“Little red dots are a hot topic in astronomy. You can compare studies of people trying to understand LRDs to the tale of blind people who are trying to describe an elephant by touching it, and each person describes it differently based on what part they’re touching,” Smadar Naoz, an astrophysicist at the University of California, Los Angeles, said in a study published October 26 in the Astrophysical Journal.
Crowded star clusters in the early Universe could forge giant stars, seed black holes, and feed mysterious little red dots, a team of astrophysicists led by University of California, Los Angeles doctoral student Claire Williams reported. The research builds a radially resolved analytic model of stellar collisions in dense, high-redshift systems, utilizing initial conditions drawn from high-resolution cosmological simulations rather than starting directly from observations.
Modeling Stellar Collisions in Dense Early Clusters
To understand how these dense environments evolve, the authors built their model and checked predictions against Monte Carlo simulations, finding good agreement across key regimes. The study encompasses the ancestors of globular clusters, nuclear star clusters like the one surrounding the Milky Way’s central black hole, and compact, reddish little red dots, known in astronomy as LRDs. The researchers developed a bottom-up framework for stellar dynamics spanning systems with and without intermediate-mass and supermassive black holes, initializing 1,000 stars for each test case within one parsec of the center.
In clusters lacking a black hole, collisions tend to be gentle mergers rather than destructive smash-ups. Merger products sink toward the center through dynamical friction, where repeated collisions build very massive stars. For clusters exceeding roughly 100,000 solar masses per cubic parsec, nearly all tested low-mass systems formed a very massive star heavier than 200 suns. In the densest cases, stars grew past 100,000 solar masses and could collapse into intermediate-mass or supermassive black holes.
Simulations Show Merging Stars Form Massive Black Holes
“Perhaps the strangest outcome of our simulations is that stars can merge repeatedly within the cluster until they form one gigantic star with a mass equal to that of an entire cluster,” Claire Williams said. When a star this massive reaches the end of its life, models suggest it has no option but to collapse into itself and form a massive black hole.
The study offers a potential solution to an ongoing scientific wrestling match regarding observations from the NASA/ESA/CSA James Webb Space Telescope. The telescope has detected supermassive black holes existing 400 million and 500 million years after the Big Bang, long before standard theory says they should. While the idea of stellar collisions driving early supermassive black holes predates the space telescope by decades, this research connects those mechanisms directly to the reddish objects discovered by JWST.
Destructive Collisions Fueling Rapid Black Hole Growth
The dynamic changes significantly when a massive black hole is already present. Its gravity accelerates stars in the inner regions so violently that collisions become destructive, ripping stars apart and spilling material into surrounding space. This process piles up dense gas around the black hole, matching or exceeding densities inferred for LRDs and potentially supplying fuel for rapid growth.
“We show that high rates of destructive collisions can rapidly build up extremely dense gaseous environments around massive black holes, potentially providing an analog to the observed population of LRDs,” the researchers concluded. If these black holes are growing quickly due to super-Eddington accretion, plentiful stellar collisions could generate the surrounding gas that acts as fuel while masking X-rays.
Frequently Asked Questions About Early Universe Stellar Dynamics
What specific simulations supplied the researchers’ initial conditions?
The research team drew initial conditions from the AREPO cosmological simulations rather than starting from observational data.
How many stars were initialized in each test case for the simulations?
The simulations initialized 1,000 stars for each test case, located within one parsec of the center.
When was the underlying study published?
The study was published on October 26 in the Astrophysical Journal under volume 1012.