The JCMT Nearby Galaxies Legacy Survey: SCUBA-2 observations of nearby galaxies

Abstract

We present 850$\mu$m observations of a sample of 8 nearby spiral galaxies, made using the SCUBA-2 camera on the James Clerk Maxwell Telescope (JCMT) as part of the JCMT Nearby Galaxies Legacy Survey (NGLS). We corrected our data for the presence of the $^{12}$CO $J=3\to 2$ line in the SCUBA-2 850$\mu$m bandwidth using NGLS HARP data, finding a typical $^{12}$CO contribution of $\sim 20$%. We measured dust column densities, temperatures and opacity indices by fitting spectral energy distributions constructed from SCUBA-2 and archival Herschel observations, and used archival GALEX and Spitzer data to make maps of surface density of star formation ($\Sigma_{\rm SFR}$). Typically, comparing SCUBA-2-derived H$_2$ surface densities ($\Sigma_{\rm H_2}$) to $\Sigma_{\rm SFR}$ gives shallow star formation law indices within galaxies, with SCUBA-2-derived values typically being sublinear and Herschel-derived values typically being broadly linear. This difference is likely due to the effects of atmospheric filtering on the SCUBA-2 data. Comparing the mean values of $\Sigma_{\rm H_2}$ and $\Sigma_{\rm SFR}$ of the galaxies in our sample returns a steeper star formation law index, broadly consistent with both the Kennicutt-Schmidt value of 1.4 and linearity. Our results show that a SCUBA-2 detection is a good predictor of star formation. We suggest that Herschel emission traces gas in regions which will form stars on timescales $\sim 5-100$ Myr, comparable to the star formation timescale traced by GALEX and Spitzer data, while SCUBA-2 preferentially traces the densest gas within these regions, which likely forms stars on shorter timescales.