[HN Gopher] A ghostly galaxy lacking dark matter
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       A ghostly galaxy lacking dark matter
        
       Author : doener
       Score  : 16 points
       Date   : 2021-12-12 20:57 UTC (2 hours ago)
        
 (HTM) web link (esahubble.org)
 (TXT) w3m dump (esahubble.org)
        
       | doener wrote:
       | Via https://www.heise.de/hintergrund/Missing-Link-Das-Raetsel-
       | Du... (German)
       | 
       | I recommend https://www.deepl.com/en/translator for translation.
        
       | foxylad wrote:
       | Can someone elucidate how astronomers know this galaxy has no
       | dark matter?
        
         | icambron wrote:
         | 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.
        
           | adgjlsfhk1 wrote:
           | 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.
        
             | icambron wrote:
             | Thanks, edited
        
             | BurningFrog wrote:
             | How do we measure the amount of dust?
        
               | dogma1138 wrote:
               | Usually by how light passes through it from both stars
               | within the galaxy and from galaxies occluded by it.
        
             | lordnacho wrote:
             | 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?
        
         | danielheath wrote:
         | At a guess, it's mass (measured by lensing) is accounted for by
         | its light output.
        
         | adgjlsfhk1 wrote:
         | 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.
        
           | jeddy3 wrote:
           | I though the haircut was an analogy at first ;D
           | 
           | Nice auto-correct
        
         | alkonaut wrote:
         | 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.
        
       | fghorow wrote:
       | Yeah, well. _That_ is an observation that is pretty bloody
       | difficult to explain with MOND type theories.
       | 
       | Any MOND proponents want to have a try?
        
       | diogenes_of_ak wrote:
       | The photino birds got clobbered by the Xeelee in that galaxy.
        
         | pharke wrote:
         | Some people seem to think this is a worthless comment, I prefer
         | to see it as a good book recommendation. It also jives with my
         | feeling that dark matter has something to do with life being
         | present in a galaxy, the effect it has on the structure of
         | galaxies seems almost... deliberate.
        
           | errcorrectcode wrote:
           | No intelligent design or superstitious nonsense here, thanks.
        
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       (page generated 2021-12-12 23:00 UTC)