A Possible Extension of the Scutum-Centaurus Arm Into the Outer Second Quadrant

Abstract

Combining HI data from the Canadian Galactic Plane Survey (CGPS) and CO data from the Milky Way Imaging Scroll Painting (MWISP) project, we have identified a new segment of a spiral arm between Galactocentric radii of 15 and 19 kpc that apparently lies beyond the Outer Arm in the second Galactic quadrant. Over most of its length, the arm is 400-600 pc thick in z. The new arm appears to be the extension of the distant arm recently discovered by Dame & Thaddeus (2011) as well as the Scutum−Centaurus Arm into the outer second quadrant. Our current survey identified a total of 72 molecular clouds with masses on the order of 10 2 -10 4 M ⊙ that probably lie in the new arm. When all of the available data from the CO molecular clouds are fit, the best−fitting spiral model gives a pitch angle of 9.3 • ±0.7 • .

Molecular Clouds in the Extreme Outer Galaxy between l = 34.°75 to 45.°25

“…Early HI emission surveys have first revealed that the disk of our Galaxy is warped beyond the Solar circle (e.g., Burke 1957;Kerr 1957;Burton 1988). This finding was confirmed by a series of studies in the Galactic disks traced by gaseous component (e.g., Oort et al 1958;Henderson et al 1982;Wouterloot et al 1990;Russeil 2003;Nakagawa et al 2005;Sun et al 2015), stellar component (e.g., Moitinho et al 2006;Momany et al 2006;Reylé et al 2009), as well as dust component (e.g., Drimmel & Spergel 2001;Drimmel et al 2003;Marshall et al 2006). However, our knowledge of the warp of the outer Galaxy to date is largely dependent on HI emission surveys.…”

“…The high-sensitivity data from MWISP survey will provide us a unique opportunity to systematically study the spiral structures and star-formation activities at the edge of the Milky Way. Using our new CO data of the MWISP, we have found dozens of new EOG clouds, which delineate a new segment of a spiral arm between Galactocentric radii of 15 and 19 kpc in the second Galactic quadrant (Sun et al 2015).…”

“…The often used face-on view of the Milky Way (R. Hurt: NASA/JPL-Caltech/SSC) superposed on all available EOG clouds are displayed in Figure 9a, including clouds (filled star) identified by Dame & Thaddeus (2011), clouds (circle) in the 2nd and 1st quadrants identified by Sun et al (2015), and this study. The white dashed line is a log spiral with a mean pitch angle of 12…”

“…And the cyan dashed line traces the fitting result of EOG clouds in Sun et al (2015) with pitch angle of 9.3…”

“…Figure 9b shows the vertical structure (in the R GC − Z space) of the Outer Scu-Cen arm traced by CO emission. We also examine the results for the Outer Scu-Cen arm in the 2nd quadrant (Sun et al 2015), the Outer arm in both 1st and 2nd quadrants (Du et al 2016;Su et al 2016). Note that all structures are derived from the l − b − v relations by applying the Reid model, with one exception, the Outer Scu-Cen arm in the 2nd quadrant, which is indicated by data points directly from observations.…”