Letters of Intent received in 2016

LoI 2018-1927
The Realm of the Low Surface Brightness Universe

Topics

1. The nature of ultra diffuse galaxies and other galaxies discovered by LSB surveys
2. Low surface brightness features around galaxies: signatures of past and on-going accretion and their constraints on dark haloes
3. The intracluster light and its role in galaxy evolution in clusters
4. The circumgalactic medium and the cosmic web of large-scale filaments
5. Properties of interstellar dust grains in cirri
6. Mass loss episodes in giant and AGB stars traced by LSB features
7. State-of-the-art in LSB instrumentation on small and large telescopes

Rationale

Preliminary list of SOC members

Roberto Abraham - Canada
Pierre-Alain Duc - France
Laura Ferrarese - Canada
Gaspar Galaz - Chile
Jin Koda - USA/Japan
David Martinez Delgado - Germany
Ignacio Trujillo - Spain
Chris Mihos - USA
Sakurako Okamoto - China
David Valls-Gabaud - France/UK
Pieter van Dokkum - USA
Rosemary Wyse - USA

Rationale

The existence of large numbers of ultra diffuse galaxies, an hitherto unknown
type of galaxies, is arguably one of the major discoveries in extragalactic
astronomy made over the past few years. This ubiquitous new population of
galaxies, which are dwarfs in terms of luminosities, but giants in terms
of sizes, constitutes a major challenge to our current understanding
of galaxy formation and evolution.

The discoveries arise thanks to both new instrumentation designed to
explore the ultra-low surface brightness universe, and to new ways to
analyse images on large telescopes, pushing to extreme limits what can
be achieved in the very challenging detection of extended objects whose
brightness is a tiny fraction of the sky background.

Surface brightness being independent of distance to first order, an
increasingly large number of diffuse galaxies are being discovered, from
the outskirts of the Magellanic Clouds all the way to the Coma cluster and
the Perseus-Pisces filament, but getting their optical spectra for example is
extremely challenging and yet essential to get distances and dynamical
constraints. The question then arises of what sort of future
ground- or space-based instrumentation is required to make progress in
this field. Will their atomic gas content be detected by FAST and SKA?
What are the prospects of resolving their
stellar populations when the ELTs will become available? How their
star formation histories compare with the ones in brighter galaxies? Can
large-scale surveys be planned to explore systematically the universe
at these faint surface brightness levels?

The exploration of the low surface brightness universe has further
revealed a wealth of structures around galaxies: from streams to
shells through tidal tails, these new features are the combined signature
of the accretion history of galactic haloes and their tidal fields.
How can they constrain the shapes of the dark haloes? Can the gaps
observed in the streams be used to measure the amount of dark matter
substructures posited by the CDM paradigm of galaxy formation?

On larger scales, the gravitational potential wells of clusters of
galaxies retain a huge fraction of the remnants of these interactions
which have not merged into galactic haloes, and give rise to the
intra-cluster light which is now being detected at different redshifts.
Predictions from numerical simulations state that, at very faint levels,
most of the optical light in the cluster actually arises from this
component. How can these measures constrain the evolution of galaxies
in the harsh environment of clusters? What are the properties of the
stars which make this component?

On even larger scales, the low surface brightness sky ought to reveal
a network of filaments which constitutes the cosmic web, but observations
are currently limited mostly to the circumgalactic medium, as probed
by new instruments at large telescopes (Keck cosmic web imager, MUSE at VLT).
These filaments could contain the reservoir of
"missing baryons" which are lacking at low redshift. Major efforts are
underway to detect them, and the Symposium will be the right place to
review them. Likewise the circumgalactic medium appears to be far more
extended than anticipated and may reveal processes which were thought to be rather
unimportant. For example, Lyman-alpha haloes at high redshift could be produced by fluorescence,
photoionisation or even gas cooling and infalling into the galaxies. Could this be late accretion or, on the contrary, a signature of early feedback?

Coming back to much smaller scales, the low surface brightness sky is
dominated by the light scattered by dust grains close to bright stars,
the so-called cirri, first detected in Schmidt plates, and then in the
infrared by IRAS. While they constitute a foreground for extragalactic
studies, they are interesting in their own right. How can the images
obtained by the current observations help understanding the properties
of these dust grains? Could their polarisation properties be used to
map the magnetic field in our Galaxy, which is a major foreground for
the measurement of the B-modes of the CMB ?

On even smaller scales, low surface brightness features appear around
red giant and AGB stars, a signature of mass loss episodes. Their study
provides us with unique constraints on the processes which enrich the
interstellar medium in metals. How frequent are they? What is their metallicity and the amount of dust in these shells?

Clearly there is a huge range of questions that will be addressed during
the Symposium
to elucidate these issues and envision the future of this
exciting field.

While the existence of these low surface brightness features
was found back in the 1970s, with the first detections on photographic plates,
it is very surprising that there has been only one IAU meeting (IAU Colloquium 171,
"The low surface brightness universe", Cardiff), back in 1998, devoted to
the subject. Twenty years later, the time is ripe to allocate a Symposium
for reviewing the properties of what remains, by and large, the last
remaining niche to be explored in observational parameter space.