Extragalactic stellar tidal streams in the Dark Energy Survey

Stellar tidal streams are a key tracer of galaxy evolution and have the potential to provide an indirect means for tracing dark matter. For the Local Group, many diffuse substructures have been identified and their link to galaxy evolution has been traced. However, the Local Group does not offer a statistically significant sample of stellar tidal streams. Thus, an analysis of a larger sample beyond the Local Group is required to better probe the frequency and characteristics of these streams to verify whether these properties are in agreement with the predictions of the Lambda CDM model and its implementation in cosmological simulations, taking into account the impact of the baryonic physics modelling. The main scope of the Stellar Stream Legacy Survey is to obtain a statistically significant sample of stellar streams in the local Universe to be able to trace and study minor mergers and their contribution to galaxy evolution with respect to the Lambda CDM theory. For that purpose, we are carrying out the first systematic survey of faint stellar debris from tidally disrupted dwarf satellites around nearby galaxies up to a distance of 100 Mpc. In this paper, we present a catalogue with the results of the first harvest of stellar tidal streams found by visual inspection in deep images of sim 700 galaxies from the Dark Energy Survey (DES). We also include, for the first time, a photometric characterisation of the streams obtained by measuring their surface brightnesses and colours. We found a total of 63 streams in our sample at distances between 40 and 100 Mpc, including 58 that were not previously reported. We measured their average surface brightness for the $g$ band, the $r$ band and the $z$ band, to be 28.35pm 0.20, 27.81pm 0.13, and 27.62pm 0.09 mag arcsec$^ $, respectively. By applying a statistical analysis to our findings, we obtained a stream detection frequency of 9.1 pm 1.1 for the given surface brightness limit of the DES image sample, in agreement with previous studies. We identified stream progenitors in 5--14 of our stream sample, depending on the confidence level. The first catalogue of streams in the local Universe presented here will be complemented by future stream surveys within the Stellar Stream Legacy Survey and can be exploited in studies pertaining to galaxy evolution and cosmological models. In this work, we have learnt that the faintest measured stream surface brightness can be significantly brighter than the surface brightness limit of an image measured at the pixel level (in our case up to sim 1 mag arcsec$^ $ for the $r$ band) mainly due to correlated noise present in the images.