Star Formation and Molecular Gas in Extremely Metal-poor Galaxies:
  Insights from the Thermal Balance in the Neutral Gas
  The apparent lack of cold molecular gas in blue compact dwarf (BCD) galaxies
is at variance with their intense star-formation episode. The CO molecule,
often used a tracer of H2 through a conversion function, is selectively
photodissociated in dust-poor environments and, as a result, a potentially
large fraction of H2 is expected to reside in the so-called CO-dark gas, where
it could be traced instead by infrared cooling lines [CI], [CII], and [OI].
Although the fraction of CO-dark gas to total molecular gas is expected to be
relatively large in metal-poor galaxies, many uncertainties remain due to the
difficulty in identifying the main heating mechanism associated to the cooling
lines observed in such galaxies. Investigations of the Herschel Dwarf Galaxy
Survey show that the heating mechanism in the neutral gas of BCDs cannot be
dominated by the photoelectric effect on dust grains below some threshold
metallicity, implying that other heating mechanisms need to be invoked, along
with a new interpretation of the corresponding infrared line diagnostics. In
the study presented here and in Lebouteiller et al. 2017, we use optical and
infrared lines to constrain the physical conditions in the HII region + HI
region of the BCD IZw18 (18Mpc; 2% solar metallicity) within a consistent
photoionization and photodissociation model. We show that the HI region is
entirely heated by a single ultraluminous X-ray source with important
consequences on the applicability of [CII] to trace the star-formation rate and
to trace the CO-dark gas. We derive stringent upper limits on the size of H2
clumps that may be detected in the future with JWST and IRAM/NOEMA. We also
show that the nature of the X-ray source can be inferred from the corresponding
signatures in the ISM. Finally we speculate that star formation may be quenched
in extremely metal-poor dwarf galaxies due to X-ray photoionization.
