Probing the Low-mass End of the Black Hole Mass Function via a Study of Faint Local Spiral Galaxies

We present an analysis of the pitch angle distribution function (PADF) for nearby galaxies and its resulting black hole mass function (BHMF) via the well-known relationship between pitch angle and black hole mass. Our sample consists of a subset of 74 spiral galaxies from the Carnegie-Irvine Galaxy Survey with absolute $B$-band magnitude ${\mathfrak{M}}_B>-19.12$ mag and luminosity distance $D_{\mathrm{L}}\le 25.4$ Mpc, which is an extension of a complementary set of 140 more luminous (${\mathfrak{M}}_B\le -19.12$ mag) late-type galaxies. We find the PADFs of the two samples are, somewhat surprisingly, not strongly dissimilar; a result that may hold important implications for spiral formation theories. Our data show a distinct bimodal population manifest in the pitch angles of the Sa-Sc types and separately the Scd-Sm types, with Sa-Sc types having tighter spiral arms on average. Importantly, we uncover a distinct bifurcation of the BHMF, such that the Sa-Sc galaxies typically host so-called “supermassive” black holes ($M_{\bullet }\gtrsim {10}^6{\mathrm{M}}_{\odot }$), whereas Scd-Sm galaxies accordingly harbor black holes that are “less-than-supermassive” ($M_{\bullet }\lesssim {10}^6{\mathrm{M}}_{\odot }$). It is amongst this latter population of galaxies where we expect fruitful bounties of elusive intermediate-mass black holes (IMBHs), through which a better understanding will help form more precise benchmarks for future generations of gravitational wave detectors.