LEO P: AN UNQUENCHED VERY LOW-MASS GALAXY

Kristen B.W. McQuinn, Evan D. Skillman, Andrew Dolphin, John M. Cannon, John J. Salzer, Katherine L. Rhode, Elizabeth A.K. Adams, Danielle Berg, Riccardo Giovanelli, Léo Girardi, Martha P. Haynes

Research output: Contribution to journalArticlepeer-review

51 Scopus citations

Abstract

Leo P is a low-luminosity dwarf galaxy discovered through the blind H i Arecibo Legacy Fast ALFA survey. The H i and follow-up optical observations have shown that Leo P is a gas-rich dwarf galaxy with active star formation, an underlying older population, and an extremely low oxygen abundance. We have obtained optical imaging with the Hubble Space Telescope to two magnitudes below the red clump in order to study the evolution of Leo P. We refine the distance measurement to Leo P to be 1.62 ± 0.15 Mpc, based on the luminosity of the horizontal branch stars and 10 newly identified RR Lyrae candidates. This places the galaxy at the edge of the Local Group, ∼0.4 Mpc from Sextans B, the nearest galaxy in the NGC 3109 association of dwarf galaxies of which Leo P is clearly a member. The star responsible for ionizing the H ii region is most likely an O7V or O8V spectral type, with a stellar mass 25 Mo. The presence of this star provides observational evidence that massive stars at the upper end of the initial mass function are capable of being formed at star formation rates as low as ∼10-5 Mo yr-1. The best-fitting star formation history (SFH) derived from the resolved stellar populations of Leo P using the latest PARSEC models shows a relatively constant star formation rate over the lifetime of the galaxy. The modeled luminosity characteristics of Leo P at early times are consistent with low-luminosity dSph Milky Way satellites, suggesting that Leo P is what a low-mass dSph would look like if it evolved in isolation and retained its gas. Despite the very low mass of Leo P, the imprint of reionization on its SFH is subtle at best, and consistent with being totally negligible. The isolation of Leo P, and the total quenching of star formation of Milky Way satellites of similar mass, implies that the local environment dominates the quenching of the Milky Way satellites.

Original languageEnglish (US)
Article number158
JournalAstrophysical Journal
Volume812
Issue number2
DOIs
StatePublished - Oct 20 2015

Keywords

  • galaxies: distances and redshifts
  • galaxies: dwarf
  • galaxies: evolution
  • galaxies: photometry
  • galaxies: stellar content
  • stars: variables: RR Lyrae

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