Probing the Optical Depth of Galaxies using Multi--waveband observations

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Session 31 -- Star-Forming Galaxies
Display presentation, Tuesday, 9:30-6:30, Heller Lounge Room

[31.20] Probing the Optical Depth of Galaxies using Multi--waveband observations

Rh. Evans (Ritter Observatory, University of Toledo)

The question of the optical depth in spiral galaxies obviously has far reaching implications, including determining the true stellar content and mass of galaxies, to the origin of the major component of the far infrared (FIR) radiation. We have investigated whether the reddening of stars can be used to determine the amount and extent of obscuring dust. In particular, we investigate how the position of the dust relative to the stars affects the reddening. We find the reddening is greatest for a geometry where the dust lies wholly between the observer and the stars (a `screen'), and least for the case where the dust has sunk into a thin layer in the mid--plane of the disk (a `thin sandwich'). For realistic geometries where the dust is either well mixed with the stars (a `slab') or lies in a layer thinner than the stars (a `sandwich') we find that the reddening reaches a maximum as we increase the optical depth of the dust, and can even start \underline{decreasing} as the optical depth is increased still further. We use the reddening vectors produced by these realistic geometries to compare models of the expected colors of galaxies to multi--waveband observations of individual $10^{\prime \prime} $ regions of a sample of galaxies. We find evidence for reddening, and with the correct reddening vectors the optical depths implied can be significant. However, the optical depths are found to decrease with increasing galactocentric radius, and are typically less than one at $\sim 2$ scale lengths from the center. We also find that the observed reddening is too great to be explained by values of the layering parameter $\zeta$ (the relative scale height of the dust to the stars) of less than $\sim 0.5$. Here we present results for the Sc galaxy NGC 6643. These reddening data and measurements of the gas column densities taken from the literature, together with a detailed investigation of the energy balance between the total stellar flux and the total FIR flux, all suggest a central optical depth for this galaxy of $\tau_{B} \approx 10$. This translates to a total B--band extinction due to internal obscuration of A$_{B} \sim 0.95^{m}$, a factor of four higher than the RC2 value.

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