The Warm Molecular Gas Around the Cloverleaf Quasar

TL;DR

This study presents the first broadband 1mm spectrum of the Cloverleaf Quasar, revealing highly-excited molecular gas, constraining its physical conditions, and exploring heating mechanisms, with implications for starburst activity and initial mass function.

Contribution

First broadband 1mm spectrum of the Cloverleaf Quasar providing high-J CO data and insights into warm molecular gas conditions and heating sources.

Findings

Detected multiple high-J CO transitions, constraining gas excitation.

Found a large mass of highly-excited molecular gas with high thermal pressure.

Suggested UV photons and X-rays both contribute to heating the molecular gas.

Abstract

We present the first broadband lambda = 1 mm spectrum toward the z=2.56 Cloverleaf Quasar, obtained with Z-Spec, a 1-mm grating spectrograph on the 10.4-meter Caltech Submillimeter Observatory. The 190-305 GHz observation band corresponds to rest-frame 272 to 444 microns, and we measure the dust continuum as well as all four transitions of carbon monoxide (CO) lying in this range. The power-law dust emission, F_nu = 14 mJy (nu/240GHz)^3.9 is consistent with the published continuum measurements. The CO J=6->5, J=8->7, and J=9->8 measurements are the first, and now provide the highest-J CO information in this source. Our measured CO intensities are very close to the previously-published interferometric measurements of J=7->6, and we use all available transitions and our 13CO upper limits to constrain the physical conditions in the Cloverleaf molecular gas disk. We find a large mass (2-50x10^9 Msun) of highly-excited gas with thermal pressure nT > 10^6 Kcm^-3. The ratio of the total CO cooling to the far-IR dust emission exceeds that in the local dusty galaxies, and we investigate the potential heating sources for this bulk of warm molecular gas. We conclude that both UV photons and X-rays likely contribute, and discuss implications for a top-heavy stellar initial mass function arising in the X-ray-irradiated starburst. Finally we present tentative identifications of other species in the spectrum, including a possible detection of the H20 2_0,2->1_1,1 transition at lambda_rest = 303 microns.