2.1.

Supermassive Black Hole Feedback

The tight correlation between supermassive black hole (SMBH) mass and stellar velocity dispersion of host galaxy bulges^2–4 reveals that black holes and galaxies have co-evolved. Foundational numerical simulations² show that accretion-powered wind feedback, equivalent to just 1 to 5% of the radiative luminosity, can reshape host galaxies and drive co-evolution, sweeping the bulge of cold gas and preventing future star formation. For accretion-driven winds to govern the coevolution of SMBHs and host galaxies, they must be relatively common and act over many orders of magnitude in physical scale, connecting regions close to the black hole to kpc scales (Fig. 1). In connecting those scales, winds must eventually entrain additional gas and dust, losing speed as they conserve momentum, and they must eventually act over a broad range in solid angle. The total mass outflow rate and kinetic power are set in the “upstream” flow, connected to the accreting SMBH. Emission from the central corona peaks at X-ray wavelengths whereas the innermost disk is brightest at FUV wavelengths; therefore, the innermost accretion flow and the winds it generates are best revealed by sensitive atomic spectra in the X-ray and FUV.

Fig. 1

Schematic diagram of an AGN central engine, including the supermassive black hole (black), the accretion disk (purple), the torus (orange), and the winds (white and blue). Arcus will reveal how winds are launched and how they are layered; this diagram assumes that the fastest winds are launched from the smallest radii. From left to right, the diagram shows the UFOs, BALs, warm absorbers, and narrow absorption lines. The diagram also depicts the different time scales (hours, day, days) at which Arcus will be able to capture the evolution of such clouds in conjunction with the variations in the AGN continuum. The ability to study the ionizing continuum and wind spectra simultaneously in X-rays and FUV is essential to all aspects of revealing wind feedback from AGN.

The X-ray grating spectrometers aboard Chandra and XMM-Newton and the UV Cosmic Origins Spectrograph aboard Hubble Space Telescope (HST) have provided a few key glimpses into wind-driven feedback in a handful of sources where sensitive spectra can be obtained. In those sources (e.g., the Seyfert 1 galaxy NGC 5548), there is evidence of similar velocity structure in UV and X-ray absorption lines, indicating a complex, multi-phase outflow that requires study in both wavelength regions.^5–7 X-ray and UV winds can be broken into two distinct velocity regimes: fast ($v_{\text{out}}>\mathrm{10,000}\mathrm{km}{\mathrm{s}}^{-1}$) and slow ($v_{\text{out}}<\mathrm{10,000}\mathrm{km}{\mathrm{s}}^{-1}$). Slower winds include “warm absorber” winds, obscurers, and “broad line region” (BLR) winds.⁸ Faster winds include ultra-fast outflows (UFOs), which are typically detected in X-rays and UV (e.g., Refs. 9 and 10), and broad absorption lines (BALs), which are only seen in the UV.¹¹ Possible origins of these fast and slow outflows are schematically depicted in Fig. 1. However, the available energy resolution and sensitivity of current X-ray spectrometers, coupled with their lack of simultaneous UV spectral coverage, make it impossible to obtain reliable measurements of the two quantities that determine the feedback delivered by time–variable accretion-driven winds: the total mass outflow rate and kinetic power at $E<3.0\mathrm{keV}$ ($\lambda >4Å$). Arcus is designed to provide these key missing data.

Arcus will conduct two primary supermassive black hole surveys to answer key questions about the launching and structure of their winds. The AGN broad sample (ABS) obtains the first meaningful census of accretion-driven winds in local SMBHs, and the AGN deep sample (ADS) will obtain the first direct constraints on wind launching radii, gas densities, and thereby true mass outflow rates and kinetic powers. These samples are selected to span key ranges in black hole mass, Eddington fraction, and viewing angle and to pair optimally with observations in optical, infrared, and sub-millimeter bands that can trace wind feedback on kpc scales.

The ABS will observe 160 AGN simultaneously with XRS and UVS to begin answering these questions and to establish a reference for General Observer investigations. It will deliver the first unbiased census of X-ray and FUV wind velocities, ionizations, and column densities in local AGN. The sample is centered on an Eddington fraction of ${\lambda }_{\mathrm{Edd}}=0.3$ but spans an order of magnitude in each direction, covering the full range where AGN feedback is expected to have the largest impact on host galaxies.^2,12 Similarly, the sample is centered on a redshift of $z=0.05$ but extends up to $z=0.3$. The UV fluxes at 1050 Å of the ABS sample are in the range $(0.1\text{to}4.5)\times {10}^{-14}\mathrm{erg}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{Å}^{-1}$, with a typical value of about $1\times {10}^{-14}\mathrm{erg}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{Å}^{-1}$.

The synergy between XRS and UVS will offer a unique view of virtually all ionization states of the most abundant elements and major products of stellar evolution: carbon, nitrogen, and oxygen with both the XRS and UVS, and iron with the XRS. At the same time, hydrogen, a major tracer of any type of wind, will be covered by UVS (via $\mathrm{Ly}\alpha$ and $\mathrm{Ly}\beta$). Exposure times on these sources are driven by the X-ray observing duration, with ABS exposure times in the range of 10 to 90 ks, enabling both UVS gratings to be used to cover the full suite of UV ions (depending on the target redshift, Sec. 3).

The ADS utilizes the same observing strategy yet acquires spectra several times deeper (with a sample size of $\sim 30\mathrm{AGNs}$) to track time variability in wind absorption lines and the corresponding changes in wind density. The ADS will study 30 bright, unobscured AGNs, selected on their X-ray characteristics using the following criteria: (1) strong variability on 10-ks timescales (producing multiple expected significant flux changes in a long observation) and (2) detection of 10% variations in O VII and O VIII absorption lines with $t_{\exp }<300\mathrm{ks}$. This preliminary list of targets was then refined to ensure that it samples the same range in black hole mass and Eddington luminosity as the ABS; however, the average AGN in the ADS will be observed for $\sim 300\mathrm{ks}$, $\sim 10\times$ deeper than sources in the ABS. Each survey has a projected total observing time duration of 8 to 9 Ms.

2.2.

Warm Gas and Metals Cycling into and Out of Galaxies

Most galactic baryons are in hot or “warm–hot” phases (${10}^5$ to $5\times {10}^6\mathrm{K}$) and reside in a circumgalactic medium (CGM) that extends to at least the virial radius (250 kpc for a Milky Way–type galaxy). The column densities are significant, but the mean density is low; detection of these “missing baryons” through absorption in the X-ray and FUV spectral regions with access to these temperatures is the most powerful technique for inferring the abundance, temperature, and kinematics of this gas in galaxy halos and at the interface with the intergalactic medium. UV absorption studies have enabled significant progress (e.g., Refs. 13 and 14), providing evidence that halos of galaxies, groups, and clusters are filled with multi-phase and metal-enriched gas clouds. However, UV observations alone cannot constrain the hottest phases of the CGM where most of the metals and baryons are expected to be found. Arcus will fill this important gap, providing sensitive X-ray and FUV spectroscopy with the gas temperature coverage and spectral resolution required to study the physics of gas outflows and inflows as well as the volume-filling hot gas of galaxy halos and beyond.

Models predict that such halos are present over a wide range of galaxy masses. In general, the gas mass increases with the total gravitating halo mass, but there are significant disagreements between models.¹⁵ For sub-L* galaxies (where L* is the characteristic luminosity of a Milky Way–like galaxy), the CGM baryon mass fraction varies by $\sim 3\times$ between recent EAGLE and IllustrisTNG cosmological hydrodynamic simulations; for galaxies above L*, differences of $\sim 2\times$ persist. Comparable discrepancies exist between observations and semi-analytic models.^16–19

Arcus will provide the first comprehensive dataset for distinguishing among models of the CGM, which have very different ion signatures across the array of species probing hot gas between $T={10}^5-{10}^7\mathrm{K}$ (Fig. 2). Observations through the Milky Way’s hot halo indicate that about half of the baryons are bound to the galaxy, but there is significant uncertainty, and other L* galaxies may be different.²² The joint X-ray and FUV spectra from Arcus will provide the first comprehensive census of these different temperature phases across a statistical sample of galaxies in the low-redshift universe. We refer the reader to the mission overview paper by Smith et al. (this volume) for additional survey descriptions.

Fig. 2

Arcus will determine the amount and temperature of hot gas in galaxy halos, groups, periphery of clusters, and cosmic web filaments over a range of redshifts. The X-ray lines sample the volume filling gas (near the virial temperature) whereas the FUV lines primarily sample transition gas, arising from radiative cooling and feedback (all species noted here are covered by Arcus at low-to-intermediate redshift). The simultaneous X-ray and UV data and the spectral coverage of UVS enable observations of lines with a range of strengths (constraining line saturation using the weaker lines) while simultaneously providing the ability to detect low-metallicity/low-density gas (using the stronger lines). Column densities of selected ions observable with Arcus assume the typical total hydrogen column density and metallicity observed in the COS-Halos survey²⁰ and the collisional ionization equilibrium ionization models from Oppenheimer and Schaye.²¹ Arcus is sensitive to all of these ions at the column densities shown.

Arcus will uniquely measure the observational signatures of feedback in the halo gas: its temperature, the shape of its density distribution, and its metallicity. The ejection of $\sim 80\%$ the metals produced by stars²³ should ensure that metal-enriched gas is extensive, a result observationally supported by a flattened distribution of lower-temperature, O VI-bearing gas, out to $\gtrsim 160\mathrm{kpc}$.²⁴ UVS will directly measure the H I and metal lines in the FUV, which will tightly constrain the metallicity of the cooler and warm–hot gas.

The range of expected thermal line widths (e.g., O VI, H I Lyman-series lines, O VII, O VIII, and C VI), potentially narrow lines arising in cool gas, and complex multi-phase component structure require velocity resolution $<18\mathrm{km}{\mathrm{s}}^{-1}$ ($R>\mathrm{17,000}$ at 1050 Å sufficient to measure the width of thermally populated O VI absorbers) and spectral coverage from 1020 to 1340 Å to access low-redshift absorption systems of O VI and $\mathrm{Ly}\beta$. Absolute wavelength calibration requirement of $15\mathrm{km}{\mathrm{s}}^{-1}$ (51 mÅ at O VI) enables robust measurements of the kinematic flows within the CGM. UVS will acquire $\mathrm{S}/\mathrm{N}>10$ per resolution element (at 1050 Å, with $R>\mathrm{20,000}$) in a characteristic 100-ks observation in the G110M grating (covering O VI and $\mathrm{Ly}\beta$ for the majority of absorption systems sampled by Arcus), for sources with GALEX FUV magnitudes $<18.8$, assuming a flat FUV spectrum. By comparison, HST-cosmic origins spectrograph (HST-COS) achieves a similar S/N at 1150 Å with $R\hspace{0.17em}\sim \hspace{0.17em}\mathrm{15,000}$ in $\approx 20\mathrm{ks}$, but without coverage of low-redshift O VI or $\mathrm{Ly}\beta$.

Type II GRB UV afterglows,²⁵ which will supplement the known list of AGN background sources, are estimated to be at a flux level of $5\times {10}^{-15}\mathrm{erg}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{Å}^{-1}$ at 10 ks post-explosion, which will permit high S/N ($\sim 20$ per resolution element) absorption line profiles for the whole range of background sources targeted by Arcus. Arcus will acquire spectroscopic GRB afterglows less than 24 h post-trigger.

2.3.

High-Energy Activity of Planet-Hosting Stars

X-ray and FUV spectra with high resolution and sensitivity can characterize the physical conditions of stellar coronae as well as the dynamical processes driving other magnetic phenomena such as flares, winds, and coronal mass ejections. Magnetic activity is the primary stellar driver for atmospheric chemistry and physics on exoplanets. In some cases, stellar X-ray and stellar extreme-UV (EUV) radiation can remove gas faster than it can be replenished,^26,27 a process that likely drives the distribution of short-period planets²⁸ and may determine the ultimate habitability of temperate, terrestrial planets.²⁹ At the same time, FUV radiation drives photochemistry and the formation of atmospheric hazes and biosignature gases.^30–32

The 2020 Astrophysics Decadal Survey’s priority area for the “worlds and suns in context” science theme, pathways to habitable worlds, includes the recommendation for the Habitable Worlds Observatory (HWO). In describing this key science theme, the decadal survey emphasized “The pathway to searching for biosignatures on habitable worlds depends strongly on the properties of their parent stars.” Characterization of HWO target stars fulfills a key NASA precursor science goal for HWO (#2 precursor observations of infrared/optical/ultraviolet (IROUV) Exoplanet Imaging Targets: “sensitive X-ray and far-UV characterization of stellar radiation environments in the target systems” (NASA Precursor to Pathways Science Gap Summary 2022), and will have a profound influence on the interpretation of exoplanet spectra observed with current [James Webb Space Telescope (JWST)] and future (HWO) great observatories.

The Arcus coronal star sample (CSS) facilitates studies of coronal structure and heating for a rich distribution of $\approx 50$ stars of different ages and types. The sample will include many of the highest priority targets for the HWO, explicitly facilitating atmospheric studies for known planets while also expanding coronal spectral studies conducted by Chandra, XMM-Newton, HST-COS, far-ultraviolet spectroscopic explorer (FUSE), and other observatories.^33–35

The broad coverage of elements and ionization states provided by joint X-ray and FUV spectroscopy will allow us to determine the emission measure distributions (EMDs; the amount of emitting plasma at a given temperature in a collisional, coronal environment) with unprecedented simultaneous temperature sampling and for stars with low-to-intermediate activity levels. Owing to biases in primary exoplanet detection techniques, low-activity stars host many of the best-studied exoplanetary systems.^35–37 The Arcus dataset enables the calculation of the EUV environments which are the dominant drivers of atmospheric mass loss on all types of planets³⁸ (Fig. 3). Arcus therefore measures two high-priority bands needed for exoplanet atmosphere modeling and provides the best constraints on the critical EUV band, where no current observational capability currently exists.

Fig. 3

Simulated Arcus data for the representative K-type star $ϵ$ Eridanus for an exposure time of 5000 s (we note that most stars are X-ray/UV fainter than $ϵ$ Eridanus, and exposure times 10 to 20 times longer are budgeted for most stellar sources). Top panels: Arcus XRS and UVS (G110M) spectra are shown at left and right, respectively, in units of total counts per angstrom in the 5000-s exposure. Simultaneous, high signal-to-noise soft X-ray and FUV inputs enable high-fidelity emission measure distribution analyses^39,40 and spectral synthesis of the key EUV emission that drives atmospheric escape on all types of exoplanets (bottom panels). Combined, the Arcus dataset enables atmospheric evolution (X-ray+EUV) and photochemistry (FUV) models for terrestrial and gaseous planets.

Arcus will answer key questions in stellar physics about coronal heating mechanisms by measuring multiple weak dielectric recombination lines in the X-ray. These data provide coronal temperatures accurate to 10% and can be used to derive the degree of ionization equilibrium when paired with EMD analyses over the ${10}^4$ to ${10}^7\mathrm{K}$ covered by XRS and UVS.⁴¹ Furthermore, the X-ray/FUV line ratio for ions that emit in both bands (e.g., Fe XIX, Fe XII, and Fe XXI) provides strong diagnostics of coronal temperatures that are independent of the specific ionization state of the plasma. Combining sensitive temperature determination and coronal abundance patterns, Arcus observations will provide empirical data to distinguish between coronal heating models.^42,43

The Arcus CSS will also include a set of M dwarfs, important for establishing any differences in magnetic activity between main sequence stars with solar-type dynamos and fully convective stars with turbulent dynamos. Surprisingly, no difference has been observed to date in their well-established rotation–activity relation,^44,45 and the planet-hosting red dwarf Proxima Cen even shows a coronal activity cycle.⁴⁶ The CSS includes $\sim 5\times$ more fully convective stars than the handful that have been observed to date with X-ray gratings, providing an unprecedented dataset for studies of magnetically heated stellar atmospheres.

Arcus will permit EMD analysis for the full sample of CSS targets. This sample consists of about half of the HWO tier A and B targets (mostly F, G, and K stars), plus the brightest ROentgen SATellite (ROSAT)-detected M dwarfs to increase the sample near the transition to fully convective stars. We note that unless the HWO telescope size is significantly increased, the current HWO target list will change very little because they are the most promising systems drawn from a volume-complete sample. Arcus CSS observations are driven by the exposure time required for high-S/N X-ray observations; UVS spectra will achieve $\mathrm{S}/\mathrm{N}>15$ per resolution element in the peak of the O VI 1032-Å emission line. As most cool stars have transition region temperature distributions peaked at cooler temperatures, this S/N metric ensures that all of the other major FUV emission lines (C III, Si III, H I $\mathrm{Ly}\alpha$, N V, C II, Si IV, and C IV) will be detected at high S/N and readily sub-divided into wavelength-dependent lightcurves for temperature-resolved flare studies (e.g., Refs. 38 and 41). The wide distribution of FUV fluxes in the galactic stars observed by Arcus drives the bright object capability of the UVS, requiring observations of sources with peak fluxes as bright as $1\times {10}^{-11}\mathrm{erg}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{Å}^{-1}$.

4.1.

Low-Starburst Galaxies: Drivers and Probes of Cosmic Feedback

The 2020 decadal report emphasized the importance of understanding how stellar feedback drives the evolution of cosmic ecosystems (Secs. 1.1.3 and 2.3 of Astro2020¹). The intense energetic radiation fields, strong stellar winds, and supernova explosions of the most massive stars (O and B stars) dominate the mass, energy, and chemical flows in galaxies. O and B stars ($\approx 5$ to $200{\mathrm{M}}_{\odot }$), born in short-lived bursts of star formation, are the strongest drivers of a galaxy’s ongoing kinematic and chemical evolution.^49–51 The FUV bandpass contains the most sensitive diagnostics for the spectral type and stellar wind properties of the most massive stars.^52,53 At the same time, these hot star populations can serve as backlights to study the gas in- and outflowing from these galaxies and lower-redshift galactic halos along the line of sight, providing a powerful complement to galactic halo studies using AGN to probe circumgalactic halos.

Arcus UVS will be able to conduct high-S/N spectral observations of low-$z$ star-forming galaxies, measuring mass outflows, stellar populations, interstellar medium of these galaxies, and line-of-sight halo gas. Figure 6 presents a simulation of the FUV spectrum of a $z\approx 0.1$ star-forming galaxy. The target flux is estimated from the low-$z$ FUSE starburst survey of Grimes et al.,⁴⁹ with the predicted S/N ($\approx 10$ per $R=24,000$ resolution element) assuming a characteristic 100-ks exposure time in the G110M grating. This simulation shows a somewhat evolved star cluster where the O-type stars have largely died out (removing the strong O VI wind lines from the simulated spectral region) and includes interstellar absorption from neutral and low-ionization metals and ${\mathrm{H}}_2$. A warm/hot gas absorption system [$T_{\mathrm{abs}}={10}^6\mathrm{K}$, $N(\mathrm{O}\mathrm{VI})=6\times {10}^{13}{\mathrm{cm}}^{-2}$, $N(\mathrm{H}\mathrm{I})=3\times {10}^{14}{\mathrm{cm}}^{-2}$] is imposed at $\mathrm{\Delta }\mathrm{v}\approx -2400\mathrm{km}{\mathrm{s}}^{-1}$ from the galaxy redshift to demonstrate the line contrast for the detection of intervening halo gas. The column densities were selected to be at the lower edge of the distributions observed in the COS halos^24,54,55 and other surveys (see, e.g., Tumlinson et al.¹³ for a review) with a galaxy-absorber velocity offset chosen arbitrarily to be visible on the same high-resolution spectral plot as prominent galactic interstellar features. These same observations would allow for off-planar gas to be observed in the rest of the UVS long slit, which will be discussed in Sec. 4.2.

Fig. 6

Simulated spectrum of a $z=0.1$ star-forming galaxy observed by Arcus UVS. Narrow atomic and molecular lines from the target galaxy ISM are labeled, and an intervening $\mathrm{Ly}\beta$ and O VI absorption system ($\mathrm{\Delta }\mathrm{v}\approx -2400\mathrm{km}{\mathrm{s}}^{-1}$) are labeled with the “abs” tag. The simulation is at the expected S/N and spectral resolution (corresponding to $R=24,000$) for a characteristic 100-ks exposure time.

4.2.

Ultraviolet Emission Line Mapping of Galaxies

The use of integral field unit spectroscopy is growing explosively in astrophysics, driven by powerful ground-based instruments (e.g., MUSE⁵⁶ and KCWI⁵⁷) as well as JWST. Arcus will uniquely contribute to this approach by providing long-slit UV spectroscopy with excellent angular and spectral resolutions (the filled-slit spectral resolution of the instrument is $R\hspace{0.17em}\sim 2000$) in the spectral region at $\lambda <1150Å$ where high-sensitivity, high-angular resolution spectroscopy has never been conducted. HST surveys such as COS Legacy Spectroscopic SurveY (CLASSY) (e.g., Refs. 58 and 59) have shown that the UV provides a rich variety of emission line diagnostics tracing abundances, physical conditions, and kinematics, but most existing work uses single apertures and does not have spatially resolved emission maps. Figure 7 shows an example of a target that could be mapped with the Arcus+UVS UV long slit. This galaxy (NGC 4631) is known to exhibit widespread O VI emission as shown (see also Refs. 47 and 60), and by sweeping a long slit across the galaxy, the O VI emission can be mapped in detail, thereby revealing the gas physics and kinematics versus location and height above the disk. This unique information directly connects to the galaxy ecosystems/baryon cycle science priority identified by the Astro2020 decadal. The interstellar medium (ISM) and halo of the Milky Way also show extensive emission from O VI and low-ionization stages such as C II* (e.g., Ref. 54), and as every Arcus observation will observe the Milky Way gas, likely more than 1000 directions would be observed, providing an extraordinary map of O VI in a variety of locations ranging from the Local Bubble out to the distant galactic halo.

Fig. 7

Sloan Digital Sky Survey g-band image of NGC 4631 with three FUSE apertures (overlaid yellow boxes) that all show O VI emission (see side panels) as well as soft X-ray emission (contours). The UVS long slit (cyan) obtains $R\sim 2500$ to 4000 spectra at arcsecond-level angular resolution along its length.

As demonstrated by Otte et al.⁶¹ and Chung et al.,⁶² typical O VI surface brightnesses around low-$z$ galaxies, as well as the halo of the Magellanic Clouds and the Milky Way,⁶³ is B(O VI) $\sim \text{few}\times {10}^{-18}\mathrm{erg}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{\mathrm{arcsec}}^{-2}$. Figure 8 illustrates the spectral mapping capability of UVS and compares it to the time required for lower angular resolution spectral mapping with FUSE. Figure 8 plots the exposure time required to map a $1\mathrm{ft.}\times 6\mathrm{ft.}$ region to a depth of S/N = 5 per $10\text{-}\mathrm{in.}\times 5\text{-}\mathrm{in.}$. angular bin. This shows that one could map the entire halo of a galaxy with a surface brightness of $\sim 1.5\times {10}^{-18}\mathrm{erg}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{\mathrm{arcsec}}^{-2}$ in approximately 1 Ms, and the bright end of the halo distribution could be mapped in the characteristic 100-ks Arcus exposure time. Also shown are examples of O VI emission from starburst galaxies⁴⁹ and galactic supernova remnants,^65,66 which can be accomplished in short observing campaigns. We compare this to the equivalent time to map this angular area with comparable resolution with the FUSE medium resolution (MDRS) aperture ($4\mathrm{arcsec}\times 20\mathrm{arcsec}$). Combining the gain in an effective area with the need to raster $15\times 18$ pointings with FUSE compared to $1\times 6$ pointings with UVS, Arcus is capable of mapping extended FUV emission sources factors of 50 to 100 times faster than FUSE (and at higher angular resolution for brighter objects) at a filled aperture resolving power of $R>2000$ to resolve the velocity structure of the outflows. The high-sensitivity and long-slit imaging capability of UVS make possible highly sought-after datasets such as complete maps of O VI ($\mathrm{Ly}\alpha$, C IV, etc.) in a way that was infeasible for previous observatories.

Fig. 8

The mapping speed to S/N = 5 per $10\mathrm{arcsec}\times 5\mathrm{arcsec}$ angular resolution element over a $1\mathrm{arcmin}\times 6\mathrm{arcmin}$ field of view (black) as a function of O VI emission line surface brightness. Arcus is capable of mapping extended FUV emission source factors of 50 to 100 times faster than FUSE (with the $4\mathrm{arcsec}\times 20\mathrm{arcsec}$ MDRS aperture, orange dashed curve). O VI surface brightness from a range of astrophysical environments is shown overplotted on the black curve, from galactic halos (red, blue, and purple points^61–64) to starburst galaxies (dark teal points⁴⁹) and galactic supernova remnants (green filled diamonds^65,66). As the mapping times become very short for bright objects, we also show the mapping time with higher signal-to-noise and angular resolution (S/N = 10 per $10\mathrm{arcsec}\times 1\mathrm{arcsec}$ angular resolution) as the open green diamonds in this figure.