LEGO II: A 3 mm molecular line study covering 100 pc of one of the most
  actively star-forming portions within the Milky Way Disc
  The current generation of (sub)mm-telescopes has allowed molecular line
emission to become a major tool for studying the physical, kinematic, and
chemical properties of extragalactic systems, yet exploiting these observations
requires a detailed understanding of where emission lines originate within the
Milky Way. In this paper, we present 60$^{\prime\prime}$ ($\sim$3pc) resolution
observations of many 3mm-band molecular lines across a large map of the W49
massive star-forming region ($\sim$100$\times$100pc at 11kpc), which were taken
as part of the "LEGO" IRAM-30m large project. We find that the spatial extent
or brightness of the molecular line transitions are not well correlated with
their critical densities, highlighting abundance and optical depth must be
considered when estimating line emission characteristics. We explore how the
total emission and emission efficiency (i.e. line brightness per H$_{2}$ column
density) of the line emission vary as a function of molecular hydrogen column
density and dust temperature. We find that there is not a single region of this
parameter space responsible for the brightest and most efficiently emitting gas
for all species. For example, we find that the HCN transition shows high
emission efficiency at high column density ($10^{22}$cm$^{-2}$) and moderate
temperatures (35K), whilst e.g. N$_2$H$^+$ emits most efficiently towards lower
temperatures ($10^{22}$cm$^{-2}$; <20K). We determine $X_{\mathrm{CO} (1-0)}
\sim 0.3 \times 10^{20} \mathrm{cm^{-2}(Kkms^{-1})^{-1}}$, and
$\alpha_{\mathrm{HCN} (1-0)} \sim 30\mathrm{M_\odot(Kkms^{-1}pc^2)^{-1}}$,
which both differ significantly from the commonly adopted values. In all, these
results suggest caution should be taken when interpreting molecular line
emission.
