Interarm islands in the Milky Way -- the one near the Cygnus spiral arm

Abstract

This study extends to the structure of the Galaxy. Our main goal is to focus on the first spiral arm beyond the Perseus arm, often called the Cygnus arm or the Outer Norma arm, by appraising the distributions of the masers near the Cygnus arm. The method is to employ masers whose trigonometric distances were measured with accuracy. The maser data come from published literature, having been obtained via the existing networks (US VLBA, the Japanese VERA, the European VLBI, and the Australian LBA). The new results for Cygnus are split in two groups: those located near a recent CO fitted global model spiral arm, and those congregating within an interarm island located halfway between the Perseus arm and the Cygnus arm. Next, we compare this island with other similar interarm objects near other spiral arms. Thus we delineate an interarm island (6 x 2 kpc) located between the two long spiral arms (Cygnus and Perseus arms); this is reminiscent of the small Local Orion arm (4 x 2 kpc) found earlier between the Perseus and Sagittarius arms, and of the old Loop (2 x 0.5 kpc) found earlier between the Sagittarius and Scutum arms. Various arm models are compared, based on observational data (masers, HII regions, HI gas, young stars, CO gas).