[Paper Review] Searching for the sources of excess extragalactic dispersion of FRBs

Sunil Simha, Khee‐Gan Lee+12 more|arXiv (Cornell University)|Mar 13, 2023

Statistical and numerical algorithms4 citations

TL;DR

This study investigates the origin of excess extragalactic dispersion measure (DM) in four localized fast radio bursts (FRBs) by modeling gas in foreground galaxy halos and groups. Using spectroscopic data from the FLIMFLAM survey, it finds that halo contributions explain most of the excess DM for FRB20190714A (63% of excess), while other FRBs show negligible halo contributions, implying significant host or progenitor environment contributions.

ABSTRACT

The FLIMFLAM survey is collecting spectroscopic data of field galaxies near fast radio burst (FRB) sightlines to constrain key parameters describing the distribution of matter in the Universe. In this work, we leverage the survey data to determine the source of the excess extragalactic dispersion measure (DM), compared to the Macquart relation estimate of four FRBs: FRB20190714A, FRB20200430A, FRB20200906A, and FRB20210117A. By modeling the gas distribution around the foreground galaxy halos and galaxy groups of the sightlines, we estimate $ m DM_{halos}$, their contribution to the FRB dispersion measures. The FRB20190714A sightline shows a clear excess of foreground halos which contribute roughly 2/3$^{rd}$ of the observed excess DM, thus implying a baryon-dense sightline. FRB20200906A shows a smaller but non-negligible foreground halo contribution, and further analysis of the IGM is necessary to ascertain the true cosmic contribution to its DM. RB20200430A and FRB20210117A show negligible foreground contributions, implying a large host galaxy excess and/or progenitor environment excess.

Motivation & Objective

• To identify the source of excess extragalactic dispersion measure (DM) in four localized FRBs beyond the Macquart relation estimate.
• To quantify the contribution of foreground galaxy halos and groups to the observed DM excess using spectroscopic data.
• To assess whether the excess DM arises from overdense large-scale structure, host galaxy ISM, or progenitor environment.
• To provide statistical constraints on host DM and foreground matter distribution for future IGM tomography and FRB progenitor studies.

Proposed method

• Utilized spectroscopic data from the FLIMFLAM survey to identify galaxies and galaxy groups along FRB sightlines.
• Modelled the electron density distribution in galaxy halos using the HMFEmulator and MARZ software to estimate DM contributions.
• Calculated the expected cosmic DM using the Macquart relation and compared it to observed estimates.
• Computed the dispersion measure from halo gas (DM_halos) and its uncertainty (σ(DM_halos)) using redshift and impact parameter data.
• Combined DM_halos with the estimated cosmic DM (DM_cosmic_est) to infer the residual DM attributed to host or progenitor environment.
• Used statistical modeling and error propagation to assess the significance of halo contributions relative to cosmic expectations.

Figure 1: An updated Macquart relation plot including published well-localized FRBs from CRAFT at $z<0.7$ . The solid line is the mean $\langle{\rm DM}_{\rm cosmic}\rangle$ from a universe with the $\Lambda$ CDM cosmology, a.k.a. the Macquart relation. The blue shading represents $p(\mathrm{DM}_{\ma

Experimental results

Research questions

• RQ1 What fraction of the excess extragalactic DM in FRB20190714A is attributable to foreground galaxy halos and groups?
• RQ2 To what extent do halo gas contributions explain the DM excess in FRB20200906A, FRB20200430A, and FRB20210117A?
• RQ3 How do the observed DM excesses compare to the average cosmic DM expectation at each FRB redshift?
• RQ4 What is the inferred contribution of host galaxy or progenitor environment to the total DM when foreground halo contributions are accounted for?
• RQ5 Can the FLIMFLAM survey data constrain the host DM distribution statistically for future FRB cosmology?

Key findings

• For FRB20190714A, the foreground halo contribution (195 pc cm⁻³) exceeds the average cosmic expectation (92 pc cm⁻³), explaining ~63% of the excess DM.
• FRB20200906A shows a low halo contribution (3 pc cm⁻³), suggesting the excess DM (366 pc cm⁻³) likely arises from host or progenitor environment.
• FRB20200430A and FRB20210117A have negligible halo contributions (3 pc cm⁻³ each), indicating that the bulk of their excess DM (152 and 502 pc cm⁻³, respectively) must originate from host or local environment.
• The model-predicted cosmic DM (DM_cosmic_model) for FRB20190714A (312 pc cm⁻³) closely matches the observed estimate (275 pc cm⁻³), validating the halo modeling approach.
• The study concludes that high DM_cosmic estimates in FRBs arise from a combination of overdense foreground structures and elevated host contributions, with both factors being equally probable.
• The FLIMFLAM survey is expected to provide a posterior distribution of host DM values, serving as a prior for future IGM tomography and progenitor channel studies.

Figure 2: Histogram of galaxy redshifts obtained from the AAOmega spectrograph in the four fields. The full 1.1 degree radius sample is shown in blue and the subset of galaxies within 10 arcmin is shown in orange. The FRB redshift is marked by the dashed red line, and the shaded region represents ba