This "galactic cannibalism" is now thought the be the mechanism that created large galaxies like our own Milky Way out of smaller building blocks. We can still see this process occurring, as these near-ultraviolet observations of NGC 4150 show.
Star formation creates stars of all sizes as gas clouds collapse upon themselves. Hot, massive stars shine brightly in ultraviolet and blue light, but these die fairly quickly. (Yes, tens to hundreds of millions of years is "quickly" in astronomical terms!) In this particular galaxy, the starburst occurred a mere billion years ago.
Personally, my favorite part of the image is the silhouetted dust that almost looks like a spiral pattern. That dust is seen because it is absorbing the high energy photons coming from the galaxy's center. The very center is yellowish, indicating an older population of stars, and it is surrounded by a blue ring of young, hot stars that likely came from the merger with a dwarf galaxy that was one-twentieth the mass of the larger galaxy.
This adds to the growing evidence from ultraviolet observatories that boring old ellipticals as seen in visible light may not be so boring after all. Galactic cannibalism, even of itty-bitty dwarf galaxies, continues to impact the evolution of large elliptical galaxies. Pretty pictures AND interesting science... what more could we have asked for from a space telescope!
Image: NGC 4150 in visible and near-ultraviolet light, taken with Hubble's Wide-Field Camera 3. Credit: NASA, ESA, R.M. Crockett (University of Oxford, U.K.), S. Kaviraj (Imperial College London and University of Oxford, U.K.), J. Silk (University of Oxford), M. Mutchler (Space Telescope Science Institute, Baltimore), R. O'Connell (University of Virginia, Charlottesville), and the WFC3 Scientific Oversight Committee This work will be published in the Astrophysical Journal, and a preprint (pdf) is available.