Molecular gas and star formation in central rings across nearby galaxies

Using high-resolution CO(2-1) observations from PHANGS-ALMA, this study characterizes the molecular gas content and star formation rates of 20 central rings in nearby galaxies, revealing that while their absolute properties vary with host galaxy characteristics, their relative contributions to global gas and star formation closely resemble those of the Milky Way's Central Molecular Zone, suggesting universal physical processes govern these structures.

The Gist

Imagine a galaxy as a giant, swirling city of stars. Usually, stars are born in the "suburbs"—the spiral arms where gas clouds drift lazily and slowly condense into new suns. But sometimes, right in the very heart of the city, there's a special, high-energy neighborhood: a central ring.

Think of this ring like a cosmic racetrack or a busy roundabout right in the middle of town. Instead of cars (gas) drifting aimlessly, they get funneled into this loop, speeding up, crashing into each other, and sparking a frenzy of new star birth.

This paper is a massive study of these "cosmic racetracks" in 20 nearby galaxies, using the world's most powerful radio telescope (ALMA) to see the invisible gas that fuels them. Here is the story of what they found, told simply:

1. The Great Gas Hunt: Finding the Invisible

For a long time, astronomers looked for these rings using visible light (like looking at a city with your eyes) or dust (looking at the smoke). But gas is invisible to the naked eye.

• The Analogy: Imagine trying to find a hidden river in a desert. You can't see the water, but you can see the green plants growing along its banks.
• The Discovery: The researchers used a special "gas detector" (CO molecules) to map the invisible rivers. They found that gas is just as good at revealing these rings as the visible light or dust was. In fact, they found rings in 25% of the galaxies they looked at, which matches what other studies found using different methods. It's like confirming a rumor by checking three different sources and getting the same answer.

2. The "Milky Way" Connection: Is Our Home Special?

Our own galaxy, the Milky Way, has a famous central ring called the Central Molecular Zone (CMZ). For years, astronomers thought our home was weird or "broken" because it has a lot of gas but isn't making stars as fast as it "should."

• The Analogy: Imagine you have a giant pizza dough (gas) in your kitchen. Most kitchens bake a pizza in an hour. Your kitchen has a huge pile of dough, but it's only making a tiny crust. You might think your oven is broken.
• The Discovery: The researchers looked at 20 other "neighborhoods" (galaxies) with similar rings. They found that our galaxy isn't actually broken. The rings in other galaxies also have huge piles of dough but bake slowly. The "slow baking" is actually normal for these high-pressure, high-speed environments. The Milky Way's CMZ fits right in with the crowd; it's not a weird outlier, just a standard member of the club.

3. The Role of the "Bar": The Cosmic Conveyor Belt

Many of these galaxies have a "bar" shape in the middle—a straight line of stars cutting through the center, like a wooden beam in a wheel.

• The Analogy: Think of the bar as a conveyor belt or a giant funnel. As the galaxy spins, the bar grabs the gas from the outer suburbs and shoves it down the middle, dumping it all onto the central racetrack.
• The Discovery:
  • Longer bars = Bigger rings. If the conveyor belt is longer, it can grab more gas and dump a bigger pile in the center.
  • Strength doesn't matter as much as you'd think. You might think a "stronger" bar (a sturdier conveyor belt) would push more gas. But the study found that the length of the bar matters more than how "strong" it is. A long, gentle push works better than a short, hard shove.

4. The "Star Factory" Efficiency

These central rings are extreme environments.

• The Analogy: If the suburbs of a galaxy are like a quiet town where one house is built every year, the central ring is like a construction site with a thousand cranes.
• The Discovery: Even though the rings are tiny (only about 1% of the galaxy's size), they hold a massive amount of gas (about 5-10% of the galaxy's total) and produce a huge chunk of the galaxy's new stars (about 13% of the total). They are incredibly efficient star factories, churning out stars much faster than the rest of the galaxy.

5. The "Massive Galaxy" Rule

You won't find these rings in small, dwarf galaxies.

• The Analogy: You need a big city with a massive population to build a giant racetrack. Small towns just don't have the resources.
• The Discovery: These rings only appear in massive galaxies. If a galaxy is too small, its gravity isn't strong enough to hold onto the gas against the explosions of dying stars (supernovae). The gas gets blown away before it can form a ring. Only the "heavyweight" galaxies can keep the gas trapped to build these rings.

The Bottom Line

This paper tells us that the center of a galaxy is a chaotic, high-pressure zone where gas gets funneled in by a "bar" structure, creating a ring that is a super-efficient star factory.

Most importantly, our own Milky Way is not special. The weird, slow-baking gas ring in our center behaves just like the rings in other massive galaxies. We aren't broken; we're just part of a universal pattern of how galaxies grow and evolve. The "cosmic racetrack" is a common feature in the universe, and we finally have a clear map of how it works.

Technical Summary

Here is a detailed technical summary of the paper "Molecular gas and star formation in central rings across nearby galaxies" by Gleis et al. (2026).

1. Problem and Scientific Context

Central rings (also known as nuclear or circumnuclear rings) are common structures in the centers of barred galaxies, typically located near the Inner Lindblad Resonance (ILR). They are characterized by high gas surface densities and intense star formation, serving as extreme environments for studying stellar birth. The Milky Way's Central Molecular Zone (CMZ) is a prime example of such a structure, yet its exact 3D geometry and physical properties remain debated due to our internal vantage point.

Previous studies of extragalactic central rings relied on tracers such as ionized gas (H$\alpha$, Pa$\alpha$), dust, or stellar light (UV/optical/NIR). However, a systematic study using molecular gas tracers (CO) at high resolution across a large sample of nearby galaxies was lacking. This study aims to:

• Characterize the molecular gas content and star formation rates (SFR) of central rings in nearby galaxies.
• Determine if molecular gas is an effective tracer for identifying these rings compared to previous methods.
• Investigate the relationship between ring properties and global galaxy parameters, specifically bar morphology and strength.
• Compare extragalactic central rings to the Milky Way CMZ to understand if the CMZ is a unique object or part of a universal population.

2. Methodology

The authors utilized data from the PHANGS-ALMA survey, which provides high-resolution ($\sim 1''$, corresponding to $\lesssim 100$ pc at distances $\le 20$ Mpc) CO(2-1) observations for 90 nearby main-sequence galaxies.

• Sample Selection: From the PHANGS-ALMA sample, the authors visually identified 20 central rings based on two criteria:
  1. Angular resolution sufficient to resolve structures $>100$ pc.
  2. Clear ring-like morphology in the molecular gas distribution (e.g., a central hole or distinct ring separate from a nuclear component).
  • Note: 14 of these 20 rings had corresponding MUSE observations providing Star Formation Rate (SFR) surface density ($\Sigma_{\text{SFR}}$) maps.
• Geometry Determination: Using SAOImageDS9, the team visually fitted elliptical annuli to the CO(2-1) moment-0 maps to define the ring's inner and outer boundaries. They derived "best," "broad" (uncertainty upper bound), and "strict" (uncertainty lower bound) masks to account for shape uncertainties.
• Property Derivation:
  • Molecular Gas Mass ($M_{\text{mol}}$): Calculated by integrating the CO(2-1) intensity over the ring area and converting to mass using a spatially varying $\alpha_{\text{CO}}$ conversion factor (Sun et al. 2025) that accounts for metallicity and excitation variations.
  • Star Formation Rate (SFR): For the 14 rings with MUSE data, SFR was derived by integrating the $\Sigma_{\text{SFR}}$ maps over the same elliptical masks, correcting for dust attenuation via the Balmer decrement.
  • Derived Metrics: Ring radii, gas fractions relative to the host galaxy, SFR fractions, and depletion times ($t_{\text{dep}} = M_{\text{mol}}/\text{SFR}$).
• Comparative Analysis: The results were compared against:
  • The AINUR catalog (Comerón et al. 2010), which identified rings using UV/optical/NIR tracers.
  • Literature values for the Milky Way CMZ.
  • Global host galaxy properties (stellar mass, position on the Star-Forming Main Sequence) and bar parameters (length, ellipticity, gravitational torque $Q_b$).

3. Key Contributions

• First Large-Scale Molecular Gas Census: This is the first study to systematically characterize central rings using high-resolution molecular gas data across a statistically significant sample (20 rings), moving beyond single-object studies or low-resolution surveys.
• Tracer Validation: The study demonstrates that molecular gas (CO) traces central rings with the same efficiency as ionized gas and dust, validating CO as a primary tool for identifying these structures in future surveys.
• MW CMZ Contextualization: By placing the Milky Way CMZ within the distribution of extragalactic central rings, the paper provides a crucial comparative framework for understanding the CMZ's properties without the bias of our internal viewing angle.
• Bar-Ring Connection: The work provides observational constraints on the link between bar morphology (specifically bar length) and the fueling of central rings, challenging the assumption that bar strength is the primary driver of gas accumulation.

4. Key Results

A. Ring Properties and Statistics

• Detection Rate: Central rings were found in 25% ($\pm 4\%$) of the PHANGS sample, consistent with previous studies using optical/UV tracers (AINUR, Knapen 2005).
• Physical Characteristics (Medians):
  • Radius: $\sim 400^{+250}_{-150}$ pc (range $\sim 120-1070$ pc).
  • Molecular Gas Mass: $\log(M_{\text{mol}}/M_\odot) \sim 8.1$ (typical range $10^{7.8} - 10^{8.3}$).
  • SFR: $\sim 0.21^{+0.15}_{-0.16} M_\odot/\text{yr}$.
  • Fractions: Rings contain $\sim 5.6\%$ of the host galaxy's total molecular gas and contribute $\sim 13\%$ to the total SFR.
  • Depletion Time: $\sim 0.58$ Gyr, which is 2–4 times shorter than typical galactic disk depletion times, indicating highly efficient star formation.
• Host Galaxy Dependence: Rings are preferentially found in massive galaxies ($\log(M_*/M_\odot) > 10$). There is a strong correlation between ring molecular gas mass and host stellar mass.

B. Comparison with the Milky Way CMZ

• The MW CMZ ($r \sim 150$ pc, $M_{\text{mol}} \sim 3 \times 10^7 M_\odot$) sits at the lower end of the radius and mass distributions of PHANGS rings.
• However, in terms of relative parameters, the CMZ is not special:
  • Its gas fraction ($\sim 4.6\%$) and SFR fraction ($\sim 3.7\%$) are consistent with the PHANGS sample.
  • Its depletion time ($\sim 0.5$ Gyr) matches the PHANGS distribution.
• Conclusion: The CMZ likely follows the same physical processes of gas inflow and star formation as extragalactic rings, suggesting it is not an outlier but rather a smaller-scale version of the same phenomenon.

C. Bar Morphology and Fueling

• Bar Length vs. Mass: A significant positive correlation exists between bar length and the molecular gas mass of the central ring. Longer bars tend to host more massive rings.
• Bar Strength vs. Mass: Surprisingly, no correlation was found between classical bar strength parameters (gravitational torque $Q_b$, ellipticity $\epsilon_{\text{bar}}$, Fourier amplitude $A_{\text{max}}^2$) and the ring's gas content.
• Implication: The ability of a bar to funnel gas to the center appears to depend more on the spatial extent (length) of the bar (sweeping up a larger disk area) than on the dynamical "strength" of the non-axisymmetric potential.

D. Early-Type Galaxies (ETGs)

• Five rings were identified in ETGs. These rings often exhibit different dynamics (smoother gas structures) and higher gas fractions (e.g., NGC 4476 contains 60% of its host's gas) compared to spiral galaxies, likely due to the dominance of the stellar potential over gas self-gravity.

5. Significance and Future Outlook

This study establishes that molecular gas is a robust tracer for central rings, confirming that the physical conditions in these extreme environments are universal across nearby galaxies. The finding that the MW CMZ shares relative properties (fractions and depletion times) with extragalactic rings suggests that the "star formation suppression" often debated for the CMZ might be a general feature of central ring physics (e.g., turbulence, shear) rather than a unique anomaly.

The lack of correlation with bar strength parameters challenges simple theoretical models where stronger bars always drive more efficient gas transport, suggesting that the geometry and length of the bar are more critical for ring fueling. Future work with higher-resolution JWST data will be essential to detect smaller rings ($r < 100$ pc) that are currently below the ALMA resolution limit, potentially revealing a population of rings that makes the MW CMZ even more typical within the distribution.