The structure and kinematics of the the Galaxy thin gaseous disc outside the solar orbit

TL;DR

This paper introduces a new method using CaII lines to measure the Galaxy's thin gaseous disc kinematics, revealing a Keplerian rotation curve outside the solar orbit, challenging the traditional flat rotation curve assumption.

Contribution

The study presents a novel observational technique that avoids common biases, providing evidence for a Keplerian rotation curve in the Galaxy's outer thin gaseous disc.

Findings

The rotation curve of the Galaxy's thin gaseous disc is Keplerian outside the solar orbit.

Traditional methods may misinterpret the rotation curve as flat due to tracer biases.

The new method offers a more accurate assessment of galactic kinematics.

Abstract

The rotation curve of the Galaxy is generally thought to be flat. However, using radial velocities from interstellar molecular clouds, which is common in rotation curve determination, seems to be incorrect and may lead to incorrectly inferring that the rotation curve is flat indeed. Tests basing on photometric and spectral observations of bright stars may be misleading. The rotation tracers (OB stars) are affected by motions around local gravity centers and pulsation effects seen in such early type objects. To get rid of the latter a lot of observing work must be involved. We introduce a method of studying the kinematics of the thin disc of our Galaxy outside the solar orbit in a way that avoids these problems. We propose a test based on observations of interstellar CaII H and K lines that determines both radial velocities and distances. We implemented the test using stellar spectra of thin disc stars at galactic longitudes of 135{\degr} and 180{\degr}. Using this method, we constructed the rotation curve of the thin disc of the Galaxy. The test leads to the obvious conclusion that the rotation curve of the thin gaseous galactic disk, represented by the CaII lines, is Keplerian outside the solar orbit rather than flat.