in regions where the radio jets decelerate and diffuse into the surrounding medium, a characteristic typical of FRI radio galaxies.




Original publication: Marek Weżgowiec, Marek Jamrozy, Krzysztof T. Chyży, Martin J. Hardcastle, Agnieszka Kuźmicz, George Heald, Timothy W. Shimwell, The twisted jets and magnetic fields of the extended radio galaxy 4C 70.19.


The research was conducted at the Department of Radioastronomy and Space Physics and the Department of Stellar and Extragalactic Astronomy of the Jagiellonian University’s Astronomical Observatory (OA UJ). The work was supported by the Polish National Science Centre through the grant 2018/29/B/ST9/01793.



Contact:

Marek Weżgowiec Astronomical Observatory Jagiellonian University M.Wezgowiec [at] oa.uj.edu.pl