# (Astro)Physics

The background is the neutral hydrogen 21-cm emission (HI) in this region. Red stands for more emission and cooler colors (blues) represent less emission. Over top the HI emission is carbon monoxide emission, which is often used to trace molecular hydrogen (H$_2$) emission.
The first (and cleanest) image has $^{13}$CO over HI emission. The second image has $^{12}$CO emission. The last image shows a comparison of the $^{12}$CO and $^{13}$CO emission. The contours here are the $^{13}$CO emission on top of the $^{12}$CO color image. Mind you, the $^{13}$CO data was taken from a different survey. It looks smoother only because it is lower spatial resolution.