Constraining the Baryon Content of Cosmic Filaments Using Localized Fast Radio Bursts and DESI Imaging Data

TL;DR

This paper investigates the baryon content of cosmic filaments by leveraging localized FRBs and DESI-imaged filamentary structures identified with DisPerSE. By splitting FRBs into sightlines that intersect filaments ('Pass') versus those that do not ('NoPass'), the authors detect a statistically significant divergence in the DM_I GM–z relation between the two groups, suggesting excess baryons in filaments. They model the filament gas with a $\beta$-model to infer a central overdensity $\delta_0 \approx 21$ and estimate the baryon fraction in filaments to be $\Omega_{b,\mathrm{fila}} \approx 0.10$–$0.17\,\Omega_b$ for $z<1$, with values decreasing toward higher redshift; these are likely lower limits due to incomplete filament catalogs at $z>0.5$. The work demonstrates an independent FRB-based method to map baryons in the cosmic web and highlights the need for larger FRB samples and deeper galaxy surveys to refine filamentary baryon inventories and address the missing baryon problem.

Abstract

Cosmic filaments are thought to host a substantial fraction of the missing baryons at redshifts $z<2$. In this study, we constraint the baryonic content of these filaments using localized Fast Radio Bursts (FRBs). Filaments are identified from the galaxy distribution in the Dark Energy Spectroscopic Instrument (DESI) imaging surveys using the DisPerSE algorithm. We find tentative evidence ($\sim 3 σ$ significance) for a divergence in the relationship between the dispersion measure (DM) contributed by the intergalactic medium and redshift for FRBs whose signals intersect cosmic filaments compared to those that do not, suggesting excess baryons in the filamentary structures. Assuming an isothermal $β$-model gas profile with $β=2/3$, this discrepancy is best explained by a central baryon overdensity of $δ_0 = 21^{+13}_{-12}$, broadly consistent with previous simulation and observational results. The inferred baryon fraction residing in filaments decreases with redshift, from approximately $0.25$-$0.30\,Ω_b$ at $z=0.02$ to $0.15$-$0.30\,Ω_b$ at $z=0.5$, and $0.03$-$0.04\,Ω_b$ at $z=0.8$. These estimates are likely lower bounds, particularly at $z>0.5$, due to the limited number of identified filaments and localized FRBs at higher redshifts. We also examine various factors that may affect the statistical significance of our results. Our method offers an independent approach to tracing baryons in cosmic filaments and underscores the importance of expanding localized FRB samples and deepening galaxy surveys, i.e., key steps toward refining these estimates and addressing the missing baryon problem.

Figures (5)

• Figure 1: Top: The sky area (blue region) in the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys and positions of localized FRBs (red pentagrams). Middle: Filaments and galaxies at $z = 0$–0.05 across the northern sky. Red lines show filament skeletons identified by DisPerSE with nsig=3 and nsig=5 thresholds; green shading indicates galaxy density. Bottom: red lines show filament counts for nsig=3 (dashed) and nsig=5 (solid); blue line shows galaxy counts per redshift bin.
• Figure 2: Upper: Example of an FRB intersecting a filament in 2D. The solid green line marks the filament spine, and 'd' indicates its angular width. Bottom: 3D illustration of an FRB-filament intersection. The black dashed line shows the filament's skeleton, the cyan cylinder marks its region with radius $\mathrm{R_{fila}}$, the blue line is the sightlines to FRB, and the red segment indicates the intersecting path.
• Figure 3: Top: Distributions of $\mathrm{DM_{obs}}$ (left) and $\mathrm{DM_{obs}} - \mathrm{DM_{MW,ISM}}$ (right) and redshifts for 37 well-localized FRBs in the DESI area, divided into ‘Pass’ (circles) and ‘NoPass’ (yellow triangles) groups. Circle color and size indicate the number of intersected filaments. Bottom Left: $\mathrm{DM_{IGM}}$ vs. redshift for the same FRBs. Dashed lines show best-fit linear trends for each group; error bars represent uncertainties of $\sim60\,\mathrm{pc\,cm^{-3}}$. Bottom Right: Same as left, but with filament contributions subtracted: $\mathrm{DM_{IGM,Pass\text{-}corr}} = \mathrm{DM_{IGM,Pass}} - \mathrm{DM_{fila}}$ (open circles), using central gas overdensity in filaments $\delta_0 = 20.78$. The black dashed line shows the corresponding linear fit.
• Figure 4: Estimated baryon mass fraction in filaments across redshift bins. Blue and orange lines show results from this work using 37 securelylocalized FRBs within DESI area and DisPerSE-identified filaments (galaxy catalog from DESI imaging surveys) with persistence thresholds nsig=3 and nsig=5, respectively. The green line and red point represent estimates from simulations by 2017ApJ...838...21Z and 2018MNRAS.473.1195L.
• Figure 5: Similar to Figure \ref{['fig:M9 S5 Phost1 res']}, the left panel presents results based on all 46 localized FRBs within the DESI sky area, using filaments identified by DisPerSE with a persistence threshold of nsig=5. In contrast, the right panel shows results for the 37 securely localized FRBs, with filaments extracted using a lower threshold of nsig=3.