Weak CS Emission in an Extremely Metal-poor Galaxy DDO 70
  In most galaxies like the Milky Way, stars form in clouds of molecular gas.
Unlike the CO emission that traces the bulk of molecular gas, the rotational
transitions of HCN and CS molecules mainly probe the dense phase of molecular
gas, which has a tight and almost linear relation with the far-infrared
luminosity and star formation rate. However, it is unclear if dense molecular
gas exists at very low metallicity, and if exists, how it is related to star
formation. In this work, we report ALMA observations of the CS
$J$=5$\rightarrow$4 emission line of DDO~70, a nearby gas-rich dwarf galaxy
with $\sim7\%$ solar metallicity. We did not detect CS emission from all
regions with strong CO emission. After stacking all CS spectra from CO-bright
clumps, we find no more than a marginal detection of CS $J$=5$\rightarrow$4
transition, at a signal-to-noise ratio of $\sim 3.3$. This 3-$\sigma$ upper
limit deviates from the $L^\prime_{\rm CS}$-$L_{\rm IR}$ and $L^\prime_{\rm
CS}$-SFR relationships found in local star forming galaxies and dense clumps in
the Milky Way, implying weaker CS emission at given IR luminosity and SFR. We
discuss the possible mechanisms that suppress CS emission at low metallicity.
