August 2019
A wealth of astrophysical evidences suggest that dark matter pervades at all scales in the Universe. The latest cosmological measurements by the Planck satellite show that dark matter is cold and non-baryonic, amounting to about 84% of the total matter content of the Universe. However, its fundamental nature remains elusive. A leading class of elementary particle candidates consists of weakly interacting massive particles (WIMPs). A worldwide experimental effort has been deployed in order to unveil the nature of dark matter. The hitherto absence of experimental hints at colliders and in direct and indirect searches for WIMPs is now pushing their mass scale to the TeV range, which makes searches with ground-based Imaging Atmospheric Cherenkov Telescopes (IACTs) crucial. The self-annihilation of TeV WIMPs would produce very-high-energy (VHE, E ≥ 100 GeV) in the final state that could be eventually detected by IACTs such as H.E.S.S.
WIMPs are expected to self-annihilate in dense regions of the sky. Due to their large dark matter content and proximity to Earth, dwarf galaxy (dSph) satellites of the Milky Way are promising environments to search for a dark matter signal. Dwarf satellite galaxies orbiting the Milky Way are small celestial objects, with fewer than 1000 stars, in contrast to the Milky Way containing billions of stars. The observational absence of conventional astrophysical VHE emitters in these objects would make the detection of dark matter annihilation signals unambiguous. Indeed, these systems harbour old populations of stars and contain little amount of gas, possibly acting as target materials for VHE cosmic rays, with low diffuse Galactic gamma-ray foregrounds expected for many of them. Therefore, dwarf Milky Way satellites are key targets for IACT observations to explore the properties of dark matter.
The Sloan Digital Sky Survey (SDSS) provided a deep and systematic coverage of the northern celestial hemisphere leading to the discovery of many Milky Way dSph satellites [1]. Among them a new population of “ultra-faint” satellites has been unveiled [2,3,4] which were found to be among the most dark-matter-dominated objects known at that time. Many new ultra-faint Milky Way satellites are being discovered by current deep wide-field optical imaging surveys such as PanSTARRS and the Dark Energy Survey (DES). DES [5] is a currently-running Southern-hemisphere survey which enabled the detection of more than twenty ultra-faint systems [6-8]. Figure 1 presents the 17 ultra-faint Milky Way satellites detected in the first two years of data collected by the DES. The photometric characteristics of the new Milky Way satellites are consistent with being dSphs. They are usually referred to as dSph candidates until the confirmation from spectroscopic measurements.
The H.E.S.S. observatory is pursuing an observation programme towards nearby dSphs since more than a decade. Searches for VHE gamma-ray emission with H.E.S.S. have been performed on classical and ultra-faint dSphs yielding no hint for significant emissions in any of the observed dSphs [9-11]. Given its location in the Southern celestial hemisphere, the H.E.S.S. instrument is ideally located to observe under favourable conditions the ultra-faint systems recently discovered by DES. An observation programme with the H.E.S.S. array including the five telescopes has been carried out in 2017 and 2018 towards a selection of the most promising DES dSphs and candidate dSphs for dark matter searches, resulting in about 80 hours of observation in total. The targeted systems were Reticulum II, Tucana II, Tucana III, Tucana IV and Grus II. The former two objects are confirmed as dSphs, while the latter three are labelled as dSph candidates in absence of sufficiently accurate spectroscopic measurements so far. The data analysis does not show any significant VHE gamma-ray excess above background in any of the objects as well as anywhere in their field of view. Using a maximum log-likelihood ratio test statistics exploiting both the spatial and spectral features of the dark matter signal with respect to the background, constraints on the velocity-weighted annihilation cross section <σ v> versus the dark matter mass have been derived. Figure 2 shows the observed upper limits at 95% confidence level (C.L.) towards Reticulum II. The mean expected limits obtained from 100 Poisson realisations of the background together with the 1 and 2σ expected containment bands are also shown. The observed upper limit at 95% C.L. on <σ v> reaches about 4×10-24 cm3 s-1 for a dark matter mass of 1.5 TeV annihilating into the W+W– channel. No significant excess has been detected towards any of the selected DES dSphs and 95% C.L. upper limits on <σ v> have been derived for each object.
A combined analysis of the five datasets has been performed. No overall significant excess has been detected. Figure 3 shows the observed limits from the combination of the five data sets from H.E.S.S. observations together with a summary of the existing constraints towards dSphs. The new H.E.S.S. results well complement those obtained by Fermi-LAT and are the most constraining in the several TeV mass range towards Milky Way dwarf satellites. The constraints obtained by H.E.S.S. are particularly relevant in the context of TeV dark matter models with enhanced spectral features such as prominent lines near or at the dark matter mass. Future searches with H.E.S.S. towards the most promising ultra-faint Milky Way satellites will strongly profit from high-quality kinematic stellar data sets in order to improve the knowledge about the dark matter distribution in these objects.
Reference: H. Abdallah et al. (H.E.S.S. Collaboration), “Search for dark matter signals towards the recently-detected DES dwarf satellites of the Milky Way with H.E.S.S.”, in preparation. Currently undergoing internal collaboration review prior to submission.
References:
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[5] http://www.darkenergysurvey.org/
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