Widespread subsonic turbulence in Ophiuchus North 1
  Supersonic motions are common in molecular clouds. (Sub)sonic turbulence is
usually detected toward dense cores and filaments. However, it remains unknown
whether (sub)sonic motions at larger scales ($\gtrsim$1~pc) can be present in
different environments or not. Located at a distance of about 110 pc, Ophiuchus
North 1 (Oph N1) is one of the nearest molecular clouds that allows in-depth
investigation of its turbulence properties by large-scale mapping observations
of single-dish telescopes. We carried out the $^{12}$CO ($J=1-0$) and C$^{18}$O
($J=1-0$) imaging observations toward Oph N1 with the Purple Mountain
Observatory 13.7 m telescope. The observations have an angular resolution of
$\sim$55\arcsec (i.e., 0.03~pc). Most of the whole C$^{18}$O emitting regions
have Mach numbers of $\lesssim$1, demonstrating the large-scale (sub)sonic
turbulence across Oph N1. Based on the polarization measurements, we estimate
the magnetic field strength of the plane-of-sky component to be
$\gtrsim$9~$\mu$G. We infer that Oph N1 is globally sub-Alfv{\'e}nic, and is
supported against gravity mainly by the magnetic field. The steep velocity
structure function can be caused by the expansion of the Sh~2-27 H{\scriptsize
II} region or the dissipative range of incompressible turbulence. Our
observations reveal a surprising case of clouds characterised by widespread
subsonic turbulence and steep size-linewidth relationship. This cloud is
magnetized where ion-neutral friction should play an important role.
