A new approach for a Galactic synchrotron polarized emission template in the microwave range

G. Bernardi 1 2 E. Carretti 2 R. Fabbri 0 C. Sbarra 2 S. Poppi 3 S. Cortiglioni 2 0 Dipartimento di Fisica, Universita` di Firenze , Via Sansone 1, I-50019 Sesto Fiorentino (FI) , Italy 1 Dipartimento di Astronomia, Universita` degli Studi di Bologna , Via Ranzani 1, I-40127 Bologna , Italy 2 CNR/IASF Bologna , Via Gobetti 101, I-40129 Bologna , Italy 3 CNR/IRA Bologna , Via Gobetti 101, I-40129 Bologna , Italy A B S T R A C T We present a new approach to modelling the polarized Galactic synchrotron emission in the microwave range (20-100 GHz), where this radiation is expected to play the leading role in contaminating the cosmic microwave background data. Our method is based on real surveys and aims at providing the real spatial distributions of both polarized intensity and polarization angles. Its main features are the modelling of a polarization horizon to determine the polarized intensity, and the use of starlight optical data to model the polarization angle pattern. Our results are consistent with several existing data, and our template is virtually free from Faraday rotation effects as required at frequencies in the cosmological window. 1 I N T R O D U C T I O N The polarized component of the diffuse background emission in the microwave range is of great interest for both Galactic structure and the cosmic microwave background (CMB). Actually, its measurement leads to probing of the structure of the interstellar medium (ISM) and the Galactic magnetic field. Moreover, the detection of CMB polarization (CMBP) allows investigation of the early Universe. CMB anisotropies and polarization are powerful tools with which to determine cosmological parameters (Sazhin & Benitez 1995; Zaldarriaga, Spergel & Seljak 1997; Kamionkowski & Kosowky 1998). However, although anisotropies have already been detected and space missions (MAP,1 Planck2) are expected to make allsky surveys down to 0.1 angular resolution, CMBP still represents a challenge for astronomers. The first detection has been just claimed by DASI (Kovac et al. 2002), and several experiments will address it soon [SPOrt3 (see Carretti et al. 2002; Cortiglioni et al. 2002), MAP, Planck, B2K2 (Masi et al. 2002), BaRSPOrt (Zannoni et al. 2002) and AMiBA (Kesteven 2002), among others]. Besides the CMBP low emission level (34 K on sub-degree scales and <1 K on large scales), difficulties in its detection are E-mail: 1 http://map.gsfc.nasa.gov/ 2 http://astro.estec.esa.nl/SA-general/Projects/Planck/ 3 http://sp0rt.bo.iasf.cnr.it:8080/ mainly related to the presence of foreground noise from Galactic and extragalactic sources. Extragalactic foregrounds essentially consist of radio and infrared discrete sources, whereas Galactic foregrounds are generated by synchrotron, freefree, thermal dust and spinning/magnetic dust emissions. Synchrotron polarized emission should represent the most relevant foreground in the microwave range: freefree is fainter [<4 K at 30 GHz in total intensity: see Reynolds & Haffner (2000)] and almost unpolarized, whereas thermal dust has a polarization degree much smaller than that of synchrotron (Prunet et al. 1998; Tegmark et al. 2000). Evidence for spinning or magnetic dust emission has been found (Kogut et al. 1996; de Oliveira-Costa et al. 1999), but it seems to play an important role only up to 50 GHz. Moreover, it should have a low polarization degree (Lazarian & Prunet 2002). In spite of its importance, synchrotron emission is scarcely surveyed: existing data mainly cover the Galactic plane area at frequencies up to 2.7 GHz, far away from the cosmological window (Duncan et al. 1997, hereafter D97; Duncan et al. 1999, hereafter D99; Uyaniker et al. 1999; Gaensler et al. 2001; Landecker, Uyaniker & Kothes 2002). The Leiden data (Brouw & Spoelstra 1976, hereafter BS76) cover high Galactic latitudes, but are limited to <1.4 GHz and are largely undersampled. This situation makes having a reliable synchrotron polarized emission template in the 20100 GHz range very important. This would allow, for instance, reliable numerical simulations to set up and test destriping techniques or foreground separation methods (Revenu et al. 2000; Tegmark et al. 2000; Sbarra et al. 2003, and references therein). At present, only toy models exist, which do not account for the real spatial distribution of both polarization intensity and polarization angles (Kogut & Hinshaw 2000; Giardino et al. 2002). In this paper we present a new approach to modelling the Galactic diffuse synchrotron polarized emission in the 20100 GHz range. It is based on real surveys and fitted to the real spatial distribution of both polarized intensity and polarization angles. Low-frequency data are used to model the polarized intensity, and optical starlight is used to model polarization angles. This allows the construction of Q and U maps covering about half of the sky with the angular resolution of the SPOrt experiment (FWHM = 7). Although our work is aimed at the SPOrt experiment, the method is general enough to be suitable also for smaller angular scales as soon as complete sets of data with sub-degree angular resolution become available. The great advantage of this new approach is to produce Q and U maps free from Faraday rotation, allowing a direct extrapolation to the cosmological window. The outline of the paper is as follows: the synchrotron polarized emission model and the procedure for building the template are presented in Section 2, results and comparisons with existing data are described in Sections 3 and 4, respectively, and Section 5 contains the conclusions. 2 T H E M O D E L 2.1 Ingredients The aim of our work is to generate template maps of the two linear Stokes parameters Q and U of the Galactic synchrotron polarized radiation in the cosmological window near 100 GHz with the angular resolution of SPOrt (FWHM = 7). We divide the problem into two parts. (i) Constructing a polarized intensity (Ip) map: it can be obtained from existing total intensity (I) sky surveys assuming a model linking the polarized to the total intensity synchrotron emission. (ii) Building a map of polarization angles not affected by the effects of Faraday rotation. At present, only optical starlight data fulfil this requirement. The Haslam map (Haslam et al. 1982) is the most complete sky survey at radio wavelengths, where synchrotron emission is dominant. It is a full-sky map at 408 MHz with a resolution of 51 arcmin, obtained by combining observations taken with different radio telescopes. However, it is not perfect for our aims, since the freefree emission is still significant, especially in the Galactic plane (Reich & Reich 1988, hereafter RR88). Consequently, identification and subtraction of this contribution are mandatory. As described in Section 2.2, we perform this separation using the Dodelson (1997) formalism, which requires a second map at different frequency. We use the Reich (1982) map at 1.4 GHz with an angular resolutio (...truncated)