Context : The Galactic Central Molecular Zone ( CMZ ) is a region containing massive and dense molecular clouds , with dynamics driven by a variety of energy sources including a massive black hole .
It is thus the nearest template for understanding physical processes in extragalactic nuclei .
The CMZ ’ s neutral interstellar gas has been mapped spectrally in many neutral atomic and molecular gas tracers , but the ionized and CO-dark H _ { 2 } regions are less well traced spectroscopically .

Aims : To identify features of the UV irradiated neutral gas , photon dominated regions ( PDRs ) and CO-dark H _ { 2 } , and highly ionized gas in the CMZ as traced by the fine structure line of C ^ { + } at 158 \mu m , [ C ii ] , and to characterize their properties .

Methods : We observed the [ C ii ] 158 \mu m fine structure line with high spectral resolution using Herschel HIFI with two perpendicular On-the-Fly strip scans , along l = –0 \aas@@fstack { \circ } 8 to +0 \aas@@fstack { \circ } 8 and b = –0 \aas@@fstack { \circ } 8 to +0 \aas@@fstack { \circ } 8 , both centered on ( l , b ) = ( 0 ^ { \circ } ,0 ^ { \circ } ) .
We analyze the spatial–velocity distribution of the [ C ii ] data , compare them to those of [ C i ] and CO , and to dust continuum maps , in order to determine the properties and distribution of the ionized and neutral gas and its dynamics within the CMZ .

Results : The longitude– and latitude–velocity maps of [ C ii ] trace portions of the orbiting open gas streams of dense molecular clouds , the cloud G0.253+0.016 , also known as the Brick , the Arched Filaments , the ionized gas within several pc of Sgr A and Sgr B2 , and the Warm Dust Bubble .
We use the [ C ii ] data to determine the physical and dynamical properties of these CMZ features .

Conclusions : The bright far–IR 158 \mu m C ^ { + } line , [ C ii ] , when observed with high spatial and spectral resolution , traces a wide range of emission features in the CMZ .
The [ C ii ] emission arises primarily from dense PDRs and highly ionized gas , and is an important tracer of the kinematics and physical conditions of this gas .