This paper focuses on the population of high-redshift radio galaxies (HzRGs; L500 MHz > 1026 W Hz−1Miley & De Breuck 2008), which are some of the most massive galaxies known at any redshift (with a narrow range in stellar masses of (1 − 6)×1011 M⊙ for 1  z  5.2; De Breuck et al. 2010). Their energetic radio jets are unique markers of concomitant powerful AGN activity, which place them amongst the most active sources at and near cosmic noon. High-redshift radio galaxies have furthermore been shown to be powerful beacons of dense (proto-)cluster environments in the early Universe (e.g., Le Fevre et al. 1996; Stern et al. 2003; Venemans et al. 2002, 2003, 2004, 2005, 2007; Wylezalek et al. 2013). The quasar-level AGN activity (Miley & De Breuck 2008) at the center is blocked by the thick dusty torus acting as the “coronograph” (Vernet et al. 2001); this makes HzRGs true obscured type-2 quasars, allowing us to probe their host galaxies and CGM without strong AGN contamination (e.g., for unobscured quasars, see Arrigoni Battaia et al. 2019, and for radio-quite type-2 sources, see Cai et al. 2017). Comprehensive studies using near-infrared integral field unit (IFU) instruments show that the ionized gas in HzRGs is highly perturbed (FWHM ∼ 1000 km s−1) at kiloparsec scales and is aligned with the radio jets (Nesvadba et al. 2006, 2007, 2008, 2017a,b; Collet et al. 2015, 2016). This implies that the energy and momentum transfer between the central quasar and their ISM is likely due to the jets. Radio-mode feedback may therefore play a fundamental role during the evolution of HzRGs. Recently, Falkendal et al. (2019) combined infrared and millimeter data and deduced a more robust result of a relatively low star formation rate (SFR) for a sample of HzRGs, suggesting evidence of rapid quenching compared to previous studies (e.g., Drouart et al. 2014). Using a small sample of HzRGs, Nesvadba et al. (2011) shows that they are going through a transition phase from active to passive. These observations indicate that HzRGs are on a different track of evolution compared to radio-quiet objects, assembling most of their stellar mass early (z ∼ 3; Seymour et al. 2007; De Breuck et al. 2010), and that radio jets may actively affect their quenching. However, there is also circumstantial evidence showing that the jet can induce star formation. Humphrey et al. (2006) found that HzRGs (z > 2 in the sample) with smaller radio sources and more perturbed gas (emission line) kinematics show lower UV continuum polarization, which could be due to the presence of more luminous young stellar populations and can possibly be explained by the interaction between radio jets and the ISM that enhances star formation. Besides, there is also an anticorrelation between the rest frame submillimeter flux density and radio size in HzRGs (Humphrey et al. 2011), although it is not clear if the physics behind this is feedback-induced star formation, a simultaneous triggering of star formation and the radio-loud AGN activity, or simply environmental effects. Some well-studied HzRGs show evidence of having high SFRs (e.g., 4C41.47 and PKS 0529−549; Nesvadba et al. 2020; Falkendal et al. 2019). In these sources, we may interestingly be witnessing both the jets compressing the gas, leading to enhanced SFRs (e.g., Fragile et al. 2017), and the feedback from the AGN and star formation quenching it (Man et al. 2019).
Citation: Cai et al. 2017
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