  We present continuum observations of the infrared dark cloud (IRDC)
G48.66-0.22 (G48) obtained with Herschel, Spitzer, and APEX, in addition to
several molecular line observations. The Herschel maps are used to derive
temperature and column density maps of G48 using a model based on a modified
blackbody. We find that G48 has a relatively simple structure and is relatively
isolated; thus this IRDC provides an excellent target to study the collapse and
fragmentation of a filamentary structure in the absence of complicating factors
such as strong external feedback. The derived temperature structure of G48 is
clearly non-isothermal from cloud to core scale. The column density peaks are
spatially coincident with the lowest temperatures (~ 17.5 K) in G48. A total
cloud mass of ~390Msun is derived from the column density maps. By comparing
the luminosity-to-mass ratio of 13 point sources detected in the Herschel/PACS
bands to evolutionary models, we find that two cores are likely to evolve into
high-mass stars (M>8 Msun). The derived mean projected separation of point
sources is smaller than in other IRDCs but in good agreement with theoretical
predications for cylindrical collapse. We detect several molecular species such
as CO, HCO+, HCN, HNC and N2H+. CO is depleted by a factor of ~3.5 compared to
the expected interstellar abundance, from which we conclude that CO freezes out
in the central region. Furthermore, the molecular clumps, associated with the
sub-millimeter peaks in G48, appear to be gravitationally unbound or just
pressure confined. The analysis of critical line masses in G48 show that the
entire filament is collapsing, overcoming any internal support.
