MEGATRON: the impact of non-equilibrium effects and local radiation fields on the circumgalactic medium at cosmic noon

TL;DR

MEGATRON presents three cosmological radiation-hydrodynamic zoom-ins of a Milky Way–mass progenitor at cosmic noon, combining on-the-fly radiative transfer with a comprehensive non-equilibrium chemistry network to follow $81$ species and resolve CGM gas down to $l_{ m min} \approx 20\,\mathrm{pc}$ (average $\sim200\,\mathrm{pc}$). Compared with PIE+UVB assumptions, the full non-equilibrium treatment with local radiation fields causes substantial shifts in ionization structure, recombination lags, and covering fractions (HI DLAs can differ by up to $\sim 40\%$), while a cooling-length refinement reveals many more, smaller cold clumps ($M \sim 10^4\,M_\odot$) and sharper ion gradients that modify emission and absorption signatures of tracers such as CIV and OIII. These results demonstrate that coupling non-equilibrium thermochemistry to radiative transfer, along with physically motivated resolution criteria, is necessary to predict CGM observables and to guide observational strategies with current and future facilities. The work highlights the importance of local, anisotropic radiation and high-resolution thermochemistry for interpreting CGM absorption and emission in a cosmological context.

Abstract

We present three cosmological radiation-hydrodynamic zoom simulations of the progenitor of a Milky Way-mass galaxy from the MEGATRON suite. The simulations combine on-the-fly radiative transfer with a detailed non-equilibrium thermochemical network (81 ions and molecules), resolving the cold and warm gas in the circumgalactic medium (CGM) on spatial scales down to 20 pc and on average 200 pc at cosmic noon. Comparing our full non-equilibrium calculation with local radiation to traditional post-processed photoionization equilibrium (PIE) models assuming a uniform UV background (UVB), we find that non-equilibrium physics and local radiation fields fundamentally impact the thermochemistry of the CGM. Recombination lags and local radiation anisotropy shift ions away from their PIE+UVB values and modify covering fractions (for example, HI damped Ly$α$ absorbers differ by up to 40%). In addition, a resolution study with cooling-length refinement allows us to double the resolution in the cold and warm CGM gas, reaching 120 pc on average. When refining on cooling length, the mass of the lightest cold clumps decreases tenfold to $\approx 10^4\,M_\odot$, their boundary layers develop sharper ion stratification, and the warm gas is better resolved, boosting the abundance of warm gas tracers such as CIV and OIII. Together, these results demonstrate that non-equilibrium thermochemistry coupled to radiative transfer, combined with physically motivated resolution criteria, is essential to predict circumgalactic absorption and emission signatures and to guide the design of targeted observations with existing and upcoming facilities.

Figures (23)

• Figure 1: We illustrate here the main features of our simulation at $z=4$. 1. Ratio between the local H i-ionizing flux (tracked on the fly) compared to the UV background. The local radiation dominates in a cone above and below the galactic disk. 2. The radiation field is coupled live to the non-equilibrium thermochemistry of $\geq 80$ species, allowing us to model self-consistently the state of CGM in absorption (e.g., H$_2$, CO and O i to O iii column densities), and emission (shown here, [O i]$\lambda6300Å$, [O ii]$\lambda3728Å$, [O iii]$\lambda1666Å$ and [O iii]$\lambda5007Å$ emission maps). 3. Our fiducial run reaches $\sim150pc$ in the cold and warm phases ($T<e5K$) of the CGM. Further refining on the cooling length over 150Myr, we reach $\sim100pc$ in the cold and warm phases and improve the definition of accretion flows and cold clumps 4. We include tracer particles to follow the Lagrangian evolution of the baryons and understand inflows and outflows in the CGM. We show the past trajectory of gas that formed three populations of stars in the main galaxy.
• Figure 2: Column densities of all the species tracked in the simulations at $z=3$. The colormap spans e18e22cm^-2 for primordial species (incl. H$_2$), and e12e16cm^-2 for metals and CO. The spatial scale is 100kpc on a side (see bottom right).
• Figure 3: Dark matter (left column) and gas density (right column) maps at $z=4$ in our three simulations. We 'genetically' modify the initial conditions so that the same initial region collapses faster or slower. From left to right: the earlier-forming, fiducial, and later-forming scenarios. In the earlier-forming scenario, a single halo of mass $\sim e11\Msun$ is already dominating the region by $z=3.6$, while its formation is delayed in the other two simulations. By $z=0$, each of the three regions assembles into a Milky Way-mass halo.
• Figure 4: Mass assembly histories of the main halo in the three sets of initial conditions compared to the typical mass growth histories of similar mass haloes from IllustrisTNG-300. The dark and light shaded regions represent the $1\sigma$ and $2\sigma$ results from IllustrisTNG-300, with the black line representing the median relation. We also include a run with increased resolution for 150Myr using a cooling length criterion, starting at $z=4$.
• Figure 5: Mass-weighted mean spatial resolution for gas at different temperatures in our fiducial simulation (earlier forming) at $z=3.7$ (top row) and when additionally refining on the cooling length (bottom row). See the Appendix (\ref{['fig:resolution_plot']}) for a map of the resolution around the main galaxy at the same redshift. Due to the Jeans length refinement, the fiducial run already reaches mean resolutions of the order of 100150pc in the cold and cool CGM ($T<e5K$). With additional cooling length refinement, the mean resolution is on average better than 130pc or better for all gas except hot gas ($T>e6K$) in the CGM. Focusing on the intergalactic medium (IGM), the cooling length refinement also improves the resolution up to radii of $\sim 3-4 R_\mathrm{vir}$.