| Description: | 4C12.50 is a nearby (z=0.122), very young radio source located within a pair of merging galaxis. One of the nuclei of these galaxies has become active in the process. Active galactic nuclei play an extremely important role in galaxy evolution: the energy released by the AGN can prevent hot gas to be accreted (i.e. stop the growth of the supermassive black hole and halt star formation). One of the possible ways of this "AGN feedback" is the interaction between the relativistic radio jet and the interstellar medium (ISM), inducing massive gas outflows from the central regions of the galaxy. Such outflows of ionized gas have been found in several objects, but more recently signatures of cold, atomic gas has been seen as well in the spectra of nearby galaxies: hydrogen absorption lines appear sometimes at high velocities (~1000 km/s).
To locate these outflows in the very center of galaxies, extremely high angular resolution and very high sensitivities are required, only achievable with global very long baseline interferometry (VLBI). The European VLBI Network (EVN plus Arecibo) and the Very Long Baseline Array (VLBA, including the Green Bank Telescope) joined up to image the nuclear region of 4C12.50 in HI absorption. The image above shows the contour radio maps of continuum emission, overlayed on colour maps of atomic hydrogen absorption at two different velocities. In the upper panel a narrow absorption feature can be seen at the systemic velocity of the galaxy, due to gas located along the line of sight to a counter-jet feature. More interestingly, the lower panel shows blueshifted absorption at the terminating point of the jet; this, and the earlier observed of very high fractional polarization in this region, indicate that there is a strong shock between the jet and the ISM, that eventually drives the massive, high velocity outflow on larger scales. This is without doubt a direct evidence for AGN feedback in the form of radio jet-cloud interaction in this fascinating object.
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