NGC 6822 as a Probe of Dwarf Galactic Evolution

Creators: Belland, Brent; Kirby, Evan; Boylan-Kolchin, Michael; Wheeler, Coral

Abstract

NGC 6822 is the closest isolated dwarf irregular galaxy to the Milky Way. Its proximity and stellar mass (10⁸ M_⊙, large for a dwarf galaxy) allow for a detailed study of its kinematic properties. The red giant branch (RGB) stars at the galaxy's center are particularly interesting because they are aligned on an axis perpendicular to the galaxy's more extended H I disk. We detected a velocity gradient among the RGB population using spectra from Keck/Deep Imaging Multi-Object Spectrograph (DEIMOS). This rotation is aligned with the H I disk, but the sense of rotation is about the major axis of the central RGB population. We measured the rotation velocity (v) and velocity dispersion (σ) of the RGB population in five metallicity bins. We found an increase of rotation support (v/σ) with increasing metallicity, driven primarily by decreasing dispersion. We also deduced an increasing radial distance for lower metallicity stars at −0.5 kpc dex⁻¹ by relating the observed stellar kinematics to position via NGC 6822's H I velocity curve. While the inverted metallicity gradient-like distribution could be interpreted as evidence for an outside-in formation scenario, it may instead indicate that stellar feedback disturbed a centrally star-forming galaxy over time.

Additional Information

© 2020 The American Astronomical Society. Received 2020 July 8; revised 2020 September 4; accepted 2020 September 5; published 2020 October 26. 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. The authors thank the anonymous referee for the input which improved this paper. This material is based upon work supported by the National Science Foundation under grant Nos. AST-1636426 and AST-1847909. E.N.K. gratefully acknowledges support from a Cottrell Scholar award administered by the Research Corporation for Science Advancement, as well as funding from generous donors to the California Institute of Technology. M.B.K. acknowledges support from NSF CAREER award AST-1752913, NSF grant AST-1910346, NASA grant NNX17AG29G, and HST-AR-15006, HST-AR-15809, HST-GO-15658, HST-GO-15901, and HST-GO-15902 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. Support for C.W. was provided by NASA through the NASA Hubble Fellowship grant HST-HF2-51449.001-A, awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. We are grateful to the many people who have worked to make the Keck Telescope and its instruments a reality and to operate and maintain the Keck Observatory. The authors 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. Facility: Keck:II (DEIMOS). -

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Published - Belland_2020_ApJ_903_10.pdf

Submitted - 2009.04555.pdf

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