Using early data from the Dark Energy Spectroscopic Instrument ( DESI ), a team of scientists - including experts from the University of Portsmouth - have compiled the largest sample ever of dwarf galaxies that host an actively feeding black hole, as well as the most extensive collection of intermediate-mass black hole candidates to date.
This dual achievement not only expands scientists' understanding of the black hole population in the Universe, but also sets the stage for further explorations regarding the formation of the first black holes to form in the Universe and their role in galaxy evolution.
These large statistical samples will allow for more in-depth studies of the dynamics between dwarf galaxy evolution and black hole growth, and open up vast discovery potential surrounding the evolution of the Universe's earliest black holes.
DESI is a state-of-the-art instrument that can capture light from 5000 galaxies simultaneously. The DESI project is an international collaboration of more than 900 researchers from over 70 institutions around the world and is managed by DOE's Lawrence Berkeley National Laboratory ( Berkeley Lab ).
Rahma Alfarsy, from the University of Portsmouth's Institute of Cosmology and Gravitation , said: "In Portsmouth, we have a team working on the physics of galaxies and active galactic nuclei using DESI data. The findings from this project illustrate DESI's potential to push the limits of the known Universe.
"Having witnessed the development and outcomes of this brilliant work, we are really excited to see what else will be unearthed from DESI's treasure trove of extra-galactic observations."
With DESI's early data, which include survey validation and 20 per cent of the first year of operations, the team, led by University of Utah postdoctoral researcher Ragadeepika Pucha, was able to obtain an unprecedented dataset that includes the spectra of 410,000 galaxies, including roughly 115,000 dwarf galaxies.
These dwarf galaxies are small, diffuse galaxies containing thousands to several billions of stars and very little gas. This extensive set would allow Pucha and her team to explore the complex interplay between black hole evolution and dwarf galaxy evolution.
While astrophysicists are fairly confident that all massive galaxies, like our Milky Way, host black holes at their centres, the picture becomes unclear as you move toward the low-mass end of the spectrum.
Finding black holes is a challenge in itself, but identifying them in dwarf galaxies is even more difficult, owing to their small sizes and the limited ability of our current instruments to resolve the regions close to these objects. An actively feeding black hole, however, is easier to spot.
Pucha said: "When a black hole at the centre of a galaxy starts feeding, it unleashes a tremendous amount of energy into its surroundings, transforming into what we call an active galactic nucleus. This dramatic activity serves as a beacon, allowing us to identify hidden black holes in these small galaxies."
From their search the team identified an astonishing 2,500 candidate dwarf galaxies hosting an active galactic nucleus (AGN) - the largest sample ever discovered. The significantly higher fraction of dwarf galaxies hosting an AGN relative to previous studies is an exciting result and suggests scientists have been missing a substantial number of low-mass, undiscovered black holes.
In a separate search through the DESI data, the team identified 300 intermediate-mass black hole candidates - the most extensive collection to date. Most black holes are either lightweight (less than 100 times the mass of our Sun) or supermassive (more than one million times the mass of our Sun).
The black holes in between the two extremes are poorly understood, but are theorised 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 centre 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 discovered by DESI, scientists now have a powerful new dataset to use to study these cosmic enigmas.
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.
Pucha added: "For example, is there any relationship between the mechanisms of black hole formation and the types of galaxies they inhabit? Our wealth of new candidates will help us delve deeper into these mysteries, enriching our understanding of black holes and their pivotal role in galaxy evolution."
The DESI instrument was constructed, and is operated, with funding from the Department of Energy (DOE) Office of Science. DESI is mounted on the U.S. National Science Foundation (NSF) Nicholas U. Mayall 4-meter Telescope at the NSF Kitt Peak National Observatory , a programme of NSF NOIRLab.
The programme is now in its fourth of five years surveying the sky and is set to observe roughly 40 million galaxies and quasars by the time the project ends.
