SPECTROSCOPIC ANALYSIS OF PICTOR II: A VERY LOW METALLICITY ULTRA-FAINT DWARF GALAXY BOUND TO THE LARGE MAGELLANIC CLOUD

We present Magellan/IMACS and Magellan/MIKE spectroscopy of the ultra-faint dwarf (UFD) galaxy Pictor II (Pic II) that is located only 12 kpc from the Large Magellanic Cloud (LMC). From the IMACS spectroscopy, we identify 13 member stars and measure a mean heliocentric velocity of 326.9±1.1 km s⁻¹, a velocity dispersion of 3.5^+1.1 −0.9^{km s−1}, a mean metallicity of [Fe/H] = −2.99±0.06, and an upper limit on the metallicity dispersion of σ_[Fe/H] < 0.18. We measure detailed elemental abundances for the brightest star, finding [Fe/H] = −3.3, high [α/Fe] ratios, and no detectable neutron capture elements, similar to stars in other UFDs. However, this star has an unusually high [Sc/Fe] ratio. The dynamical mass-to-light ratio (M/L = 760⁺⁹¹⁰ −420^M⊙ L⁻¹ ⊙), size, and chemical abundances confirms that Pic II is a dark matter-dominated dwarf galaxy. We perform detailed orbit modeling of Pic II in a combined Milky Way (MW) and LMC potential and find that Pic II is highly likely to be a long-term LMC satellite. Furthermore, we find that Pic II is likely still bound to the LMC today. Pic II is the seventh LMC-associated UFD and among the most metal-poor UFDs known. We further update the morphological parameters with deeper Dark Energy Camera (DECam) photometry, compute the dark matter properties for dark matter indirect detection searches, verify the extremely low metallicity with narrowband CaHK imaging, and briefly discuss tidal influences of the LMC and MW.