Introducing the search for intermediate-mass black holes in Nearby Galaxies (SIBLING) Survey

Intermediate-mass black holes (IMBHs) have masses between 10(2) and 10(6) M<sub and are key to our understanding of the formation of massive black holes. The known population of IMBHs 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 <= 0.35). Active IMBHs accreting at low rates show small amplitude variability with timescales of hours, as is seen in one of the known IMBHs, NGC 4395. We found a sample of similar to 500 galaxies demonstrating fast and small amplitude variation in their week-based light curves. We estimate an average occupancy fraction of 4% and a surface density of similar to 3 deg(-2), which represent an increase by a factor of similar to 40 compared to previous searches. A large fraction (78%) of the candidates are in spiral galaxies. We preliminarily confirm the active galactic nucleus nature of 22 sources via Baldwin, Phillips, and Terlevich diagrams using Sloan Digital Sky Survey legacy spectra. Further confirmation of these candidates will require multiwavelength observations, especially in X-ray and radio bands.Comision Nacional de Investigacion Cientifica y Tecnologica (CONICYT) PCHA/Doctorado-Nacional/2014-21140892 Ministry of Economy, Development, and Tourism's Millennium Science Initiative IC120009 Comision Nacional de Investigacion Cientifica y Tecnologica (CONICYT) CONICYT FONDECYT 1161184 11130228 Center of Excellence in Astrophysics and Associated Technologies PFB 06 Basal Project, Centro de Modelamiento Matemaico, Universidad de Chile PFB-03 High Performance Computing infrastructure of the National Laboratory for High Performance Computing (NLHPC), CONICYT PIA ECM-02 United States Department of Energy (DOE) National Science Foundation (NSF) Spanish Government Science & Technology Facilities Council (STFC) Higher Education Funding Council for England National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign Ohio State University Mitchell Institute for Fundamental Physics and Astronomy at Texas AM University Ciencia Tecnologia e Inovacao (FINEP) Fundacao Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro National Council for Scientific and Technological Development (CNPq) German Research Foundation (DFG) United States Department of Energy (DOE) University of Chicago University of California at Santa Cruz University of Cambridge Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas-Madrid University of Chicago University College London DESBrazil Consortium University of Edinburgh ETH Zurich United States Department of Energy (DOE) University of Chicago University of Illinois at UrbanaChampaign Institut de Ciencies de l'Espai (IEEC/CSIC) Institut de Fisica d'Altes Energies United States Department of Energy (DOE) Ludwig-Maximilians Universitat Munchen associated Excellence Cluster Universe University of Michigan System National Science Foundation (NSF) NSF - Directorate for Mathematical & Physical Sciences (MPS) University of Nottingham Ohio State University OzDES Membership Consortium University of Pennsylvania University of Portsmouth SLAC National Accelerator Laboratory Stanford University University of Sussex Texas AM University Kavli Institute of Cosmological Physics at the University of Chicago Ministerio da Ciencia, Tecnologia e Inovaca