We identify six ultra-diffuse galaxies (UDGs) outside clusters in three nearby isolated groups (0.014 < z < 0.026) using very deep imaging in three different Sloan Digital Sky Survey filters (g, r and i bands) from the IAC Stripe82 Legacy Project. By comparing with the abundance of UDGs in rich galaxy clusters, we find that the density of UDGs (i.e. their number per unit mass of the host structure where they are located) decreases towards the most massive systems. This is compatible with a scenario where UDGs are formed preferentially outside clusters. In the periphery (D > 250 kpc) of our three groups, we identify a population of potential UDG progenitors (two of them confirmed spectroscopically). These progenitors have similar masses, shapes and sizes but are bluer, g − i ∼ 0.45 (and for this reason brighter, µ g (0) < 24 mag arcsec −2 ) than traditional UDGs (g − i ∼ 0.76). Passive evolution of these progenitors will transform them into regular (i.e. µ g (0) > 24 mag arcsec −2 ) UDGs after ∼6 Gyr. If confirmed, our observations support a scenario where UDGs are old, extended, low surface brightness dwarf galaxies (M ∼ 10 8 M ) born in the field, are later processed in groups and, ultimately, infall into galaxy clusters by group accretion.

Taking advantage of the Sloan Digital Sky Survey Stripe82 data, we have explored the spatial distribution of ultra-diffuse galaxies (UDGs) within an area of 8×8 Mpc 2 centred around the galaxy cluster Abell 168 (z = 0.045). This intermediate massive cluster (σ = 550 km s −1 ) is surrounded by a complex large-scale structure. Our work confirms the presence of UDGs in the cluster and in the large-scale structure that surrounds it, and it is the first detection of UDGs outside clusters. Approximately 50 per cent of the UDGs analysed in the selected area inhabit the cluster region (∼11 ± 5 per cent in the core and ∼39 ± 9 per cent in the outskirts), whereas the remaining UDGs are found outside the main cluster structure (∼50 ± 11 per cent). The colours and the spatial distribution of the UDGs within this large-scale structure are more similar to the dwarf galaxies than to L galaxies, suggesting that most of UDGs could be bona fide dwarf galaxies.

The ubiquitous presence of Galactic cirri in deep optical images represents a major obstacle to study the low surface brightness features of extragalactic sources. To address this issue, we have explored the optical properties of cirri using g, r, i, and z bands in the Sloan Digital Sky Survey (SDSS) Stripe82 region. Using state-of-the-art, custom made, image processing techniques, including the modeling and removal of the instrumental scattered light produced by the stars, we managed to isolate the optical diffuse emission by the cirri, allowing their photometric characterization. We find that their optical colors are driven by the dust column density: The cirri become redder as their 100 μm emission increases. Remarkably, the optical colors of the Galactic cirri differ significantly from those of extragalactic sources, with a characteristic bluer r-i color for a given g-r, allowing one to detect these by using a simple color relation. Our results show the high potential of deep multi-band optical photometry, on its own, identifying the presence of cirri at a higher spatial resolution than those provided by far-infrared observations. The combination of very deep data and multi-band photometry (as the one produced by LSST and Euclid) would make it possible to build dust maps of unprecedented quality.