Galaxy Zoo Talk

Straight line

  • Alice by Alice

    Is it just me, or is there a very very straight line at 11 o'clock? Is it a distant edge-on?

    I went to SDSS to have a look and it isn't there, but then it's very faint and SDSS is much less in focus. GAMA I think shows something, but I'm not at all sure!

    Posted

  • Budgieye by Budgieye moderator

    Hi Alice, I'm not sure either. Dr Lee Kelvin, what do asteroids look like in this project?

    A typical SSO track in the KiDs survey.

    enter image description here

    1. SEARCHING FOR SOLAR SYSTEM OBJECTS (SSO) IN THE KIDS SURVEY https://www.cosmos.esa.int/web/esac-trainees/2016-projects

    Posted

  • leeskelvin by leeskelvin scientist

    Hi Alice, Hi Budgieye,

    What a beautiful galaxy and a fascinating image! I've done some sleuthing around our data sets to try and find out exactly what this straight line could be exactly. I hope my answers below aren't too long!

    So, first off, is this a distant edge-on disk? Galaxy disks are comprised of billions of stars orbiting around a central point. Not all of those stars will lie in exactly in the same plane, in fact, most of them will 'wobble' up and down on their orbits around the galaxy. This is why most edge-on disk galaxies do not appear completely flat, but instead the disk has a certain 'thickness'. If you try to keep squashing all the stars in a disk into an ever thinner and flatter disk then the stars will eventually push back due to gravitational interactions and 'puff' the disk thickness back up again. It's a bit like squashing a wet sponge between the palms of your hands - you can never make the sponge perfectly flat, it will eventually push back out again! And so, perfectly flat disk galaxies that look like this straight line do not exist in nature, so we can rule this option out.

    Another thing we can quickly rule out is a satellite. Satellite streaks are very long and would cut across this image in its entirety.

    Could this be an asteroid? I must admit, SSOs are not exactly my field of expertise so I can't rule this out, but I've spoken with colleagues of mine here at LJMU and here are my thoughts. I first however have to explain a little bit how we take the observations of this galaxy. When taking a picture of a galaxy such as this in the KiDS survey, what we do is attach a colour filter to the telescope camera to only look at one particular colour of the galaxy. For example, we attach a 'green' filter to the camera, and see how green the galaxy looks. After we have taken that image, we then change the filter to a different colour and take another picture. Building up a picture of a galaxy this way gives us extra information, allowing us to use this colour information to determine other properties of the galaxy such as its age. In the KiDS survey we have 4 different filters. For convenience, we give them the shorthand names of u, g, r and i which are the first letters of ultra-violet, green, red and infra-red. In addition to that, we don't just take 1 'green' picture, 1 'red' and so on, we actually take 5 pictures with each filter (only 4 with the u filter) and then we add them together afterwards. This allows us to remove some additional imaging artefacts after-the-fact.

    So, the reason I mention all of this is because the image you linked to showing a typical SSO track has 5 'bumps' along the line. This corresponds to the 5 pictures taken of that region through that filter. The asteroid has moved between each 5 exposures, and when we add all 5 exposures together it produces a straight line. However, the straight line feature in our image doesn't have such bumps, which is strange. Furthermore, the straight line feature only appears in 1 filter (the 'r' filter). This could be an asteroid which is tumbling through space, and so some of the 'bumps' I mentioned above would be fainter than others, perhaps causing us to miss the distinctive 5-bump pattern.

    Finally, it could be a cosmic ray. Cosmic rays are highly energetic particles from space which fly straight through the earth's atmosphere and right through the telescope camera leaving a distinctive bright spot. Usually the cosmic rays hit the camera head on and so only leave a bright dot, however, sometimes the cosmic rays come in at just the right angle to graze the surface of the camera sensor, leaving a straight line artefact such as this one. There are some nice example images of cosmic ray artefacts at this link: http://darkerview.com/CCDProblems/particlehit.php

    So in conclusion, I can't say for certain what this is. My first instinct was a cosmic ray, but it could also be an asteroid. We can rule out that this is a distance edge-on disk galaxy however.

    A fascinating image - it's been fun thinking about the possibilities! Thank you for such an interesting question!

    Good luck finding more unusual things out there!

    All the best,
    Lee

    Posted

  • Budgieye by Budgieye moderator

    Thank you for your extensive answer, Dr Lee Kelvin. I hope to learn more about artifacts on these images.

    Posted