Metal enrichment of the neutral gas of blue compact dwarf galaxies: the compelling case of Pox 36

Abstract

Context. Evidence has grown over the past few years that the neutral phase of blue compact dwarf (BCD) galaxies may be metaldeficient as compared to the ionized gas of their H ii regions. These results have strong implications for our understanding of the chemical evolution of galaxies, and it is essential to strengthen the method, as well as to find possible explanations. Aims. We present the analysis of the interstellar spectrum of Pox 36 with the Far Ultraviolet Spectroscopic Explorer (FUSE). Pox 36 was selected because of the relatively low foreground gas content that makes it possible to detect absorption-lines weak enough that unseen components should not be saturated. Methods. Interstellar lines of Hii, Ar i, and Fe ii are detected. Column densities are derived directly from the observed line profiles except for H i, whose lines are contaminated by stellar absorption, thus needing the stellar continuum to be removed. We used the TLUSTY models to remove the stellar continuum and isolate the interstellar component. The best fit indicates that the dominant stellar population is B0. The observed far-UV flux agrees with an equivalent number of ∼300 B0 stars. The fit of the interstellar H i line gives a column density of 10 20.3±0.4 cm −2 . Chemical abundances were then computed from the column densities using the dominant ionization stage in the neutral gas. Our abundances are compared to those measured from emission-line spectra in the optical, probing the ionized gas of the H ii regions.Results. Our results suggest that the neutral gas of Pox 36 is metal-deficient by a factor ∼7 as compared to the ionized gas, and they agree with a metallicity of ≈1/35 Z . Elemental depletion is not problematic because of the low dust content along the selected lines of sight. In contrast, the ionized gas shows a clear depletion pattern, with iron being strongly depleted. Conclusions. The abundance discontinuity between the neutral and ionized phases implies that most of the metals released by consecutive star-formation episodes mixes with the H i gas. The volume extent of the enrichment is so large that the metallicity of the neutral gas increases only slightly. The star-forming regions could be enriched only by a small fraction (∼1%), but it would greatly enhance its metallicity. Our results are compared to those of other BCDs. We confirm the overall underabundance of metals in their neutral gas, with perhaps only the lowest metallicity BCDs showing no discontinuity.

Star formation sustained by gas accretion

“…2.4), and it is lower than the metallicity in the disk, the HI is likely cosmic gas being accreted. Lebouteiller et al (2009) find an even larger discrepancy between the HI and HII metallicities of Pox 36, where the HI metallicity is around 0.03 Z .…”

The dust content of high-zsubmillimeter galaxies revealed byHerschel

“…When computing M Z,gas , we assume the same metal abundance for the neutral and ionized gas phases. This can be false, as the former may be characterized by lower (by a factor of ∼7, Lebouteiller et al 2009) abundances than the latter. However, this effect does not change our results.…”

Chemical enrichment and physical conditions in I Zw 18

“…Since we use the information from all available lines in the wavelength range, the error bar can be significantly smaller than when considering a single line. We refer to L04 and Lebouteiller et al (2006Lebouteiller et al ( , 2009) for more details. The comparison between the column densities we derive and the FUSE results asks for caution.…”

“…The neutral gas chemical composition was derived in several BCDs, spanning a wide range in ionized gas metallicity, from 1/30 to 1/3 Z , with I Zw 18 (Aloisi et al 2003, hereafter A03; Lecavelier des Etangs et al 2004, hereafter L04), SBS 0335-052 , I Zw 36 (Lebouteiller et al 2004), Pox 36 (Lebouteiller et al 2009), Mark 59 (Thuan et al 2002), NGC 625 (Cannon et al 2005), and NGC 1705 (Heckman et al 2001;Lee & Skillman 2004). These investigations showed that the neutral gas of BCDs is not pristine; it has already been enriched with metals, with a minimal metallicity of ∼1/50 Z (see the FUSE sample analysis in Lebouteiller et al 2009). The second most important result is that the metallicity of the neutral gas is generally lower than that of the ionized gas − except for the two lowest-metallicity BCDs, I Zw 18 and SBS 0335-052E − implying that the H  region has been probably less processed than the H  region.…”