Schedule Feb 17, 2012
The UV Properties of Star Forming Galaxies
Stephen Wilkins (Univ. of Oxford)

The installation of WFC3, with its superb sensitivity in the Near-IR, on the Hubble Space Telescope has permitted us to identify large numbers of candidate galaxies at z=7 and beyond. This deep NIR imaging also allows us to study the wider properties, and in particular the UV continuum (UVC) slope, of high-redshift (4<z<8) galaxies. While the UVC is sensitive to a range of properties, the UVC has most often been employed as a diagnostic of the dust attenuation (in star forming galaxies). This is particularly important at high-redshift where alternative diagnostics (such as Balmer Decrement/FIR/1.2 GHz observations) are typically inaccessible except for the most vigorously star forming systems. Thus the UVC slope allows us to the probe the dust-obscured fraction of star formation allowing us to produce more accurate estimates of the true intrinsic star formation rate density at high-redshift.

While the UVC is principally affected by dust (insofar as it is dust that has the greatest effect on the UVC of star forming galaxies over cosmic history) it is also sensitive to properties intrinsic to the stellar population including the metallicity, recent star formation history and initial mass function. Thus the UVS potentially offers the opportunity to probe exotic stellar populations, especially at high-redshift where the effects of dust are likely diminished. Indeed recent results presented in the literature have indicated the existence of extremely blue UVC slopes in z=7 galaxies.

However, results from our analysis of ACS+WFC3 observations at z=7 contest this conclusion and we find no significant evidence for extreme stellar populations. Extending our analysis to lower-redshift we find evidence of both a luminosity and redshift dependence of the mean UVC slope insofar as galaxies at lower redshift and higher luminosities are typically redder.


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