The Cetus-Palca stream: A disrupted small dwarf galaxy. A prequel to the science possible with WEAVE with precise spectro-photometric distances

Abstract

We present a new fully data-driven approach to derive spectro-photometric distances based on artificial neural networks. The method was developed and tested on SEGUE data and will serve as a reference for the $Contributed$ $Data$ $Product$ SPdist of the WEAVE survey. With this method, the relative precision of the distances is of $\sim 13 \%$. The catalogue of more than 300,000 SEGUE stars for which we have derived spectro-photometric distances will soon be publicly available on the Vizier service of the Centre de Donn\'ees de Strasbourg. With this 6D catalogue of stars with positions, distances, line-of-sight velocity, and $Gaia$ proper motions, we were able to identify stars belonging to the Cetus stellar stream in the integrals of motion space. Guided by the properties we derived for the Cetus stream from this 6D sample, we searched for additional stars from the blue horizontal and from the red giant branches in a 5D sample. We find that the Cetus stream and the Palca overdensity are actually two parts of the same structure, which therefore we propose to rename the Cetus-Palca stream. We found that the Cetus-Palca stream has a stellar mass of $\simeq 1.5 \times 10^6$ M$_\odot$ and presents a prominent distance gradient of 15 kpc over the $\sim 100 \deg$ that it covers on the sky. Additionally, we also report the discovery of a second structure, almost parallel to the Cetus stream, covering $\sim 50 \deg$ of the sky, that could potentially be a stellar stream formed by the tidal disruption of a globular cluster that was orbiting around the Cetus stream progenitor.