2025ApJ...990..221M

We present Atacama Large Millimeter/submillimeter Array Total Power CO(1-0) mapping of Stephan's Quintet (SQ), a prototypical compact galaxy group, with a uniform noise level at a spatial scale of ∼25 kpc. These observations provide the first complete view of molecular gas across the whole system. Molecular gas is found to spread over a wide area (∼120 × 80 kpc), mainly over the two main member galaxies (NGC 7318B and 7319), but also in the shocked ridges between these galaxies, the tidal tail, and also in intergalactic regions north of the tail. The total CO(1-0) luminosity is (2.47 ± 0.12) × 10⁹ K km s^–1 pc², corresponding to a molecular gas mass of (1.07 ± 0.05) × 10¹⁰M_⊙ assuming the Galactic CO-to-H₂ conversion factor. The global star formation efficiency of SQ is estimated at 0.29-0.70 Gyr^–1, comparable to or lower than that of nearby star-forming galaxies. Molecular gas spans a velocity range of ∼1300 km s^–1, which can be divided into three components (low, mid, and high). The low- and mid-velocity components, linked to NGC 7318B and the ridge, show relatively active star formation, whereas the high-velocity component, associated with NGC 7319, shows suppressed star formation. Our mapping reveals molecular gas extending ∼100 kpc in projection along the inner tail and north of it, containing (1.64 ± 0.08) × 10⁹M_⊙ (15% of total) with low velocity dispersion (∼20 km s^–1) and ongoing star formation. While previous studies suggested in situ molecular gas formation in the tail, our data suggest an additional contribution from gas stripped from NGC 7319.