Black holes with masses equivalent to millions of suns do put a brake on the
birth of new stars, say astronomers. Using machine learning and three
state-of-the-art simulations to back up results from a large sky survey, the
researchers resolve a 20-year long debate on the formation of stars. Joanna
Piotrowska, a PhD student at the University of Cambridge, presented the new
work on July 20, 2021, at the virtual National Astronomy Meeting (NAM 2021).
Star formation in galaxies has long been a focal point of astronomy
research. Decades of successful observations and theoretical modeling
resulted in our good understanding of how gas collapses to form new stars
both in and beyond our own Milky Way. However, thanks to all-sky observing
programs like the Sloan Digital Sky Survey (SDSS), astronomers realized that
not all galaxies in the local Universe are actively star-forming — there
exists an abundant population of “quiescent” objects which form stars at
significantly lower rates.
The question of what stops star formation in galaxies remains the biggest
unknown in our understanding of galaxy evolution, debated over the past 20
years. Piotrowska and her team set up an experiment to find out what might
be responsible.
Using three state-of-the-art cosmological simulations — EAGLE, Illustris,
and IllustrisTNG — the astronomers investigated what we would expect to see
in the real Universe as observed by the SDSS, when different physical
processes were halting star formation in massive galaxies.
The astronomers applied a machine learning algorithm to classify galaxies
into star-forming and quiescent, asking which of three parameters: the mass
of the supermassive black holes found at the center of galaxies (these
monster objects have typically millions or even billions of times the mass
of our Sun), the total mass of stars in the galaxy, or the mass of the dark
matter halo around galaxies, best predicts how galaxies turn out.
These parameters then enabled the team to work out which physical process:
energy injection by supermassive black holes, supernova explosions or shock
heating of gas in massive halos is responsible for forcing galaxies into
semi-retirement.
The new simulations predict the supermassive black hole mass as the most
important factor in putting the brakes on star formation. Crucially, the
simulation results match observations of the local Universe, adding weight
to the researchers’ findings.
Piotrowska says: “It’s really exciting to see how the simulations predict
exactly what we see in the real Universe. Supermassive black holes — objects
with masses equivalent to millions or even billions of Suns — really do have
a big effect on their surroundings. These monster objects force their host
galaxies into a kind of semi-retirement from star formation.”
Meeting: Royal Astronomical Society National Astronomy Meeting
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Space & Astrophysics