Abstract
We present 0.″3 (band 6) and 1.″5 (band 3) ALMA observations of the (sub)millimeter dust continuum emission for 25 radio galaxies at 1 < z < 5.2. Our survey reaches a rms flux density of ∼50 μJy in band 6 (200–250 GHz) and ∼20 μJy in band 3 (100–130 GHz). This is an order of magnitude deeper than single-dish 850 μm observations, and reaches fluxes where synchrotron and thermal dust emission are expected to be of the same order of magnitude. Combining our sensitive ALMA observations with low-resolution radio data from ATCA, higher resolution VLA data, and infrared photometry from Herschel and Spitzer, we have disentangled the synchrotron and thermal dust emission. We determine the star-formation rates and AGN infrared luminosities using our newly developed Multi-resolution and multi-object/origin spectral energy distribution fitting code (MR-MOOSE). We find that synchrotron emission contributes substantially at λ ∼ 1 mm. Through our sensitive flux limits and accounting for a contribution from synchrotron emission in the mm, we revise downward the median star-formation rate by a factor of seven compared to previous estimates based solely on Herschel and Spitzer data. The hosts of these radio-loud AGN appear predominantly below the main sequence of star-forming galaxies, indicating that the star formation in many of the host galaxies has been quenched. Future growth of the host galaxies without substantial black hole mass growth will be needed to bring these objects on the local relation between the supermassive black holes and their host galaxies. Given the mismatch in the timescales of any star formation that took place in the host galaxies and lifetime of the AGN, we hypothesize that a key role is played by star formation in depleting the gas before the action of the powerful radio jets quickly drives out the remaining gas. This positive feedback loop of efficient star formation rapidly consuming the gas coupled to the action of the radio jets in removing the residual gas is how massive galaxies are rapidly quenched.
Highlights
The connection between active galactic nuclei (AGN), their host galaxies and environments has been one of the central questions in extra-galactic astrophysics for over 30 years (Balick & Heckman 1982)
We model the MIR through radio spectral energy distributions with three components: a component representing dust heated by an AGN which we model as a power law with a slope, γ, and an exponential cut-off at νcut Eq (1); a component representing dust heated by the young stellar population of the host galaxy or companion which we model as a modified blackbody Eq (2); and a component representing synchrotron emission which we modeled as a simple power law with constant slope, α, with no cut-off at high frequencies Eq (3)
We have disentangled the IR luminosity into components heated by the stellar populations and that heated by the luminous AGN that reside in these host galaxies
Summary
The connection between active galactic nuclei (AGN), their host galaxies and environments has been one of the central questions in extra-galactic astrophysics for over 30 years (Balick & Heckman 1982). To gain a deeper understanding about how the growth of host galaxy and the SMBH are intertwined, it is important to study the characteristics of the star formation occurring in the host galaxies of actively fueled black holes Within this context, powerful radio galaxies generally, and high-redshift radio galaxies (HzRGs) in particular, are important test beds of our ideas on the physics underlying AGN feedback. Because each component potentially makes a significant contribution to the over all SED, it requires an analysis of photometry covering an order-of-magnitude range in spatial scales – sub arcsecond to 10s of arc seconds To this end, we developed the Multi-resolution and multiobject/origin spectral energy distribution fitting procedure MrMoose (Drouart & Falkendal 2018).
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