arXiv subjects

Konstantinos Topalakis

Publications and source records attributed to Konstantinos Topalakis.

3 recordsLinked to original sources

arXiv2026

Little Red Dots are Direct-Collapse Black Hole-Forming Galaxies

"Little Red Dots" (LRDs) at $4 < z < 8$ are one of the most challenging discoveries by JWST to date because their distinctive V-shaped spectra and compact morphologies (100 - 200 pc) defy conventional astrophysical interpretation. Previous attempts to explain LRDs as compact stellar systems, heavily-cocooned black holes with differential flows, supermassive stars, or more exotic objects like 'black-hole stars' either cannot show how they formed, explain the origin of the dense shells needed for the absorption features in their spectra, or account for their observed abundances or inferred lifetimes. Here we show that LRDs are simply direct-collapse black hole galaxies in which the BH is still shrouded by the massive disk that created it. Our cosmological simulations yield spectra that are good matches to those of LRDs because high densities at the center of the disk trap X-rays from the BH and produce the observed Balmer absorption features while allowing UV, optical and reprocessed IR flux to partly escape. The host galaxy forms a dense 10$^8$ M$_{\odot}$ cluster of stars with a radius of 150 pc next to the BH, consistent with observations of LRDs. Our models reproduce a wide variety of LRD spectra from typical objects like RUBIES-EGS-42046 at $z = 5.28$ to those with the strongest Balmer breaks such as MoM-BH$^*$-1 at $z = 7.76$ and those at the highest redshifts like CAPERS-LRD-z9 at $z = 9.29$.

astro-ph.GA↗

arXiv2025

The Evolution of Pop III.1 Protostars Powered by Dark Matter Annihilation. II. Dependence on WIMP Properties

The rapid appearance of supermassive black holes (SMBHs) at $z\gtrsim7$ requires efficient pathways to form massive black hole seeds. We investigate whether annihilation of weakly interacting massive particles (WIMPs) can alter primordial (Pop III.1) protostellar evolution sufficiently to enable formation of such `heavy'' seeds. Using the one-dimensional Geneva stellar-evolution code (GENEC) with an implemented Gould single-scatter capture module, we compute a grid of protostellar evolution models covering ambient WIMP mass densities $ρ_χ=10^{12}$-$10^{16}\ \mathrm{GeV\,cm^{-3}}$, WIMP masses $m_χ=30$-$3000\ \mathrm{GeV}$, spin-dependent cross sections $σ_{\rm SD}=10^{-42}$-$10^{-40}\ \mathrm{cm^2}$, and baryonic accretion rates $\dot{M_*}=(1-3)\times10^{-3}\, M_\odot \,{\rm yr}^{-1}$. We find a robust bifurcation of outcomes. For sufficiently high ambient dark matter density ($ρ_χ\gtrsim5\times10^{14}\ \mathrm{GeV\,cm^{-3}}$) and capture efficiency ($σ_{\rm SD}\gtrsim10^{-41}\ \mathrm{cm^2}$) WIMP annihilation supplies enough energy to inflate protostars onto extended, cool (Hayashi-track) configurations that dramatically suppress ionizing feedback and permit uninterrupted growth to $\sim10^{5}\,M_\odot$. Lighter WIMPs and larger $σ_{\rm SD}$ favour earlier and stronger annihilation support; heavier WIMPs delay the effect. For our fiducial case, WIMP masses $<$3 TeV are essential for allowing growth to the supermassive regime, otherwise the protostar evolves to the compact, feedback-limited regime that results in `light'' seeds. These results indicate that, under plausible halo conditions, DM annihilation provides a viable channel for forming heavy black hole seeds.

astro-ph.GA↗

arXiv2025

The Evolution of Pop III.1 Protostars Powered by Dark Matter Annihilation. I. Fiducial model and first results

The existence of billion-solar-mass quasars at redshifts $z \gtrsim 7$ poses a formidable challenge to theories of black hole formation, requiring pathways for the rapid growth of massive seeds. Population III.1 stars, forming in pristine, dense dark matter (DM) minihalos, are compelling progenitors. This study presents a suite of stellar evolution models for accreting Pop III.1 protostars, calculated with the \textsc{GENEC} code. We systematically explore a wide parameter space, spanning ambient WIMP densities of $ρ_χ\sim 10^{12}\mbox{-}10^{16}\,\mathrm{GeV\,cm^{-3}}$ and gas accretion rates of $10^{-3}\mbox{-}10^{-1}\,M_\odot\,\mathrm{yr^{-1}}$, to quantify the effects of DM annihilation. A central finding is that for a protostar to grow to supermassive scales ($\gtrsim 10^5 \, M_{\odot}$), the ambient DM density in the immediate vicinity of the star must exceed a critical threshold of $ρ_χ \gtrsim 5 \times 10^{14} \, \text{GeV cm}^{-3}$. The energy injected by WIMP annihilation inflates the protostar, lowering its surface temperature, which suppresses the ionizing feedback that would otherwise halt accretion and significantly delays the onset of hydrogen fusion. This heating also governs the star's final fate: in dense halos ($ρ_χ\gtrsim 10^{15}\,\mathrm{GeV\,cm^{-3}}$), stars remain stable against general relativistic instability beyond $10^6 \, M_{\odot}$, whereas at lower densities ($ρ_χ\lesssim 10^{13}\,\mathrm{GeV\,cm^{-3}}$), they collapse at masses of $\sim 5 \times 10^5 \, M_{\odot}$. Once the DM fuel is exhausted and core burning commences, the protostar contracts and its ionising photon output can reach very high levels $\sim 10^{53} s^{-1}$. These distinct evolutionary phases offer clear observational signatures for the JWST, providing a robust, physically-grounded pathway for forming heavy black hole seeds in the early universe.

astro-ph.SR↗