Simulating Atomic Dark Matter in Milky Way Analogs

Abstract

Dark sector theories naturally lead to multicomponent scenarios for dark matter where a subcomponent can dissipate energy through self-interactions, allowing it to efficiently cool inside galaxies. We present the first cosmological hydrodynamical simulations of Milky Way analogs where the majority of dark matter is collisionless cold dark matter (CDM) but a subcomponent (6%) is strongly dissipative minimal atomic dark matter (ADM). The simulations, implemented in GIZMO and utilizing FIRE-2 galaxy formation physics to model the standard baryonic sector, demonstrate that the addition of even a small fraction of dissipative dark matter can significantly impact galactic evolution despite being consistent with current cosmological constraints. We show that ADM gas with roughly standard model–like masses and couplings can cool to form a rotating “dark disk” with angular momentum closely aligned with the visible stellar disk. The morphology of the disk depends sensitively on the parameters of the ADM model, which affect the cooling rates in the dark sector. The majority of the ADM gas gravitationally collapses into dark “clumps” (regions of black hole or mirror star formation), which form a prominent bulge and a rotating thick disk in the central galaxy. These clumps form early and quickly sink to the inner ∼kiloparsec of the galaxy, affecting the galaxy’s star formation history and present-day baryonic and CDM distributions.

Electromagnetic Signatures of Mirror Stars

“…Furthermore, mirror stellar relics are expected to give rise to distinctive gravitationalwave signals from merger events, including mirror white dwarfs (Ryan & Radice 2022), mirror neutron stars (Hippert et al 2022(Hippert et al , 2023, and black holes with unusual masses sourced by ADM collapse (Pollack et al 2015;Shandera et al 2018;Singh et al 2021;Gurian et al 2022b;Fernandez et al 2024). simulated in Roy et al (2023) We now briefly summarize the most important points of the analyzes by Curtin & Setford (2020a, 2020b, which demonstrated the existence of observable optical and X-ray signatures of mirror stars. 7 If ADM or something like it exists, then it is highly likely that the massless dark photon kinetically mixes with the SM photon.…”

“…ADM is not only self-interacting: It can form dark atomic bound states and is dissipative, meaning it can cool by emitting dark photons and collapse to form structure. While ADM would therefore leave its imprint in early-Universe cosmology (Cyr-Racine et al 2014;Bansal et al 2022Bansal et al , 2023Zu et al 2023) and galactic dynamics (Fan et al 2013a;Ghalsasi & McQuinn 2018;Gemmell et al 2023;Roy et al 2023), 4 one of its most spectacular predictions is the existence of mirror stars.…”

Dark black holes in the mass gap

“…Here, t dyn = 1 √ 8πGρm is the approximate timescale associated with doubling of the halo mass through mergers and accretion. Different processes are efficient at cooling the JCAP01(2024)064 halos at different times and the exact fraction of halos that cool has to be understood through a merger tree [46] or a hydrodynamical simulation [63,64]. Merger tree or hydrodynamical simulations will be necessary to model the distribution of black holes which is an important first step in understanding the merger rates of the black holes.…”

The effect of multiple cooling channels on the formation of dark compact objects

“…For instance, atomic dark matter [31], in which the dark sector is comprised of a dark proton, a dark electron, and a dark photon, is a model that permits both atomic and molecular processes -resulting in a plethora of relevant interactions one must account for when considering the formation histories of the associated astrophysical structures. Recent studies have investigated the complicated structure formation from atomic dark matter by either studying the range of possible simulation results by interpolating between a range of assumptions regarding density evolution [28,[32][33][34] or using numerical hydrodynamics simulation suites [35,36] to study these objects [29].…”

“…A dark sector able to form dark compact objects would fundamentally alter our understanding of the distribution of small-scale astrophysical objects. Several different types of dark structures and formation mechanisms have already been proposed; for example, primordial black holes [12,13], axion stars [14,15], and dissipative dark sectors [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. The last case arises from the natural possibility that DM is not just one particle, but rather a sector comprised of a variety of dark particles, analogous to the photons, electrons, and baryons of the Standard Model.…”