The H i hole and AGB stellar population of the Sagittarius dwarf irregular galaxy

HST proper-motion decontamination^⋆,⋆⋆

¹ European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile
² INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
e-mail: yazan.almomany@oapd.inaf.it
³ Centro de Astrofisica, Universidade do Porto, Rua das Estrelas, 4150-762, Porto, Portugal
⁴ Division of Astronomy, University of California, 8979 Math Sciences, Los Angeles, CA 90095-1562, USA
⁵ W. M. Keck Observatory, 65-1120 Mamalahoa Highway, Waimea, HI 96743, USA

Received: 24 April 2014
Accepted: 21 August 2014

Abstract

Using two HST/ACS data sets that are separated by about two years has allowed us to derive the relative proper motion for the Sagittarius dwarf irregular (SagDIG) and reduce the heavy foreground Galactic contamination. The proper-motion decontaminated SagDIG catalog provides a much clearer view of the young red-supergiant and intermediate-age asymptotic giant branch populations. Previously identified carbon- and oxygen-rich star samples, which are based on narrow-band filter photometry, were complemented by membership criteria. We report identifying three Milky Way carbon-rich dwarf stars, which probably belong to the thin disk, and pointing to the high incidence of this class at low Galactic latitudes. A subgroup of four oxygen-rich candidate stars depicts a faint, red extension of the well-defined SagDIG carbon-rich sequence. The origin of these oxygen-rich candidate stars remains unclear, reflecting the uncertainty in the ratio of carbon- and oxygen-rich stars. Finally, SagDIG is a gas-rich galaxy characterized by a single large cavity in the gas disk (H i hole), which is offset by ~360 pc from the optical center of the galaxy. We nonetheless investigate the stellar feedback hypothesis by comparing the proper-motion cleaned stellar populations within the H i hole with appropriately selected comparison regions that have higher H i densities external to the hole. The comparison shows no significant differences. In particular, the center of the H i hole (and the comparison regions) lack stellar populations younger than ~400 Myr, which are otherwise abundant in the inner body of the galaxy. We conclude that there is no convincing evidence that the SagDIG H i hole is the result of stellar feedback and that gravitational and thermal instabilities in the gas are the most likely mechanism for its formation.