Deblending galaxies with Variational Autoencoders: a joint multi-band,
  multi-instrument approach
  Blending of galaxies has a major contribution in the systematic error budget
of weak lensing studies, affecting photometric and shape measurements,
particularly for ground-based, deep, photometric galaxy surveys, such as the
Rubin Observatory Legacy Survey of Space and Time (LSST). Existing deblenders
mostly rely on analytic modelling of galaxy profiles and suffer from the lack
of flexible yet accurate models. We propose to use generative models based on
deep neural networks, namely variational autoencoders (VAE), to learn
probabilistic models directly from data. We train a VAE on images of centred,
isolated galaxies, which we reuse, as a prior, in a second VAE-like neural
network in charge of deblending galaxies. We train our networks on simulated
images including six LSST bandpass filters and the visible and near-infrared
bands of the Euclid satellite, as our method naturally generalises to multiple
bands and can incorporate data from multiple instruments. We obtain median
reconstruction errors on ellipticities and $r$-band magnitude between
$\pm{0.01}$ and $\pm{0.05}$ respectively in most cases, and ellipticity
multiplicative bias of 1.6% for blended objects in the optimal configuration.
We also study the impact of decentring and prove the method to be robust. This
method only requires the approximate centre of each target galaxy, but no
assumptions about the number of surrounding objects, pointing to an iterative
detection/deblending procedure we leave for future work. Finally, we discuss
future challenges about training on real data and obtain encouraging results
when applying transfer learning. Our code is publicly available on GitHub
(https://github.com/LSSTDESC/DeblenderVAE).
