By Dario Fadda and Joan Schmelz

Paper: [C II] and CO Emission along the Bar and Counter-arms of NGC 7479
Fadda, Dario, et al., 2021/03, ApJ, 909, 204.

SOFIA observations of the barred spiral galaxy, NGC 7479, were able to separate shock-enhanced ionized carbon [CII] emission from that associated with star formation, reminding us that it is important to consider multiple mechanisms when studying the [CII] emission in galaxies with active nuclei.

Infrared observations reveal that 70% of spiral galaxies exhibit barred structures. Since this percentage declines at higher redshifts, the bar is likely to form late in the life of a galaxy. Merging with smaller galaxies plays a major role in the evolution of bars, which efficiently channel gas to the nuclei and fuels the growth of central black holes. These processes lead to the formation of Active Galactic Nuclei (AGN). As the nucleus start energizing the surrounding medium via winds, jets, and radiation, it triggers star formation and suppresses further inflow by blowing the gas out, a process generally known as AGN feedback.

In the case of NGC 7479, the asymmetric shape of the arms is probably due to an episode of merging revealed by remnants along the bar and the dust lanes across the bar. However, the most peculiar feature of this galaxy is the presence of the so-called “counter-arms,” which are clearly visible in the radio observations with a curvature opposite to the normal spiral structure seen in the optical image (see figure above).

The morphology of the counter-arms is probably linked to jets from the hidden AGN at the center of the bar. Matter flows along the jets and collides with dense clumps of gas. Momentum is transferred to the gas along the axis of the bar, gradually changing the direction of the jet as the component of velocity along the bar decreases. As the jet exits the bar and enters the less dense disk region, the direction of the jet will remain constant.

SOFIA observed the entire bar and counter-arm structure of NGC 7479 with the Far Infrared Field-Imaging Line Spectrometer (FIFI-LS) to study the distribution and intensity of the [CII] line at 157.7µm. This is the first complete [CII] observation of a galaxy bar ever done. The SOFIA study concludes that most of the [CII] emission comes from the molecular gas along the bar. However, the emission from the ends of the counter-arm structure is more complex.

Each lateral panel of the image (see figure above) shows many blue young stars, but the relative positions of these stars are different with respect to the tip of the radio emission. The northern end (right panel) shows a bubble-like structure of bright young stars, but the southern end (left panel) is only partially surrounded. This suggests that the southern end of the jet was able to escape the disk of the galaxy and expand into the lower-density molecular gas of the halo where it is much less likely to trigger new star formation.

In addition, infrared diagnostics show that the [CII] emission is compatible with star formation in the northern region but much more intense than expected from star formation in the southern region (see above plot). This excess [CII] emission probably originates in warm shocked molecular gas heated by the interaction of the radio jet forming the counter-arms with the interstellar medium in the galaxy.

These SOFIA observations are helping to interpret studies of distant galaxies where the [CII] line is redshifted into the radio band and generally appear as point sources when observed with ALMA. Without the structural context provided by nearby galaxies like NGC 7479, the [CII] line strength might be used as a simplistic proxy for the star formation rate, unaware of how other energy sources may be contributing to the [CII] emission.