Nuclear Radio Jet From a Low-Luminosity Active Galactic Nucleus in NGC 4258

Abstract

The nearby low-luminosity active galactic nucleus (LLAGN) NGC 4258 has a weak radio continuum component at the galactic center. We investigate its radio spectral properties on the basis of our new observations using the Nobeyama Millimeter Array at 100 GHz and archival data from the Very Large Array at 1.7-43 GHz and the James Clerk Maxwell telescope at 347 GHz. The NGC 4258 nuclear component exhibits (1) an intra-month variable and complicated spectral feature at 5-22 GHz and (2) a slightly inverted spectrum at 5-100 GHz (α ∼ 0.3; F ν ∝ ν α ) in time-averaged flux densities, which are also apparent in the closest LLAGN M81. These similarities between NGC 4258 and M81 in radio spectral natures in addition to previously known core shift in their AU-scale jet structures produce evidence that the same mechanism drives their nuclei. We interpret the observed spectral property as the superposition of emission spectra originating at different locations with frequency-dependent opacity along the nuclear jet. Quantitative differences between NGC 4258 and M81 in terms of jet/counter jet ratio, radio loudness, and degree of core shift can be consistently understood by fairly relativistic speeds (Γ 3) of jets and their quite different inclinations. The picture established from the two closest LLAGNs is useful for understanding the physical origin of unresolved and flat/inverted spectrum radio cores that are prevalently found in LLAGNs, including Sgr A*, with starved supermassive black holes in the present-day universe.

Evidence of Parsec-Scale Jets in Low-Luminosity Active Galactic Nuclei

“…Other LLAGN not included in the Palomar Sample (e.g., Arp 299-A, NGC 1052, NGC 1961, NGC 4258, NGC 7217) also show extended pc-scale radio emission (e.g., Kadler et al 2004;Krips et al 2007;Pérez-Torres et al 2010;Doi et al 2013), and in this work we report the first detection of extended pc-scale jets in the LIN-ERs NGC 1097 and NGC 4594 (also not included in the Palomar Sample). All these sources cannot be added to the statistics as they do not belong to a flux-limited complete sample, but indicate that the number of LLAGN with pc-scale jets is probably much larger than the statistical values reported above.…”

The origin of radio emission from radio-quiet active galactic nuclei

“…Significant improvement in this field could already be achieved with the current radio telescopes in a shortterm future, ideally requiring: i) a complete and statistically meaningful sample of RQ AGN which covers a large range of radio luminosities and accretion rates, achievable by reaching µJy flux-density levels with short exposure times (i.e., ∼ 30 min of total time with array-A VLA to reach a sensitivity of 10µJy beam −1 and 0.4 resolution at 5 GHz; ∼ 1 hour for eMERLIN at 5 GHz for a sensitivity of 25µJy beam −1 and 0.04 resolution); ii) a full spectral coverage from cm band to FIR to discriminate between the different emission mechanisms (i.e., combining cm-band VLA and mm-band ALMA data, see Doi et al 2011Doi et al , 2013Inoue & Doi 2018); iii) multiscale (arcsec to mas-resolution) imaging to map and resolve the different emission structures from the galaxy to the innermost regions and trace their connection. In this respect, very-long baseline radio arrays have been implemented in the recent years with the possibility to combine them together in the Global VLBI network 6 , allowing us to reach unprecedented angular resolution (up to 45 µarcsec) and uv coverage.…”

Water masers in Compton-thick AGN

“…Furthermore, a period of about 20 years has passed between the NVSS and our EVN observations. This long time-lag makes the comparison of the VLA and VLBI flux densities controversial, because significant variations (with time-scales of years) in the nuclear radio flux density of Seyferts and other radio quiet AGN have been often found (e.g., Mundell et al 2009;Doi et al 2013).…”