Extended Hα over compact far-infrared continuum in dusty submillimeter galaxies

Abstract

By using data from the Atacama Large Millimeter/submillimeter Array and near-infrared (NIR) integral field spectrographs, including both Spectrograph for INtegral Field Observations in the Near Infrared and K-band Multi Object Spectrograph on the Very Large Telescope, we investigate the two-dimensional distributions of Hα and rest-frame far-infrared (FIR) continuum in six submillimeter galaxies (SMGs) at z ∼ 2. At a similar spatial resolution (∼0.″5 FWHM; ∼4.5 kpc at z = 2), we find that the half-light radius of Hα is significantly larger than that of the FIR continuum in half of the sample, and on average Hα is a median factor of 2.0 ± 0.4 larger. Having explored various ways to correct for the attenuation, we find that the attenuation-corrected Hα-based star-formation rates (SFRs) are systematically lower than the infrared (IR)-based SFRs by at least a median factor of 3 ± 1, which cannot be explained by the difference in half-light radius alone. In addition, we find that in 40% of cases the total V-band attenuation (AV) derived from energy balance modeling of the full ultraviolet (UV)-to-FIR spectral energy distributions (SEDs) is significantly higher than what is derived from SED modeling using only the UV-to-NIR part of the SEDs, and the discrepancy appears to increase with increasing total infrared luminosity. Finally, in considering all of our findings along with the studies in the literature, we postulate that the dust distributions in SMGs, and possibly also in less IR luminous z ∼ 2 massive star-forming galaxies, can be decomposed into the following three main components: the diffuse dust heated by older stellar populations, the more obscured and extended young star-forming H II regions, and the heavily obscured central regions that have a low filling factor but dominate the infrared luminosity in which the majority of attenuation cannot be probed via UV-to-NIR emissions.