Letters of Intent received in 2017

LoI 2019-2014
The Realm of the Low Surface Brightness Universe

Date: 8 July 2019 to 12 July 2019
Location: Tenerife, Spain
Contact: David Valls-Gabaud (david.valls-gabaud@obspm.fr)
Coordinating division: Division H Interstellar Matter and Local Universe
Co-Chairs of SOC: David Valls-Gabaud (Observatoire de Paris)
Sakurako Okamoto (Kavli/Peking University)
Ignacio Trujillo (IAC)
Eva Grebel (ARI)
Co-Chairs of LOC: Ignacio Trujillo (IAC)
Johan Knapen (IAC)



1. State-of-the-art in present and future instrumentation for LSB observations
2. Massive database management of LSB images
2. The nature of interplanetary dust grains in the Zodiacal light
3. Mass loss episodes, shocks and debris discs as traced by LSB features
around stars
4. Low surface brightness features around galaxies: signatures of past
and on-going accretion and their constraints on dark haloes
5. The nature of ultra diffuse galaxies and other galaxies
discovered by LSB surveys
6. The intracluster light and its role in galaxy evolution in clusters
7. The circumgalactic medium of high-redshift galaxies
8. The cosmic web of large-scale filaments
9. The UV/optical/IR cosmological background radiation and its fluctuations
10. The role of amateurs in LSB observations and citizen science LSB projects



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. Low surface brightness imaging has also
discovered recently a wealth of structures around galaxies, which possibly
are the remnants and signatures of hierarchical galaxy formation.
Amateurs have remarkably contributed to this field, which is also a perfect
tool for citizen science.

These discoveries, among others, 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. This applies to
all wavelengths considered (UV/optical/IR/radio) and new ground-
and space-based projects aim at mapping the sky at unprecedented
deep surface brightness limits. These maps will challenge not only
the way to perform photometry (galaxies appear to have no edges,
trailing off in the intergalactic medium) but also require
novel ways to deal with the management of massive databases.

With these instruments, and given that surface brightness is, to first
order, independent of distance, 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 current LCDM 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
appear to be 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 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 light scattered by planetary dust grains in the Zodiacal cloud,
and also by the so-called cirri, interstellar grains close to bright stars,
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 physical 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? Disc debris are likewise detected
rather close to their parent stars, but their extent is such that they can
be detected at much larger angular separations through scattering. How far
do they actually extend? How are they related to planet formation in the
outskirts of these systems?

The Zodiacal light changes subtly due to meteoritic and cometary streams, but
measuring such changes requires sensitive instrumentation. New optical designs
are being explored to survey the sky at extremely low surface brightness
levels, both from the ground and from space.

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.
The topics addressed in this Symposium go well beyond previous (somewhat)
related symposia such as IAU S321 "Formation and evolution of galaxy outskirts"
(2016), and IAU S317 "The General Assembly of Galaxy Halos: Structure, Origin
and Evolution" (2015), and not have been addressed before comprehensively.

In fact, while the existence of these low surface brightness features
was found back in the 1970s, with the first detections on photographic plates,
it is extraordinarily surprising that there has been only one single
IAU meeting (Colloquium 171,"The low surface brightness universe", Cardiff),
back in 1998, devoted to the subject.

Over 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.