Cosmic-ray (CR) antihelium is an important observable for dark matter (DM) indirect searches due to extremely low secondary backgrounds towards low energies. In a given DM model, the predicted CR antihelium flux is expected to be strongly correlated with that of CR antiprotons.In this work, we use the AMS-02p/p data to constrain the DM annihilation cross section, and the ALICE data on the 3 He and T productions to determine the parameters in the coalescence model for anti-nucleus formation. The hadronic cross sections are estimated using Monte-Carlo event generators including EPOS-LHC and DPMJET. Based on these constraints, we make predictions for the maximal antihelium flux for typical DM annihilation final states, and perform a detailed analysis on the uncertainties due to the DM density profiles and CR propagation models. We find that the results are highly insensitive to both of them, but still significantly depend on the hadronization models in event generators. The prospects of detecting antihelium for the AMS-02 experiment is discussed. We show that with very optimistic assumptions, CR 3 He is within the reach of the AMS-02 experiment. The 3 He events which can be detected by AMS-02 are likely to have kinetic energy T 30 GeV, which is consistent with the preliminary AMS-02 search results. The events which can be observed by AMS-02 are likely to arise dominantly from secondary backgrounds rather than DM interactions. * dingyucheng@itp.ac.cn

The galactic 511 keV gamma-ray line has been observed since 1970's, and was identified as the result of electron-positron annihilation, but the origin of such positrons is still not clear. Apart from the astrophysical explanations, the possibilities that such positrons come from dark matter (DM) annihilation are also widely studied. Primordial black hole (PBH) is also an extensively studied candidate of DM. If PBHs exist, the DM particles may be gravitationally bound to the PBHs and form halo around PBHs with density spikes. DM annihilation in these density spikes can enhance the production rate of positrons from DM particles, but the signal morphology is similar to the decaying DM. We consider such a mixed model consisting of DM particles and PBHs and obtain the upper limit from the data of 511 keV gamma-ray line from INTEGRAL/SPI on the decaying component of DM particles and the constraint on the PBH abundance. These constraints are general and independent of particle DM models. For the mixed model consisting of excited DM and PBHs, the constraints on the PBH abundance can be down to O(10-17) for DM particle with mass around 1, which is more stringent than that obtained from the extragalactic gamma-ray background.

We report the discovery of ancient massive merger events in the early-type galaxies NGC 1380 and NGC 1427, members of the Fornax galaxy cluster. Both galaxies have been observed by the MUSE integral-field-unit instrument on the VLT as part of the Fornax3D project. By fitting recently developed population-orbital superposition models to the observed surface brightness, stellar kinematic, age, and metallicity maps, we obtain the stellar orbits, age, and metallicity distributions of each galaxy. We then decompose each galaxy into multiple orbital-based components, including a dynamically hot inner stellar halo component that is identified as the relic of past massive mergers. By comparing to analogs from cosmological galaxy simulations, chiefly TNG50, we find that the formation of such a hot inner stellar halo requires the merger with a now-destroyed massive satellite galaxy of $ 3.7_{-1.5}^{+2.7} \times 10^{10} $ M⊙ (about one-fifth of its current stellar mass) in the case of NGC 1380 and of $ 1.5_{-0.7}^{+1.6} \times10^{10} $ M⊙ (about one-fourth of its current stellar mass) in the case of NGC 1427. Moreover, we infer that the last massive merger in NGC 1380 happened ∼10 Gyr ago based on the stellar age distribution of the regrown dynamically cold disk, whereas the merger in NGC 1427 ended t ≲ 8 Gyr ago based on the stellar populations in its hot inner stellar halo. The major merger event in NGC 1380 is the first one with both merger mass and merger time quantitatively inferred in a galaxy beyond the local volume. Moreover, it is the oldest and most massive merger uncovered in nearby galaxies so far.