
Researchers solve mystery of the galaxy with no dark matter - curtis
https://phys.org/news/2019-06-mystery-galaxy-dark.html
======
privong
It's worth nothing that the authors of the original NGC1052-DF4 article
claiming little to no dark matter have disputed the conclusions of the
Trujillo+2019 (i.e., the paper the phys.org article is based on):
[https://ui.adsabs.harvard.edu/abs/2019RNAAS...3b..29V/abstra...](https://ui.adsabs.harvard.edu/abs/2019RNAAS...3b..29V/abstract)

I think it's safe to say that the jury is still out on this one.

Edit: One reason this is difficult is that you can think of galaxies as having
two major contributions to the line-of sight velocity we observe from earth:
expansion of the Universe ("Hubble flow") and "peculiar" velocity (motion due
to the local gravitational field in which the galaxy lives). The latter
velocities are typically a few hundred km/s (or later if the galaxy is in a
cluster). NGC 1052 has an observed recession velocity of 1510 km/s
([https://ned.ipac.caltech.edu/byname?objname=NGC%201052&hcons...](https://ned.ipac.caltech.edu/byname?objname=NGC%201052&hconst=67.8&omegam=0.308&omegav=0.692&wmap=4&corr_z=1)).
Assuming a peculiar velocity of 300 km/s, that means the peculiar velocity is
on the order of 20% of the total velocity. But we don't know what component of
the peculiar velocity is along the line of sight or whether it's positive or
negative. So with these numbers it's "hubble flow" velocity could be from 1200
km/s to 1800 km/s. If you naive plug those two recession velocities into a
cosmology model (e.g., the latest model derived from Planck observations) the
distance range you get is 17.3-26.1 Mpc, or a ~40% uncertainty. So we cannot
rely just on the Universe's measured expansion to get a sufficiently reliable
distance estimate.

So one must use other techniques to measure distances. They each have their
own advantages and disadvantages. In general most distance estimates are build
on the "Cosmic distance ladder" and accumulate uncertainty as you move up the
ladder:
[https://en.wikipedia.org/wiki/Cosmic_distance_ladder](https://en.wikipedia.org/wiki/Cosmic_distance_ladder)

~~~
_Codemonkeyism
What I don't understand here is: You have a galaxy where mass doesn't add up.
There are rivaling explanations, one being that you estimated the distance
wrong, the other that there is no Dark Matter. Isn't the first one the more
likely one you should investigate?

~~~
privong
> There are rivaling explanations, one being that you estimated the distance
> wrong, the other that there is no Dark Matter. Isn't the first one the more
> likely one you should investigate?

If you look back at the original paper claiming the discovery of the dark
matter free galaxy, they do explore several different ways of estimating the
distance and argue that they're consistent and imply the more distant value.
So that's something they had considered and attempted to cross-check between
different distance indicators. Later came the Trujillo paper saying
essentially, hey you didn't consider "X" which argues that the galaxy is
closer. The authors of the original paper then published a Research Note (that
I'd liked in my original post) saying that "X" doesn't agree with the other
data.

So people did consider distance errors before claiming the absence of dark
matter. But getting accurate galaxy distances Is Hard and there are multiple
ways to do it. The original paper considered the distance estimates they could
think of and they were essentially consistent. So they did investigate that
first.

------
sriku
A naive question (not read the actual paper or know much about the field) - if
requiring dark matter to explain galactic physics can be dispensed with by a
distance correction, doesn't "we've been underestimating distances all this
time" serve as a simpler fact of galactic physics than needing "there is
mysterious dark matter everywhere" to explain galactic physics?

~~~
idlewords
There are several different features of galaxies that dark matter explains.
One of them is the speed with which stars orbit the center of a galaxy—this
"rotation curve" is independent of how far the stars are from the galactic
center, where if there were no dark matter, you would expect the outermost
stars to go slower. That is an effect that is not affected by errors in
measuring the distance to remote galaxies. We can see it in our own galaxy!
[https://www.e-education.psu.edu/astro801/content/l8_p8.html](https://www.e-education.psu.edu/astro801/content/l8_p8.html)

In general, if you can think of an explanation that does away with the need
for dark matter, you can also assume experts have thought hard about it and
rejected it for good reason. Nobody likes dark matter as a hypothesis,
everyone remembers analogous complications in the history of science that
proved to be signposts to a better theory, and everyone knows that a Nobel
prize waits for anyone who can find the correct explanation.

~~~
trhway
>One of them is the speed with which stars orbit the center of a galaxy

do we really orbit it? I mean is it a stable orbit (that is the option which
requires DM ) or a just a spiral trajectory (no DM required)? The spiral
galaxies look like a fireworks wheel
[https://depositphotos.com/69366109/stock-video-fireworks-
pin...](https://depositphotos.com/69366109/stock-video-fireworks-pinwheel-
catherine-wheel.html) , and without DM our "orbital" speed being higher than
otherwise expected Keplerian orbital speed means that we're just moving away
on a spiral trajectory from the center the same way like the sparks fly away
on a spiral trajectory from the center of the fireworks wheel.

~~~
kryptiskt
If they are flying away, something would need to add kinetic energy to the
stars over time. Also, galaxies should then start small and disperse as they
age.

~~~
HyperTalk2
Would the spinning motion of the galaxy affect us any differently than the way
the rotation of a planet affects a spacecraft during a gravity assistance
boost maneuver?

~~~
ncmncm
You mean, not?

It's not the rotation that assists, it's the planet's linear -- well, bent,
but that doesn't help -- that does it.

There is this notion of "frame dragging" in general relativity, but its effect
is practically impossible to measure. They lofted a satellite to measure its
effect over several years because it took that long to accumulate enough to
measure.

------
whatshisface
If MOND was dealt a near-lethal blow with the discovery of a galaxy that had
no dark matter (implying that dark matter was a thing you could put in and
take away, not a universal law like MOND), now that the anomalous galaxy is
gone, does that mean MOND is back?

~~~
cwmma
MOND has other issues, not least of which it _only_ explains galaxy rotational
curves, it doesn't explain the larger structures of the universe (which dark
matter does explain)

~~~
ncmncm
Large-scale structure has been attributed to plasma-dynamic effects. It's not
a popular hypothesis, apparently in large part because plasma dynamics is
hard. So plasma is generally agreed to be there (to the exclusion of
everything else, really -- except, y'know, dark matter), but is not allowed to
(be said to) do anything dynamical and hard to model.

It will take a future generation of plasma-dynamics-loving astrophysicists to
change the ... er, dynamic. Presuming we live that long.

------
8bitsrule
_Using five independent methods to estimate the distance of the object, they
found that all of them coincided in one conclusion..._

I wish they had listed the five methods, I can't think of that many ... (?!)

~~~
T-A
[https://arxiv.org/abs/1806.10141](https://arxiv.org/abs/1806.10141)

------
cobbzilla
Weren’t these all sparse elliptic galaxies? Didn't they all lack a SMB core?

Is it possible they are whisps of parts of galaxies thrown off in a galactic
collision? Like two spiral arms synchronizing and flying off, then coalescing
far from the dark matter, which consolidates in the merged galaxy. These
degenerate galaxies might then understandably have little/no dark matter of
their own.

~~~
ncmncm
That's called a "just so" story. Scientists are not gentle about those.

It's all, "show me the math or GTFO!". Understandable, really.

~~~
cobbzilla
christ it was an honest/curious question. i luv HN, really

~~~
ncmncm
It was a valid question. But the details matter too much for it to have an
answer.

------
p1necone
I want to know more about how it was measured incorrectly the first time.

~~~
pdonis
This paper is not based on new measurements. It's based on a new analysis of
existing data.

~~~
thedudeabides5
looks like they overshot by around 50%

Anyone able to explain what the new analysis was that led that existing data
to be interpreted so differently?

