| Summary: | 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 <i>B</i>-band magnitude <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi mathvariant="fraktur">M</mi><mi>B</mi></msub><mo>></mo><mo>−</mo><mn>19.12</mn></mrow></semantics></math></inline-formula> mag and luminosity distance <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>D</mi><mi mathvariant="normal">L</mi></msub><mo>≤</mo><mn>25.4</mn></mrow></semantics></math></inline-formula> Mpc, which is an extension of a complementary set of 140 more luminous (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi mathvariant="fraktur">M</mi><mi>B</mi></msub><mo>≤</mo><mo>−</mo><mn>19.12</mn></mrow></semantics></math></inline-formula> 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 (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>M</mi><mo>•</mo></msub><mo>≳</mo><msup><mn>10</mn><mn>6</mn></msup><mspace width="0.166667em"></mspace><msub><mi mathvariant="normal">M</mi><mo>⊙</mo></msub></mrow></semantics></math></inline-formula>), whereas Scd–Sm galaxies accordingly harbor black holes that are “less-than-supermassive” (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>M</mi><mo>•</mo></msub><mo>≲</mo><msup><mn>10</mn><mn>6</mn></msup><mspace width="0.166667em"></mspace><msub><mi mathvariant="normal">M</mi><mo>⊙</mo></msub></mrow></semantics></math></inline-formula>). 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.
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