MNRAS 000,1–22 (2017) Preprint 25 October 2018 Compiled using MNRAS LATEX style file v3.0 Gravitational lensing reveals extreme dust-obscured star formation in quasar host galaxies H. R. Stacey,1;2? J. P. McKean,1;2 N. C. Robertson,3 R. J. Ivison,4;5 K. G. Isaak,6 D. R. G. Schleicher,7 P. P. van der Werf,8 Willem Baan,1 A. Berciano Alba,1;8 M. A. Garrett9, E. Loenen8 1Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands 2Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands 3Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK 4Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK 5European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei München, Germany 6Science Support Office, ESTEC/SCI-S, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands 7Departamento de Astronomía, Facultad Ciencias Físicas y Matemàticas, Universidad de Concepción, Av. Esteban Iturra s/n Barrio Universitario, Casilla 160-C Concepción, Chile 8Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, The Netherlands 9Jodrell Bank Centre for Astrophysics, University of Manchester, Manchester M13 9PL, UK Accepted 2017. Received 2017 ; in original form 2017 ABSTRACT We have derived dust temperatures, dust masses, star formation rates and far-infrared lumi- nosities for 104 gravitationally-lensed quasars at z∼1–4 observed with Herschel/SPIRE, the largest such sample ever studied. By targeting gravitational lenses we probe intrinsic luminosi- ties more typical of the population than the extremely luminous sources otherwise accessible. We detect 87 (84 percent) of the sample with SPIRE: 82 (79 percent) quasars have spectra characteristic of dust emission, and we find evidence for dust-obscured star formation in at +840 −1 least 72 (69 percent). We find a median magnification-corrected SFR of 220−130 M yr and +25:5 11 LFIR of 6:7−4:1 × 10 L . The results are in line with current models of quasar evolution, but suggest that most quasars exist in a transitional phase between a dusty star-forming galaxy and an AGN dominated system. This further indicates that AGN feedback does not quickly quench star formation in these sources. Additionally, we find no significant difference in dust luminosities between radio-loud and radio-quiet quasars, implying that radio mode feedback has no significant effect on host galaxy properties. Key words: gravitational lensing – quasars: general – galaxies: evolution – galaxies: star formation – submillimeter: galaxies – infrared: galaxies 1 INTRODUCTION 2006) and various observational studies (Page et al. 2012; Stevens 2005; Coppin et al. 2008) support a unified model, initially pro- Key to the study of galaxy formation and evolution is understanding posed by Sanders et al.(1988) and more recently by Hopkins et al. the physical processes that drive the growth of active galactic nuclei arXiv:1705.10530v1 [astro-ph.GA] 30 May 2017 (2008), in which quasars are formed as a result of gas-rich major (AGN) and star formation. The symbiosis of these phenomena is mergers. According to this scenario, luminous dusty star-forming thought to be driven by radio-mode feedback from the AGN (Bick- galaxies (DSFGs) are merger-driven starbursts that represent a tran- nell et al. 2000; Klamer et al. 2004), which may quench or induce sition phase into dust-obscured quasars. Over time, feedback effects star-formation in the host galaxy through interactions with the in- strip the dusty quasars of gas and dust, and they evolve into ultra- terstellar medium, although this process is currently not well under- violet (UV) luminous quasars, and later become passive spheroidal stood. Hydrodynamical simulations of galaxy formation (Di Mat- galaxies. teo, Springel & Hernquist 2005; Hopkins et al. 2005; Bower et al. Quasars that are luminous in the far-infrared (FIR) to mm regime are predicted to be in the dust-obscured transition phase of ? [email protected] their evolution, where most of the emission is assumed to originate c 2017 The Authors 2 H. R. Stacey et al. from star formation. Therefore, studying the properties of these and less biased towards high intrinsic luminosities1. In combina- sources can provide important information about the evolutionary tion, these methodologies allow for a more complete view of the process, particularly when compared to the large population of ex- quasar population to be constructed. treme starburst galaxies that were discovered with the Submillime- In this paper, we have targeted a sample of strong gravitation- tre Common-User Bolometer Array (SCUBA), Herschel Space Ob- ally lensed quasars with the Herschel Space Observatory (Pilbratt servatory and now the Atacama Large Millimetre/sub-millimetre et al. 2010) and derive their dust temperatures, intrinsic FIR lumi- Array (ALMA). nosities, dust-obscured SFRs and dust masses. Previous work in Studies of the brightest FIR-luminous quasars, such as those this area has been undertaken by Barvainis & Ivison(2002), who in the SCUBA Bright Quasar Survey (Isaak et al. 2002; Priddey et detected 23 of 40 gravitationally-lensed quasars in their sample at al. 2003) and MAMBO/IRAM-30 m Survey (Omont et al. 2001, 850 µm with SCUBA. They found dust emission broadly compa- 2003), have found that these quasars are embedded within gas- and rable to radio galaxies, in line with the AGN unification model, dust-rich starbursting galaxies, with star formation rates of ∼1000 and no statistically significant difference between radio-quiet and −1 M yr , which are comparable to DSFGs. The low spatial den- radio-loud sources, as would be expected if they have the same host sity of FIR-luminous quasars, relative to DSFGs and UV-luminous galaxy properties. We have observed 104 quasars, including 37 of quasars, argues for a quick transition from starbursting DSFG to an the Barvainis & Ivison sample, detecting 78 sources in at least one AGN-dominated quasar, with the IR-luminous quasar phase being band with the Herschel/SPIRE, but as our data cover a lower wave- less than 100 Myr, and perhaps as short as ∼1 Myr (Simpson et al. length range, we are also able to determine the dust temperatures 2012). for the first time and infer whether the heated dust is due to star- Spectral line observations of CO reveal different physical formation or AGN activity. properties for the molecular gas distribution in DSFGs and UV- In Section2, we present our sample selection, the relevant luminous quasars. Based on observations of CO (1-0), DSFGs are properties of the quasars in our sample, the parameters of the ob- found to have reservoirs of cold gas that is extended over 5-25 kpc, servations, and our data reduction methods. In Section3, we report which fuels the starburst activity (Ivison et al. 2011), whereas IR- the results of the photometric measurements and the analysis of the luminous quasars consist of a compact component, < 2 kpc in size radio-to-FIR spectral energy distributions (SEDs) of the sources. In (Riechers et al. 2006). Observations of higher level transitions of Section4, we show that the SEDs are consistent with dust heating CO reveal the spectral line energy distributions (SLEDs) of quasars due to star formation in the quasar host galaxies, and we compare peak at high excitations, J = 5 to 9, corresponding to warmer gas our results with a sample of DSFGs at similar redshifts. Here, we temperatures (Weiß et al. 2007, APM 08279+5255). These sources also consider the contribution to the total radio emission from star- are typically characterized by a colder (60-80 K) gas component, formation processes for these AGN by considering the FIR–radio consistent with excitation by star formation, and a warmer (100- correlation. Finally, in Section5 we present a summary of our re- 200 K) component that is thought to be close to the AGN (Ao et al. sults and discuss the future work that we will carry out with this 2008; Scott et al. 2011). sample. While there are significant previous studies revealing high lev- Throughout, we assume the Planck 2015 instance of a flat −1 −1 els of star formation in quasar host galaxies, these studies have in- ΛCDM cosmology with H0 = 67:8 km s Mpc , ΩM = 0:31 and evitably focused on significantly bright sources due to limitations in ΩΛ = 0:69. sensitivity or source confusion. For most of the quasar population, even basic properties like the amount of dust or the star-formation rate (SFR) are therefore not well known. So, it is not clear whether 2 SAMPLE AND OBSERVATIONS these brighter sources represent the high end of star formation in the population or if they are representative of the quasar popula- In this section, we describe our sample of gravitationally lensed tion as a whole. Thus, the next step requires an investigation of quasars and present the observations that were carried out using lower surface-brightness sources. While some recent progress has the Herschel Space Observatory. been made with the improved sensitivity and resolution of ALMA (Harrison et al. 2016; Banerji et al. 2016), resolutions of 100-pc are required to spatially resolve regions of star formation and AGN- 2.1 Sample heating which are still difficult to attain in for the high-redshift Uni- Our sample is drawn from all of the gravitationally lensed quasars verse. Many of these issues can be mitigated by studying quasars that were observed with the Herschel Spectral and Photometric that have been magnified by a gravitational lens. Imaging Receiver (SPIRE) instrument (Griffin et al. 2010), the vast The advantages of observing strong gravitationally-lensed majority of which came from our own open time project (Pro- quasars are three-fold.