Gas and dust cooling along the major axis of M 33 (HerM33es). Herschel/PACS [C II] and [O I] observations

Context. M 33 is a gas rich spiral galaxy of the Local Group. Its vicinity allows us to study its interstellar medium (ISM) on linear scales corresponding to the sizes of individual giant molecular clouds. <BR /> Aims: We investigate the relationship between the two major gas cooling lines and the total infrared (TIR) dust continuum. <BR /> Methods: We mapped the emission of gas and dust in M 33 using the far-infrared lines of [C II] and [O I](63 μm) and the total infrared continuum. The line maps were observed with the PACS spectrometer on board the Herschel Space Observatory. These maps have 50 pc resolution and form a ∼370 pc wide stripe along its major axis covering the sites of bright H II regions, but also more quiescent arm and inter-arm regions from the southern arm at 2 kpc galacto-centric distance to the south out to 5.7 kpc distance to the north. Full-galaxy maps of the continuum emission at 24 μm from Spitzer/MIPS, and at 70 μm, 100 μm, and 160 μm from Herschel/PACS were combined to obtain a map of the TIR. <BR /> Results: TIR and [C II] intensities are correlated over more than two orders of magnitude. The range of TIR translates to a range of far ultraviolet (FUV) emission of G<SUB>0, obs</SUB> ∼ 2 to 200 in units of the average Galactic radiation field. The binned [C II]/TIR ratio drops with rising TIR, with large, but decreasing scatter. The contribution of the cold neutral medium to the [C II] emission, as estimated from VLA H I data, is on average only 10%. Fits of modified black bodies 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 [O I] data before comparing it with models of photon dominated regions. Most of the ratios of [C II]/[O I] and ([C II]+[O I])/TIR are consistent with two model solutions. The median ratios are consistent with one solution at n ∼ 2 × 10<SUP>2</SUP> cm<SUP>-3</SUP>, G<SUB>0</SUB> ∼ 60, and a second low-FUV solution at n ∼ 10<SUP>4</SUP> cm<SUP>-3</SUP>, G<SUB>0</SUB> ∼ 1.5. <BR /> Conclusions: 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. <P />Maps of TIR, [C II], [O I] shown in Figs. 2 and 3 are only available at the CDS via anonymous ftp to <A href='http://cdsarc.u-strasbg.fr/'>http://cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href='http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/639/A61'>http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/639/A61</A> <P />Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.