Artistic representation of the Milky
Way, where the innermost stars move at near relativistic speeds (defined as
velocities that constitute a significant fraction of the speed of light,
typically considered to be 10% or more) around a dense core of dark matter,
with no black hole at the centre. At greater distances, the halo part of the
same invisible dark matter distribution continues to shape the motions of stars
in the outskirts of our galaxy, tracing the characteristic rotation curve.
Credit: Valentina Crespi et al. License type Attribution (CC BY 4.0)
Our Milky Way galaxy may not have a
supermassive black hole at its center but rather an enormous clump of
mysterious dark matter exerting the same gravitational influence, astronomers
say. They believe this invisible substance—which makes up most of the universe's
mass—can explain both the violent dance of stars just light-hours (often used
to measure distances within our own solar system) away from the galactic center
and the gentle, large-scale rotation of the entire matter in the outskirts of
the Milky Way.
The new study has been published
today in Monthly Notices of the Royal
Astronomical Society.
Rethinking the Milky Way's dark heart
It challenges the leading theory
that Sagittarius A* (Sgr A*), a proposed black hole at the heart of our galaxy,
is responsible for the observed orbits of a group of stars, known as the
S-stars, which whip around at tremendous speeds of up to a few thousand
kilometers per second.
The international team of
researchers have instead put forward an alternative idea—that a specific type
of dark matter made up of fermions, or light subatomic particles, can create a
unique cosmic structure that also fits with what we know about the Milky Way's
core.
It would in theory produce a
super-dense, compact core surrounded by a vast, diffuse halo, which together
would act as a single, unified entity.
The inner core would be so compact
and massive that it could mimic the gravitational pull of a black hole and
explain the orbits of S-stars that have been observed in previous studies, as
well as the orbits of the dust-shrouded objects known as G-sources which also
exist nearby.
Gaia's detailed map of the outer halo
Of particular importance to the new
research is the latest data from the European Space Agency's Gaia DR3 mission, which has meticulously mapped the
rotation curve of the Milky Way's outer halo, showing how stars and gas orbit
far from the center.
It observed a slowdown of our
galaxy's rotation curve, known as the Keplerian decline, which the researchers say can be explained by their
dark matter model's outer halo when combined with the traditional disk and
bulge mass components of ordinary matter.
This, they add, strengthens the
"fermionic" model by highlighting a key structural difference. While
traditional cold dark matter halos spread out following an extended "power
law" tail, the fermionic model predicts a tighter structure, leading to
more compact halo tails.
The research has been carried out
by an international collaboration involving the Institute of Astrophysics La
Plata in Argentina, International Centre for Relativistic Astrophysics Network
and National Institute for Astrophysics in Italy, Relativity and Gravitation
Research Group in Colombia and Institute of Physics University of Cologne in
Germany.
"This is the first time a dark
matter model has successfully bridged these vastly different scales and various
object orbits, including modern rotation curve and central stars data,"
said study co-author Dr. Carlos Argüelles, of the Institute of Astrophysics La
Plata.
"We are not just replacing the
black hole with a dark object; we are proposing that the supermassive central
object and the galaxy's dark matter halo are two manifestations of the same,
continuous substance."
Mimicking a black hole's shadow
Crucially, this fermionic dark
matter model had already passed a significant test. A previous study by Pelle and team, also published in MNRAS, showed that
when an accretion disk illuminates these dense dark matter cores, they cast a
shadow-like feature strikingly similar to the one imaged by the Event Horizon
Telescope (EHT) collaboration for Sgr A*.
"This is a pivotal
point," said lead author Valentina Crespi, of the Institute of
Astrophysics La Plata.
"Our model not only explains
the orbits of stars and the galaxy's rotation but is also consistent with the
famous 'black hole shadow' image. The dense dark matter core can mimic the
shadow because it bends light so strongly, creating a central darkness
surrounded by a bright ring."
Testing the new dark matter scenario
The researchers statistically
compared their fermionic dark matter model to the traditional black hole model.
They found that while current data
for the inner stars cannot yet decisively distinguish between the two
scenarios, the dark matter model provides a unified framework that explains the
galactic center (central stars and shadow), and the galaxy at large.
The new study paves the way
for future observations. More precise data from instruments such as the
GRAVITY interferometer, on the Very Large Telescope in Chile, and the search
for the unique signature of photon rings—a key feature of black holes and
absent in the dark matter core scenario—will be crucial to test the predictions
of this new model, the authors say.
The outcome of these findings could potentially reshape our understanding of the fundamental nature of the cosmic behemoth at the heart of the Milky Way.
Provided by Royal
Astronomical Society
Source: Dark matter, not a black hole, could power Milky Way's heart
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