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Low Surface Brightness structures

The study of the morphology of galaxies is essential to constrain models of galaxy evolution. Indeed, in the standard model, galaxies are formed through successive mergers with galaxies and other phenomena such as dark matter accretion or continuous process of cold gas. Vestiges of these past interactions between galaxies remain today, and they are very important as their shape and number informs us about the past merging history of a galaxy, and may change its apparent morphology.

However, these structures, these collisional debris, are very faint so their detection is complicated. They are called Low Surface Brightness (LSB) structures, as their flux per unit area is low. That is the reason why they have not been studied much outside the Local Group, until the use of powerful enough telescopes made their studies possible.

In particular, the Canada-France-Hawaii Telescope (CFHT) provided deep enough images of some galaxies that were studied with the MATLAS survey: many LSB structures around elliptical galaxies were discovered and classified. More recently, CFIS (Canada-France Imaging Suvey) is a large CFHT program aiming at mapping a large part of the Northern hemisphere (5,000 square degrees) with deep images, where MATLAS only performed pointed observations of galaxies. CFIS images are interesting by their extended spatial coverage on the sky and their depth, making possible the detection and classification of LSB structures around a large number of galaxies.

Aim of this website

The aim of this server is to enable people from the scientific community to easily identify and classify LSB structures around galaxies in CFIS,MATLAS and NGVS images. In particular, this website enables users to draw with precision the shapes of LSB structures , overplotted on the images. This precise delimitation of the structures will enable scientists to retrieve quantitative measurements about them, which is a feature that is not possible on MATLAS website.

As explained in more details in the tutorial, for each galaxy, the user will delimitate the boundary of each LSB structure as well as the galaxy, the shape of its halo and its potential companion galaxy. Each shape drawn is then properly labeled. Additional information such as the presence of galactic cirrus or contaminants (like ghosted halos, satellite trails, high background) can be added.


MATLAS (Mass Assembly of early-Type GaLAxies with their fine Structures) investigates the mass assembly of Early-Type Galaxies (ETGs) and the build-up of their scaling relations, with extremely deep optical images. The stellar populations in the outermost regions of ETGs, the fine structures (tidal tails, stellar stream, and shells) around them, the Globular Cluster (GCs) and dwarf satellites, preserve a record of past merger events and more generally of the evolution and transformation of galaxies.

This Large Programme of the Canada-France-Hawaii Telescope capitalizes on several timely developments:
(1) The unique capabilities of the MegaCam camera on the CFHT. Dedicated imaging procedures and a LSB-optimized pipeline allow us to detect low surface brightness structures, as faint as 29 mag/arcsec2 in the g band.
(2) A stunning ancillary dataset provided by the ATLAS3D project giving complementary information on their dynamics, gas and stellar content.
(3) Numerical simulations that predict the various types of fine-structures and stellar halos depending on the mechanism driving the mass assembly: major or minor, wet or dry mergers; secular evolution.

The database collected as part of this Large Programme complemented by earlier observations with the NGVS, consists of multi-color images for a volume limited sample of 238 nearby ETGs and 116 Late Type galaxies located in their field (belonging to the Atlas3D parent sample), together with about 2000 dwarf galaxy satellites.


CFIS (Canada-France Imaging Survey) is a CFHT Large Program whose aim is to address some questions in cosmology, such as the assembly of the Milky Way, the properties of dark matter and dark energy, or the growth of structures in the Universe. Thanks to its large area covered and high image quality, CFIS data will be very useful as a complement of the future space mission EUCLUD that will tackle the dark matter and dark energy issues.

In addition, CFIS large spatial coverage (5,000 square degrees in the Northern hemisphere), and excellent r-band image quality can also be used to study with precision low surface brightness structures around galaxies. Therefore, fine structures can be studied around many ETGs and LTGs. By combining CFIS and MATLAS data, it is possible to create a reference sample of nearby massive galaxies around which fine structures can be studied.