A Heavy Seed Black Hole Mass Function at High Redshift – Prospects for LISA

The advent of new and near-future observatories probing the earliest epochs of the Universe has opened the opportunity to investigate the formation and growth of the first massive black holes (MBHs). Additionally, the use of high resolution cosmological simulations to investigate these high-redshift environments is needed to predict the dark matter halos in which these MBH seeds will form. We use the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mtext mathvariant="italic">𝑅𝑒𝑛𝑎𝑖𝑠𝑠𝑎𝑛𝑐𝑒</mml:mtext> </mml:math> simulations to analyse the formation and growth of so-called heavy seed black holes. Other past work has investigated the formation and growth of light (black hole) seeds with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mtext mathvariant="italic">𝑅𝑒𝑛𝑎𝑖𝑠𝑠𝑎𝑛𝑐𝑒</mml:mtext> </mml:math> and found that these black holes do not grow in the environments in which they reside. In this work we seed MBHs, in post-processing, and track accretion onto the MBHs as well as mergers with other MBHs at high-redshift. We show that the heavy seeds struggle to achieve high accretion rates with only the most massive black holes ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>≳</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>5</mml:mn> </mml:msup> <mml:msub> <mml:mtext mathvariant="normal">M</mml:mtext> <mml:mo>⊙</mml:mo> </mml:msub> </mml:mrow> </mml:math> ) growing at close to the Eddington limit under optimistic conditions. Despite the lack of significant growth for these early MBHs, the signals from their merger events will be sufficiently strong (SNR <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>2</mml:mn> </mml:msup> </mml:mrow> </mml:math> ) to be probed by the next generation of gravitational wave observatories, such as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mtext mathvariant="italic">𝐿𝐼𝑆𝐴</mml:mtext> </mml:math> . We predict that <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mtext mathvariant="italic">𝐿𝐼𝑆𝐴</mml:mtext> </mml:math> will observe of the order of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mn>10</mml:mn> </mml:math> MBH merger events per year where the mergers occur at z <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mo>≳</mml:mo> </mml:math> 10 or at least begin their early inspiral phase at z <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mo>≳</mml:mo> </mml:math> 10.