Low surface brightness (LSB) galaxies are known since the 80s when they were initially studied in the Virgo and Fornax clusters (see e.g. Sandage & Binggeli 1984). With the discovery of a large population of particularly extended LSB galaxies in the Coma cluster, named ultra-diffuse galaxies (UDGs, van Dokkum et al. 2015), the interest in the study of these objects has significantly accelarated.
UDGs are among the lowest surface brightness and largest dwarf galaxies known in the universe, with a central surface brightness of μ₀,_g ≥ 24 mag/arcsec², and an effective radius Re ≥ 1.5 kpc. Being so diffuse, UDGs should host a large amount of dark matter (DM) to survive cluster tides. This hypothesis triggered an ever-increasing attention to the detection and study of UDGs and gave these galaxies a special role in the realm of the LSB universe. Since 2015, numerous observational campaigns have been carried out to get deep images and spectroscopic data to constrain the characteristics of LSB galaxies, including UDGs, in different environments from isolation to clusters of galaxies. UDGs turn out to be the most intriguing systems.

This special session aims to bring together junior and senior researchers working across different areas of imaging and spectroscopic observations, modeling and simulations, in order to trigger a timely discussion on the nature of LSB galaxies, mainly UDGs.
In particular, we expect to tackle the following open questions emerging from observational results and simulations:
- Based on the observed colors, kinematics, star formation history, do different classes of UDGs exist?
- How do the observed properties of UDGs reconcile with the different formation scenarios?
- Can we confirm that UDGs also differ in terms of DM content? And that this is related to the formation channel?
- How do the UDGs compare with the classical dwarf galaxies? In particular, can we establish an evolutionary link between UDGs and other dwarf galaxies through a comparison of their stellar population and structural properties?
- Do the observed properties of UDGs correlate with the environment?
- Being so diffuse, are the UDGs with dwarf-like DM halo long-lived systems?

Programme

The goal of this session is to offer a comprehensive program with talks covering the main field of research on UDGs, as listed below:

• Structural properties based on imaging & spectroscopic data (i.e. light distribution, colors, GCs content, stellar kinematics, age and metallicity, gas content)
• Dynamics and masses: DM content
• Simulations: formation scenarios versus observations

Invited speakers

• Jonah Gannon Swinburne University, Australia
• Anna Ferre-Mateu IAC, Sapin
• Laura V. Sales University of California, USA

Scientific organisers

• Enrichetta Iodice (INAF-Astronomical Observatory of Capodimonte, Italy, Chair)
• Shany Danieli (Princeton University, USA)
• Arianna Di Cintio (Instituto de Astrofísica de Canarias, Spain)
• Duncan A. Forbes (Swinburne University, Australia)
• Michael Hilker (European Southern Observatory, Germany)
• Francine Marleau (University of Innsbruck, Austria)
• Oliver Müller (Ecole Polytechnique Fédérale de Lausanne, Switzerland)
• Dennis Zaritsky (University of Arizona, USA)

Contact

Enrichetta Iodice - email: enrichetta.iodice @ inaf.it