COS-EDGES: Controlled CGM Survey
- COS-EDGES is a targeted HST/COS survey that uses near-edge-on, major-axis sightlines to isolate the multiphase circumgalactic medium of star-forming disk galaxies.
- The survey finds that low-ionisation gas in the inner halo co-rotates with the disk, while high-ionisation gas in the outer regions displays weaker rotational alignment and reduced velocities.
- Detailed spectroscopy and galaxy rotation curve measurements enable constraints on angular-momentum retention and radial kinematic stratification, informing models of inflow and recycling.
COS-EDGES is a deliberately geometry-controlled HST/COS absorption-line survey of the multiphase circumgalactic medium around isolated, near-edge-on, star-forming disk galaxies. Its first results are based on nine galaxy–quasar pairs at (z\sim 0.2), with each background quasar placed close to the projected major axis at impact parameters of (13)–(38) kpc, so that the line-of-sight component of disk rotation is maximized and the sign of the absorber velocity directly tests co-rotation. In this configuration, the survey targets the kinematic connection between the ISM and the cool, warm, and warm-hot CGM, with particular emphasis on angular-momentum retention, rotational lag, and radial stratification across ionization state [2507.11613].
1. Definition and scientific rationale
The name denotes the “COS (Cosmic Origins Spectrograph) – EDGE-on Survey” [2507.11613]. Its central premise is that edge-on, major-axis sightlines are the cleanest absorption-line configuration for testing whether circumgalactic gas shares the angular momentum of the galactic disk. Along the projected major axis of an edge-on system, the line of sight is nearly parallel to the disk plane, the projected rotation amplitude is large, and the observed velocity sign unambiguously identifies whether the gas lies on the same kinematic side as the disk.
The survey is designed to isolate the regime in which extended co-rotating structures, cold-mode inflow, and recycled accretion should be strongest. Previous CGM surveys established broad absorber demographics across wide ranges of azimuth, inclination, and halo-centric distance, but COS-EDGES intentionally narrows the geometry to near-edge-on disks, major-axis sightlines, and (D/R_{\rm vir}\lesssim 0.3). This controlled selection is intended to suppress projection ambiguity and to expose any ionization-dependent transition from rotation-dominated cool gas to more weakly coupled warm-hot halo gas [2507.11613].
A further motivation is the expectation, from the simulation and observational context summarized in the survey, that low-ionisation gas should retain stronger rotational alignment with disks than high-ionisation gas. COS-EDGES therefore treats co-rotation not as a binary property of “the CGM,” but as a phase-dependent kinematic observable.
2. Sample definition and geometric configuration
The first COS-EDGES sample comprises nine galaxies, labeled G1–G9, drawn from earlier galaxy–quasar compilations and selected to be isolated, late-type, emission-line disks with quasar sightlines near the projected major axis [2507.11613]. Isolation is defined operationally by the absence of a comparably bright neighbour within at least (\sim 130) kpc in the spectroscopic surveys used for target selection.
The geometric and host-galaxy parameter space is compact and intentionally structured.
| Quantity | Range or characteristic value |
|---|---|
| Galaxy redshift | (z_{\rm gal}\approx 0.16)–0.29 |
| Median redshift | (z\sim 0.2) |
| Inclination | (60\circ \le i \le 85\circ) |
| Mean inclination | (70\circ \pm 9\circ) |
| Azimuthal angle | (1\circ \le \Phi \le 35\circ) |
| Mean azimuthal angle | (15\circ \pm 12\circ) |
| Impact parameter | (D = 13.0)–37.5 kpc |
| Scaled impact parameter | (D/R_{\rm vir} = 0.12)–0.31 |
| Stellar mass | (\log(M_\star/M_\odot)=9.4)–10.6 |
| Halo mass | (\log(M_{\rm h}/M_\odot)\approx 11.1)–11.9 |
| Star-formation rate | (\mathrm{SFR} \sim 0.1)–1.2 (M_\odot\,\mathrm{yr}{-1}) |
The sample is further divided into an inner subsample with (0.12 \le D/R_{\rm vir} \le 0.20) and an outer subsample with (0.21 \le D/R_{\rm vir} \le 0.31) [2507.11613]. This radial split is central to the survey’s main result, namely that the CGM kinematics change around (0.2R_{\rm vir}).
Virial radii are computed using the Bryan & Norman formalism in a flat (\Lambda)CDM cosmology with (H_0=70~\mathrm{km\,s{-1}\,Mpc{-1}}), (\Omega_M=0.3), and (\Omega_\Lambda=0.7). In the notation used by the survey,
[
R_{\rm vir} = \left(\frac{3 M_{\rm h}}{4\pi \Delta_c \rho_c}\right){1/3}.
]
Galaxy inclination is inferred from the GALFIT axis ratio through
[
\cos2 i = \frac{q2 - q_02}{1-q_02},\quad q=b/a,\quad q_0\sim 0.1.
]
3. Observational data and kinematic diagnostics
COS-EDGES combines high-resolution optical spectroscopy of low ions with far-UV spectroscopy of H I and higher ions, together with galaxy rotation curves from optical emission lines [2507.11613].
| Instrument | Principal role |
|---|---|
| VLT/UVES | Mg I, Mg II, Fe II absorption |
| HST/COS G130M | H I Lyman series, C III, O VI |
| Magellan/MagE | Galaxy rotation curves for G1, G8, G9 |
| Keck/LRIS | Galaxy rotation curves for G2–G7 |
The UVES spectra use Blue CCD1 over (3000)–(4000) Å with a (1.2{\prime\prime}) slit and FWHM (\sim 6~\mathrm{km\,s{-1}}). COS G130M provides (R\sim 20{,}000), corresponding to FWHM (\sim 18~\mathrm{km\,s{-1}}), with multiple central wavelength settings to cover the relevant transitions at the galaxy redshifts. Galaxy kinematics are measured from H(\alpha) or [O II] along the major axis, using stepped apertures and Gaussian centroiding to recover the line-of-sight rotation curve and systemic redshift [2507.11613].
The survey’s core observables are defined from the absorption profiles. The equivalent-width co-rotation fraction is
[
f_{\rm EW,corot} \;=\; \frac{\displaystyle \int_{v_{\rm sys}}{v_{\rm max}} \bigl[1 - F(v)\bigr]\,\mathrm{d}v}
{\displaystyle \int_{v_{\rm min}}{v_{\rm max}} \bigl[1 - F(v)\bigr]\,\mathrm{d}v},
]
where (F(v)) is the continuum-normalized flux and the numerator includes only velocities on the same side of systemic as disk rotation toward the quasar. The optical-depth-weighted median velocity (v_{\rm abs}) is defined by
[
\int_{-\infty}{v_{\rm abs}} \tau(v)\, dv
= \frac{1}{2}\,\int_{-\infty}{+\infty} \tau(v)\, dv,\quad \tau(v)=-\ln F(v).
]
Velocity widths are characterized by (\Delta v_{50}) and (\Delta v_{90}), containing 50% and 90% of the total optical depth, respectively [2507.11613].
These measurements are evaluated both in absolute units and normalized by the maximum observed line-of-sight rotation speed (v_{\rm rot,los}). Uncertainties are estimated from 1000 bootstrap resamplings of the stacked spectra in each (D/R_{\rm vir}) bin [2507.11613].
4. Principal empirical results
The first COS-EDGES result is a clear radial and ionisation-dependent split in CGM kinematics. At lower (D/R_{\rm vir}) ((D/R_{\rm vir}\leq 0.2)), over 80% of absorption in all ions lies on the side of systemic velocity matching disk rotation, and (v_{\rm abs}) is consistent with the peak rotation speed [2507.11613]. In the stacked analysis, all ions in the inner subsample have (v_{\rm abs}\sim 110~\mathrm{km\,s{-1}}), and the normalized quantity (v_{\rm abs}/v_{\rm rot}) is approximately unity.
At higher (D/R_{\rm vir}) ((D/R_{\rm vir} > 0.2)), the kinematics diverge by ionisation state. For low ionisation gas, the amount of co-rotating absorption remains (>80\%), yet (v_{\rm abs}) drops to 60% of the galaxy rotation speed. For high ionisation gas traced by O VI, only 60% of the absorption is consistent with co-rotation and (v_{\rm abs}) drops to 20% of the rotation speed [2507.11613]. In the stacked outer sample, O VI has (v_{\rm abs} \approx 23\pm 18~\mathrm{km\,s{-1}}), substantially lower than the inner-halo value.
The velocity-width behavior is similarly stratified. For low ionisation gas, (\Delta v_{50}) is 1.8 times larger in the inner halo than at larger radii, whereas for C III and O VI (\Delta v_{50}) remains unchanged with distance [2507.11613]. Thus the cool CGM narrows kinematically with increasing (D/R_{\rm vir}), while the warm and warm-hot phases retain broad profiles.
A compact summary of the radial split is as follows.
| Regime | Low-ionisation gas | High-ionisation gas |
|---|---|---|
| (D/R_{\rm vir}\leq 0.2) | (>80\%) co-rotating; (v_{\rm abs}) consistent with peak rotation | (>80\%) co-rotating; (v_{\rm abs}) consistent with peak rotation |
| (D/R_{\rm vir}>0.2) | (>80\%) co-rotating; (v_{\rm abs}\approx 0.6\,v_{\rm rot}) | O VI (\sim 60\%) co-rotating; (v_{\rm abs}\approx 0.2\,v_{\rm rot}) |
The escape-velocity comparison adds an important dynamical constraint. At (R_{\rm vir}), low ions lie below (v_{\rm esc}(R_{\rm vir})) in 8/9 galaxies, while high ions do so in 6/9 galaxies; the named exceptions include G7 for low ions and G7, G1, and G3 for some high-ion absorption. At the projected radius (D), however, all observed velocities for all ions in all galaxies lie below (v_{\rm esc}(D)) [2507.11613]. The detected gas is therefore gravitationally bound at the radius where it is observed.
5. Physical interpretation
The survey interprets these measurements as evidence for a radially dependent CGM kinematic structure: the inner halo hosts cool, dynamically broad gas tightly coupled to disk rotation, whereas beyond (0.2R_{\rm vir}), particularly traced by O VI and H I, the CGM shows weaker rotational alignment and lower velocity dispersion [2507.11613]. This is the core physical conclusion of the first COS-EDGES paper.
In this framework, low-ionisation gas likely traces extended co-rotating gas, inflows and/or recycled accretion, while high-ionisation gas reflects a mixture of co-rotating, lagging, discrete collisionally ionised structures, indicating a kinematic stratification of the multi-phase CGM [2507.11613]. The survey therefore does not present “the CGM” as a single dynamical component. Instead, it resolves a phase hierarchy in which the cool gas preserves disk-like angular momentum more efficiently than the warm-hot phase.
The inner-halo behavior is consistent with the presence of extended co-rotating structures sometimes described in the simulation context as cold-flow disks or rotating inflow layers. The outer-halo O VI behavior, by contrast, is consistent with a more dispersion-supported halo component that retains only partial rotational memory. This suggests that angular-momentum coupling weakens with radius and with ionisation state.
Because all detected gas is bound at (D), the survey argues that these major-axis sightlines are more naturally interpreted in terms of inflow and recycling than escape. A plausible implication is that the survey is isolating the region where accreting and reaccreting gas is dynamically assimilated into the rotating baryonic halo before joining the disk.
6. Place within HST/COS absorber surveys, limitations, and future development
COS-EDGES occupies a distinct niche within the broader HST/COS absorber-survey landscape. Large COS programs such as the low-redshift IGM survey catalogued 2610 distinct redshift systems at (z_{abs}<0.75) along 82 AGN sightlines [1402.2655], while CASBaH combined high-S/N quasar spectroscopy with a deep galaxy survey around 9 sightlines and measured an O VI–galaxy cross-correlation with ((r_0,\gamma)=(6.00 \pm 1.09,\ 1.25 \pm 0.18)\ h{-1}\ {\rm Mpc}) for systems with (\log N({\rm O\,VI}) \ge 13.5) [1908.07675]. COS-EDGES is narrower by design: it sacrifices sample size and environmental breadth in order to enforce a major-axis, edge-on configuration that is maximally sensitive to co-rotation.
The present survey also has explicit limitations. The sample size is nine galaxies. The geometry is restricted to major-axis sightlines. The radial lever arm is confined to (D/R_{\rm vir}<0.32). The first paper focuses on kinematics rather than detailed metallicity or ionisation modeling. DECaLS imaging provides the structural basis for inclination and azimuth, but not the highest available morphological fidelity [2507.11613].
These limitations define the natural future program. The survey identifies cloud-by-cloud ionisation and metallicity modelling as the next step for separating pristine inflow from recycled material. It also points toward larger samples, minor-axis and intermediate-azimuth sightlines, higher-resolution imaging, emission-line mapping with facilities such as KCWI or MUSE, and more detailed comparison to synthetic absorption in modern zoom simulations [2507.11613].
Taken as a survey concept, COS-EDGES establishes a controlled observational benchmark for the angular-momentum structure of the multiphase CGM. Its first result is not merely that major-axis CGM gas often co-rotates, but that this co-rotation is radially stratified and ionisation-dependent: strong and disk-like in the inner cool halo, weaker and lagging in the outer warm-hot halo, with all phases remaining bound at the observed radii [2507.11613].