Krezinger Máté
Active galactic nuclei (AGNs) are powered by black holes as massive as billons of the Sun. Young and powerful radio AGNs are often compact symmetric objects (CSOs) with double-lobed structures confined to within 1 kpc. They represent the earliest evolutionary phase of AGNs that launch relativistic plasma jets. These jets interact with the surrounding material of the black hole and create the radio emitting-lobes. Studying CSOs is a useful tool for understanding the evolution of the galaxies and the interaction between the jets and the medium of the host galaxy. While achieving the finest imaging resolution using the technique of very long baseline interferometry (VLBI), it is not always straightforward to identify a compact double-lobed structure with complete certainty. The radio source named J1110+4817 was considered a CSO candidate in the literature earlier, but because of the lack of clear evidence, it could not be securely classified as a CSO. Here we present a comprehensive analysis of archival multi-frequency VLBI observations combined with accurate Gaia optical astrometric information to prove that J1110+4817 is indeed a CSO. Gaia is an optical astrometry space mission with a goal to measure the celestial positions of as many sources as possible with the highest accuracy. VLBI radio observations also reveal an extended radio feature nearly perpendicular to the main jet structure in J1110+4817, apparently emanating from the brighter northern hot spot, that is atypical among the known CSOs. The combination of high-resolution VLBI radio imaging with Gaia optical data could be the key to identify more and more CSOs in the future.
Máté Krezinger, Sándor Frey, Tao An, Sumit Jaiswal and Yingkang Zhang
Active galactic nuclei (AGNs) are powered by black holes as massive as billons of the Sun. Young and powerful radio AGNs are often compact symmetric objects (CSOs) with double-lobed structures confined to within 1 kpc. They represent the earliest evolutionary phase of AGNs that launch relativistic plasma jets and may evolve into larger-scale radio galaxies. Most CSOs show very little relativistic beaming effect suggesting their jet orientation is close to the plane of the sky with high inclination. The jets interact with the surrounding material of the black hole and create radio-emitting lobes. Studying CSOs is a useful tool for understanding the evolution of the galaxies and the interaction between the jets and the medium of the host galaxy.
By connecting various radio antennas across the Earth, we can create a virtual radio telescope as large as the distance between the elements of the network. While achieving the finest imaging resolution using this technique called very long baseline interferometry (VLBI), it is not always straightforward to identify a compact double-lobed structure with complete certainty. With the advent of the optical astrometric space mission Gaia, accurate optical positions are becoming available for some AGNs as well. The main goal of Gaia is to measure the celestial positions of as many sources as possible with the highest accuracy. This may help localise the weak or even undetected radio core of certain CSOs.
A radio source at redshift z = 0.74, named J1110+4817, was considered a CSO candidate in the literature earlier, but because of the lack of clear evidence, it could not be securely classified as a CSO. The source was imaged multiple times with two different radio telescope networks, the Very Large Array (VLA) as well as the Very Long Baseline Array (VLBA) at several frequencies (2.3, 4.8, 8.4 GHz). These images show nearly symmetric radio structure extending to ~30 milliarcsec (mas) in the NE¬−SW direction, with the NE feature being the brighter. The two dominant components appear connected with a weak jet-like emission. The radio structure could be interpreted in many ways, e.g., a core−jet structure or even a binary AGN. The observations also reveal an extended radio feature nearly perpendicular to the main jet structure, apparently emanating from the brighter hot spot, that is atypical among other CSOs.
We analysed published and unpublished archival multi-frequency VLBI data of J1110+4817 and, based also on the most accurate Gaia optical position of the AGN, we proved that the object is indeed a CSO. As the VLBI images show, at each frequency, the optical position falls between the NE and the SW features. The projected linear distance between these two brightest components at this redshift is nearly 200 pc and it is growing by ~0.4 pc annually. With a deduced kinematic age of ~530 years. They represent two radio lobes of a young jetted AGN. From the literature, we collected total radio flux density measurements of J1110+4817 to determine its broad-band radio spectrum. The peak frequency and flux density of the best log-parabolic fit also suggest a young radio source.
All the evidence from the morphology, the spectral index distribution, and the radio spectrum support the classification of J1110+4817 as a CSO rather than a compact core–jet source. As demonstrated here, the availability of accurate Gaia positional measurements opens a promising new avenue in confirming CSO candidates and studying CSOs.