arXiv subjects

N. Hoyer

Publications and source records attributed to N. Hoyer.

3 recordsLinked to original sources

arXiv2024

Efficiency of black hole formation via collisions in stellar systems: An analysis of data from simulations and observations

This paper explores the theoretical relation between star clusters and black holes within, focusing on the potential role of nuclear star clusters (NSCs), globular clusters (GCs), and ultra compact dwarf galaxies (UCDs) as environments that allow for black hole formation via stellar collisions. This study aims to identify the optimal conditions for stellar collisions across a range of stellar systems leading to the formation of very massive stars that subsequently collapse into black holes. We analyze data from numerical simulations and observations of diverse stellar systems, encompassing various initial conditions, initial mass functions, and evolution scenarios. We computed a critical mass, determined by the interplay of collision time, system age, and initial properties of the star cluster. The efficiency of black hole formation ($\epsilon_{\mathrm{BH}}$) is defined as the ratio of initial stellar mass divided by critical mass. We find that stellar systems with a ratio of initial stellar mass over critical mass above 1 exhibit a high efficiencies in terms of black hole formation, ranging from $30-100\%$. While there is some scatter, potentially attributed to complex system histories and the presence of gas, the results highlight the potential for achieving high efficiencies via a purely collisional channel in black hole formation. In conclusion, this theoretical exploration elucidates the connection between star clusters and black hole formation. The study underscores the significance of UCDs, GCs, and NSCs as environments conducive to the black hole formation scenario via stellar collisions. The defined black hole formation efficiency ($\epsilon_{\mathrm{BH}}$) is shown to be influenced by the ratio of the initial stellar mass to the critical mass.

astro-ph.GA↗

arXiv2023

Demographics of Tidal Disruption Events with L-Galaxies: I. Volumetric TDE rates and the abundance of Nuclear Star Clusters

Stars can be ripped apart by tidal forces in the vicinity of a massive black hole (MBH), causing luminous flares known as tidal disruption events (TDEs). These events could be contributing to the mass growth of intermediate-mass MBHs, and new samples from transient surveys can provide useful information on this growth channel. This work aims to study the demographics of TDEs by modeling the co-evolution of MBHs and their galactic environments in a cosmological framework. We use the semi-analytic galaxy formation model \emph{L-Galaxies}BH, which follows the evolution of galaxies as well as of MBHs, including multiple scenarios for MBH seeds and growth, spin evolution, and binary MBH dynamics. Time-dependent TDE rates are associated with each MBH depending on the stellar environment, following the solutions to the 1-D Fokker Planck equation solved with \textsc{PhaseFlow}. Our model produces volumetric rates that are in agreement with the latest optical and previous X-ray samples. This agreement requires a high occupation fraction of nuclear star clusters with MBHs since these star reservoirs host the majority of TDEs at all mass regimes. We predict that TDE rates are an increasing function of MBH mass up to $\sim\, 10^{5.5}$M$_\odot$, beyond which the distribution flattens and eventually drops for $>\,10^{7}$M$_\odot$. In general, volumetric rates are predicted to be redshift-independent at $z\,{<}\,1$. We discuss how the spin distribution of MBHs around the event horizon suppression can be constrained via TDE rates and what is the average contribution of TDEs to the MBH growth. In our work, the majority of low-mass galaxies host nuclear star clusters that have their loss-cone depleted by $z\,=\,0$, explaining why TDEs are rare in these systems. This highlights that time-dependent TDE rates are essential for any model to be in good agreement with observations at all mass regimes.

astro-ph.HE↗

arXiv2019

3C 294 revisited: Deep Large Binocular Telescope AO NIR images and optical spectroscopy

Context. High redshift radio galaxies are among the most massive galaxies at their redshift, are often found at the center of protoclusters of galaxies, and are expected to evolve into the present day massive central cluster galaxies. Thus they are a useful tool to explore structure formation in the young Universe. Aims. 3C~294 is a powerful FR II type radio galaxy at z = 1.786. Past studies have identified a clumpy structure, possibly indicative of a merging system, as well as tentative evidence that 3C~294 hosts a dual active galactic nucleus (AGN). Due to its proximity to a bright star, it has been subject to various adaptive optics imaging studies. Method. In order to distinguish between the various scenarios for 3C~294 we performed deep, high-resolution adaptive optics near-infrared imaging and optical spectroscopy of 3C~294 with the Large Binocular Telescope. Results. We resolve the 3C~294 system into three distinct components separated by a few tenths of an arcsecond on our images. One is compact, the other two are extended, and all appear to be non-stellar. The nature of each component is unclear. The two extended components could be a galaxy with an internal absorption feature, a galaxy merger, or two galaxies at different redshifts. We can now uniquely associate the radio source of 3C~294 with one of the extended components. Based on our spectroscopy, we determined a redshift of z = 1.784+-0.001, which is similar to the one previously cited. In addition we found a previously unreported emission line at $λ$6749.4 Å in our spectra. It is not clear that it originates from 3C~294. It could be the Ne [IV] doublet lambda 2424/2426 AA at z = 1.783, or belong to the compact component at a redshift of z ~ 4.56. We thus cannot unambiguously determine whether 3C~294 hosts a dual AGN or a projected pair of AGNs.

astro-ph.GA↗