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

Kavli Affiliate: Luis C. Ho

| First 5 Authors: Jun Yang, Xiaolong Yang, Joan M. Wrobel, Zsolt Paragi, Leonid I. Gurvits

| Summary:

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 ~10$^4$ and ~10$^5$ 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 x 10$^{33}$ erg s$^{-1}$ at 5 GHz and
indicate no evidence of a disc-jet coupling on sub-pc scales.

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