Star Formation and Molecular Gas in Extremely Metal-poor Galaxies: Insights from the Thermal Balance in the Neutral Gas

TL;DR

This study investigates the heating mechanisms and molecular gas properties in extremely metal-poor dwarf galaxies, revealing the dominant role of X-ray sources and implications for star formation and gas tracers.

Contribution

It introduces a combined optical and infrared analysis of IZw18, highlighting the impact of X-ray heating on molecular gas and star formation in metal-poor environments.

Findings

HI region heated by ultraluminous X-ray source

Constraints on H2 clump sizes for future detection

X-ray source signatures inform ISM and star formation processes

Abstract

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.