The Radio Synchrotron Background: Conference Summary and Report
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PASP, 2018, 130, 985 A Preprint typeset using LTEX style emulateapj v. 12/16/11 THE RADIO SYNCHROTRON BACKGROUND: CONFERENCE SUMMARY AND REPORT J. Singal1, J. Haider1, M. Ajello2, D. R. Ballantyne3, E. Bunn1, J. Condon4, J. Dowell5, D. Fixsen6, N. Fornengo7, B. Harms8, G. Holder9, E. Jones1, K. Kellermann4, A. Kogut6, T. Linden10, R. Monsalve11,12,13, P. Mertsch14, E. Murphy4, E. Orlando15, M. Regis7, D. Scott16, T. Vernstrom17, L. Xu16 PASP, 2018, 130, 985 ABSTRACT We summarize the radio synchrotron background workshop that took place July 19–21, 2017 at the University of Richmond. This first scientific meeting dedicated to the topic was convened because current measurements of the diffuse radio monopole reveal a surface brightness that is several times higher than can be straightforwardly explained by known Galactic and extragalactic sources and processes, rendering it by far the least well understood photon background at present. It was the conclusion of a majority of the participants that the radio monopole level is at or near that reported by the ARCADE 2 experiment and inferred from several absolutely calibrated zero level lower frequency radio measurements, and unanimously agreed that the production of this level of surface brightness, if confirmed, represents a major outstanding question in astrophysics. The workshop reached a consensus on the next priorities for investigations of the radio synchrotron background. Subject headings: radio continuum: general; diffuse radiation; galaxies: halos; Galaxy: halo; ISM: magnetic fields; galaxies: luminosity function 1. INTRODUCTION tragalactic radio sources, which are the explanations for The radio synchrotron background is an astrophysi- the observed monopoles at infrared, optical/UV, X-ray, cal phenomenon that has been a subject of interest to and gamma-ray wavelengths. In addition, various pos- many in the community in recent years. The measured sible Galactic and extragalactic production mechanisms surface brightness level of the radio monopole is several are highly constrained. times that implied by the source counts of discrete ex- Although a bright high Galactic latitude diffuse ra- dio monopole had been reported as early as the 1960s (Costain 1960; Bridle 1967) and was seen in data from [email protected] the 1980s (Phillipps et al. 1981; Beuermann et al. 1985), 1 Department of Physics, University of Richmond 28 Westhampton Way, Richmond, VA 23173, USA early estimates of the high-latitude sky simply assumed 2 Department of Physics & Astronomy, Clemson University, that the observed intensity was some mixture of an extra- 118 Kinard Laboratory, Clemson, SC 29634-0978, USA 3 galactic background from radio point sources with the re- School of Physics, Georgia Institute of Technology, Atlanta, mainder allocated to a Galactic contribution, and neither GA 30332-0430, USA 4 National Radio Astronomy Observatory, 520 Edgemont of these was particularly well constrained at the time. Road, Charlottesville, VA 22903-2475, USA Renewed interest came with the publication of results 5 Department of Physics & Astronomy, University of New from the ARCADE 2 experiment (Fixsen et al. 2011; Mexico, 1919 Lomas Blvd. NE, Albuquerque, NM 87131-0001, Kogut et al. 2011a; Singal et al. 2011). Combining the USA 6 NASA Goddard Space Flight Center, 8800 Greenbelt Rd, ARCADE 2 balloon-based absolute spectrum data from Greenbelt, MD 20771, USA 3–90 GHz with absolutely calibrated zero level single- 7 Department of Theoretical Physics, University of Torino and dish degree-scale resolution radio surveys at lower fre- INFN, Via Giuria 1, 10125 Turin, Italy 8 quencies (e.g Haslam et al. 1982) re-confirmed a bright Department of Physics & Astronomy, University of Alabama, 514 University Blvd. Tuscaloosa, AL 35487-0324, USA radio synchrotron monopole level. This level is several arXiv:1711.09979v2 [astro-ph.HE] 5 Feb 2018 9 Department of Physics, University of Illinois at Urbana- times brighter than had widely been predicted, as esti- Champaign, 1110 West Green Street, Urbana, IL 61801-3003, mates of the Galactic monopole level and combined ex- USA 10 Department of Physics, The Ohio State University, 191 tagalactic point sources have become more firmly con- West Woodruff Avenue, Columbus, OH 43210, USA strained in recent decades (e.g. Vernstrom et al. 2011). 11 Center for Astrophysics and Space Astronomy, University of These results, although consistent between ARCADE 2 Colorado at Boulder, 389 UCB, Boulder, Colorado 80309-0389, in the low GHz range and lower frequency absolutely cal- USA 12 School of Earth and Space Exploration, Arizona State Uni- ibrated zero level radio surveys in the MHz range, are not versity, Tempe, AZ 85287, USA without controversy. ARCADE 2 was designed as an ab- 13 Facultad de Ingenier´ıa, Universidad Cat´olica de la Santsima solute measurement but had limited sky coverage, while Concepci´on, Alonso de Ribera 2850, Concepci´on, Chile the lower frequency radio surveys were not designed with 14 Theoretical Particle Physics and Cosmology, Niels Bohr In- stitute, Blegdamsvej 17, 2100 Copenhagen Ø, Denmark such an absolute zero-level calibration as a primary goal, 15 Department of Physics, Stanford University, 382 Via Pueblo and did not tend to stress the intricacies of their zero- Mall, Stanford, CA 94305-4060, USA 16 level calibration when reporting results. Still, the agree- Department of Physics & Astronomy, University of British ment between ARCADE 2 and absolutely calibrated zero Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada level MHz radio surveys necessitates investigation, be- 17 Dunlap Institute for Astronomy & Astrophysics, University cause such a result, if true, would potentially imply very of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada interesting consequences for our understanding of Galac- 2 Singal et al. diffuse radio emission at different frequencies does not support such a large halo (Fornengo et al. 2014). Lastly, although galaxies with large radio emitting halos do ex- ist (e.g. Weigert et al. 2015), a halo of the necessary size and emissivity would make our Galaxy anomalous among nearby similar spiral galaxies (Singal et al. 2015a). However an extragalactic origin for the radio syn- chrotron background also presents many challenges. Sev- eral authors have considered deep radio source counts Fig. 1.— Spectrum of the sky monopole in radiometric temper- ature units, reproduced from Seiffert et al. (2011), as measured by (Vernstrom et al. 2011, 2014; Condon et al. 2012) and several different instruments or surveys. The spectrum shows a concluded that the background would require an incred- clear power-law rise at frequencies below ∼10 GHz, above the oth- ibly numerous new population of radio sources far below erwise dominant cosmic microwave background (CMB) level. Di- the flux densities currently probed by deep interferomet- amonds are measurements from ARCADE 2 (Fixsen et al. 2011), the square point is the measurement from Reich & Reich (1986), ric surveys. These results are in agreement with others the triangle point is the measurement of Haslam et al. (1982), the who have probed whether active galactic nuclei (AGN — star point is the measurement of Maeda et al. (1999), and the plus Draper et al. 2011) or other objects (Singal et al. 2010) point is the measurement of Roger et al. (1999). Higher frequency are numerous enough. Some authors have noted that the measurements from COBE-FIRAS (Fixsen & Mather 2002) are represented as the thick solid line. far-infrared background would be overproduced above observed levels if the radio synchrotron background were tic and/or extragalactic radio emission. produced by sources that follow the known correla- The radio synchrotron monopole level as reported by tion between radio and far-infrared emission in galax- ARCADE 2 and lower frequency surveys is spatially uni- ies (Ponente et al. 2011; Ysard & Lagache 2012), while form enough to be considered a “background,” thus it others have observed that the correlation may evolve would join the astrophysical backgrounds known in all with redshift and have noted the implications for the other regions of the electromagnetic spectrum. As there radio background (Ivison et al. 2010a,b; Magnelli et al. are many reasons to believe the diffuse radio monopole is 2015). Other authors have investigated the anisotropy extragalactic and in fact cosmological, many have taken of the radio synchrotron background and shown that ar- to referring to it as the “cosmic radio background.” As cminute scale fluctuations are sufficiently small that if we strive for neutrality on this question this work will the sources trace the large scale structure of the Uni- refer to it as the radio synchrotron background. The re- verse the background must be produced by objects at ported radio synchrotron monopole spectrum as a func- redshifts greater than z ∼5 (Holder 2014), and some of tion of frequency in radiometric temperature units is the simplest high redshift mechanisms have been ruled shown in Figure 1, while a comparison with the photon out (Cline & Vincent 2014). backgrounds in other wavebands is shown in spectral en- Such constraints have led authors to consider ergy surface brightness units in Figure 2. origins such as annihilating dark matter in ha- If it is indeed at the level determined from ARCADE 2 los or filaments (Fornengo et al. 2014; Hooper et al. and lower frequency absolutely calibrated zero level radio 2012; Fang & Linden 2015) or ultracompact halos surveys as reported by Fixsen et al. (2011), the origin of (Yang et al. 2013), “dark” stars in the early uni- the radio synchrotron background would be one of the verse (Spolyar et al. 2009), supernovae of massive mysteries of contemporary astrophysics. It is difficult population III stars (Biermann et al. 2014), emission to produce the observed level of surface brightness by from Alf´en re-acceleration in merging galaxy clusters known processes without violating existing constraints. (Fang & Linden 2016), an enhancement in the local A brief review of some recent literature on the subject bubble (Sun et al.