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Overcoming the Dominant Foreground of Inflationary B-modes: Tomography of Galactic Magnetic Dust via Measurements of Starlight Polarization

Periodic Reporting for period 2 - PASIPHAE (Overcoming the Dominant Foreground of Inflationary B-modes: Tomography of Galactic Magnetic Dust via Measurements of Starlight Polarization)

Reporting period: 2019-12-01 to 2021-05-31

PASIPHAE (Polar-Areas Stellar-Imaging in Polarization High-Accuracy Experiment) is a unique Astrophysics experiment taking place at Skinakas Observatory in Crete, Greece, and at the South African Astronomical Observatory, located near the city of Sutherland in South Africa. The experiment is a collaborative effort of the Astrophysics Group in Crete (the joint Astrophysics group at the Foundation of Research and Technology-Hellas and the University of Crete) with the Inter-University Center for Astronomy and Astrophysics (IUCAA) in Pune, India, the California Institute of Technology in the USΑ, the South African Astronomical Observatory, and the Oslo University in Norway.

PASIPHAE will use unique, innovative polarimeters that are designed specifically for this purpose, to map, with unprecedented accuracy, the polarization of millions of stars at areas of the sky away from the Galactic plane, in both the Northern and the Southern hemispheres. Combined with stellar distances provided by ESA’s Gaia mission, this data will allow us, for the very first time, to construct a tomographic map of the Galactic magnetic field, and the dust that resides in our own Galaxy. This magnetized dust produces polarized microwaves that obscure our view of the polarization of the cosmic microwave background, the radiation that has been left behind by our Universe's dense and hot past. The polarization of the microwave background is believed to contain evidence of a period of inflationary expansion that took place only moments after the Big Bang. By removing the "veil" of Galactic dust, our results will allow accurate estimates of the polarization of the radiation emitted during the Big Bang in order to probe the first instants of the Universe, as well as the, yet-unknown, physics of Gravity at unprecedentedly high densities and temperatures.
During the first reporting period, work has focused on preliminary polarimetric data taking with the RoboPol polarimeter in small patches of sky to demonstrate the new techniques that will be used in the project (Galactic magnetic tomography, measurements of extremely low and unexpectedly high starlight polarization) and to establish new methods for the calibration of our and other teams' polarimeters, including establishing a new, highly reliable set of stars with standard (known and unchanging) optical polarization, against which different instruments can be cross-calibrated.

Our demonstration of the Galactic tomography technique (Panopoulou, Tassis, Skalidis et al. 2019, https://arxiv.org/abs/1809.09804) showed how the combination of measurements of the optical polarization of stars with measurements of stellar distances from ESA's Gaia mission can allow us to disentangle the polarization signal imparted by different clouds along the line of sight. In the same paper, we also demonstrated how the magnetic fields in clouds that reside one behind another can be severely misaligned - a situation which is particularly problematic for measurements of the polarization of the cosmic microwave background. Our work showed how such cases can be identified and, if necessary, removed from the processing of microwave data. Subsequent datataking by PASIPHAE will allow us to identify most such problematic cases, allowing for a much improved "dust cleaning" of the microwave polarization sky.

Our demonstration that an average direction of the magnetic field can be established even for clouds with very little dust emission (Skalidis et al. 2018 https://arxiv.org/abs/1802.04305) showed that PASIPHAE can help "clean" the dust contamination even in regions where the amount of dust does not allow a confident measurement of its effects in any other way. At the opposite end of starlight polarization strengths, Panopoulou, Hensley, Skalidis et al. (2019, https://arxiv.org/abs/1903.09684) showed that a region where the Planck mission had found puzzling strong polarized dust emission is also characterized by unusually high starlight polarization, further cementing the tight correlation between polarization of stars (measured by PASIPHAE) and polarized emission from dust (obscuring polarization measurements of cosmic microwaves). This connection is at the heart of PASIPHAE's power to clear the way towards detecting the signature of inflation.

The effect of the local bubble on the polarized microwave sky was shown to be significant, but possible to account for with a combination of polarization and Gaia data (Skalidis & Pelgrims 2019, Pelgrims et al. 2020).

Finally, our (yet unpublished) data taking to establish new optopolarimetric standards has shown that many of the standards currently in use are not stable enough to ensure the high level of accuracy needed for experiments such as PASIPHAE. However, the new standard stars we have been observing, verified for brightness and polarization stability over several years and with many measurements, will cover this need, not only for PASIPHAE, but also for any future polarimetric experiments.
Already our project has pushed forward the state of the art in using polarimetry to map the properties of dust and magnetic fields in the interstellar medium. As discussed above, we have demonstrated that magnetic tomography can work and does work, with sensitive enough measurements of stellar polarizations and stellar distances. We have showed that, even in clouds where very little dust is present, this dust does have a polarization signature, and that this signature is measurable with an experiment such as PASIPAHE. And we have also demonstrated that the intrinsic polarizing ability of dust grains at optical wavelengths has been underestimated, and indications from the Planck mission that some regions exhibit unexpectedly high polarization also maps in optical wavelengths. Our future, wider-field data-taking will extend these measurements over large sections of the sky. We will measure, with unprecedented accuracy, the polarization of millions of stars. We will provide a wealth of polarimetric data with which to study the properties of dust and magnetic fields in interstellar clouds. And we will add an invaluable tool in our collective efforts, as astronomical community, to measure the polarization of the cosmic microwave background, including a potential signature from the beginning of the Universe.
Magnetic field directions in the region of magnetic tomography demonstration (Panopoulou et al. 2019
Optical polarizations in the region of magnetic tomography demonstration (Panopoulou et al. 2019)