Author Topic: Discussion: The Ultraviolet Attenuation Law in Backlit Spiral Galaxies  (Read 1334 times)


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Just accepted for publication in the Astronomical Journal (arxiv preprint of accepted version):

The Ultraviolet Attenuation Law in Backlit Spiral Galaxies

William C. Keel, Anna M. Manning, Benne W. Holwerda, Chris J. Lintott,  and Kevin Schawinski

The effective extinction law (attenuation behavior) in galaxies in the emitted ultraviolet regime is well known only for actively star-forming objects and combines effects of the grain properties, fine structure in the dust distribution, and relative distributions of stars and dust. We use GALEX, XMM Optical Monitor, and HST data to explore the UV attenuation in the outer parts of spiral disks which are backlit by other UV-bright galaxies, starting with the candidate list of pairs provided by Galaxy Zoo participants. New optical images help to constrain the geometry and structure of the target galaxies. Our analysis incorporates galaxy symmetry, using non-overlapping regions of each galaxy to derive error estimates on the attenuation measurements. The entire sample has an attenuation law across the optical and UV which is close to the Calzetti et al. (1994) form; the UV slope for the overall sample is substantially shallower than found by Wild et al. (2011), which is a reasonable match to the more distant galaxies in our sample but not to the weighted combination including NGC 2207. The nearby, bright spiral NGC 2207 alone gives accuracy almost equal to the rest of our sample, and its outer arms have a very low level of foreground starlight. Thus, this widespread, fairly ``grey" law can be produced from the distribution of dust alone, without a necessary contribution from differential escape of stars from dense clouds. Our results indicate that the extrapolation needed to compare attenution between backlit galaxies at moderate redshifts from HST data, and local systems from SDSS and similar data, is mild enough to allow use of galaxy overlaps to trace the cosmic history of dust in galaxies. For NGC 2207, HST data in the near-UV F336W band show that the covering factor of clouds with small optical attenuation becomes a dominant factor farther into the ultraviolet, which opens the possibility that widespread diffuse dust dominates over dust in star-forming regions deep into the ultraviolet. Comparison with published radiative-transfer models indicates that the role of dust clumping dominates over differences in grain populations, at this coarse spatial resolution.

This was partly a prelude to tell how well a comparison between dust measures from the SDSS pairs and Hubble Zoo overlaps (many of which are at much higher redshift, giving leverage for tracing the evolution of dust in galaxies) could be compared, and the results are encouraging that we do know how to extrapolate back from near-ultraviolet data to what we'd see in visible light. As often happens, getting the results was the quick part, and deriving the uncertainty ranges (error bars) took about 80% of the effort. Unlike many other uses of overlapping pairs, for this one we wanted both pair members to be spirals, so there would be enough ultraviolet from the background for a useful measurement. Taking that, having to have a long enough UV exposure from the GALEX survey mission, and then winnowing galaxies that either show no evidence for dust attenuation (so that the other galaxy may be in front) or have a UV-bright star-forming region in just the wrong place, meant that even starting with the Galaxy Zoo catalog, we wound up with only a handful of pairs to analyze. Of these, the nearest and best-resolved is the NGC 2207/IC 2163 pair, outside the SDSS region (and once featured as an OOTD). Some images will show up in an OOTD shortly.

(Edited to include arxiv link)
« Last Edit: January 07, 2014, 03:18:38 am by NGC3314 »