The Spiral Arms of the Milky Way: The Relative Location of Each Different Arm Tracer Within a Typical Spiral Arm Width

Abstract

From the Sun's location in the Galactic disk, different arm tracers (CO, H i, hot dust, etc.) have been employed to locate a tangent to each spiral arm. Using all various and different observed spiral arm tracers (as published elsewhere), we embark on a new goal, namely the statistical analysis of these published data (data mining) to statistically compute the mean location of each spiral arm tracer. We show for a typical arm cross-cut, a separation of 400 pc between the mid-arm and the dust lane (at the inner edge of the arm, toward the Galactic center). Are some arms major and others minor? Separating arms into two sets, as suggested by some, we find the same arm widths between the two sets. Our interpretation is that we live in a multiple (four-arm) spiral (logarithmic) pattern (around a pitch angle of 12• ) for the stars and gas in the Milky Way, with a sizable interarm separation (around 3 kpc) at the Sun's location and the same arm width for each arm (near 400 pc from mid-arm to dust lane).

A guided map to the spiral arms in the galactic disk of the Milky Way

“…Note the tracer reversal across the galactic center: at right (l > 0°) the hot dust is at a smaller galactic longitude than the CO, while at left (l < 0°) the hot dust is at a greater galactic longitude than the CO. Table 2 shows the statistics on the linear separations of each tracer, with 18 entries averaged over 6 half-arms. It confirms earlier efforts [18,22,52]. In Table 2, the first four arms are in Galactic quadrant IV, and the hot dust is separated from CO by a positive value toward the GC; the last row shows its mean separation from CO to be between 200 and 400 pc toward the GC.…”

“…Thus the angular distance of a tracer (dust, say) from the arm center (CO) is positive toward the arm's inner edge (toward the Galactic Center), and negative otherwise [18,52]. This observed reversal, around the Sun-GC line, is the first concrete, necessary and sufficient proof for the basic galactic density wave model with shocks.…”

“…Once one does a proper dust correction, then other arms show up. Taking all wavelengths into account, the crosscut of alternating spiral arms shows all arm widths to be nearly equal [18]. A sizable minority of nearby spiral galaxies possess a four-arm symmetry along with a bar, such as NGC 5970, NGC 0180, NGC 3346, NGC 6744.…”

“…The Catalog has a master table of the mean longitude for each arm and for each tracer (Table 3), followed by individual tables (one table for each arm tracer) listing individual observational data for each arm [22]. In its first publication, this Catalog had 43 entries (Table 3 in [18]). In the most recent publication, it grew to 215 entries (Tables 3-10 in [22]).…”

MALT90 Kinematic Distances to Dense Molecular Clumps

“…Sources with near neighbors at other velocities are concentrated in a few regions, in particular near l≈332°and l≈338°; the first is near tangency to the Norma Arm, and the second is near the start of the Perseus Arm or the end of the Long Bar (see, e.g., Vallée 2014). We will show that we see evidence for pileup and distance confusion among the 22% of sources that have near neighbors at different velocities, illustrating an intrinsic vulnerability of the method.…”