The relationship between radio power at 22 and 43 GHz and black hole
  properties of AGN in elliptical galaxies
  We investigate the relationship between radio power and properties related to
active galactic nuclei (AGNs). Radio power at 1.4 or 5 GHz, which has been used
in many studies, can be affected by synchrotron self-absorption and free-free
absorption in a dense region. On the other hand, these absorption effects get
smaller at higher frequencies. Thus, we performed simultaneous observations at
22 and 43 GHz using the Korean VLBI Network (KVN) radio telescope based on a
sample of 305 AGN candidates residing in elliptical galaxies from the overlap
between the Sloan Digital Sky Survey (SDSS) Data Release 7 and Faint Images of
the Radio Sky at Twenty-Centimeters (FIRST). About 37% and 22% of the galaxies
are detected at 22 and 43 GHz, respectively. Assuming no flux variability
between the FIRST and KVN observation, spectral indices were derived from FIRST
and KVN data and we found that over 70% of the detected galaxies have flat or
inverted spectra, implying the presence of optically thick compact regions near
the centres of the galaxies. Core radio power does not show a clear dependence
on black hole mass at either low (1.4 GHz) or high (22 and 43 GHz) frequencies.
However, we found that the luminosity of the [OIII] $\lambda$5007 emission line
and the Eddington ratio correlate with radio power more closely at high
frequencies than at low frequencies. This suggests that radio observation at
high frequencies can be an appropriate tool for unveiling the innermost region.
In addition, the luminosity of the [OIII] $\lambda$5007 emission line and the
Eddington ratio can be used as a tracer of AGN activity. Our study suggests a
causal connection between high frequency radio power and optical properties of
AGNs.
