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Press Release
Open Access
Issue
A&A
Volume 710, June 2026
Article Number A342
Number of page(s) 8
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/202660151
Published online 25 June 2026

© The Authors 2026

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1. Introduction

Ultra-faint dwarf galaxies (UFDs; MV > −7.7, Simon 2019) represent the extreme low-mass end of galaxy formation (M* ≤ 105 M⊙). Their shallow potential wells make them particularly sensitive to processes that regulate galactic growth, such as galactic outflows driven by stellar feedback (e.g. Bovill & Ricotti 2009; Applebaum et al. 2021; Collins & Read 2022; Rey et al. 2025) and reionisation at early cosmic times (e.g. Efstathiou 1992; Bullock et al. 2000; Brown et al. 2014; Weisz et al. 2014). If these tiny galaxies formed the majority of their stars prior to the epoch of reionisation (and have undergone little to no evolution since) then they may be considered reionisation fossils and offer precious constraints on pre-ionisation galaxy formation (e.g. Gnedin & Kravtsov 2006). Additionally, UFDs are some of the most dark matter-dominated systems known (e.g. Bullock & Boylan-Kolchin 2017), and their abundance and spatial distribution places observational constraints on the population of dark matter subhaloes around massive galaxies (e.g. Simon & Geha 2007). However, as a significant majority of these constraints have come from the Milky Way’s (MW) own UFD population, studying external systems is thus important to avoid biasing our understanding of galaxy evolution and dark matter.

Given its proximity (776 kpc, Savino et al. 2022) and a massive dark matter halo ( M 200 M 31 ∼ 2.0 ± 0.5 × 10 12 M ⊙ Mathematical equation: $ M_{200}^{M31}\sim 2.0 \pm 0.5 \times10^{12}\, M_\odot $; Fardal et al. 2013) expected to host plenty of substructures, the Andromeda galaxy (M31) provides an excellent test bed for identifying and characterising extremely low-luminosity systems. More than 40 dwarf galaxies are now known in the M31 system (Pace 2025), of which ∼16 are classified as UFDs. This is a significant increase from the roughly ten M31 dwarfs found by the end of the last century, which were mainly discovered through visual inspection of photographic plates (e.g, van den Bergh 1972; Armandroff et al. 1998; Karachentsev & Karachentseva 1999). The advent of wide-field photometric surveys, such as the Sloan Digital Sky Survey (SDSS; Abazajian et al. 2009), the Panoramic Survey Telescope and Rapid Response System (PanSTARRS; Chambers et al. 2016), the DESI Legacy Imaging Survey (DESI LS; Dey et al. 2019), and UNIONS (Gwyn et al. 2025), has greatly accelerated the discovery of dwarf galaxies within and beyond the M31 halo (∼150 kpc in projection). Thanks to their contiguous wide-field coverage, new dwarf galaxies have been discovered through both visual inspection of stacked images (e.g. Slater et al. 2011; Martin et al. 2013c,b) and searches for resolved red giant branch (RGB) star overdensities (e.g. Zucker et al. 2004; Martin et al. 2013a), which were later confirmed as real systems through deeper photometric or spectroscopic follow-up (e.g. Zucker et al. 2007; Bell et al. 2011; McQuinn et al. 2023).

Yet the most substantial expansion of the known M31 satellite population has come from the Pan-Andromeda Archaeological Survey (PAndAS; e.g. McConnachie et al. 2009, 2018), carried out using the MegaCam wide-field imager on top of the Canada-France-Hawaii Telescope (CFHT). Contiguously mapping ∼150 kpc of M31 and ∼50 kpc of M33 to depths reaching three magnitudes below the tip of the RGB, its depth and wide-field view accelerated galactic archaeology of M31’s halo, leading to the discovery of more than 20 dwarf satellites, down to luminosities of MV ∼ −6 (e.g, Martin et al. 2006; Ibata et al. 2007; McConnachie et al. 2008; Martin et al. 2009; Richardson et al. 2011; Martin et al. 2016). Despite its depth, however, ground-based imaging of galaxies at the distance of M31 typically reaches only the red clump region of the colour-magnitude diagram (CMD). This makes it increasingly difficult to detect systems fainter than MV ∼ −6, as the number of RGB stars becomes considerably low, although a few discoveries have been made recently (Arias et al. 2025; Smith et al. 2025). Identifying diffuse or partially resolved overdensities with visual inspection and following up on candidates with deeper observations therefore remains a complementary approach to extending M31’s observed satellite luminosity function (Pace 2025, their figure 2). This approach is more sensitive to partially resolved (and unresolved) systems that could be otherwise missed by match-filter searches.

In an effort to identify new M31 satellites, we extended our visual search campaign to archival PAndAS data to locate semi-resolved candidates that may have been previously missed by automated searches. Our campaign has already identified UFDs in the outskirts of M31, such as Pisces VII (Psc VII; Martínez-Delgado et al. 2022; Collins et al. 2024) and Pegasus V (Peg V; Collins et al. 2022), initially identified in shallower DESI LS data. Their star formation histories (SFHs) have since been found to suggest that both systems may have been quenched by cosmic reionisation (Jones et al. 2026), a scenario more commonly observed in MW satellites and potentially linked to differences in the accretion histories of the MW and M31 (e.g. Sacchi et al. 2021; Savino et al. 2025). These emerging differences between the MW and M31 satellite populations highlight the importance of expanding the census of UFDs across the Local Group.

In this paper, we present the discovery of Andromeda XXXVI (And XXXVI). In Sect. 2 we describe our follow-up imaging with the OSIRIS+ instrument. In Sect. 3 we characterise And XXXVI, including its structural properties and luminosity. Finally, in Sect. 4 we discuss And XXXVI and present our conclusions.

2. Observations and data reduction

Andromeda XXXVI (And XXXVI) was discovered by amateur astronomer Giuseppe Donatiello during a systematic visual inspection of public images from the full PAndAS footprint (Figure 1). The search followed the same approach as our previous studies with the DESI LS (M31: Martínez-Delgado et al. 2022; Collins et al. 2022, Do I: Martínez-Delgado et al. 2018, NGC 253: Martínez-Delgado et al. 2021, 2026). A total of 11 candidates were shortlisted, and we selected the two most conspicuous semi-resolved overdensities for follow-up observations with the Director’s Discretionary Time (GTC2025-292.227, PI: Martínez-Delgado) using the 10.4-m Gran Telescopio Canarias (GTC; Roque de Los Muchachos Observatory, La Palma, Spain). In this work, we present our results for one of the two candidates.

Thumbnail: Fig. 1. Refer to the following caption and surrounding text. Fig. 1.

Location of And XXXVI (marked in red) within PAndAS (McConnachie et al. 2018). And XXXVI is located approximately ∼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 was adapted from the And450 ultra-deep astrophotography survey (Donatiello 2025).

We took deep images of And XXXVI on January 22, 2026, with the Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS+)1 instrument on top of the GTC. We used an un-vignetted field of view of 7.8′×7.8′ and a scale of 0.254″ pixel−1. The total exposure time was 20 × 150 s = 3600 s and 20 × 149 s = 2980 s for the g′ and r′ photometric bands, respectively. A ten-point dithering pattern (10″ offsets) was applied to correct for chip defects and improve image sampling. The seeing was around 0.9″ in g′ and r′.

Images were processed using SAUSERO2, a Python pipeline developed specifically to reduce OSIRIS+@GTC broadband imaging that is included in the standard GTC pipeline3. Scientific images were pre-processed in the standard way (i.e. performing over-scan correction, bias subtraction, flat field division, and astrometrisation via Gaia-DR3 stars) to produce a single final stacked image for each of the observed bands. The resulting image is shown in Figures 2 and 3- the overdensity is significantly more resolved in the GTC image than in the CFHT data belonging to the PAndAS survey. Prior to PAndAS, And XXXVI was almost invisible in PanSTARRS and SDSS due to its low-surface brightness.

Thumbnail: Fig. 2. Refer to the following caption and surrounding text. Fig. 2.

Stacked OSIRIS+ image of And XXXVI with a field of view of 7.8 × 7.8 arcmin. In the inset we show a negative colour zoom-in on And XXXVI. The overdensity can be clearly seen between the two bright foreground stars. North is up and east is left.

Thumbnail: Fig. 3. Refer to the following caption and surrounding text. Fig. 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 it only becomes apparent with in CFHT and GTC.

The photometric reduction was performed using the DAOPHOT suite of codes (Stetson 1987), performing the typical sequence DAOPHOT-ALLSTAR-ALFRAME. Aperture photometry in individual images was realised adopting a 3σ threshold. The point spread function (PSF) was estimated by selecting stars using automatic routines. These routines pick up bright sources with a low photometric error and a good parameter shape while covering the full field of view to properly model the PSF spatial variations. After running ALLSTAR on individual images, ALLFRAME was used for the simultaneous reduction of all images, assuming an input list of stars obtained by merging the ALLSTAR catalogues. The PSF extraction and ALLFRAME steps were repeated two more times to improve the fit quality as well as the geometric transformation between images. Special care was devoted to cleaning the input list from background unresolved sources and bad detections around saturated stars.

The photometric calibration curve was derived by a linear fit using stars in common with PanSTARRS. We assumed a relation of the form

g _ cal = g _ instr + 8.856 + 0.043 ∗ ( g − r ) _ instr i _ cal = i _ instr + 8.884 + 0.088 ∗ ( g − r ) _ instr Mathematical equation: $$ \begin{aligned}&g\_{cal} = g\_{instr} + 8.856 + 0.043*(g-r)\_{instr} \nonumber \\&i\_{cal} = i\_{instr} + 8.884 + 0.088*(g-r)\_{instr} \end{aligned} $$(1)

with rms ∼ 0.04 mag. We corrected the data for extinction using the Schlegel et al. 1998 reddening maps, recalibrated by Schlafly & Finkbeiner 2011. For our star-galaxy separation criteria, we selected stars that are measured by DAOPHOT to have |0.2|< SHARP. In Figure 4, we recover the overdensity using this cut (left, ‘stars’ panel), showing that it blends into the field when |0.2|> SHARP (middle, ‘galaxies’ panel).

Thumbnail: Fig. 4. Refer to the following caption and surrounding text. Fig. 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.

3. Characterising And XXXVI

In Figure 5 (left), we present the CMD of And XXXVI, selecting stars within a 0.284 arcmin (blue) and 0.568 arcmin (black) radius. To help disentangle And XXXVI from field stars, in the same Figure we show a CMD within an equally sized (0.568 arcmin) area centred approximately ∼1 arcmin away (right). The RGB is clearly seen in the overdensity and not seen in the field stars. As horizontal branch stars cannot be clearly identified, in red we overlay an old (12.5 Gyr) metal-poor ([Fe/H] = −2.5) isochrone from the PARSEC library (Bressan et al. 2012) at the average distance of M31 (776 kpc, Savino et al. 2022). While it is difficult to say whether the brightest stars (r0 ≤ 22) are probable members or contaminating halo stars, the isochrone fits the RGB well overall.

Thumbnail: Fig. 5. Refer to the following caption and surrounding text. Fig. 5.

Colour-magnitude diagram of And XXXVI (left) within 1rh (blue points, 0.284 arcmin radius) and 2rh (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 (2rh) area near the overdensity, selected to represent our field contamination for visual reference (see text for details on field contamination determination).

To measure the luminosity and structural parameters of And XXXVI, we followed the methodology described in Martínez-Delgado et al. (2022), Collins et al. (2022, 2024). First, we applied the EMCEE code (Foreman-Mackey et al. 2013) to determine the structural properties of And XXXVI, adapting the methods from Martin et al. (2016). Given EMCEE does not use distance or metallicity information, our structural parameter measurements are unaffected by different distance and metallicity assumptions. This approach assumes that the overdensity contains N* observed member stars with a radial density profile, ρdwarf(r), that can be described as

ρ dwarf ( r ) = 1 . 68 2 2 π r h 2 ( 1 − ϵ ) N ∗ exp ( − 1.68 r r h ) , Mathematical equation: $$ \begin{aligned} \rho _{\rm {dwarf}}(r) = \frac{1.68^2}{2\pi r_{\rm h}^2(1-\epsilon )}N^*\exp {\left(\frac{-1.68r}{r_{\rm h}}\right)} , \end{aligned} $$(2)

with ϵ representing its ellipticity (ϵ = 1 − (b/a)) and rh as the half-light radius. The elliptical radius (r) is defined as

r = ( ( 1 1 − ϵ ( ( x − x 0 ) cos θ − ( y − y 0 ) sin θ ) ) 2 + ( ( x − x 0 ) sin θ − ( y − y 0 ) cos θ ) 2 ) 1 2 , Mathematical equation: $$ \begin{aligned} r=&\Bigg ( \Big (\frac{1}{1-\epsilon }((x-x_{0})\cos {\theta }-(y-y_{0})\sin {\theta })\Big )^2 \nonumber \\&+\Big ((x-x_{0})\sin {\theta }-(y-y_{0})\cos {\theta }\Big )^2 \Bigg ) ^{\frac{1}{2}} , \end{aligned} $$(3)

where x0 and y0 are coordinates for the candidate’s centre, x and y are the coordinates on the plane tangent to the sky at the centre of the field, and θ is the position angle of the major axis. Throughout the analysis we describe the background contamination (Σb) as constant. We note that Σb is determined by subtracting dwarf galaxy stars (calculated using equation (2)) from the total number of possible member stars:

Σ b = ( n − ∫ A ρ dwarf d A ) ∫ A d A . Mathematical equation: $$ \begin{aligned} \Sigma _{b} = \frac{\left(n - \int _{A} \rho _{\rm dwarf} \,dA\right)}{\int _{A} dA} . \end{aligned} $$(4)

Finally, we combined equations (2), (3), and (4) to construct the likelihood function to be sampled by EMCEE:

ρ model ( r ) = ρ dwarf ( r ) + Σ b . Mathematical equation: $$ \begin{aligned} \rho _{\rm model}(r) = \rho _{\rm dwarf}(r) + \Sigma _{b} . \end{aligned} $$(5)

The EMCEE sampler employs an iterative Bayesian Markov chain Monte Carlo approach. We only input sources that fall within a colour range of −0.2 < g0 − r0 < 1.6 and above our completeness magnitude cuts of g0 < 25.5, r0 < 25. To avoid over-constraining the solution, we used broad, uniform flat priors – we set 0.5 arcmin for the half-light radius, ±1 arcmin for the central RA and Dec, 0 < θ < π for the ellipticity, and N* ≥ 0. We also used 12 walkers, over a total of 30 000 iterations, with a burning stage of 20 000. In the Fig. A.1 in the Appendix, we present the final corner plot provided by EMCEE. And XXXVI is localised at RA = 19.168 and Dec = 47.656, with a half-light radius of r h = 0 . 284 − 0.084 + 0.132 Mathematical equation: $ r_h = 0.284 ^{+0.132}_{-0.084} $ arcmin, ellipticity ϵ = 0 . 015 − 0.012 + 0.032 Mathematical equation: $ \epsilon = 0.015^{+0.032}_{-0.012} $, position angle of the major axis θ = 78 . 6 − 28.5 + 29.7 Mathematical equation: $ \theta = 78.6^{+29.7}_{-28.5} $ degrees, and the number of stars corresponding to our CMD selection being N ∗ = 46 − 11 + 14 Mathematical equation: $ N_{\mathrm{*}} = 46^{+14}_{-11} $. The structural parameters obtained from EMCEE are summarised in Table 1.

Table 1.

Structural properties of And XXXVI derived from our analysis.

3.1. Luminosity of And XXXVI

To calculate the luminosity of And XXXVI, we obtained a theoretical luminosity function from the PARSEC library representing a stellar population with an age of 12.5 Gyrs, metallicity [Fe/H] = − 2.5, and α enhancement [α/Fe]= + 0.4 dex, following our CMD result in Figure 5. Using this information, we defined the probability distribution function, which describes the expected RGB star count per magnitude bin. We modelled And XXXVI to have N* stars (taken from Table 1), and assuming a distance of 776 kpc, we employed a probability-weighted random sampling of stars from the PDF above our completeness magnitude cuts. Once we had acquired N* stars, we converted all the g, r magnitudes to a luminosity and sum to get the final measurement. The average luminosity of And XXXVI was thus obtained by repeating this sampling process 1000 times. We recovered Mg ∼ −5.4 ± 0.1 and Mr ∼ −6.2 ± 0.1, respectively, calculating the error bar from the standard deviation of the individual results. By applying colour transformations of Jordi et al. 2006, we converted these values to absolute V-band magnitudes and obtained MV ∼ −5.9 ± 0.1, which equates to a total luminosity of L = ( 2 . 15 − 0.36 + 0.43 ) × 10 4 L ⊙ Mathematical equation: $ L = (2.15^{+0.43}_{-0.36}) \times 10^{4} L_{\odot} $ for And XXXVI. The final luminosity and size parameters are summarised in Table 2, including the results obtained after shifting the distance assumption by ±100 kpc.

Table 2.

Luminosity and size of And XXXVI according to the distance assumption.

4. Discussion and conclusions

We report the discovery of a new semi-resolved object near M31 (And XXXVI) found using archival PAndAS CFHT data. Our follow-up GTC data vastly improved the CFHT image of And XXXVI (Fig. 3), resolving enough stars to construct its CMD and characterise its structural and luminosity properties. While we do not identify probable horizontal branch stars, which would help anchor the distance of the object, by overlapping an old (12.5 Gyr) metal-poor ([Fe/H] = −2.5) isochrone, we demonstrate that the RGB locus is consistent with the distance to M31 (776 kpc). Given its projected distance of ∼119 kpc from M31, And XXXVI would be located well within the estimated virial radius of M31 (r200 ∼ 260 kpc, assuming an NFW profile of average concentration), suggesting it could be a bound satellite of this galaxy.

Assuming a distance of 776 kpc, we measured MV ∼ −5.9 ± 0.1 ( L = 1 . 90 − 0.13 + 0.12 × 10 4 L ⊙ Mathematical equation: $ L = 1.90^{+0.12}_{-0.13} \times 10^{4} L_{\odot} $) and showed that And XXXVI is relatively small, with r h ∼ 64 − 19 + 30 Mathematical equation: $ r_{\mathrm{h}} \sim 64 ^{+30}_{-19} $ pc and ϵ ∼ 0 . 015 − 0.012 + 0.032 Mathematical equation: $ \epsilon \sim 0.015^{+0.032}_{-0.012} $. By comparing its derived size and luminosity to other confirmed compact systems around the MW and M31 (Fig. 6), we conclude that And XXXVI is most likely one of the faintest M31 dwarfs discovered to date. Having varied our distance assumption by ±100 kpc (Table 2), we demonstrate that And XXXVI lies securely on the size-luminosity relation as an UFD.

Thumbnail: Fig. 6. Refer to the following caption and surrounding text. Fig. 6.

Left: Absolute visual magnitude MV versus half-light radius rh (pc) for MW and M31 dwarf spheroidal galaxies, globular clusters, and MW-only hyper-faint compact stellar systems. We overlay our measurements for And XXXVI. Right: Position of And XXXVI versus other M31-only dwarf spheroidals on the same graph. In black we indicate the position of And XXXVI after varying the distance estimate by ±100 kpc. And XXXVI fits securely on the size-luminosity relation as a UFD. Data were taken from the Local Volume database (Pace 2025), and we only selected systems confirmed to be real according to their criteria.

Follow-up kinematic data are required to decisively constrain And XXXVI’s member stars and structural parameters. Targeting the brightest stars close to the tip of the RGB would help constrain its distance (e.g. Collins et al. 2024) and confirm its MV measurement. If And XXXVI is confirmed to be old and metal poor, it would be interesting to constrain the quenching time through a detailed SFH. If And XXXVI is found to likely be a reionisation fossil, it would question whether the SFH differences between satellite populations of M31 and the MW are driven by their environments. This would require deep space-based photometry (such as from the Hubble Space Telescope) to obtain a CMD reaching down to the oldest main-sequence turn-off (e.g. Savino et al. 2023; Jones et al. 2026) to understand when and how this system was quenched.

M31 is predicted to host as many as ∼92 dwarf galaxies (MV < −5.5, Doliva-Dolinsky et al. 2023), yet ∼43 are currently known, suggesting that our census of M31 satellites is far from complete. Given that upcoming wide-field telescopes, such as the Legacy Survey of Space and Time (Ivezić et al. 2019) and Euclid (Euclid Collaboration: Mellier et al. 2025), will not observe M31, follow-up observations of archival data with large ground-based telescopes (8+ meter class, under excellent seeing conditions) or space telescopes remains a promising avenue for improving our knowledge of the M31 system. Indeed, the discovery of And XXXVI highlights that visual inspection remains very complementary to automatic and machine learning approaches when using resolved and/or semi-resolved data. This combination of methods thus remains crucial in the work towards constructing a complete picture of Andromeda.

Acknowledgments

We thank the anonymous referee for helping improve the manuscript, and Antonio L. Cabrera-Lavers for his support during the DDT GTC observations. Data is available upon request to the corresponding author. JS and DMD acknowledge financial support from project PID2022-138896NB-C53. JS acknowledges financial support from the Severo Ochoa grant CEX2021-001131-S funded by MCIN/AEI/ 10.13039/501100011033. DMD thanks financial support for a visiting researcher stay at the Astronomy and Astrophysics Department of the University of Valencia within the framework of the ≪Talent Attraction≫ programme implemented by the Office of the Vice-Principal for Research (INV25-01-15). MLMC acknowledges support from STFC grants ST/Y002857/1 and ST/Y002865/1. MM acknowledges support from the Agencia Estatal de Investigación del Ministerio de Ciencia e Innovación (AEI-MCINN) under grants “At the forefront of Galactic Archaeology: evolution of the luminous and dark matter components of the MW and LG dwarf galaxies in the Gaia era” with references PID2020-118778GB-I00/10.13039/501100011033 and PID2023-150319NB-C21/10.13039/501100011033. ADD acknowledges support from STFC grants ST/Y002857/1. Based on observations made with the GTC telescope, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, under Director’s Discretionary Time GTC2025-292.

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Appendix A: The EMCEE sampler analysis outputs

Thumbnail: Fig. A.1. Refer to the following caption and surrounding text. Fig. A.1.

Results of the structural analysis performed with EMCEE on And XXXVI- the central coordinates (RA, dec), half-light radius rh (arcmin), ellipticity ϵ, the position angle of the major axis θ (degrees), and the number of stars N* based on the CMD selection cuts. We use dashed lines to indicate the average result and 1σ error boundaries.

All Tables

Table 1.

Structural properties of And XXXVI derived from our analysis.

In the text
Table 2.

Luminosity and size of And XXXVI according to the distance assumption.

In the text

All Figures

Thumbnail: Fig. 1. Refer to the following caption and surrounding text. Fig. 1.

Location of And XXXVI (marked in red) within PAndAS (McConnachie et al. 2018). And XXXVI is located approximately ∼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 was adapted from the And450 ultra-deep astrophotography survey (Donatiello 2025).
In the text
Thumbnail: Fig. 2. Refer to the following caption and surrounding text. Fig. 2.

Stacked OSIRIS+ image of And XXXVI with a field of view of 7.8 × 7.8 arcmin. In the inset we show a negative colour zoom-in on And XXXVI. The overdensity can be clearly seen between the two bright foreground stars. North is up and east is left.
In the text
Thumbnail: Fig. 3. Refer to the following caption and surrounding text. Fig. 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 it only becomes apparent with in CFHT and GTC.
In the text
Thumbnail: Fig. 4. Refer to the following caption and surrounding text. Fig. 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.
In the text
Thumbnail: Fig. 5. Refer to the following caption and surrounding text. Fig. 5.

Colour-magnitude diagram of And XXXVI (left) within 1rh (blue points, 0.284 arcmin radius) and 2rh (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 (2rh) area near the overdensity, selected to represent our field contamination for visual reference (see text for details on field contamination determination).
In the text
Thumbnail: Fig. 6. Refer to the following caption and surrounding text. Fig. 6.

Left: Absolute visual magnitude MV versus half-light radius rh (pc) for MW and M31 dwarf spheroidal galaxies, globular clusters, and MW-only hyper-faint compact stellar systems. We overlay our measurements for And XXXVI. Right: Position of And XXXVI versus other M31-only dwarf spheroidals on the same graph. In black we indicate the position of And XXXVI after varying the distance estimate by ±100 kpc. And XXXVI fits securely on the size-luminosity relation as a UFD. Data were taken from the Local Volume database (Pace 2025), and we only selected systems confirmed to be real according to their criteria.
In the text
Thumbnail: Fig. A.1. Refer to the following caption and surrounding text. Fig. A.1.

Results of the structural analysis performed with EMCEE on And XXXVI- the central coordinates (RA, dec), half-light radius rh (arcmin), ellipticity ϵ, the position angle of the major axis θ (degrees), and the number of stars N* based on the CMD selection cuts. We use dashed lines to indicate the average result and 1σ error boundaries.
In the text

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