Vertical position of the Sun with γ-rays

Abstract

We illustrate a method for estimating the vertical position of the Sun above the Galactic plane by γ-ray observations. Photons of γ-ray wavelengths are particularly well suited for geometrical and kinematic studies of the Milky Way because they are not subject to extinction by interstellar gas or dust. Here, we use the radioactive decay line of 26 Al at 1.809 MeV to perform maximum likelihood fits to data from the spectrometer SPI on board the INTEGRAL satellite as a proof-of-concept study. Our simple analytic 3D emissivity models are line-of-sight integrated, and varied as a function of the Sun's vertical position, given a known distance to the Galactic centre. We find a vertical position of the Sun of z 0 = 15 ± 17 pc above the Galactic plane, consistent with previous studies, finding z 0 in a range between 5 and 29 pc. Even though the sensitivity of current MeV instruments is several orders of magnitude below that of telescopes for other wavelengths, this result reveals once more the disregarded capability of soft γ-ray telescopes. We further investigate possible biases in estimating the vertical extent of γ-ray emission if the Sun's position is set incorrectly, and find that the larger the true extent, the less is it affected by the observer position. In the case of 26 Al with an exponential scale height of 150 pc (700 pc) in the inner (full) Galaxy, this may lead to misestimates of up to 25 %.

Revisiting Galactic Disk and Spiral Arms Using Open Clusters

“…Numerous studies have estimated z e using a wide variety of celestial sources, and most of these studies obtained z e in the range of 10-25 pc to the north of the Galactic plane (e.g., Joshi 2007;Majaess et al 2009;Buckner & Froebrich 2014;Yao et al 2017;Siegert 2019;Griv et al 2021). Though a direct comparison of z e in different studies is difficult, it is found that the choice of the data sample and method of determination account for most of the disagreements among z e values (Joshi 2007;.…”

“…Moreover, the wide range of ages of OCs, from a few million years to nearly 10 billion years, provides a unique opportunity to study the evolution of the Galactic disk and trace the star formation history across the Milky Way. In recent years, the number of OCs has grown manyfold (Cantat-Gaudin 2022;Castro-Ginard et al 2022;Chi et al 2023), paving the way for a better understanding of the Galactic disk, thanks to large-scale surveys and machinelearning methods (Cantat-Gaudin et al 2018, 2019Ryu & Lee 2018;Sim et al 2019;Liu & Pang 2019;Dias et al 2021;He et al 2021;Hao et al 2022b;Castro-Ginard et al 2022). Accurate physical parameters for OCs are obtained through analysis of precise membership identification of cluster stars facilitated through their astrometric and kinematic study (Dias et al 2014(Dias et al , 2021Cantat-Gaudin et al 2018;Castro-Ginard et al 2022;Zhong et al 2022).…”