Space Warps Talk

Distant Arclets and Dark Matter

  • pljordaan by pljordaan

    Hello,

    As I understand there is long time search going on in order to find Dark Matter. As a volunteer, I am searching for Gravitational Lenses here at SpaceWarps.org. After looking at more than 900 slides, I could not help noticing a pattern. I have seen many blue galaxies situated in the form of arcs, each containing about 3-5 galaxies around dark, circular pieces of the sky. On average, there are 3-5 blue or orange galaxies in an arc. There are no galaxies or stars visible just many tiny red, green and blue dots (artifacts?). You can see a good example on the Home Page: 6 blue galaxies in an arc from 3-4 o'clock, although the sky here is not completely empty.

    In the Spotter’s Guide they are called Distant Arclets and must be seen as False Positives, since there are no massive galaxies or lenses present. I was wondering, since in Space most objects are spheres or have at least rounded shapes, isn’t possible that Dark Matter can be shaped roughly as a ball and can act as a weak lens and thus show a number of distant galaxies around its rim while being invisible itself? In most of the slides I have seen those circular gaps in all sizes, some very clear, others very faint. It is hard to believe that this is just a coincidence.

    I have been experimenting with the lens toy, there I noticed the difference in distance between the cursor and the small attached lensed galaxy when the cursor is being moved around over the slide. Close to the bigger galaxies, the distance to the cursor is small and the galaxy looks much distorted. While going farther away from the bigger galaxies, the lensed galaxy is being pushed far away from the cursor while there is no distortion at all. Is the app on the website made just to show the distortion around the big galaxies and is what I mentioned just a side effect and does not resemble the real thing? If not, could a ball of Dark Matter in the same manner push the image of a distant galaxy away as a lens without distortion?

    Peter Jordaan

    Posted

  • Budgieye by Budgieye moderator

    The short answer is: Since we know so little about Dark Matter, you may be right.

    A longer answer: Dark Matter has been mapped to be around galaxies. My mental image in thinking about Dark Matter is that large subatomic particles are synthesized in quasars in the nucleus of galaxies and squirted out in galactic jets around most galaxies. Most galaxies were quasars in the past. (Real scientists are allowed to disavow any of this, and recoil in horror). Can we find Dark Matter without a galaxy? Have we looked yet?

    Finding circles everywhere:

    1. Distant Arclets: These blue galaxies are just forming. In Galaxy Zoo 2, these galaxies are being analyzed to see how they come together to form larger galaxies. I don't know the results of their modelling. Maybe they form arcs as they merge. If the little galaxies are to be as a result of one blue galaxy being lensed, each lensed image of the galaxy must all be the same colour and form discrete narrow arcs.

    2. Our brains tend to "join the dots" and make patterns where none exist.

    3. The images are heavily processed, and there are curves somehow added in the clean-up process, which can help the brain to see more curves.

    I'm not sure what you mean with lens toy, and I can't get it to work right now, so I can't answer about that.

    Any scientists care to add an an even longer answer?

    Posted

  • Tom_Collett by Tom_Collett scientist

    Hi guys,

    Lot's of interesting points raised by both of you. Hopefully I can clear up a few things here.

    Firstly, 3 points will always lie on an arc, so seeing three galaxies in an arc is guaranteed - the presence of extra galaxies on the arc is not going to be rare given the frequency of blue galaxies in spacewarps images, and the large number of images we are looking at (your brain is ignoring all the cases where 4 galaxies don't form on an arc). Budgieye's point 2 is correct - our brains are very good at seeing pattens that aren't there. Despite the fact that these patterns aren't the result of lensing, your questions about dark matter lenses are still interesting.

    Indeed we expect dark matter to end up forming into 'balls' which we call halos, these halos form because the dark matter falls in on itself due to its own gravity, but because dark matter only interacts via gravity, the dark matter particles struggle to shed their angular momentum, and the halo doesn't collapse into an ultra dense object, but remains quite extended. Now these dark matter halos CAN act as weak lenses, and indeed every line of sight in the universe goes through the outskirts of at least a few dark matter halos; the light rays get slightly perturbed by their weak lensing effect.

    So why do I think your distant arclets are coincidence? Like Budgieye says dark matter is found around galaxies (although it certainly isn't created in any great quantities by quasars), and the reason for that is due to how the big bang 'seeded' our universe. There is overwhelming evidence that the big bang didn't create a perfectly smooth universe, but some parts were ever so slightly over/under dense. Wind forward a billion years or so and it's the over-densities that collapsed to form the galaxies we see today. What is intriguing, is that the original over-densities in normal matter were in the same place as the over-densities in dark matter (whatever high-energy processes were going on in the big bang seem to have treated the two types of matter in the same way), so where the normal matter was collapsing to form galaxies, the dark matter was collapsing into halos around those galaxies. The quick summary is that, if there's a massive dark matter halo, then it will host a massive galaxy (the biggest halos host the groups and clusters). Only the least massive dark matter halos don't host galaxies, but these don't produce a noticeable lensing effect.

    I've done a lot of work on trying to infer the lensing effect of dark matter halos based on the visible galaxies within them; you've asked a proper research question, and a few paragraphs can't really do it justice, but I hope I got some of the picture across.

    Posted

  • anupreeta by anupreeta scientist, admin

    Quick answers here: The lenstoy on the landing page is not working properly. The tech team is trying to fix it.

    Even though Universe is very empty with not much stuff in it, it is also extremely large. The reason why you see the effect of lensing is when a distant object seems to be almost perfectly aligned to an object in the foreground and us. This has less chance of occurring than other objects that appear to be closely or nearly aligned. That's to say, more often you will see objects to be close to each other (close in projection but not physically close) than objects that are perfectly aligned.

    For lensing effects to be strong enough, you need some critical density of mass. As far as we've probed the Universe, the density of dark matter (found typically, around galaxies) alone is found not be high enough to cause lensing effect on its own. This is found to be generally true in the Universe based on both theory and observations but there could be some places where dark matter alone is compact enough to cause lensing effects on its own. Such cases have not been found in my knowledge.
    Some info on DM and lensing -
    http://www.ifa.hawaii.edu/~ger/ASTRO-110_sp08/Lecture28_DarkMatter.pdf
    and on weak lensing
    http://www.cfhtlens.org/public/what-gravitational-lensing

    Re. your last question on "ball of DM pushing the image of a distant galaxy .... "
    Lensing causes light of distant galaxy to come towards us instead of getting pushed away and that is why we are able to see the images of the distant galaxy - in spite of a massive object right in front of it.
    e.g. see some ray diagrams here - http://spiff.rit.edu/classes/phys240/lectures/grav_lens/grav_lens.html

    Posted

  • Budgieye by Budgieye moderator

    My updated mental imagery regarding Dark Matter.

    Balls of Dark Matter are formed during the Big Bang. Then somehow, matter (galaxies) appears inside them. We measure Dark Matter as a halo around the galaxy ( even though it is inside the galaxy, even in the room with us, even inside us)

    There can be small balls of dark matter without matter in them, but the small balls of dark matter won't be "heavy" enough to warp space to cause strong gravitational lenses, but may do some weaker lensing not relevant to SpaceWarps.

    An analogy : holes on a road surface have water in them. Small depressions may not have water in them.

    I'll start a separate discussion on LensToy in Bugs.

    Posted

  • anupreeta by anupreeta scientist, admin in response to Budgieye's comment.

    No need for that @Budgieye, Amit's working on it right now.. its almost done getting fixed!! 😃

    Posted