Andromeda’s young star clusters show signs of rapid early expansion

Measuring clusters by their light

Karolis Daugevičius and colleagues analysed 1,918 clusters younger than 800 million years in the north-eastern disc of Andromeda, also called M31. Their sample spans masses of roughly 300 to 30,000 Suns, extending the investigation to relatively low-mass clusters that are harder to study in more distant galaxies.

The researchers used a catalogue based on the Panchromatic Hubble Andromeda Treasury survey. Cluster ages, masses and dust obscuration were estimated using stellar-population models and measurements of stars’ brightnesses and colours. These estimates assumed solar metallicity: the Sun’s abundance of elements heavier than helium.

Their main size measure was the half-light radius, the projected radius enclosing half a cluster’s measured light. Across the full sample, its median was about 1.8 parsecs. This measures the distribution of brightness, not directly the distribution of stellar mass or a cluster’s outer boundary.

A fast start, then slower growth

Not every apparent stellar grouping is held together by gravity. Some are loose associations whose members will disperse. To reduce this contamination, the team compared each object’s age with its estimated crossing time—the characteristic time for stars to travel across the system.

Objects older than one crossing time were treated as likely bound clusters, leaving 1,815 objects. Within this selection, the median half-light radius rose from approximately 0.6 to 1.4 parsecs during the first roughly 20 million years.

After that, growth was much slower. Fitting median sizes in age bins, the researchers found that radius scaled with age to the power 0.12, with a formal uncertainty of 0.01. Typical radii reached around two parsecs over the following approximately 700 million years.

These are population comparisons, not repeated measurements of the same clusters over millions of years. Interpreting them as an evolutionary sequence therefore depends on how representative the different age groups are.

What could make clusters expand?

The authors favour a sequence beginning with the removal of leftover star-forming gas. Subsequent mass loss from evolving stars, together with gravitational interactions among stars, could weaken or redistribute the cluster’s gravitational binding and allow it to expand.

They compared the observations with previously published simulations that include gas dynamics, stellar evolution and gravitational encounters. Those models show a similarly rapid early expansion followed by slower growth.

The agreement is qualitative, however. The simulated clusters were less massive and formed in a dwarf galaxy with a much lower heavy-element abundance than assumed for the Andromeda sample. Their resemblance supports a possible explanation; it does not establish which process dominates in M31.

The team also found no significant relationship between cluster size and distance from Andromeda’s centre in the region examined. That is consistent with internal processes playing the larger early role, but does not demonstrate that galactic environmental effects are absent.

The selection problem

The strongest caveat concerns the youngest objects. The likely-bound selection itself excludes many young, extended groupings, potentially creating an apparent increase in size with age. The criterion is also less reliable below about 10 million years. Without it, the full sample shows no clear overall growth trend.

The authors nevertheless identify supporting evidence: clusters younger than 30 million years have a size-distribution peak near one parsec, versus about 1.7 parsecs for older clusters. That younger distribution remains broadly similar without the bound-cluster cut. They also argue that the absence of very compact objects at later ages is unlikely to be entirely a detection effect.

Other uncertainties remain. Older, low-mass clusters become harder to detect, potentially tilting age–size trends towards larger systems. Chance contributions from bright stars can affect light-based structural measurements.

Mass matters, modestly

For clusters older than 100 million years, radius scaled with mass to the power 0.18, with an estimated uncertainty of 0.01. More massive clusters were therefore somewhat larger, but the relationship was shallow. For younger clusters, selection effects and rapid structural changes make the relationship less secure.

The study is available as an arXiv preprint; the supplied manuscript also records acceptance by Astronomy & Astrophysics. Its central contribution is evidence for rapid early structural change in low-mass Andromeda clusters—not a definitive measurement of individual expansion histories or proof of a single driving mechanism.