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The e-MERGE Survey (e-MERLIN Galaxy Evolution Survey): overview and survey description Article (Accepted Version) Muxlow, T W B, Thomson, A P, Radcliffe, J F, Wrigley, N H, Beswick, R J, Smail, Ian, McHardy, I M, Garrington, S T, Ivison, R J, Jarvis, M J, Prandoni, I, Bondi, M, Guidetti, D, Argo, M K, Sargent, M T et al. (2020) The e-MERGE Survey (e-MERLIN Galaxy Evolution Survey): overview and survey description. Monthly Notices of the Royal Astronomical Society, 495 (1). pp. 1188- 1208. ISSN 0035-8711 This version is available from Sussex Research Online: http://sro.sussex.ac.uk/id/eprint/92394/ This document is made available in accordance with publisher policies and may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher’s version. Please see the URL above for details on accessing the published version. Copyright and reuse: Sussex Research Online is a digital repository of the research output of the University. Copyright and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners. To the extent reasonable and practicable, the material made available in SRO has been checked for eligibility before being made available. Copies of full text items generally can be reproduced, displayed or performed and given to third parties in any format or medium for personal research or study, educational, or not-for-profit purposes without prior permission or charge, provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. http://sro.sussex.ac.uk MNRAS 000, 1{23 (2020) Preprint 7 May 2020 Compiled using MNRAS LATEX style file v3.0 The e-MERLIN Galaxy Evolution Survey (e-MERGE): Overview and Survey Description T. W. B. Muxlow1, A. P. Thomson1;2?, J. F. Radcliffe1;3, N. H. Wrigley1, R. J. Beswick1, Ian Smail4, I. M. McHardy2, S. T. Garrington1, R. J. Ivison5;6, M. J. Jarvis7;8, I. Prandoni9, M. Bondi9, D. Guidetti9, M. K. Argo10, David Bacon11, P. N. Best6, A. D. Biggs5, S. C. Chapman12, K. Coppin13, H. Chen1;14;15, T. K. Garratt13, M. A. Garrett1;16, E. Ibar17, Jean-Paul Kneib18;19, Kirsten K. Knudsen20, L. V. E. Koopmans21, L. K. Morabito4, E. J. Murphy22, A. Njeri1, Chris Pearson23, M. A. P´erez-Torres24, A. M. S. Richards1, H. J. A. R¨ottgering16, M. T. Sargent25, Stephen Serjeant26, C. Simpson27, J. M. Simpson28, A. M. Swinbank4, E. Varenius20;1, T. Venturi9 Author affiliations shown in Appendix A Accepted 2020 May 5; Received 2020 May 5; in original form 2020 February 29 ABSTRACT We present an overview and description of the e-MERLIN Galaxy Evolution survey (e-MERGE) Data Release 1 (DR1), a large program of high-resolution 1.5 GHz radio observations of the GOODS-N field comprising ∼ 140 hours of observations with e- MERLIN and ∼ 40 hours with the Very Large Array (VLA). We combine the long baselines of e-MERLIN (providing high angular resolution) with the relatively closely- packed antennas of the VLA (providing excellent surface brightness sensitivity) to produce a deep 1.5 GHz radio survey with the sensitivity (∼ 1:5µJy beam−1), angular resolution (000: 2{000: 7) and field-of-view (∼ 150 × 150) to detect and spatially resolve star-forming galaxies and AGN at z & 1. The goal of e-MERGE is to provide new constraints on the deep, sub-arcsecond radio sky which will be surveyed by SKA1- mid. In this initial publication, we discuss our data analysis techniques, including steps taken to model in-beam source variability over a ∼ 20 year baseline and the development of new point spread function/primary beam models to seamlessly merge e-MERLIN and VLA data in the uv plane. We present early science results, including measurements of the luminosities and/or linear sizes of ∼ 500 galaxes selected at 1.5 GHz. In combination with deep Hubble Space Telescope observations, we measure a mean radio-to-optical size ratio of reMERGE/rHST ∼ 1:02±0:03, suggesting that in most high-redshift galaxies, the ∼GHz continuum emission traces the stellar light seen in optical imaging. This is the first in a series of papers which will explore the ∼kpc-scale arXiv:2005.02407v1 [astro-ph.GA] 5 May 2020 radio properties of star-forming galaxies and AGN in the GOODS-N field observed by e-MERGE DR1. Key words: Galaxies: evolution { Galaxies: high-redshift { Galaxies: radio continuum { Astronomical instrumentation, methods and techniques: interferometric 1 INTRODUCTION Historically, optical and near-infrared surveys have played a leading role in measuring the integrated star formation history of the Universe (e.g. Lilly et al. 1996; Madau et al. ? E-mail: [email protected] 1996), however in recent years a pan-chromatic (i.e. X-ray © 2020 The Authors 2 T. W. B. Muxlow et al. { radio) approach has become key to achieving a consensus galaxy population in general, and to obtain unequivocal ra- view on galaxy evolution (e.g. Scoville et al. 2007; Driver dio counterparts for close merging systems requires sensitive −1 et al. 2009). Since the pioneering work in the far-infrared (σrms ∼ 1 µJy beam ) radio imaging over representative ar- 0 0 (FIR) and sub-millimetre wavebands undertaken with the eas (& 10 × 10 ) with ∼kpc (i.e. sub-arcsecond) resolution. Submillimeter Common-User Bolometer Array (SCUBA) on The Karl G. Jansky Very Large Array (VLA) is currently the James Clerk Maxwell Telescope (JCMT), it has been es- capable of delivering this combination of observing goals in tablished that a significant fraction of the integrated cosmic S-band (3 GHz), X-band (10 GHz), and at higher frequen- star formation (up to ∼ 50% at z ∼ 1{3; Swinbank et al. 2014; cies. However by z ∼ 2 these observations probe rest-frame Barger et al. 2017) has taken place in heavily dust-obscured frequencies νrest & 10{30 GHz, a region of the radio spec- environments, which can be difficult (or impossible) to mea- trum in which the effects of spectral curvature may become sure fully with even the deepest optical/near-infrared data important due to the increasing thermal free-free compo- (e.g. Barger et al. 1998; Seymour et al. 2008; Hodge et al. nent at high-frequencies (e.g. Murphy et al. 2011), and/or 2013; Casey et al. 2014). Within this context, deep inter- spectral steepening due to cosmic ray effects (Galvin et al. ferometric radio continuum observations are an invaluable 2018; Thomson et al. 2019) and free-free absorption (Ti- complement to studies in other wavebands, providing a dust- sani´cet al. 2019). This potential for spectral curvature com- unbiased tracer of star formation (e.g. Condon 1992; Smolˇci´c plicates efforts to measure the rest-frame radio luminosities et al. 2009), allowing us to track the build-up of stellar pop- (conventionally, L1:4 GHz) of high redshift galaxies from these ulations through cosmic time without the need to rely on higher-frequency observations. uncertain extinction corrections. Moreover, radio continuum Furthermore, the instantaneous field of view (FoV) of observations also provide a direct probe of the synchrotron an interferometer is limited by the primary beam, θPB, which emission produced by active galactic nuclei (AGN), which scales as λ/D, with D being the representative antenna di- are believed to play a crucial role in the evolution of their ameter. At 1.4 GHz the FoV of the VLA's 25 m antennas is 0 host galaxies via feedback effects (Best et al. 2006; Schaye θPB ∼ 32 , while the angular resolution offered by its rela- 00 et al. 2015; Harrison et al. 2018). tively compact baselines (Bmax = 36:4 km) is θres ∼ 1: 5. This The radio spectra of galaxies at & 1 GHz frequencies corresponds to ∼ 12 kpc at z ∼ 2, and is therefore insufficient are typically thought to result from the sum of two power- to morphologically study the bulk of the high-redshift galaxy law components (e.g. Condon 1992; Murphy et al. 2011). population, which have optical sizes of only a few kpc (van At frequencies between νrest ∼ 1{10 GHz, radio observations der Wel et al. 2014). At 10 GHz, in contrast, the angular α 00 trace steep-spectrum (α ∼ −0:8, where Sν / ν ) synchrotron resolution of the VLA is θres ∼ 0: 2 (∼ 1:5 kpc at z = 2), but 0 emission, which can be produced either by supernova explo- the FoV shrinks to θPB ∼ 4: 5. This large (a factor ∼ 50×) sions (in which case it serves as a dust-unbiased indicator of reduction in the primary beam area greatly increases the the star-formation rate, SFR, over the past ∼ 10{100 Myr: cost of surveying deep fields over enough area to overcome Bressan et al. 2002) or from accretion processes associated cosmic variance (e.g Murphy et al. 2017), particularly given with the supermassive black holes (SMBHs) at the centres that the positive k-correction in the radio bands means that of AGN hosts. At higher frequencies (νrest & 10 GHz), radio these observations probe an intrinsically fainter region of observations trace flatter-spectrum (α ∼ −0:1) thermal free- the rest-frame radio SEDs of high-redshift galaxies to begin free emission, which signposts the scattering of free-electrons with. in ionised Hii regions around young, massive stars, and thus Over the coming decade the SKA1-mid and its pre- is considered to be an excellent tracer of the instantaneous cursor instruments (including MeerKAT and ASKAP) will SFR.
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