Chemical enrichment and physical conditions in I Zw 18

Abstract

Context. Low-metallicity star-forming dwarf galaxies are prime targets to understand the chemical enrichment of the interstellar medium. The H  region contains the bulk of the mass in blue compact dwarfs, and it provides important constraints on the dispersal and mixing of heavy elements released by successive star-formation episodes. The metallicity of the H  region is also a critical parameter to investigate the future star-formation history, as metals provide most of the gas cooling that will facilitate and sustain star formation. Aims. Our primary objective is to study the enrichment of the H  region and the interplay between star-formation history and metallicity evolution. Our secondary objective is to constrain the spatial-and time-scales over which the H  and H  regions are enriched, and the mass range of stars responsible for the heavy element production. Finally, we aim to examine the gas heating and cooling mechanisms in the H  region. Methods. We observed the most metal-poor star-forming galaxy in the Local Universe, I Zw 18, with the Cosmic Origin Spectrograph onboard Hubble. The abundances in the neutral gas are derived from far-ultraviolet absorption-lines (H , C , C *, N , O , ...) and are compared to the abundances in the H  region. Models are constructed to calculate the ionization structure and the thermal processes. We investigate the gas cooling in the H  region through physical diagnostics drawn from the fine-structure level of C + . Results. We find that H  region abundances are lower by a factor of ∼2 as compared to the H  region. There is no differential depletion on dust between the H  and H  region. Using sulfur as a metallicity tracer, we calculate a metallicity of 1/46 Z (vs. 1/31 Z in the H  region). From the study of the C/O, [O/Fe], and N/O abundance ratios, we propose that C, N, O, and Fe are mainly produced in massive stars. We argue that the H  envelope may contain pockets of pristine gas with a metallicity essentially null. Finally, we derive the physical conditions in the H  region by investigating the C * absorption line. The cooling rate derived from C * is consistent with collisions with H 0 atoms in the diffuse neutral gas. We calculate the star-formation rate from the C * cooling rate assuming that photoelectric effect on dust is the dominant gas heating mechanism. Our determination is in good agreement with the values in the literature if we assume a low dust-to-gas ratio (∼2000 times lower than the Milky Way value).

Hubble Space Telescope Observations of Accretion-Induced Star Formation in the Tadpole Galaxy Kiso 5639

“…I Zw 18 is one of the lowest metallicity local galaxies (Searle & Sargent 1972;Izotov et al 2009). Our estimates for supernovae clearing in Kiso 5639 are consistent with similar conclusions about star formation in I Zw 18 based on HST WFPC2 images (Hunter & Thronson 1995;Annibali et al 2013;Lebouteiller et al 2013).…”

Star formation sustained by gas accretion

“…2.2) and therefore large velocities must be associated with outflows. The only direct way of assessing accretion, however, is by observing the gas in absorption in the starlight spectrum emitted by the galaxy (e.g., Lebouteiller et al 2013)-since the gas is in between galaxy and observer, redshift implies infall.…”

Gas Accretion and Star Formation Rates

“…Sometimes the metallicity of the H I gas can be measured directly using UV lines in absorption against the stellar light of the galaxy. In the case of IZw 18, Lebouteiller et al (2013) find that H I region abundances are also around 1 % of the solar value.…”

“…Thus, gas inflows have been detected in absorption against stellar spectra (e.g., Rubin & MaNGA Team, 2016), but they are probably associated with metal rich gas returning to the galaxy after being ejected by a starburst or an AGN (e.g., Sancisi et al, 2008;Marasco et al, 2012;Fraternali, 2014). Neutral gas has been studied in absorption in several metal-poor objects (Aloisi et al, 2003;Cannon et al, 2005;Lebouteiller et al, 2009Lebouteiller et al, , 2013. However, the relative velocity between the gas absorption and the systemic velocity of the galaxy is too small to tell whether the gas is falling in or going out.…”

“…However, the relative velocity between the gas absorption and the systemic velocity of the galaxy is too small to tell whether the gas is falling in or going out. If anything, the gas seems to be flowing out with a mild velocity between 10 and 20 km s −1 (Lebouteiller et al, 2013). The origin of this putative metal-poor outflow is puzzling.…”