Spectroscopic Confirmation of the Milky Way Satellites Boötes V and Leo Minor I and the LMC Satellite DELVE 2: Three Small Ultra-Faint Dwarf Galaxies
Abstract
We present the first multi-object spectroscopy of Boötes V, Leo Minor I, and DELVE 2 -- three small (20 pc ≤ r1/2≤ 26 pc), ultra-faint (−3.2≤ MV≤−2.1) stellar systems in the outer Galactic halo (71 kpc ≤ D⊙≤ 102 kpc) with heretofore ambiguous classifications. Leveraging at least two epochs of Keck/DEIMOS or Magellan/IMACS observations targeting each satellite, we identify 13, 7, and 7 spectroscopic members in the three systems, respectively. We tentatively resolve a σv=2.1+1.5−0.9 km s−1 velocity dispersion for DELVE 2 but place upper limits of σv<2.7 km s−1 and σv<7.8 km s−1 for Boötes V and Leo Minor I. We find that all three systems are very metal-poor, with mean iron abundances −2.85≤[Fe/H]̅≤−2.15. Both Boötes V and Leo Minor I display unambiguous metallicity spreads (σ[Fe/H]=0.46+0.18−0.12 dex and σ[Fe/H]=0.57+0.23−0.18 dex), while DELVE 2 displays a modestly significant spread (σ[Fe/H]=0.52+0.29−0.25 dex) across three stars. Based primarily on these metallicity measurements, we confidently classify Boötes V and Leo Minor I as dwarf galaxies and determine that DELVE 2 is most likely a dwarf galaxy as well. We explore the orbit of each satellite in a joint Milky Way + LMC potential, confirming for the first time that DELVE 2 is a long-term bound satellite of the LMC. Finally, we review the empirical Milky Way satellite classification landscape and find that nearly every spectroscopically characterized satellite with azimuthally averaged half-light radius r1/2>15 pc and surface brightness μV,1/2>26.5 mag arcsec−2 is likely a dwarf galaxy. This result can guide initial probabilistic classifications for satellites discovered by Rubin, Roman, and Euclid.
Related
- The Study of Detailed Morphology of Milky Way Dwarf Galaxies: Draco and Boötes I
- Investigating the formation of small Solar System objects using stellar occultations by satellites: present, future and its use to update satellite orbits
- The Halo Gas of Local Spiral Galaxies and the Link to Gaseous Satellites
- The Merian Survey: A Statistical Census of Bright Satellites of Sub-Milky Way-Mass Galaxies
- The origin of UV emission in extremely low-mass satellite galaxies: Insights from Leo T
BibTeX
@misc{cerny2026spectroscopic,
title = {{Spectroscopic Confirmation of the Milky Way Satellites Boötes V and Leo Minor I and the LMC Satellite DELVE 2: Three Small Ultra-Faint Dwarf Galaxies}},
author = {W. Cerny and M. Geha and J. D. Simon and T. S. Li and A. B. Pace and R. P. van der Marel and N. Kallivayalil and A. P. Ji and A. Drlica-Wagner and D. Crnojević and G. Limberg and N. E. D. Noël and A. H. Riley and D. J. Sand and G. S. Stringfellow and Y. Choi and P. S. Ferguson and C. E. Martínez-Vázquez and P. Massana and G. E. Medina and B. Mutlu-Pakdil and J. D. Sakowska and C. Y. Tan and A. K. Vivas and J. Bruce},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.07182},
eprint = {2610.07182},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.07182},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Spectroscopic Confirmation of the Milky Way Satellites Boötes V and Leo Minor I and the LMC Satellite DELVE 2: Three Small Ultra-Faint Dwarf Galaxies
AU - W. Cerny
AU - M. Geha
AU - J. D. Simon
AU - T. S. Li
AU - A. B. Pace
AU - R. P. van der Marel
AU - N. Kallivayalil
AU - A. P. Ji
AU - A. Drlica-Wagner
AU - D. Crnojević
AU - G. Limberg
AU - N. E. D. Noël
AU - A. H. Riley
AU - D. J. Sand
AU - G. S. Stringfellow
AU - Y. Choi
AU - P. S. Ferguson
AU - C. E. Martínez-Vázquez
AU - P. Massana
AU - G. E. Medina
AU - B. Mutlu-Pakdil
AU - J. D. Sakowska
AU - C. Y. Tan
AU - A. K. Vivas
AU - J. Bruce
PY - 2026
DA - 2026/10/05
PB - arXiv
DO - 10.48550/arxiv.2610.07182
UR - https://arxiv.org/abs/2610.07182
AB - We present the first multi-object spectroscopy of Boötes V, Leo Minor I, and DELVE 2 -- three small (20 pc ≤ r_1/2≤ 26 pc), ultra-faint (−3.2≤ M_V≤−2.1) stellar systems in the outer Galactic halo (71 kpc ≤ D_⊙≤ 102 kpc) with heretofore ambiguous classifications. Leveraging at least two epochs of Keck/DEIMOS or Magellan/IMACS observations targeting each satellite, we identify 13, 7, and 7 spectroscopic members in the three systems, respectively. We tentatively resolve a σ_v=2.1^+1.5_−0.9 km s^−1 velocity dispersion for DELVE 2 but place upper limits of σ_v<2.7 km s^−1 and σ_v<7.8 km s^−1 for Boötes V and Leo Minor I. We find that all three systems are very metal-poor, with mean iron abundances −2.85≤[Fe/H]̅≤−2.15. Both Boötes V and Leo Minor I display unambiguous metallicity spreads (σ_[Fe/H]=0.46^+0.18_−0.12 dex and σ_[Fe/H]=0.57^+0.23_−0.18 dex), while DELVE 2 displays a modestly significant spread (σ_[Fe/H]=0.52^+0.29_−0.25 dex) across three stars. Based primarily on these metallicity measurements, we confidently classify Boötes V and Leo Minor I as dwarf galaxies and determine that DELVE 2 is most likely a dwarf galaxy as well. We explore the orbit of each satellite in a joint Milky Way + LMC potential, confirming for the first time that DELVE 2 is a long-term bound satellite of the LMC. Finally, we review the empirical Milky Way satellite classification landscape and find that nearly every spectroscopically characterized satellite with azimuthally averaged half-light radius r_1/2>15 pc and surface brightness μ_V,1/2>26.5 mag arcsec^−2 is likely a dwarf galaxy. This result can guide initial probabilistic classifications for satellites discovered by Rubin, Roman, and Euclid.
ER -