Free Astronomy Magazine May-June 2025

18 MAY-JUNE 2025 ASTRO PUBLISHING times the mass of our Sun). The black holes in between the two ex- tremes are poorly understood, but are theorized to be the relics of the very first black holes formed in the early Universe, and the seeds of the supermassive black holes that lie at the center of large galaxies today. Yet they remain elusive, with only around 100–150 intermediate-mass black hole candidates known until now. With the large population dis- covered by DESI, scientists now have a powerful new dataset to use to study these cosmic enigmas. “The technological design of DESI was important for this project, par- ticularly its small fiber size, which al- lowed us to better zoom in on the center of galaxies and identify the subtle signatures of active black holes,” says Stephanie Juneau, asso- ciate astronomer at NSF NOIRLab and co-author of the paper. “With other fiber spectrographs with larger fibers, more starlight from the galaxy’s outskirts comes in and di- lutes the signals we’re searching for. This explains why we managed to find a higher fraction of active black holes in this work relative to previ- ous efforts.” Typically, black holes found in dwarf galaxies are expected to be within the intermediate-mass regime. But intriguingly, only 70 of the newly discovered intermediate-mass black hole candidates overlap with dwarf AGN candidates. This adds another layer of excitement to the findings and raises questions about black hole formation and evolution within galaxies. “For example, is there any relation- ship between the mechanisms of black hole formation and the types of galaxies they inhabit?” Pucha said. “Our wealth of new candidates will help us delve deeper into these mysteries, enriching our understand- ing of black holes and their pivotal role in galaxy evolution.” T his mosaic shows a series of images featuring intermediate-mass black hole candidates, arranged in increasing order of stellar mass, captured with the Sub- aru Telescope’s Hyper Suprime-Cam. [Legacy Surveys/D. Lang (Perimeter Institute)/ NAOJ/HSC Collaboration/D. de Martin (NSF NOIRLab) & M. Zamani (NSF NOIRLab)] T his mosaic shows a series of images featuring candidate dwarf galaxies host- ing an active galactic nucleus, captured with the Subaru Telescope’s Hyper Suprime-Cam. [Legacy Surveys/D. Lang (Perimeter Institute)/NAOJ/HSC Collabo- ration/D. de Martin (NSF NOIRLab) & M. Zamani (NSF NOIRLab)] nucleus,” says Pucha. “This dramatic activity serves as a beacon, allowing us to identify hidden black holes in these small galaxies.” From their search the team identi- fied an astonishing 2500 candidate dwarf galaxies hosting an active galactic nucleus (AGN) — the largest sample ever discovered. The sig- nificantly higher fraction of dwarf galaxies hosting an AGN (2%) rela- tive to previous studies (about 0.5%) is an exciting result and suggests scientists have been missing a sub- stantial number of low-mass, undis- covered black holes. In a separate search through the DESI data, the team identified 300 intermediate-mass black hole candi- dates — the most extensive collec- tion to date. Most black holes are either lightweight (less than 100 times the mass of our Sun) or su- permassive (more than one million !