Argon, silicon and iron abundances in the damped Ly-αsystem I Zw 18

Abstract

Abstract. We show how the ratio of the abundances of Ar, Si and Fe as derived from FUSE absorption spectra of I Zw 18 are affected by large-scale stochastic velocity fields (mesoturbulence). 21-cm observations with high spatial resolution show the presence of velocity fields with large correlation lengths. FUSE observations were performed with a large entrance aperture which fully covered the galaxy. This means that the observed profiles are averaged over the full body of I Zw 18. Taking this into account, we show that the observations are consistent with the abundance ratios [α-chain/Fe] being similar to those derived from Galactic metal-poor stars. Using a mesoturbulent approach, we found that the observed profiles can be reproduced well with IntroductionThe blue compact galaxy (BCG) I Zw 18 (Mrk 116) has been an intensively studied object for the last three decades since the first spectroscopic observations by Zwicky (1966). I Zw 18 shows a powerful and recent burst of star formation which makes this galaxy an attractive target for studies of the history of star formation. The neutral gas in I Zw 18 has been observed in the 21-cm line (see van Zee et al. 1998 and references cited therein) as well as in several UV lines (Kunth et al. 1994;Pettini & Lipman 1995;Vidal-Madjar et al. 2000). High velocity and high spatial resolution radio observations have shown that the overall kinematics of the H i gas associated with I Zw 18 is very complex and that the neutral gas velocity dispersion σ equals 13-14 km s −1 , or b ≡ √ 2σ = 18-20 km s −1 (van Zee et al. 1998). The H i column density in front of the bright star clusters in I Zw 18 deduced from the Ly-α absorption profile by Kunth et al., N H i = (3.5 ± 0.5) × 10 21 cm −2 , is comparable to the peak H i surface density found by van Zee et al., N H i 3.0 × 10 21 cm −2 . This high value is such that the neutral gas in I Zw 18 can be considered as a local damped Ly-α system (DLA), similar to high-redshift DLAs observed in the light of many background quasars.The first HST measurement of the (O/H) abundance in the H i region has indicated a possible discrepancy between the metal content in the neutral gas and in the H ii regions (Kunth et al. 1994). This, however, was not confirmed in later studies by Pettini & Lipman (1995) and by van Zee et al. (1998) who have shown that both the neutral and ionized gas in I Zw 18 may have the same oxygen abundance.Article published by EDP Sciences and available at http://www.aanda.org or http://dx

Multiple starbursts in Blue Compact Galaxies

“…Results by Pettini & Lipman (1995) and van Zee et al (1998) seem to indicate indeed a metallicity comparable to the abundance in the H ii regions. A recent work of Levshakov et al (2001) about argon and silicon emission line profiles in IZw18 also reveals a good degree of mixing between hot and cold regions. Future FUSE abundance determinations will certainly shed more light on the problem.…”

“…Recently, Levshakov et al (2001) with their mesoturbulent approach, found higher overabundances of α elements (they found [Si, Ar/Fe] = 0.7 ± 0.1). Since there are some indications that the WW yields of iron are overestimated by a factor of 2 (Timmes et al 1995), we tried to calculate α/Fe ratios with reduced iron yields from massive stars and the results for [Mg/Fe] and [Si/Fe] evolution are shown in Fig.…”

“…However this effect, already present in the one-burst case, is less strong here. Levshakov et al (2001). -Finally, we computed the abundances of metals which should pertain to the HI gas and estimated that they are roughly lower by a factor of 2 compared to the abundances in the star forming region (H ii), a prediction which should be tested on new FUSE data.…”

Continuous star formation in IZw18

“…established that these authors overestimated the solar oxygen composition (Holweger 2001). We keep this value since, due to the difficulties in the estimation of the iron content in IZw18 (see e.g., Izotov et al 1997b;Levshakov et al 2001) …”

Nitrogen-deficient and iron-rich associated absorbers with oversolar metallicities towards the quasar HE0141-3932

“…Since these systems arise in gas ejected from the circumnuclear region, careful analysis of the element abundances can produce important clues about the enrichment mechanisms and physical processes leading to the QSO/AGN phenomena. The numbers presented in Table 1 are obtained with the Monte Carlo Inversion (MCI) method -an advanced computational procedure designed to estimate the physical parameters of optically thin absorbers with varying gas density and turbulent velocity field (Levshakov et al 2000).We conclude the following:• The analyzed absorption systems show high iron content, [Fe/C] = 0.15 ± 0.1, [Fe/Mg] = 0.0 ± 0.1 (z abs =1.78), but at the same time nitrogen is strongly underabundant, [N/C] < ∼ −0.5 (z abs =1.68). These abundances are estimated in different associated systems but they are representative for the bulk of circumnuclear gas since the mass of the stellar population involved in the enrichment of quasar's circumnuclear region is high, > 10 4 M , (Baldwin et al 2003) and, hence, large metallicity gradients and sharp discontinuities due to enrichment by only a few stars are unlikely.…”