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