The mass function of nearby black hole candidates

Abstract

<i>Context. <i/>The mass function of supermassive black holes in our cosmic neighborhood is required to understand the statistics of their activity and consequently the origin of ultra high energy particles.<i>Aims. <i/>We determine a mass function of supermassive black hole candidates from the entire sky except for the Galactic plane.<i>Methods. <i/>Using the 2MASS catalogue as a starting point, and the well-established correlation between black hole mass and the bulge of old population of stars, we derive a list of nearby black hole candidates within the redshift range <i>z<i/> <i><<i/> 0.025, then perform an additional selection based on the Hubble type. We present our resulting catalogue elsewhere. The final list of black hole candidates above a mass of <i>M<i/><sub>BH<sub/> <i>><i/> 3 × 10<sup>6<sup/> has 5829 entries. We perform a Hubble-type correction to account for selection effects, which reduces this number to 2919 black hole candidates. Here we use this catalogue to derive the black-hole mass function. We also correct for volume, so that this mass function is a volume-limited distribution to redshift 0.025.<i>Results. <i/>The differential mass function of nearby black hole candidates is a curved function, with a straight simple power-law of index -3 above 10<sup>8<sup/> that becomes progressively flatter towards lower masses, turns off towards a gap below 3 × 10<sup>6<sup/> , and then extends into the range where nuclear star clusters replace black holes. The shape of this mass function can be explained in a simple merger picture. Integrating this mass function over the redshift range for which it has been derived, infers a total number of black holes with <i>z<i/> <i><<i/> 0.025, and <i>M<i/><sub>BH<sub/> <i>><i/> 10<sup>7<sup/> of about 2.4 × 10<sup>4<sup/>, or, if we average uniformly, 0.6 for every square degree on the sky.