[Paper Review] Fueling the central engine of radio galaxies. III. Molecular gas and star formation efficiency of 3C 293

Á. Labiano, S. García‐Burillo+7 more|arXiv (Cornell University)|Feb 28, 2014

Galaxies: Formation, Evolution, Phenomena142 references16 citations

TL;DR

This study investigates molecular gas and star formation in the evolved radio galaxy 3C~293 using high-resolution CO(1-0) and CO(2-1) observations from the IRAM Plateau de Bure Interferometer. It finds no fast outflows in the cold molecular gas, a high star formation efficiency consistent with normal galaxies (SFE = 0.18 Gyr⁻¹), and suggests that low SFE values in evolved radio galaxies may stem from underestimated SFRs or overestimated H₂ densities, particularly when relying on 7.7 μm PAH emission tracers.

ABSTRACT

Aims. We investigate the different manifestations of AGN feedback in the evolved, powerful radio source 3C293 and their impact on the molecular gas of its host galaxy, which harbors young star-forming regions and fast outflows of HI and ionized gas. Methods. We study the distribution and kinematics of the molecular gas of 3C293 using high spatial resolution observations of the CO(1-0) and CO(2-1) lines, and the 3 and 1mm continuum taken with the IRAM PdBI. We mapped the molecular gas of 3C293 and compared it with the dust and star-formation images of the host. We searched for signatures of outflow motions in the CO kinematics, and reexamined the evidence of outflowing gas in the HI spectra. We also derived the star formation rate (SFR) and efficiency (SFE) of the host with all available SFR tracers from the literature, and compared them with the SFE of young and evolved radio galaxies and normal star-forming galaxies. Results. The CO(1-0) emission line shows that the molecular gas in 3C293 is distributed along a massive (2.2E10 Msun) warped disk with diameter of 21 kpc that rotates around the AGN. Our data show that the dust and the star formation are clearly associated with the CO disk. The CO(2-1) emission is located in the inner 7 kpc (diameter) region around the AGN, coincident with the inner part of the CO(1-0) disk. Both the CO(1-0) and CO(2-1) spectra reveal the presence of an absorber against the central regions of 3C293 that is associated with the disk. We do not detect any fast (>500 km/s) outflow motions in the cold molecular gas. The host of 3C293 shows an SFE consistent with the Kennicutt-Schmidt law. The apparently low SFE of evolved radio galaxies may be caused by an underestimation of the SFR and/or an overestimation of the molecular gas densities in these sources. We find no signatures of AGN feedback in the molecular gas of 3C293.

Motivation & Objective

• To investigate the role of AGN feedback in the molecular gas and star formation of the evolved radio galaxy 3C~293.
• To determine whether fast outflows in ionized and H I gas are mirrored in the cold molecular gas phase.
• To re-evaluate the star formation efficiency (SFE) of 3C~293 using multiple SFR tracers and compare it with normal and young radio galaxies.
• To assess whether the low SFEs reported for evolved radio galaxies are real or artifacts of SFR and H₂ mass estimation.

Proposed method

• High spatial resolution interferometric observations of CO(1-0) and CO(2-1) lines were conducted with the IRAM Plateau de Bure Interferometer to map molecular gas distribution and kinematics.
• The CO data were used to derive the systemic velocity (v_sys^CO = 13,434 ± 8 km s⁻¹), inclination, and position angle of the rotating disk.
• Spectral analysis identified CO absorption features toward the AGN and jet, consistent with H I absorption and disk kinematics.
• Star formation rate (SFR) was estimated using multiple tracers: [Ne II], 11.3 μm PAH, and 24/70 μm continuum, with AGN contamination assessed.
• The star formation efficiency (SFE) was calculated as SFR / M(H₂), comparing 3C~293 to the Kennicutt-Schmidt law and other radio galaxies.
• Fourier decomposition of the velocity field was applied to detect warp signatures in the CO disk.

Experimental results

Research questions

• RQ1 Is there evidence of fast (≥500 km s⁻¹) molecular outflows in the cold gas of 3C~293, as seen in H I and ionized gas?
• RQ2 What is the true star formation efficiency (SFE) of 3C~293, and how does it compare to normal star-forming galaxies and other radio galaxies?
• RQ3 Why do evolved radio galaxies typically show low SFEs, and is this due to systematic biases in SFR or H₂ mass estimates?
• RQ4 Are the CO absorption features in the central regions of 3C~293 associated with the rotating molecular disk?
• RQ5 Does the kinematic structure of the CO disk indicate a warped morphology?

Key findings

• The molecular gas in 3C~293 forms a massive, warped disk with a total H₂ mass of ~2.2 × 10¹⁰ M☉, extending over ~21 kpc (24 arcsec).
• The CO(1-0) and CO(2-1) emissions trace a rotating disk, with the CO(2-1) line confined to the inner 7 kpc, consistent with the central AGN region.
• No fast (≥500 km s⁻¹) outflowing motions were detected in the cold molecular gas, despite clear outflows in H I and ionized gas.
• The star formation rate (SFR) of 3C~293 is 4.0 ± 1.5 M☉ yr⁻¹, derived from reliable tracers such as [Ne II] and 11.3 μm PAH emission.
• The star formation efficiency (SFE) of 3C~293 is 0.18 Gyr⁻¹, consistent with the Kennicutt-Schmidt law for normal and young radio galaxies.
• The SFE of 3C~293 is 10–50 times higher than values derived from 7.7 μm PAH emission in other evolved radio galaxies, suggesting that low SFEs in such systems may be due to underestimated SFRs or overestimated H₂ masses.