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キーワード:
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要旨:
We explore evidence for maintenance-mode feedback in non-starforming (quiescent) galaxies through the incidence of radio-loud AGN, both at low and high redshift (z 1). Drawing from SDSS, FIRST and NVSS we construct a sample of radio-loud AGN at z 0.1 and find that these prefer hosts with intrinsically round shapes even at a fixed stellar velocity dispersion (i.e., black-hole mass). Since the majority of quiescent galaxies are relatively flat (i,e, disk-like) this result raises important questions: is maintenance-mode feedback a generally valid explanation for quiescence; and, if so, how does that feedback avoid manifesting as a radio-loud AGN at least in disk-like quiescent galaxies? We extend this work to z 1 by constructing a sample of radio- loud AGN from the spectroscopic LEGA-C survey with VLT/VIMOS and the VLA-COSMOS 3 GHz Project. We find that radio-loud AGN preferentially reside in galaxies with large stellar velocity dispersions and round shapes even though the sample is too small to confirm a significant dependence on flattening. We also find that even at z 1 the host galaxies have old stellar populations, much older than the life times of radio AGN; these AGN are not responsible for quenching star formation, and the observations are consistent with radio-AGN `switching on' roughly every Gyr to provide the type of maintenance mode feedback that is implemented in all galaxy formation models to prevent excessive star formation in massive galaxies.
