The birth of the intracluster medium: the evolution of multiphase gas and Lyman-$α$ haloes in a simulated $z\sim3$ protocluster

Abstract

Galactic haloes host a complex, multiphase circumgalactic medium (CGM), and at high redshift are fed by cold, filamentary inflows. In contrast, mature galaxy clusters are dominated by a hot, enriched, X-ray emitting intracluster medium (ICM), with cold gas largely confined to member galaxies. However, the transition between these regimes remains poorly constrained. We present a cosmological zoom-in simulation of a massive cluster progenitor evolved to $z=2.7$, with enhanced CGM resolution to better trace the accretion, mergers and feedback events that precede the birth of the ICM. We connect this evolution to mock MgII and OVII absorption, tracing low and high ionisation gas phases. We also study Lyman-$α$ (Ly$α$) and Balmer-$α$ (H$α$) haloes in emission, using radiative transfer in post-processing. Between $z\sim4.4$ and $2.7$, a major merger and AGN feedback drive an inside-out transformation, redistributing gas to larger radii and flattening density, temperature and metallicity profiles. Intermediate column MgII absorbers are rapidly destroyed, leaving a clumpier cold gas distribution associated with satellites, while gas is ionised beyond OVII as the inner halo enters the X-ray regime. An extended Ly$α$ halo remains detectable even without AGN photoionisation, and evolves from filamentary to more spherical as inflowing gas is disrupted. Our fiducial model underpredicts observed central Ly$α$ emission - we likely require more efficient Ly$α$ production in the nuclear region, either through more effective escape of stellar Ly$α$ photons or through enhanced conversion of AGN-powered ionisation into Ly$α$ emission. H$α$ haloes are dimmer and smaller than Ly$α$, but with JWST may provide a complementary probe of the evolving CGM at this critical epoch.

Figures (13)

• Figure 1: Maps of gas number density (left column), temperature (centre column) and metallicity (right column), at four redshifts $z = 4.4, 3.7, 3.3,$ and $2.7$ (from top to bottom). All maps show mass-weighted average quantities. The density maps are zoomed in further to highlight the transition from a filamentary to clumpy cool gas structure in the CGM as the halo grows. Dashed circles in all panels denote $R_{200}$.
• Figure 2: Volume-weighted average profiles of gas density (top panel), temperature (middle panel) and metallicity (bottom panel) for the protocluster from $z=8$ (purple) to $z=2.7$ (yellow). Coloured markers at the top and bottom of each panel show the position of $R_{200}$ at each time. As the halo grows we can see how the density profile flattens, the hot halo expands, and the forming ICM is enriched to a near-uniform metallicity.
• Figure 3: Radial profiles of gas fractions in different phases for the protocluster from $z=8$ (purple) to $z=2.7$ (yellow). The panels show the fractions for, from top to bottom, gas with temperature $T<10^{5.5}\,\mathrm{K}$ (including all star-forming gas), $10^{5.5}\,\mathrm{K}<T<10^7\,\mathrm{K}$, $10^{7}\,\mathrm{K}<T<10^{7.5}\,\mathrm{K}$, and $T>10^{7.5}\,\mathrm{K}$. Coloured markers at the top and bottom of each panel show the position of $R_{200}$ at each time. The gaseous halo heats up from the inside out as it grows, with the increasing temperature affecting the ionisation state of the nascent ICM.
• Figure 4: Evolution of the covering fraction of Mg ii, for column densities $N_{\mathrm{Mg\,II}} > 10^{6}$ cm$^{-2}$ (top panel), and for columns $N_{\mathrm{Mg\,II}} > 10^{11}$ cm$^{-2}$ (bottom panel) as a function of radius. Coloured markers at the top and bottom of each panel show the position of $R_{200}$ at each time. The covering fraction of Mg ii drops with time as the halo grows, with the $50-250\,$kpc range particularly affected for high column densities as intermediate density gas is destroyed.
• Figure 5: Evolution of the distribution of Mg ii column densities within 500 kpc of the forming cluster's centre. Intermediate columns of Mg ii are gradually destroyed, while the highest columns remain unchanged, leading to a clumpier distribution dominated by satellite galaxies.