Heavy‐Element Abundances in Blue Compact Galaxies

Abstract

We present high-quality ground-based spectroscopic observations of 54 supergiant H II regions in 50 low-metallicity blue compact galaxies with oxygen abundances 12 ] log O/H between 7.1 and 8.3. We use the data to determine abundances for the elements N, O, Ne, S, Ar, and Fe. We also analyze Hubble Space T elescope (HST ) Faint Object Spectrograph archival spectra of 10 supergiant H II regions to derive C and Si abundances in a subsample of seven BCGs. The main result of the present study is that none of the heavy elementÈtoÈoxygen abundance ratios studied here ( This constancy (Z ¹ Z _ /20). implies that all of these heavy elements have a primary origin and are produced by the same massive (M º 10 stars responsible for O production. The dispersion of the ratios C/O and N/O in these M _ ) galaxies is found to be remarkably small, being only^0.03 and^0.02 dex, respectively. This very small dispersion is strong evidence against any time-delayed production of C and primary N in the lowest metallicity BCGs (secondary N production is negligible at these low metallicities). The absence of a timedelayed production of C and N is consistent with the scenario that galaxies with 12 ] log O/H ¹ 7.6 are now undergoing their Ðrst burst of star formation, and that they are therefore young, with ages not exceeding 40 Myr. If very low metallicity BCGs are indeed young, this would argue against the commonly held belief that C and N are produced by intermediate-mass (3 stars at very M _ ¹ M ¹ 9 M _ ) low metallicities, as these stars would not have yet completed their evolution in these lowest metallicity galaxies. In higher metallicity BCGs (7.6 \ 12 ] log O/H \ 8.2), the abundance ratios Ne/O, Si/O, S/O, Ar/O, and Fe/O retain the same constant value they had at lower metallicities. By contrast, there is an increase of C/O and N/O along with their dispersions at a given O. We interpret this increase as due to the additional contribution of C and primary N production in intermediate-mass stars, on top of that by high-mass stars. The above results lead to the following timeline for galaxy evolution : (1) all objects with 12 ] log O/H ¹ 7.6 began to form stars less than 40 Myr ago ; (2) after 40 Myr, all galaxies have evolved so that 12 ] log O/H [ 7.6 ; (3) by the time intermediate-mass stars have evolved and released their nucleosynthetic products (100È500 Myr), all galaxies have become enriched to 7.6 \ 12 ] log O/H \ 8.2. The delayed release of primary N at these metallicities greatly increases the scatter in N/O ; (4) later, when galaxies get enriched to 12 ] log O/H [ 8.2, secondary N production becomes important. BCGs show the same O/Fe overabundance with respect to the Sun (D0.4 dex) as Galactic halo stars, suggesting the same chemical enrichment history. We compare heavy elements yields derived from the observed abundance ratios with theoretical yields for massive stars and Ðnd general good agreement. However, the theoretical models are unable to reproduce the observed N/O and Fe/O. Further theoretical developments ...

Star Forming Dwarf Galaxies

“…One of the reasons may be that the stellar population in these galaxies manages to ionize a large part of the gas in the central region and consequently the measured metallicity will be lower than for a milder starburst. The most metal poor galaxies, the low luminosity dwarfs IZw18 and SBS0335-052, have metallicities of a few % solar.This is close to the metallicity derived for the neutral gas in these and other BCGs using UV spectroscopy [49,63,63,83,84,156]. It appears as if there is a minimum plateau defined by the mean metallicity of the intergalactic medium, i.e.…”

“…The sample has been collected from various publications [4,13,37,50,63,95,129,137,161,Bergvall et al,unpublished]. The classification is therefore not homogeneous and the sample is not in any sense complete.…”

The Sulfur Abundance Anomaly in Planetary Nebulae

“…However, S vs. O is an exception. (Izotov & Thuan 1999). In addition, we show our data for Type I and Type II PNe as open circles and squares, respectively.…”

The ESO UVES advanced data products quasar sample – III. Evidence of bimodality in the [N/α] distribution

“…The N-production duality is revealed when the measurements of N in H ii regions of spiral (van Zee et al 1998), BCD (Izotov & Thuan 1999, 2004, and emissionline galaxies (Nava et al 2006;Izotov et al 2006;Pérez-Montero & Contini 2009) are compared. In low metallicity environments N goes in lockstep with O, so that (N/O) remains approximately constant.…”

“…Measurements of nitrogen have been performed in different astrophysical sites. In H ii regions of spiral and dwarf-irregular galaxies c 2013 RAS (van Zee et al 1998;van Zee & Haynes 2006), metal-poor emission-line galaxies (Nava et al 2006;Izotov et al 2006;Pérez-Montero & Contini 2009), and H ii regions in blue compact dwarf (BCD) galaxies (Izotov & Thuan 1999, 2004, (N/O) ratios show a primary plateau at low oxygen abundances and a secondary behaviour for [O/H] > −1. Nitrogen abundances were also measured in low-metallicity galactic halo stars (Spite et al 2005) and H ii regions in Large Magellanic Cloud (Bekki & Tsujimoto 2010).…”