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Absolute magnitude as a function of half-light radius. MW globular clusters ('+' symbols; Harris 2010), recently found MW star clusters ('×' symbols; Koposov et al. 2007; Belokurov et al. 2010; Muñoz et al. 2012; Balbinot et al. 2013; Laevens et al. 2014, 2015b; Kim & Jerjen 2015a; Kim et al. 2015, 2016a; Luque et al. 2016, 2017; Koposov et al. 2017), MW dwarf galaxies (blue squares; McConnachie 2012; Bechtol et al. 2015, Drlica-Wagner et al. 2015; Koposov et al. 2015a; Kim et al. 2016b; Torrealba et al. 2016a,b), M 31 dwarf galaxies (blue triangles; McConnachie 2012), previously reported dwarf galaxy candidates in the DES footprint (red circles; Bechtol et al. 2015; Drlica-Wagner et al. 2015; Koposov et al. 2015a; Kim & Jerjen 2015b; Luque et al. 2017), other recently reported dwarf galaxy candidates (green diamonds; Laevens et al. 2015a,b; Martin et al. 2015; Drlica-Wagner et al. 2016; Homma et al. 2016, 2017), DES 3 (red star), and DES J0222.7−5217 (green star) are shown. The red circles connected with a line represent the two previous DES J0222.7−5217 estimates. Note that DES 3 and DES J0222.7−5217 lie inside the region inhabited by faint star clusters. The uncertainties of both objects are comparable to the symbol size. The dashed lines indicate contours of constant surface brightness at µ = {25, 27.5, 30} mag arcsec −2 .
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We report the discovery of a new star cluster, DES 3, in the constellation of Indus, and deeper observations of the previously identified satellite DES J0222.7$-$5217 (Eridanus III). DES 3 was detected as a stellar overdensity in first-year Dark Energy Survey data, and confirmed with deeper photometry from the 4.1 metre Southern Astrophysical Resea...
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Context 1
... procedure yields an absolute magnitude of MV = −1.8 +0.4 −0.3 for the exponencial model and MV = −2.0 +0.4 −0.3 for the Plummer model. Therefore, in the size-luminosity plane DES 3 lies in the faint star cluster region (see Fig. 7). The luminosity of DES 3 is comparable to Koposov 1 (MV ∼ −2; Koposov et al. 2007), DES 1 (MV ∼ −2.21; Luque et al. 2016), and Gaia 2 (MV ≃ −2; Koposov et al. 2017). How- ever, the small size (r h ∼ 6.5 pc) of DES 3 is comparable to Balbinot 1 (r h ≃ 7.24 pc; Balbinot et al. 2013), Kim 1 (r h ≃ 6.9 pc; Kim & Jerjen 2015a) and Laevens ...
Context 2
... a half-light radius of r h ∼ 10.5 pc and a luminosity of MV ∼ −2.7, DES J0222.7−5217 lies in a region of size- luminosity space occupied by faint star clusters (see Fig. 7). Interestingly, the half-light radius, ellipticity and absolute magnitude of DES J0222.7−5217 are comparable to those for the star cluster DES 1 (r h ≃ 9.88 pc, ǫ ≃ 0.53 and MV ≃ −2.21; Luque et al. 2016). ...
Context 3
... distance of ≃ 76.2 kpc for DES 3. It is consistent with being dominated by an old (≃ 9.8 Gyr) and metal-poor ([Fe/H] ≃ −1.88) population, as commonly observed in MW GCs found in the Galactic halo. However, its small physical size (r h ∼ 6.5 pc) and low luminosity (MV ∼ −1.9) place DES 3 in the region occupied by faint star clusters as observed in Fig. 7. In fact, DES 3 is also one of the faint star clusters with smallest angular size known so ...
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Spectral energy distribution (SED) fitting of stellar population synthesis models is an important and popular way to constrain the physical parameters ---e.g., the ages, metallicities, masses for stellar population analysis. The previous works suggest that both blue-bands and red-bands photometry works for the SED-fitting. Either blue-domained or r...
Citations
... In the process of characterizing these systems, chemical abundance analysis has proven to be a valuable tool for discerning the physical nature of the associations and sometimes reveals unusual chemical abundance patterns. Although further observations of a few of these overdensities have determined that they are not genuine physical objects (Cantu et al. 2021), nearly all of the overdensities have been found to be either star clusters or ultrafaint dwarf (UFD) galaxies (Laevens et al. 2014;Luque et al. 2018;Simon 2019). ...
We present a detailed chemical abundance analysis of the brightest star in the ultrafaint dwarf (UFD) galaxy candidate Cetus II from high-resolution Magellan/MIKE spectra. For this star, DES J011740.53-173053, abundances or upper limits of 18 elements from carbon to europium are derived. Its chemical abundances generally follow those of other UFD galaxy stars, with a slight enhancement of the α -elements (Mg, Si, and Ca) and low neutron-capture element (Sr, Ba, and Eu) abundances supporting the classification of Cetus II as a likely UFD. The star exhibits lower Sc, Ti, and V abundances than Milky Way (MW) halo stars with similar metallicity. This signature is consistent with yields from a supernova originating from a star with a mass of ∼11.2 M ⊙ . In addition, the star has a potassium abundance of [K/Fe] = 0.81, which is somewhat higher than the K abundances of MW halo stars with similar metallicity, a signature that is also present in a number of UFD galaxies. A comparison including globular clusters and stellar stream stars suggests that high K is a specific characteristic of some UFD galaxy stars and can thus be used to help classify objects as UFD galaxies.
... We follow similar methods to other studies that have confirmed the status of many MW satellites as dwarf galaxies (see, e.g., Sand et al. 2012;Crnojević et al. 2016;Kim et al. 2016;Luque et al. 2016;Carlin et al. 2017;Conn et al. 2018aConn et al. , 2018bLuque et al. 2018;Mutlu-Pakdil et al. 2018). Our data complements other studies by utilizing a larger FOV (necessary for the potentially larger extents), while still resolving magnitudes ∼3 mags deeper than the discovery papers. ...
We present deep g- and r-band Magellan/Megacam photometry of two dwarf galaxy candidates discovered in the Dark Energy Survey (DES), Grus i and Indus ii (DES J2038–4609). For the case of Grus i, we resolved the main sequence turn-off (MSTO) and ~2 mags below it. The MSTO can be seen at g0 ~ 24 with a photometric uncertainty of 0.03 mag. We show Grus i to be consistent with an old, metal-poor (~13.3 Gyr, [Fe/H] ~ −1.9) dwarf galaxy. We derive updated distance and structural parameters for Grus i using this deep, uniform, wide-field data set. We find an azimuthally-averaged halflight radius more than two times larger (~151+21−31 pc; ~$4\buildrel{\,\prime}\over{.} {16}_{-0.74}^{+0.54}$) and an absolute V-band magnitude ~−4.1 that is ~1 magnitude brighter than previous studies. We obtain updated distance, ellipticity, and centroid parameters that are in agreement with other studies within uncertainties. Although our photometry of Indus ii is ~2–3 magnitudes deeper than the DES Y1 public release, we find no coherent stellar population at its reported location. The original detection was located in an incomplete region of sky in the DES Y2Q1 data set and was flagged due to potential blue horizontal branch member stars. The best-fit isochrone parameters are physically inconsistent with both dwarf galaxies and globular clusters. We conclude that Indus ii is likely a false positive, flagged due to a chance alignment of stars along the line of sight.
... We opt for this value over other studies 19,74 after performing several tests. First, we compared the half-light radii reported in each study with those determined from deeper follow-up photometry for seven systems [75][76][77][78][79][80] . These seven systems were chosen as they were the only systems in ref. 18 that have published deeper photometric studies. ...
The Milky Way is surrounded by dozens of ultrafaint (<105 L☉) dwarf satellite galaxies1–3. They are the remnants of the earliest galaxies4, as confirmed by their ancient5 and chemically primitive6,7 stars. Simulations8–10 suggest that these systems formed within extended dark matter halos and experienced early galaxy mergers and feedback. However, the signatures of these events would lie outside their core regions11, where spectroscopic studies are challenging12. Here we identify members of the Tucana II ultrafaint dwarf galaxy out to nine half-light radii, demonstrating the system to be markedly more spatially extended and chemically primitive than previously found. The distant stars in this galaxy are, on average, extremely metal poor (≲1/1000 of the solar iron abundance), affirming Tucana II as the most metal-poor known galaxy. We observationally establish an extended dark matter halo surrounding an ultrafaint dwarf galaxy out to 1 kpc, with a total mass of >107 M☉, consistent with a generalized Navarro–Frenk–White density profile. The extended nature of Tucana II suggests that it may have undergone strong bursty feedback or been the product of an early galactic merger10,11. We demonstrate that spatially extended stellar populations in ultrafaint dwarf galaxies13,14 are observable, opening up the possibility for detailed studies of the stellar halos of relic galaxies. Stars in the Tucana II ultrafaint dwarf galaxy observed out to nine half-light radii reveal the presence of an extended dark matter halo with a total mass of >107 solar masses, consistent with a generalized Navarro–Frenk–White density profile and suggestive of past strong bursty feedback or an early galactic merger.
... Furthermore, the advent of the Dark Energy Camera (DECam; Flaugher et al. 2015) on the 4 m Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile has resulted in the discovery of a multitude of faint satellite galaxies and compact star clusters orbiting the Milky Way at surface brightnesses inaccessible to previous photographic surveys and SDSS. The DECam searches covering ∼5000 deg 2 of the southern sky using data from the Dark Energy Survey (DES; DES Collaboration et al. 2005 resulted in the discovery of more than 20 new star cluster and dwarf galaxy satellites (e.g., Bechtol et al. 2015;Drlica-Wagner et al. 2015;Kim & Jerjen 2015a;Koposov et al. 2015a;Luque et al. 2016Luque et al. , 2018. These efforts have continued through a number of recent community-led DECam surveys, including the Survey of the MAgellanic Stellar History (SMASH; e.g., Martin et al. 2015;Nidever et al. 2017), the Magellanic SatelLites Survey (MagLiteS; e.g., Drlica-Wagner et al. 2016;Torrealba et al. 2018), and the Magellanic Edges Survey (e.g., Koposov et al. 2018), all of which have contributed to the census of Milky Way satellites, especially in the region of sky in the periphery of the Magellanic Clouds. ...
... We further discuss methods for the classification of DELVE 2 in the following subsection. Harris 1996), and recently discovered halo star clusters with D e > 10 kpc (red circles; Fadely et al. 2011;Muñoz et al. 2012;Balbinot et al. 2013;Belokurov et al. 2014;Laevens et al. 2014Laevens et al. , 2015aKim & Jerjen 2015b;Kim et al. 2016;Luque et al. 2016Luque et al. , 2017Luque et al. , 2018Koposov et al. 2017;Mau et al. 2019Mau et al. , 2020Torrealba et al. 2019). DELVE 2 is displayed as a yellow star. ...
We report the discovery of a new ultra-faint stellar system found near the Magellanic Clouds in the DECam Local Volume Exploration Survey. This new system, DELVE J0155−6815 (DELVE 2), is located at a heliocentric distance of D ⊙ = 71 ± 4 kpc, which places it at a 3D physical separation of 12 ± 3 kpc from the center of the Small Magellanic Cloud and from the center of the Large Magellanic Cloud (LMC). DELVE 2 is identified as a resolved overdensity of old ( τ > 13.3 Gyr) and metal-poor ( dex) stars with a projected half-light radius of and an absolute magnitude of . The size and luminosity of DELVE 2 are consistent with both the population of recently discovered ultra-faint globular clusters and the smallest ultra-faint dwarf galaxies. However, its photometrically derived age and metallicity would place it among the oldest and most metal-poor globular clusters in the Magellanic system. In the absence of spectroscopic measurements of the system’s metallicity dispersion and internal kinematics, we are unable to conclusively classify this system at this time. DELVE 2 is detected in Gaia DR2 with a clear proper-motion signal, with multiple blue horizontal-branch stars near the centroid of the system with proper motions consistent with the systemic mean. We measure the system proper motion to be = mas yr ⁻¹ . We compare the spatial position and proper motion of DELVE 2 with simulations of the accreted satellite population of the LMC and find that it is very likely to be associated with the LMC.
... We assume a half-light radius of 7.2' for Tucana II [18] in this analysis. We opt for this value over others [19,74] due to better agreement (within 2 sigma) between the structural parameters in ref [18] and those obtained from deeper follow-up photometry [75,76,77,78,79,80] of seven other dwarf galaxies with such available data. The structural parameters from refs [19,74] for these seven systems show more scatter when compared to the deeper imaging studies. ...
The Milky Way is surrounded by dozens of ultra-faint (< $10^5$ solar luminosities) dwarf satellite galaxies. They are the surviving remnants of the earliest galaxies, as confirmed by their ancient (~13 billion years old) and chemically primitive stars. Simulations suggest that these systems formed within extended dark matter halos and experienced early galaxy mergers and supernova feedback. However, the signatures of these events would lie outside their core regions (>2 half-light radii), which are spectroscopically unstudied due to the sparseness of their distant stars. Here we identify members of the Tucana II ultra-faint dwarf galaxy in its outer region (up to 9 half-light radii), demonstrating the system to be dramatically more spatially extended and chemically primitive than previously found. These distant stars are extremely metal-poor (<[Fe/H]>=-3.02; less than ~1/1000th of the solar iron abundance), affirming Tucana II as the most metal-poor known galaxy. We observationally establish, for the first time, an extended dark matter halo surrounding an ultra-faint dwarf galaxy out to one kiloparsec, with a total mass of >$10^7$ solar masses. This measurement is consistent with the expected ~2x$10^7$ solar masses using a generalized NFW density profile. The extended nature of Tucana II suggests that it may have undergone strong bursty feedback or been the product of an early galactic merger. We demonstrate that spatially extended stellar populations, which other ultra-faint dwarfs hint at hosting as well, are observable in principle and open the possibility for detailed studies of the stellar halos of relic galaxies.
... edition; McConnachie 2012; Drlica-Wagner et al. 2015;Kim & Jerjen 2015;Koposov et al. 2015;Luque et al. 2018), along with nearby galaxies partially resolved into stars in the DES images and catalogues (IC5152, ESO294-G010, NGC 55, NGC 300, NGC 1399, NGC 247, IC1613, ESO410-G005). The stars from those objects represent a potential contamination to Galactic fields and these fields contained positive residuals in initial iterations ofMWFITTING. ...
We present a technique to fit the stellar components of the Galaxy by comparing Hess Diagrams (HDs) generated from TRILEGAL models to real data. We apply this technique, which we call MWFITTING, to photometric data from the first 3 yr of the Dark Energy Survey (DES). After removing regions containing known resolved stellar systems such as globular clusters, dwarf galaxies, nearby galaxies, the Large Magellanic Cloud, and the Sagittarius Stream, our main sample spans a total area of ∼2300 deg2. We further explore a smaller subset (∼1300 deg2) that excludes all regions with known stellar streams and stellar overdensities. Validation tests on synthetic data possessing similar properties to the DES data show that the method is able to recover input parameters with a precision better than 3 per cent. We fit the DES data with an exponential thick disc model and an oblate double power-law halo model. We find that the best-fitting thick disc model has radial and vertical scale heights of 2.67 ± 0.09 kpc and 925 ± 40 pc, respectively. The stellar halo is fit with a broken power-law density profile with an oblateness of 0.75 ± 0.01, an inner index of 1.82 ± 0.08, an outer index of 4.14 ± 0.05, and a break at 18.52 ± 0.27 kpc from the Galactic centre. Several previously discovered stellar overdensities are recovered in the residual stellar density map, showing the reliability of MWFITTING in determining the Galactic components. Simulations made with the best-fitting parameters are a promising way to predict Milky Way star counts for surveys such as the LSST and Euclid.
... Moreover, as Fig. 6 illustrates, another clue is the close alignment between the direction of the dwarf's motion (as measured using the GDR2 data, black filled circle with error bars) and the elongation of its iso-density contours (dashed line). In addition, the radial velocity gradient (grey filled circle with error bars) does not fully match the Gaia proper motions, possibly indicating (Belokurov et al. 2010;Harris 2010;Muñoz et al. 2012;Balbinot et al. 2013;Kim et al. , 2016aLaevens et al. 2015;Luque et al. 2016Luque et al. , 2017Martin et al. 2016a;Weisz et al. 2016;Koposov et al. 2017;Luque et al. 2018); the M31 satellites are shown as black open circles and the other LG galaxies are shown as black crosses (both from McConnachie 2012). The position of Antlia 2 is shown with a red filled circle. ...
We report the discovery of a Milky Way satellite in the constellation of Antlia. The Antlia 2 dwarf galaxy is located behind the Galactic disc at a latitude of b ∼ 11° and spans 1.26°, which corresponds to ∼2.9 kpc at its distance of 130 kpc. While similar in spatial extent to the Large Magellanic Cloud, Antlia 2 is orders of magnitude fainter at MV = −9 mag, making it by far the lowest surface brightness system known (at ∼31.9 mag arcsec−2), ∼100 times more diffuse than the so-called ultra diffuse galaxies. The satellite was identified using a combination of astrometry, photometry, and variability data from Gaia Data Release 2, and its nature confirmed with deep archival DECam imaging, which revealed a conspicuous BHB signal. We have also obtained follow-up spectroscopy using AAOmega on the AAT, identifying 159 member stars, and we used them to measure the dwarf’s systemic velocity, 290.9 ± 0.5 km s−1, its velocity dispersion, 5.7 ± 1.1 km s−1, and mean metallicity, [Fe/H] = −1.4. From these properties we conclude that Antlia 2 inhabits one of the least dense dark matter (DM) haloes probed to date. Dynamical modelling and tidal-disruption simulations suggest that a combination of a cored DM profile and strong tidal stripping may explain the observed properties of this satellite. The origin of this core may be consistent with aggressive feedback, or may even require alternatives to cold dark matter (such as ultra-light bosons).
... Ultra-faint galaxies are characterized by their large physical sizes, relative to their low luminosities, and their large stellar velocity dispersions, relative to their small observed stellar masses. The discovery of ultra-faint galaxies has come hand-in-hand with the discovery of a class of ultrafaint halo star clusters (e.g., Fadely et al. 2011;Muñoz et al. 2012;Balbinot et al. 2013;Belokurov et al. 2014;Laevens et al. 2014Laevens et al. , 2015Kim et al. 2015Kim et al. , 2016Luque et al. 2016Luque et al. , 2017Luque et al. , 2018Koposov et al. 2017;Torrealba et al. 2019). These clusters have physical sizes consistent with the population of globular clusters (a few parsecs), but can have luminosities hundreds of times fainter (M 2.5 V -). ...
... It is also possible that BLISSJ0321+0438 (BLISS 1) has a common origin with other Milky Way open or globular clusters, and that it is merely an outlier in distance and luminosity. The rapidly growing catalog of faint outer-halo star clusters (e.g., Fadely et al. 2011;Muñoz et al. 2012;Balbinot et al. 2013;Belokurov et al. 2014;Laevens et al. 2014Laevens et al. , 2015Kim et al. 2015Kim et al. , 2016Luque et al. 2016Luque et al. , 2017Luque et al. , 2018Koposov et al. 2017;Torrealba et al. 2019), emphasizes the incompleteness of current surveys. As deep, homogeneous sky coverage improves (i.e., with the Large Synoptic Survey Telescope), it is likely that a much larger number of similar faint, small systems will be discovered. ...
We present the discovery of a faint, resolved stellar system, BLISS J0321+0438 (BLISS 1), found in Dark Energy Camera data from the first observing run of the Blanco Imaging of the Southern Sky (BLISS) survey. BLISS J0321+0438 (BLISS 1) is located at with a heliocentric distance of . It is a faint, , and compact, , system consistent with previously discovered faint halo star clusters. Using data from the second data release of the Gaia satellite, we measure a proper motion of mas yr ⁻¹ . Combining the available positional and velocity information with simulations of the accreted satellite population of the Large Magellanic Cloud (LMC), we find that it is unlikely that BLISS J0321+0438 (BLISS 1) originated with the LMC.
... There have been a plethora of new candidate satellites in recent years, especially in the southern sky (e.g., Laevens et al. 2015;Martin et al. 2015;Drlica-Wagner et al. 2016;Torrealba et al. 2016Torrealba et al. , 2018Homma et al. 2018). Many have been found in the footprint of the Dark Energy Survey (DES), a 5 yr, 5000 deg 2 survey (Bechtol et al. 2015;Drlica-Wagner et al. 2015a;Kim & Jerjen 2015;Koposov et al. 2015a;Luque et al. 2016Luque et al. , 2017Luque et al. , 2018. Many of these objects remain candidates, and deeper photometry (e.g., Carlin et al. 2017) and/or spectroscopy (e.g., Simon et al. 2015;Li et al. 2018b) is required to verify the stellar overdensity and uncover their nature as star clusters or dwarf galaxies (Willman & Strader 2012). ...
... While several objects have been confirmed via spectroscopy to be ultrafaint dwarf spheroidal galaxies (Eridanus II, Horologium I, Reticulum II, and Tucana II; Koposov et al. 2015b;Simon et al. 2015;Walker et al. 2015Walker et al. , 2016Li et al. 2017), others remain ambiguous (Grus I and Tucana III; Walker et al. 2016;Simon et al. 2017;Li et al. 2018a;Mutlu-Pakdil et al. 2018). In addition, several of the satellites are thought to be star clusters (Eridanus III and Kim 2; Kim et al. 2015b;Conn et al. 2018a;Luque et al. 2018). Several of the candidates (Tucana V and Cetus II) have been argued to be false positives from deeper data (Conn et al. 2018a(Conn et al. , 2018b. ...
... Interestingly, the proper motion overlaps with the Kallivayalil et al. (2018) LMC satellite accretion models and as a satellite co-orbiting in the VPOS (Pawlowski et al. 2015). Of the satellites with new measurements, it is the only satellite considered to be a star cluster (Conn et al. 2018a;Luque et al. 2018). ...
We present a new, probabilistic method for determining the systemic proper motions of Milky Way (MW) ultra-faint satellites in the Dark Energy Survey (DES). We utilize the superb photometry from the first public data release (DR1) of the DES to select candidate members and cross-match them with the proper motions from the Gaia DR2. We model the candidate members with a mixture model (satellite and MW) in spatial and proper motion space. This method does not require prior knowledge of satellite membership and can successfully determine the tangential motion of 13 DES satellites. With our method, we present measurements of the following satellites: Columba I, Eridanus III, Grus II, Phoenix II, Pictor I, Reticulum III, and Tucana IV. This is the first systemic proper motion measurement for several of these satellites, and the majority lack extensive spectroscopic follow-up studies. We compare these to the predictions of Large Magellanic Cloud satellites and the vast polar structure. With the high-precision DES photometry, we conclude that most of the newly identified member stars are very metal-poor ([Fe/H] ≲ −2), similar to other ultra-faint dwarf galaxies, while Reticulum III is likely more metal-rich. We also find potential members in the following satellites that might indicate their overall proper motion: Cetus II, Kim 2, and Horologium II. However, due to the small number of members in each satellite, spectroscopic follow-up observations are necessary to determine the systemic proper motion in these satellites.
We present the discovery of Ursa Major III/UNIONS 1, the least luminous known satellite of the Milky Way, which is estimated to have an absolute V -band magnitude of + 2.2 − 0.3 + 0.4 mag, equivalent to a total stellar mass of 16 − 5 + 6 M ⊙ . Ursa Major III/UNIONS 1 was uncovered in the deep, wide-field Ultraviolet Near Infrared Optical Northern Survey (UNIONS) and is consistent with an old ( τ > 11 Gyr), metal-poor ([Fe/H] ∼ −2.2) stellar population at a heliocentric distance of ∼10 kpc. Despite its being compact ( r h = 3 ± 1 pc) and composed of few stars, we confirm the reality of Ursa Major III/UNIONS 1 with Keck II/DEIMOS follow-up spectroscopy and identify 11 radial velocity members, eight of which have full astrometric data from Gaia and are co-moving based on their proper motions. Based on these 11 radial velocity members, we derive an intrinsic velocity dispersion of 3.7 − 1.0 + 1.4 km s ⁻¹ but some caveats preclude this value from being interpreted as a direct indicator of the underlying gravitational potential at this time. Primarily, the exclusion of the largest velocity outlier from the member list drops the velocity dispersion to 1.9 − 1.1 + 1.4 km s ⁻¹ , and the subsequent removal of an additional outlier star produces an unresolved velocity dispersion. While the presence of binary stars may be inflating the measurement, the possibility of a significant velocity dispersion makes Ursa Major III/UNIONS 1 a high-priority candidate for multi-epoch spectroscopic follow-ups to deduce the true nature of this incredibly faint satellite.
