In a lonely patch of the universe, five tightly grouped galaxies engage in a leisurely dance. For its 32nd birthday 🥳, #Hubble captured these galaxies at a very special moment in their lifetimes as they fall together and continue their dance, but before they merge. (1/7) 🧵 
Called the Hickson Compact Group 40 (HCG 40), this eclectic bunch includes three spiral-shaped galaxies, an elliptical galaxy and a lenticular (lens-like) galaxy. In about 1 billion years, they will collide and merge to form a giant elliptical galaxy. (2/7)
Scientists have cataloged more than 100 such compact galaxy groups, but HCG 40 is one of the most densely packed. Tight groups like this may have been more common in the early universe when their superheated. (3/7)
Though such cozy galaxy groupings can be found in the heart of huge galaxy clusters, these galaxies are notably isolated in their own small patch of the universe, in the direction of the constellation Hydra. (4/7)
One possible explanation is that there's a lot of dark matter—an unknown and invisible form of matter—associated with these galaxies. If they come close together, then the dark matter can form a big cloud within which the galaxies are orbiting. (5/7)
As the galaxies plow through the dark matter they feel a resistive force due to its gravitational effects. This slows their motion and makes the galaxies lose energy, so they fall together. (6/7)
Studying nearby groups like HCG 40 helps astronomers learn about how galaxies formed: hubblesite.org/contents/news-… (7/7)
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