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Proposal Information for 2009B-0360


PI: Kimberly A. Herrmann, Lowell Observatory, herrmann@lowell.edu
Address: 1400 West Mars Hill Road, Flagstaff, AZ 86001, USA

CoI: Robin Ciardullo, Penn State University

Title: The Stellar Kinematics of Outer Disks: Evidence for Halo Substructure?

Abstract: \centerlineLook at a spiral; what do you see? Stars zooming `round in the galaxy! \centerlineTheir motions indicate total mass, but how much is DM, stars, and gas? \centerlineStudy motions in and out; first find monochromatic stars- that's my kind. \centerlineFind us, get our velocities, then: determine disk mass! I'm a PN! Rotation curves indicate the total mass of spirals, but halo mass profiles cannot be decoupled from the visible disk mass using rotation curves alone. To break this disk-halo degeneracy, we have been using planetary nebulae (PNe) to measure the z-component of the stellar velocity dispersion in the disks of face-on spirals. These measurements of \sigma_z, coupled with straightforward assumptions, have yielded disk surface mass estimates over several scale lengths (h_R) in 6 spirals. We find that in the inner regions of galaxies (R < 3.5 h_R), the values of \sigma_z are consistent with those expected from a constant mass-to-light ratio (M/L), constant scale-height disk. However, in the two galaxies with data beyond this limit, \sigma_z flattens and becomes constant with radius. We have interpreted this as evidence for the existence of discrete cold-dark-matter subhalos which heat the outer disk and cause it to flare, but an increasing disk M/L may also contribute to the behavior. To investigate this phenomenon more thoroughly, we are proposing to extend the surveys of two additional galaxies, M74 and IC 342, to R > 8 h_R. Last year we imaged M74 and two fields of IC 342 with the wide-field Mosaic camera on the KPNO 4-m telescope. This proposal is to perform follow-up spectroscopy on ~500 outer PNe in the two galaxies with WIYN/Hydra.


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