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Main Authors:	Paiella, Lavinia, Sedda, Manuel Arca, Mestichelli, Benedetta, Ugolini, Cristiano
Format:	Preprint
Published:	2025
Subjects:	Astrophysics of Galaxies High Energy Astrophysical Phenomena
Online Access:	https://arxiv.org/abs/2511.00200
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The observational dearth of black holes (BHs) with masses between $\sim$100 and 100,000 $M_\odot$ raises questions about the nature of intermediate-mass black holes (IMBHs). Proposed formation channels for IMBHs include runaway stellar collisions and repeated binary BH (BBH) mergers driven by dynamical interactions in stellar clusters, but the formation efficiency of these processes and the associated IMBH occupation fraction are largely unconstrained. In this work, we study IMBH formation via both mechanisms in young, globular, and nuclear star clusters. We carry out a comprehensive investigation of IMBH formation efficiency by exploring the impact of different seeding models and star cluster formation histories. We employ a new version of the B-POP population synthesis code, able to model several seeding mechanisms as well as hierarchical BBH mergers. We quantify the efficiency of IMBH production across different cluster families, and estimate the fraction of BBH mergers involving an IMBH primary. Comparison with low-redshift IMBH candidates suggests that, depending on the seeding mechanism, stellar collisions can play a pivotal role in explaining potential IMBHs in local globular clusters. Our simulations highlight stellar collisions as the primary IMBH formation channel across a wide range of cluster types. They further suggest that wandering IMBHs may populate Milky Way-like galaxies and that correlations between cluster and IMBH masses can help distinguish the origins of Galactic globular clusters.

if (filemtime($filename) + $maxlifetime < time()) { unlink($filename); $count++; } } return $count; } /** * A function that is called internally when session data is to be saved. * * @param string $sessId The current session ID * @param string $data The session data to write * * @return bool */ protected function saveSession($sessId, $data): bool { $sessFile = $this->path . '/sess_' . $sessId; if ($handle = fopen($sessFile, 'w')) { $return = false; // Lock the file for exclusive access to avoid issues with multiple // processes writing session simultaneously: if (flock($handle, LOCK_EX)) { $return = fwrite($handle, $data); // Make sure that there's no trailing data by truncating the file to // the correct length: ftruncate($handle, strlen($data)); fflush($handle); flock($handle, LOCK_UN); } fclose($handle); if ($return !== false) { return true; } } // If we got this far, something went wrong with the file output... // It is tempting to throw an exception here, but this code is called // outside of the context of exception handling, so all we can do is // echo a message.

// Anecdotal testing Today and Yesterday seems to indicate destroy() // is called by the garbage collector and everything is good. // Something to keep in mind though. return 0; } /** * Write function that is called when session data is to be saved. * * @param string $sessId The current session ID * @param string $data The session data to write * * @return bool */ public function write($sessId, $data): bool { if ($this->writesDisabled) { return true; } return $this->saveSession($sessId, $data); } /** * A function that is called internally when session data is to be saved. * * @param string $sessId The current session ID * @param string $data The session data to write * * @return bool */ abstract protected function saveSession($sessId, $data): bool; }

* * @return void */ public function writeClose() { // The assumption is that we're using PHP's ext/session. // session_write_close() will actually overwrite $_SESSION with an // empty array on completion -- which leads to a mismatch between what // is in the storage object and $_SESSION. To get around this, we // temporarily reset $_SESSION to an array, and then re-link it to // the storage object. // // Additionally, while you _can_ write to $_SESSION following a // session_write_close() operation, no changes made to it will be // flushed to the session handler. As such, we now mark the storage // object isImmutable. $storage = $this->getStorage(); if (! $storage->isImmutable()) { $_SESSION = $storage->toArray(true); session_write_close(); $storage->fromArray($_SESSION); $storage->markImmutable(); } } /** * Attempt to set the session name * * If the session has already been started, or if the name provided fails * validation, an exception will be raised. * * @param string $name * @return SessionManager * @throws Exception\InvalidArgumentException */ public function setName($name) { if ($this->sessionExists()) { throw new Exception\InvalidArgumentException( 'Cannot set session name after a session has already started'

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