2025ApJ...985..186B

We present SOFIA/HAWC+ continuum polarization data on the magnetic fields (B fields) threading 17 pc-scale massive molecular clumps at the western end of the η Carinae giant molecular cloud (Region 9 of the Galactic Census of High- and Medium-mass Protostars, or CHaMP, representing all stages of star formation from pre-stellar to dispersing via feedback), revealing important details about the field morphology and role in the gas structures of this clump sample. We performed Davis-Chandrasekhar-Fermi (DCF) and histogram of relative orientation (HRO) analyses tracing column densities 25.0 < log(N/m^–2) < 27.2. With HRO, B fields change from mostly parallel to column density structures to mostly perpendicular at a threshold N_crit = (3.7 ± 0.6) × 10²⁶ m^–2, indicating that gravitational forces exceed magnetic forces above this value. The same analysis in 10 individual clumps gives similar results, with the same clear trend in alignments and a threshold N_crit = (1.9${}_{-0.8}^{+1.5}$) × 10²⁶ m^–2. In the other seven clumps, the alignment trend with N is much flatter or even reversed, inconsistent with the usual HRO pattern. Instead, these clumps' fields reflect external environmental forces from the nearby H II region NGC 3324. DCF analysis reveals field strengths somewhat higher than typical of nearby clouds, with the B-n data lying mostly above the R. M. Crutcher relation. The mass-to-flux ratio λ across all clumps has a Gaussian distribution, logλ_DCF = -0.75 ± 0.45; only small areas are dominated by gravity. However, a significant trend of rising logλ with falling T_dust parallels R. L. Pitts et al.'s result: T_dust falls as ${N{}_{{\rm{H}}}}_{2}$ rises toward clump centres. In this massive clump sample, B fields provide enough support against gravity to explain their overall low star formation rate.