Impact of dark matter-baryon relative velocity on the 21-cm forest

We study the effect of the relative velocity between the dark matter (DM) and the baryon on the 21-cm forest signals. The DM-baryon relative velocity arises due to their different evolutions before the baryon-photon decoupling epoch, and it gives an additional anisotropic pressure that can suppress the perturbation growth. It is intriguing that the scale $k\sim \mathcal{O}(10\sim {10}^3)h/\mathrm{Mpc}$ at which the matter power spectrum is affected by such a streaming velocity turns out to be the scale which the 21-cm forest signal is sensitive to. We demonstrate that the 21-cm absorption line abundance can decrease by more than a factor of a few due to the small-scale matter power spectrum suppression caused by the DM-baryon relative velocity.

Figure 1

Matter power spectrum at $z=40$, 10 without (solid line) and with (dashed line) the effect of relative velocity.

Figure 1

Matter power spectrum at $z=40$, 10 without (solid line) and with (dashed line) the effect of relative velocity.

Figure 2

Halo mass abundance at $z=10$ without (solid line) and with (dashed line) the effect of relative velocity.

Figure 3

Schematic picture of a minihalo.

Figure 4

Optical depth as a function of $\alpha /r_{\mathrm{vir}}$ for a given minihalo mass.

Figure 5

Maximum impact parameter $r_{\mathrm{tau}}$ as a function of minihalo mass for a given optical depth.

Figure 6

Abundance of the 21-cm absorption lines at $z=10$ without relative velocity (solid line) and with relative velocity (dashed line).

Figure 7

Abundance of the 21-cm absorption lines at different redshifts.

Figure 8

Abundance of the 21-cm absorption lines for different IGM temperatures. We show the number of 21-cm absorption lines for no relative velocity ($T_{\mathrm{IGM}}=2\mathrm{K}$) (solid line), including relative velocity ($T_{\mathrm{IGM}}=2\mathrm{K}$) (dashed-dot line), no relative velocity ($T_{\mathrm{IGM}}=T_{\mathrm{CMB}}$) (dotted line) and including relative velocity ($T_{\mathrm{IGM}}=T_{\mathrm{CMB}}$) (dashed line).

Figure 9

Abundance of the 21 -cm absorption lines for different x-ray heating efficiency $f_X$. To evaluate the pure impact of $f_X$ on the 21-cm absorption lines, we do not include the effect of relative velocity for different $f_X$ cases. For comparison, we also show the abundance of the 21-cm absorption lines with and without relative velocity for $T_{\mathrm{IGM}}=2\mathrm{K}$.