July 2021
In 1937 Fritz Zwicky measured the velocities of galaxies in the Coma cluster of galaxies and deduced that the total mass of this cluster is considerably higher than the combined mass of all the stars in all the galaxies that can be detected with telescopes [1]. Ever since his publication, the evidence has grown that about 85% of the total mass in the universe is invisible and does not radiate light or other electromagnetic waves.
Despite intense studies, the nature of this dark matter (DM) remains a mystery and very different suggestions have been explored. One possible explanation assumes that DM may be composed of elementary particles different from those explained in the Standard Model of particle physics (i.e. particles, which have been detected or produced in experiments on Earth). Since DM interacts mostly gravitationally, it is not easy to detect directly with observatories. Theoretical models suggest that hypothetical, so-called Weakly Interactive Massive Particles (WIMPs), which might have been produced in the early Universe, are promising candidates. WIMPs might be detectable if they annihilate in collisions and produce gamma-rays in the process. Such gamma-rays could eventually be detected by telescopes like the Fermi Large Area Telescope (LAT) or H.E.S.S.
Searches for DM signals have been conducted in the Galactic Center (GC) region [2], [3] and nearby dwarf galaxies [4-7] (H.E.S.S. SOM 8/2019 or H.E.S.S. SOM 2/2018). Cosmological N-body simulations suggest that other compelling and complementary targets are self-gravitating DM clumps populating the Galactic Halo [8], which are predominantly composed of DM and thus radiate gamma-rays. However, they would be dark (invisible) when observed with radio-, optical- or X-ray telescopes.
In its all-sky survey, the satellite-borne gamma-ray instrument Fermi-LAT has indeed detected sources that cannot be identified in other wavebands [9]. These objects referred to as ‘Unidentified Fermi Objects’ (UFOs) may hence indeed be observable for gamma-ray telescopes only, radiating gamma-rays as a result of DM annihilation. A smoking gun signature for DM detection is a very distinct cut-off in the gamma-ray energy spectrum close to the DM particle mass. For a high DM particle mass, i.e. above a few hundred GeV, the energy cut-off would be too high in energy to be measurable by the LAT. The combination of Fermi-LAT and H.E.S.S. observations is therefore mandatory for searching DM dominated objects among the unidentified sources detected by the Fermi-LAT.
Four of the most promising UFOs, 3FHL J0929.2-4110, 3FHL J1915.2-1323, 3FHL J2030.2-5037 and 3FHL J2104.5+2117, have been observed by H.E.S.S. A total amount of 49 hours of data was gathered between 2018 and 2019. The observed sources do not show any changes in their brightness over the 12 years of Fermi-LAT observations, do not have an obvious conventional counterpart at other wavelengths, and are located outside of the Galactic plane. The results of this study have now been published by the HESS collaboration [10].
The expected flux of gamma rays from the annihilation of WIMPs depends on the amount of DM in the source and its distance to the observer on Earth, as well as the interaction cross-section between two DM particles. The amount of DM present in the source is expressed by the so-called J-factor. No significant signal is detected from H.E.S.S., neither in the data from the individual observations of the UFOs nor in the data obtained by their combination. When no signal is detected, we can estimate upper limits on the DM properties, i.e., on the product between the J-factor and the cross-section of the DM particles. In Figure 2, these upper limits are shown. The computation of the limits assumes the annihilation of DM particles into W + W – particles, one of several possible end-products of DM annihilation. The upper limits represent the sensitivity of our telescopes to these types of annihilations. Every value above the lines is excluded; if such an annihilation signal from DM exists in these objects, we would have detected it with the HESS telescopes.
Since no significant excess is found in the combined dataset, upper limits from the latter are computed as well. In Figure 3 we show these upper limits, considering DM models for the W + W – annihilation channel. The green line shows the limits computed with the H.E.S.S. observations while the colour bands represent the strength of the signal detected by Fermi-LAT as a function of the J-factor on the y-axis and the mass of the DM particle (mDM) on the x-axis (assuming that the gamma-ray emission detected with Fermi-LAT results from DM annihilation). The H.E.S.S. upper limits considerably restrict the allowed range of J-factors as a function of the DM mass mDM. Assuming a model of the DM distribution within the Galaxy, the distribution of the J-factors can be derived. From these predictions, we could further constrain the allowed range of J-factors. However, simulations are subject to large uncertainties which weaken the constraints derived from them. Since H.E.S.S. constraints are derived from observations only and do not depend on assumed parameters in simulations, they are more robust for the interpretation of the UFOs as Galactic substructures of annihilating DM.
References
[1] Zwicky, Fritz, 1937. On the Masses of Nebulae and of Clusters of Nebulae. Astrophys. J. Vol. 86, 217–246. DOI = 10.1086/143864
[2] Abdalla, H. et al. 2016, Search for Dark Matter Annihilations towards the Inner Galactic Halo from 10 Years of Observations with H.E.S.S., Phys. Rev. Lett., 117, 111301
[3] Abdalla, H. et al. 2018, Search for γ-ray line signals from dark matter annihilations in the inner Galactic halo from ten years of observations with H.E.S.S, Phys.Rev.Lett. 120 (2018) 20, 201101
[4] Abdalla, H. et al. 2018, Searches for gamma-ray lines and ‘pure WIMP’ spectra from Dark Matter annihilations in dwarf galaxies with H.E.S.S, JCAP, 11, 037
[5] Abdalla, H. et al. 2008, Observations of the Sagittarius dwarf galaxy by the HESS experiment and search for a dark matter signal, Astropart. Phys., Vol. 29, 55-62, DOI = 10.1016/j.astropartphys.2007.11.007
[6] Abramowski, A. et al., 2010, H.E.S.S. constraints on Dark Matter annihilations towards the Sculptor and Carina Dwarf Galaxies, Astropart. Phys., DOI = 10.1016/j.astropartphys.2010.12.006
[7] Abdalla, H. et al. 2020, Search for dark matter signals towards a selection of recently-detected DES dwarf galaxy satellites of the Milky Way with H.E.S.S., Phys.Rev.D 102 (2020) 6, 062001
[8] Kamionkowski, M., Koushiappas, S. M., & Kuhlen, M. 2010, Galactic substructure and dark-matter annihilation in the Milky Way halo, Phys. Rev. D, 81, 043532
[9] Abdollahi, S. et al. 2020, Fermi Large Area Telescope Fourth Source Catalog, Astrophys. J. Suppl., 247, 33
[10] Abdalla, H. et al., 2021, Search for dark matter annihilation signals from unidentified Fermi-LAT objects with H.E.S.S, accepted in The Astrophysical Journal (2021)
