AzTEC survey of the central molecular zone: increasing spectral index of dust with density

Abstract

ABSTRACT The central molecular zone (CMZ) of our Galaxy hosts an extreme environment analogous to that found in typical starburst galaxies in the distant Universe. In order to understand dust properties in environments like our CMZ, we present results from a joint spectral energy distribution analysis of our AzTEC/Large Millimeter Telescope survey, together with existing Herschel far-IR data on the CMZ, from a wavelength range of 160 $\mu {\rm m}$ to 1.1 mm. We include global foreground and background contributions in a novel Bayesian modelling that incorporates the point spread functions of the different maps, which enables the full utilization of our high-resolution (10.5 arcsec) map at 1.1 mm and reveals unprecedentedly detailed information on the spatial distribution of dusty gas across the CMZ. There is a remarkable trend of increasing dust spectral index β, from 2.0 to 2.4, towards dense peaks in the CMZ, indicating a deficiency of large grains or a fundamental change in dust optical properties. This environmental dependence of β could have a significant impact on the determination of dust temperature in other studies. Depending on how the optical properties of dust deviate from the conventional model, dust temperatures could be underestimated by $10\!-\!50{{\ \rm per\ cent}}$ in particularly dense regions.