Deep Imaging of Grus II and Horologium. II. Structure and Extent of Two Ultrafaint Milky Way Satellites*

@article{prabhu2026deep,
  title = {{Deep Imaging of Grus II and Horologium. II. Structure and Extent of Two Ultrafaint Milky Way Satellites*}},
  author = {Deepthi S. Prabhu and David J. Sand and Anirudh Chiti and Burçin Mutlu-Pakdil and Sasha N. Campana and J. L. Carlin and A. P. Ji and Jaclyn Jensen and C. E. Martínez-Vázquez and Dennis Zaritsky and A. B. Pace and A. H. Riley and D. Crnojević and G. Limberg and Laura Congreve Hunter and Kristine Spekkens and Michael G. Jones and Amandine Doliva-Dolinsky and Paul Bennet and V. M. Placco and Quinn O. Casey and Guinevere Herron and W. Cerny and Nitya Kallivayalil and Y. Choi and Guy S. Stringfellow and D. L. Nidever and J. A. Carballo-Bello and M. Navabi and P. Massana and G. E. Medina and N. E. D. Noël and D. J. James and (MAGIC \& DELVE Collaborations)},
  journal = {The Astrophysical Journal},
  year = {2026},
  month = oct,
  doi = {10.3847/1538-4357/aea977},
  url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea977},
  note = {CC BY 4.0},
}

TY  - JOUR
TI  - Deep Imaging of Grus II and Horologium. II. Structure and Extent of Two Ultrafaint Milky Way Satellites*
AU  - Deepthi S. Prabhu
AU  - David J. Sand
AU  - Anirudh Chiti
AU  - Burçin Mutlu-Pakdil
AU  - Sasha N. Campana
AU  - J. L. Carlin
AU  - A. P. Ji
AU  - Jaclyn Jensen
AU  - C. E. Martínez-Vázquez
AU  - Dennis Zaritsky
AU  - A. B. Pace
AU  - A. H. Riley
AU  - D. Crnojević
AU  - G. Limberg
AU  - Laura Congreve Hunter
AU  - Kristine Spekkens
AU  - Michael G. Jones
AU  - Amandine Doliva-Dolinsky
AU  - Paul Bennet
AU  - V. M. Placco
AU  - Quinn O. Casey
AU  - Guinevere Herron
AU  - W. Cerny
AU  - Nitya Kallivayalil
AU  - Y. Choi
AU  - Guy S. Stringfellow
AU  - D. L. Nidever
AU  - J. A. Carballo-Bello
AU  - M. Navabi
AU  - P. Massana
AU  - G. E. Medina
AU  - N. E. D. Noël
AU  - D. J. James
AU  - (MAGIC & DELVE Collaborations)
PY  - 2026
DA  - 2026/10/14
JO  - The Astrophysical Journal
DO  - 10.3847/1538-4357/aea977
UR  - https://iopscience.iop.org/article/10.3847/1538-4357/aea977
AB  - 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 M V = −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 M V = −2.10 ± 0.44 mag, and is more compact with r h = 2 . ′ ⁡ 1 ± 0 . ′ ⁡ 2 ( 4 4 − 5 + 6 pc) and ϵ = 0.3 2 − 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 r h 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.
SN  - 0004-637X
SN  - 1538-4357
ER  -