Introducing the Search for Intermediate-mass Black-hole In Nearby Galaxy (SIBLING) Survey

TL;DR

This paper introduces a new optical variability method to identify intermediate-mass black hole candidates in nearby galaxies, significantly increasing the known candidate population and providing a basis for future multiwavelength confirmation.

Contribution

The study presents a novel variability-based selection technique for IMBH candidates, expanding the candidate pool by a factor of 40 over previous methods and highlighting their prevalence in spiral galaxies.

Findings

Identified ~500 galaxies with small amplitude, fast variability suggestive of IMBH activity.

Estimated an IMBH candidate occupancy fraction of 4%.

Confirmed 22 candidates as AGN via spectral analysis.

Abstract

Intermediate mass black holes (IMBHs) have masses between the $10^2\!-\!10^6$ M$_\odot$ and are key to our understanding of the formation of massive black holes. The known population of IMBH remains small, with a few hundred candidates and only a handful of them confirmed as bona-fide IMBHs. Until now, the most widely used selection method is based on spectral analysis. Here we present a methodology to select IMBH candidates via optical variability analysis of the nuclear region of local galaxies ($z \leqslant 0.35$). Active IMBH accreting at low rates show small amplitude variability with time scales of hours, as it is seen in one of the known IMBH NGC4395. We found a sample of $\sim \!500$ galaxies evidencing fast and small amplitude variation in their weekly based light curves. We estimate an average occupancy fraction of 4\% and a surface density of $\sim \!3$ deg$^{-2}$, which represent an increase by a factor of $\sim\!40$ compared to previous searches. A large fraction ($78\%$) of the candidates are in spiral galaxies. We preliminary confirm the AGN nature of 22 sources via BPT diagrams using SDSS legacy spectra. Further confirmation of these candidates will require multiwavelength observations, especially in X-ray and radio bands.