Inferring the mass of the circumgalactic medium using X-ray resonant scattering

Abstract

The circumgalactic medium (CGM) regulates galaxy growth and retains the imprint of feedback from supernovae and supermassive black holes. However, the bulk of the hot CGM produces little X-ray emission and is challenging to study with X-ray telescopes. We propose a novel method for evaluating the CGM mass using resonant scattering of the helium-like oxygen (OVII) resonant line at $E=574$ eV. In a spherically symmetric and static CGM halo with a sharp central X-ray peak, the number of OVII ions within an outer radial shell can be calculated from the ratio of the two directly observable quantities: the OVII flux from the bright inner region and the scattered OVII flux from the shell (where the scattered flux can be much higher than the intrinsic emission). To evaluate the accuracy of this geometric estimate for realistic galaxies -- with satellites, asymmetries, and gas velocities -- we use a sample of galaxies from the TNG50 cosmological simulation. We find that, when the most irregular systems are excluded based on their X-ray observables, we accurately predict the OVII mass in the outer halo (e.g., in an $r=R_{\rm 500c}-R_{\rm 200c}$ shell) from the ratio of the fluxes in the corresponding annulus and the central peak region ($r<0.2R_{\rm 500c}$), with only a 10% bias and an rms scatter of $\sim 0.2$ dex. As OVII mass strongly correlates with the total oxygen and gas mass, this direct OVII-counting method enables indirect estimates of those quantities by future X-ray microcalorimeter missions, such as NewAthena and HUBS.

Figures (12)

• Figure 1: Resonant scattering can be used to deduce the mass of O VII ions a CGM halo. Most O VII r photons are emitted within the inner CGM. These photons scatter off O VII ions in the outer CGM, where the intrinsic O VII r emission is negligible in comparison. The observer sees both the direct O VII r emission from the central (source) region and photons from the same source scattered off the O VII ions in the outer region. The ratio of these fluxes gives the total number of O VII ions in the outer region. Both regions are reasonably optically thin (except for small fractions occupied by the disk and bulge), which means that ions throughout the outskirts see the same flux from the central region as does the observer.
• Figure 2: Definition of the inner (source) and outer CGM regions based on O VII r emission profiles and scattering enhancement. Left: The O VII r cumulative intrinsic (i.e., not considering resonant scattering) emission profiles, normalized to total emission within $R_{\rm 500c}$, are shown as a function of projected radius. Individual profiles are shown by thin lines, and the thick black line shows the sample median. The vertical dotted line marks the median radius within which $90\%$ of ${L_{\rm X}(<R_{\rm 500c})}$ originates (denoted as $r_{\rm 90}$). Most of the O VII r emission originates within ${\sim 0.2R_{\rm 500c}}$. Right: The O VII r surface brightness enhancement factor due to the resonant scattering. The vertical dotted lines indicate the sample median values of $R_{\rm 500c}$ and $R_{\rm 200c}$. On average, the scattering-driven enhancement in the O VII r surface brightness increases toward the galaxy outskirts and plateaus at $r>R_{\rm 500c}$. The clean subsample (red curves), which excludes irregular galaxies identified based on X-ray observables (yellow curves), is defined in Section \ref{['sec:systematics']}.
• Figure 3: Effects of resonant scattering on the relation between the O VII mass in the 3D $R_{500c}-R_{200c}$ shell and the O VII r flux ratio between the outer and inner projected annuli. Each symbol represents a simulated galaxy in the full TNG50 sample; red and gray symbols show the flux ratios with and without resonant scattering, respectively. Shaded bands show the ${\rm 16^{th}-84^{th}}$ percentile ranges and the red line is the red sample median. For reference, dashed lines show linear relations with different normalizations, theoretically expected from geometric arguments. With scattering included, the two quantities are tightly correlated with the expected linear slope, except at the high flux ratios. As we show below, those large deviations can be identified and excluded based on X-ray observables.
• Figure 4: Impact of satellite contamination on the scattering $M_{\rm OVII}$-flux ratio relation. (a), (b), and (c): Examples of individual galaxies with satellite contamination. Shown are O VII r surface brightness maps, each 500 kpc on a side. The white circles mark the radii $R_{\rm 500c}$ and $R_{\rm 200c}$. (d): cumulative distribution of the ratio of satellite to core O VII r emission (see text for details). The vertical dotted line indicates a $15\%$ threshold. (e): The $M_{\rm OVII}$-flux ratio relation for satellite-contaminated (red) versus non-contaminated (green) galaxies. The solid line shows the running median. Dashed lines represent the theoretically-expected linear relations with different normalizations. Galaxies with significant satellite contamination systematically flatten the mass-flux ratio relation relative to the expected linear trend.
• Figure 5: Impact of azimuthal anisotropy in O VII r emission on the $M_{\rm OVII}$-flux ratio relation. (a), (b), and (c): Examples of individual galaxies that exhibit significant anisotropy in their O VII r surface brightness maps. The map size and annotations follow the same conventions as Figure \ref{['fig:satellite']}. (d): Histogram of the standard deviation of O VII r emission across four azimuthal quadrants, normalized by the mean emission across the four quadrants. The quadrants are defined within an annulus bounded by $0.2R_{\rm 500c}$ and $R_{\rm 200c}$. The vertical dotted line indicates the threshold value of 0.3. (e): The scaling relation for both symmetric (green) and asymmetric (red) galaxies---after excluding satellite-contaminated systems---where asymmetric galaxies are defined as those with a normalized standard deviation exceeding the threshold. Despite a few outliers among the asymmetric galaxies, both subsets are broadly consistent with the expected linear relation.