A Patchy Galaxy: Unraveling the Flocculent Structure of the Milky Way’s Inner Disk

The Milky Way is often imagined as a grand spiral, with bright arms wrapping neatly around its center. But Dana S. Balser and W. B. Burton challenge this classic image. Using modern data from the HI4PI survey, they argue that the inner regions of our Galaxy, the parts lying inside the Sun’s orbit, are far more irregular than majestic. Their work revisits and strengthens an idea first hinted at more than half a century ago: our Galaxy may not be a “Grand-design” spiral with well-defined arms, but rather a “Flocculent” spiral, a galaxy whose structure is fragmented and disordered.

Background

Balser and Burton begin by revisiting a long history of attempts to map the Milky Way’s spiral structure. Early hydrogen (H I) observations in the 1950s, like those by Westerhout and Schmidt, inspired generations of astronomers to search for the Galaxy’s spiral arms. Yet, as Shane and Bieger-Smith noted in 1966, clear “shoulders” or gaps in hydrogen spectra, signatures of empty interarm regions, were missing. Burton himself had tested this idea in 1971 using data from the Dwingeloo telescope, finding no signs of these expected features. Together, these studies hinted that the Milky Way’s inner disk might lack the cleanly separated spiral arms typical of Grand-design galaxies.

Data and Analysis

To revisit this question, Balser and Burton use the HI4PI survey, which combines northern and southern sky observations from the Effelsberg 100 m and Parkes 64 m telescopes. This all-sky dataset provides high-sensitivity measurements of neutral hydrogen across the Milky Way. The authors extracted 122 averaged spectra along the Galactic plane, spanning longitudes from 20°–80° in the north and 280°–340° in the south, and fit Gaussian curves to the high-velocity edges of each spectrum. By analyzing the width (σ) of these fits, they looked for the subtle shoulders expected if the Galaxy contained distinct interarm gaps in hydrogen density.

Results

The results were striking in their simplicity: no evidence for interarm regions was found. The velocity dispersions (σ) remained consistent, showing no high values that would suggest broad, weak shoulders in the spectral lines. Similarly, when the authors examined the total hydrogen brightness integrated across velocities, they found no “valleys” or dips that would mark the emptier spaces between spiral arms. Instead, both the northern and southern Galactic quadrants showed a relatively smooth distribution, interrupted only by small fluctuations likely caused by local events, such as supernova explosions or expanding shells of gas. These findings confirm and expand on the results from earlier, lower-quality data.

Discussion

Balser and Burton place their findings in the wider context of how galaxies are classified. Some galaxies, like the well-known Whirlpool Galaxy, display symmetrical, sweeping arms and are called Grand-design spirals. Others, the Flocculent spirals, have patchy, irregular patterns of gas and stars without a clear spiral form. The Milky Way’s outer disk, where data are less confused by motion effects, still shows relatively organized spiral features. But the new evidence suggests that the inner disk is chaotic and fragmented. The authors review several decades of studies using various tracers, from ionized hydrogen (H II) regions to molecular gas (CO), infrared stars, and dust maps, concluding that none provide strong, consistent evidence for a well-defined spiral structure near the Galactic center.

The authors also discuss subtle trends in their data, such as slightly decreasing velocity dispersion with distance from the Galactic center, which might arise from geometric effects or differences in gas temperature and star formation. Even asymmetries between the northern and southern Galactic hemispheres, likely caused by the tilt of the central bar, do not alter the overall picture: the inner Galaxy lacks the strong arm–interarm contrast characteristic of a Grand-design spiral.

Summary

Balser and Burton’s study reinforces an emerging view of the Milky Way as a patchwork rather than a pattern. Using the most sensitive all-sky H I survey to date, they find no evidence for the distinct spiral arms long assumed to exist inside the Sun’s orbit. Instead, the data reveal a disorganized mix of gas clouds and filaments, what they describe as a “shambles” of loosely connected features. Their conclusion is clear: the inner Milky Way is flocculent. This finding reshapes how astronomers picture our Galaxy’s structure and invites a rethinking of the processes that shape spiral galaxies across the universe.

Source: Balser