The dependence of Circumgalactic Medium properties on halo assembly histories in the IllustrisTNG simulations

Abstract

While halo mass is the dominant factor shaping the embedded galaxies, the properties of the circumgalactic medium (CGM) also depend on halo assembly history. To investigate this, we calculate the formation times for TNG50 halos with masses between $10^{10.5}$ and $10^{12.5} M_\odot$, classifying them into `early-' and `late-forming' populations. It is found that across all mass bins, early-formed halos generally host galaxies with higher stellar mass and higher metallicity, with lower CGM gas mass and lower specific star formation rate (sSFR) at $z\sim0$. For the CGM metallicity, `early' halos with masses below $10^{12}\mathrm{M_\odot}$ show systematically higher gas phase metallicities, whereas in the $10^{12-12.5}\mathrm{M_\odot}$ bin the trend reverses. When examining the origins of the CGM gas, it is found that fresh accretion is insensitive to assembly history, whereas the `late' galaxies experience more wet mergers. These differences in gas properties arise from processes after the formation time, given that the CGM gas masses show no significant differences at formation time. Finally, our analysis of CGM kinematics shows that for halos below $10^{12}\mathrm{M_\odot}$, the cold gas in late-forming halos carries higher specific angular momentum and simply has a higher degree of rotational support, while the same properties in the $10^{12-12.5}\mathrm{M_\odot}$ bin shows no significant dependence on assembly history.