2023A&A...680A..81R

Aims. The arm tangent direction provides a unique viewing geometry, with a long path in relatively narrow velocity ranges and lines of view that cross the arm perpendicular to its thickness. The spiral arm tangent regions are therefore the best directions for studying the interstellar medium within spiral density waves in the Milky Way, probing the internal structure in the arms. We focus here on the gas kinematics and star formation in the Galactic plane zone with longitudes of between 281° and 285.5° and latitudes of between ∼-2.5° and ∼1°, respectively, which contains the Carina arm tangency.
Methods. The Carina arm tangent direction was observed as part of a velocity-resolved Hα survey of the southern Milky Way using a scanning Fabry-Perot mounted on a telescope, which makes it possible to obtain data cubes containing kinematic information. Our detailed analysis of the resultant Hα profiles reveals the presence of several layers of ionized gas with different velocities over the surveyed region. We combine the Hα data with multi-wavelength information in order to assign velocity and distance to the H II regions in the probed area and to study the star-formation activity in the Carina arm tangency.
Results. We find that the Carina arm tangency is at l = 282°, and that it spreads from 2 to 6 kpc with a V_LSR range of between -20 and +20 km s^–1. We deduce an arm width of ∼236 pc. We also probe the star formation on a scale of ∼1 kpc^–2, showing that the star-formation activity is intermediate in comparison with the quiescient Solar neighborhood and the most active Galactic central molecular zone. From our analysis of the stellar motions extracted from the Gaia DR3 catalog, we observe that stars around 2.5 kpc are tracing the trailing and the leading sides of the arm, while stars at greater distances more closely trace the inner part of the arm. In parallel, we studied the Hα velocity structure of the H II regions RCW48 and RCW49 in detail, confirming the expansion velocity of ∼20 km s^–1 for RCW 49 and the double-shell structure of RCW 48, which is in agreement with a wind interaction with a previous mass-loss episode.