Kinematics of Extremely Metal-Poor Galaxies: Evidence for Stellar Feedback

Abstract

The extremely metal-poor (XMP) galaxies analyzed in a previous paper have large star-forming regions with a metallicity lower than the rest of the galaxy. Such a chemical inhomogeneity reveals the external origin of the metal-poor gas fueling star formation, possibly indicating accretion from the cosmic web. This paper studies the kinematic properties of the ionized gas in these galaxies. Most XMPs have rotation velocity around a few tens of km s −1 . The star-forming regions appear to move coherently. The velocity is constant within each region, and the velocity dispersion sometimes increases within the star-forming clump towards the galaxy midpoint, suggesting inspiral motion toward the galaxy center. Other regions present a local maximum in velocity dispersion at their center, suggesting a moderate global expansion. The Hα line wings show a number of faint emission features with amplitudes around a few percent of the main Hα component, and wavelength shifts between 100 and 400 km s −1 . The components are often paired, so that red and blue emission features with similar amplitudes and shifts appear simultaneously. Assuming the faint emission to be produced by expanding shell-like structures, the inferred mass loading factor (mass loss rate divided by star formation rate) exceeds 10. Since the expansion velocity exceeds by far the rotational and turbulent velocities, the gas may eventually escape from the galaxy disk. The observed motions involve energies consistent with the kinetic energy released by individual core-collapse supernovae. Alternative explanations for the faint emission have been considered and discarded.

Subaru/FOCAS IFU revealed the metallicity gradient of a local extremely metal-poor galaxy

“…More than 90% of EMPGs have diffuse structures in their immediate vicinity (Isobe et al 2021). Preceding studies suggest that EMPGs are star-forming regions in such diffuse host galaxies whose metallicities are higher than the metal-poor parts, indicating positive metallicity gradients when considered their whole structures (Sánchez Almeida et al 2013, Sánchez Almeida et al 2016, Olmo-García et al 2017. Our result suggests possible diversity of metallicity gradients in EMPGs.…”

Characterization of star-forming dwarf galaxies at 0.1 ≲z ≲ 0.9 in VUDS: probing the low-mass end of the mass-metallicity relation

“…Differently from high-mass (M * > 10 9 M ) galaxies, which show a continuous rate of star formation (SF), the most common scenario for dwarf galaxies is the cyclic bursty mode, as pointed out by theoretical models (e.g., Hopkins et al 2014;Sparre et al 2017) and observations (Guo et al 2016b). Intense SF episodes produce stellar feedback through strong winds and supernova, which heat and expel the surrounding gas in outflows, eventually resulting in a temporary quenching of SF on timescales of tens of Myr (e.g., Olmo-Garcia et al 2017;Pelupessy et al 2004). Then, the metal-enriched gas may be accreted back to the galaxy triggering new SF episodes.…”

Gas Accretion and Star Formation Rates

“…Figure 7 summarizes these results. Independent observations proof that the XMP galaxies rotate, and that the star-forming clumps of low metallicity are dynamically decoupled from the underlying disk (Olmo-García et al, 2016).…”

“…It is not uncommon to infer factors up to 10 in local dwarfs (Martin, 1999;Veilleux et al, 2005). An extreme case is presented by Olmo-García et al (2016), where faint multiple components in the wings of Hα are interpreted as produced by SN driven outflows with η often larger than 20.…”