摘要：We serendipitously discovered in theHerschelReference Survey an extremely bright infrared source withS500 ∼ 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 ± 0.6 × 1014L⊙. At the position of theHerschelemission, 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. InSpitzerand deep optical observations, two sources, High-zLens 1 (HL1) West and HL1 East, are detected at the center of the three VLA/NOEMA blobs. These two sources are placed atz = 1.48 with X-shooter 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″ ± 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-zsubmillimeter galaxies, this source exhibits one of the lowestL[CI]toLIRratios, 3.2 ± 0.9 × 10−6, suggesting an extremely short gas depletion time of only 14 ± 5 Myr. It also shows a relatively highL[CI]toLCO(4−3)ratio (0.7 ± 0.2) and lowLCO(5−4)toLIRratio (only ∼50% of the value expected for normal galaxies) hinting at 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-zgalaxies, or a rapid decrease of star formation, which would bias the estimate of the depletion time toward an artificially low value.
关键词：engalaxies: high-redshiftgalaxies: ISMgalaxies: starburstgalaxies: star formationsubmillimeter: galaxies
