Optical Detection of Star Formation in a Cold Dust Cloud in the Counterjet Direction of Centaurus A

TL;DR

This study detects optical emission-line features in a cold dust cloud near Centaurus A, revealing star formation likely triggered or influenced by galactic outflows, with implications for understanding AGN feedback and star formation processes.

Contribution

It presents the first optical spectroscopic evidence of star formation in a cold dust cloud in Centaurus A, linking it to potential outflow interactions and AGN activity.

Findings

Optical emission lines indicate star formation in a cold dust cloud.

Line ratios suggest photoionization by OB stars, obscured along line of sight.

Limited evidence for shocks, implying weak gas compression from outflows.

Abstract

We identify optical emission-line features 700" (12 kpc) southwest of the nucleus of Centaurus A, roughly opposite the radio jet and well-known optical emission filaments associated with the northern radio structure. These regions are spatially associated with far-infrared emission peaks in a cold dust cloud identified using Herschel and Spitzer data, and there may be a mismatch between the low temperature of the dust and the expected heating effect of young stars. We use integral-field optical spectroscopy to trace the ratios of strong emission lines. Their ratios are consistent with photoionization in normal H II regions, by modest numbers of OB stars; they must be obscured along our line of sight. The location raises the question of whether this star-forming episode was enhanced or triggered by an outflow from the central parts of Centaurus A. Optical emission-line ratios and line widths limit the role of shocks on the gas, so any interaction with an outflow, either from the radio source or star formation in the gas-rich disk, can at most have compressed the gas weakly. We speculate that the presence of similar star-forming regions on both sides of the galaxy, contrasted with the difference in the character of the emission-line clouds, reflects the presence of a collimated radio jet to the northeast and perhaps anisotropic escape of ionizing radiation from the AGN. Star formation on the southwestern side of Cen A could be enhanced by a broad outflow, distinct from the radio jet and lobes. (abridged)