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Comment by foxylad | Hacker News Reader

Can someone elucidate how astronomers know this galaxy has no dark matter?

It’s not revealed in the article for this particular case, but the usual way of determining the dark matter content works by estimating the total mass or mass distribution, and subtracting the visible mass of stars.

The total mass can be estimated from gravitational lensing, and for rotating galaxies the distribution of mass within the galaxy can be estimated using the speed of rotation of stars at different distances from the center.

"Rotating" meaning sighted at an angle so you can use Doppler shifts of familiar spectra to figure out the tangential velocity of parts of the galaxy. In this case they used what they called "gas", meaning I suppose interstellar plasma, as the reference for the Doppler-shifted waves. They could check emission lines from the plasma, or reflections off it, or gaps in transmitted light representing absorption of that light by the plasma.

Dark matter is inferred by the difference in the amount of gravity a galaxy appears to have, and the gravity we'd expect from the mass we observe inside of it. So in this case, the observed gravity is what all the observed mass would predict it to be, instead of being far more, like most galaxies.

it's not actually star mass. only about 1% of the normal matter in a galaxy is from stars. the rest is low density dust.

Thanks, edited

How do we measure the amount of dust?

Usually by how light passes through it from both stars within the galaxy and from galaxies occluded by it.

Couldn't dark matter just be dust that acts differently then? Like what if it were little clumps of dust instead, shouldn't that alter how light passes?

Cold baryonic dark matter candidates have been pretty much all but excluded.

The dark matter most people are looking for only interacts with gravity which means other than gravitational lensing it would not interact with light in any way (and the lensing is technically not an interaction either).

My own personal opinion is that whilst MOND is probably wrong I think the idea of it might be closer to reality that not.

I don’t think that dark matter as in matter that interacts only with gravity is going to be discovered.

Keep in mind this isn’t my field so it’s not a professional opinion it just feels like we are chasing elegance rather than going back to the drawing board.

We are still really bad at measuring distances and thus anything else at those scales the distance of the galaxy in question went from 20 something to 50 something to over 70M light years with consecutive measurements and I’m still not sure we got it right.

We are heavily reliant on “standard candles” for measuring distances and in the end it’s all about calibration which is somewhat still based on guess work as in we hope that we got the objects absolute luminance correct before comparing it to the absorbed luminance.

I should also mention that we not only using standard candles, parallax measurements are also used and those are probably relatively accurate however we can usually only use them for relatively near by stelar objects so they aren’t useful for measuring distances to galaxies or even remote stars within our own galaxy.

We did use parallax to check some of our standard candles but for galaxies many of our standard candles are supernovae not stars so there is still a bunch of variance there.

The galaxy in question makes it even harder to be confident in our ability to measure its distance accurately it’s very sparse, the stars are in it are mostly about 10B years old and are very dim and it doesn’t have much if any star formation.

That dust would invariably collide, heat up a bit, emit that heat as thermal radiation and end up with a bit less energy. Thus slowly but surely its orbit would decay and it would move towards the core like normal matter, because it is normal matter.

However to explain the rotation curves the extra mass must have a different density profile, and it has to be present far beyond the edge of the visible galaxy.

If one assumes a particle which can't emit thermal radiation as it does not interact with electromagnetism, then it won't cool down and will remain in a diffuse blob around the galaxy.

Turns out one can get good fits to galaxy rotation curves with a huge diffuse blob of such a particles around a galaxy.

That raises a question. If there's a second variable, the dust %, how can we solve for the amount of dark matter from the visible mass?

Not an astronomer but i think it would still glow at lower wavelengths (infra-red)

At a guess, it’s mass (measured by lensing) is accounted for by its light output.

TFA suggests its mass was estimated by rotation curves, not lensing.

tldr is that the rate a haircut rotates is proportional to its mass (and mass distribution). most galaxies spin faster than they "should", so there has to be a lot of mass that we can't observe using telescopes. this one doesn't.

I though the haircut was an analogy at first ;D

Nice auto-correct

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