The Universal Stellar Mass-Stellar Metallicity Relation for Dwarf Galaxies

Creators: Kirby, Evan N.; Cohen, Judith G.; Guhathakurta, Puragra; Cheng, Lucy; Bullock, James S.; Gallazzi, Anna

Abstract

We present spectroscopic metallicities of individual stars in seven gas-rich dwarf irregular galaxies (dIrrs), and we show that dIrrs obey the same mass-metallicity relation as the dwarf spheroidal (dSph) satellites of both the Milky Way and M31: Z_* ∝ M_*^(0.30±0.02). The uniformity of the relation is in contradiction to previous estimates of metallicity based on photometry. This relationship is roughly continuous with the stellar mass-stellar metallicity relation for galaxies as massive as M_* = 10^(12) M_☉. Although the average metallicities of dwarf galaxies depend only on stellar mass, the shapes of their metallicity distributions depend on galaxy type. The metallicity distributions of dIrrs resemble simple, leaky box chemical evolution models, whereas dSphs require an additional parameter, such as gas accretion, to explain the shapes of their metallicity distributions. Furthermore, the metallicity distributions of the more luminous dSphs have sharp, metal-rich cut-offs that are consistent with the sudden truncation of star formation due to ram pressure stripping.

Additional Information

© 2013 American Astronomical Society. Received 2013 August 15; accepted 2013 October 1; published 2013 November 27. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We are grateful to the many people who have worked to make the Keck Telescopes and their instruments a reality and to operate and maintain the Keck Observatory. The authors wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations presented herein would have been possible. We extend a special note of gratitude to Keck support astronomers Luca Rizzi, Greg Wirth, and Marc Kassis. We thank the anonymous referee for reviewing our manuscript. We also thank Yuichi Matsuda and Brenda Frye for obtaining the Subaru images of Aquarius presented in Section 2.2.5, Andrew Cole and Mike Irwin for sharing the INTWide Field Survey photometric catalog of Leo A, Edouard Bernard for sharing his photometric catalog of IC 1613, and Josh Simon and Marla Geha for sharing their DEIMOS spectroscopy of Leo T and other faint galaxies. E.N.K. thanks Evan Skillman, Leslie Hunt, Laura Magrini, and Jose Oñorbe for helpful conversations. We thank Edouard Bernard for a careful reading of the manuscript and helpful comments. We also thank Namrata Anand for early contributions to the coaddition of DEIMOS spectra. E.N.K. acknowledges support from the Southern California Center for Galaxy Evolution, a multicampus research program funded by the University of California Office of Research, and partial support from NSF grant AST-1009973. J.G.C. thanks NSF grant AST-0908139 for partial support. P.G. acknowledges support from NSF grant AST-10-10039. He thanks the staff of the Aspen Center for Physics for their generous hospitality during his visit. L.C. was supported by UCSC's Science Internship Program (SIP). A.G. acknowledges support from the EU FP7/2007-2013 under grant agreement No. 267251 AstroFIt. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web site is http://www.sdss.org/. IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. Facility: Keck:II (DEIMOS)

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