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Multifrequency observations explore radio galaxy 3C 111 and its jet

Using the Very Long Baseline Array (VLBA), European astronomers have carried out multi-wavelength radio observations of a radio galaxy designated 3C 111. Results of the observational campaign, March 24 on the arXiv preprint server, shed more light on the properties of this galaxy and its relativistic jet.

Radio galaxies (RGs) emit huge amounts of radio waves from their central cores. Black holes at the centers of these galaxies accrete gas and dust, generating high-energy jets visible in radio wavelengths, which accelerate electrically charged particles to high velocities.

Based on their morphology and radio luminosity, astronomers divide radio galaxies into FR I and FR II RGs. FR I radio galaxies have lower power and their emission is dominated by their jet; on the other hand, the ones of the FR II class have high power and are dominated on large scales by hotspots, i.e., regions in which a jet terminates due to an interaction with the intergalactic medium.

Located some 640 million light years away, 3C 111 is an RG showcasing a typical FR II morphology. It exhibits radio lobes, a strong core and a prominent one-sided jet, terminating in the hot spot of the northeast lobe. The jet has a blazar-like behavior, showing apparent superluminal motion.

A team of astronomers led by Vieri Bartolini of the Max Planck Institute for Radio Astronomy in Bonn, Germany, decided to take a close look at 3C 111, focusing on its radio emission and properties of the jet.

"In this paper, we present a set of multi-frequency, high angular resolution observations of pc-scale radio emission in the broad line radio galaxy 3C 111. The images are obtained in full polarization with the VLBA, and as such our work is the first study of this jet with simultaneous data from 5 GHz up to 87.6 GHz," the researchers explained.

3C 111 was detected by Bartolini's team at all the frequencies used. The obtained images appear to be dominated by a compact core of brightness ranging between 3.0 Jy and 1.0 Jy (at 8.4 GHz and 87.6 GHz, respectively).

The observations found that a one-sided jet emerges from the core of 3C 111. The jet is remarkably straight, being oriented at a position angle of about 65 degrees and shows little to no evidence of bending over its full extension.

According to the study, the spectral index for the core of 3C 111 varies from 1.5 in the lower frequency pair 5−8.4 GHz to about 0 in the higher frequency pair 43.8−87.6 GHz. The spectral index along the jet turned out to be generally steep, reaching a minimum value of approximately -3.0 in the highest frequency pair.

Furthermore, the brightness temperature distribution across 3C 111 varies from 10 million to about 1 trillion K. The equipartition magnetic field was estimated to be within the range of 1−100 mG across the radio galaxy.

The observations also found that the polarized emission lies mostly in the jet at all the frequencies, and with increasing frequency, this emission also arises from regions progressively closer to the jet base.