Is there a sub-parsec-scale jet base in the nearby dwarf galaxy NGC 4395?

Abstract

NGC 4395 is a dwarf galaxy at a distance of about 4.3 Mpc (scale: ∼0.021 pc mas−1). It hosts an intermediate-mass black hole (IMBH) with a mass between ∼104 and ∼105 solar masses. The early radio observations of NGC 4395 with the very long baseline interferometry (VLBI) network, High Sensitivity Array (HSA), at 1.4 GHz in 2005 showed that its nucleus has a sub-mJy outflow-like feature (E) extending over 15 mas. To probe the possibility of the feature E as a continuous jet with a base physically coupled with the accretion disc, we performed deep VLBI observations with the European VLBI Network (EVN) at 5 GHz, and analysed the archival data obtained with the HSA at 1.4 GHz in 2008, NSF’s Karl G. Jansky Very Large Array (VLA) at 12–18 GHz and the Atacama Large Millimetre/submillimetre Array (ALMA) at 237 GHz. The feature E displays more diffuse structure in the HSA image of 2008 and has no compact substructure detected in the EVN image. Together with the optically thin steep spectrum and the extremely large angular offset (about 220 mas) from the accurate optical Gaia position, we explain the feature E as nuclear shocks likely formed by the IMBH’s episodic ejection or wide-angle outflow. The VLA and ALMA observations find a sub-mJy pc-scale diffuse feature, possibly tracing a thermal free–free emission region near the IMBH. There is no detection of a jet base at the IMBH position in the VLBI maps. The non-detections give an extremely low luminosity of ≤4.7 × 1033 erg s−1 at 5 GHz and indicate no evidence of a disc-jet coupling on sub-pc scales.

Radio Observations of Four Active Galactic Nuclei Hosting Intermediate-mass Black Hole Candidates: Studying the Outflow Activity and Evolution

“…For NGC 4395, the Eddington ratio is 1.2 × 10 −3 (Wrobel & Ho 2006), and the black hole mass is ≈10 4 M e (Woo et al 2019). The 5 GHz radio luminosity of 1.3 × 10 34 erg s −1 is estimated from the VLA A array 15 GHz observations (Saikia et al 2018a) which probes a region size of 4 pc (taking the in-band 15 GHz spectral index of −0.07, see also Yang et al 2022c), and the X-ray luminosity is 10 39.9 erg s −1 (Moran et al 2005). For NGC 404, the Eddington ratio is 1.5 × 10 −6 (Paragi et al 2014), and the black hole mass ≈5 × 10 5 M e (Davis et al 2020).…”

“…2003) with the High Sensitivity Array (HSA) at 1.4 GHz reveal a radio-quiet nucleus and an elongated sub-pc scale structure, which is indicative of an outflow (Wrobel & Ho 2006). The recent study of NGC 4395 by Yang et al (2022c) finds an undetected source from European VLBI Network (EVN) 5 GHz high-resolution observations. Very large array (VLA) 15 GHz observations detect a core (coincident with the Gaia optical position) and eastern (E) components.…”

A compact symmetric ejection from the low mass AGN in the LINER galaxy NGC 4293

“…On studying of a sample of high accretion rate IMBHs show that the ejection process in IMBHs is likely episodic, and in some cases the radio core is visible and in other cases, it is not (Yang et al 2022, In preparation). A recent study of the IMBH candidate NGC 4395 reveals both fossil radio ejecta and diffuse and flat-spectrum radio emission in the nucleus (Yang et al 2022), and a thermal mechanism in the nucleus is proposed correspondingly.…”

“…The VLBI detection of compact pc-scale radio-emitting structures (core/core-jet/jetknot) in the nuclear regions of dwarf galaxies can directly probe the jet/outflow activity enabled by an accreting, potential IMBH. To date, high-resolution VLBI observational studies are limited to only a few IMBH candidate hosts: NGC 4395 (Wrobel & Ho 2006;Yang et al 2022), Henize 2-10 (Reines & Deller 2012), NGC 404 (Paragi et al 2014) and RGG 9 (Yang et al 2020a). In order to understand the ejection process in actively accreting IMBHs, we present an observational study of an IMBH candidate in NGC 4293.…”

Intermediate-mass black holes: finding of episodic, large-scale and powerful jet activity in a dwarf galaxy

“…To date, we know little about IMBH jets because of their weakness and low detection rate (e.g. Greene et al 2020;Reines 2022;Yang et al 2022a). Some nearby low-mass galaxies have been found to host AGNs likely resulting from accreting IMBHs.…”