2025A&A...695A.118B

Context. In recent years, the jet formation region in active galaxies has been imaged through millimeter very long baseline interferometry (mm-VLBI) for a few ideal targets, in particular, M 87. An important leap forward for understanding jet launching could be taken by identifying a larger number of suitable objects, characterized by different accretion modes and jet powers.
Aims. In this article, we present 1 cm and 7 mm VLBI data of a sample of 16 poorly explored radio galaxies, comprising both high-excitation (HEGs) and low-excitation galaxies (LEGs), spanning a broad range in terms of radio power. There are several γ-ray emitters among this sample. The sources proximity (z < 0.1) combined with a high black hole mass (log M_BH ≳ 8.5) implies a high spatial resolution in units of Schwarzschild radii (< 10³ - 10⁴R_S), necessary for probing the region where the jet is initially accelerated and collimated. We aim to identify the best candidates for follow-up observations with current and future VLBI facilities.
Methods. The observations were performed with the High Sensitivity Array (HSA), including the Effelsberg telescope and the phased Very Large Array (VLA). The addition of elements with a large collecting area has allowed us to characterize the sub-parsec properties of these faint jets and to estimate their core brightness temperature and orientation.
Results. The number of sources imaged on scales ≲10³R_S has more than doubled thanks to the present study. All targets were detected at both frequencies, with several of them presenting two-sided jet structures. Several LEG jets show hints of limb brightening. The core brightness temperatures are generally below the equipartition value, indicating that equipartition has not yet been reached and/or that the emission is de-boosted. Among the LEGs in the sample, we identified 3C 31, 3C 66B, and 3C 465 as the most promising, as they combine a relatively high flux density (> 50 mJy) with a superb spatial resolution (< 500 R_S) at 7 mm. The powerful HEG 3C 452 is interesting as well, due to its highly symmetric, two-sided jet base. Most sources are expected to become prime targets for future experiments with the next generation Event Horizon Telescope (ngEHT) and next generation Very Large Array (ngVLA).