Modeling the Interstellar Medium of Low Surface Brightness Galaxies: Constraining Internal Extinction, Disk Color Gradients, and Intrinsic Rotation Curve Shapes

Abstract

We use a combination of three-dimensional Monte Carlo radiative transfer techniques and multiwavelength (BRHK, Hα) imaging data to investigate the nature of the interstellar medium (ISM) in the edge-on, low surface brightness (LSB) galaxy UGC 7321. Using realistic models that incorporate multiple scattering effects and clumping of the stars and the interstellar material, we explore the distribution and opacity of the interstellar material (gas+dust) and its effects on the observed stellar disk luminosity profiles, color gradients, and rotation curve shape. We find that UGC 7321 contains a small but nonnegligible dusty component to its ISM, yielding a B-band optical depth e,B ~ 4.0 from disk edge to center. A significant fraction (~50% ± 10%) of the interstellar material in the innermost regions of UGC 7321 appears to be contained in a clumpy medium, indicating that LSB galaxies can support a modest, multiphase ISM structure. In spite of the clear presence of dust, we conclude that the large radial optical color gradients observed in UGC 7321 and other similar LSB spiral galaxies cannot be accounted for by dust and must result primarily from significant stellar population and/or metallicity gradients. We show that realistic optical depth effects will have little impact on the observed rotation curves of edge-on disk galaxies and cannot explain the linear, slowly rising rotation curves seen in some edge-on LSB spirals. Projection effects create a far larger uncertainty in recovering the true underlying rotation curve shape of galaxies viewed at inclinations i ≳ 85°.