Comment by burnerRhodov2
1 day ago
In this context, “dark object” really does mean a localized blob of dark matter, not a black hole or a dim, normal-matter object.
The research team detected it only through its gravitational lensing effect — the way it slightly distorted the light from a more distant galaxy. There’s no emission at any wavelength (optical, infrared, or radio), and its gravitational signature matches a million-solar-mass clump of invisible mass rather than a compact point source like a black hole.
They specifically interpret it as a dark matter subhalo — one of the small, dense lumps that simulations of “cold dark matter” predict should pepper the universe’s larger halos. It’s too massive to be a single star, far too diffuse to be a stellar remnant, and not luminous enough to be a faint galaxy.
So “dark” here isn’t just shorthand for “too dim to see at this distance” — it’s used in the literal physical sense: matter that doesn’t emit or absorb light at all, detectable only via gravity.
Eventually, all the dark matter clumps into rings around galaxies, but since this one is so distant, ~10B light years, so we are seeing that clump as it was that long ago before it difused into it's ring shape we can see in the galaxies around us.
Why does dark matter form halos/rings around galaxies. Why isn't it attracted to the centre of the galaxy like 'normal' matter?
All matter (stuff interacting with gravity) is attracted toward other gravitational centres, however all matter also has momentum, which may tend to carry it away from that centre. Objects don't merely fall directly toward a gravitational centre, but, subject to their initial velocity, orbit it. You may find yourself thankful for this on reflection, as the body you're likely resting on has been in such an orbit for roughly 4.5 billions of years, and will continue to be so for roughly a similar period of time.
If you're sufficiently close to the mass, and/or its radius (relative to your own and your distance from it) is large, as with, say, a stone tossed from ground level on Earth, that orbit will intersect the surface rather quickly.
At astronomical distances, ranging from some significant fraction of the distance between the Earth and Moon to interstellar and intergallactic distances, it's far more likely that an attraction will result in some other form, generally an ellipse (typical of a captured orbit), circle (a perfectly non-eccentric ellipse), a parabola (object moving at escape velocity), or hyperbola (object moving faster than escape velocity).
Ring systems form as multiple masses interact around a larger mass, be that a moon, planet, star / quasi-stellar object, galaxy, or other mass. Until the tangential velocity is lost, the particles within the ring will continue their orbit. Occasional interactions and collisions, as well as radiated energy (including gravitational radiation) may cause a given particle to spiral inwards, or be ejected from, the ring system.
<https://en.wikipedia.org/wiki/Ring_system#Formation>
It is attracted to the center of the galaxy.
Normal matter also makes halos or rings around the center of the galaxy. That's how gravity works. And since dark matter interacts less, it stays more spread.
Halo implies empty (or low density) at the center. The 'normal' matter is denser at the center of a galaxy. I'm trying to understand why the difference.
>since dark matter interacts less
With electromagnetism or gravity?
5 replies →
I believe that you have the order of operations misunderstood.
I probably don't know that much more than you about the subject, but from what I understand, the prevailing model suggests that these Halos formed early in the formation of the universe when spacetime had varying "pockets" of density that naturally led to these halos - the formation of the galactic disk therein was actually supported by the halo existing first, because baryonic matter (aka non-dark matter, the stuff that makes up planets, stars, etc) was still too energetic from the formation of the universe to become gravitationally bound to itself.
Does the dark matter not move under the influence of gravity like 'normal' matter?
2 replies →