Research
What are extreme galaxies?
When people think of galaxies, the image of a normal, large, majestic galaxy likely comes to mind. It turns out that most normal galaxies fall under two broad categories, spiral or elliptical (shown below), based on their morphologies. Other features such as color and structures (e.g. spiral arms) provide more bases of comparison needed to classify the many galaxies in our universe.
NGC 4535 (spiral galaxy) and M49 (elliptical galaxy)
(Image Credit: NSF-DOE Vera C. Rubin Observatory)
However, some galaxies don't resemble spiral or elliptical galaxies at all. They might be extremely faint and difficult to spot, or have features so irregular that none of the normal structures we associate with galaxies are discernible. In this project, we will refer to these objects as extreme galaxies, and the ones that we are interested in are known as "blue blobs" and diffuse galaxies.
Blue Blobs
Blue blobs are unique stellar systems because they have highly irregular, clumpy shapes and consist almost entirely of young, blue stars. There are only a handful of examples known (most of which were identified by Zooniverse volunteers) and they are all in the Virgo galaxy cluster, which is an extremely hostile environment to the cold gas needed to form new stars. Yet, blue blobs have somehow survived under these conditions and appear to be isolated from any other galaxy or companion, so the fact that these isolated systems exist with young blue stars is a bit of a conundrum!
Hubble Space Telescope images of three of the first blue blobs known: SECCO1, BC1, and BC3.
(Image Credit: NASA/ESA Hubble Space Telescope)
Diffuse Galaxies
Diffuse galaxies contain roughly the same number of stars as dwarf galaxies (about one million to one billion stars), but they are spread over an area comparable in size to the Milky Way, which contains hundreds of billions of stars. A galaxy’s effective radius is the radius that encloses half of its total light, and diffuse galaxies have unusually large effective radii for their relatively small stellar populations. This is why they are called diffuse galaxies: their stars, which are mostly old, are spread out over a much larger volume than in typical galaxies. As a result, diffuse galaxies emit less light per area on the sky (which astronomers call "surface brightness") than normal galaxies, causing them to appear as faint, blurry patches of light.
The almost transparent galaxy LSBVCC437 as seen in LSST DP2:
(Image Credit: NSF-DOE Vera C. Rubin Observatory)
Why are we interested in these types of extreme galaxies?
Just 10 years ago, very few extreme galaxies were known. As astronomical imaging surveys improved, the number of known examples grew dramatically. Today, we know of many more of these objects, but we still cannot explain why so many of them exist. Blue blobs, for example, were discovered only recently and may not even be galaxies. Instead, they could be star clusters that formed outside their parent galaxies. The origins of both blue blobs and diffuse galaxies cannot be fully explained by the processes thought to form normal galaxies. By studying these unusual objects, we hope to fill important gaps in our understanding of how galaxies form and evolve.
Why LSST, and why DP2?
Diffuse galaxies and blue blobs are, almost by definition, objects that fall just below the detection limit of most surveys. The telescopes and cameras of the past weren't sensitive enough to pick out something so faint and so spread out against the glow of the night sky. This is where the NSF-DOE Vera C. Rubin Observatory changes the picture. With its 8.4-meter mirror, 3,200-megapixel camera, and repeated deep exposures of the same patch of sky, Rubin's Legacy Survey of Space and Time (LSST) is designed to reach surface brightness limits fainter than anything achieved before. That extra depth matters enormously for diffuse galaxies in particular: since their light is smeared out over such a large area, most of it hides below the noise floor of shallower images and only accumulates into a recognizable "smudge" once you stack up enough exposures. Rubin Data Preview 2 (DP2) gives us an early, real look at that improvement in action. The image below shows why this leap matters: the same patch of sky around VCC 1249 (a diffuse dwarf embedded in the halo of M49), imaged across four decades of survey technology - going from a barely-there smudge in 1990s photographic plates to a resolved, diffuse galaxy in Rubin's DP2 data. Rubin Observatory's own team also highlighted this same kind of view for the compact galaxy group RSCG 55 in a recent social media post proving that this "glow-up" isn't unique to one object, but a preview of what Rubin will reveal about faint, low-surface-brightness structures over the next ten years. That's exactly the kind of newly visible structure we need your eyes to help us find.
The following compilation of images of VCC 1249 shows the evolution of ground-based sky surveys spanning more than three decades!
Explore this galaxy in Skyviewer!
Why do we need your help?
The first step to understanding extreme galaxies is finding more of them across all kinds of environments. For more regularly shaped galaxies, astronomers can employ special computer algorithms to efficiently identify these objects in a large dataset. While there are algorithms developed to identify diffuse galaxies, it's difficult to know how reliable they are without checking their findings against human identification, since diffuse galaxies appear more like smudges than distinguishable objects. Blue blobs, on the other hand, are harder for computers to identify because no two objects look alike, and they can be exceptionally faint. Therefore, the most reliable method is to identify both of these objects with the human eye. Surveys of nearby galaxy clusters, though, correspond to thousands of images, far too many for any one person to look through. In DP2, the M49 region of the Virgo Cluster corresponds to about 10,000 subjects! However, with a large team of citizen scientists, we can tackle this challenge together, and by doing so, you will be playing a vital role in helping us solve the mystery of extreme galaxies!
The following map shows the entire M49 field from the DP2 SkyViewer preview
(Image Credit: NSF-DOE Vera C. Rubin Observatory)
The following image shows an annotated First Look image of the Virgo Cluster
(Image Credit: NSF-DOE Vera C. Rubin Observatory)
NSF-DOE Vera C. Rubin Observatory is funded by the US National Science Foundation and the US Department of Energy’s Office of Science.
How will we use your classifications?
Your classifications help us narrow down where in a galaxy cluster we should be looking in order to study extreme galaxies more closely on an individual basis. We will review all of the classifications made by citizen scientists and select the best candidates to follow up with other telescopes to look for any signs of gas and to study their stellar populations. With this larger sample and the new information from follow-up observations, we aim to better understand how these objects formed and how they will evolve in the future.