Seminars and Colloquia at ESO Garching and on the campus

Hydra I cluster hosts a large population of Ultra Diffuse Galaxies, which

are defined by their low central surface brightness(μg,0 ≥ 24 mag/arcsec2)

and large effective radii (Re ≥ 1.5kpc). The combination of photometry from

OmegaCAM@VST and VIRCAM@VISTA with integral-field spectroscopy from

MUSE@VLT allowed a detailed study of Globular Cluster (GCs) in the Hydra

I UDGs. In this presentation I will outline the photometric, morphometric and spec-

troscopic criteria for selection of GCs, the analysis of their physical properties

and use to trace the dark matter content and structure of UDGs. We found

that Hydra I UDGs can be divided into two groups: one that is GC-poor, and

the other containing brigth and potentially numerous GCs. Exploiting the es-

tablished relation between GC number and host halo mass we derived the dark

matter content of the UDGs. An independent probe of the dark matter was ob-

tained from the deep H-band observations. The GC counts and stellar masses

indicate that these low-surface brightness galaxies are dark matter dominated

with dynamical to stellar mass ratios of ∼10-1000.

Axions and other Weakly Interacting Sub-eV Particles (WISPs) are compelling candidates for physics beyond the Standard Model, offering solutions to the strong-CP problem and viable dark matter components. Astrophysical X-ray observations provide a powerful and complementary approach to searching for these particles through their conversion into photons in large-scale magnetic fields.Using data from NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR), we pursue multiple strategies for axion detection. We search for axion–photon conversion in the Sun’s atmospheric magnetic field during the 2020 solar minimum, and for axion production via the nuclear M1 transition of 57Fe in the cores of nearby supergiant stars such as Betelgeuse. In addition, NuSTAR observations of Betelgeuse, following the work of Gianotti et al., probe axion production through the Primakoff process and axion–electron interactions, providing sensitivity to axion–photon, axion–nucleon, and axion–electron couplings. These studies yield the most stringent astrophysical constraints to date, improving previous solar and stellar limits by up to 1–1.5 orders of magnitude. Looking ahead, wide-field X-ray surveys such as eROSITA, with improved effective area and a complementary energy range, offer strong potential to further extend astrophysical axion searches across a broad range of masses and couplings. Together with laboratory experiments such as CAST and the forthcoming IAXO, these results underscore the growing role of X-ray astronomy in probing axions, WISPs, and the dark sector.

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