Free Astronomy Magazine January-February 2024

32 JANUARY-FEBRUARY 2024 ASTRO PUBLISHING N GC 3628, sometimes nicknamed the Hamburger Galaxy or Sarah’s Galaxy, is an unbarred spiral galaxy about 35 million light-years away in the constellation Leo. Extending to the left of NGC 3628 for around 300,000 light-years is a ‘tidal tail’ — an elongated region of stars that arises as a result of gravitational in- teraction with another galaxy. Em- bedded within this tidal tail is the ultra-compact dwarf galaxy known as NGC 3628-UCD1. [CTIO/NOIRLab/ DOE/NSF/AURA. Image processing: T.A. Rector (University of Alaska An- chorage/NSF’s NOIRLab), M. Zamani (NSF’s NOIRLab), & D. de Martin (NSF’s NOIRLab)] mal galaxies that are located farther away beyond the Virgo Cluster. To distinguish the candidate UCD progenitors from the background galaxies, the team performed fol- low-up spectroscopic studies with Gemini North to obtain more con- crete measurements of their dis- tances. These observations allowed the astronomers to eliminate all of the background galaxies from their samples until only the UCDs within the Virgo Cluster remained. Scattered among this vast survey are many dwarf galaxies that contain ultra-compact central star clusters. These galaxies represent the early stages of the transformation process and suggest that after neighboring massive galaxies strip these dwarfs of their outer layers of stars and gas, what remains will be an object identical to the late-stage UCDs that have already been identified. The researchers also found many objects with very extended and dif- fuse stellar envelopes around them, indicating that they are currently in the throes of transitioning as their stars and dark matter is stripped away. Within their extensive sample the team identified objects at sev- eral other stages of the evolution- ary process that, when placed in sequence, tell a compelling story of the morphology of UCDs.