arXiv subjects

Daisy Bissonette

Publications and source records attributed to Daisy Bissonette.

3 recordsLinked to original sources

arXiv2026

The Metallicity Distribution of the Ultra-Faint Dwarf Galaxy Segue 1

Ultra-faint dwarf galaxies (UFDs, $M_* < 10^5 M_\odot$) offer unique insights into early chemical evolution in low-mass systems. However, interpreting their metallicity distribution functions (MDFs) has been challenging due to limited spectroscopic samples, especially beyond the red giant branch. We present metallicities from the Ca II K absorption feature, measured from low-resolution ($R \sim 1000$) Keck/LRIS spectroscopy of 40 stars in the UFD Segue 1 ($M_* \approx 500 M_\odot$), including both red giant branch and main-sequence turnoff stars, resulting in a metallicity sample more than six times larger than previously published data for Segue 1. The resulting MDF has an average [Fe/H] $= -2.52 \pm 0.10$ dex and a dispersion of $\sigma = 0.59 \pm 0.06$ dex, with no evidence for distinct subpopulations. This is consistent with a continuous, short-duration ($\lesssim 1$ Gyr) episode of star formation and chemical enrichment prior to reionization. The nonzero metallicity spread reaffirms its classification as a galaxy. Segue 1 highlights the rich chemical enrichment histories present even in the least massive galaxies, and underscores the importance of deep spectroscopic follow-up to fully characterize these ancient stellar systems.

astro-ph.GA↗

arXiv2026

A Chemodynamical Census of the Milky Way's Ultra-Faint Compact Satellites. I. A First Population-Level Look at the Internal Kinematics and Metallicities of 19 Extremely-Low-Mass Halo Stellar Systems

Deep, wide-area photometric surveys have uncovered a population of compact ($r_{1/2} \approx$ 1-15 pc), extremely-low-mass ($M_* \approx$ 20-4000 $M_{\odot}$) stellar systems in the Milky Way halo that are smaller in size than known ultra-faint dwarf galaxies (UFDs) and substantially fainter than most classical globular clusters (GCs). Very little is known about the nature and origins of this population of "Ultra-Faint Compact Satellites" (UFCSs) owing to a dearth of spectroscopic measurements. Here, we present the first spectroscopic census of these compact systems based on Magellan/IMACS and Keck/DEIMOS observations of 19 individual UFCSs, representing $\sim$2/3 of the known population. We securely measure mean radial velocities for all 19 systems, velocity dispersions for 15 (predominantly upper limits), metallicities for 17, metallicity dispersions for 8, and $\textit{Gaia}$-based mean proper motions for 18. This large new spectroscopic sample provides the first insights into population-level trends for these extreme satellites. We demonstrate that: (1) the UFCSs are kinematically colder, on average, than the UFDs, disfavoring very dense dark matter halos in most cases, (2) the UFCS population is chemically diverse, spanning a factor of $\sim$300 in mean iron abundance ($\rm -3.3 \lesssim [Fe/H] \lesssim -0.8$), with multiple systems falling beneath the "metallicity floor" proposed for GCs, and (3) while some higher-metallicity and/or younger UFCSs are clearly star clusters, the dynamical and/or chemical evidence allows the possibility that up to $\sim$50% of the UFCSs in our sample (9 of 19) may represent the smallest and least-massive galaxies yet discovered.

astro-ph.GA↗

arXiv2025

No Observational Evidence for Dark Matter Nor a Large Metallicity Spread in the Extreme Milky Way Satellite Ursa Major III / UNIONS 1

The extremely-low-luminosity, compact Milky Way satellite Ursa Major III / UNIONS 1 (UMaIII/U1; $L_V = 11 \ L_{\odot}$; $a_{1/2} = 3$ pc) was found to have a substantial velocity dispersion at the time of its discovery ($\sigma_v = 3.7^{+1.4}_{-1.0} \rm \ km \ s^{-1}$), suggesting that it might be an exceptional, highly dark-matter-dominated dwarf galaxy with very few stars. However, significant questions remained about the system's dark matter content and nature as a dwarf galaxy due to the small member sample ($N=11$), possible spectroscopic binaries, and the lack of any metallicity information. Here, we present new spectroscopic observations covering $N=16$ members that both dynamically and chemically test UMaIII/U1's true nature. From higher-precision Keck/DEIMOS spectra, we find a 95% confidence level velocity dispersion limit of $\sigma_v< 2.3 \rm \ km \ s^{-1}$, with a $\sim$120:1 likelihood ratio now favoring the expected stellar-only dispersion of $\sigma_* \approx 0.1 \rm \ km \ s^{-1}$ over the original $3.7 \rm \ km \ s^{-1}$ dispersion. There is now no observational evidence for dark matter in the system. From Keck/LRIS spectra targeting the Calcium II K line, we also measure the first metallicities for 12 member stars, finding a mean metallicity of $\rm [Fe/H] = -2.65 \; \pm \, 0.1$ (stat.) $\pm \,0.3$ (zeropoint) with a metallicity dispersion limit of $\sigma_{\rm [Fe/H]} < 0.35$ dex (at the 95% credible level). Together, these properties are more consistent with UMaIII/U1 being a star cluster, though the dwarf galaxy scenario is not fully ruled out. Under this interpretation, UMaIII/U1 ranks among the faintest and most metal-poor star clusters yet discovered.

astro-ph.GA↗