INTEGRAL measures the vertical position of the Sun with respect to the Milky Way
For many areas of astronomical measurement, the knowledge about the
absolute location of the solar system within the Milky Way galaxy is
of utmost importance: In order to understand the kinematics of stars
and gas on small and large scales, the observer's view point is one
of the main drivers of systematic differences in studies of Galactic
dynamics.
Especially in the era of microarcsecond angular resolution, the celestial
coordinate system defined by the International Astronomical Union (IAU)
still carries uncertainties of the order of 0.03 degrees. This is, among
other things, based on the difficulty to determine the relative 3D position
of the solar system inside our Galaxy. While the cosmic distance ladder
provides good estimates along sight-lines, even the smallest deviations
in vertical position let objects (and their properties) appear skewed.
The vertical position of the Sun above the Galactic mid plane (galactic
latitude b=0) is typically based on counting the number of stars in
certain populations, and correcting the IAU value until the populations
appear symmetric. However, dust and gas can absorb the optical and
infrared light from these stars, which might have an influence on the
result.
Gamma-rays from the radioactive decay of 26Al that are
ejected by massive stars and supernovae, carry more than a million
times more energy than optical photons (MeV vs. eV), meaning that
their propagation is not hindered by interstellar absorption.
The idea hence stays the same: placing the Sun as observing point above
or the Galactic mid plane (see top panel of the figure) should result
in a better match between the observations made by INTEGRAL/SPI and the
appearance of the 26Al sky (bottom left panel). Using more
than 13 years of data from the gamma-ray decay line of 26Al
at 1.809 MeV photon energies, it is possible to narrow down the vertical
position of the Sun to values between -2 and +32 pc with respect to the
Galactic plane (bottom right panel).
While this range is consistent with other measurements, the uncertainties
are still several tens of per cent larger. Keeping in mind that the
sensitivity of MeV telescopes is several orders of magnitude below those
of optical telescopes, but also that INTEGRAL/SPI is capable of seeing
large portions of the sky at once, such high-precision measurements are
already possible now.
"Revised geometric estimates of the North Galactic Pole and the Sun's height above the Galactic mid-plane",
M.T. Karim,E.E. Mamajek
Monthly Notices of the Royal Astronomical Society, Volume 465, Issue 1, p.472-481
https://ui.adsabs.harvard.edu/abs/2017MNRAS.465..472K/abstract
