ALFALFA Discovery of the Nearby Gas-Rich Dwarf Galaxy Leo P. IV. Distance Measurement from LBT Optical Imaging

摘要

Leo P is a low-luminosity dwarf galaxy discovered through the blind HIArecibo Legacy Fast ALFA (ALFALFA) survey. The HI and follow-up opticalobservations have shown that Leo P is a gas-rich dwarf galaxy with both activestar formation and an underlying older population, as well as an extremely lowoxygen abundance. Here, we measure the distance to Leo P by applying the tip ofthe red giant branch (TRGB) distance method to photometry of the resolvedstellar population from new Large Binocular Telescope (LBT) V and I bandimaging. We measure a distance modulus of 26.19 (+0.17/-0.50) mag correspondingto a distance of 1.72 (+0.14/-0.40) Mpc. Although our photometry reaches 3magnitudes below the TRGB, the sparseness of the red giant branch (RGB) yieldshigher uncertainties on the lower limit of the distance. Leo P is outside theLocal Group with a distance and velocity consistent with the local Hubble flow.While located in a very low-density environment, Leo P lies within ~0.5 Mpc ofa loose association of dwarf galaxies which include NGC 3109, Antlia, SextansA, and Sextans B, and 1.1 Mpc away from its next nearest neighbor, Leo A. Leo Pis one of the lowest metallicity star-forming galaxies known in the nearbyuniverse, comparable in metallicity to I Zw 18 and DDO 68, but with stellarcharacteristics similar to dwarf spheriodals (dSphs) in the Local Volume suchas Carina, Sextans, and Leo II. Given its physical properties and isolation,Leo P may provide an evolutionary link between gas-rich dwarf irregulargalaxies and dSphs that have fallen into a Local Group environment and beenstripped of their gas.