Probing the Cosmic Baryon Distribution and the Impact of Active Galactic Nuclei Feedback with Fast Radio Bursts in CROCODILE Simulation

Using CROCODILE hydrodynamic simulations with GADGET3/4-OSAKA, this study investigates the cosmic baryon distribution and the impact of AGN feedback by generating FRB light cones to constrain the diffuse baryon mass fraction up to z=1 and quantify how AGN-driven gas ejection reshapes the circumgalactic and intergalactic media.

The Gist

Here is an explanation of the paper, translated into everyday language with creative analogies.

The Cosmic "Missing" Puzzle

Imagine the universe is a giant, invisible ocean. We know exactly how much water (matter) should be in this ocean based on the Big Bang theory. But when we look around with our telescopes, we can only find about half of it. The rest is the "Missing Baryon Problem."

Where did the rest of the water go? Scientists suspect it's hiding in the vast, hot, invisible spaces between galaxies, called the Intergalactic Medium (IGM). But because this gas is so hot and thin, it's like trying to find a specific drop of water in a steam cloud.

The Cosmic "Flashlight": Fast Radio Bursts (FRBs)

Enter Fast Radio Bursts (FRBs). Think of these as cosmic lighthouses or flashlights that flash for a split second from billions of light-years away.

As the light from these flashes travels through the universe to reach us, it passes through that invisible "steam cloud" of gas. The gas slows down the radio waves slightly. The more gas the light hits, the more it gets delayed. Scientists call this delay the Dispersion Measure (DM).

By measuring how much the signal is delayed, we can count how much gas the light passed through. It's like walking through a crowded room: if you get bumped a lot, you know the room was crowded. If you get bumped a little, the room was empty.

The Simulation: Building a Digital Universe

The authors of this paper didn't just look at the real sky; they built a digital universe inside a supercomputer. They used a code called GADGET3/4-OSAKA (think of it as a very advanced video game engine for physics).

They created a virtual universe filled with:

• Dark Matter Halos: Invisible scaffolding that holds galaxies together.
• Galaxies: Where stars are born.
• Feedback: The "traffic control" of the universe.

The Villain and the Hero: AGN Feedback

The paper focuses heavily on AGN Feedback.

• The Analogy: Imagine a galaxy is a house. The center of the house has a super-massive black hole (the AGN). Sometimes, this black hole gets hungry, eats gas, and then burps out massive jets of energy.
• The Effect: These "burps" (feedback) act like a powerful leaf blower. They blow the gas out of the center of the galaxy and push it into the surrounding neighborhood (the IGM).

The researchers ran two versions of their simulation:

1. The "Fiducial" Model: The black holes have their leaf blowers turned on (AGN feedback is active).
2. The "NoBH" Model: The black holes are asleep (no feedback).

The Result: In the model with the leaf blowers, the center of the galaxies was much emptier, and the surrounding neighborhood was slightly denser. The feedback successfully moved the "missing water" from the galaxy centers out into the open space.

The "Forensic" Work: Tracing the Path

The team created Light Cones. Imagine taking a long, thin slice of the universe from the present day all the way back to the beginning. They sent thousands of virtual FRB "flashlights" through this slice.

They found that:

• The "Missing" Baryons are Found: By $z=1$ (a time when the universe was about half its current age), they found that about 86% of the missing baryons were accounted for in the diffuse gas between galaxies and around them.
• The "Halos" are Tricky: Some of the gas is stuck in the "halos" (the atmosphere) of galaxies. If you don't account for this, you might think the gas is in the open space when it's actually stuck to a galaxy. The paper shows that AGN feedback helps move this gas out, making it easier to find.

The Host Galaxy: Where the Flashlight Starts

The paper also looked at where the FRBs actually come from (the "Host Galaxy").

• Dwarf Galaxies: Small, quiet galaxies. If an FRB happens here, the gas delay is tiny (like walking through a quiet library).
• Spiral Galaxies (like the Milky Way): Medium-sized, busy. The delay is moderate (like walking through a busy mall).
• Galaxy Clusters: Massive groups of thousands of galaxies. If an FRB happens here, the delay is huge (like walking through a mosh pit).

Key Insight: If an FRB comes from a massive galaxy cluster, the gas inside that cluster contributes so much to the delay that it can drown out the signal from the rest of the universe. It's like trying to hear a whisper from across the ocean while standing next to a jet engine.

The Big Takeaway

This paper is a success story for digital detective work.

1. We found the missing water: Using simulations and FRBs, we confirmed that most of the missing matter is indeed in the hot gas between galaxies.
2. Black holes are movers: Supermassive black holes act as cosmic gardeners, blowing gas out of galaxies and spreading it into the universe.
3. We need better maps: To get the exact numbers right, we need to know exactly which galaxies the FRB signals passed through, so we can subtract their contribution and see the "pure" universe.

In short, the universe isn't missing its ingredients; they are just hiding in the steam between the stars, and Fast Radio Bursts are the perfect tool to find them.

Technical Summary

Here is a detailed technical summary of the paper "Probing the cosmic baryon distribution and the impact of AGN feedback with FRBs in CROCODILE simulation."

1. Problem Statement

The paper addresses the "missing baryon problem," where a significant fraction (estimated at 30–50%) of the universe's baryons remains unaccounted for in observations of the local universe. Current theories suggest these baryons reside in the Warm-Hot Intergalactic Medium (WHIM) ($10^5 - 10^7$ K) or within the Circumgalactic Medium (CGM) of dark matter halos.

While Fast Radio Bursts (FRBs) offer a promising tool to trace these baryons via their Dispersion Measure (DM) (the integrated column density of free electrons), a major challenge remains: disentangling the DM contributions from the smooth Intergalactic Medium (IGM) versus the clumpy foreground halos (CGM) and host galaxies. Furthermore, the role of Active Galactic Nucleus (AGN) feedback in redistributing baryons between the CGM and IGM, and how this affects DM measurements, requires precise theoretical modeling.

2. Methodology

The authors utilize the CROCODILE simulation suite, employing the GADGET3/4-OSAKA smoothed particle hydrodynamics (SPH) code. The study combines two distinct simulation approaches:

• Large-Scale Structure (LSS) Simulations:
  • Setup: Multiple box sizes ranging from $25 h^{-1}$Mpc to$500 h^{-1}$ Mpc.
  • Physics: Includes star formation, supernova (SN) feedback, and thermal AGN feedback.
  • Comparisons: The authors compare a fiducial model (with AGN feedback) against a NoBH model (without black holes/AGN feedback) to isolate the impact of AGN.
  • Light Cone Generation: Randomly rotated simulation boxes are stitched together to create continuous light cones from $z=0$ to $z \approx 1$, generating 10,000 sight lines to compute DM profiles and electron density distributions.
• Zoom-in Cosmological Hydrodynamic Simulations:
  • Targets: High-resolution simulations of three distinct environments: a Dwarf Galaxy ($M_* \sim 10^7 M_\odot$), a Milky Way-like Spiral Galaxy ($M_* \sim 10^{11} M_\odot$), and a Galaxy Cluster ($M_{200} \sim 10^{15} M_\odot$).
  • Purpose: To quantify the DM contribution from host galaxies ($DM_{HG}$) and satellite galaxies, analyzing how source location (central vs. off-center) affects the total DM.

3. Key Contributions

1. Quantification of AGN Feedback on Baryon Distribution: The study explicitly measures how AGN feedback reshapes the boundary between the CGM and IGM, specifically focusing on how it expels gas from halo centers.
2. Decoupling $f_{diff}$ and $f_{IGM}$: The authors clarify the distinction between the diffuse baryon fraction ($f_{diff} = f_{IGM} + f_{Halos}$), which is observationally constrained by FRBs, and the true IGM fraction ($f_{IGM}$), which excludes halo gas. They provide a semi-analytic framework to reconstruct $f_{IGM}$ from observational $f_{diff}$ constraints.
3. Halo Profile Modeling: The paper validates the use of modified NFW (mNFW) profiles over standard NFW profiles to better capture the effects of baryonic feedback on dark matter and gas density profiles.
4. Host Galaxy DM Characterization: By simulating specific galaxy types, the paper provides a detailed breakdown of $DM_{HG}$ contributions, showing how they vary by orders of magnitude depending on the host environment (dwarf vs. cluster).

4. Key Results

A. AGN Feedback and Baryon Redistribution

• Central Density Suppression: AGN feedback significantly reduces central gas densities in halos. The effect is most pronounced in halos with masses between $10^{12.5} - 10^{13.5} M_\odot$.
  • In this mass range, AGN feedback reduces the central gas density (at 15 kpc) by 86.1% and suppresses the central DM contribution by 73% compared to the NoBH model.
• Redistribution: While AGN feedback expels gas from the centers, much of it remains within the virial radius ($R_{200}$), effectively moving baryons from the inner CGM to the outer CGM/IGM interface.
• Impact on DM: The fiducial model (with AGN) shows a slightly higher average DM at $z=1$ compared to the NoBH model because expelled gas increases the electron density in the diffuse IGM, despite the reduction in central halo densities.

B. Constraints on Baryon Fractions ($f_{diff}$ and $f_{IGM}$)

• Diffuse Fraction ($f_{diff}$): Using the DM–redshift ($z$) relation up to $z=1$, the study constrains the diffuse baryon mass fraction:
  • Fiducial Model: $f_{diff} = 0.865^{+0.101}_{-0.165}$
  • NoBH Model: $f_{diff} = 0.856^{+0.101}_{-0.162}$
  • These values align with recent observational constraints (e.g., Connor et al. 2024) and serve as upper limits for the true IGM fraction.
• True IGM Fraction ($f_{IGM}$): When halo gas (CGM) is subtracted using various cutoff radii ($R_{200}$, $2R_{200}$, FoF), the true$f_{IGM}$ is lower.
  • At $z=1$, $f_{IGM}$ ranges from 0.56 (using $3R_{200}$cutoff) to **0.87** (using$R_{200}$ cutoff).
  • The study suggests that current FRB observations likely probe a mix of IGM and CGM, with the effective boundary lying between $R_{200}$ and $2R_{200}$.

C. Host Galaxy DM Contributions

The zoom-in simulations reveal a massive variance in host DM contributions ($DM_{HG}$):

• Dwarf Galaxies: Central DM is low, typically < 10 pc cm$^{-3}$ (median ~4.1 pc cm$^{-3}$). Off-center sources contribute negligibly.
• Milky Way-like Galaxies: Central DM is significant, with a median of ~163 pc cm$^{-3}$ (range 83–813 pc cm$^{-3}$). This is comparable to the Galactic ISM contribution.
• Galaxy Clusters: The DM contribution is dominant.
  • Central: Median DM $\approx$ 1670 pc cm$^{-3}$.
  • Satellite: Median DM $\approx$ 1473 pc cm$^{-3}$.
  • In cluster environments, the host contribution can exceed the total DM from the IGM and all other components combined.

D. Redshift Evolution

• The paper provides fitting models (C-Exp, CPL-Exp, DPL-Exp) for the redshift evolution of $f_{IGM}$ and $f_{diff}$.
• $f_{diff}$ increases with redshift more steeply than intrinsic $f_{IGM}$ due to the increasing probability of FRB sight lines intersecting massive foreground halos at higher $z$.

5. Significance

• Resolving the Missing Baryons: The study confirms that a large fraction of baryons resides in the diffuse medium ($f_{diff} \approx 0.86$ at $z=1$), supporting the WHIM scenario.
• AGN Feedback Validation: It provides quantitative evidence that AGN feedback is the primary driver of baryon redistribution in halos of mass $10^{12.5} - 10^{13.5} M_\odot$, effectively flattening density profiles and altering the CGM-IGM boundary.
• Observational Strategy: The results highlight the critical need to map foreground galaxies to subtract their CGM contributions when using FRBs to constrain cosmological parameters. Without this, $f_{IGM}$ is systematically overestimated.
• Modeling Framework: The semi-analytic framework developed to reconstruct intrinsic $f_{IGM}$ from cumulative $f_{diff}$ observations offers a robust tool for future FRB cosmology, bridging the gap between raw observational data and theoretical $\Lambda$CDM predictions.

In conclusion, this paper establishes that while FRBs are powerful probes of cosmic baryons, accurate interpretation requires sophisticated modeling of AGN feedback and precise subtraction of foreground halo contributions, particularly in the $10^{12.5} - 10^{13.5} M_\odot$ mass range where feedback is most efficient.