The Resolved Structure and Dynamics of an Isolated Dwarf Galaxy: A VLT and Keck Spectroscopic Survey of WLM

Creators: Leaman, Ryan; Venn, Kim A.; Brooks, Alyson M.; Battaglia, Giuseppina; Cole, Andrew A.; Ibata, Rodrigo A.; Irwin, Mike J.; McConnachie, Alan W.; Mendel, J. Trevor; Tolstoy, Eline

Abstract

We present spectroscopic data for 180 red giant branch (RGB) stars in the isolated dwarf irregular galaxy Wolf-Lundmark-Mellote (WLM). Observations of the calcium II triplet lines in spectra of RGB stars covering the entire galaxy were obtained with FORS2 at the Very Large Telescope and DEIMOS on Keck II, allowing us to derive velocities, metallicities, and ages for the stars. With accompanying photometric and radio data we have measured the structural parameters of the stellar and gaseous populations over the full galaxy. The stellar populations show an intrinsically thick configuration with 0.39 ≤ q_0 ≤ 0.57. The stellar rotation in WLM is measured to be 17 ± 1 km s^(–1); however, the ratio of rotation to pressure support for the stars is V/σ ~ 1, in contrast to the gas, whose ratio is seven times larger. This, along with the structural data and alignment of the kinematic and photometric axes, suggests we are viewing WLM as a highly inclined oblate spheroid. Stellar rotation curves, corrected for asymmetric drift, are used to compute a dynamical mass of (4.3 ± 0.3) × 10^8 M_☉ at the half-light radius (r_h = 1656 ± 49 pc). The stellar velocity dispersion increases with stellar age in a manner consistent with giant molecular cloud and substructure interactions producing the heating in WLM. Coupled with WLM's isolation, this suggests that the extended vertical structure of its stellar and gaseous components and increase in stellar velocity dispersion with age are due to internal feedback, rather than tidally driven evolution. These represent some of the first observational results from an isolated Local Group dwarf galaxy that can offer important constraints on how strongly internal feedback and secular processes modulate star formation and dynamical evolution in low-mass isolated objects.

Additional Information

© 2012 American Astronomical Society. Received 2011 September 30; accepted 2012 February 18; published 2012 April 13. We thank the anonymous referee for comments that helped improve this paper and Dr. Amanda Kepley for generously sharing her Hi data. R.L. acknowledges support from NSERC Discovery Grants to Don VandenBerg and K.V. The authors acknowledge support from the International Space Science Institute and useful discussions with associated team members. A.B. acknowledges support from the Sherman Fairchild Foundation. R.I. gratefully acknowledges support from the Agence Nationale de la Recherche though the grant POMMME (ANR 09-BLAN-0228). The authors thank M. Rejkuba, S. Cote, E. Skillman, J. Peñarrubia, and J. Navarro for useful discussions. FORS2 observations were collected at the ESO, proposal 072.B-0497. Some photometric data are based on observations obtained at Cerro Tololo Inter-American Observatory, a division of the National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. This research also uses services or data provided by the NOAO Science Archive. VLA and the National Radio Astronomy Observatory are facilities of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Additional 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 wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Facilities: VLT:Yepun (FORS2), Keck:II (DEIMOS), Blanco (MOSAICII).

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