We serendipitously discovered in the Herschel Reference Survey an extremely bright infrared source with S _ { 500 } \sim 120 mJy in the line of sight of the Virgo cluster which we name Red Virgo 4 ( RV4 ) .
Based on IRAM/EMIR and IRAM/NOEMA detections of the CO ( 5-4 ) , CO ( 4-3 ) , and [ CI ] lines , RV4 is located at a redshift of 4.724 , yielding a total observed infrared luminosity of 1.1 \pm 0.6 \times 10 ^ { 14 } L _ { \odot } .
At the position of the Herschel emission , three blobs are detected with the VLA at 10 cm .
The CO ( 5-4 ) line detection of each blob confirms that they are at the same redshift with the same line width , indicating that they are multiple images of the same source .
In Spitzer and deep optical observations , two sources , High-z Lens 1 ( HL1 ) West and HL1 East , are detected at the center of the three VLA/NOEMA blobs .
These two sources are placed at z = 1.48 with XSHOOTER spectra , suggesting that they could be merging and gravitationally lensing the emission of RV4 .
HL1 is the second most distant lens known to date in strong lensing systems .
Constrained by the position of the three VLA/NOEMA blobs , the Einstein radius of the lensing system is 2.2″ \pm 0.2 ( 20 kpc ) .
The high redshift of HL1 and the large Einstein radius are highly unusual for a strong lensing system .
In this paper , we present the insterstellar medium properties of the background source RV4 .
Different estimates of the gas depletion time yield low values suggesting that RV4 is a starburst galaxy .
Among all high-z submillimeter galaxies ( SMGs ) , this source exhibits one of the lowest L _ { [ CI ] } to L _ { IR } ratios , 3.2 \pm 0.9 \times 10 ^ { -6 } , suggesting an extremely short gas depletion time of only 14 \pm 5 Myr .
It also shows a relatively high L _ { [ CI ] } to L _ { CO ( 4 - 3 ) } ratio ( 0.7 \pm 0.2 ) and low L _ { CO ( 5 - 4 ) } to L _ { IR } ratio ( only \sim 50 \% of the value expected for normal galaxies ) hinting a low density of gas .
Finally , we discuss the short depletion time of RV4 .
It can be explained by either a very high star formation efficiency , which is difficult to reconcile with major mergers simulations of high- z galaxies , or a rapid decrease of star formation , which would bias the estimate of the depletion time toward artificially low value .