The vertical distribution of molecular gas in the fourth quadrant of the Milky Way

Abstract

The vertical distribution of molecular gas in the Milky Way disk is poorly studied, especially in the 4th quadrant and the far side of the Galaxy. While surveys over 20 years ago suffered from coarse beam and non-Nyquist sampling, most of the recent surveys are limited to $|b|<1^\circ$ and/or focus on the Northern Galactic plane. We examine the molecular gas&#39;s vertical distribution and properties in the fourth quadrant to check the impact of Galactic structure on current star formation. We mapped two arm tangent regions ($l=[329^\circ.0,327^\circ.0]$ for the Norma-Outer arm, and $l=[310^\circ.0,308^\circ.5]$ for the Scutum-Centaurus arm) with the $^{12}$CO 3-2 line using the 7-pixel LASMA receiver of the APEX 12 m telescope, covering up to $b=[-2^\circ,+2^\circ]$ (or $|z|\sim200$ pc near the tangent points). The noise level of the $^{12}$CO 3-2 line is about 0.4 K [$T_{MB}$] in velocity channels of 0.5 km s$^{-1}$. The vertical distribution around the tangent regions of the spiral arm is fitted with two Gaussian components, the narrow one with an FWHM thickness of 25 pc and the wide one of about 100 pc. Two explanations are proposed: they are two sub-components of the thin disk (the narrowest tracing the spiral arm and the other the inter arms components), or they trace both thin and thick disks that show a smaller thickness when observed with CO 3-2 than when observed with CO 1-0. The latter indicates that the density distribution of the interstellar medium exhibits a steeper gradient at larger heights above the plane. Clumps in the two tangent regions are searched and divided into groups by relative height. Compared with the low relative height clumps ($|z|<50$ pc), the median relative height clumps ($50~\mathrm{pc}<|z|<100~\mathrm{pc}$) have lower excitation temperature and surface density, with fewer high values in the distribution. A similar tentative trend is found for the mass.