Robust velocity-induced acoustic oscillations at cosmic dawn

Abstract

The redshifted 21-cm line of hydrogen holds great potential for the study of cosmology, as it can probe otherwise unobservable cosmic epochs. In particular, measurements of the 21-cm power spectrum during cosmic dawn-the era when the first stars were formed-will provide us with a wealth of information about the astrophysics of stellar formation, as well as the origin of fluctuations in our Universe. In addition to their usually considered density fluctuations, dark matter and baryons possess large relative velocities with respect to each other, due to the baryon acoustic oscillations (BAOs) suffered by the latter, which suppress the formation of stars during cosmic dawn, leaving an imprint on 21-cm observables during this era. Here we present 21cmvFAST, a version of the publicly available code 21cmFAST modified to account for this effect. Previous work has shown that the inclusion of relative velocities produces an acoustic modulation on the large-scale 21-cm power spectrum during cosmic dawn. By comparing analytic calculations with simulations from 21cmvFAST, here we demonstrate that this modulation takes the form of robust velocity-induced acoustic oscillations (VAOs), during both the Lyman-α coupling era and the subsequent epoch of heating. The unique shape of these VAOs, which is determined by the acoustic physics that generated the relative velocities, can be analytically computed and is easily distinguishable from the usual density-sourced fluctuations. We find that, under a broad range of astrophysical assumptions, the VAOs are detectable at high significance by the upcoming HERA interferometer, which could therefore confirm the presence of acoustic oscillations at cosmic dawn.

Probing the small-scale matter power spectrum with large-scale 21-cm data

“…Given our ignorance about the wedge, we will show results under three foreground assumptions, mirroring those of Ref. [47] (based on Refs. [121,122]), with values of a = {0, 0.05, 0.1} h Mpc −1 for optimistic, moderate, and pessimistic foregrounds, respectively; and b given by the horizon limit in all cases except optimistic, where it is smaller by a factor of sin(θ b /2), where θ b is the beam full-width half maximum.…”

“…The main quantity that will determine the progress of 21-cm evolution in the universe is the collapsed fraction F coll of baryons onto star-forming haloes (with mass M > M cool ), defined as [47,48,52] where f g is the fraction of gas collapsed into haloes of mass M , ρ b is the cosmic baryonic density, and…”

“…[53] (for reference, it corresponds to the case of regular feedback strength in Ref. [47]). Given this flux, and including the effect of DM-baryon relative velocities, by z = 25 (when we realistically can start observing the 21-cm line), M mol = 2 × 10 6 M (an order of magnitude larger than the starting molecular-cooling haloes), and LW feedback is complete by z ≈ 15 (i.e., M cool = M atom ).…”

“…This is because the power at these scales is dominated by the VAOs, which have opposite effects during the LCE and EoH, as large velocities produce fewer stars, reducing the 21-cm absorption during the LCE but increasing it during the EoH (for more details see Ref. [47]). As before, large-k matter fluctuations only affect the signal at high redshifts, and these derivatives change sign whenever the fiducial ∆ 2 21 (k21) has a local extremum.…”

Physical Cosmology from the 21 cm Line

“…For example, if the cooling is triggered by scattering between the baryons and a fraction of the dark matter that has a modest charge, the scattering rate will be modulated by the relative streaming velocity of dark matter and baryons. Such a model therefore leaves distinct features in the 21-cm power spectrum that trace the velocity structure [52,51]. The detailed implications of most of these exotic models are mostly unexplored, but any model in which (1) the energy exchange depends on local density, velocity, or temperature; or (2) in which background radio energy deposition is inhomogeneous, should generically leave signatures in the power spectrum.…”