A Centiparsec-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 (SMBH) has a mass of M _BH ∼ 4.5 × 10 ^9 M _⊙ and a Schwarzschild radius corresponding to R _S ∼ 5.4 μ 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 ∼20 mJy at both frequencies, a size of ≤0.27 mas (99.7% confidence level), about 0.022 pc (50 R _S ) at 7.6 GHz, and a brightness temperature of ≥6 × 10 ^9 K. This suggests that the observed centiparsec-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 broadband very long baseline interferometry observations at millimeter wavelengths to probe ADAF scenarios and tightly constrain the potential photon ring (about 28 μ as) around its SMBH.

Keywords

• Supermassive black holes
• Low-luminosity active galactic nuclei
• Schwarzschild radius
• Very long baseline interferometry
• Radio cores
• Jets