Dwarf galaxies stripped of stars prove to be the missing link in the formation of rare ultra-compact dwarf galaxies

Astronomers using the Gemini North telescope, one-half of the International Gemini Observatory operated by NSF's NOIRLab, have captured the eroding remains of more than 100 dwarf galaxies as they transition into ultra-compact ...
Ultra-compact dwarf galaxies (UCDs) are among the densest stellar groupings in the universe. Being more compact than other galaxies with similar mass, but larger than —the objects they most closely resemble—these mystifying objects have defied classification. The missing piece to this puzzle has been a lack of sufficient transitional (intermediate) objects to study. A new galaxy survey, however, fills in these missing pieces to show that many of these enigmatic objects are likely formed from the destruction of dwarf galaxies.
The work is published in the journal Nature.

The idea that UCDs are remnants of disrupted dwarf galaxies has been proposed since they were discovered over two decades ago. However, previous searches have not revealed the large population of galaxies in transition that you would expect to find.

So, an international team of conducted a systematic search for these intermediate-stage objects around the Virgo Cluster, a grouping of thousands of galaxies in the direction of the constellation Virgo. Using the Gemini North telescope near the summit of Mauna Kea in Hawaiʻi, the team identified more than 100 of these missing-link galaxies that show every stage of the transformation process.

Strongly nucleated dEs, eUCDs and UCDs in the Virgo Cluster core. a, u*g′i′ color composite images of representative strongly nucleated dEs (left column) and eUCDs (right column). Each cutout image is 0.5 arcmin × 0.5 arcmin. b, BSDVS imaging of low-surface-brightness features in the Virgo core region. Yellow and dark red circles represent UCDs and eUCDs, respectively. Blue circles and squares are strongly nucleated dEs with and without velocity measurements. u*g′i′ co-added and Gaussian-smoothed deep NGVS imaging of four disrupting dE,Ns (VCC 1672, NGVSJ 1229 + 1343, NGVS 2078, VLSB-A) and NGVS-UCD509 are shown in the insets. The arrow indicates the 70-kpc-long tidal stream that probably originates from NGVS-UCD509. Credit: Nature (2023). DOI: 10.1038/s41586-023-06650-z
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This illustration shows a dwarf galaxy in the throes of transitioning to an ultra-compact dwarf galaxy as it’s stripped of its outer layers of stars and gas by a nearby larger galaxy. Ultra-compact dwarf galaxies are among the densest stellar groupings in the Universe. Being more compact than other galaxies with similar mass, but larger than star clusters — the objects they most closely resemble — these mystifying objects have defied classification. The missing piece to this puzzle has been a lack of sufficient transitional, or intermediate objects to study. A new galaxy survey, however, fills in these missing pieces to show that many of these enigmatic objects are likely formed from the destruction of dwarf galaxies. Credit: NOIRLab/NSF/AURA/M. Zamani
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A continuum of galaxies captured at different stages of the transformation process from a dwarf galaxy to an ultra-compact dwarf galaxy (UCD). These objects are located near the supergiant elliptical galaxy M87, the dominant member of the neighboring Virgo Cluster. Credit: NOIRLab/NSF/AURA/NASA/R. Gendler/K. Wang/M. Zamani
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NGC 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 interaction with another galaxy. Embedded within this tidal tail is the ultra-compact dwarf galaxy known as NGC 3628-UCD1. This image was captured by the US Department of Energy's (DOE) Dark Energy Camera mounted on the National Science Foundation's (NSF) Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory in Chile, a Program of NSF's NOIRLab. NGC 3628 was not part of the survey of the Virgo Cluster. Credit: CTIO/NOIRLab/DOE/NSF/AURA; Image processing: T.A. Rector (University of Alaska Anchorage/NSF's NOIRLab), M. Zamani (NSF's NOIRLab), & D. de Martin (NSF's NOIRLab)
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