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| - Quasar induced galaxy formation: a new paradigm?
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| - Aims. We discuss observational evidence that quasars play a key role in the formation of galaxies, starting from the detailed study of the quasar HE0450-2958 and extending the discussion to a series of converging evidence that radio jets may trigger galaxy formation. Methods. We use mid infrared imaging with VISIR at the ESO-VLT to model the mid to far infrared energy distribution of the system and the stellar population of the companion galaxy using optical VLT-FORS spectroscopy. The results are combined with optical, CO, radio continuum imaging from ancillary data. Results. The direct detection with VISIR of the 7 kpc distant companion galaxy of HE0450-2958 allows us to spatially separate the sites of quasar and star formation activity in this composite system made of two ultra-luminous infrared galaxies (ULIRGs), where the quasar generates the bulk of the mid infrared light and the companion galaxy powered by star formation dominates in the far infrared. No host galaxy has yet been detected for this quasar, but the companion galaxy stellar mass would bring HE0450-2958 in the local $M_{\rm{BH}} - M_{\star}^{\rm bulge}$ relation if it were to merge with the QSO. This is bound to happen because of their close distance (7 kpc) and low relative velocity (~60-200 km s -1). We conclude that we may be witnessing the building of the $M_{\rm{BH}} - M_{\star}^{\rm bulge}$ relation, or at least of a major event in that process. The star formation rate (~340 $M_{odot}$ yr -1), age (40-200 Myr) and stellar mass (~[5-6] $\times$10 10 $M_{odot}$) are consistent with jet-induced formation of the companion galaxy. We suggest that HE0450-2958 may be fueled by fresh material from cold gas accretion from intergalactic filaments. We map the projected galaxy density surrounding the QSO as a potential tracer of intergalactic filaments and discuss a putative detection. Comparison to other systems suggest that an inside-out formation of quasar host galaxies and jet-induced galaxy formation may be a common process. Two tests are proposed for this new paradigm: (1) the detection of offset molecular gas or dust emission with respect to the position of distant QSOs; (2) the delayed formation of host galaxies as a result of QSO activity, hence the two step building of the $M_{\rm BH}/M_{\star}^{\rm bulge}$ ratio.
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