February 2026 - Picture of the Month
Ultra-Diffuse Galaxy (UDG) 32 in Hydra
My feature object this month is the ultra-diffuse galaxy UDG 32 in Hydra, which can just be seen as a smudge to the lower right, although I have to admit that the more spectacular NGC 3314 in the upper left certainly lifts the image.
Ultra-diffuse galaxies (UDGs) are very faint, as this image clearly shows, much like dwarf galaxies. In fact, they might be consider as the same class of object given their similar masses, except that UDGs are the same size as galaxies like the Milky Way, and far from being dwarf in their physical extent. Their faintness arises from their sparse old stellar populations that contain 100—1000 times fewer stars than similar sized galaxies.
The cause of such large yet sparse galaxies remains an open question. They're a diverse group of objects with some comprised of almost nothing but dark matter and others having very little. The main contenders appear to be that UDGs are either failed galaxies or dwarf galaxies that have grown in size, possibly inflated by the action of supernovae. They may represent separate populations of both types of object. 1
There's suggestion that this month's feature, UDG 32, might have been formed from the stellar filaments from NGC 3314A, one of the pair of galaxies lending spice to this image, as it lies about 40 kpc away in its trail. 2
NGC 3314 is a pair of overlapping and non-interacting galaxies imaginatively designated as NGC 3314A, the foreground face-on spiral galaxy of about 16.8 kpc (55,000 light-years) diameter, and NGC 3314B, which is a much larger spiral galaxy whose diameter is about 83 kpc (270,000 light-years). The Hubble Space Telescope has an excellent close-up of the NGC 3314 pair and for their apparent sizes to be so close there must be a large distance between them, about 26 Mpc (85 million light-years). 3
The very nature of UDG 32 makes it unlikely to be a target for imagers, let alone visual observers, without the benefit of a very large telescope. However the NGC 3314 pairing is a possible target for both imaging and visual observation, although the latter will need a large scope. It will be challenging since it's faint and rises less than 10° above the horizon in the UK; those further south may have more luck.
James Whinfrey - Website Administrator.
References
- Maria Luisa Buzzo, Duncan A Forbes, Thomas H Jarrett, Francine R Marleau, Pierre-Alain Duc, Jean P Brodie, Aaron J Romanowsky, Anna Ferré-Mateu, Michael Hilker, Jonah S Gannon, Joel Pfeffer, Lydia Haacke, “The multiple classes of ultra-diffuse galaxies: can we tell them apart?”, Monthly Notices of the Royal Astronomical Society, Volume 536, Issue 3, January 2025, Pages 2536–2557, https://doi.org/10.1093/mnras/stae2700
- “Formation of an ultra-diffuse galaxy in the stellar filaments of NGC 3314A: Caught in the act?”, Enrichetta Iodice, Antonio La Marca, Michael Hilker, Michele Cantiello, Giuseppe D’Ago, Marco Gullieuszik, Marina Rejkuba, Magda Arnaboldi, Marilena Spavone, Chiara Spiniello, Duncan A. Forbes, Laura Greggio, Roberto Rampazzo, Steffen Mieske, Maurizio Paolillo, Pietro Schipani, A&A 652 L11 (2021), DOI: 10.1051/0004-6361/202141086
- Data for the NGC 3314 pair of galaxies taken from NASA/IPAC Extragalactic Database (NED) on 27 January 2026. The dodgy maths of the conversions to light-years is my own.
