Radio Galaxy Zoo Talk

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

  • JeanTate by JeanTate

    Keel+ (2018):

    We have identified a set of optical emission-line features 700′′ (12 kpc) to the southwest of the nucleus of Centaurus A, roughly opposite to the radio jet and well-known optical emission filaments associated with the northern radio structure (along the axis of the southwestern radio lobes, although there is no coherent counterjet at this radius). We use integral-field optical spectroscopy to trace the ratios of strong emission lines, showing changes in excitation across the region, and significant local reddening. The emission regions are spatially associated with far-infrared emission peaks in one of two cold dust clouds identified using Herschel and Spitzer data, and there may be a mismatch between the low temperature of the du
    st and the expected heating effect of young stars. The strong emission lines have ratios consistent with photoionization in
    normal H II regions, requiring only modest numbers of OB stars; these stars and their cooler accompanying populations must be obscured along our line of sight. These data fit with a picture of fairly ordinary formation of clusters in a large giant molecular cloud, or network of such clouds. The location, projected near the radio-source axis and within the radius where a starburst wind has been inferred on the other side of the galaxy, raises the question of whether this star-forming
    episode was enhanced or indeed triggered by an outflow from the central parts of Centaurus A. However, optical emission-line ratios and line widths limit the role of shocks on the gas, so any interaction with an outflow, associated either with the radio source or star formation in the gas-rich disk of Centaurus A, 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 as well. In this view, the star formation on the southwestern side of Cen A could be enhanced by a broad outflow (whether originated by a starburst or AGN), distinct from the radio jet and lobes.

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