Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

Axions are a potential dark matter candidate, which may condense and form self-gravitating compact objects, called axion stars (ASs). In this work, we study for the first time head-on collisions of relativistic ASs with black holes (BHs) and neutron stars (NSs). In the case of BH-AS mergers we find that, in general, the largest scalar clouds are produced by mergers of low compactness ASs and spinning BHs. Although in most of the cases which we study the majority of the mass is absorbed by the BH within a short time after the merger, in favorable cases the remaining cloud surrounding the final BH remnant can be as large as 30% of the initial axion star mass, with a bosonic cloud mass of O ( 10 − 1 ) M BH and peak energy density comparable to that obtained in a superradiant buildup. This provides a dynamical mechanism for the formation of long lived scalar hair, which could lead to observable signals in cases where the axion interacts with baryonic matter around the BH, or where it forms the seed of a future superradiant buildup in highly spinning cases. Considering NS-AS collisions we find two possible final states: (i) a BH surrounded by a (small) scalar cloud, or (ii) a stable NS enveloped in an axion cloud of roughly the same mass as the initial AS. While for low mass ASs the NS is only mildly perturbed by the collision, a larger mass AS gives rise to a massive ejection of baryonic mass from the system, purely due to gravitational effects. Therefore, even in the absence of a direct axion coupling to baryonic matter, NS-AS collisions could give rise to electromagnetic observables in addition to their gravitational wave signatures.

Abstract:

Intragalaxy signals contain a wealth of information on fundamental physics, both the dark sector and the nature of gravity. While so far largely unexplored, such probes are set to rise dramatically in importance as upcoming surveys provide data of unprecedented quantity and quality on galaxy structure and dynamics. In this paper, we use warping of stellar disks to test the chameleon- or symmetron-screened fifth forces which generically arise when new fields couple to matter. We take r -band images of mostly late-type galaxies from the Nasa Sloan Atlas and develop an automated algorithm to quantify the degree of U-shaped warping they exhibit. We then forward model the warp signal as a function of fifth-force strength, ΔG/GN, and range, λC, and the gravitational environments and internal properties of the galaxies, including full propagation of the non-Gaussian uncertainties. Convolving this fifth-force likelihood function with a Gaussian describing astrophysical and observational noise and then constraining ΔG/GN and λC by Markov chain Monte Carlo, we find the overall likelihood to be significantly increased (Δlog(L)≃20) by adding a screened fifth force with λC≃2 Mpc and ΔG/GN≃0.01. The variation of Δlog(L) with λC is quantitatively as expected from the correlation of the magnitude of the fifth-force field with the force’s range, and a similar model without screening achieves no increase in likelihood over the General Relativistic case ΔG=0. Although these results are in good agreement with a previous analysis of the same model using offsets between galaxies’ stellar and gas mass centroids [H. Desmond et al., Phys. Rev. D 98, 064015 (2018).], we caution that the effects of confounding baryonic and dark matter physics must be thoroughly investigated for the results of the inference to be unambiguous.