The outer spiral arm of the Milky Way using red clump stars - tracing the asymmetry across the disc

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 (RC) stars. The spiral arms, warp structure, and other asymmetries present in the disc are revisited using a systematic study of RC star counts over the disc of the Galaxy. Methods. We developed a method for systematically extracting the RC stars from a 2MASS (J - Ks, J) colour-magnitude diagram of 1° ×1° bins in ℓ × b, covering the range 40° ≤ ℓ ≤ 320° and -10° ≤ b ≤ 10°. 2MASS data continue to be important because we are able to identify and trace the RC stars to much farther distances than in any optical survey of the Galactic disc. The foreground star contamination in the selected sample was removed using the accurate astrometric data from Gaia EDR3. We analysed the spiral arms and asymmetry in the Galaxy above and below the Galactic plane in galactocentric coordinates. 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 RC 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 RC stars. Through this study, we are able to trace the Outer arm well into the third Galactic quadrant for the first time. In addition to 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 RC 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.