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

We use the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) galaxy formation simulations to make predictions for the baryonic budget in present day galaxies ranging from dwarf (M₂₀₀ ∼ 10¹⁰ M_☉) to Milky Way (M₂₀₀ ∼ 10¹² M_☉) 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 versus halo mass and cold gas versus stellar mass relations. Thus they make plausible predictions for the baryon budget. We present the mass fractions of stars, cold gas (T < 10⁴ K), cool gas (10⁴ < T < 10⁵ K), warm-hot gas (10⁵ < T < 5 × 10⁶ K) and hot gas (T > 5 × 10⁶ K), inside the virial radius, R₂₀₀. Compared to the predicted baryon mass, using the dark halo mass and the universal baryon fraction, f_b ≡ Ω_b /Ω_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 (M₂₀₀ ≲ 3 × 10¹¹ M_☉) while the warm-hot gas dominates at high mass (M₂₀₀ ≿ 3 × 10¹¹ M_☉). Haloes of mass M₂₀₀ ∼ 10¹⁰ M_☉ are missing ∼90 per cent of their baryons. More massive haloes (M₂₀₀ ∼ 10¹² M_☉) retain a higher fraction of their baryons, with ∼30 per cent missing, consistent with recent observational estimates. Moreover, these more massive haloes reproduce the observed fraction of cold, warm-hot and hot gases. The fraction of cool gas we predict (0.11 ± 0.06) is significantly lower than the observation from COS-Halos (0.3-0.47), but agrees with the alternative analysis of Stern et al. (2016).