Category: Fermi LAT

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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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Dark Matter search in dwarf irregular galaxies with the Fermi Large Area Telescope

Written by Viviana Gammaldi.

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

arXiv: 2109.11291

Almost a century ago, first astrophysical hints came to light pointing to the existence of new physics: the gravitational interaction was failed to describe the kinematics of extragalactic objects. The gap between theory and observation could be filled both on astrophysical and cosmological scale, by assuming the existence of a new kind of non-luminous, yet gravitationally-interacting matter, i.e. the Dark Matter (DM). Currently, the abundance of DM has been estimated to be the 27% of the total content of the Universe, although its nature remains still unknown. Among other, the Weakly Interactive Massive Particle (WIMP) represents one plausible candidate beyond the Standard Model (SM) of particle physics. WIMPs particle can annihilate in astrophysical objects producing SM particles, whose decay processes generate secondary fluxes of gamma rays, among others detectable fluxes. Searching for DM hints in secondary fluxes produced in astrophysical sources is what we call indirect searches for DM.


Recently, dwarf irregular galaxies have been proposed as astrophysical targets of interest for indirect searches of DM. In fact, they are DM dominated astrophysical objects, with a negligible astrophysical background in gamma-rays. Nonetheless, dwarf irregular galaxies also represent an interesting laboratory from the point of view of the DM distribution. In fact, although the rotation curves of spiral galaxies have been the first observational evidence of the existence of DM, there is still a lot to learn about the distribution of DM in galaxies. From the benchmark LCDM cosmology and N-body DM-only simulation (N-body DM-only simulation are a good approximation due to the abundance of DM in the Universe – 27% of the total content – with respecto to the visible baryon matter, i.e. stars, gas, etc. – which represent only the 5% of the total content of the Universe), we know that the DM distribution generally follows the well-known Navarro-Frenk-White (NFW) profile, i.e. a profile with a “cusp” in the center of big structures. On the other hand, the experimental data of the rotation curves of smaller objects, point to “core” DM distribution profile. The reason of such a discrepancy is still unknown, and represent the so called “cusp/core problem”, an open question in physics, astrophysics and particle physics. Dwarf irregular galaxies are indeed an example of the objects where the cusp/core problem is observed.


In this work we remain agnostic about the cusp/core problem: we analysed the rotation curves of 7 dwarf irregular galaxies (see e.g. the rotation curve of the IC10 galaxy, Fig. 1).

The Burkert core profile (full blue line) corresponds to the best fit of the rotation curves. Instead, the NFW cusp profile has been theoretically constructed, and the corresponding rotation curve has been created and compared with the data. With these two different DM distributions, we have constructed the DM modelling, including the enhancement in the expected gamma-ray flux due to the existence of substructures. Substructures in DM halo are over density, sub clumps in the DM distribution profiles, relics of the bottom-up formation history of the galaxy. Their effect is to enhance the DM annihilation rate, i.e. the gamma-ray production, due to the enhance of the DM density locally. We have created the two-dimensional spacial templete for our targets. In Fig. 2 the spatial template of the IC10 galaxy is shown for four different DM density distribution profiles. Indeed, the MIN model corresponds to a Burket core profile without any substructure, the MED model is the Burkert profile with a medium contribution from substructures. The maximum contribution from substructures has been calculated for both the Burkert core and the cusp NFW profile (respectively, Bur-MAX and NFW-MAX).

Finally, we have analysed the data of the Fermi Large Area Telescope. We didn’t find any gamma-ray signal from these objects, indeed we can exclude some region of the DM mass and annihilation cross section parameter space. The exclusion limit for the DM mass and annihilation cross-section are presented in Fig. 3. The blue line corresponds to the results of this work for the combined analysis of all the 7 targets with a MED model, i.e. considered as extended sources. These results are compared with the thermally averaged annihilation cross section (dotted black line), the result of the previous proof-of-concepts paper (yellow dashed line), the results of 100 simulation with a null signal from DM (yellow band) and the exclusion limit from well known dwarf spheroidal galaxy obtained with Fermi-LAT (green dashed line). The results of our study are dominated by the constraints obtained by IC10 and NGC6822, and dimly depend on the considered DM profile.