The Central Molecular Zone (CMZ), the ~300 pc central reservoir of dense molecular gas in the Milky Way, forms stars at an efficiency roughly an order of magnitude below that of Galactic disk clouds, and whether magnetic fields contribute to this suppression is unresolved. Lu et al. (2024) mapped polarized dust emission in 11 CMZ clouds with JCMT/POL-2 at ~0.5 pc resolution, estimated plane-of-sky field strengths of 0.1-1.7 mG via the angular dispersion function method, and found weak correlations of field strength and mass-to-flux ratio, and a moderate correlation of the magnetized virial parameter, with star formation rate (SFR) or dense gas fraction, concluding that self-gravity and turbulence dominate. Because that analysis rested on a small sample (9 clouds with SFR data) and field strengths carrying factor-of-~2 systematic uncertainties, they called for a full-CMZ polarization survey to establish statistically robust magnetic field-star formation correlations (including whether correlations analogous to those in external galaxies exist) and for interferometric polarimetry to test whether the random distribution of field-orientation differences between large (~2.5 pc) and small (~0.5 pc) scales persists down to core scales (<~0.1 pc). The problem remains open: the enabling full-CMZ polarization surveys now exist (SOFIA/HAWC+ FIREPLACE; JCMT BISTRO Galactic Center), first ALMA/SMA core-scale polarimetry and ALMA external-pressure measurements have appeared, but no published work has performed the robust correlation analysis across a full-CMZ cloud sample, the cloud-population multi-scale orientation test, or the core-scale mass-to-flux versus fragmentation test in the CMZ.

Background

The Central Molecular Zone (CMZ) is the innermost ~300 pc of the Milky Way, containing roughly 10710^7 solar masses of dense, warm, highly turbulent molecular gas. Despite this large gas reservoir, the CMZ forms stars at a rate about an order of magnitude lower than expected from standard dense-gas star formation relations, making it the nearest laboratory for suppressed star formation in galactic nuclei. The physical cause of this suppression - turbulence, tidal shear, high temperature, feedback, or magnetic fields - is actively debated.

Plane-of-sky magnetic field strengths in molecular clouds are commonly estimated with the Davis-Chandrasekhar-Fermi (DCF) technique and its refinements such as the angular dispersion function (ADF) method, which relate the dispersion of field orientations, inferred from polarized thermal dust emission, to the ratio of turbulent to mean field strength, combined with gas density and line-of-sight velocity dispersion σv\sigma_v. Key derived quantities are the mass-to-flux ratio λ\lambda (the cloud's mass per unit magnetic flux divided by the critical value; λ<1\lambda<1 clouds are magnetically subcritical, meaning the field can support the gas against gravity, while λ>1\lambda>1 clouds are supercritical and can collapse), and the magnetized virial parameter αvir,B\alpha_{\mathrm{vir,B}} (ratio of kinetic plus magnetic support to self-gravity; values above ~2 suggest an unbound cloud unless external pressure confines it).

Lu et al. (2024) used the James Clerk Maxwell Telescope (JCMT) with its POL-2 polarimeter at 850 μ\mum (14 arcsec, ~0.55 pc resolution) to observe 11 CMZ clouds, then the largest sub-parsec-resolution polarimetric sample in the CMZ, and derived field strengths of 0.1-1.7 mG. They found moderate correlation of the virial parameter with star formation rate (SFR) and dense gas fraction (CDGF, the fraction of cloud mass in compact dense substructures on 0.1-2 pc scales), weak correlation of mass-to-flux ratio with SFR, and weak anti-correlation of field strength with CDGF - overall suggesting a minor role for magnetic fields relative to self-gravity and turbulence at cloud scales. Two systematic limitations framed their conclusions. First, cloud-scale velocity dispersions, measured from single-dish spectra, are overestimated by a factor of ~1.6 on average because about 1.6 distinct velocity components overlap along each line of sight in the CMZ (Henshaw et al. 2016), biasing DCF field strengths; high-resolution spectral line decomposition is needed. Second, comparing JCMT (~0.5 pc) fields with large-scale fields from the Atacama Cosmology Telescope (ACT) 1-arcmin (~2.5 pc) polarization maps, the distribution of position-angle differences between scales is statistically uniform (random), unlike the bi-modal distribution found in Galactic disk high-mass star-forming regions (Zhang et al. 2014), where it signals dynamically important fields channeling core formation. The random CMZ behavior could reflect strong turbulence decoupling scales, but may also be contaminated by foreground polarization in the ACT data.

Adjacent results frame the open questions. In the central kiloparsec of the nearby barred galaxy NGC 1097, radio-continuum analyses, with field strengths inferred from the non-thermal radio emission under the assumption of equipartition between the magnetic-field and cosmic-ray energy densities, show that most molecular clouds are magnetically critical or supported against collapse and that the star formation efficiency of the clouds drops as the total field strength rises (Tabatabaei et al. 2018); whether an analogous field-star formation relation holds within the CMZ is untested with adequate samples. In Galactic disk massive cores, the mass-to-flux ratio correlates tentatively with the level of fragmentation (Palau et al. 2021); whether this extends to CMZ core scales is unknown. Several CMZ clouds have virial parameters above 2, implying they should disperse unless external pressure confines them; analysis of the CMZ clump population indicates such pressure is likely critical (Myers et al. 2022), but cloud-by-cloud quantification is lacking. Finally, Lu et al. noted the correlation pattern could reflect evolution - quiescent clouds being magnetically subcritical and unbound, evolving toward supercritical, bound, star-forming states - a scenario requiring time-resolved or simulation-based tests.

Since 2024 the observational landscape has changed substantially. The FIREPLACE survey (Far-infrared Polarimetric Large-area CMZ Exploration), carried out with the HAWC+ far-infrared imaging polarimeter on board the Stratospheric Observatory for Infrared Astronomy (SOFIA), delivered full-CMZ far-infrared (214 μ\mum, 19.6 arcsec) polarimetry (~64,000 polarization pseudovectors; Paré et al. 2024), and the JCMT BISTRO survey (B-fields In STar-forming Region Observations) mapped the inner CMZ at 450/850 μ\mum (Yang et al. 2025; Karoly et al. 2025), revealing an ordered field aligned with the orbital gas flows. Core-scale interferometric polarimetry has begun (observations of Sgr B2, the CMZ's most massive star-forming complex, with the Submillimeter Array (SMA) by Pan et al. 2024; observations of two massive clumps in the 20 km s−1^{-1} cloud at 2000 au resolution with the Atacama Large Millimeter/submillimeter Array (ALMA) by Liu et al. 2026), and ALMA continuum and spectral surveys of hundreds of dense cores in CMZ clouds quantify external-pressure confinement (Zhang et al. 2025). These datasets make the statistical questions raised by Lu et al. (2024) answerable, but the analyses themselves remain to be done.

Problem Statement

Determine whether and how magnetic fields regulate the dynamics and star formation of molecular clouds in the Central Molecular Zone, by answering the following linked questions on a statistically robust, full-CMZ sample of clouds, superseding the 11-cloud (9 with star formation data) analysis of Lu et al. 2024: (1) Do the magnetic-field properties of CMZ clouds - plane-of-sky field strength BB, mass-to-flux ratio λ\lambda, and magnetized virial parameter αvir,B\alpha_{\mathrm{vir,B}} - correlate with star formation indicators (star formation rate and dense gas fraction), and does a field-star formation relation analogous to that found in external galaxies hold within the CMZ (in the central kiloparsec of NGC 1097, the star formation efficiency of molecular clouds drops with increasing total field strength; Tabatabaei et al. 2018)? (2) Does the statistically random distribution of magnetic-field position-angle differences between large (~2.5 pc) and small (~0.5 pc) scales persist down to dense-core scales (<~0.1 pc) in the CMZ clouds for which interferometric (e.g. ALMA/SMA) polarimetry resolves individual cores, and to what extent that limited core-scale sample constrains the CMZ cloud population; and does the tentative correlation between mass-to-flux ratio and fragmentation level seen in Galactic disk cores also hold at CMZ core scales? (3) What external pressure is required to confine the clouds with virial parameters above 2 and bring them into dynamical equilibrium, and is that pressure actually present? (4) Do the observed correlations (or their absence) reflect an evolutionary sequence in which quiescent clouds are gravitationally unbound and magnetically subcritical and evolve toward bound, supercritical, star-forming states? These inferences are limited by systematic uncertainties: cloud-scale velocity dispersions overestimated by a factor of ~1.6 from blended line-of-sight velocity components, the factor-of-~2 or larger systematic uncertainty of DCF/ADF field strengths, and foreground contamination of large-scale polarization data. Each of the four questions is independently resolvable and stands on its own, and together they serve the single objective of establishing the magnetic field's role.

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