STAR FORMATION IN 3CR RADIO GALAXIES AND QUASARS AT z < 1*

Abstract

Using the Herschel Space Observatory we have observed a representative sample of 87 powerful 3CR sources at redshift z < 1. The far-infrared (FIR, 70-500 µm) photometry is combined with mid-infrared (MIR) photometry from the Wide-Field Infrared Survey Explorer (WISE) and catalogued data to analyse the complete spectral energy distributions (SEDs) of each object from optical to radio wavelength. To disentangle the contributions of different components, the SEDs are fitted with a set of templates to derive the luminosities of host galaxy starlight, dust torus emission powered by active galactic nuclei (AGN) and cool dust heated by stars. The level of emission from relativistic jets is also estimated, in order to isolate the thermal host galaxy contribution. The new data are in line with the orientation-based unification of highexcitation radio-loud AGN, in that the dust torus becomes optically thin longwards of 30 µm. The low excitation radio galaxies and the MIR weak sources represent MIR-and FIR-faint AGN population different from the high-excitation MIR-bright objects; it remains an open question whether they are at a later evolutionary state or an intrinsically different population. The derived luminosities for host starlight and dust heated by star formation are converted to stellar masses and star formation rates (SFR). The host-normalized SFR of the bulk of the 3CR sources is low when compared to other galaxy populations at the same epoch. Estimates of the dust mass yield a 1-100 times lower dust/stellar mass ratio than for the Milky Way, indicating that these 3CR hosts have very low levels of interstellar matter explaining the low level of star formation. Less than 10% of the 3CR sources show levels of star formation above those of the main sequence of star forming galaxies.

AGN feedback in the FR II galaxy 3C 220.1

“…The SFR of the BCG is then estimated to be about 60 -80 M /yr from the calibrations of Lee et al (2013) and Rieke et al (2009), assuming a Kroupa IMF. This is consistent with the estimate from Westhues et al (2016) and comparable to the SFR of another high redshift cluster IRAS 09104+4109 (z = 0.44) also with a luminous central AGN (O'Sullivan et al 2012). Substantial SF in the BCG of 3C 220.1 is also implied by its distorted morphology as seen with HST (Fig.…”

“…The cluster hosts a large X-ray cool core, with a central cooling time less than 1 Gyr (Worrall et al 2001). The BCG is still actively forming stars, with HST data suggesting a star formation rate of ∼ 21 − 67 M yr −1 (Westhues et al 2016). We sum-marize the cluster properties of 3C 220.1 in Table 1.…”

Hubble Space Telescope Emission-line Images of Nearby 3CR Radio Galaxies: Two Photoionization, Accretion, and Feedback Modes

“…Once the radiation field is produced either by the jet or the accretion disk, the ISM in the host galaxies reprocesses the ionizing light into emission lines depending on their available cold gas amount. The dust and cold gas present in a large amount in HEGs are visible in the optical band and detected either in the IR band or in molecular lines (10 7 -10 8 M ⊙ , Smolčić & Riechers 2011;Tadhunter et al 2014;Westhues et al 2016). These are usually tracers of the location of the emission-line gas (Dicken et al 2009).…”

“…These sources are all classified as HEGs. Indeed, this ionization mechanism may contribute more for the HEGs which show evidence of ongoing star formation rather than more passive host galaxies of LEGs (e.g., Herbert et al 2010;Janssen et al 2012;Hardcastle et al 2013;Gürkan et al 2015;Westhues et al 2016). Post-AGB stars which also emit in UV can also play a minor role in the photoionization.…”

“…On one hand, HEGs, which are commonly interpreted as quasar-like AGN, have strong radiation field since they are powered by a standard accretion disk (Hardcastle et al 2009). Optical and infrared studies indicate that HEGs show a large amount of cold gas Dicken et al 2009;Tadhunter et al 2014;Westhues et al 2016), which channels towards the center and fuels the BH, establishing a cold accretion mode (Hardcastle et al 2007;Best & Heckman 2012;Heckman & Best 2014). On the other hand, LEGs are fuelled by the hot phase of ISM and show evidence for the presence of a radiatively inefficient accretion disk (Balmaverde et al 2008;Hardcastle et al 2007).…”