Probing the Active Massive Black Hole Candidate in the Center of NGC 404 with VLBI

Abstract

Recently Nyland et al. (2012) argued that the radio emission observed in the center of the dwarf galaxy NGC 404 originates in a low-luminosity active galactic nucleus (LLAGN) powered by a massive black hole ($M\sim <10^6$ M${}_{\odot }$). High-resolution radio detections of MBHs are rare. Here we present sensitive, contemporaneous Chandra X-ray, and very long baseline interferometry (VLBI) radio observations with the European VLBI Network (EVN). The source is detected in the X-rays, and shows no long-term variability. If the hard X-ray source is powered by accretion, the apparent low accretion efficiency would be consistent with a black hole in the hard state. Hard state black holes are known to show radio emission compact on the milliarcsecond scales. However, the central region of NGC 404 is resolved out on 10 milliarcsecond (0.15-1.5 pc) scales. Our VLBI non-detection of a compact, partially self-absorbed radio core in NGC 404 implies that either the black hole mass is smaller than $3_{-2}^{+5}\times 10^5$ M${}_{\odot }$, or the source does not follow the fundamental plane of black hole activity relation. An alternative explanation is that the central black hole is not in the hard state. The radio emission observed on arcsecond (tens of pc) scales may originate in nuclear star formation or extended emission due to AGN activity, although the latter would not be typical considering the structural properties of low-ionization nuclear emission-line region galaxies (LINERs) with confirmed nuclear activity.

Accepted for publication in the Astrophysical Journal. 7 pages, 2 figures, 1 table

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• NGC5252: a pair of radio-emitting active galactic nuclei?
• Compact jets dominate the continuum emission in low-luminosity active galactic nuclei
• Intermediate-mass black holes: finding of episodic, large-scale and powerful jet activity in a dwarf galaxy
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• Is there a sub-parsec-scale jet base in the nearby dwarf galaxy NGC 4395?
• Radio observations of four active galactic nuclei hosting intermediate-mass black hole candidates: studying the outflow activity and evolution
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