Gas and dust cooling along the major axis of M33 (HerM33es) --
  Herschel/PACS [CII] and [OI] observations
  M33 is a gas rich spiral galaxy of the Local Group. We investigate the
relationship between the two major gas cooling lines and the total infrared
(TIR) dust continuum. We mapped the emission of gas and dust in M33 using the
far-infrared lines of [CII] and [OI](63um) and the TIR. The line maps were
observed with Herschel/PACS. These maps have 50pc resolution and form a ~370pc
wide stripe along its major axis covering the sites of bright HII regions, but
also more quiescent arm and inter-arm regions from the southern arm at 2kpc
galacto-centric distance to the south out to 5.7kpc distance to the north.
Full-galaxy maps of the continuum emission at 24um from Spitzer/MIPS, and at
70um, 100um, and 160um from PACS were combined to obtain a map of the TIR. TIR
and [CII] intensities are correlated over more than two orders of magnitude.
The range of TIR translates to a range of far ultraviolet (FUV) emission of
G0,obs~2 to 200 in units of the average Galactic radiation field. The binned
[CII]/TIR ratio drops with rising TIR, with large, but decreasing scatter. Fits
of modified black bodies (MBBs) to the continuum emission were used to estimate
dust mass surface densities and total gas column densities. A correction for
possible foreground absorption by cold gas was applied to the [OI] data before
comparing it with models of photon dominated regions (PDRs). Most of the ratios
of [CII]/[OI] and ([CII]+[OI])/TIR are consistent with two model solutions. The
median ratios are consistent with one solution at n~2x10^2 cm-3, G0~60, and and
a second low-FUV solution at n~10^4 cm-3, G0~1.5. The bulk of the gas along the
lines-of-sight is represented by a low-density, high-FUV phase with low beam
filling factors ~1. A fraction of the gas may, however, be represented by the
second solution.
