Nuclear Radio Continuum Emission of Low-Luminosity Active Galactic Nucleus NGC 4258

TL;DR

This study investigates the nuclear radio emission of the LLAGN NGC 4258 across multiple frequencies, revealing complex spectral features and variability that suggest multiple jet components, drawing parallels to the Galactic center Sgr A*.

Contribution

The paper provides the first multi-frequency, quasi-simultaneous radio observations of NGC 4258, uncovering spectral complexity and variability indicative of multiple nuclear jet blobs.

Findings

Inverted spectra observed across 5-22 GHz with intra-month variability.

Detection of large amplitude variability at 100 GHz.

Spectral features suggest multiple nuclear jet components.

Abstract

The nearby low-luminosity active galactic nucleus (LLAGN) NGC 4258 has a weak radio continuum emission at the galactic center. Quasi-simultaneous multi-frequency observations using the Very Large Array (VLA) from 5 GHz (6 cm) to 22 GHz (1.3 cm) showed inverted spectra in all epochs, which were intra-month variable, as well as complicated spectral features that cannot be represented by a simple power law, indicating multiple blobs in nuclear jets. Using the Nobeyama Millimeter Array (NMA), we discovered a large amplitude variable emission at 100 GHz (3 mm), which had higher flux densities at most epochs than those of the VLA observations. A James Clerk Maxwell Telescope (JCMT) observation at 347 GHz (850 micron) served an upper limit of dust contamination. The inverted radio spectrum of the nucleus NGC 4258 is suggestive of an analogy to our Galactic center Sgr A*, but with three orders of magnitude higher radio luminosity. In addition to the LLAGN M 81, we discuss the nucleus of NGC 4258 as another up-scaled version of Sgr A*.