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Education
Lensed Galaxies
Lensed Quasars
Group-scale lenses
Lens Impostors:Disk galaxies
Lens Impostors:Loners or Edge-on Disks
Lens Impostors:Neighbors
Ambiguous cases
Stars
Artifacts
Build your own lens
Gravitational Lenses
See the definition of some lensing related terms in the FAQ.
Lensed Galaxies
A strong lens consists of at least two galaxies: one in the foreground, doing the lensing, and one in the background, the image of which is distorted. Lens galaxies are typically very massive, yellow/red in color, with an elliptical shape. Most of the images you'll see feature one such galaxy in the center. A lensed galaxy is usually, but not necessarily, blue, and is often distorted into multiple images that roughly trace a circle around the lens.
Above are clear examples of strong lenses, recognizable from the big blue arc (with 3 compact peaks) and the small blue "counter-image", located on the other side of the lens galaxy, opposite to the arc.
The presence of counter-images can help a lot in determining whether a galaxy is a lens or not: look out for them! Most of the lenses we're looking for, however, are not as obvious to spot as the ones above. And, in some cases, only one arc is visible, with no obvious signs of counter-image (the counter-image may be there but could be too faint to show up in the data).
Above examples show systems where the lensed galaxies in the background may have very compact shape (left) or reddish colors (right). Depending on redshift and the nature of the background galaxy the images can appear blue, green or red. These are not very common but you may discover some!
Above are examples of almost complete einstein rings produced by yellowish/reddish massive foreground galaxies. These foreground galaxies are almost round in shape creating a symmetry when deflecting the light from a background galaxy.
Lensed Quasars
Lensing does not always produce arcs. When the background source is small in size, such as a quasar or a compact galaxy, it can be lensed into multiple circular images, like the examples above. Four-image systems (called quads) are quite common. Two-image systems (called doubles) are even more common, but are not as obvious to identify as a "quad".
Note that the quad system here is unusual. There is a red lensing galaxy at the center and three bluish compact lensed images of the quasar are obvious. Now, instead of a fourth bluish image, a small reddish image can be seen right below one of the blue images . This faint fourth image is almost overlapping with another reddish galaxy making the combined appearance of that a reddish object. Sometimes, such unusual cases occur and turn out to be real lenses.
Group scale Lenses
Galaxy groups and clusters are systems containing several tens to hundreds of galaxies, gas, dark matter held together by gravity. They are the most massive systems in the Universe. As such these are the most powerful cosmic telescopes (owing to the magnifying effect due to lensing) and can produce large, spectacular arcs and multiple images.
The above two examples show "group scale lenses" where the foreground lens comprise of a group of galaxies (usually of similar colors). The on sky projected distance between the multiple arcs produced by group scale lenses is much larger than those by individual galaxies. This is because it is the combined mass of the group that contributes in producing the lensing effect. The number of arcs or the arrangement of arcs around the galaxy groups can also be non-standard or complex.
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Lens Impostors
Lens impostors are astronomical objects that look like gravitational lens systems but are physically different. Below are some typical examples of lens impostors that you can familiarize yourself with to aid your classification.
Disk Galaxies
Many galaxies have yellow "bulges" in the centers, surrounded by a disk of gas and blue "spiral arms.". We call these "disk galaxies". Unlike gravitationally lensed arcs, the arms don't trace a rough circle around the bulge and they are usually accompanied by some fuzzy distribution of light. In these examples, you can see fuzzy arms that either don't trace out a well-defined shape, or a hint of a "bar" that connects the arms to the bulge.
Some galaxies are surrounded by a blue belt of young stars. When looking at one of these "ring galaxies" face-on, it can be difficult to distinguish it from an Einstein ring of a gravitational lens. Above are examples of ring galaxies with the ring showing fuzzy distribution of light. However, because of their random orientation and inclination with respect to the plane of the sky, most ring galaxies appear to be flattened/elliptical with the bluish "ring" following the ellipse-like shape (see below).
In contrast, the bluish ring-like lensed images produced by yellowish lens galaxies are usually rather round owing to the circularly symmetric distribution of mass in the foreground lens galaxy.
Loners or Edge-on Disks
In the images above, there are elongated features that could resemble gravitationally lensed galaxies. However, there are no plausible nearby lensing galaxies, suggesting that they are not lensed arcs. Even if there was a yellowish lensing galaxy in the vicinity, notice the light distribution in these features is concentrated towards the center and tapers towards the edges. This is a sign of a disk galaxy seen edge-on rather than a gravitationally lensed galaxy.
Neighbors
Images taken of the sky are a flattened 2D projection of a 3D universe. This means that physically unrelated and far-away galaxies appear next to each other in the 2D images - this is a chance or random alignment. Sometimes the shapes or patterns of these neighbouring galaxies are such that they can mimic lenses, but there could be clear signs that these are not gravitational lenses. For lenses, remember to look for an arc that is well-defined and curved around a plausible lensing galaxy, and also look for hints of a counter-image. If things look very extended and fuzzy, or curved the wrong way, or too straight, they are most likely not lenses. Some examples follow.
Left: There is a yellow galaxy which could be a foreground lensing galaxy but the neighboring blue feature is most likely an edge-on galaxy because of the light profile (see explanation in the previous category), the shape is too straight (not curved enough) and obviously is not circling around the yellow lensing galaxy. Hence, this is most likely a chance alignment of a yellow and a blue galaxy possibly located at different distances. Similarly, is the case for the red blob to the right of the edge-on galaxy.
Right: Here you can see a very elliptical or a disky galaxy and a reddish blob juxtaposed by chance. There's no sign of any other fainter reddish blob on the opposite side of the galaxy. Therefore, this is unlikely to be a lens system.
Ambiguous cases
In a lot of cases, it's hard to tell whether a galaxy is a lens or not, as in the two examples shown above.
The object on the left shows a blue image very close to a galaxy. The blue feature is slightly elongated and aligned in the same way a lensed arc would be, but it looks boxy rather than arc-like. The boxy shape, however, could be the result of the image quality.
The image on the right shows some characteristic features of a gravitational lens, such as a nice round arc oriented towards the central galaxy, as well as a fuzzy distribution of light more typical of spiral galaxies.
In cases like these, we would like you to be inclusive and label them as lenses. Further screening process will decide whether or not to include such systems but at this stage of inspection, it is preferable to include them.
Stars
In addition to galaxies, there are also stars from our own galaxy that are visible in our images.
Young stars appear bluish white and older stars appear reddish yellow. See two examples below which show how a reddish star (left) could look different from a reddish galaxy (right). Both have a bluish object near them. The left object should definitely be rejected whereas for object on the right, it may be worth looking at images after subtracting out the light from the lensing galaxy to check for presence of a counter-image to further decide possibility of lensing.
However, quasars are also bright point sources, so can be hard to distinguish from stars but quasars are distant enough to be lensed by an intervening galaxy. If you see two or four 'stars' of the same colour on opposite sides of a red/orange galaxy then it might be a lensed quasar.
Some stars are easier to spot than the others. For example, brighter stars cause saturation in the camera pixels (see bluish circle in the image on the right below) and/or show diffraction spikes (see bluish / reddish cross-like pattern). If you see any objects in the vicinity, these are most likely distant background galaxies in the line of sight.
The mass of an individual star is too small for it to produce multiple images of a distant galaxy. These stars, however, could act as a lens for other distant stars within our own galaxy, if they appear to cross each other from our perspective. This is called Microlensing. You won't be seeing any such microlensing in the SpaceWarps images.
Artifacts
The process of recording astronomical images into the nice maps of the sky you see here can lead to some interesting artifacts in the data that aren't real. In addition, there are real objects in the sky that are moving (either fast or slow) that can leave strange features in the data.
The images presented here are colour images made up of three channels: red, green and blue. Some of these artifacts only come from one of the channels, leading to pure red, green, or blue features. The actual astronomical filters we use in these colour channels are red: i-band (just outside the visible range that our eyes are sensitive to), green: r-band (middle wavelength, the reddest light our eyes can see), and blue: g-band (what our eyes would see as green). There are some examples of moving target artifacts below.
Fast-Moving Objects
These are examples of fast moving objects, such as satellites, asteroids, airplanes, etc. As the camera observes the sky for a fixed amount of time per exposure, fast-moving objects leave trails in the data. The object in the left panel was only in the field when the image was taken in the blue filter, therefore it appears blue. The object in the right panel is similar, but the image was taken in the green filter.
Slow-moving Objects
In the image above, you see several spots in a line. This is a slow-moving object, such as a distant asteroid or a slow-moving satellite. The object appears multiple times because it has moved between exposures taken in the filters that make up this image. As the sky images are collected filter by filter (not necessarily on consecutive nights), moving objects like these get smeared out or appear as discrete images. Each image in each filter is further made up of several exposures that have been stacked and stitched onto each other. The right image is similar, except the object is present in the green filter.
More examples
Here are some more examples where differences in filters produce artifacts. These end up obstructing or contaminating emission from astrophysical sources.
Fun activity
Build your own lens
You can create images similar to gravitational lenses using a wine glass. Try looking at a light source, like a small flashlight or a candle, through the base of the glass. Play around with the glass to produce arcs and rings.
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