Figuring Out Gas & Galaxies In Enzo (FOGGIE) XI: Circumgalactic O VI Emission Traces Clumpy Inflowing Recycled Gas

TL;DR

This study uses the FOGGIE cosmological zoom-in simulations to predict O VI emission from the inner CGM of edge-on, Milky-Way–mass galaxies across $z=1\rightarrow0$. By post-processing with CLOUDY emissivities and assuming ionization equilibrium, the authors show that O VI SB is brightest in $T\sim10^{5.5}$ K, collisionally ionized, clumpy inflows near the galaxy, enriched by stellar feedback and likely tracing galactic fountain flows rather than pristine accretion or hot outflows. The emission is modestly sensitive to halo mass and CGM density, strongly tied to the central SFR, and represents a lower limit to O VI columns due to model limitations and non-equilibrium effects. These results imply that upcoming CGM emission surveys, notably with the Aspera SmallSat, can map the recycled baryon cycle in nearby galaxies, highlighting O VI as a diagnostic of metal-enriched inflows and fountain-like recycling processes.

Abstract

The circumgalactic medium (CGM) is host to gas flows into and out of galaxies and regulates galaxy growth, but the multiphase, diffuse gas in this region is challenging to observe. We investigate the properties of gas giving rise to O VI emission from the CGM that upcoming missions, such as the Aspera SmallSat, will be able to map in local galaxies. We use the FOGGIE simulations to predict the O VI emission from edge-on galaxies across the redshift range $z=1\rightarrow0$. O VI emission is brightest surrounding small, clumpy structures near the galaxy where the gas density is high. Most of the O VI surface brightness originates from collisionally ionized, $T\sim10^{5.5}$ K, inflowing gas and is not preferentially aligned with the major or minor axis of the galaxy disk. Simulated galaxies with higher halo masses, higher median CGM gas density, and higher star formation rates produce brighter and more widespread O VI emission in their CGM. We show that while O VI emission primarily originates in inflowing gas, turning off outflows in a simulation without star formation feedback eliminates most of the O VI emission. Enrichment from feedback is necessary to mix with the inflowing gas and allow it to glow in O VI. Collectively, our findings point towards a picture where O VI emission traces warm, ionized envelopes of cooler clouds that are accreting onto the galaxy in a metal-enriched galactic fountain. Finally, we show that the detection limit of Aspera is sufficient to detect O VI emission tens of kpc from the galaxy center for $\sim L^\star$ galaxies.

Figures (16)

• Figure 1: Projected gas density (left) and surface brightness of O6 emission (right) from the six galaxies in the FOGGIE suite, each oriented edge-on and shown at $z=0$. The morphology of the emission is highly variable from one galaxy to the next, and shows both large, smooth structures and small, clumpy structures.
• Figure 2: (continued)
• Figure 3: The average of the surface brightness profiles of all six galaxies' O6 emission as a function of galactocentric distance (black curve) compared to the O6 SB profile of only that gas along the major axis (green dashed curve) or along the minor axis (pink dotted curve), averaged over $z=0-0.1$. Shaded regions show the standard deviation of the SB profile among the six galaxies and over time. There is no strong difference in O6 SB profile between the major axis and the minor axis, suggesting no strongly preferred orientation for O6-emitting gas.
• Figure 4: The average of the surface brightness profiles of all six galaxies' O6 emission as a function of galactocentric distance (black curve) compared to the O6 SB profile of only that gas that is radially inflowing with $v_r<-100$ km s$^{-1}$ (blue dashed curve), radially outflowing with $v_r>100$ km s$^{-1}$ (red dotted curve), or "slow-flow" gas with $-100 < v_r < 100$ km s$^{-1}$ (green dot-dashed curve) averaged over $z=0-0.1$. The black curve is the same as in Figure \ref{['fig:sb_axes_z0']}. Shaded regions show the standard deviation of the SB profiles among the six simulated galaxies and over time. The O6 surface brightness originates roughly equally from inflowing gas and slow-flow gas, with only a minor contribution from outflowing gas.
• Figure 5: A temperature-density phase plot of the Maelstrom CGM at $z=0$, color-coded by mass-weighted average O6 emissivity (black colors indicate where there is gas mass but no O6 emission). While there is some emission from cooler gas phases near $T\sim10^4$, the majority of the emission comes from $T\sim10^5$--$10^6$ K gas.