A centi-pc-scale compact radio core in the nearby galaxy M60

Abstract

M60, an elliptical galaxy located 16.5~Mpc away, has an active nucleus with a very low luminosity and an extremely low accretion rate. Its central supermassive black hole has a mass of $M_{\mathrm{BH}}\sim 4.5\times 10^9{M}_{\odot }$ and a Schwarzschild radii corresponding to $R_{\mathrm{S}}\sim 5.4\mu \mathrm{as}$. To investigate the nature of its innermost radio nucleus, data from the Very Long Baseline Array (VLBA) at 4.4 and 7.6~GHz were reduced. The VLBA images reveal a compact component with total flux densities of $\sim$20~mJy at both frequencies, a size of $\le$0.27~mas (99.7$\%$ confidence level), about 0.022~pc ($50R_{\mathrm{S}}$) at 7.6~GHz, and a brightness temperature of $\ge 6\times 10^9$~K. This suggests that the observed centi-parsec-scale compact core could be attributed to a nonthermal jet base or an advection-dominated accretion flow (ADAF) with nonthermal electrons. The extremely compact structure also supports the presence of an SMBH in the center. Our results indicate that M60 is a promising target for broad-band VLBI observations at millimeter wavelengths to probe ADAF scenarios and tightly constrain the potential photon ring (about 28\,$\mu$as) around its SMBH.

15 pages, 5 figures, 3 tables, accepted for publication in Astrophysical Journal