Spiral Galaxies as Gravity Lense

I was reading a post on 'Starts With a Bang' where they were discussing the merits of Dark Matter over MOND and there was a picture of a large spiral galaxy with many starts in the field behind it. What struck me was that the background seemed to be in perfect clarity. There was no distortion in the field of stars in the background. However, Spiral Galaxies must have incredibly huge Dark Matter Halos in order to have such flat velocity profiles. I believe I read somewhere that the Milky Way is supposed to have 6 to 8 times the visible mass in its halo. Is there any evidence of Spiral Galaxies acting as Gravity Lenses proportional to the amount of Black Matter needed to flatten the velocity profiles?
Those were surely foreground stars* and/or other galaxies. Do you have a link to the image in question?

* Stars in our own galaxy in the field of view


Weak Lensing

If the lensing effect is strong enough to be seen by the human eye on an astronomical image, like in Abell 2218, we call this strong lensing. Strong lensing only happens when a massive cluster of galaxies lies between us and some other galaxies - it is the further-away galaxies that have their shapes changed by lensing. In this case, it is easy to see and measure the effects of lensing. However, there are not that many clusters in the sky that are so big that they cause such a large lensing effect - most of the time, we don’t see galaxies stretched into arcs or multiply-imaged. So these instances of strong lensing are very useful - and pretty - but rare.

However, the fact that there is some dark matter in between us and every distant galaxy we see means that ALL galaxies are lensed - even if it is only slightly. In fact, most galaxies are lensed such that their shapes are altered by only 1%, an effect we call weak gravitational lensing.
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The actual picture is not important as it could very well be foreground information. Here's the link:


The picture simply got me to thinking, if there was a halo of dark matter that was powerful enough to flatten the velocity curve out to hundreds of thousands of light years, that must surely create enough of a distortion to be distinct from a lens created by visible matter. I've seen reports of elliptical galaxies causing lensing that indicates matter that can't be seen (which isn't a surprise, since we can't really see the matter inside an elliptical galaxy), but I've not been able to find any studies that demonstrate the presence of a dark matter halo in a Spiral Galaxy through gravitational lensing.


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If I understand you correctly then I think you are on the right track. Gravitational lensing is one of the ways DM is detected and the lensing effect is used to MAP clouds of DM.

Google "bullet cluster" to see mapping of density contours of DM cloud obtained by lensing of background.

Good thing to realize, though, is that galaxies tend to come in clusters and the DM cloud extends throughout the cluster. This means the whole ensemble is largely transparent (the way a swarm of gnats is transparent, only scattered specks block the view) so it is easier to detect and map the optical distortion effect in the case of a cluster.

A swarm of galaxies with its huge mass of dark matter in and around it can act like a big MAGNIFYING GLASS and allow astronomers to see dimmer more distant objects behind it, and this is in fact how the "most distant galaxies" are being discovered and imaged these days. Every couple of years they find a new "most distant" object with the highest redshift found to date, and as I recall it recently as been the case that the latest candidates have all been found with the help of this magnification, they are in the background of a much closer (lower redshift) cluster with its DM cloud.

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