Oxygen and Nitrogen in Isolated Dwarf Irregular Galaxies

Abstract

We present long slit optical spectroscopy of 67 HII regions in 21 dwarf irregular galaxies to investigate the enrichment of oxygen, nitrogen, neon, sulfur, and argon in low mass galaxies. Oxygen abundances are obtained via direct detection of the temperature sensitive emission lines for 25 HII regions; for the remainder of the sample, oxygen abundances are estimated from strong line calibrations. The direct abundance determinations are compared to the strong-line abundance calibrations of both McGaugh (1991) and Pilyugin (2000). Global oxygen and nitrogen abundances for this sample of dwarf irregular galaxies are examined in the context of open and closed box chemical evolution models. While several galaxies are consistent with closed box chemical evolution, the majority of this sample have an effective yield ~1/4 of the expected yield for a constant star formation rate and Salpeter IMF, indicating that either outflow of enriched gas or inflow of pristine gas has occurred. The effective yield strongly correlates with M_H/L_B in the sense that gas-rich galaxies are more likely to be closed systems. However, the effective yield does not appear to correlate with other global parameters such as dynamical mass, absolute magnitude, star formation rate or surface brightness. A correlation is found between the observed nitrogen-to-oxygen ratio and the color of the underlying stellar population; redder dwarf irregular galaxies have higher N/O ratios than blue dwarf irregular galaxies. The relative abundance ratios are interpreted in the context of delayed release of nitrogen and varied star formation histories.Comment: 47 pages, 11 figures. Accepted to Ap

THE PRIMORDIAL DEUTERIUM ABUNDANCE OF THE MOST METAL-POOR DAMPED Lyα SYSTEM∗

“…We also note that the [N/O] ratio is well-determined in this DLA, since both N I and O I trace the H I bearing gas due to charge transfer reactions (Field & Steigman 1971;Steigman et al 1971). Our value of [N/O] is consistent with, or slightly lower than, the primary N/ O plateau (Izotov & Thuan 1999;Centurión et al 2003;van Zee & Haynes 2006;Petitjean et al 2008;Pettini et al 2008;Pettini & Cooke 2012b;Zafar et al 2014).…”

Oxygen Abundance Measurements of Shield Galaxies

“…The lines represent fits described in the text and Table 6. Tremonti et al 2004;Salzer et al 2005b;Lee et al 2006;van Zee & Haynes 2006;Berg et al 2012;Haurberg et al 2013;Skillman et al 2013). The underlying cause of this relationship is not fully understood, but it is believed to trace a more fundamental M-Z relationship, where luminosity is representative of stellar mass.…”

“…A promising alternative method for finding XMD galaxies has been to study very-low-luminosity dwarfs since there is a historically well-established correlation between metallicity and luminosity among low-redshift dwarf galaxies (Lequeux et al 1979;Skillman et al 1989a;Pilyugin 2001;Melbourne & Salzer 2002;Tremonti et al 2004;Salzer et al 2005a; Lee et al 2006;van Zee & Haynes 2006). This method has already led to the discovery of several XMD galaxies (Skillman et al 1989a(Skillman et al , 1989bvan Zee 2000;Pustilnik et al 2005Pustilnik et al , 2011Berg et al 2012;Skillman et al 2013).…”

THE FIRST OBSERVATIONS OF LOW-REDSHIFT DAMPED Lyα SYSTEMS WITH THE COSMIC ORIGINS SPECTROGRAPH: CHEMICAL ABUNDANCES AND AFFILIATED GALAXIES

“…Abundance measurements of N have been measured previously using optical spectra of H ii regions of nearby lowluminosity galaxies, where N is mainly produced from primary production, by Van Zee et al (2006) and show a flat plateau in [N/O] ratio with respect to metallicity. This ratio was also measured in higher metallicity H ii regions in nearby spiral galaxies, where more secondary production occurs, by Van Zee et al (1998) and these show a steady increase in [N/O] for larger metallicities.…”

“…An L * galaxy with a diameter of 30 kpc placed at a redshift of z abs = 0.1 would be very bright, m i = 17.1, and have sufficient angular separation (13. 4) to be distinguishable from the large point-spread function [α/H] for the eight systems discussed in this paper (red points), high-redshift DLAs from Pettini et al (2008), H ii regions of nearby dwarf galaxies from Van Zee et al (2006), the low-redshift sub-DLA system from Tripp et al (2005), the low-redshift DLA from Bowen et al (2005), and H ii regions in nearby spiral galaxies from Van Zee et al (1998). Correction factors have been applied here for the sub-DLAs.…”