Figuring Out Gas & Galaxies In Enzo (FOGGIE). XIV. The Observability of Emission from Accretion and Feedback in the Circumgalactic Medium with Current and Future Instruments

TL;DR

This study uses high-resolution FOGGIE simulations to predict the observability of CGM emission across multiple ionic lines around Milky Way–mass halos at $z\approx0.5$. Through CLOUDY-based emissivity post-processing and a disk-removal procedure, the authors generate mock emission maps for eight lines, revealing a rich multiphase CGM morphology and distinct radial profiles. They show that instrument sensitivity is the dominant constraint on recovering CGM mass and that achieving $\Delta v\lesssim 30$ km s$^{-1}$ is crucial to disentangle inflow, outflow, and phase-specific kinematics. The work also evaluates current and planned UV/optical instruments, illustrating trade-offs between spatial, spectral, and sensitivity capabilities, and provides concrete guidance for future instrument design to map CGM structure and dynamics in emission. Overall, the results underscore the need for high-sensitivity, high-kinematic-resolution observations across multiple ions to reconstruct the full multiphase CGM and its role in galaxy evolution.

Abstract

Observing the circumgalactic medium (CGM) in emission lines from ionized gas enables direct mapping of its spatial and kinematic structure, offering new insight into the gas flows that regulate galaxy evolution. Using the high-resolution Figuring Out Gas & Galaxies In Enzo (FOGGIE) simulations, we generate mock emission-line maps for six Milky Way-mass halos. Different ions (e.g., HI, OVI) trace distinct CGM phases and structures, highlighting the importance of observations in multiple species. We quantify the observable CGM mass fraction as a function of instrument spatial resolution and surface brightness sensitivity, finding that sensitivity is the dominant factor limiting detectability across all ions. At fixed sensitivity, higher spatial resolution reveals more structures; at fixed spatial resolution, higher sensitivity recovers a higher percentage of the total mass. We explore CGM kinematics by constructing emissivity-weighted projected velocity maps and comparing line-of-sight velocities between ions. OVI shows the largest kinematic deviation from HI, while MgII and SiII most closely follow HI velocities. Distinguishing these phases out to 50kpc from the galaxy center requires spectral resolution better than 30km/s for most ion pairs. Additionally, separating inflowing from outflowing gas based on projected kinematics also requires high spectral resolution: at 30km/s, more than 80% of gas above the emission detection threshold can be distinguished kinematically, but this fraction drops to <40% with a resolution of 200km/s. Our results provide predictions for future UV and optical instruments, showing that recovering the multiphase structure and kinematics of circumgalactic emission will require both high sensitivity and fine kinematic resolution.

Figures (17)

• Figure 1: Face-on H1 column density maps for the FOGGIE Squall halo within a 100 kpc box. Top: All gas contributing to the H1 column density. Bottom: Map after removing the H1 disk component. Some H1 remains visible in the inner region after removing the disk; this gas is not part of the disk but lies in front of or behind it along the line of sight. This figure illustrates how removing the dense H1 disk reveals surrounding gas structures and helps isolate CGM features from the disk.
• Figure 2: Face-on surface brightness maps of the Squall halo for eight ions with emission lines in the UV/optical: H$\alpha$, Mg2, Si2, Si3, C3, C4, and O6.. The box sizes cover 100 kpc fields of view. The H1 disk has been removed to isolate CGM emission; green contours indicate where the disk was. These maps highlight the multiphase nature of the CGM, as different ions trace different physical structures and temperatures. The zoom-in panels emphasize key differences in morphology across ions. These comparisons demonstrate that no single emission line fully captures the CGM’s complexity. Each ion offers a distinct view of its structure and thermodynamic state. Observing multiple UV lines is essential for building a complete, multiphase picture of the circumgalactic medium.
• Figure 3: Radial surface brightness profiles for eight ions across all FOGGIE halos (face-on, 0.27 kpc resolution). Solid lines show median profiles including all gas; dashed lines show CGM-only emission after H1 disk removal. Shaded regions indicate the range of halo-to-halo variation. Most ions follow power-law profiles, while H$\alpha$, Mg2, and Si2 show a break in slope around $0.1~R_{\rm vir}$—reflecting a transition from disk to CGM-dominated emission. CGM-only profiles flatten at small radii for low-ionization species.
• Figure 4: Surface brightness maps for three UV emission lines for the Blizzard halo, shown edge-on. Each column corresponds to a different spatial resolution: 0.27 kpc, 1, 3 and 6 kpc. These correspond at $z = 0.5$ ($z = 0$, 10 Mpc) to $\sim$ 0.04 (5.6), 0.16 (21), 0.5 (62), and 1 (123) arcsec, respectively. The galactic disk has been removed to isolate CGM emission. The colormap represents surface brightness in units of photons s$^{-1}$ cm$^{-2}$ sr$^{-1}$, with grayscale indicating regions below a representative instrument sensitivity threshold. Colored regions are potentially detectable by instruments with corresponding sensitivity. For reference, at $z = 0.5$, 6 kpc subtends $\sim 1$ arcsec, similar to the limits of seeing-limited ground-based observations, while a space-based telescope or AO-assisted ground-based telescope could reach $\sim 0.1$ arcsec, or $\lesssim$ 1 kpc spatial resolution.
• Figure 5: Observable CGM mass fraction as a function of spatial resolution (x-axis) and surface brightness sensitivity (y-axis). The spatial resolutions of 0.27, 1, 3, and 6 kpc, correspond at $z = 0.5$ ($z = 0$, 10 Mpc) to $\sim$ 0.04 (5.6), 0.16 (20), 0.5 (61), and 1 (120) arcsec, respectively. These maps are for three representative emission lines, averaged over six FOGGIE halos and for edge-on projections (see Appendix \ref{['sec:appendixA']} for maps of all eight ions' face-on and edge-on results). Each cell shows the percentage of total CGM mass detectable above the given sensitivity limit at the specified resolution, with values color-coded and overlaid. As expected, observability decreases with lower sensitivity and coarser resolution, though sensitivity plays a stronger role: at fixed resolution, increasing the detection threshold from 100 to 500 reduces the detectable mass by more than 50% in several ions. Achieving high sensitivity is critical for maximizing CGM detection, particularly for tracing diffuse, highly ionized gas.