The MALATANG Survey: Dense Gas and Star Formation from High Transition
  HCN and HCO+ maps of NGC253
  To study the high-transition dense-gas tracers and their relationships to the
star formation of the inner $\sim$ 2 kpc circumnuclear region of NGC253, we
present HCN $J=4-3$ and HCO$^+ J=4-3$ maps obtained with the James Clerk
Maxwell Telescope (JCMT). With the spatially resolved data, we compute the
concentration indices $r_{90}/r_{50}$ for the different tracers. HCN and
HCO$^+$ 4-3 emission features tend to be centrally concentrated, which is in
contrast to the shallower distribution of CO 1-0 and the stellar component. The
dense-gas fraction ($f_\text{dense}$, traced by the
velocity-integrated-intensity ratios of HCN/CO and HCO$^+$/CO) and the ratio
$R_\text{31}$ (CO 3-2/1-0) decline towards larger galactocentric distances, but
increase with higher SFR surface density. The radial variation and the large
scatter of $f_\text{dense}$ and $R_\text{31}$ imply distinct physical
conditions in different regions of the galactic disc. The relationships of
$f_\text{dense}$ versus $\Sigma_\text{stellar}$, and SFE$_\text{dense}$ versus
$\Sigma_\text{stellar}$ are explored. SFE$_\text{dense}$ increases with higher
$\Sigma_\text{stellar}$ in this galaxy, which is inconsistent with previous
work that used HCN 1-0 data. This implies that existing stellar components
might have different effects on the high-$J$ HCN and HCO$^+$ than their low-$J$
emission. We also find that SFE$_\text{dense}$ seems to be decreasing with
higher $f_\text{dense}$, which is consistent with previous works, and it
suggests that the ability of the dense gas to form stars diminishes when the
average density of the gas increases. This is expected in a scenario where only
the regions with high-density contrast collapse and form stars.
