Abstract
We present the first far infrared (FIR) dust emission polarization map covering the full extent of Milky Way’s central molecular zone (CMZ). The data, obtained with the PILOT balloon-borne experiment, covers the Galactic center region − 2° < ℓ < 2°, − 4° < b < 3° at a wavelength of 240 μm and an angular resolution of 2.2′. From our measured dust polarization angles, we infer a magnetic field orientation projected onto the plane of the sky (POS) 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 orthogonally 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 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 and 50 km s−1 molecular clouds. We find no evidence that the magnetic field orientation is related to the 100 pc twisted ring structure within the CMZ. The low polarization fraction in the Galactic center region measured with Planck at 353 GHz combined with a highly ordered projected field orientation is unusual. This feature actually extends to the whole inner Galactic plane. We propose that it could be caused by the increased number of turbulent cells for the long lines of sight towards the inner Galactic plane or to dust properties specific to the inner regions of the Galaxy. Assuming equipartition between magnetic pressure and ram pressure, we obtain magnetic field strength estimates of the order of 1 mG for several CMZ molecular clouds.
Highlights
The interstellar medium (ISM) near the Galactic center (GC) is dominated by the central molecular zone (CMZ), a large reservoir of dense molecular gas with a total mass of ∼107 M and typical gas densities of a few times 104 cm−2 (e.g, Ferrière et al 2007)
The data were obtained during 30 min of observations during the second flight of the PILOT balloon-borne experiment, which was launched from Alice Springs, Australia in April 2017
We develop a Galactic model with ordered and turbulent magnetic field components and a realistic density distribution to demonstrate quantitatively that the observed polarization properties cannot be explained by the magnetic field of the Galactic disk along the line-of-sight to the Galactic center region. – The PILOT polarization angle measurements show that the structure of the magnetic field in the plane of the sky is remarkably uniform over the full extent of our mapped region
Summary
The interstellar medium (ISM) near the Galactic center (GC) is dominated by the central molecular zone (CMZ), a large reservoir of dense molecular gas with a total mass of ∼107 M and typical gas densities of a few times 104 cm−2 (e.g, Ferrière et al 2007). Novak et al (2003) observed a much larger, 170 pc × 30 pc, area around Sgr A∗ Their 450 μm polarization map clearly shows that the magnetic field threading molecular clouds is, on the whole, approximately parallel to the Galactic plane. Each of the observing tiles was configured to scan the target region at a different angle, with a median scan speed of 11.8 /s and a median scan leg duration of 13 s These scan directions were chosen to be preferentially perpendicular to the Galactic plane, while at the same time providing sufficiently varied directions for efficient destriping during map-making We estimate that the accuracy of the pointing from the differences between pointing reconstruction solutions obtained with different Estadius offsets computed on the various sources used in L0 is 15 arcsec
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