Molecular gas content of gravitational-lensed quasars at cosmic noon

TL;DR

This study leverages gravitational lensing to probe the molecular gas content of high-redshift quasars with moderate to low intrinsic luminosities. Using IRAM-30m CO observations of 17 quadruply imaged quasars, the authors detect CO in five sources (~30%) and derive magnification-corrected gas properties, showing that these hosts are gas-rich but exhibit milder star formation and lower star formation efficiencies than the most luminous high-z quasars. The CO SLEDs for a subset indicate low to moderate excitation, consistent with less intense starbursts, while one Cloverleaf-like source shows a starburst-level SLED. Collectively, the results imply substantial diversity in quasar host ISM conditions at cosmic noon and demonstrate that lensing enables studying a population of high-z quasars in weaker starburst environments that would be challenging to observe otherwise.

Abstract

Star-forming activity in the host galaxies of high-redshift quasars is crucial to understanding the connection between supermassive black hole (SMBH) activity and galaxy evolution. While most existing studies are biased toward luminous quasars, we conduct carbon monoxide (CO) observations of 17 gravitationally lensed quasars that have four images using the IRAM 30m telescope to investigate the molecular gas content of moderate- to low-luminosity quasars. CO emissions are detected in five out of 17 quasars, corresponding to a detection rate of about 30\%. Analysis of their star formation activity reveals that these quasars live in gas-rich environments but exhibit weaker starbursts and lower star formation efficiencies compared to other luminous high-redshift quasars. In addition, the CO spectral line energy distributions of the two quasars (SDSS J0924+0219, SDSS J1330+1810) are also consistent with mild star formation instead of extreme starbursts. These results suggest that these lensed quasars reside in weaker starburst environments.

Figures (6)

• Figure 1: Observed spectra at a velocity range from $\rm -2500\ km\;s^{-1}$ to $\rm 2500\ km\;s^{-1}$ of all objects, where the zero velocity is the optical redshift. The background blue curves are the stacked and smoothed spectra. The orange shadow represents the 1-$\sigma$ standard deviation of the continuum, while the grey region marks the CO emission line. The best-fitted Gaussian profile is marked by the red curve.
• Figure 2: CO spectral line energy distribution. The flux of other CO J ladders of our sample is collected from the literature. For J0924+0219, we collect the CO J=5-4 Badole2020 and J=8-7 Stacey2021. For J1330+1810, we collect the CO J=7-6 Stacey2021. For H1413+117, we collect the CO J=4-3, J=5-4, J=6-5, J=7-6, J=8-7, J=9-8 Barvainis1997Bradford2009. We compare our sample with several representative galaxy samples and theoretical simulations, including the inner disk of the Milky Way Fixsen1999, the average SLED of local ULIRGs Papadopoulos2012, SMGs Bothwell2013, and luminous high-redshift quasars Carilli2013, and the simulation-predicted SLEDs of star-forming galaxies with the $\Sigma_{\rm SFR} = 1-10 {\rm M_{\odot}\;yr^{-1}\;kpc^{-2}}$Narayanan2014. Except for H1413+117 (cloverleaf), other quasars show different SLED shapes, indicating distinct physical conditions of the interstellar medium within their host galaxies.
• Figure 3: Comparison of star-forming activity between our sample and other representative galaxy samples. (a) CO J=1-0 vs. SFR. The black solid line shows the star-forming main sequence (SFMS) with 1-$\sigma$ scatter, while the black dotted line shows the starburst trend Sargent2014. (b) Star formation efficiency (${\rm SFE} \equiv {\rm SFR} / L_{\rm CO\ J=1-0}^{'}$) as a function of redshift. Various galaxy samples include the near-infrared selected (Bzk) galaxies Daddi2010, SMGs Greve2005Daddi2009aDaddi2009b, luminous high-redshift quasars Solomon2005Riechers2006, local PG quasars Shangguan18Shangguan2020, local ULIRGs Solomon1997, local spirals Leroy2008Leroy2009Wilson2009, and Quads lensed quasars from literature Barvainis1997Barvainis2002Ao2008Bradford2009Riechers2011Deane2013Paraficz2018Stacey2020Stacey2021Stacey2022Castillo2024.
• Figure 4: SFE as a function of redshift, adding the inferred SFE cosmic evolution of star-forming main sequence from Sargent2014 as indicated with the black dotted line with 1-$\sigma$ scatter. Compared to other high-z quasars Solomon2005Circosta2021, the host galaxies of our quasars show a lower SFE.
• Figure 5: Comparison of the SMBHs mass and bolometric luminosity with local Shangguan18 and distant quasars Circosta2021. The dashed lines represent the constant Eddington ratios. The black hole masses and bolometric luminosities are corrected with magnification. The black hole mass of our sample is distributed between the local PG quasars and luminous quasars at cosmic noon, while bolometric luminosities show a similar distribution.