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MNRAS has recently published an article by Ilin Lazar and collaborators (including Sugata Kaviraj and Aaron Watkins) describing the morphological diversity of nearby dwarf galaxies.

The paper, The morphological mix of dwarf galaxies in the nearby Universe, performsa novel, unbiased statistical study of dwarf morphology at z < 0.08. It offers both an unprecedented view of dwarfs using the deep Hyper Suprime-Cam surveys (the precursor to LSST) and a demonstration of the type of science that will be possible in the forthcoming LSST era.

Typical dwarfs are too faint to be visible, outside the local neighbourhood, in past surveys like the SDSS. Unbiased studies of dwarfs requires surveys that are both deep and wide. Lazar et al. use a complete, unbiased sample of 257 dwarf (10⁸ M_⊙ < M_⋆ < 10^9.5 M_⊙) galaxies at z < 0.08 in the COSMOS field, where deep ultraviolet to mid-infrared imaging is available, to study the morphological mix of dwarfs in low-density environments.

The study uses visual inspection of extremely deep optical images from the Hyper Surpime-Cam and their unsharp-masked counterparts to reveal three principal dwarf morphological classes (as shown in the figure). 43 and 45 per cent of dwarfs exhibit the traditional 'early-type' (elliptical/S0) and 'late-type' (spiral) morphologies respectively.

However, 10 per cent populate a 'featureless' class, that lacks both the central light concentration seen in early-types and any spiral structure - this class is missing in the massive-galaxy regime. 14, 27 and 19 per cent of early-type, late-type, and featureless dwarfs respectively show evidence for interactions, which drive around 20 per cent of the overall star formation activity in the dwarf population.

Lazar et al. find that, compared to their massive counterparts, dwarf early-types show a much lower incidence of interactions, are significantly less concentrated and share similar rest-frame colours as dwarf late-types. This suggests that the formation histories of dwarf and massive early-types are different, with dwarf early-types being shaped less by interactions and more by secular processes. The lack of large groups or clusters in COSMOS at z < 0.08, and the fact that the dwarf morphological classes show similar local density, suggests that featureless dwarfs in low-density environments are created via internal baryonic feedback, rather than by environmental processes.

Finally, the study finds that, while interacting dwarfs can be identified using the asymmetry parameter, it is challenging to cleanly separate early and late-type dwarfs using traditional morphological parameters, such as 'CAS', M20, and the Gini coefficient (unlike in the massive-galaxy regime).