Fundamental CO(1–0) Transition Survey (FACTS)

• FACTS is a CO(1–0)-anchored survey that minimizes excitation bias by directly tracing cold molecular gas in galaxies.
• It leverages ALMA’s 12‑m, 7‑m, and Total Power arrays to produce maps from kiloparsec scales down to ~50–200 pc resolution.
• The survey calibrates higher-J CO measurements by quantifying the CO(2–1)/CO(1–0) ratios, highlighting radial and structural variations linked to galactic dynamics.

Fundamental CO(1–0) Transition Survey (FACTS) denotes a survey framework that takes the ground-state CO $J=1\!\rightarrow\!0$ line as the reference observable for molecular-gas studies. In its current nearby-galaxy implementation, ALMA-FACTS maps CO(1–0) in a complete subset of galaxies common to PHANGS–ALMA CO(2–1), Spitzer SINGS, and Herschel KINGFISH, using ALMA 12‑m, 7‑m, and Total Power data; the survey is designed to place higher-$J$ CO measurements on a CO(1–0) footing and to resolve how the CO(2–1)/CO(1–0) ratio varies with galactic structure from kiloparsec to $100$–$200$ pc scales (Komugi et al., 13 May 2025, Lee et al., 17 Jul 2025).

1. Fundamental rationale for a CO(1–0)-anchored survey

CO(1–0) occupies a privileged role in molecular-gas astronomy because it is the ground-state rotational transition and therefore the least excitation-biased CO observable. High-redshift survey work explicitly describes CO(1–0) as “the most robust tracer of the overall molecular gas content (including the wide-spread, low-density and subthermally excited component),” and emphasizes that the luminosity of the ground transition is least affected by excitation conditions of the gas (Emonts et al., 2011). In the same sense, targeted studies of $z\sim2$ submillimetre galaxies argue that ground-state CO(1–0) avoids uncertain excitation corrections and directly constrains the largest, coolest reservoir (Huynh et al., 2017).

This logic is central to FACTS. Much of the recent extragalactic CO literature has been built around CO(2–1) or higher-$J$ lines because those transitions are observationally convenient, but the substitution is not exact. Galaxy-integrated analyses across 122 systems show that the use of CO(2–1) “depends on a knowledge of the ratio between CO(2–1) and CO(1–0) luminosities, $r_{21}$,” and that a constant $r_{21}$ introduces biases into molecular-gas scaling relations (Keenan et al., 2024). FACTS is therefore not merely another mapping survey; it is a calibration program for the excitation ladder, with CO(1–0) functioning as the molecular-gas anchor.

A broader implication, consistent with blind and targeted CO(1–0) work, is that CO(1–0) surveys define galaxy samples by cold molecular-gas mass more directly than dust continuum or mid-$J$ CO surveys. Blind deep-field programs state this explicitly: CO(1–0) “uniformly selects starbursts and massive Main Sequence galaxies based on their cold molecular gas masses” (Pavesi et al., 2018). FACTS applies the same principle in the nearby-galaxy regime, where resolved structure and dynamics can be examined directly.

2. Survey architecture, sample definition, and observational scope

ALMA-FACTS is an ALMA mapping survey of 12 nearby galaxies in ${}^{12}\mathrm{CO}(J=1\!-\!0)$. The sample is a complete subset of galaxies that have PHANGS–ALMA CO(2–1) maps, Spitzer SINGS, and Herschel KINGFISH coverage; NGC 4569 is excluded because its negative recession velocity places CO(1–0) near the edge of the optimal ALMA Band 3 frequency range (Komugi et al., 13 May 2025). The resulting sample spans barred and unbarred spirals, flocculent systems, and a range of star-formation activity (Lee et al., 17 Jul 2025).

The survey has been analyzed at two complementary scales. The large-scale study uses only the ALMA Total Power cubes for both CO(1–0) and CO(2–1), specifically to avoid interferometric imaging systematics and missing flux. In that mode, CO(1–0) is mapped over roughly half the optical disk, typically $J$0, with an effective beam of $J$1; at the mean sample distance of $J$2 Mpc, this corresponds to $J$3 kpc (Komugi et al., 13 May 2025). The high-resolution study re-images PHANGS CO(2–1) and combines it with new FACTS CO(1–0) observations at a common angular resolution of $J$4, corresponding to $J$5–$J$6 pc depending on distance (Lee et al., 17 Jul 2025).

Two survey conventions are structurally important. First, the optical radius is defined as $J$7, and radial trends are usually normalized by $J$8. Second, bar radii $J$9 are taken from near-infrared Fourier decompositions, specifically the radius where the $100$0 amplitude peaks (Komugi et al., 13 May 2025).

FACTS component	Scale and data basis	Principal use
ALMA FACTS II	TP-only, $100$1, $100$2 kpc	Large-scale $100$3, radial gradients, disk/outlier decomposition
ALMA FACTS III	$100$4, $100$5–$100$6 pc	Structure-resolved $100$7 maps
Survey parent design	ALMA 12‑m, 7‑m, TP + PHANGS, SINGS, KINGFISH overlap	CO(1–0) reference for multi-wavelength nearby-galaxy studies

The survey design makes FACTS a bridge between single-dish style flux-complete CO(1–0) measurements and high-resolution excitation mapping. That dual character is unusual: the same program addresses global calibration and internal molecular-ISM structure.

3. Measurement framework: $100$8, $100$9, and kinematic decomposition

In the published FACTS analyses, the CO(2–1)/CO(1–0) ratio appears in two closely related notations. The large-scale TP study defines

$200$0

with integrated intensities in brightness-temperature units (Komugi et al., 13 May 2025). The high-resolution paper uses $200$1 for the same physical quantity and analyzes its variation at $200$2–$200$3 pc scales (Lee et al., 17 Jul 2025).

The TP analysis constructs luminosity-weighted ratios from matched CO(1–0) and CO(2–1) maps and then moves to major-axis position–velocity space. A slit is placed along the major axis, PV diagrams are extracted, and a tilted-ring model is fitted with 3DBarolo to represent the pure circularly rotating disk. The model PV diagram defines a “disk locus”; observed pixels inside the corresponding contour are assigned to the rotating disk, while pixels at or outside it are classified as kinematic outliers (Komugi et al., 13 May 2025). This disk/outlier partition is essential because it separates normal rotation from forbidden-velocity or non-circular components without requiring a fully resolved dynamical model.

The large-scale study then builds three classes of radial profile: disk only, outliers only, and the combined distribution. The high-resolution study instead works directly with $200$4 maps and empirical structural classifications derived from optical and molecular morphologies (Lee et al., 17 Jul 2025). Taken together, these two approaches allow FACTS to ask the same question at different scales: whether the excitation ratio is primarily set by global morphology, by local star formation, or by non-circular gas dynamics.

4. Large-scale $200$5 phenomenology from ALMA FACTS II

The large-scale Total Power analysis establishes the basic empirical baseline for nearby galaxies. The luminosity-weighted $200$6 of the 11 analyzed sample galaxies ranges from $200$7 to $200$8, with an average of $200$9 (Komugi et al., 13 May 2025). This places the sample securely in the regime expected for cool to moderately excited disk molecular gas, but the central result is that the ratio is not spatially constant.

Kinematic outliers are systematically more excited than the rotating disk. Averaged over PV pixels, the disk has $z\sim2$0, whereas outliers have $z\sim2$1 (Komugi et al., 13 May 2025). The outlier population therefore occupies the high-$z\sim2$2 tail. The study interprets these components as gas in strong non-circular motions, bar-driven streaming, overshooting gas in barred potentials, or nuclear structures that appear at forbidden velocities in PV space.

By contrast, the average $z\sim2$3 does not differ significantly among SA, SAB, and SB systems. The decisive morphology dependence appears instead in the radial gradient. FACTS II finds a bimodal large-scale behavior: a group containing all SA galaxies prefers constant or very shallow $z\sim2$4 gradients out to $z\sim2$5 of the optical radius, while another group containing all SB galaxies has a steep $z\sim2$6 gradient, decreasing by $z\sim2$7 before $z\sim2$8 of the optical radius, which also corresponds to the radius of the stellar bar (Komugi et al., 13 May 2025). Beyond that radius, the barred galaxies become consistent with a constant or shallow trend.

This result is methodologically consequential. It shows that a galaxy can have a perfectly ordinary luminosity-weighted $z\sim2$9 and still host strong radial excitation structure. It also means that the common expedient of converting CO(2–1) to CO(1–0) with a single ratio is not merely noisy; in barred galaxies it is systematically wrong in a morphology-dependent way.

5. High-resolution structure dependence from ALMA FACTS III

The high-resolution analysis extends the FACTS program from kiloparsec-scale radial averages to $J$0–$J$1 pc maps. These early results already show that $J$2 varies systematically with galactic structure, dynamics, and star formation activity (Lee et al., 17 Jul 2025). The survey makes empirical classifications based on optical and molecular morphologies and finds clear structural regularities.

Barred spirals follow a characteristic sequence. $J$3 is high in the center, low along the bar, increases at the bar ends, and then declines in the outer parts of the disk (Lee et al., 17 Jul 2025). This pattern is more detailed than the TP result and is entirely consistent with it: the bar imprints a specific excitation geography, not just a global radial decrement. The structure dependence suggests the importance of galactic dynamics on molecular-gas evolution and, consequently, on star formation.

Spiral arms are not uniform excitation features. $J$4 fluctuates in the spiral arms for both barred and unbarred galaxies, indicating that arm/interarm structure alone does not define a single excitation state (Lee et al., 17 Jul 2025). H II regions increase $J$5 locally in their surrounding gas and are often associated with galactic structures, so the ratio is sensitive to both dynamical placement and recent star formation.

A plausible implication is that FACTS is beginning to isolate at least three superposed controls on low-$J$6 excitation: central concentration, orbit family and bar dynamics, and local feedback around H II regions. The published result stops short of a full physical inversion, but it establishes the morphological phase space in which such inversions become meaningful.

6. Broader survey lineage, interpretive reach, and intrinsic limitations

FACTS did not emerge in isolation. Before the nearby-galaxy ALMA implementation, the AT-LESS CO(1–0) programme was already described as a pathfinder for a Fundamental CO(1–0) Transition Survey, showing in the high-redshift regime that compact configurations and natural weighting are crucial to capture total flux, that wide instantaneous bandwidth is indispensable, and that $J$7–$J$8 per source are required to robustly detect $J$9 at $r_{21}$0 with current facilities (Huynh et al., 2017). Blind deep-field work generalizes the same principle: COLDz demonstrates that CO(1–0) can be used to constrain the CO luminosity function and molecular-gas density at $r_{21}$1–3, while also finding CO-traced gas reservoirs up to $r_{21}$2 kpc in size (Pavesi et al., 2018). In that sense, FACTS belongs to a wider CO(1–0) programmatic lineage that spans nearby-galaxy structure, high-redshift targeting, and blind cosmological surveys.

The survey also participates in an active debate over whether higher-$r_{21}$3 CO can stand in for CO(1–0). AMISS shows strong trends between galaxy-integrated $r_{21}$4 and SFR, SFR surface density, star-formation efficiency, and distance from the star-formation main sequence, and shows that the assumption of a constant $r_{21}$5 biases the recovery of the Kennicutt–Schmidt law and other scaling relations (Keenan et al., 2024). FACTS II reaches the same conclusion from a different angle: the ratio varies systematically with bars, radius, and kinematic outliers (Komugi et al., 13 May 2025). The combined message is that CO(2–1) is not a universal surrogate for CO(1–0); it requires a survey-specific or population-specific excitation calibration.

At the same time, CO(1–0) itself is not an infallible tracer in every environment. Low-metallicity dwarf-galaxy work finds that CO suffers from significant selective photodissociation, that $r_{21}$6, and that $r_{21}$7 can be $r_{21}$8–$r_{21}$9 the Galactic value, with $r_{21}$0 (Cormier et al., 2014). Resolved ALMA observations of Arp 220 add a different limitation: the CI/CO(1–0) luminosity ratio is almost constant only up to $r_{21}$1 and then increases with $r_{21}$2; in the putative outflow, the CI/CO(1–0) ratio reaches $r_{21}$3, suggesting a CI-rich and CO-poor gas phase (Ueda et al., 2022). These cases do not negate the rationale for FACTS, but they delimit it. CO(1–0) is the most robust low-$r_{21}$4 anchor for total molecular gas, not a guarantee of environmental invariance.

The enduring significance of FACTS lies precisely in that balance. It institutionalizes CO(1–0) as the reference line, quantifies how higher-$r_{21}$5 maps depart from it, and exposes where the remaining systematics—metallicity, optical depth, chemistry, non-circular dynamics, and local feedback—must be confronted rather than assumed away.