Blue BX galaxies breaking bad : allegations of a 2175Å feature at z ~ 2.2 in violation of the Calzetti dust law

Abstract

The dust properties of star forming galaxies at z ~ 2 are examined using a sample of BX objects selected from the Keck Deep Fields (KDF) using colour-colour selection criteria. Spectral energy distribution (SED) fitting is used to infer physical properties of these galaxies from UnGRIHKs photometric data. Results suggest that the spectra of many of the BX objects with blue rest-frame UV colours include a 2175Å absorption feature not conventionally considered in high redshift analyses. The redshift ranges over which SEDs exhibiting the feature satisfy the BX selection criteria are computed and found to vary significantly between models including different dust laws. Clustering analyses are shown to be sensitive to the assumed dust attenuation law. Inappropriate dust model choices may lead to factor-of-two overestimates of spatial correlation lengths. The possibility of using the 2175Å feature as a redshift indicator is investigated through tests on synthetic galaxy photometry with promising results.