Supermassive black-hole seeds

Telescopes find massive black holes in galaxies from the Universe’s first billion years. What were their starting points, and how did they grow so early? A small black hole left by a star could gain mass by swallowing gas; a much larger starting black hole would need less growth. The starting object is called a seed.[1] [2] [3]

Massive black holes are inferred in galaxies seen within the first billion years. Which seeds and growth histories built that population? Light seeds are remnants of stars; heavy-seed models begin with much larger objects. Accretion, the addition of surrounding gas, can partly conceal this difference in the final mass.

A light-seed simulation follows growth in dense gas. A separate heavy-seed simulation follows gas near a luminous neighbour: its radiation suppresses early star formation, letting gas accumulate before supermassive stars collapse. The latter model also reproduces properties of compact red sources called little red dots. These are possible histories, while observed masses and host properties still require inference.[1] [2] [3]

Their starting masses and growth routes remain uncertain.[1] [2] [3]

2026 studies demonstrate possible growth from light seeds and heavy-seed formation resembling little red dots. Neither match uniquely determines the observed population’s ancestry.[1] [3]

The two routes tell different stories about the first galaxies: whether stars formed first and left small remnants, or gas accumulated into a much larger starting object.[1] [3]

Seeds connect the birth of stars with the assembly of galaxies. Identifying their histories would constrain when gas became stars, when it fed black holes, and which environments supported rapid growth. Similar present masses can otherwise hide very different beginnings.[1] [2] [3]