Why Some Galaxies Never Puff Up: Indian Researchers Decode The Mystery of Superthin Galaxies
Researchers from IISER Tirupati and RRI have discovered that dark matter dominance prevents bar formation, helping superthin galaxies maintain their remarkably flat structures.
By Anubha Jain
Published : October 17, 2025 at 7:46 PM IST
We live in a largely invisible universe—about 69 per cent of it is dark energy, 26 per cent is dark matter, and only about 5 per cent is ordinary (baryonic) matter, which forms stars, planets, and life. Within this small fraction of visible matter, disk galaxies take shape, typically featuring a thick, star-filled outer disk and a thinner inner disk. In our own Milky Way, for example, the thick disk extends about 3,000 light-years above and below the galactic plane, while the thin disk is roughly 1,000 light-years thick. The visible component of the galaxies is usually made up of three principal segments:
- The disk, where the spiral arms and most of the gas and dust are concentrated
- The halo, a rough and sparse sphere around the disk that contains little gas, dust, or star formation
- The central bulge at the heart of the disk, formed by a large concentration of ancient stars surrounding the Galactic Centre.
Yet, scattered across the universe are galaxies that defy this structure, known as superthin galaxies—incredibly flat systems that seem to have resisted puffing up over billions of years—leading to the question: How have these cosmic blades managed to stay so impossibly slim?
A new study, conducted by researchers K Aditya of the Raman Research Institute and Arunima Banerjee of the Indian Institute of Science Education and Research (IISER), has uncovered why these galaxies remain incredibly flat and thin. Their research, combining state-of-the-art observations and theoretical modelling, explores why these galaxies—technically known as low-surface-brightness (LSB) ultra-thin galaxies—are faint, spread out, and are often difficult to detect.
The hidden world of superthin galaxies
The team focused on FGC 2366, one of the ultra-thin galaxies in their sample. Due to its edge-on orientation, features like spiral arms or bars are obscured, prompting the researchers to construct a detailed model of the galaxy. They simulated its stellar evolution over five gigayears using a supercomputer. When the model incorporated a higher stellar mass fraction, it produced a rectangular, bar-like structure—suggesting that FGC 2366’s low surface brightness plays a key role in preserving its remarkably thin stellar disk.
“It serves as a typical example of this class of dark matter–dominated, superthin galaxies,” Arunima Banerjee said. "To ensure precision, the team deliberately selected one of the thinnest galaxies available for their study."
“Using HI 21-cm observations with the Giant Metrewave Radio Telescope (GMRT) near Pune, facilities for radio astronomy, and stellar photometry from the Palomar sky survey, we mapped the overall structure and distribution of neutral hydrogen gas and stars within these galaxies,” said K Aditya.
Photometry measures the light emitted by stars—that is, the photons from stellar populations—while the HI 21-cm line traces atomic hydrogen gas, a key component of galaxies. Since all galaxies are primarily composed of stars and gas, these two serve as diagnostic tracers of the underlying gravitational potential. Although dark matter itself cannot be observed directly, it shapes this potential, influencing the motion of stars and gas. Dark matter affects the shape of these galaxies, and they model galaxies with different types of dark matter halos (the massive invisible component that holds a galaxy together).
The study’s most surprising findings
Researchers found that no matter the type of dark matter halo, if the galaxy started out thin, it stayed thin. They also discovered that these galaxies don't grow thick over time because they don't form strong central bars—elongated features that usually stir things up in other galaxies and make them puffier. But when the team artificially increased the density of stars in the galaxy, the bar did form and the galaxy thickened—but only if the dark matter halo was less concentrated.
Bars refer to coherent, elongated stellar structures that maintain their shape over long periods. These bars scatter nearby stars, disturbing their orbits and increasing their velocity dispersion, which causes the galactic disk to thicken over time.
“In galaxies with bars, this scattering makes the disk puff up,” Banerjee explained. “But in superthin galaxies, the deep dark matter potential keeps the disk stable and prevents bar formation. Without bars, there’s no scattering, so the disk stays thin.”
In a conversation with ETV Bharat, Arunima Banerjee and K Aditya discussed ultrathin galaxies, their formation, and other aspects of their study.
Why study superthin galaxies
For Banerjee, these galaxies serve as natural laboratories to test fundamental cosmological models. “Superthin galaxies are excellent test beds for studying the influence of dark matter,” she said. “They contain a very small proportion of visible matter compared to their dark matter content. This makes them ideal for examining predictions of the ΛCDM (Lambda Cold Dark Matter) model—the most widely accepted framework explaining the universe’s large-scale structure and evolution.”
Despite ΛCDM’s success, most large-scale cosmological simulations struggle to produce ultra-thin disk galaxies like those observed. “We know a few thousand of such flat systems in the observable universe, yet current ΛCDM-based simulations rarely generate disks this flat,” Banerjee noted. “Incorporating detailed baryonic physics—the behaviour of ordinary matter—within ΛCDM may help resolve this inconsistency.”
Challenges
Aditya explained that studying superthin, faint, low-surface-brightness galaxies is challenging because they require long observations to capture complete photometric data. Measuring the dark matter content requires mapping the gas distribution at multiple spatial points across each galaxy. It requires deep, extended observations to track how gas and stars move under the influence of dark matter.
Banerjee noted that advances in modern observational facilities now allow astronomers to study them in much greater detail. “Observing these galaxies remains both challenging and expensive,” she said.
Decoding the cosmic flatness: The role of PCA and angular momentum
The study uses principal component analysis (PCA) to examine which of these factors—dark matter dominance, disk stability, or combinations thereof—most significantly contribute to the remarkably thin structure of these galaxies.
Banerjee explained that PCA, a commonly used machine learning technique, helps identify which physical parameters—or combinations of them—most strongly affect a galaxy’s thin structure. When galaxies face disturbances, their disks usually thicken. But in dark matter–dominated galaxies, the strong dark matter gravity keeps the disk stable and prevents it from puffing up. This stabilising factor is captured by a property called compactness.
Illustrating the role of angular momentum by comparing two rotating disks of the same mass—one spinning faster and the other slower—she said that the faster-spinning disk spreads out more radially, making its vertical thickness smaller relative to its radial extent. A similar phenomenon occurs in superthin galaxies, where high specific angular momentum causes the disk to extend outward, resulting in low surface brightness and a small vertical scale height.
Additionally, she noted that velocity dispersion—the random motion of stars and gas—is another crucial factor. If the vertical velocity dispersion is large, the disk becomes thicker. Superthin galaxies likely have lower vertical velocity dispersion compared to other galaxies of similar mass. Simulations have shown that thin galaxies tend to remain thin regardless of the nature of their dark matter halos.
Superthin galaxies as 'under-evolved'
Banerjee described superthin galaxies as “under-evolved” galaxies in terms of their star formation history and chemical evolution, as they have all the ingredients needed for growth but have evolved slowly and incompletely.
Aditya explained that galaxy evolution typically begins with a dark matter halo into which gas accumulates. Over time, this gas undergoes instabilities, leading to the formation of a stellar disk. “In the case of underdeveloped or superthin galaxies, much of the gas within the dark matter halo has not yet been converted into stars,” he said. “As a result, these galaxies exhibit low surface brightness, indicating that they contain relatively few stars compared to typical spiral galaxies.”
Lonely and unperturbed
When asked whether these galaxies are expected to remain thin indefinitely or if future events—such as mergers or bar formation—could alter their structure, Banerjee explained that another reason for their thinness lies in their isolation. Most superthin galaxies exist in low-density field environments, far from bustling galactic clusters where interactions are frequent.
“Since they reside in sparse regions, the likelihood of major mergers is very low,” said Banerjee. “If a merger were to occur, the disk would thicken. But because these galaxies are dark matter–dominated and live in quiet neighbourhoods, they can remain thin for long periods—certainly not puffing up within one or two dynamical times, or full rotations.”
The Milky Way connection
When asked whether studying superthin galaxies could shed light on the formation of the Milky Way’s thin and thick discs, Aditya described that ESA's Gaia Satellite measures the distribution of stars, and through photometric data, one can observe that the Milky Way has both a thin and a thick disc component.
The thin disc stars are relatively scattered and fewer in number, while the thick disc stars are more widely distributed and typically belong to an older stellar population. In contrast, when we observe ultra-thin galaxies in the optical waveband, there is no clear distinction between the thick and thin disc components—all the stars appear confined to a single, flat plane. It shows a thick central component, and it may not be the thick disk.
Next step: Observations and simulations
The researchers emphasise the need to study a larger sample of superthin galaxies to confirm their findings. Detecting bars—especially in edge-on galaxies—is particularly challenging. Banerjee pointed out that HI gas observations could reveal a characteristic “figure-eight” pattern in position–velocity diagrams, indicating the presence of a central bar. “Such observational confirmation from a large sample of super-thins is crucial to validate theory,” she said.
Future studies using powerful instruments like the James Webb Space Telescope (JWST) and next-generation radio arrays could provide even deeper insights into how dark matter and baryonic physics shape galactic evolution.
For Banerjee, the field offers both scientific and educational promise. “This field is evolving quickly and bridges analytical theory with real galaxies, making learning more engaging for students,” Banerjee said. “Superthin galaxies aren’t just cosmic curiosities—they’re valuable case studies in galactic dynamics.”
Aditya added, “JWST is raising new questions. The persistence of these ultra-thin galaxies over billions of years remains a profound cosmic mystery.”
