October
2026
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2026ApJ..1010..150P
Authors
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Prabhu, Deepthi S.
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Sand, David J.
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Chiti, Anirudh
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Mutlu-Pakdil, Burçin
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Campana, Sasha N.
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Carlin, J. L.
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Ji, A. P.
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Jensen, Jaclyn
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Martínez-Vázquez, C. E.
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Zaritsky, Dennis
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Pace, A. B.
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Riley, A. H.
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Crnojević, D.
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Limberg, G.
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Hunter, Laura Congreve
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Spekkens, Kristine
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Jones, Michael G.
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Doliva-Dolinsky, Amandine
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Bennet, Paul
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Placco, V. M.
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Casey, Quinn O.
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Herron, Guinevere
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Cerny, W.
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Kallivayalil, Nitya
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Choi, Y.
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Stringfellow, Guy S.
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Nidever, D. L.
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Carballo-Bello, J. A.
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Navabi, M.
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Massana, P.
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Medina, G. E.
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Noël, N. E. D.
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James, D. J.
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(Magic
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Delve Collaborations)
Abstract
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We present deep, wide-field Magellan/Megacam imaging of the ultrafaint Milky Way (MW) satellites Grus II (Gru II) and Horologium II (Hor II), with the aim of deriving improved constraints on their distances, luminosities, and structural parameters, while also searching for possible signs of tidal disturbance. Our photometry reaches ~3 mag deeper than the discovery data, enabling robust measurements of these quantities. Both systems exhibit color--magnitude diagrams consistent with old (~12.5 Gyr), very metal-poor stellar populations. We find Gru II to be at a distance of 52.3 ± 1.9 kpc, with absolute magnitude MV = -4.07 ± 0.50 mag, while fitting an exponential profile to the stellar distribution yields a half-light radius of 6.'8±0.'5 (103 ± 9 pc), and ellipticity ϵ = 0.25 ± 0.07. Hor II is further away at a distance of 72.4-5.5+5.9 kpc, with MV = -2.10 ± 0.44 mag, and is more compact with rh = 2.'1±0.'2 ( 44-5+6 pc) and ϵ = 0.32-0.16+0.20 , adopting an exponential profile. Both galaxies lie within the typical size--luminosity locus of MW ultrafaint dwarfs. Gru II shows an asymmetric morphology including multidirectional clumpy features, some of which could be suggestive of tidal disturbance. We further identify and spectroscopically confirm a new distant member just outside 3 rh in Gru II, providing independent evidence for member stars at large projected radii. In contrast to Gru II, Hor II appears regular, with no significant extended structure detected to the surface-brightness limits of our data. *This paper includes data gathered with the 6.5 m Magellan Telescopes at Las Campanas Observatory, Chile.
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