As you may have noticed over the years, we like to celebrate the anniversary of Hubble’s launch with a special image that showcases the orbiting observatory’s fantastic light-gathering abilities and tack-sharp views of the cosmos. This year, to mark 32 years in orbit, we pointed Hubble towards an obscure grouping of galaxies known as Hickson Compact Group 40 (HCG 40), so named for its inclusion as number 40 in astronomer Paul Hickson’s eponymous catalog of groupings of galaxies. Hickson’s 1989 paper detailing the catalog of 100 galaxy groupings can be found here.
We chose HCG 40 over other targets because it had never been observed with Hubble and promised to create quite a spectacle with Hubble’s keen vision–and it certainly delivered! What we’re seeing in this image is a very compact grouping of five different types of galaxies. Indeed, these galaxies are so tightly packed that all five of them would fit within a space not much larger than our own Milky Way galaxy! Here we see classic spiral galaxies, as well as older elliptical galaxies whose lanes of gas and dust have been stripped away. It’s difficult to tell without much more thorough analysis, but it does appear that there are interactions between the galaxies based on what is known as tidal stretching between them. This is most clearly seen in the distorted dust lanes of the spiral galaxy, seen in the upper left around 11 o’clock. This is caused by the mutual gravitational interactions of these galaxies stretching and distorting their shapes. Let’s take a closer look at how the image and subsequent visualization of the data were put together.
Assembling the Image
As with most Hubble images, the telescope performs much of the work in creating a vivid, full-color image, and this was no different for the case of HCG 40. Hubble captured HCG 40 in exquisite detail with the workhorse Wide Field Camera 3 (WFC3) detector in four different filters including F435W, F606W, F665N, and F814W. This combination of filters produces what we call a “natural image,” where the wide-band filters closely match what our eyes perceive as red, green, and blue. As per usual, we assign colors to each image in chromatic order, where the shortest broadband filter is prescribed blue, the longest filter is prescribed red and the filter in between is assigned green. The narrow-band filter, F665N, which captures hot hydrogen and nitrogen gas, also is assigned red (as it appears on the spectrum) which highlights any star-forming regions. Once the hues are applied, the filters are combined additively to get the full-color image.
After the color image has been assembled, we apply adjustments in color, contrast, and detail, while reducing noise. At this time, we also remove any artifacts not intrinsic to the source, using a process we described in a previous blog post. As with any Hubble image, the goal is to produce a compelling image which accentuates the science and features of a particular object. For HCG 40, this includes emphasizing the structure within the spiral galaxies such as the dark dust lanes, bright core and subtle, but present star-forming regions, as well as distinguishing it from the uniform, spherical appearance of the elliptical galaxies. Steps are taken also to highlight the rich assortment of background galaxies that are captured in just a tiny fraction of the sky, showcasing the breadth and number of galaxies in our universe.
Anatomy of a Visualization
As with all Hubble images, the data received on the ground are 2D images, but of course these galaxies exist within a 3D environment and we can help bring depth and dimension to the images through a visualization. In this case, we decided to try to illustrate the compact nature of this galaxy grouping by creating a pseudo 3D fly-through of the galaxies. The “pseudo” moniker here stems from the fact that there are no volumetric 3D models of the galaxies in the visualization; everything that is shown is 2D. The first step in producing such a visualization is to deconstruct the 2D image into its component parts (i.e., a sky background, distant background galaxies, and individual layers for each of HCG 40’s members). Of course, this is easier said than done and requires a skilled hand and keen insight into the actual structure of these objects and filling in background around them when they are separated.
Once everything is cleaned and separated, it needs to be arranged in 3D space with enough distance between objects to cause parallax as the camera traverses the environment. We consulted astronomers including Tyler Desjardins at the Space Telescope Science Institute and Paul Hickson himself to verify that the arrangement of the galaxies could be plausible given our current understanding of the movement of the galaxies within HCG 40. This visualization represents our scientifically informed best guess of the 3D arrangement of this group of galaxies and helps to bring the image to life.
Soundtrack of the Universe
Finally, the appropriate mood music really helps to set the scene with this visualization. I’ve been somewhat obsessed with analog and generative synthesis lately and wanted to make use of some new gear for the music in this visualization. I settled on a theme and used a generative sequencer to create two complementary bouncing/cascading lines of notes sitting atop an undercurrent of synth swells. The opening scene is backed by a long swell as the constellation artwork appears on the screen, and then we begin our journey as the first set of notes kicks in. Soon after, just as the background shifts into the higher resolution of the digitized sky survey, the ticking beat kicks in to reflect this change and emphasize the enormous amount of time and space elapsing as we travel to HCG 40. A key change signals the Hubble image coming into focus, but we soon revert back to the familiar sound of the original key as we settle into this new view of the universe. Another swell signals the shift from 2D to 3D as we begin our deep dive into HCG 40. We then thread a course through the group of galaxies leading up to the final scene, a closeup view of the large elliptical galaxy looming overhead as the music slowly fades.