Effects of dark matter-baryon scattering on redshifted 21 cm signals

We demonstrate that elastic scattering between dark matter (DM) and baryons can affect the thermal evolution of the intergalactic medium at early epochs and discuss the observational consequences. We show that, due to the interaction between DM and baryons, the baryon temperature is cooled after decoupling from the CMB temperature. We illustrate our findings by calculating the 21 cm power spectrum in coexistence with a velocity-dependent DM elastic scattering cross section. For instance, for a DM mass of 10 GeV, the 21 cm brightness temperature angular power spectrum can be suppressed by a factor of 2 within the currently allowed DM-baryon cross section bounded by the CMB and large-scale structure data. This scale-independent suppression of the angular power spectrum can be even larger for a smaller DM mass with a common cross section (for instance, as large as a factor of 10 for $m_d\sim 1\mathrm{GeV}$), and such an effect would be of great interest for probing the nature of DM in view of forthcoming cosmological surveys.

Figure 1

The baryon and dark matter temperature evolution for different values of DM-baryon coupling (the DM-baryon elastic scattering cross section is parametrized as $\sigma ={\sigma }_0{v}^{-4}$, with ${\sigma }_0={\sigma }_{17}{m}_{\mathrm{H}}10^{-17}{\mathrm{cm}}^2/\mathrm{g}$). We set $m_d=m_{\mathrm{H}}$ in the left panel and $m_d=10m_{\mathrm{H}}$ in the right panel. The solid and dotted lines represent the baryon and dark matter temperatures, respectively. The CMB temperature is plotted as the dashed line. The magenta, red, green and blue lines are for ${\sigma }_{17}=0.01$, 0.1, 1.0 and 10, respectively. The black solid line shows the baryon temperature evolution without DM-baryon coupling (${\sigma }_{17}=0$).

Figure 2

The spin temperature evolution for different values of DM-baryon coupling. The solid, dotted and dashed lines represent the spin, baryon and CMB temperatures, respectively. We set $m_d=m_{\mathrm{H}}$ in the left panel and $m_d=10m_{\mathrm{H}}$ in the right panel. The magenta, red, green and blue lines are for ${\sigma }_{17}=0.01$, 0.1, 1.0 and 10, respectively. The temperatures evolution without DM-baryon coupling (${\sigma }_{17}=0$) is plotted in black.

Figure 3

The evolution of the angular power spectrum $C_{\ell }$ for different values of DM-baryon coupling. We set $m_d=m_{\mathrm{H}}$ and $10m_{\mathrm{H}}$ in the left and right panels, respectively. In both left and right panels, the redshifts are set to $z=40$, 30 and 20 from the top to bottom, respectively.

Figure 4

The evolution of the ionization fraction for different values of DM-baryon coupling.