arXiv subjects

Cosmin Ilie

Publications and source records attributed to Cosmin Ilie.

At least 19 recordsLinked to original sources

arXiv2026

JWST's Little Red Dots as collapsed Supermassive Dark Stars

The nature of the ``Little Red Dots'' (LRDs) is one of the most profound mysteries posed by the JWST data. One promising class of models that can reproduce the observed LRDs spectra and morphology are quasi-stars: massive envelopes surrounding accreting black holes formed via the collapse of supermassive stars (SMSs). However, the canonical SMS pathway relies on a highly restricted set of environmental and structural conditions: strong Lyman--Werner (LW) backgrounds to suppress H$_2$ cooling, high and sustained gas inflow rates to enforce entropy stratified envelopes, and assume non-zero rotational support in order to prevent GR instability collapse before $\sim 10^6 M_{\odot}$. Here we show that supermassive dark stars (SMDSs), powered by dark matter (DM) annihilation rather than nuclear burning, naturally satisfy the key structural and energetic requirements for quasi-star (QS) formation while relaxing {\it all} of those restrictive conditions listed above. Moreover, quasi-stars formed through the SMDS pathway are born with prompt BH masses ($\gtrsim 10\%$) of the progenitor mass. They therefore enter directly into a late-stage quasi-star regime; subsequently the envelope expands and cools until its photosphere reaches the zero-metallicity opacity limit $(T_{\rm eff}\sim3000$-$6000\,{\rm K}$). Those cool, optically thick, unresolved photospheres can reproduce key features of many JWST LRDs.

astro-ph.CO↗

arXiv2026

Supermassive Dark Stars and their remnants as a possible solution to three recent cosmic dawn puzzles

The James Webb Space Telescope (JWST) has begun to revolutionize our view of the Cosmos. The discovery of Blue Monsters (i.e., ultra-compact yet very bright high-z galaxies) and the Little Red Dots (i.e., very compact dustless strong Balmer break cosmic dawn sources) pose significant challenges to pre-JWST era models of the assembly of first stars and galaxies. In addition, JWST data further strengthen the problem posed by the origin of the supermassive black holes that power the most distant quasars observed. Stars powered by Dark Matter annihilation (i.e., Dark Stars) can form out of primordial gas clouds during the cosmic dawn era and subsequently might grow via accretion and become supermassive. In this paper we argue that Supermassive Dark Stars (SMDSs) offer natural solutions to the three puzzles mentioned above.

astro-ph.GA↗

arXiv2025

Early Formation of Supermassive Black Holes via Dark Star Gravitational Instability

We show that dark stars, which are dark-matter-powered stars in the early universe, can grow by accretion to masses in the range $\mathscr{O}\left ({10}^4\right )-\mathscr{O}\left ({10}^7\right)\,{M_\odot}$ before the general-relativistic Feynman-Chandrasekhar instability causes their dynamical collapse to black holes. These accreting dark star configurations avoid standard stellar nuclear- and weak-interaction evolution that would lead to their demise long before they reached this supermassive size. Remarkably, this mechanism for supermassive black hole (SMBH) genesis is relatively robust to initial dark star mass, formation epoch, accretion rate and its history. The SMBHs produced this way can serve as seeds for even larger SMBHs $({\gtrsim}10^9\,M_\odot)$ that have been discovered at high redshift.

astro-ph.CO↗

arXiv2025

Neural Network identification of Dark Star Candidates. II. Spectroscopy

Some of the first stars in the Universe might be powered by Dark Matter (DM) annihilations, rather than nuclear fusion. Those objects, i.e. Dark stars (DS), offer a unique window into understanding DM via the observational study of the formation and evolution of the first stars and their Black Hole (BH) remnants. In \cite{NNSMDSPhot} (Paper~I) we introduced a feedforward neural network (FFNN) trained on synthetic DS photometry in order to detect and characterize dark star {\it photometric} candidates in the early universe based on data taken with the NIRCam instrument onboard the James Webb Space Telescope (JWST). In this work we develop a FFNN trained on synthetic DS spectra in order to identify {\it spectroscopic} dark star candidates in the data taken with JWST's NIRSpec instrument. In order to validate our FFNN model we apply it to real data for the four spectroscopic Supermassive Dark Star (SMDS) candidates recently identified in \cite{ilie2025spectroscopicsupermassivedarkstar} and reconfirm that indeed \JADESeleven, \JADESzthirteen, \JADESfz, and \JADESfo have spectra that are consistent with those of Supermassive Dark Stars. The main advantage of our FFNN model, in comparison to the Nedleaer-Mead Monte Carlo parameter estimator used in \cite{ilie2025spectroscopicsupermassivedarkstar}, is that the approach introduced here predicts parameters in milliseconds, over 10,000 times faster than the traditional method used in \cite{ilie2025spectroscopicsupermassivedarkstar}. With this in mind, the FFNN model we developed and validated in this work will be adapted for Bayesian uncertainty analyses and automatic analyses of NIRSpec publicly available data for high redshift objects. This study establishes a robust and efficient tool for probing Dark Stars and understanding their role in cosmic evolution.

astro-ph.IM↗

arXiv2025

Neural Network identification of Dark Star Candidates. I. Photometry

The formation of the first stars in the universe could be significantly impacted by the effects of Dark Matter (DM). Namely, if DM is in the form of Weakly Interacting Massive Particles (WIMPs), it could lead to the formation (at $z\sim 25-10$) of stars that are powered by DM annihilations alone, i.e. Dark Stars (DSs). Those objects can grow to become supermassive ($M\sim 10^6 M_\odot$) and shine as bright as a galaxy ($L\sim 10^8 L_\odot)$. Using a simple $\chi^2$ minimization, the first three DSs photometric candidates (i.e. JADES-GS-z11, JADES-GS-z12, and JADES-GS-z13) were identified by Ilie et al. (2023). Our goal is to develop tools to streamline the identification of such candidates within the rather large publicly available high redshift JWST data sets. We present here the key first step in achieving this goal: the development and implementation of a feed-forward neural network (FFNN) search for Dark Star candidates, using data from the JWST Advanced Deep Extragalactic Survey (JADES) photometric catalog. Our method reconfirms JADES-GS-z13 and JADES-GS-z11 as dark star candidates, based on the chi-squared goodness of fit test, yet they are $\sim10^4$ times faster than the Neadler-Mead $\chi^2$ minimization method used in Ilie et al. (2023). We further identify six {\it new photometric} Dark Star candidates across redshifts $z \sim 9$ to $z \sim 14$. These findings underscore the power of neural networks in modeling non-linear relationships and efficiently analyzing large-scale photometric surveys, advancing the search for Dark Stars.

astro-ph.CO↗

arXiv2025

Constraining Asymmetric DM Properties by Black Hole Formation in Neutron Stars and Population III Stars

In this work we explore the potential for Neutron Stars (NSs) at the Galactic center and Population~III stars to constrain bosonic Asymmetric Dark Matter (ADM). We demonstrate that for NSs in an environment of sufficiently high DM density ($ρ_χ\gtrsim10^{9}\text{GeV/cm}^3$), the effects of both multiscatter capture and DM evaporation cannot be neglected. Conversely, for Pop~III stars, we find they are excellent at probing low-mass ADM. For instance, the most easily observable Population III stars could be highly effective at constraining high-$σ$ low-$m_χ$ DM, maintaining efficacy below $m_χ=10^{-15}\text{GeV}$ (assuming a Bose Einstein Condensate(BEC) forms) thanks to their far lower value of $m_χ$ at which capture saturates to the geometric limit. Finally, we derive closed-form approximations for the evaporation rate of DM from arbitrary polytropic objects and from DM particles in a BEC state.

astro-ph.CO↗

arXiv2025

Reconstructing PTA measurements via early seeding of supermassive black holes

Motivated by recent findings that PTA's nHz signal may be dominated by supermassive black hole (SMBH) binaries ($M \gtrsim 10^9 M_\odot$), and high redshift quasar observations revealing unexpectedly massive SMBHs, we calculate the implications of early seeded SMBHs for the PTA signal. As an application, we explore two prominent scenarios of high-$z$ SMBHs seeding mechanisms: direct collapse black holes (DCBHs) and collapse of Dark Stars. We show that Dark Star seeded SMBHs, with comoving seed number density of $\mathcal{O}(10^{-3}) \, {\rm Mpc}^{-3}$ can be the dominant contributor to the PTA signal while the DCBH channel may contribute sub-dominantly. We also suggest ways to place an upper bound on the seed number density.

astro-ph.CO↗

arXiv2025

Spectroscopic Supermassive Dark Star candidates

Dark Stars, i.e. early stars composed almost entirely of hydrogen and helium but powered by Dark Matter, could form in zero metallicity clouds located close to the center of high redshift Dark Matter halos. In 2023 three of us identified (in a PNAS work) the first three photometric Dark Star candidates: JADES-GS-z11-0, JADES-GS-z12-0, and JADES-GS-z13-0. We report here our results of a followup analysis based on available NIRSpec JWST data. We find that JADES-GS-z11-0 and JADES-GS-z-13-0 are spectroscopically consistent with a Dark Star interpretation. Moreover, we find two additional spectroscopic Dark Star candidates: JADES-GS-z14-0 and JADES-GS-z-14-1, with the former being the most distant luminous object ever observed. We furthermore identify a feature in its spectrum indicative of the smoking gun signature of Dark Stars: the He II$λ$1640 absorption line. In view ALMA's recent identification of a probable OIII nebular emission line in the spectrum of JADES-GS-z14-0, the simple interpretation of this object as an isolated Dark Star is unlikely. If both spectral features survive follow-up observations it would imply a Dark Star embedded in a metal rich environment, requiring theoretical refinements of the formation of evolution of Dark Stars, which in previous studies were assumed to form in isolation, without any companions.

astro-ph.CO↗

arXiv2024

Dark Sector Tunneling Field Potentials for a Dark Big Bang

All of the significant evidence for dark matter observed thus far has been through its gravitational interactions. After 40 years of direct detection experiments, the parameter space for Weakly Interacting Massive Particles (WIMPs) as dark matter candidates is rapidly approaching the neutrino floor. In this light, we consider a dark sector that is strongly decoupled from the visible sector, interacting exclusively through gravity. In this model, proposed by Freese and Winkler (arXiv:2302.11579), dark matter can be produced through a first-order phase transition in the dark sector dubbed the Dark Big Bang. In this study we fully determine the allowed region of parameter space for the tunneling potential that leads to the realization of a Dark Big Bang and is consistent with all experimental bounds available.

hep-ph↗

arXiv2024

Detectability of Supermassive Dark Stars with the Roman Space Telescope

The first bright objects to form in the Universe at redshift $z \sim 10-20$ might have been Dark Stars, made primarily of hydrogen and helium but powered by dark matter. In this study, we investigate the detectability of Supermassive Dark Stars (SMDS) by the Roman Space Telescope. RST will be able to detect SMDSs at redshifts as high as $z\simeq 14$. In cases with gravitational lensing factors of $μ\sim 100$, RST will be able to find SMDS as small as $\sim10^4 M_{\odot}$ at $z\sim 12$ with $\sim 10^6$ s of exposure. To differentiate SMDS from early galaxies containing zero metallicity stars at similar redshifts, we compare their spectra, photometry in RST bands, color indexes and image morphology. With RST alone, the differentiation is possible only for limited cases: SMDS formed via "adiabatic contraction" (DM pulled into the star via gravity alone) with $M\gtrsim 10^5M_{\odot}$ and lensed by $μ\gtrsim 30$ have distinct photometric signatures from those of the first galaxies. For SMDSs formed via "dark matter capture," their spectra are degenerate to those of many galaxies with little to no nebular emission. Thus with RST alone, the only way to tell them apart from first galaxies would be via image morphology: i.e. point object (SMDSs) vs. extended object (sufficiently magnified galaxies). However, if the same objects are further examined by JWST spectroscopy, a "smoking gun" for detection of SMDS is the HeII $λ$1640 absorption line. While RST does not cover the wavelength band required to find this line (for $z_{\rm emi}\gtrsim 10$), JWST does. Hence the two detectors can be used together in identifying SMDS. The confirmed detection of any SMDSs will provide evidence for a new type of star, powered by dark matter. Moreover, such massive stars can also be natural progenitors of the supermassive black holes powering the extremely bright quasars observed at $z\gtrsim 6$.

astro-ph.GA↗

arXiv2024

Closed-form expressions for multiscatter Dark Matter capture rates

Any astrophysical object can, in principle, serve as a probe of the interaction between Dark Matter and regular, baryonic matter. This method is based on the potential observable consequences annihilations of captured Dark Matter has on the surface temperature of the object itself. In a series of previous papers we developed and validated simple analytic approximations for the total capture rates of Dark Matter (DM) valid in four distinct regions of the DM-nucleon scattering cross section ($σ$) vs. DM particle mass ($m_X$) parameter space. In this work we summarize those previous results and extend them significantly, by deriving a completely general, closed form solution for the total capture rate of Dark Matter in the multiscatter regime. Moreover, we demonstrate the existence of a region in the $σ$ vs. $m_X$ parameter space where the constraining power of any astrophysical object heated by annihilations of captured DM is lost. This corresponds to a maximal temperature ($T_{crit}$) any astrophysical object can have, such that it can still serve as a DM probe. Any object with observed temperature $T_{obs}>T_{crit}$ loses its DM constraining power. We provide analytic formulae that can be used to estimate $T_{crit}$ for any object.

astro-ph.CO↗

arXiv2023

UHZ1 and the other three most distant quasars observed: possible evidence for Supermassive Dark Stars

The James Webb Space Telescope (JWST) has recently uncovered a new record-breaking quasar, UHZ1, at a redshift of $z\sim10$. This discovery continues JWST's trend of confronting the expectations from the standard $Λ$CDM model of cosmology with challenges. Namely, too many very massive galaxies and quasars have been observed at very high redshifts, when the universe was only a few hundred million years old. We have previously shown that Supermassive Dark Stars (SMDSs) may offer a solution to this puzzle. These fascinating objects would be the first stars in the universe, growing to be $\sim 10^5-10^7 M_{\odot}$ and shining as bright as $10^9$ suns. Unlike Population III stars (the major alternative proposed model for the first stars in the universe, which would also have zero metallicity and would be powered by nuclear fusion), SMDSs would be powered by dark matter heating (e.g. dark matter annihilation) and would be comparatively cooler. At the ends of their lives (when they run out of dark matter fuel), SMDSs would directly collapse into black holes, thus providing possible seeds for the first quasars. Previous papers have shown that to form at $z\sim10$, UHZ1 would require an incredibly massive seed ($\sim 10^4 -10^5 M_{\odot}$), which was assumed to be a Direct Collapse Black Hole (DCBH). In this paper, we demonstrate that Supermassive Dark Stars (SMDSs) offer an equally valid solution to the mystery of the first quasars, by examining the four most distant known quasars: UHZ1, J0313-1806, J1342+0928, and J1007+2115, with particular emphasis on UHZ1.

astro-ph.GA↗

arXiv2023

The effectiveness of exoplanets and Brown Dwarfs as sub-GeV Dark Matter detectors

In this work we demonstrate that Dark Matter (DM) evaporation severely hinders the effectiveness of exoplanets and Brown Dwarfs as sub-GeV DM probes. Moreover, we find useful analytic closed form approximations for DM capture rates for arbitrary astrophysical objects, valid in four disticnt regions in the $σ-m_X$ parameter space. As expected, in one of those regions the Dark Matter capture saturates to its geometric limit, i.e. the entire flux crossing an object. As a consequence of this region, which for many objects falls within the parameter space not excluded by direct detection experiments, we point out the existence of a DM parameter dependent critical temperature ($T_{crit}$), above which astrophysical objects lose any sensitivity as Dark Matter probes. For instance, Jupiters at the Galactic Center have a $T_{crit}$ ranging from $700$ K (for a $3 M_J$ Jupiter) to $950$ K (for $14 M_J$). This limitation is rarely (if ever) considered in the previous literature of indirect Dark Matter detection based on observable signatures of captured Dark Matter inside celestial bodies.

astro-ph.CO↗

arXiv2023

Supermassive Dark Star candidates seen by JWST?

The first generation of stars in the Universe is yet to be observed. There are two leading theories for those objects that mark the beginning of the cosmic dawn: hydrogen burning Population~III stars and Dark Stars, made of hydrogen and helium but powered by Dark Matter heating. The latter can grow to become supermassive ($M_\star\sim 10^6\Msun$) and extremely bright ($L\sim 10^9L_\odot$). We show that each of the following three objects: JADES-GS-z13-0, JADES-GS-z12-0, and JADES-GS-z11-0 (at redshifts $z\in[11,14]$) are consistent with a Supermassive Dark Star interpretation, thus identifying, for the first time, Dark Star candidates.

astro-ph.CO↗

arXiv2022

Analytic Approximations for the Velocity Suppression of Dark Matter Capture

Compact astrophysical objects have been considered in the literature as dark matter (DM) probes, via the observational effects of annihilating captured DM. In this respect, Population III (Pop III) stars are particularly interesting candidates, since they form at high redshifts, in very high DM density environments. It is customary to assume such a star would form roughly at the center of a DM halo, and, as such, have no rotational velocity. In this paper, we break from this assumption and explore the effects we can expect to observe if a Pop III star forms at some distance away from the center of the halo and thus has a non-zero rotational velocity. The capture rate of DM in such a star is suppressed by a predictable amount. We develop and validate an analytical expression for the capture rate suppression factor and re-evaluate the bounds placed on the DM-nucleon cross section as a result of DM capture. We find that our previous results, obtained under the assumption of star formed within the central 10 AU of the DM mini-halo are essentially unchanged, even when considering the possible rotational velocities for those central stars.

astro-ph.CO↗

arXiv2022

Constraining Dark Matter properties with the first generation of stars

Dark Matter (DM) can be trapped by the gravitational field of any star, since collisions with nuclei in dense environments can slow down the DM particle below the escape velocity ($v_{esc}$) at the surface of the star. If captured, the DM particles can self-annihilate, and, therefore, provide a new source of energy for the star. We investigate this phenomenon for capture of DM particles by the first generation of stars [Population III (Pop III) stars], by using the multiscatter capture formalism. Pop III stars are particularly good DM captors, since they form in DM-rich environments, at the center of$~\sim 10^6 M_\odot$ DM minihalos, at redshifts $z\sim 15$. Assuming a DM-proton scattering cross section ($σ)$ at the current deepest exclusion limits provided by the XENON1T experiment, we find that captured DM annihilations at the core of Pop III stars can lead, via the Eddington limit, to upper bounds in stellar masses that can be as low as a few $M_\odot$ if the ambient DM density ($ρ_X$) at the location of the Pop III star is sufficiently high. Conversely, when Pop III stars are identified, one can use their observed mass ($M_\star$) to place bounds on $ρ_Xσ$. Using adiabatic contraction to estimate the ambient DM density in the environment surrounding Pop III stars, we place projected upper limits on $σ$, for $M_\star$ in the $100-1000~M_\odot$ range, and find bounds that are competitive with, or deeper than, those provided by the most sensitive current direct detection experiments for both spin independent and spin dependent interactions, for a wide range of DM masses. Most intriguingly, we find that Pop III stars with mass $M_\star \gtrsim 300 M_\odot$ could be used to probe the SD proton-DM cross section below the "neutrino floor," i.e. the region of parameter space where DM direct detection experiments will soon become overwhelmed by neutrino backgrounds.

astro-ph.CO↗

arXiv2021

Multicomponent multiscatter capture of Dark Matter

In recent years, the usefulness of astrophysical objects as Dark Matter (DM) probes has become more and more evident, especially in view of null results from direct detection and particle production experiments. The potentially observable signatures of DM gravitationally trapped inside a star, or another compact astrophysical object, have been used to forecast stringent constraints on the nucleon-Dark Matter interaction cross section. Currently, the probes of interest are: at high red-shifts, Population III stars that form in isolation, or in small numbers, in very dense DM minihalos at $z\sim 15-40$, and, in our own Milky Way, neutron stars, white dwarfs, brown dwarfs, exoplanets, etc. Of those, only neutron stars are single-component objects, and, as such, they are the only objects for which the common assumption made in the literature of single-component capture, i.e. capture of DM by multiple scatterings with one single type of nucleus inside the object, is valid. In this paper, we present an extension of this formalism to multi-component objects and apply it to Pop III stars, thereby investigating the role of He on the capture rates of Pop III stars. As expected, we find that the inclusion of the heavier He nuclei leads to an enhancement of the overall capture rates, further improving the potential of Pop III stars as Dark Matter probes.

astro-ph.CO↗

arXiv2021

Probing below the neutrino floor with the first generation of stars

We show that the mere observation of the first stars (Pop III stars) in the universe can be used to place tight constraints on the strength of the interaction between dark matter and regular, baryonic matter. We apply this technique to a candidate Pop III stellar complex discovered with the Hubble Space Telescope at $z \sim 7$ and find bounds that are competitive with, or even stronger than, current direct detection experiments, such as XENON1T, for dark matter particles with mass ($m_X$) larger than about $100$ GeV. We also show that the discovery of sufficiently massive Pop III stars could be used to bypass the main limitations of direct detection experiments: the neutrino background to which they will be soon sensitive.

astro-ph.CO↗