To Stack Or Not To Stack: Spectral Energy Distribution Properties Of Ly Alpha-Emitting Galaxies At Z=2.1

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Vargas, Carlos J.
Bish, Hannah
Acquaviva, Viviana
Gawiser, Eric
Finkelstein, Steven L.
Ciardullo, Robin
Ashby, Matthew L. N.
Feldmeier, John
Ferguson, Henry
Gronwall, Caryl

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We use the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS) GOODS-S multi-wavelength catalog to identify counterparts for 20 Ly alpha emitting (LAE) galaxies at z = 2.1. We build several types of stacked spectral energy distributions (SEDs) of these objects. We combine photometry to form average and median flux-stacked SEDs, and postage-stamp images to form average and median image-stacked SEDs. We also introduce scaled flux stacks that eliminate the influence of variation in overall brightness. We use the SED fitting code SpeedyMC to constrain the physical properties of individual objects and stacks. Our LAEs at z = 2.1 have stellar masses ranging from 2 x 10(7) M-circle dot to 8 x 10(9) M-circle dot (median = 3 x 10(8) M-circle dot), ages ranging from 4 Myr to 500 Myr (median = 100 Myr), and E(B - V) between 0.02 and 0.24 (median = 0.12). Although still low, this represents significantly more dust reddening than has been reported for LAEs at higher redshifts. We do not observe strong correlations between Ly alpha equivalent width (EW) and age or E(B - V). The Ly alpha radiative transfer (q) factors of our sample are predominantly close to one and do not correlate strongly with EW or E(B - V). The absence of strong correlations with EW or q implies that Ly alpha radiative transfer is highly anisotropic and/or prevents Ly alpha photons from scattering in dusty regions. The SED parameters of the flux stacks match the average and median values of the individual objects, with the flux-scaled median SED performing best with uncertainties reduced by a factor of two. Median image-stacked SEDs provide a poor representation of the median individual object, and none of the stacking methods capture the large dispersion of LAE properties.



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Vargas, Carlos J., Hannah Bish, Viviana Acquaviva, Eric Gawiser, Steven L. Finkelstein, Robin Ciardullo, Matthew LN Ashby et al. "To Stack or Not to Stack: Spectral Energy Distribution Properties of Ly?-emitting Galaxies at z= 2.1." The Astrophysical Journal, Vol. 783, No. 1 (Mar., 2014): 26.