Pisces VII: Discovery of a Possible Satellite of Messier 33 in the Dark Energy Survey

MNRAS 000,1–8 (2021) Preprint 9 April 2021 Compiled using MNRAS LATEX style ﬁle v3.0

Pisces VII: Discovery of a possible satellite of Messier 33 in the Dark Energy Survey

David Martinez Delgado1★, Noushin Karim2, Walter Boschin3 4 5, Emily J. E. Charles2, Matteo Monelli 4 5, Michelle L. M. Collins 2, Giuseppe Donatiello 6, Emilio J. Alfaro 1 1Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía, E-18080, Granada, Spain 2Physics Department, University of Surrey, Guildford, GU2 7XH, UK 3Fundación G. Galilei - INAF (Telescopio Nazionale Galileo), Rambla J. A. Fernández Pérez 7, E-38712 Breña Baja (La Palma), Spain 4Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, E-38205 La Laguna, Tenerife; Spain 5Facultad de Física, Universidad de La Laguna, Avda. Astrofísico Fco. Sánchez s/n, 38200La Laguna, Tenerife, Spain. 6UAI – Unione Astroﬁli Italiani /P.I. Sezione Nazionale di Ricerca Profondo Cielo, 72024 Oria, Italy

9 April 2021

ABSTRACT We report deep imaging observations with DOLoRes@TNG of an ultra-faint dwarf satellite candidate of the Triangulum galaxy (M33) found by visual inspection of the public imaging data release of the Dark Energy Camera Legacy Survey. Pisces VII/Triangulum (Tri) III is found at a projected distance of 72 kpc from M33, and using the tip of the red giant branch method +195 − +0.8 we find a distance to this faint system of 퐷 = 820−190 kpc. We estimate an absolute magnitude of 푀푉 = 4.4−0.7 and a half-light radius of 푟half = 100 ± 14 pc for the galaxy, consistent with similarly faint galaxies around the Milky Way. As the tip of the red giant branch is sparsely populated, constraining a precision distance is difficult, but if Pisces VII/Tri III can be confirmed as a true satellite of M33, it is a significant finding. Firstly, it would be the faintest dwarf galaxy detected to-date outside of the Milky Way. With only one potential satellite detected around M33 previously (Andromeda XXII/Tri I), it lacks a significant satellite population in stark contrast to the similarly massive Large Magellanic Cloud. The detection of more satellites in the outskirts of M33 could help to better illuminate if this discrepancy between expectation and observations is due to a poor understanding of the galaxy formation process, or if it is due to the low luminosity and surface brightness of the M33 satellite population which has thus far fallen below the detection limits of previous surveys. Key words: galaxies: Local Group – galaxies: formation – galaxies:dwarf – surveys

1 INTRODUCTION To date, no brighter satellites have been found outside this radius in shallower surveys such as the SDSS. There is also a notable With a stellar mass of 푀 = 3 × 109 푀 (McConnachie 2012), and ∗ dearth of globular clusters around M33 compared to other spiral a halo mass of ∼ 1011 푀 (Corbelli et al. 2014), the Triangulum galaxies (Cockcroft et al. 2011). This negligible satellite population spiral galaxy (M33) is the most massive satellite of the Andromeda is in stark contrast to that of the similarly massive Large Magellanic galaxy (M31) and one of the most massive galaxies of the Local Cloud (LMC), which has upwards of 7 known satellite galaxies (e.g. Group. With this mass, the ΛCDM cosmological paradigm predicts Jethwa et al. 2016; Fritz et al. 2019; Erkal & Belokurov 2020). Part

arXiv:2104.03859v1 [astro-ph.GA] 8 Apr 2021 that M33 should host a number of its own satellites. Cosmological of this is due to the difference in limiting magnitude that can be simulations find it should have between 9-25 companions with 3 probed in M33 versus the far nearer LMC. But even so, the lack of stellar mass > 10 푀 (e.g. Dooley et al. 2017; Patel et al. 2018; 4 6 bright companions (푀∗ > 10 푀 ) is surprising. Bose et al. 2018), and at least 1 with 푀∗ > 10 푀 (Deason et al. 2013). However, to date, only one potential satellite has been uncovered: Andromeda (And) XXII/Triangulum (Tri) I, which has a Previously, this lack of satellite galaxies and far flung globular stellar mass of 2×104 푀 (Martin et al. 2009, 2016). This was found clusters of M33 was attributed to its dynamical evolution. Warps to in the framework of the Pan-Andromeda Archaeological Survey its outer stellar and HI disks were thought to originate from a prior (PAndAS; McConnachie et al. 2009), an observing program which interaction with M31 occurring ∼2 Gyr ago, which would have has conducted a deep survey of the M33 halo out to ∼1/3 of its virial stripped much of M33’s stellar halo and satellites (McConnachie radius. et al. 2009; Cockcroft et al. 2011). Nevertheless, newer studies which include up-to-date proper motions for M31 and M33 suggest that the latter is more likely on its first infall to the M31 system (e.g. Patel et al. 2017; van der Marel et al. 2019). If this scenario is ★ Talentia Senior Fellow correct, M33’s satellite system should extend beyond a single, low

© 2021 The Authors 2 Martinez Delgado et al. mass satellite. density in the Pisces constellation. It was found by visual inspection of the available DES-DR1 images of an extensive area of 20◦ × 30◦ This dramatic gap between theoretical expectations and observa- in the surroundings of the Triangulum galaxy (M33), outside the tions could imply our understanding of the formation of low mass PAndAs footprint (see Fig. 1 left panel). The detection was subse- galaxies is flawed. Perhaps the feedback recipes used in hydro- quently confirmed by a visual inspection of the SDSS DR9 images dynamical surveys are incorrect, or we are wrong about the nature of and follow-up observations using the Italian Telescopio Nazionale dark matter itself. Or, it could be that the majority of M33 satellites Galileo (TNG) described in Sec. 2.1 . The position of the center of have luminosities and surface brightnesses that lie just below the de- this new dwarf galaxy is given in Table 1. tection limits of previous surveys. In any event, given the paucity of known M33 satellites, even a single new detection or exclusion of a companion has the potential to completely change our understanding of the M33 system, and galaxy formation more widely. 2.1 TNG imaging observations In the last decades, the discoveries of Andromeda satellites have been made by means of visual inspection or automatic algorithm We used deep images of a 8.6’x8.6’ field around the candidate searches in stellar density maps of resolved red giant branch (RGB) galaxy obtained with the focal reducer instrument DOLoRes (see stars, counted in selected areas of the color-magnitude diagrams http://www.tng.iac.es/instruments/lrs/) of the 3.58-m TNG taken on (CMDs) from large scale photometric survey data, such as the Sloan November 17 2020 (program A42DDT2; P.I.: W. Boschin). Digital Sky Survey (SDSS, Abazajian et al. 2009) and the Panoramic These observations include 41x180 sec unbinned (scale Survey Telescope and Rapid Response System (Pan-STARRs; Cham- 0.25200/pixel) exposures in the g’-band and 20x180 sec unbinned bers et al. 2016; Martin et al. 2013a,b). However, the main contri- exposures in the r’-band, with a median seeing of 1.15" and 0.85", bution to the dwarf census of M31 satellites came from PAndAS respectively. with the wide-field imager on the Canada French Hawaii Telescope The raw data were preprocessed in a routine way using standard (CFHT) (McConnachie et al. 2009; Martin et al. 2009; Richardson IRAF tasks, i.e. dividing the trimmed and bias-subtracted images by et al. 2011). a master flat field produced from multiple twilight sky-flat exposures. Although the M31 stellar halo photometry from the PAndAS is Images were reduced using the DAOPHOT/ALLFRAME suite significantly deeper than those from the SDSS or Pan-STARRs, this of programmes (Stetson 1987, 1994), largely following the method ground-based data can only reach the red clump locus in the CMDs outlined by Monelli et al.(2010). For each individual image, we at the distance of Andromeda. Thus, low-mass systems with absolute performed the initial steps: i) search for stellar sources, ii) aperture magnitude fainter than 푀푉 ∼ −6 are very hard to detect because of photometry, iii) PSF derivation, iv) PSF photometry with ALLSTAR. the lack of enough RGB star tracers in their CMDs, yielding a cut-off Then images were registered with DAOMASTER and stacked on a in the luminosity function of satellites of M31 (see Brasseur et al. median image. This was used to extract a deeper list of stars, which 2011; their Fig. 1). An alternative approach is to search for partially was fed to ALLFRAME. This provides individual catalogues with resolved stellar over-densities in the public deep images recently better determined position and instrumental magnitude of the input available from the Dark Energy Survey (DES-DR1; Abbott et al. sources. The updated photometry allowed us to refine the PSF (using 2018). These DES-DR1 data were obtained with the Dark Energy improved list of stars) and the geometric transformations (leading Camera (DECam) mounted on the Blanco 4-m telescope, located at to a better coadded image and cleaner input list). A final run of the Cerro Torrolo Inter-American Observatory (Flaugher et al. 2015), ALLFRAME provided the final photometry. The list of sources was − which reach surface brightness as faint as ∼ 29 mag arcsec 2. This cleaned using the sharpness parameter, and further polished by re- low surface regime allows us to detect the underlying, unresolved moving object after a visual inspection on the stacked image. population of these systems as a diffuse light round over-density The photometric calibration was performed using local standard overlapping a small (∼ 1-2 arcmin) clump of faint stars. from the PanSTARRs 1 survey (Chambers et al. 2016), using 121 Using this approach, we have very recently identified a partial re- stars in common. The mean magnitudes were calibrated with a linear solved dwarf galaxy candidate outside the PAndAS survey footprint. relation for the 푔 band, and a zero point for the 푟 band. The calibrated Placed at the distance of M33, it would be located at a projected local standard agree well below 0.01 mag with the tabulated ones, distance of &70 kpc from the Triangulum galaxy. Unfortunately, the with standard deviation of the order of 0.03 mag. DES photometry is too shallow to allow the measurement of an ac- curate distance, making it impossible to reject it as a background isolated dwarf situated a few Mpcs behind Andromeda (as e.g. the Do I dwarf; Martínez-Delgado et al. 2018). In this paper we present follow-up photometric observations, a distance estimate and structural analysis of this stellar system that suggests its likely association 3 METHODS to M33. 3.1 Identifying Probable Members To determine the structural properties of the dwarf candidate, we 2 OBSERVATIONS AND DATA REDUCTION identify stars that are most likely to be associated with the dwarf from the CMD. We select stars that lie on or near the RGB of the Pisces VII/Tri III1 was discovered in the Pisces constellation by the system. Two selection criteria were used to filter the foreground amateur astronomer Giuseppe Donatiello as a partially resolved over- and background contaminants out of the data. First the morphology of each datum was assessed and only stellar (point-like) sources 1 Following the naming convention suggested in the Appendix of Martin et were included. Secondly, we use an old, metal-poor isochrone to al. (2009), we suggest for this galaxy the dual naming introduced with the trace the RGB (10 Gyrs, [훼/Fe]=0.0 dex, [Fe/H]=-2.0 dex taken discovery of Pegasus/And VI, Cassiopeia/And VII or Pisces VI/And XXII. from the Dartmouth isochrones, Dotter et al. 2008), and measure the

MNRAS 000,1–8 (2021) Pisces VII 3

Figure 1. Left panel: Image of the dwarf galaxy Pisces VI/Tri III from the DES-DR1. Right panel: TNG 푟-band image of the galaxy obtained from TNG follow-up observations (see Sec. 2). The total ﬁeld of view of both images is 8.60 × 8.60. North is up, East is left.

minimum distance, 푑푚푖푛, of each datum to the isochrone to calculate Table 1. The priors used in the emcee routine. All values are in radians unless a probability of membership deﬁned by: otherwise stated.

" 2 !# MCMC Parameter Value 푑푚푖푛 푃푖푠표 = exp − (1) 푥 0.3 푥 0.4 2휂2 0 6 0 6 푦0 0.45 6 푦0 6 0.5 푟 0 푟 < 5 [0] where 휂 is a free parameter used to account for scatter about the ℎ 6 ℎ푎푙 푓 휖 0 6 휖 6 1 isochrone. We set 휂 = 0.05 and use a probability cut of 푃 > 푖푠표 휃 0 6 휃 6 휋 0.25 to isolate the most likely RGB candidates. This allows a broad selection of stars, without including too many obvious foreground contaminants. If a datum satisfied these criteria it is considered a the minor-to-major axis ratio, 휖 = 1 − (푏/푎) and 푟h is the half-light probable member of the dwarf galaxy. A total of 65 probable member radius. 푟 is the elliptical radius such that: stars were identified within ∼ 10 of the centre of the overdensity, as illustrated in figure2.

1 2 푟 = ((푥 − 푥 ) cos 휃 − (푦 − 푦 ) sin 휃) 1 − 휖 0 0

1 (3) 3.2 Structural Properties ! 2 2 + (푥 − 푥 ) sin 휃 − (푦 − 푦 ) cos 휃 The structural properties were determined using the iterative 0 0 Bayesian approach of Markov Chain Monte Carlo (MCMC) analysis and we use the emcee code developed by Foreman-Mackey et al. where 휃 is the position angle of the major axis, 푥0 and 푦0 are co- (2013) to implement this. Our methodology followed the procedure ordinates for the centre of the candidate dwarf galaxy and 푥 and 푦 the outlined in §3.1 of (Martin et al. 2016), specifically using Equations coordinates of each datum, both in right ascension and declination 2 and3 below. respectively. Equations2 and3 combine together to give the likelihood func- The radial density profile of the dwarf galaxy, 휌dwarf (푟), can be described by: tion, used in the MCMC analysis. Uniform flat priors were used to constrain the parameter space to physical solutions but were kept broad to ensure the analysis wasn’t over-constrained, see Table1. 2 1.68 ∗ −1.68푟 The emcee routine used 100 walkers, over a total of 10,000 itera- 휌dwarf (푟) = 푁 exp (2) 2 ( − ) 푟 tions with a burn in stage of 5,000. Fig.3 is the resulting corner plot 2휋푟h 1 휖 h from the MCMC analysis and the derived structural parameters are where 푁∗ is the number of likely member stars, determined by summarised in Table2. the selection criteria outlined in § 3.1, 휖 the ellipticity, defined by Next we plot a stellar radial density profile using the parameters

MNRAS 000,1–8 (2021) 4 Martinez Delgado et al.

21.5 26.46

22.0 26.44

22.5

26.42 23.0

23.5 26.40 r

24.0 Dec [deg] 26.38

24.5

26.36 25.0

25.5 26.34

26.0 0.5 0.0 0.5 1.0 1.5 2.0 20.34 20.36 20.38 20.40 20.42 20.44 20.46 20.48 20.50 (g-r) RA [deg]

Figure 2. Left: Colour magnitude diagram within 1’ of the centre of Pisces VII/Tri III. The grey data points indicate the complete data set observed. The red data points are the sources deemed likely members of the candidate dwarf galaxy. The solid black line is an isochrone from the Dartmouth isochrone database with [Fe/H] = -2.0, dex [훼/Fe] = 0.0 dex and age = 10 Gyr. Right: Spatial density plot of the observed data, with red points showing the likely members of the dwarf galaxy which pass the probability cut described in § 3.1. The ellipses show 1× and 2 × 푟h.

Table 2. The ﬁnal structural and photometric properties for the dwarf. x0[°] = 20.422±0.001

Property Value

ℎ 푚 푠 푠 RA 1 21 41.3 ± 0.2 y0[°] = 26.392±0.001 0 dec 26°23 31.78” ± 3.5” 26.396 ] °

푟 (arcmin) 0.43 ± 0.06 [

ℎ 26.394 0 퐷 (kpc) 820+195 y −190 26.392 ± 푟ℎ (pc) 100 14 26.390 rh['] = 0.43±0.06 +0.8 − 26.388 푀푉 4.4−0.7 0.75

휇 28.4 ± 0.8 ] 0 ' [

0.60

+5.3 3 h 퐿 (퐿 ) 4.92 × 10 r −2.0 0.45 휖 0.47 +0.08 −0.09 0.30 = 0.47+0.08 휃 (°) 51 +9 0.09 −11 0.8

0.6

0.4

0.2 [°] = 50+9 that were deduced from the MCMC analysis, which we show in Fig.4. 11

The error bars on the data points represent the Poisson uncertainties 160

] 120 ° [

for each point. We overplot the exponential profile deduced using MCMC, which agrees very well with the observed profile. In addition 80 40 we perform a basic chi-squared fit of the exponential profile shown 0.2 0.4 0.6 0.8 40 80 0.30 0.45 0.60 0.75 120 160 20.41820.42020.42220.42420.42626.388 26.390 26.392 26.394 26.396 in Equation2 to this binned data. We recover a very similar value for x [°] r ['] [°] 0 0 y0 [°] h 푟h = 0.48 , completely consistent with the MCMC approach.

Figure 3. 2D and marginalized PDFs for the central coordinates of the system, 푥0 and 푦0, the half-light radius, 푟ℎ, the ellipticity, 휖 and the position angle 3.3 The distance to the dwarf of the major axis, 휃. The dashed lines represent the mean value and 1휎 To determine the distance to Pisces VII/Tri III, we ﬁrst use the Tip uncertainties. of the Red Giant Branch (TRGB) method. We construct a luminosity function for our dwarf galaxy (see Fig.5), using all sources within 2 × 푟h of the galaxy’s centroid and colours of 0.3 < 푔 − 푟 < 1.5 source. The gray shaded area in Fig.5 shows the region below which to minimise foreground contamination. From this, we subtract an our photometry is < 90% complete. area-normalised background luminosity function constructed from The TRGB in the 푟−band should have an absolute magnitude of sources with the same colour cut, between 2.5-3.5 arcmin from our 푀푟,TRGB = −3.01 ± 0.1 (Sand et al. 2014). Using a Sobel edge

MNRAS 000,1–8 (2021) Pisces VII 5

630 kpc 820 kpc 1.15 Mpc 7

102 6 ] 2 n

i 5 m c r a / s r a

t 4 s [

101 * N

0 1 2 3 4 5 Radius [arcmin] 1

Figure 4. Stellar radial density profile with the observed data binned in ellip- 0 tical annuli with the favoured structural properties of ellipticity, the position 21 22 23 24 25 26 r mag angle, the structural centre and the number of stars. The solid black fit uses the MCMC values and the dashed grey line is the chi-squared fit, which yields 0 a value for 푟h = 0.48 . Figure 5. A background corrected luminosity function for all stars within 2 × 푟h of the galaxy centre. The gray shaded region shows the point at which detection filter (Lee et al. 1993) we find two possible values for our completeness drops below 90%. Using a Sobel edge detection filter, two the TRGB at 푟 = 21.9 and 푟 = 22.9, which are marked as the potential peaks in the luminosity function are found at 푟 = 20.9 and 푟 = 22.3, black solid and dashed line in Fig.5. These correspond to physical which are indicated as the solid and dashed black lines. The location of the distance estimates of 630 kpc and 1.15 Mpc. The dot-dashed line TRGB of M33 is shown as a dot-dashed line. The grey lines show the predicted represents the TRGB location for an object at the distance of M33 locations for the HB in the LF for these three distances. (820 kpc, Conn et al. 2012). This large spread in distances is due to the paucity of RGB stars in this faint system. Given its intrinsically capture the top few magnitudes of the RGB, we use PAndAS photom- low luminosity, the TRGB is not well-populated, and so this method etry of 2 M31 dwarf galaxies to estimate how much of the flux we are cannot return a precision distance estimate. In Fig.6, we show the missing (McConnachie et al. 2018). We select 1 bright satellite (And CMD for all stars witin 2 × 푟h of the dwarf galaxy with isochrones = − ± = − +0.9 XXI, 푀푉 9.1 0.3) and one faint (And XXVI, 푀푉 5.8−1.0) of varying metallicity overlaid at each of these proposed distances (Martin et al. 2016) for comparison. We determine that on average we (Dotter et al. 2008). For a distance of 820 kpc, the dwarf appears capture 85% of their reported flux using our method, and correct our metal poor, with [Fe/H]∼ −1.5. findings for Pisces VII/Tri III accordingly. Using the average value As a second step, we also look for a peak in the luminosity function to background correct our luminosity, we calculate 푀푟 = −4.6 ± 0.7 at the location of the horizontal branch (HB). The HB is a secure and 푀푔 = −3.9±0.7 (where the uncertainty comes from our distance distance estimator and it is typically better populated than the RGB, uncertainty and the scatter in the background luminosity correction). allowing a better constraint. In Fig.5, we also show the predicted We then convert this into an absolute magnitude in the 푉-band using location of the HB assuming the two Sobel distance estimates, as +0.7 the colour transforms of Jordi et al.(2006), giving 푀푉 = −4.4 . well as the HB for an object at the distance of M33. As can be −0.8 seen, we tentatively detect the HB at 푟 ∼ 25.1, implying a distance consistent with M33. However, as our photometry is incomplete at this magnitude we cannot be definitive. As such we assume that our 4 DISCUSSION AND CONCLUSIONS candidate is located at the distance of M33 and we use the Sobel filter We report the discovery of Pisces VII/Tri III, a new dwarf galaxy in detections as upper and lower bounds, giving 퐷 = 820+195 kpc. −190 the surroundings of M33 by visual inspection of the public available Deeper imaging is needed to confirm the distance to this galaxy. image of the DES-DR1 outside the PAndAs survey footprint. Using Combined with our MCMC parameters, this also gives a physical deeper follow-up imaging from DOLoRes@TNG we measure its value for 푟h = 100 ± 14 pc assuming a distance of 820 kpc. = +195 distance to be 퐷 820−190 kpc, making it a likely satellite of M33. = − +0.7 = It is extremely faint and compact with 푀푉 4.4−0.8 and 푟h 100 ± 14 pc, comparable to UFDs of the Milky Way (e.g. Columba 3.4 Luminosity I, 푀푉 = −4.2 ± 0.2, 푅h = 117 ± 17 pc, Carlin et al. 2017). This We determine the luminosity of Pisces VII/Tri III using aperture pho- luminosity would make it the faintest dwarf galaxy detected beyond tometry. We calculate the luminosity within an aperture covering one the Milky Way. half-light radius of our source (which is by definition, half of the total Armed with the structural and photometric properties of our dwarf, luminosity). To calculate the average contamination from foreground we can place it in context with the other dwarf galaxies of the Local and background sources, we repeat the process for 10 apertures of group. In Fig.7, we show the relationship between luminosity and equal size randomly placed throughout our field of view. As we only half-light radius (left) and surface brightness (right). The light grey

MNRAS 000,1–8 (2021) 6 Martinez Delgado et al.

21 630 kpc 820 kpc 1.1 Mpc Field 21

22 22

23 23 0 r

24 24

25 25

26 26 0 1 0 1 0 1 0 1 (g r)0 (g r)0 (g r)0 (g r)0

Figure 6. The first 3 panels show the CMD for all sources within 2 × 푟h of the centre of the dwarf. In each panel, Dotter et al.(2008) isochrones with an age of 10 Gyr, [훼/Fe]=+0.0 and metallicities of [Fe/H]= −1.0, −1.5, −2.0 and − 2.5 are overlaid for three different distances: The upper and lower bounds determined by the Sobel filter (630 kpc and 1.15 Mpc) and the distance to M33 (820 kpc). The final panel shows a control field located away from the centre of the dwarf of equal area.

3 10 24

26 f 0 l , a V h r 27 102 28

101 31 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 MV MV

Figure 7. Left: 푀푉 vs 푟h for dwarf spheroidal galaxies of the Milky Way (light grey triangles) and M31 (dark gray circles). Our new candidate is highlighted as a large red circle, and ﬁts comfortably within the size-luminosity relation for Local Group dwarfs. The point with the red shading is the only other likely satellite of M33, Andromeda XXII/Triangulum I. Right: 푀푉 vs central surface brightness, 휇푉 ,0, for Local Group dwarfs.

MNRAS 000,1–8 (2021) Pisces VII 7

spirals are very rare in cosmological simulations based on the ΛCDM paradigm (e.g. Pawlowski 2018). Some solutions proposed for this possible small-scale problem for the ΛCDM theory are the accretion of dwarf galaxies along filaments of the cosmic web (e.g. Buck et al. 2015, infall of satellites in groups (e.g. Samuel et al. 2020) or a possible tidal dwarf galaxy origin for some of the satellites of Andromeda (Hammer et al. 2018). The available DES imaging data outside the PAndAs footprint also allows us to explore the existence of additional members of this possible GPoA at larger projected distances from M31. Fig.8 shows the position of the Pisces VII/Tri III with respect to M33 and the GPoA (Pawlowski 2018). It clearly lies off the plane, and in projection seems likely associated to the outer M33 halo. The discovery of Pisces VII/Tri III by visual inspection of a limited area around M33 using the DES-DR1 deep imaging suggests that there is still room for discovery of low surface brightness dwarf galaxies lurking in the outskirts of Andromeda, beyond the bounds of the PAndAS survey. To confirm whether it is a bona-fide satellite of M33, a precision distance should be measured using deep imaging. Follow-up studies using the Hubble Space Telescope would allow the HB and main sequence of the dwarf to be resolved, allowing us to distinguish between the distances measured from the scarce RGB stars. In addition, spectroscopy of its brightest member stars would allow us to determine whether it is dynamically bound to M33. Given Figure 8. Position of Pisces VI/Tri III (green triangle) with respect to M31 the faint nature of the stars, an 8–10 m class telescope, such as Keck, (black ellipse) and M33 and the GPoS. The dashed and dotted lines indicate the would be required. orientation and width of the best-fit GPoS respectively. The white footprint corresponds to the region explored by the PAndAS survey. Satellites with measured radial velocities are showed as color-coded triangles, according to whether they are approaching (blue) or receding (red) relatively to M31. ACKNOWLEDGEMENTS Adapted from Fig. 1 in Pawlowski (2018). We thank to the TNG director, Dr Ennio Poretti, for the telescope time he kindly granted us to perform the photometric follow-up of triangles show Milky Way satellites, while dark grey circles show this dwarf galaxy. DMD acknowledges financial support from the M31 satellites. These values are compiled from a number of sources Talentia Senior Program (through the incentive ASE-136) from Sec- (McConnachie 2012; Koposov et al. 2015; Bechtol et al. 2015; Drlica- retaría General de Universidades, Investigación y Tecnología, de Wagner et al. 2015; Martin et al. 2016; Weisz et al. 2019). Our la Junta de Andalucía. DMD and EJA acknowledge funding from candidate is highlighted as a large red point. It is only the second the State Agency for Research of the Spanish MCIU through the potential satellite galaxy of M33 discovered. The other, Andromeda “Center of Excellence Severo Ochoa" award to the Instituto de As- XXII, is highlighted with red shading2. trofísica de Andalucía (SEV-2017-0709). This publication is based Our derived properties for the new dwarf are perfectly consistent on observations made on the island of La Palma with the Italian with other ultra-faint dwarf galaxies, and is the faintest candidate Telescopio Nazionale Galileo, which is operated by the Fundación dwarf discovered in the M31-M33 system. If this galaxy was con- Galileo Galilei-INAF (Istituto Nazionale di Astrofisica) and is lo- firmed as a satellite of M33, it would alleviate the current tension cated in the Spanish Observatorio of the Roque de Los Muchachos between the observed and predicted number of satellites around M33 of the Instituto de Astrofísica de Canarias. This project used public (Patel et al. 2018). Given its low luminosity, it also suggests it may archival data from the Dark Energy Survey (DES). Funding for the be the tip of the iceberg in terms of finding more ultra-faint dwarfs DES Projects has been provided by the U.S. Department of Energy, in the M31-M33 system. the U.S. National Science Foundation, the Ministry of Science and The new census of dwarf galaxies and the homogeneous distance Education of Spain, the Science and Technology FacilitiesCouncil measurements for all the known M31 satellites from the PAndAS of the United Kingdom, the Higher Education Funding Council for survey suggest the possible existence of a coherent flattened galaxy England, the National Center for Supercomputing Applications at plane of 15 satellite galaxies in that survey volume (Ibata et al. 2013). the University of Illinois at Urbana-Champaign, the Kavli Institute This Great Plane of Andromeda (GPoA, see Fig8) is almost edge-on of Cosmological Physics at the University of Chicago, the Center orientated and extends more than 400 kpc from the center of M31. for Cosmology and Astro-Particle Physics at the Ohio State Univer- Interestingly, this plane seems to be aligned with the Giant Stellar sity, the Mitchell Institute for Fundamental Physics and Astronomy Stream in the M31 halo and, in contrast with a similar satellite plane at Texas A&M University, Financiadora de Estudos e Projetos, Fun- found in the Milky Way (Pawlowski et al. 2012), it is not perpen- dação Carlos Chagas Filho de Amparo à Pesquisa do Estado do dicular to its galactic disk but inclined ∼ 50 degrees. The existence Rio de Janeiro, Conselho Nacional de Desenvolvimento Científico e of these kinematically correlated satellite planes in the Local Group Tecnológico and the Ministério da Ciência, Tecnologia e Inovação, the Deutsche Forschungsgemeinschaft, and the Collaborating Insti- tutions in the Dark Energy Survey. The Collaborating Institutions are 2 Laevens 2/Tri II is a Milky Way globular cluster situated at 30 kpc (Laevens Argonne National Laboratory, the University of California at Santa et al. 2015). Cruz, the University of Cambridge, Centro de Investigaciones En-

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This paper has been typeset from a TEX/LATEX ﬁle prepared by the author. DATA AVAILABILITY The data underlying this article will be shared on reasonable request to the corresponding author.

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