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 "+ = 4.4−0.7 and a half-light radius of Ahalf = 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 " = 3 × 109 " (McConnachie 2012), and ∗ dearth of globular clusters around M33 compared to other spiral a halo mass of ∼ 1011 " (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 " (e.g. Dooley et al. 2017; Patel et al. 2018; 4 6 bright companions ("∗ ¡ 10 " ) is surprising. Bose et al. 2018), and at least 1 with "∗ ¡ 10 " (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 " (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 "+ ∼ −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.

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