Dust and Cold Gas Properties of Starburst HyLIRG-Quasars at $z\sim 2.5$

Abstract

Some high-z active galactic nuclei (AGNs) are found to reside in extreme star-forming galaxies, such as hyper-luminous infrared galaxies (HyLIRGs), with AGN-removed $L_{\mathrm{IR}}$ of $>10^{13}{L}_{\odot }$. In this paper, we report NOEMA observations of six apparent starburst HyLIRGs associated with optical quasars at $z\sim 2-3$ in the Stripe 82 field, to study their dust and molecular CO properties. Five out of the six candidates are detected with CO(4-3) or CO(5-4) emission, and four in 2mm dust continuum. Based on the linewidth-$L_{\mathrm{CO}(1-0)}^{\prime }$ diagnostics, we find that four galaxies are likely unlensed or weakly lensed sources. The molecular gas mass is in the range of $\mu M_{{\mathrm{H}}_2}\sim 0.8-9.7\times 10^{10}{M}_{\odot }$ (with $\alpha =0.8M_{\odot }(\mathrm{Kkm}{\mathrm{s}}^{-1}\mathrm{p}{\mathrm{c}}^2{)}^{-1}$ and $\mu$ is the unknown possible gravitational magnification factor). We fit their SEDs, after including the observed 2mm fluxes and upper limits, and estimate their apparent (uncorrected for possible lensing effect) star formation rates ($\mu$SFRs) to be $\sim 400-2500$ $M_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$ with depletion time of $\sim 20-110$ Myr. We notice interesting offsets, of $\sim 10-40$ kpc spatially or $\sim 1000-2000$ km s${}^{-1}$ spectroscopically, between the optical quasar and the mm continuum or CO emissions. The observed velocity shift is likely related to the blueshifted broad-emission-line region of quasars, though mergers or recoiling black holes are also possible causes, which can explain the spatial offset and the high intrinsic SFRs in the HyLIRG-quasar systems.

22 pages, 6 figures, 7 tables. Accepted for publication in The Astrophysical Journal