A census of baryons in the Universe from localized fast radio bursts (2005.13161v1)

Abstract: More than three quarters of the baryonic content of the Universe resides in a highly diffuse state that is difficult to observe, with only a small fraction directly observed in galaxies and galaxy clusters. Censuses of the nearby Universe have used absorption line spectroscopy to observe these invisible baryons, but these measurements rely on large and uncertain corrections and are insensitive to the majority of the volume, and likely mass. Specifically, quasar spectroscopy is sensitive either to only the very trace amounts of Hydrogen that exists in the atomic state, or highly ionized and enriched gas in denser regions near galaxies. Sunyaev-Zel'dovich analyses provide evidence of some of the gas in filamentary structures and studies of X-ray emission are most sensitive to gas near galaxy clusters. Here we report the direct measurement of the baryon content of the Universe using the dispersion of a sample of localized fast radio bursts (FRBs), thus utilizing an effect that measures the electron column density along each sight line and accounts for every ionised baryon. We augment the sample of published arcsecond-localized FRBs with a further four new localizations to host galaxies which have measured redshifts of 0.291, 0.118, 0.378 and 0.522, completing a sample sufficiently large to account for dispersion variations along the line of sight and in the host galaxy environment to derive a cosmic baryon density of $\Omega_{b} = 0.051_{-0.025}^{+0.021} \, h_{70}^{-1}$ (95% confidence). This independent measurement is consistent with Cosmic Microwave Background and Big Bang Nucleosynthesis values.

• The paper introduces a novel method using FRB dispersion measures paired with precise redshifts to directly estimate cosmic baryon density.
• It employs ASKAP's sub-arcsecond localization to identify four FRB host galaxies, allowing for a detailed breakdown of dispersion contributions from various cosmic structures.
• The findings confirm that the missing baryons predominantly reside in the ionized intergalactic medium, aligning with CMB and BBN cosmological models.

A Census of Baryons in the Universe from Localized Fast Radio Bursts

This paper presents an analysis of the baryon content of the universe by utilizing the dispersion measures of fast radio bursts (FRBs). The research addresses a longstanding issue in observational cosmology: the so-called "missing baryon problem," wherein a substantial fraction of baryonic matter is not accounted for in observable structures like stars and galaxies. It explores the use of FRBs as a tool to directly measure these elusive baryons dispersed in the intergalactic medium (IGM).

Methodology and Observations

The paper involves the precise localization of FRBs to their host galaxies using the Australian Square Kilometre Array Pathfinder (ASKAP), allowing for the measurement of their redshifts. The ASKAP's ability to localize FRBs with sub-arcsecond precision is crucial for associating them with their host galaxies and determining the corresponding redshifts. This pursuit extends the sample of known localized FRBs by adding four new localizations at redshifts of 0.291, 0.118, 0.378, and 0.522.

Analysis and Results

Through measurements of the electron column density along each line of sight towards the FRBs, the paper calculates a cosmic baryon density of Ωb = 0.051{-0.025}^{+0.021} h_{70}^{-1} (95% confidence level), which aligns well with the values derived from the Cosmic Microwave Background (CMB) and Big Bang Nucleosynthesis (BBN) observations. This independent measurement provides a robust cross-verification of earlier findings from different methodologies and suggests that the missing baryons are predominantly in the ionized IGM.

The research models the dispersion measures as being comprised of four main contributions: from the Milky Way's interstellar medium (ISM), the Galactic halo, the FRB host galaxy, and the IGM itself. The model emphasizes the use of the DM-redshift relation for directly measuring diffuse cosmic baryons, circumventing some systematic biases inherent in previous estimation techniques that rely heavily on assumptions or indirect observations.

Implications and Future Work

The findings confirm that the baryons missing from traditional astronomical surveys are indeed present in the IGM, providing a resolution to the missing baryon problem. This has significant implications for our understanding of cosmic structure formation and the distribution of matter in the universe. Additionally, with modest expansions in the FRB sample size, FRBs can serve as effective cosmological probes, offering insights into cosmic expansion history, baryon distribution specifics, and feedback mechanisms in galaxy formation.

This method paves the way for refining cosmological models and improving parameters associated with cosmic matter distributions. As more FRBs are localized with precise redshifts and dispersion measures, the constraints on Ω_b and the dynamics of baryonic matter across cosmic scales are expected to become even sharper.

Conclusion

The paper shows that FRBs can be employed as valuable cosmological tools to measure cosmic baryons. The methodology developed here for using FRB dispersion measures provides a promising avenue for addressing complex questions about the large-scale structure of the universe and the evolution of baryonic matter. This work underlines the integral role of high-resolution radio astronomy in cosmology, potentially steering future research into the utilization of transient celestial phenomena as cosmological probes.