Post-inflationary axion constraints from the Lyman-$α$ forest

Abstract

Among the most compelling cold dark matter candidates, the axion has recently been subject to a wide range of astrophysical studies aiming to constraints its properties. We present updated bounds on the isocurvature fraction, $f_{\rm{iso}}$, which parameterizes the contribution of isocurvature perturbations induced by post-inflationary produced axion-like particles (ALPs) to the ordinary power spectrum. We use new simulations based on the Sherwood-Relics suite to fit high-resolution Lyman-$α$ forest flux power spectrum data. With the published noise model of the Lyman-$α$ forest data, we find a tentative detection of $f_{\rm{iso}}$ = ${0.0064^{+0.0012}_{-0.0014}}$ (68% C.L), after accounting for the degenerate effect of IGM thermal evolution. With a more conservative modelling of the residual noise in the data, the upper bound is weakened to $f_{\rm{iso}}< 0.0084$ (95% C.L), which translates into an ALP temperature-independent mass $m_a > 1.73 \times 10^{-18}$eV. Our constraints are stronger than bounds derived from large-scale structure probes at higher and lower redshifts and are competitive with those derived from UV luminosity function data. Interestingly, the best current Lyman-$α$ forest data prefers a non-zero contribution from isocurvature modes.

Figures (4)

• Figure 1: Constraints on the linear matter power spectrum at $z$=0. Bounds from below the CDM (black) line are inferred from WDM (irsic23) and CWDM cosmologies (gg25) in red and blue solid lines, respectively. For the latter, we further show in varying blue line styles the models that delineate the 2$\sigma$ allowed region, corresponding to the blue shaded area (see Fig.2 in gg25). Similarly, we show in green the bounds from this work above the CDM line. The solid green line corresponds to the fraction of isocurvature fluctuations allowed with a conservative noise treatment at 95% C.L, and corresponding allowed region of models also in shaded green.
• Figure 2: 1D posterior distribution for $f_{\rm{iso}}$ for our default analysis ($\tau_{\rm{CMB}}$ priors) in blue. We also show the analysis where the last three $k-$bins are excluded in the dashed orange line. The dotted red line shows the analysis where a noise term in the flux power spectrum prediction per redshift bin is included, following irsic23. For the analysis shown in dash-dot green, we include the contribution from Si-III, following ma25.
• Figure 3: Best-fit plot at each redshift bin. The top left panel shows the Default analysis in different colored lines. Both the top left and right panels show in dashed black lines the theory model with the same parameters as the Default analysis except for $f_{\rm{iso}}$=0. The top right panel shows the best-fit from the CDM analysis with the same color coding for each redshift. The contours for this analysis in the $u_0^{5.0}-T_0^{5.0}$ and $u_0^{5.0}-f_{\rm iso}$ planes are shown in Fig. \ref{['u0t0']} of the Supplemental Material. The bottom panels shows the percentage residual of the data over each model for Default (left) and CDM (right). We have shifted the redshift points $z$=4.6 (orange) and $z$=5.0 (green) along the x-axis on the bottom two panels for visual clarity.
• Figure S1: 2D posterior in ${u_0}-{T_0}$ (left) and ${u_0}-{f_{\rm iso}}$ plane (right) at $z$= 5.0, for the Default analysis (with $\tau_{\rm{CMB}}$ priors + $R_{s}$) in blue, with patchy and thermal-dependent resolution correction in orange, and for $f_{\rm iso} =0$ (CDM) in red. Constraints for the same parameters from villasenor22 in red, rogers21 in green, boera19 in purple and irsic23 in brown are also included.