The Outer spiral arm of the Milky Way using Red Clump stars

Abstract

Aims: Our aim is to provide an observational view of the old Disc structure of the Milky Way galaxy using the distribution of red clump stars. The spiral arms, warp structure, and other asymmetries present in the Disc are re-visited using a systematic study of red clump star counts over the disc of the Galaxy. Methods: We developed a method to systematically extract the red clump stars from 2MASS ($J-K_s, ~J$) colour-magnitude diagram of $1^\circ \times 1^\circ$ bins in $\ell \times b$ covering the range $40^\circ \le \ell \le 320^\circ$ and $-10^\circ \le b \le 10^\circ$. 2MASS data continues to be important since it is able to identify and trace the red clump stars to much farther distances than any optical survey of the Disc. The foreground star contamination in the selected sample is removed by utilising the accurate astrometric data from Gaia EDR3. Results: We have generated a face-on-view (XY-plane) of the Galaxy depicting the density distribution and count ratio above and below the Galactic plane. The resulting over-density of red clump stars traces the continuous morphology of the Outer arm from the second to the third Galactic quadrant. This is the first study to map the Outer arms across the disc using red clump stars. Through this study, we are able to trace the Outer arm well into the 3rd Galactic quadrant for the first time. Apart from the spiral structures, we also see a wave-like asymmetry above and below the Galactic plane with respect to longitudes indicating the warp structure. The warp structure is studied systematically by tracing the ratio of red clump stars above and below the Galactic plane. We provide the first direct observational evidence of the asymmetry in the Outer spiral arms confirming that the spiral arms traced by the older population are also warped, similar to the Disc.