[Paper Review] Radio jets and outflows of cold gas

R. Morganti|arXiv (Cornell University)|Dec 21, 2011

Galaxies: Formation, Evolution, Phenomena1 references3 citations

TL;DR

This paper investigates the role of radio jets in driving multiphase outflows of cold gas—both atomic (HI) and molecular (CO)—in AGN-host galaxies. Using high-sensitivity radio observations, it finds that cold gas outflows, particularly in the molecular phase, have high mass outflow rates (up to 600 M☉ yr⁻¹ in Mrk231), suggesting AGN jets are key drivers. These outflows likely represent a major feedback mechanism, with HI possibly acting as an intermediate cooling phase before forming molecular gas.

ABSTRACT

Massive gas outflows are considered a key component in the process of galaxy formation and evolution. It is, therefore, not surprising that a lot of effort is going in quantifying their impact via detailed observations. This short contribution presents recent results obtained from HI and CO observations of different objects where the AGN - and in particular the radio jet - is likely playing an important role in producing the gas outflows. These preliminary results are reinforcing the conclusion that these outflows have a complex and multiphase structure where cold gas in different phases (atomic and molecular) is involved and likely represent a major component. These results will also provide important constraints for establishing how the interaction between AGN/radio jet and the surrounding ISM occurs and how efficiently the gas should cool to produce the observed properties of the outflowing gas. HI likely represents an intermediate phase in this process, while the molecular gas would be the final stage. Whether the estimated outflow masses match what expected from simulations of galaxy formation, it is still far from clear.

Motivation & Objective

• To investigate the role of radio jets in driving outflows of cold gas in AGN-host galaxies.
• To quantify the mass outflow rates of atomic (HI) and molecular (CO) gas in radio-loud AGN.
• To determine whether cold gas outflows are consistent with feedback models in galaxy formation simulations.
• To explore the kinematic and structural complexity of multiphase outflows in different AGN types.
• To assess whether HI acts as an intermediate phase in the cooling and conversion of atomic to molecular gas during outflows.

Proposed method

• Conducted deep HI 21 cm absorption line observations using single-dish and interferometric radio telescopes to detect blueshifted velocity components indicative of outflows.
• Performed CO(1-0), CO(2-1), and higher-order line observations with APEX and other submillimeter facilities to trace molecular gas kinematics.
• Analyzed spectral profiles for asymmetric features, such as blueshifted wings, to identify outflowing gas components.
• Combined multi-wavelength data (radio, FIR, mid-IR) to correlate outflow signatures with radio jet structures and hotspots.
• Used mass outflow rate calculations based on line flux, velocity dispersion, and covering factor estimates to quantify feedback impact.
• Compared observed outflow properties with theoretical models of AGN feedback and cooling processes in outflows.

Experimental results

Research questions

• RQ1 What is the contribution of atomic (HI) and molecular (CO) gas to the total mass outflow rate in radio-loud AGN?
• RQ2 How do the kinematics of HI and CO outflows correlate with the location of radio jets and hotspots?
• RQ3 Is the observed cold gas outflow consistent with AGN-driven feedback models, particularly in terms of energy and mass budget?
• RQ4 Does HI act as an intermediate phase in the cooling and conversion of atomic gas into molecular gas during outflows?
• RQ5 Why are such massive cold outflows rare in early-type galaxies despite their high outflow rates?

Key findings

• In Mrk231, a blueshifted wing in the HI absorption profile was detected, indicating the presence of outflowing atomic gas associated with a previously known molecular outflow.
• The molecular gas outflow in Mrk231 has a mass outflow rate ranging from >600 M☉ yr⁻¹ (under standard assumptions) to below 100 M☉ yr⁻¹ (if optically thin correction is applied), suggesting a potentially dominant AGN origin.
• The HI mass outflow rate in Mrk231 is modest (~10 M☉ yr⁻¹), significantly lower than the molecular component, indicating molecular gas dominates the mass outflow.
• In IC 5063, a blueshifted wing in the CO(2-1) profile confirms a molecular outflow, with a total mass of ~10⁷ M☉, comparable to the H I outflow mass (~3×10⁶ M☉), both exceeding warm ionized gas outflows.
• In 3C 305, enhanced CO velocity dispersion and a weak blue wing at the radio hot spot location suggest jet-ISM interaction drives molecular outflows.
• In NGC 1266, a massive molecular outflow of ~2.4×10⁷ M☉ with a mass outflow rate of ~13 M☉ yr⁻¹ was found, implying a short depletion timescale (~85 Myr), consistent with AGN-driven origin despite low radio luminosity.