Andromeda XXXVI: discovery of a new ultra-faint dwarf galaxy towards M31

Abstract

We present deep imaging of Andromeda XXXVI (And XXXVI), a dwarf galaxy discovered through visual inspection of the Pan-Andromeda Archaeological Survey, using observations obtained with the OSIRIS+@GTC instrument. The colour-magnitude diagram of And XXXVI shows a well-defined red giant branch (RGB). However, constraining a distance is challenging because the tip of the RGB is sparsely populated and no horizontal branch stars are found. The RGB is nevertheless well matched by an old (12.5 Gyr), metal-poor ([Fe/H] = - 2.5) isochrone shifted to the distance of Andromeda (776 kpc). With a projected distance of 119 kpc from M31, And XXXVI is therefore likely a satellite of Andromeda. With $M_{V} = -6.0 \pm 0.2$, half-light radius $r_{h} = 64 ^{+30}_{-19}$ pc and an ellipticity $ε= 0.015^{+0.032}_{-0.012}$ And XXXVI is one of the faintest, and potentially the second most compact, of ultra-faint M31 dwarfs discovered to date. The discovery of And XXXVI adds to the faint end of M31's satellite luminosity function, suggesting the presence of an even larger population of very faint satellites. Deeper space-based imaging and/or spectroscopic observations are needed to better constrain its position within M31's halo. Combined with a detailed star formation history, such data would help determine whether its old, metal-poor stellar population indicates early quenching, similar to the trends seen in Milky Way satellites, and whether And XXXVI could be considered a reionisation fossil.

Figures (7)

• Figure 1: Location of And XXXVI (marked in red) within the Pan-Andromeda Archaeological Survey (PAndAS; McConnachie2018), located approximately $\sim$ 119 kpc in projected distance from M31. We additionally mark Peg V and Psc VII, which were discovered following the same approach used in this work. The presented map is adapted from the And450 ultra-deep astrophotography survey (Donatiello2025).
• Figure 2: Stacked OSIRIS+ image of And XXXVI with a field of view of 7.8 $\times$ 7.8 arcmin. In the inset we show a negative colour zoom-in on And XXXVI. The overdensity is clearly evident in-between two bright foreground stars. North is up, east is left.
• Figure 3: Image of And XXXVI across four different datasets. From left to right: SDSS DR9, PanSTARRS DR1, CFHT (PAndAS) and GTC (this work). The overdensity is almost invisible in SDSS and PanSTARRS, and only becomes apparent with the CFHT and GTC.
• Figure 4: Filtered maps of all sources in our GTC image categorised as 'stars' (left) and 'galaxies' (right) following our star/galaxy separation criteria. The overdensity clearly stands out in the stars panel.
• Figure 5: Colour-magnitude diagram of And XXXVI (left) within 1$r_{h}$ (blue points, 0.284 arcmin radius) and 2$r_{h}$ (blue and black points, 0.568 arcmin). A clear RGB can be delineated. In red we overlay an old (12.5 Gyr), metal poor ([Fe/H] = $-$ 2.5) PARSEC isochrone using the distance to M31 (776 kpc). The isochrone matches the RGB locus well. On the right we show the CMD of an equal sized (2$r_{h}$) area near the overdensity, selected to represent our field contamination.