Out in the dark depths of space, our models of the Universe get messy. A new
study looking at the ultra-diffuse dwarf galaxy AGC 114905 has revived a
controversial theory (or more accurately a hypothesis) of gravity, and given
us more questions than answers about what's making our galaxies tick.
It all starts with dark matter – or in this case, no dark matter. Although
most cosmologists agree there's something out there called 'dark matter',
causing spiral galaxies to rotate faster than they should, even dark matter
doesn't answer all the questions we need it to.
So, it's not a bad idea to look at some alternative options. You know, just
in case we are never able to find the stuff.
One alternative hypothesis to dark matter is called Modified Newtonian
dynamics (MOND) or Milgromian dynamics framework. This hypothesis – first
published in 1983 by physicist Mordehai Milgrom – suggests that we don't
need dark matter to fill in the Universe's gravity gaps, if we calculate the
gravitational forces experienced by stars in outer galactic regions in a
different manner to how Newtonian laws suggest.
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| Radio image of the neutral hydrogen gas in the galaxy AGC 114905. (Mancera Pina et al., MNRAS, 2021) |
To test this idea, which involves working with proportionality to the star's
radius or centripetal acceleration, we need to be looking at the speeds of
galaxies – specifically weird ones like ultra-diffuse galaxies.
These very faint, ugly ducklings of the galaxy world have a habit of not
acting like a galaxy should. For example, some ultra diffuse galaxies seem
to be made almost entirely of dark matter, whilst others are almost
completely dark matter-less.
This is where AGC 114905 comes in. This ultra-diffuse dwarf galaxy around
250 million light years away had recently been looked at in detail in a
paper published in 2021 investigating how fast it spins.
But this team found that the galaxy's spin was extremely slow – slow enough
that not only did they not need dark matter to confirm the models, but the
rotation curve of the galaxy also cast huge doubt on the MOND framework. It
doesn't fit with either hypothesis.
"The very low reported rotation speed of this galaxy is inconsistent with
both MOND and the standard approach with dark matter," says University of St
Andrews physicist and one of the researchers of the new paper, Hongsheng
Zhao.
"But only MOND is able to get around this apparent contradiction."
The new paper has 'un-debunked' the 2021 finding, suggesting that the issue
isn't with MOND, but instead with the inclination of the galaxy itself.
When we look at galaxies far away in the depths of space, it can sometimes
be hard to confirm which angle we're seeing. The original team found that
AGC 114905 looked elliptical, suggesting that we're looking at the galaxy
from an angle.
But using simulations, researchers now suggest the galaxy could appear
elliptical even when it's facing us straight on. A change in the angle of
the galaxy to us would also change how fast the galaxy is rotating, making
all the MOND math add up after all.
"Our simulations show that the inclination of AGC 114905 might be
significantly less than reported, which would mean the galaxy is actually
rotating much faster than people think, in line with MOND expectations,"
says lead author of the new paper, physicist Indranil Banik, also from the
University of St Andrews.
Now, this is still an open question. We don't know whether this new paper,
or the 2021 paper is going to be crowned victorious – or at least most
correct.
In the meantime, if this new finding holds, it seems that the MOND framework
might live on for another day. As wild as MOND might be, with dark matter
still elusive, and many other questions still to be answered, we need all
the options we can get.
The research has been published in the Monthly Notices of the Royal
Astronomical Society.
References:
Indranil Banik et al, Overestimated inclinations of Milgromian disc
galaxies: the case of the ultradiffuse galaxy AGC 114905, Monthly Notices of
the Royal Astronomical Society (2022).
DOI: 10.1093/mnras/stac1073
Pavel E Mancera Piña et al, No need for dark matter: resolved kinematics of
the ultra-diffuse galaxy AGC 114905, Monthly Notices of the Royal
Astronomical Society (2021).
DOI: 10.1093/mnras/stab3491
Tags:
Physics