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

Abstract

We present the result of an unbiased CO survey in Galactic range of 34.75• ≤l≤ 45.25• and -5.25• ≤b≤ 5.25• , and velocity range beyond the Outer arm. A total of 168 molecular clouds (MCs) are identified within the Extreme Outer Galaxy (EOG) region, and 31 of these MCs are associated with 13 CO emission. However, none of them show significant C 18 O emission under current detection limit. The typical size and mass of these MCs are 5 pc and 3×10 3 M ⊙ , implying the lack of large and massive MCs in the EOG region. Similar to MCs in the outer Galaxy, the velocity dispersions of EOG clouds are also correlated with their sizes, however, are well displaced below the scaling relationship defined by the inner Galaxy MCs. These MCs with a median Galactocentric radius of 12.6 kpc, show very different distributions from those of the MCs in the Outer arm published in our previous paper, while roughly follow the Outer Scutum-Centaurus arm defined by Dame & Thaddeus (2011). This result may provide a robust evidence for the existence of the Outer Scutum-Centaurus arm. The lower limit of the total mass of this segment is about 2.7×10 5 M ⊙ , which is about one magnitude lower than that of the Outer arm. The mean thickness of gaseous disk is about 1.45• or 450 pc, and the scale height is about 1.27• or 400 pc above the b = 0 • plane. The warp traced by CO emission is very obvious in the EOG region and its amplitude is consistent with the predictions by other warp models using different tracers, such as dust, HI and stellar components of our Galaxy.

Clumpy shocks as the driver of velocity dispersion in molecular clouds: the effects of self-gravity and magnetic fields

“…where Γ ≈ 0.5 (Larson 1981). The universal proportionality of this relationship is supported by a series of observations of CO linewidths (Heyer & Brunt 2004; Rice 2016; Sun et al 2017), and interpreted observations of dust polarisation (e.g. Poidevin et al 2013).…”

First Detection of the Molecular Cloud Population in the Extended Ultraviolet Disk of M83

“…M83 XUV clouds are blue (and the blue arrows are the upper limits in radius, while they are resolved in velocity). Reference points in the background are for the MW's inner disk (Solomon et al 1987), outer disk (r gal ∼ 12-14 kpc; Sun et al 2017), and far outer disk (∼14-22 kpc; Sun et al 2015), and for the Large and Small Magellanic Clouds (Bolatto et al 2008;Wong et al 2011) and the galaxy WLM (Rubio et al 2015). The solid line is a fit to the inner disk clouds (Solomon et al 1987).…”

“…It is crucial to study whether the same is true in XUV disks. The Milky Way's outer disk hosts small molecular clouds (10 2-4 M e ; Sun et al 2015Sun et al , 2017, but unlike XUV disks there is not much SF above late B-type stars (Izumi et al 2017). Numerous efforts have been made to detect CO emission in XUV disks, but they have rarely succeeded (Watson & Koda 2017, for review including unpublished efforts that resulted in nondetections).…”

The Galactic spiral structure as revealed by O- and early B-type stars

“…Most of the current Galactic spiral models are based on the kinematic distances of ionized H II regions (Georgelin & Georgelin 1976;Downes et al 1980;Caswell & Haynes 1987;Russeil 2003;Hou et al 2009), H I neutral gas (Kerr 1969;Weaver 1970;McClure-Griffiths et al 2004;Kalberla & Kerp 2009;Koo et al 2017) and CO molecular clouds (Solomon & Rivolo 1989;May et al 1997;Heyer et al 2001;Dame & Thaddeus 2011;Hou & Han 2014;Sun et al 2015Sun et al , 2017. However, due to the large errors of those kinematic distances, the identification of the spiral arms suffers from inevitable large ambiguity.…”