Abstract
We serendipitously discovered in the Herschel Reference Survey an extremely bright infrared source with S500 ∼ 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 × 1014 L⊙. 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 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-z submillimeter galaxies, this source exhibits one of the lowest L[CI] to LIR ratios, 3.2 ± 0.9 × 10−6, suggesting an extremely short gas depletion time of only 14 ± 5 Myr. It also shows a relatively high L[CI] to LCO(4−3) ratio (0.7 ± 0.2) and low LCO(5−4) to LIR ratio (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-z galaxies, or a rapid decrease of star formation, which would bias the estimate of the depletion time toward an artificially low value.
Highlights
In the local Universe, a third of the total bolometric luminosity of galaxies is emitted in the infrared (IR) and submillimetric domains by dust grains, which reprocess the energy absorbed from the stars and active galactic nuclei (AGN; Soifer & Neugebauer 1991)
The IRAC fluxes measured from PSF fitting for High-z Lens 1 (HL1)-W and High-z Lens 1 East (HL1-E) are consistent with its spectral energy distribution (SED), indicating no particular excess of flux that could be attributed to Red Virgo 4 (RV4)
It has been found that ∼20% of SMGs host an AGN; we investigate if our data allow us to rule out the presence of an AGN contributing to the IR luminosity of RV4
Summary
In the local Universe, a third of the total bolometric luminosity of galaxies is emitted in the infrared (IR) and submillimetric (submm) domains by dust grains, which reprocess the energy absorbed from the stars and active galactic nuclei (AGN; Soifer & Neugebauer 1991). The ISM of high-z sources can be studied through the easier detection of lines, thanks to flux boosting (for instance, Weiß et al 2013; Alaghband-Zadeh et al 2013; Béthermin et al 2015; Bothwell et al 2017; Yang et al 2017; Cunningham et al 2019) Another aspect of gravitational lensing is that the deflection of the light emitted by a background source allows us to probe the mass distribution of the foreground source acting as the lens, constraining dark matter (DM) sub-halo structures (Hezaveh et al 2016) as well as the initial mass function (IMF; Cañameras et al 2017), for instance. We describe the set of data that we obtained for the characterization of the lensing system composed of RV4 and HL1
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