On the Oxygen and Nitrogen Chemical Abundances and the Evolution of the “Green Pea” Galaxies

Abstract

We have investigated the oxygen and nitrogen chemical abundances in extremely compact starforming galaxies (SFGs) with redshifts between ∼0.11 and 0.35, popularly referred to as "green peas". Direct and strong-line methods sensitive to the N/O ratio applied to their Sloan Digital Sky Survey (SDSS) spectra reveal that these systems are genuine metal-poor galaxies, with mean oxygen abundances ∼ 20% solar. At a given metallicity these galaxies display systematically large N/O ratios compared to normal galaxies, which can explain the strong difference between our metallicities measurements and previous ones. While their N/O ratios follow the relation with stellar mass of local SFGs in the SDSS, we find that the mass-metallicity relation of the "green peas" is offset 0.3 dex to lower metallicities. We argue that recent interaction-induced inflow of gas, possibly coupled with a selective metal-rich gas loss, driven by supernova winds, may explain our findings and the known galaxy properties, namely high specific star formation rates, extreme compactness, and disturbed optical morphologies. The "green pea" galaxy properties seem to be not common in the nearby universe, suggesting a short and extreme stage of their evolution. Therefore, these galaxies may allow us to study in great detail many processes, such as starburst activity and chemical enrichment, under physical conditions approaching those in galaxies at higher redshifts.

On the universality of luminosity–metallicity and mass–metallicity relations for compact star-forming galaxies at redshifts 0 < z < 3

“…The mass-metallicity relation by Tremonti et al (2004) is often used as a benchmark for comparisons with similar relations obtained for low-z and high-z galaxies (e.g. Lee et al 2006;Maiolino et al 2008;Amorín et al 2010;Manucci et al 2010;Maier et al 2014;Steidel et al 2014;Zahid et al 2011Zahid et al , 2012Zahid et al , 2013Zahid et al , 2014a. However, we note that Tremonti et al (2004) selected all star-forming galaxies, including galaxies with low-excitation H ii regions, while only galaxies with high-excitation H ii regions are present in high-z samples.…”

Star formation sustained by gas accretion

“…They have some of the highest specific SFRs in the local Universe, able to double their stellar masses in a fraction of Gyr. GPs are low-metallicity outliers of the mass-metallicity relationship (Amorín et al 2010(Amorín et al , 2012a. Detailed analyses of their emission lines reveal complex kinematical structures with several components coexisting in only a few kpc (Amorín et al 2012b).…”

“…Even though GPs have low oxygen metallicity, they present an overabundance of N/O which is typical of aging stellar populations. This puzzling observation is naturally explained if GPs have recently received a major flood of low-metallicity gas (Amorín et al 2010(Amorín et al , 2012a. Then the mixing with metalpoor gas reduces the metallicity (i.e., O/H), but the ratio between metal species (N/O) remains as in the original high-metallicity ISM.…”

Evolution of N/O abundance ratios and ionization parameters from z ∼ 0 to 2 investigated by the direct temperature method

“…Here, Queyrel et al (2009) and Yabe et al (2015) have derived the N/O values based on the z ∼ 0 empirical relation with the ratios of the strong emission lines of [N II] λ6584/[S II] λλ6716,6731 (N2S2). This N2S2 index is calibrated by and Amorín et al (2010). Masters et al (2014) derive the N/O ratio, assuming that the electron temperature (Te) is 10,000 K. However, with local SDSS galaxies, the strong line method gives N/O values systematically higher than the reliable direct temperature (Te) method (see Section 5 for more details).…”