We report the detection of HCO ^ { + } ( J =1 \to 0 ) emission towards the Cloverleaf quasar ( z = 2.56 ) through observations with the Very Large Array .
This is the first detection of ionized molecular gas emission at high redshift ( z > 2 ) .
HCO ^ { + } emission is a star formation indicator similar to HCN , tracing dense molecular hydrogen gas ( n ( { H _ { 2 } } ) \simeq 10 ^ { 5 } cm ^ { -3 } ) within star–forming molecular clouds .
We derive a lensing–corrected HCO ^ { + } line luminosity of L ^ { \prime } _ { HCO ^ { + } } = 3.5 \times 10 ^ { 9 } K km s ^ { -1 } pc ^ { 2 } .
Combining our new results with CO and HCN measurements from the literature , we find a HCO ^ { + } /CO luminosity ratio of 0.08 and a HCO ^ { + } /HCN luminosity ratio of 0.8 .
These ratios fall within the scatter of the same relationships found for low– z star–forming galaxies .
However , a HCO ^ { + } /HCN luminosity ratio close to unity would not be expected for the Cloverleaf if the recently suggested relation between this ratio and the far–infrared luminosity were to hold .
We conclude that a ratio between HCO ^ { + } and HCN luminosity close to 1 is likely due to the fact that the emission from both lines is optically thick and thermalized and emerges from dense regions of similar volumes .
The CO , HCN and HCO ^ { + } luminosities suggest that the Cloverleaf is a composite AGN–starburst system , in agreement with the previous finding that about 20 % of the total infrared luminosity in this system results from dust heated by star formation rather than heating by the AGN .
We conclude that HCO ^ { + } is potentially a good tracer for dense molecular gas at high redshift .