Molecular interstellar medium of distant quasar host galaxies

One of the best means for studying the molecular interstellar medium in distant galaxies is through the rotational transition of the carbon monoxide (CO) molecule. CO has emission lines that span the far-infrared to mm wavelength and is a good tracer of cold molecular gas (H2) in galaxies.

For my research, we have embarked a survey with the Atacama Large Millimeter/submm Array to study the molecular gas in distant quasar (1.3<z<2.6) host galaxies for which we have exquisite near-infrared IFS observations with Keck OSIRIS. Particularly, we started with sources in which we found powerful ionized outflows encompassing a large swath of the host galaxy. We don’t see evidence for recent star formation as traced with ionized Hα over these outflow regions. Hence we would like to understand the state of the molecular gas over these outflow regions. If the molecular gas is in a turbulent phase or heavily depleted, that would imply that the outflows are stirring and removing the molecular ISM directly affecting current and future star formation. On the other hand, if we detect molecular gas in the non-turbulent gravitationally bound state that would imply star formation may still be ongoing and merely is not traced by Hα as its heavily obscured.


Figure 1: Molecular gas distribution in the host galaxy of 3C 298. White contours represent the faintest Hα emission detected with Keck OSIRIS.

On the left, we have an example of ALMA cycle 3 observations of molecular gas in the host galaxy of 3C 298 traced with the CO molecule. Majority of the molecular gas is distributed in a disc centered on the quasar (labeled with a white star) and in a star-forming region 21 kpcs away. We detect a molecular outflow emanating from the disc. Below is a spectrum of the outflow as seen through the CO (5-4) transition, where it’s particularly bright. The line width near zero intensity is nearly 1200 km/s. This gas is moving above the escape velocity of the disc. Our work showcases direct evidence that quasar driven outflows are capable of regulating star formation by stirring and expelling the molecular reservoir necessary for star formation.

Figure 2: Spectrum of CO (5-4) emission from the molecular outflow in host galaxy of the radio-loud quasar 3C 298.