Zusammenfassung
(abridged) The dominant thermal mechanisms in the neutral interstellar
medium, which acts as a star-forming gas reservoir, are uncertain in extremely
metal-poor galaxies. Our objective is to identify the heating mechanisms in one
such galaxy, IZw18, and assess the diagnostic value of fine-structure cooling
lines. We also seek to constrain the mass of H$_2$, which, despite being an
important catalyst and tracer of star formation, remains elusive in this
object. Building on a previous photoionization model within a multi-sector
topology, we provide additional constraints from the CII and OI lines and
the dust mass recently measured with Herschel. The heating of the HI region
appears to be mainly due to photoionization by radiation from a bright X-ray
binary source, while photoelectric effect (PE) is negligible. The CII and
OI lines imply an average X-ray luminosity of $4\times10^40$ erg s$^-1$,
while the NeV upper limits bring strong constraints to the soft X-ray flux
arising from the binary. A negligible amount of H$_2$ is predicted, but
$łesssim10^7$ M$_ødot$ of H$_2$ may be hidden in sufficiently dense clouds of
order $łesssim10$ pc in size. Regardless of the presence of significant
amounts of H$_2$, CII and OI do not trace the so-called CO-dark gas, but
the almost purely atomic medium. Although the CII+OI/TIR ratio is close to
values found in more metal-rich sources, it cannot be safely used as a PE
heating efficiency proxy. This ratio seems to be kept stable due to a
correlation between the X-ray luminosity and the star-formation rate. We
propose that X-ray heating is an important process in extremely metal-poor
sources. The weak PE heating due to the low dust-to-gas ratio could be
compensated for by the larger occurrence and power of X-ray binaries in
low-metallicity galaxies. We speculate that X-ray heating may quench star
formation.
Nutzer