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Abstract

In this thesis we study the energy balance that sets the structure of the interstellar medium (ISM) phases, which in turn control the processes of star-formation (SF). Understanding the energy balance of the ISM is an essential step towards understanding the processes which shape evolution of galaxies across the cosmic time, and lead to the diversity of galaxy properties in the Universe. This work focuses on M31, an ideal laboratory to study the ISM due to its proximity, external perspective, and as it is a representative of the large, star-forming, metal-rich galaxies where most of the SF in the local Universe occurs. We present the Survey of Lines in M31 (SLIM) with contributing new Integral Field Spectroscopic Herschel [CII] and Calar Alto Halpha data, which allow us to study the dominant neutral gas coolant – [CII] on ~50 pc scales. We combine archival ISO [CII] measurements in the bulge of M31 with ancillary data to identify potential heating sources responsible for gas heating in the absence of young stars. For the first time in extragalactic studies, we directly constrain the ISRF intensity and spectral shape, based on PHAT modelling of the spectral energy distributions (SEDs) and extinction of the individual stars in M31 from the PHAT. We find that a significant amount (~20–90%) of the [CII] emission comes from outside SF regions. We find that the [CII] – SFR correlation holds even at the small scales (~50 pc), although the relation typically has a flatter slope than found at larger ~kpc) scales, where [CII] traces the SFR approximately linearly. The photoelectric (PE) heating mechanism seems as the largest contributor to gas heating (where FUV flux is generated by evolved stellar populations), however photoionization and cosmic rays heating mechanisms are likely to be responsible for the observed [CII] “excess” in the bulge. We also find that the attenuated UV energy relative to the total attenuated energy correlates well with the [CII]/TIR ratio, suggesting that it is the soft photon heating of dust that is driving the variation in the [CII]/TIR ratio across this disk. We propose that a better method to approximate the PE heating efficiency (than [C ii]/TIR) is an estimate of the energy absorbed by dust in the PE heating wavelength range (UVatt), that together with the [CII] emission allows us to determine the “true” PE efficiency [CII]=UVatt. Our preliminary results suggest that the UVatt is generated predominantly by B0–B3 type stars (~60%), and only approximately 30% is coming from O-stars. This means that in M31, SFR tracers such as UV & [C ii] might be dominated by heating generated by stars ~10 Myr old, not only recent SF, which would lead to the overestimate of SFRs.