The geometry of the magnetic field in the central molecular zone measured by PILOT

Abstract

We present the first far infrared (FIR) dust emission polarization map covering the full extent Milky Way's Central molecular zone (CMZ). The data, obtained with the PILOT balloon-borne experiment, covers the Galactic Center region −2 • < < 2 • , −4 • < b < 3 • at a wavelength of 240 µm and an angular resolution 2.2 . From our measured dust polarization angles, we infer a magnetic field orientation projected onto the plane of the sky that is remarkably ordered over the full extent of the CMZ, with an average tilt angle of 22 • clockwise with respect to the Galactic plane. Our results confirm previous claims that the field traced by dust polarized emission is oriented nearly orthogonal to the field traced by GHz radio synchrotron emission in the Galactic Center region. The observed field structure is globally compatible with the latest Planck polarization data at 353 GHz and 217 GHz. Upon subtraction of the extended emission in our data, the mean field orientation that we obtain shows good agreement with the mean field orientation measured at higher angular resolution by the JCMT within the 20 km/s and 50 km/s molecular clouds. We find no evidence that the magnetic field orientation is related to the 100 pc twisted ring structure within the CMZ. We propose that the low polarization fraction in the Galactic Center region and the highly ordered projected field orientation can be reconciled if the field is strong, with a 3D geometry that is is mostly oriented 15 • with respect to the line-of-sight towards the Galactic center. Assuming equipartition between the magnetic pressure and ram pressure, we obtain magnetic field strengths estimates as high as a few mG for several CMZ molecular clouds.

Simulations of the Milky Way’s central molecular zone – I. Gas dynamics

“…The conditions in the CMZ are extreme. The CMZ has average densities (Longmore et al 2017;Mills et al 2018), temperatures (Ginsburg et al 2016;Immer et al 2016;Krieger et al 2017;Oka et al 2019), velocity dispersions (Shetty et al 2012;Federrath et al 2016), and estimated magnetic field strengths (Chuss et al 2003;Crocker et al 2010;Morris 2015;Mangilli et al 2019) several orders of magnitudes higher than in the Galactic disc.…”

“…Star formation (SF) and gas dynamics in the CMZ have been the subject of intense study during the past decade, which has produced vast advancements both on the observational (e.g. Molinari et al 2011;Immer et al 2012Immer et al , 2016Jones et al 2012;Ginsburg et al 2016;Henshaw et al 2016;Kauffmann et al 2017a, b;Krieger et al 2017;Longmore et al 2017;Mills et al 2018;Mangilli et al 2019;Oka et al 2019) and theoretical (e.g. Kruijssen, Dale & Longmore 2015;Krumholz & Kruijssen 2015;Sormani, Binney & Magorrian 2015a, c;Krumholz, Kruijssen & Crocker 2017;Ridley et al 2017;Sormani et al 2018Sormani et al , 2019Armillotta et al 2019;Dale, Kruijssen & Longmore 2019;Kruijssen et al 2019a;Armillotta, Krumholz & Di Teodoro 2020;Li, Shen & Schive 2020) sides.…”

Simulations of the Milky Way’s Central Molecular Zone – II. Star formation

“…The main reason is that the environmental conditions in which stars are born are more extreme than anywhere else in the Galaxy. Indeed, the physical properties of the interstellar medium (ISM) in the CMZ are substantially different from those in the Galactic disc: average gas volume densities (Guesten & Henkel 1983;Walmsley et al 1986;Longmore et al 2017;Mills et al 2018), temperatures (Ginsburg et al 2016;Immer et al 2016;Krieger et al 2017;Oka et al 2019), velocity dispersions E-mail: mattia.sormani@alumni.sns.it (Shetty et al 2012;Federrath et al 2016), and magnetic field strengths (Morris 2015;Mangilli et al 2019) are all much higher than in the disc. The interstellar radiation field and higher cosmic ray ionization rate (Clark et al 2013;Ginsburg et al 2016;Oka et al 2019) are also much stronger.…”

Statistical description of dust polarized emission from the diffuse interstellar medium

“…Planck has provided us with the first all-sky survey of polarized dust emission, opening the path to statistical studies (Planck Collaboration XII 2020). This broad view is being complemented by observations at higher angular resolution, which are carried out by the balloon-borne experiments BLASTPOL (Fissel et al 2016) and PILOT (Mangilli et al 2019), the far-IR HAWC+ camera onboard SOFIA (Chuss et al 2019) and imaging at sub-mm/mm wavelengths from large single-dish telescopes (Ritacco et al 2020) and ALMA (Hull et al 2017). These observations all contribute to a common scientific goal: understanding the role turbulence and magnetic fields play along the star formation process, from the diffuse interstellar medium to molecular clouds and protostellar cores.…”