Quasar feedback revealed by giant molecular outflows

Abstract: In the standard scenario for galaxy evolution young star-forming galaxies transform into red bulge-dominated spheroids, where star formation has been quenched. To explain such a transformation, a strong negative feedback generated by accretion onto a super-massive black hole is often invoked. The depletion of gas resulting from quasar-driven outflows should eventually stop star-formation in the host galaxy and lead the black hole to "suicide" for starvation. Direct observational evidence for a major quasar feedback onto the host galaxy is still missing, since outflows previously observed in quasars are generally associated with the ionized component of the gas, which only accounts for a minor fraction of the total gas content, and typically occurring in the central regions. We used the IRAM PdB Interferometer to observe CO(1-0) in Mrk 231, the closest QSO known. Thanks to the wide band we detect broad wings of the CO line, with velocities up to 750 km/s and spatially resolved on the kpc scale. Such broad CO wings trace a giant molecular outflow of about 700 MSun/yr, far larger than the ongoing star-formation rate (~200 MSun/yr) observed in the host galaxy. This wind will totally expel the cold gas reservoir in Mrk 231 in about 1e7 yrs, therefore halting the star-formation activity on the same timescale. The inferred kinetic energy in the molecular outflow is ~1.2e44 erg/s, corresponding to a few percent of the AGN bolometric luminosity, very close to the fraction expected by models ascribing quasar feedback to highly supersonic shocks generated by radiatively accelerated nuclear winds. The direct observational evidence for quasar feedback reported here provides solid support to the scenarios ascribing the observed properties of local massive galaxies to quasar-induced large scale winds.

• The paper provides compelling observational evidence of quasar-driven molecular outflows, with CO(1-0) wings reaching velocities up to 750 km/s.
• Using the IRAM Plateau de Bure Interferometer, the study measured an outflow rate of approximately 700 M⊙/year, far exceeding the star formation rate.
• The findings demonstrate that quasar feedback can expel cold gas in about 10 million years, significantly affecting the evolutionary paths of massive galaxies.

Insights into Quasar Feedback through Molecular Outflows

The paper presents a compelling empirical study of quasar feedback through the investigation of molecular outflows in Mrk 231, a nearby quasar and Ultra-Luminous Infrared Galaxy (ULIRG). This research provides critical evidence supporting the theory that quasars exert significant negative feedback on their host galaxies, influencing their evolutionary pathways.

Context and Methodology

In the context of galaxy evolution, it is postulated that star-forming galaxies transition into bulge-dominated systems where star formation is suppressed. One key driver of this transformation is the feedback mechanism associated with active galactic nuclei (AGN), particularly quasars. Such feedback processes are thought to deplete gas reserves, thereby halting star formation. However, direct observational evidence of widespread molecular outflows resulting from quasar activity has been scarce.

The authors employed the IRAM Plateau de Bure Interferometer to observe the CO(1-0) transition in Mrk 231, focusing on detecting and characterizing broad wings of the CO line. These observations revealed molecular outflows in a kpc-scale region, expanding our understanding of the dynamics in quasar-hosting galaxies.

Significant Findings

The study identified broad CO wings with velocities up to 750 km/s, indicative of a giant molecular outflow with a rate of about 700 M$_\odot$/year. This rate surpasses the concurrent star-formation rate in Mrk 231, estimated at approximately 200 M$_\odot$/year. The outflow's kinetic energy, estimated at 1.2×10$^{44}$ erg/s, aligns with predictions for quasar feedback involving supersonic shocks propelled by nuclear wind acceleration, substantiating the energy transport models within galactic environments.

The paper also contrasts the contribution of supernovae to the outflow's kinetic energy, which falls significantly short of accounting for the observed effects, underscoring the predominant influence of quasar activity.

Implications for Galactic Evolution

The findings from Mrk 231 hold substantial implications for our understanding of AGN feedback's role in galactic evolution. The molecular outflows observed suggest that quasars can indeed expel cold gas on relatively short timescales—on the order of 10$^7$ years—potentially leading to rapid quenching of star formation. This supports models that argue quasar feedback is a decisive factor in shaping the properties of massive galaxies, including their red color from older stellar populations and their relatively low abundance compared to predictions from models excluding AGN feedback.

Future Directions

Future research, with enhanced sensitivity and bandwidth from upcoming millimeter interferometers, will enable the exploration of quasar feedback across a more extensive sample of quasars, allowing for a broader understanding of its ubiquity and impact. Such studies will refine our theoretical models and may reveal new dynamics of quasar-host interaction.

In conclusion, the paper provides significant observational backing for theoretical models positing quasar feedback as a critical process in galaxy evolution, offering a more nuanced comprehension of how active galactic nuclei can regulate the life cycle of galaxies.