Velocity resolved [CII] spectroscopy of the center and the BCLMP302
  region of M33 (HerM33es)
  We aim to understand the contribution of the ionized, atomic and molecular
phases of the ISM to the [CII] emission from clouds near the dynamical center
and the BCLMP302 HII region in the north of the nearby galaxy M33 at a spatial
resolution of 50pc. We combine high resolution [CII] spectra taken with the
HIFI spectrometer onboard the Herschel satellite with [CII] Herschel-PACS maps
and ground-based observations of CO(2-1) and HI. All data are at a common
spatial resolution of 50pc. Typically, the [CII] lines have widths intermediate
between the narrower CO(2-1) and broader HI line profiles. We decomposed the
[CII] spectra in terms of contribution from molecular and atomic gas detected
in CO(2-1) and HI, respectively. We find that the relative contribution of
molecular and atomic gas traced by CO(2-1) and HI varies depends mostly on the
local physical conditions and geometry. We estimate that 11-60% and 5-34% of
the [CII] intensities in the center and in BCLMP302, respectively, arise at
velocities showing no CO(2-1) or HI emission and could arise in CO-dark
molecular gas. The deduced strong variation in the [CII] emission not
associated with CO and HI cannot be explained in terms of differences in A_v,
far-ultraviolet radiation field, and metallicity between the two studied
regions. Hence the relative amounts of diffuse (CO-dark) and dense molecular
gas possibly vary on spatial scales smaller than 50pc. Based on the emission
measure observed at radio wavelengths we estimate the contribution of ionized
gas at a few positions to lie between 10-25%. The correlations between the
intensities of tracers corresponding to the same velocity range as [CII],
differ from the correlation derived from PACS data. The results in this paper
emphasize the need for velocity-resolved observations to discern the
contribution of different components of the ISM to [CII] emission. (abridged)
