On the baryon budget in the X-ray-emitting circumgalactic medium of Milky Way-mass galaxies

Abstract

Recent observations with SRG/eROSITA have revealed the average X-ray surface brightness profile of the X-ray-emitting circumgalactic medium (CGM) around Milky Way (MW)-mass galaxies, offering valuable insights into the baryon mass in these systems. However, the estimation of the baryon mass depends critically on several assumptions regarding the gas density profile, temperature, metallicity, and the underlying halo mass distribution. Here, we assess how these assumptions affect the inferred baryon mass of the X-ray-emitting CGM in MW-mass galaxies, based on the stacked eROSITA signal. We find that variations in temperature profiles and uncertainties in the halo mass introduce the dominant sources of uncertainty, resulting in X-ray-emitting baryon mass estimates that vary by nearly a factor of four ($0.8-3.5\times10^{11} M_\odot$). Assumptions about metallicity contribute an additional uncertainty of approximately $50\%$. We emphasize that accurate X-ray spectral constraints on gas temperature and metallicity, along with careful modeling of halo mass uncertainty, are essential for accurately estimating the baryon mass for MW-mass galaxies. Future X-ray microcalorimeter missions will be crucial for determining the hot CGM properties and closing the baryon census at the MW-mass scale.

Figures (2)

• Figure 1: Derived baryon mass fraction of the X-ray-emitting gas within $R_{\rm vir}$ based on the mean X-ray surface brightness profile of MW-mass central galaxies from Zhang2024profile. We evaluate the effects of different assumptions on the inferred baryon fraction, including: $n_{\rm H}$ profile (Sect. \ref{['Sec_nh']}); isothermal temperature value (Sect. \ref{['Sec_T']}); log-normal distribution of temperature, ExpCGM model, and TNG-based forward model (Sect. \ref{['Sec_Tlog']}); halo mass uncertainty (Sect. \ref{['Sec_Mh']}); and metallicity (Sect. \ref{['Sec_Z']}). The vertical dashed line indicates the cosmological baryon fraction ($f_{\rm b}=$0.157).
• Figure 2: Baryon mass detected through different tracers as summarized in Table \ref{['Table_MWb']} and Appendix \ref{['Sec_MWb']}. The baryon masses are measured for the MW or $L_*$ galaxies (noting that the definition of $L_*$ varies across studies, spanning a broad range in stellar and halo mass). For reference, we take a stellar mass of $5.5\times10^{10}\,M_\odot$. Dark and light purple bars indicate the lower and upper limits of detected baryon mass, respectively. The upper limit derived in this work excludes the case of an isothermal $T=0.08\,\rm keV$. The gray line (and shaded bar) denotes the expected baryon mass of galaxies with $M_{\rm h}=1.3\times10^{12}\,M_\odot$ ($1-1.6\times10^{12}\,M_\odot$).