Explain the formation and population of almost-dark galaxies

Characterize the formation mechanisms, abundance, and environmental conditions of almost-dark galaxies with very low surface brightness and large effective radii.

Background

Almost-dark galaxies are described as systems with exceptionally faint central surface brightness, often combined with effective radii comparable to those of ultra-diffuse galaxies but with much lower luminosities. The paper notes that current galaxy simulations cannot reproduce this combination of properties.

The discovery of TTT J1237327+143535 supports the possibility that a substantial population of such galaxies exists in the Virgo Cluster. The authors therefore identify unresolved questions concerning how these galaxies form, how numerous they are, and the environments in which they survive.

References

This raises many unanswered questions on the formation, abundance, and environments where those galaxies stay.

— Serendipitous discovery of an almost-dark galaxy in the Virgo Cluster  (2608.19326 - Le et al., 19 Aug 2026) in Section 1, Introduction, page 1

Although UDGs are commonly regarded as an extreme subset of the broader LSB population, it remains unclear whether UDGs simply represent the high-size or high-spin tail of LSB galaxies or instead constitute a physically distinct class.

— An HI study of a large sample of ultra-diffuse galaxies  (2608.24225 - Zhou et al., 25 Aug 2026) in Section 1, Introduction