NIHAO VII: Predictions for the galactic baryon budget in dwarf to Milky Way mass haloes

TL;DR

This study uses high-resolution simulations to predict the distribution of baryons in galaxies from dwarf to Milky Way sizes, revealing significant baryon loss beyond the virial radius and varying gas phase dominance with galaxy mass.

Contribution

It provides detailed predictions of baryon fractions across different gas phases in galaxies, highlighting baryon loss and its dependence on galaxy mass, based on a large set of high-resolution simulations.

Findings

All halos are missing baryons compared to the universal fraction.

Cool gas dominates in low-mass halos, warm-hot in high-mass halos.

Massive halos retain about 30% of their baryons, consistent with observations.

Abstract

We use the NIHAO galaxy formation simulations to make predictions for the baryonic budget in present day galaxies ranging from dwarf to Milky Way masses. The sample is made of 88 independent high resolution cosmological zoom-in simulations. NIHAO galaxies reproduce key properties of observed galaxies, such as the stellar mass vs halo mass and cold gas vs stellar mass relations. Thus they make plausible predictions for the baryon budget. We present the mass fractions of stars, cold gas ($T<10^4$K), cool gas ($10^4 < T < 10^5$K), warm-hot gas ($10^5 < T < 5\times10^6$K), and hot gas (T$> 5\times10^6$K) inside the virial radius, $R_{200}$. Compared to the predicted baryon mass, using the dark halo mass and the universal baryon fraction, $f_{\rm b}\equiv \Omega_{\rm b}/\Omega_{\rm m}=0.15$, we find that all of our haloes are missing baryons. The missing mass has been relocated past 2 virial radii, and cool gas dominates the corona at low mass (M$_{200} < 3 \times 10^{11} M_\odot$) while the warm-hot gas dominates at high mass (M$_{200} > 3 \times 10^{11} M_\odot$). Haloes of mass $\sim 10^{10} M_\odot$ are missing $\sim 90\%$ of their baryons. More massive haloes ($\sim 10^{12} M_\odot$) retain a higher fraction of their baryons, with $\sim 30\%$ missing, consistent with recent observational estimates. Moreover, these more massive haloes reproduce the observed fraction of cold, warm-hot and hot gas. The fraction of cool gas we predict ($0.11\pm0.06$) is significantly lower than the observation from COS-HALOs (0.3-0.47), but agrees with the alternative analysis of Stern et al. 2016.