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 \sim 7 \% 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 } _ { CS } - L _ { IR } and L ^ { \prime } _ { 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 .