# High-Resolution Radio Observations of Five Optically Selected Type 2 [PERSON]
###### Abstract
At GHz frequencies, obscuration effects are minimal compared to the infrared or ultra-violet regimes, and sensitive high-resolution radio maps of a source can be produced with typically less than 1-2 h of observation. The arcsec-scale multi-frequency VLA imaging can provide essential spectral and structural information [14; 15], which can help selecting targets for follow-up VLBI observations of the mas-scale radio structure. With this much finer angular resolution, the extended radio emission from the host galaxy is resolved out in general, thus only the compact emission from the AGN remains to be detected. AGN cores typically have flat radio spectra, as opposed to extended jet or hot spot features. Therefore, arcsec-scale radio imaging is a useful tool to identify candidate multiple AGN systems. Sources with multiple flat-spectrum features can then be targeted with VLBI observations that provide mas-scale resolution. A compact, high brightness temperature (\(T_{\rm b}>10^{6}\) K) component that can only be revealed with VLBI is a clear indication of AGN core emission ([16; 17] and references therein). This method could offer an excellent means for finding multiple AGNs with kpc separation. Dual AGNs are expected at stages of galaxy evolution driven by mergers throughout the history of the Universe. While their observational identification is challenging, their study is important in many areas of current cosmology and multi-messenger astrophysics (see [16; 17] and references therein). VLBI observations of a large sample of arcsec-scale double radio AGNs with flat/inverted overall spectra would provide an opportunity to estimate the prevalence of dual radio-emitting AGNs in an unbiased way. Synchrotron self-absorbed jets, and emission from a magnetically-heated corona in an accretion disc can show flat/inverted spectrum [18; 19]. In either case, the compact double structure would indicate dual AGN activity since the detection of mas-scale emission at GHz frequencies is directly related to AGN activity in sources above redshift \(z\approx 0.1\)[17]. As compact cores are relatively common in LLAGNs, they are good candidates for finding dual AGNs. This would provide an efficient selection method that can be applied for future sub-arcsec resolution interferometer surveys, such as the ones using the SKA [20]. Securely confirming dual AGN candidates is not a trivial task that can solely rely on radio measurements. It often benefits from observations at multiple wavebands, from optical to X-rays, using both spatially resolved spectroscopic and imaging techniques [17; 21]. In this paper, we report on the observations of five LLAGNs with the EVN at two frequencies, 1.7 and 5 GHz. Section 2 presents the sample and its selection. Section 3 gives details about the radio interferometric observations. The data analysis and the result are described in Section 4, and our findings are discussed in Section 5. We conclude the paper in Section 6. Throughout this study, we adopt a standard flat \(\Lambda\) Cold Dark Matter cosmology with \(\Omega_{\rm m}=0.3\), \(\Omega_{\Lambda}=0.7\), and \(H_{0}=70\) km s\({}^{-1}\) Mpc\({}^{-1}\) for calculating luminosities and projected linear sizes [22].