AzTEC Survey of the Central Molecular Zone: Increasing Spectral Index of Dust with Density

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 SED analysis of our AzTEC/Large Millimeter Telescope survey, together with existing Herschel far-IR data on the CMZ, from a wavelength range of 160 µm to 1.1 mm. We include global foreground and background contributions in a novel Bayesian modeling that incorporates the Point Spread Functions (PSFs) of the different maps, which enables the full utilization of our high resolution (10.5 ′′ ) 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 − 2.4, toward 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% in particularly dense regions.

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“…Additionally, we have assumed a spectral index, 𝛽, of 1.75 (Battersby et al 2011). It should be noted that recent estimates by Tang et al (2020) determine 𝛽 in the CMZ to be in the range 2.0 -2.4 on scales of 10.5 . Using the upper value of this range of 𝛽 = 2.4 instead of our assumed value of 1.75 increases our reported masses by a factor of ∼1.69.…”

“…We also need to assume the dust temperature in order to estimate masses. We use a dust temperature of ∼20 K, based on estimates by Tang et al (2020). The uncertainties in the dust temperature and opacity mean that systematic uncertainties in the mass estimates are a factor of ∼2 (Kauffmann et al 2008).…”