Category: subhalos

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Stellar streams from dwarf galaxies could be key to unveiling the nature of dark matter

  • A new study led by researchers from IFT and UAM is the first to propose searching for dark matter signals in stellar streams originating from dwarf galaxies.
  • The goal is to detect gamma rays produced by the annihilation of WIMPs, hypothetical particles that are extremely difficult to detect and are the most promising candidates for dark matter.
  • NASA’s Fermi Gamma-ray Space Telescope (Fermi-LAT) is the key instrument for conducting these measurements.

One of the greatest mysteries and challenges in modern physics is uncovering the nature of dark matter. It is called “dark” because it neither emits nor absorbs light, and we can only infer its presence through its gravitational influence on visible objects around it. Although it has not yet been directly detected, there is extensive evidence suggesting its existence in the universe, accounting for 85% of all matter (with the remaining 15% being the ordinary matter that makes up everything we can see).

Now, a recent study led and entirely developed by researchers from the Institute for Theoretical Physics (IFT UAM-CSIC) and the Department of Theoretical Physics at the Autonomous University of Madrid proposes stellar streams as a novel and promising tool for detecting dark matter. These streams are tubular structures made of stars that move together, resembling “rivers of stars” flowing through galaxies like our own.

Searching for WIMPs in stellar streams

Specifically, this study aims to use stellar streams to detect interactions from one of the leading candidates to explain dark matter: Weakly Interacting Massive Particles (WIMPs). Scientists worldwide have been trying for nearly a century to uncover this enigmatic form of matter, and among all the proposed theories and candidates, WIMPs are one of the best-motivated and most extensively studied in recent decades.

WIMPs are hypothetical, very massive particles that, in addition to experiencing gravity, may also interact via the weak force—one of the four known fundamental interactions between subatomic particles, responsible for certain types of radioactivity. These particles are extremely difficult to detect, making indirect detection one of the most promising approaches. This technique involves capturing the products of their interactions, as WIMPs may decay or annihilate each other when they meet, producing detectable particles such as gamma rays, the most energetic form of light.

Currently, scientists search for gamma-ray signals from dark matter in regions where we expect high concentrations of it, such as the center of our galaxy, galaxy clusters in the nearby universe, and small satellite galaxies of the Milky Way.

This innovative study, published in arXiv:2502.15656v1, proposes looking for gamma-ray signals from WIMP annihilations in the cores of stellar streams originating from dwarf galaxies. This marks the first time such an approach has been used for dark matter detection.

Stellar streams from dwarf galaxies

Stellar streams form when a galaxy, such as the Milky Way, gravitationally tears apart a globular cluster or a dwarf galaxy orbiting around its center. As a result of intense tidal forces, the object stretches into an elongated, tubular stellar structure, losing mass as it continues orbiting. Over hundreds of millions to billions of years, it may even completely dissolve.

This study focuses on stellar streams originating from dwarf satellite galaxies of the Milky Way, which are the most dark matter-dominated astrophysical objects known. Specifically, a significant amount of dark matter is expected to have survived in the central regions of these objects despite the stretching and mass loss they undergo as they are captured by our galaxy and evolve under the resulting intense tidal forces.

Sky map, in Galactic coordinates and Hammer projection, of the celestial tracks of stellar streams used in the study

The Fermi-LAT Telescope: a key tool

To search for dark matter signals in the form of gamma rays from WIMP annihilations in the core of these stellar streams, the research team used the Large Area Telescope (LAT) aboard NASA’s Fermi satellite.

The authors analyzed data collected by the telescope in the direction of a sample of stellar streams, which were specifically selected for this research based on their known properties. Although the data analysis did not reveal a significant signal from these objects, it did allow the researchers to set competitive limitson the mass and interaction rate of dark matter.

According to Cristina Fernández-Suárez, PhD student and lead author of the study: “Even finding no signal brings us closer to discovering what dark matter is, as it helps us rule out what it is not.”

The study’s co-author, Dr. Miguel A. Sánchez-Conde, who is also the current Scientific Coordinator of the entire Fermi-LAT collaboration, added: “During its more than 16 years of operation, the Fermi-LAT gamma-ray space telescope has revolutionized high-energy astrophysics, opening a new window for exploring the most violent aspects of the universe and improving our understanding of it. This study once again demonstrates its potential to lead the search for dark matter today.”

This study represents the first attempt to search for dark matter signals in the form of gamma rays from stellar streams, demonstrating their viability as competitive targets in the search for dark matter. It also highlights their complementarity with other astrophysical objects traditionally used in such searches.

Fernández-Suárez, C., & Sánchez-Conde, M. A. (2025). A search for dark matter annihilation in stellar streams with the Fermi-LAT. arXiv. https://doi.org/10.48550/arXiv.2502.15656

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A search for dark matter among Fermi-LAT unidentified sources with systematic features in Machine Learning

Written by Viviana Gammaldi.

Summary of the paper with the same title published in MNRAS.

arXiv: 2207.09307

The recent 4FGL Fermi-LAT catalogue, the result of 8 years of telescope operation, is a collection of sources with associated gamma-ray spectra, containing important information about their nature. As shown in Fig. 1, somehow surprisingly, an important fraction of objects in the Fermi-LAT catalogs, ca. 1/3 of the total, remain as unidentified (unIDs), i.e., objects lacking a clear single association to a known object identified at other wavelengths, or to a well-known spectral type emitting only in gamma rays, e.g. certain pulsars. Indeed, there is the exciting possibility that some of these sources could be a DM signal. Among other prospective sources of gamma rays from DM annihilation events, dark satellites or subhalos in the Milky Way, with no optical counterparts, are the preferred candidates, as they are expected to exist in high number according to standard cosmology and they would not be massive enough to retain gas/stars. Further, main galaxies in local Universe, e.g. dwarf irregular galaxies, may also represent good candidates for unIDs.

Fig. 1: Fermi-LAT detected sources.

We propose a new approach to solve the standard, Machine Learning (ML) binary classification problem of disentangling prospective DM sources (simulated data) from astrophysical sources (observed data) among the unIDs of the 4FGL Fermi-LAT catalogue.

In particular, we are interested in one of the parametrizations of the gamma-ray spectrum used in the 4FGL, known as the Log-Parabola (LP), which allow us to identify different astrophysical sources of gamma rays by means of at least two parameters, the emission peak ,Epeak, and the spectral curvature, beta. Indeed, we introduce the DM sample in the parameter space by fitting the simulated DM gamma-ray spectrum with the same LP functional form (Fig. 2, left panel). Furthermore, we artificially build two systematic features for the DM data which are originally inherent to observed data: the detection significance and the relative uncertainty on the spectral curvature, beta_rel. We do it by sampling from the observed population of unIDs, assuming that the DM distributions would, if any, follow the latter. In Fig. 2 we show the parameter space without the uncertainty on beta (left panel) and by including the uncertainty on beta, created for the DM sample as systematic feature.

Fig. 2: beta-Epeak parameter space. Left panel: Astrophysical (yellow), DM (magenta) and unIDs (red) sources are shown. Right panel: Same as left panel, but including the uncertainty on beta for the training/test set (grey data) and the unIDs sources to be classified (red data point).

Finally, we consider different ML models for the classification task: Logistic Regression, Neural Network (NN), Naive Bayes and Gaussian Process, out of which the best, in terms of classification accuracy, is the NN, achieving around 93% performance. Applying the NN to the unIDs sample, we find that the degeneracy between some astrophysical and DM sources (visible as overlapping region in Fig. 2) can be partially solved within by including systematic features in the classification task (Fig. 3). Nonetheless, due to strong statistical fluctuations, we conclude that there are no DM source candidates among the pool of 4FGL Fermi-LAT unIDs.

Fig. 3: Probability for each unIDs to be a DM source. Left panel: results adopting only two feature beta-Epeak for classification. Right panel: results for the four-features (beta, Epeak, sigma, beta_rel) classification.

Further details can be found in https://doi.org/10.1093/mnras/stad066 .

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ΛCDM halo substructure properties revealed with high resolution and large volume cosmological simulations

Written by Angie Moliné.

Summary of the paper with the same title submitted to MNRAS.

arXiv: 2110.02097

In the current standard model of cosmology, ΛCDM, the structure of the Universe is formed via a hierarchical, bottom-up scenario with small primordial density perturbations growing to the point where they collapse into the filaments, walls and eventually dark matter (DM) haloes that form the underlying large-scale-structure filamentary web of the Universe. Galaxies are embedded in these massive, extended DM haloes teeming with self-bound substructure, the so-called subhaloes.

The study of the statistical and structural properties of the subhalo population is of prime importance because subhaloes represent important probes of the mass accretion history and dynamics of host haloes and accordingly, of the underlying cosmological model. In addition to representing a cosmological test by themselves, understanding both the statistical and structural properties of subhaloes plays a key role for many other diverse studies, such as gravitational lensing, stellar streams and indirect or direct DM detection experiments.

Studying the complicated dynamics of these subhaloes within their hosts requires numerical simulations, which have proven to be crucial for understanding structure formation in the Universe. By making use of data at different cosmic times from the Phi-4096 and Uchuu suite of high-resolution N-body cosmological simulations, in this work we improve upon previous studies aimed at characterize the subhalo population. More precisely, the superb numerical resolution and halo statistics of these simulations allow for a careful and dedicated study – for the first time consistently over more than seven decades in ratio of subhalo-to-host-halo mass – of the dependency of subhalo abundance with halo host mass as a function of subhalo mass, the maximum circular velocity of particles within the subhalo, Vmax, and distance to the host halo centre. We also dissect the evolution of these dependencies over cosmic time.

Subhalo structural properties are codified via a concentration parameter that does not depend on any specific, pre-defined density profile and relies only on Vmax. We derive such relation in the range 7-1500 km/s and find an important dependence on distance of the subhalo to the host halo centre, as already described in Moliné et al. (2017) for subhaloes in Milky-Way-like hosts. Interestingly, we also find subhaloes of the same mass to be significantly more concentrated when they reside inside more massive hosts. We provide accurate fits that take into account all mentioned dependencies. In addition, the study of the evolution of subhalo concentrations with cosmic time is very scarce in the literature as of today. We investigate such redshift evolution of concentrations and provide an accurate fit.

Our results offer an unprecedented detailed characterization of the subhalo population, consistent over a wide range of subhalo and host halo masses, as well as cosmic times. Our analyses enables precision work in any future research involving dark matter halo substructure.

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Sensitivity of the Cherenkov Telescope Array to dark subhalos

Written by Javier Coronado-Blázquez.

Summary of the paper with the same title submitted to PDU.

arXiv: 2101.10003

In this work, we study the potential of the Cherenkov Telescope Array (CTA) for the detection of Galactic dark matter (DM) subhalos, focusing on low-mass subhalos – not massive enough to retain any baryonic content – therefore lacking any multiwavelength counterpart. As in previous papers, devoted to the Fermi-LAT and HAWC instruments, If the DM is made of weakly interacting massive particles (WIMPs), these dark subhalos may appear in the gamma-ray sky as unidentified sources. We perform a detailed characterization of CTA instrumental response to dark subhalos, using the ctools analysis software, simulating CTA observations under different array configurations and pointing strategies.

We distinguish three different observational modes: i) a key science project, the extragalactic survey (codename EGAL). This will observe a fourth of sky at high-latitudes with uniform exposure, providing unprecedented coverage at very high energies; ii) a proposed deep-field campaign (DEEP), which would point at a blank spot of the sky aiming to serendipitously find new sources, such as dark subhalos, due to the extreme sensitivity; and iii) a chance of finding a dark subhalo in the field of view of any of CTA’s science operations through accumulated exposure the years, so-called overall exposure (EXPO).

To be able to compute the latter strategy, one has to estimate the sky coverage in, e.g., 10 years of operation, as well as the median exposure time. We did so by extrapolating the MAGIC telescope operations, which share location with the CTA-North. In this way, we get a realistic estimation of the accumulated observations, which turn out to be a factor 2 more area and a factor 10 more time than the EGAL survey. This, together with information on the subhalo population as inferred from N-body cosmological simulations, allows us to predict the CTA detectability of dark subhalos, i.e., the expected number of subhalos in each of the considered observational scenarios.

In the absence of detection, for each observation strategy we set competitive limits to the annihilation cross section as a function of the DM particle mass, that are between one and two orders of magnitude away from the thermal cross section, for the bb and ττ annihilation channels. This way, CTA will offer the most constraining limits from subhalo searches in the intermediate range between 1−3 TeV, complementing previous results with Fermi-LAT and HAWC at lower and higher energies, respectively, as well as an independent probe of DM.

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Constraints to dark matter annihilation from high-latitude HAWC unidentified sources

Written by Javier Coronado-Blázquez.

Summary of the paper with the same title published by Galaxies.

arXiv: 2001.02536

Unidentified gamma-ray sources (unIDs) have proven to be competitive targets for indirect DM detection, via the ΛCDM-predicted subhalos in our own Galaxy, not massive enough to retain baryons and become visible. These may shine in gamma rays provided that the dark matter (DM) is made of weakly interacting massive particles (WIMPs), that would self-annihilate and would appear as unIDs.

In previous works, we used Fermi-LAT due to the all-sky coverage, which maximizes the probability of finding a DM subhalo within the unIDs pool. One can play the same game for Very High Energy (VHE) observatories, such as the Imaging Atmospheric Cherenkov Telescopes (IACTs). Some of them are MAGIC, H.E.S.S., VERITAS or MILAGRO. The “problem” of these observatories is that, unlikely Fermi-LAT, are pointing telescopes, i.e., have a very narrow field of view, and therefore, the sky coverage is very small.

Another interesting observatory is HAWC, which works very similar to IACTs, but using water as medium for the Cherenkov radiation. HAWC is not a pointing telescope, and due to the rotation of the Earth it observes ~2/3 of the sky with uniform exposure. Combining all these observatories produces the TeVCat, a VHE source catalog. As we are interested in high-latitude sources, due to the Galactic astrophysical confusion at low latitudes.

In the TeVCat we find three interesting high-latitude (|b|≥10º) sources present in HAWC’s 2HWC catalog, with no associations at other wavelengths. Indeed, only one of these sources, 2HWC J1040+308, is found to be above the HAWC detection threshold when considering 760 days of data, a factor 1.5 more exposure time than in the original 2HWC catalog. Other instruments such as Fermi-LAT or VERITAS at lower energies do not detect this source.

Also, this unID is reported as spatially extended, which is supposed to be a “smoking gun” in a DM search context. While waiting for more data that may shed further light on the nature of this source, we can set competitive upper limits on the annihilation cross section by comparing this HAWC unID to expectations based on state-of-the-art N-body cosmological simulations of the Galactic subhalo population. We find these constraints to be particularly competitive for heavy WIMPs.

Although far from the thermal relic cross section value, the obtained limits are independent and nicely complementary to those from radically different DM analyses and targets, demonstrating again the high potential of this DM search approach.

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Spectral and spatial analysis of the dark matter subhalo candidates among Fermi Large Area Telescope unidentified sources

[Javier Coronado-Blázquez, Miguel A. Sánchez-Conde, Mattia Di Mauro, Alejandra Aguirre-Santaella, Ioana Ciucă, Alberto Domínguez, Daisuke Kawata, Néstor Mirabal]

Written by Javier Coronado-Blázquez.

Summary of the paper with the same title accepted by JCAP.

arXiv: 1910.14429

Fermi-LAT unidentified sources (unIDs) are very promising targets for indirect dark matter (DM) searches of subhalos, which would appear as this kind of objects in the gamma-ray sky. In a previous work, we found that among the 1235 unIDs in Fermi-LAT catalogs (3FGL, 2FHL and 3FHL) only 44 are DM subhalos candidates (see https://projects.ift.uam-csic.es/damasco/?p=61). Here, we perform a detailed spectral analysis to test whether these remaining sources are compatible or not with a DM origin. This analysis is executed using almost 10 years of Pass 8 Fermi-LAT data, which maximizes the response and sensitivity of the instrument.

As a result, none of the unIDs are found to significantly prefer DM-induced emission compared to more conventional astrophysical sources. Previous constraints on the DM annihilation cross section are updated and improved with the new number of remaining DM subhalo candidates among unIDs, becoming closer to the famous LAT dSphs limits.

Additionally, in order to discriminate between pulsar and DM sources, which have a very similar spectrum, we developed a new method based on the source’s spectral curvature, peak energy, and its detection significance, which turns out to be specially useful for bright sources.

We also look for spatial extension, which may be a hint for a DM origin. In fact, according to our N-body simulation studies of the subhalo population, the brightest subhalos should appear as extended in the sky, maybe as large as 10º. Nevertheless, we did find no evidence of spatial extension for any of our best candidates.

Finally, we used Gaia DR2 data to search for a potential stellar counterpart to our best DM subhalo candidates, if one of them once hosted a dwarf galaxy. Due to tidal forces induced by gravitational interaction with the DM host halo, the subhalos can be torn into a stellar stream, a filament of stars orbiting the Milky Way halo. Finding evidence of an unID in one of these streams would reinforce the DM hypothesis. Although no firm associations could be found, one of our candidates coincides with the Sagittarius stellar stream.

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Properties of subhalos in the interacting dark matter scenario

[Moliné, Ángeles; Schewtschenko, Jascha A.; Sánchez-Conde, Miguel A.; Aguirre-Santaella, Alejandra; Cora, Sofía A.; Abadi, Mario]

Written by A. Moliné

Summary of the paper with the same title published in Galaxies.

arXiv: 1907.12531

The current standard model of cosmology is based on a cosmological constant to explain the late-time accelerated expansion of the Universe and a cold dark matter (CDM) component to account for the required additional gravitational attraction to form and support the galaxies and larger structures we observe today. In this framework, the structure of the Universe is formed via a hierarchical, bottom-up scenario with small primordial density perturbations growing to the point where they collapse into the filaments, walls and eventually dark matter (DM) halos that form the underlying large-scale-structure filamentary web of the Universe. The galaxies are embedded in these massive, extended DM halos teeming with self-bound substructure. Any viable cosmological model has to successfully predict both the abundance and internal properties of these structures and their substructures, and match the observational data on a wide range of scales.

One natural deviation from the collisionless CDM in the standard model is the assumption of the existence of interactions between DM and the standard model (SM) particles we know about, in particular, photons or neutrinos. This does not only affect the formation of DM structures on small scales, but also provides an explanation for the exact relic abundance of DM found in the Universe today. Such possible interacting dark matter (IDM) model would imply a suppression of small-scale structures due to a large collisional damping effect, even though the weakly interacting massive particle (WIMP) can still be the DM candidate. Because of this, IDM models can help alleviate alleged tensions between standard CDM predictions and observations at small mass scales.

Using a high-resolution cosmological N-body simulation specifically run within this alternative model, we investigate the properties of DM halo substructure or subhalos formed in. We also run its CDM counterpart, which allowed us to compare subhalo properties in both cosmologies. We show that, in the lower mass range covered by our simulation runs, both subhalo concentrations and abundances as a function of the distance to the host halo center and subhalo mass (or, alternatively maximum circular velocity) are systematically lower in IDM compared to the CDM scenario. Yet, as in CDM, we find that median IDM subhalo concentration values increase towards the innermost regions of their hosts for same mass subhalos. Also similarly to CDM, we find IDM subhalos to be more concentrated than field halos of the same mass.

Our work has a direct application on studies aimed at the indirect detection of DM where subhalos are expected to boost the DM signal of their host halos significantly. From our results, we conclude that the role of halo substructure in DM searches will be less important in interacting scenarios than in CDM, but is nevertheless far from being negligible.

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Unidentified gamma-ray sources as targets for indirect dark matter detection with the Fermi Large Area Telescope

Written by Javier Coronado-Blázquez.

Summary of the paper with the same title accepted by JCAP.

arXiv: 1906.11896

If the dark matter (DM) is made of WIMP particles, they should yield a gamma-ray annihilation flux. Provided the DM clumps, which are predicted by the bottom-up collapse scenario for structure formation within LCDM, are light enough, these are not expected to retain any baryons and, therefore to remain completely dark to standard telescopes.

Only in gamma rays we should detect this emission, with a highly curved and distinctive spectrum. Fermi-LAT, a space-based gamma-ray telescope, has roughly one third of detected sources of unknown nature, the so-called unidentified sources (unIDs). Are there subhalos hidden in the Fermi-LAT catalogs, just waiting to be properly classified? To answer this question we propose a ‘filtering’ of the unIDs, according to the expected characteristics of a DM subhalo. For example, a DM subhalo should be detected as a steady gamma-ray source, so if an unID is reported as variable, is removed from our pool of candidates. With these ‘filters’, we are able to reduce the DM candidates from 1235 to just 44 unIDs.

A correct computation of the expected annihilation fluxes (encoded in the so-called J-factor) requires a good understanding of the DM subhalo population in our Galaxy. To do so, we use a state-of-the-art N-body cosmological simulation, Via Lactea II (VL-II). Unfortunately, the resolution of this, and any, simulation is limited, meaning we are unable to resolve the smallest members of the subhalo population (which can be as light as the Earth). Nevertheless, some of these subhalos may be close enough to us and still be bright enough to be detected by Fermi-LAT. To take them into account, we ‘repopulate’ the parent simulation with subhalos, reaching 3 orders of magnitude better resolution in mass. As the Earth position is arbitrary, to ensure a proper statistical treatment of the J-factors, 1000 realizations are done.

Also, we must compute the sensitivity of the Fermi-LAT to this kind of objects. The reported instrumental sensitivity is for a power-law source, i.e., with a non-curved spectrum, while we deal with highly-curved spectra, which also vary with the annihilation channel and mass of the WIMP. Furthermore, taking into account the diffuse gamma-ray emission and the considered catalog, this sensitivity also varies with the sky position (e.g. a source close to the Galactic plane, where the diffuse emission is more intense, will be harder to detect than a high-latitude source) and observation parameters (such as the energy threshold, total exposure time, instrumental response functions, etc.). We compute a huge grid of configurations, changing all the above mentioned parameters and generating a sensitivity skymap for each annihilation channel, WIMP mass, and catalog.

Finally, we are able to set constraints in the cross section vs. WIMP mass parameter space. We do so comparing the predictions of our repopulated simulations with the actual gamma-ray candidates. The obtained constraints are competitive with other targets such as the CMB or the dSphs, but they can still be improved via new rejections of candidates.