Catalog of spiral arm tangents (Galactic longitudes) in the Milky Way,and the age gradient based on various arm tracers

An updated catalog of 205 observed tangents to the spiral arms (in Galactic longitudes) since 1980 is presented. This represents an addition of 80 arm tangents in 6 years (since 2016). Most arm tangents are observed at telescopes in the radio régime.In this study, the separation of each arm tracer from the dust lane is analysed to obtain the relative speed away from the dust lane (an age gradient). Each arm tracer is observed to be separated from the dust lane, showing an age gradient of about 11.3 ±2 Myr/kpc across the spiral arm – a relative speed away from the dust lane of about 87 ±10 km/s.