Optical constraints on the coldest metal-poor population

The coldest metal-poor population made of T and Y dwarfs are archaeological tracers of our Galaxy because they are very old and have kept the pristine material. The optical properties of these objects are important to characterise their atmospheric properties. We aim at characterising further the optical properties of ultracool metal-poor population with deep far-red optical images and parallax determinations. With a two-year baseline, we solved trigonometric parallaxes of the five metal-poor T dwarf candidates using Calar-Alto 3.5-m telescope. We obtained $z'$-band photometry for the other 12 metal-poor T dwarf candidates using 10.4-m Gran Telescopio Canarias, the 8.2-m European Southern Observatory Very Large Telescope, and the Dark Energy Survey, increasing the sample of T subdwarfs with optical photometry from 12 to 24. We report a 3-$\sigma$ limit for the only potential metal-poor Y dwarf, a.k.a., the Accident in five optical bands using the Gran Telescopio Canarias. We compared these objects with a known subdwarf benchmark and solar-metallicity dwarfs in colour-magnitude and colour-colour diagrams, as well as with state-of-the-art theoretical ultracool models. We confirm three more T subdwarfs and show that the Accident is subluminous compared to the current Y dwarf limit. Additionally, we propose two more Y subdwarf candidates. We emphasise that the $z_{PS1} - W1$ colour combining with the $W1 - W2$ colour could break the metallicity-temperature degeneracy for T and possibly for Y dwarfs. The $z_{PS1} - W1$ colour shifts redward when metallicity decreases for a certain temperature, which is not predicted by models. The Accident has the reddest $z_{PS1} - W1$ colour among our sample. The $z_{PS1} - W1$ colour will be useful to search for other examples of this cold and old population in upcoming and existing deep optical and infrared large-area surveys.