The CGM with local universe FRBs: evidence of strong AGN feedback in a massive elliptical galaxy

Modern cosmology and galaxy formation rely on an understanding of how cosmic baryons are distributed, a significant portion of which exist in the diffuse gas confined to halos. Fast Radio Bursts (FRBs) are a promising probe of the Universe’s ionized gas. At low redshift, the contribution to the dispersion measure (DM) from the intergalactic medium (IGM) and intervening halos is subdominant, allowing us to study the circumgalactic media (CGM) of the host galaxies. We select a sample of five local universe FRBs whose host interstellar medium (ISM) DM is negligible and use these to constrain the mass of the CGM in each halo. We find that one of our sources, the only massive elliptical host galaxy, has been evacuated of its baryons ($M_\mathrm{gas}=0.02^{+0.02}{-0.02}M\mathrm{h}$, corresponding to $\sim$10$%$ of the cosmological average $\frac{Ω_b}{Ω_m}$). This galaxy shows evidence of a past episode of AGN activity, consistent with the picture of strong AGN feedback in galaxy group-scale halos. The other sources are consistent with existing multiwavelength data and tentatively support more baryon retention in $L_*$ galaxies compared to group-scale halos. We show that FRBs can measure the halo gas fraction $f_\mathrm{gas}$ in halos of mass $M_\mathrm{h}\sim10^{11-13}M_\odot$, and up to $\sim10^{14}M_\odot$ if galaxy cluster hosts are included, which is a larger range than other gas probes can access. Finally, we demonstrate that a large sample of local universe FRBs, such as those expected from upcoming all-sky radio telescopes, will enable precision measurements of halo gas, which is crucial for understanding the effects of feedback.

💡 Research Summary

This paper presents a novel method for measuring the gas content of galaxy halos by exploiting the dispersion measures (DM) of fast radio bursts (FRBs) in the local universe (z ≲ 0.2). At these low redshifts the contribution from the intergalactic medium (IGM) and intervening halos to the total DM is modest, allowing the host galaxy’s circum‑galactic medium (CGM) to be isolated. The authors first decompose the observed DM into five components: Milky Way interstellar medium (MW‑ISM), Milky Way CGM (MW‑CGM), cosmic IGM+intervening halos, host ISM, and host CGM. Their goal is to extract the host CGM term, which directly traces the electron column of the halo gas.

To minimize the host‑ISM contribution, the authors assemble a sample of 67 well‑localized FRBs with spectroscopic redshifts below 0.2, then apply a series of empirical cuts based on host galaxy inclination, scattering timescale (τ_exc), rotation measure (RM), and impact parameter (b⊥). These observables correlate with the amount of ISM a burst traverses; low τ_exc, low RM, large b⊥, and face‑on orientations suggest negligible host‑ISM DM. After this selection, five FRBs remain, four hosted by L*‑mass spirals or irregulars and one by a massive elliptical galaxy (halo mass ≈10^13 M⊙).

The probability density functions (PDFs) for each DM component are constructed using state‑of‑the‑art models. Cosmic DM is modeled with a modified normal distribution featuring a power‑law tail, calibrated on the IllustrisTNG simulation (Konietzka et al. 2025). Milky Way ISM is estimated by combining three independent approaches: the NE2001 and YMW16 electron‑density models, HI4PI neutral‑hydrogen column densities, and WHAM Hα emission measures, with a hard lower limit set by the nearest pulsar DM. Milky Way CGM follows the Yamasaki & Totani (2020) model with a 0.2 dex log‑normal scatter. Convolution of these PDFs yields a posterior for the host CGM DM after subtracting the other components.

The host CGM DM is converted to a gas mass by assuming an electron temperature of 8000 K and relating DM to emission measure (EM). For the massive elliptical, the inferred gas fraction is f_gas = 0.02^{+0.02}_{-0.02}, i.e., only ~2 % of the halo mass, corresponding to ~10 % of the universal baryon fraction Ω_b/Ω_m. This extreme depletion is interpreted as the result of strong AGN feedback that expelled most of the halo gas. The four L* galaxies show higher gas fractions (f_gas ≈ 0.2–0.4), consistent with recent simulations that incorporate strong feedback mechanisms. The results therefore support a picture where feedback efficiency rises with halo mass, leading to reduced baryon retention in group‑scale halos.

Beyond the specific measurements, the study demonstrates that FRBs can probe halo gas fractions across a wide mass range (M_h ≈ 10^{11}–10^{14} M⊙), surpassing the reach of X‑ray, thermal Sunyaev‑Zel’dovich (tSZ), and kinetic SZ (kSZ) techniques, which are limited to higher‑mass systems or require assumptions about temperature and metallicity. The authors argue that upcoming all‑sky radio facilities such as CHIME/FRB, DSA‑2000, and the SKA will deliver thousands of low‑z FRBs, enabling statistical uncertainties on f_gas to shrink dramatically. This will allow precise tests of galaxy‑formation models, especially the role of AGN‑driven outflows in shaping the baryon budget of halos.

The paper acknowledges current limitations: the small sample size, uncertainties in Milky Way CGM/ISM models, and the assumption that DM components are independent. Future work should combine FRB data with multi‑wavelength observations (X‑ray, UV absorption, optical spectroscopy) and improve simulations of the IGM/CGM to refine the cosmic DM PDF. Nonetheless, the work establishes FRBs as a powerful, complementary probe of the diffuse baryons that dominate the mass budget of galaxies and groups, with significant implications for cosmology, dark‑matter clustering, and the interpretation of weak‑lensing surveys.