# High-Resolution Radio Observations of Five Optically Selected Type 2 [PERSON]
###### Abstract  
Many low-luminosity active galactic nuclei (AGNs) contain a compact radio core which can be observed with high angular resolution using very long baseline interferometry (VLBI). Combining arcsec-scale structural information with milliarcsec-resolution VLBI imaging is a useful way to characterise the objects and to find compact cores on parsec scales. VLBI imaging could also be employed to look for dual AGNs when the sources show kpc-scale double symmetric structure with flat or inverted radio spectra. We observed five such sources at redshifts \(0.36<z<0.58\) taken from an optically selected sample of Type 2 quasars with the European VLBI Network (EVN) at 1.7 and 5 GHz. Out of the five sources, only one (SDSS J1026-0042) shows a confidently detected compact VLBI core at both frequencies. The other four sources are marginally detected at 1.7 GHz only, indicating resolved-out radio structure and steep spectra. Using first-epoch data from the ongoing Karl G. Jansky Very Large Array Sky Survey, we confirm that indeed all four of these sources have steep radio spectra on arcsec scale, contrary to the inverted spectra reported earlier in the literature. However, the VLBI-detected source, SDSS J1026\(-\)0042, has a flat integrated spectrum. Radio AGNs that show kpc-scale symmetric structures with truly flat or inverted spectra could still be promising candidates of dual AGNs, to be targeted with VLBI observations in the future. active galactic nuclei; quasars; radio continuum; radio spectrum; interferometry; imaging 2020, 12, 527; doi:10.3390/sym12040527 Issue 1 [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]  
such as the European VLBI Network (EVN), are up to a few thousand kilometers long, providing milliarcsec (mas) or sub-mas resolution, depending on the observing frequency. Because of the high resolution, a VLBI detection of a radio source at GHz frequencies typically implies high brightness temperatures (\(T_{\rm b}>10^{6-7}\) K) that can be produced by the non-thermal (synchrotron) emission of AGNs [6; 7]. A radio power exceeding \(10^{21}\) W Hz\({}^{-1}\) in a source unresolved by the Karl G. Jansky Very Large Array (VLA) on arcsec scales is already indicative of accretion-powered nuclear activity [7] or maybe circumnuclear starburst. Detection of a compact core with VLBI in a high-luminosity source is a very strong AGN indicator. In LLAGNs, compact radio emission with flat (i.e., with spectral index \(-0.5\leq\alpha\leq 0\), where the flux density is \(S\propto\
u^{\alpha}\), and \(\
u\) is the frequency) or inverted (\(\alpha>0\)) radio continuum spectrum can indicate radio emission that is fueled by a synchrotron self-absorbed jet base coupled to an underluminous accretion disk (e.g., [4]) or an accretion inflow onto the black hole [8]. When observed at high frequencies, magnetized plasma in the accretion disc corona can also produce compact sub-pc scale optically thick flat-spectrum radio emission ([9] and references therein). On the other hand, external free-free absorption of non-thermal radio emission can cause flat spectrum by flattening the steep-spectrum (\(\alpha<-0.5\)) external emission [10; 11], although it would have brightness temperature \(T_{\rm b}\sim 10^{4}\) K [12]. There is a phenomenon analogous to coronal mass ejections in active stars that could also occur in AGNs, in the form of outflowing blobs of highly magnetized plasma [13]. However, similar to jet activity, it produces extended optically thin emission with steep radio spectrum. At highly subluminal speeds, the magnetized corona could be considered as the base for jet launching ([9] and references therein). The combination of radio interferometric measurements with the VLA and VLBI is an effective tool to identify compact cores in LLAGNs (see [3; 5] and references therein).