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2006ApJ...637..269V - Astrophys. J., 637, 269-282 (2006/January-3)

Oxygen and nitrogen in Leo A and GR 8.


Abstract (from CDS):

We present elemental abundances for multiple H II regions in Leo A and GR 8 obtained from long-slit optical spectroscopy of these two nearby low-luminosity dwarf irregular galaxies. As expected from their luminosities, and in agreement with previous observations, the derived oxygen abundances are extremely low in both galaxies. High signal-to-noise ratio (S/N) observations of a planetary nebula in Leo A yield 12+log(O/H)=7.30±0.05; semiempirical calculations of the oxygen abundance in four H II regions in Leo A indicate 12+log(O/H)=7.38±0.10. These results confirm that Leo A has one of the lowest ISM metal abundances of known nearby galaxies. Based on results from two H II regions with high S/N measurements of the weak [O III] λ4363 line, the mean oxygen abundance of GR 8 is 12+log(O/H)=7.65±0.06; using ``empirical'' and ``semiempirical'' methods, similar abundances are derived for six other GR 8 H II regions. Similar to previous results in other low-metallicity galaxies, the mean log(N/O)=-1.53±0.09 for Leo A and -1.51±0.07 for GR 8. There is no evidence of significant variations in either O/H or N/O in the H II regions. The metallicity-luminosity relation for nearby (D<5 Mpc) dwarf irregular galaxies with measured oxygen abundances has a mean correlation of 12+log(O/H)=5.67MB-0.151MB, with a dispersion in oxygen about the relationship of σ=0.21. These observations confirm that gas-rich, low-luminosity galaxies have extremely low elemental abundances in the ionized gas phase of their interstellar media. Although Leo A has one of the lowest metal abundances of known nearby galaxies, detection of tracers of an older stellar population (RR Lyrae variable stars, horizontal branch stars, and a well-populated red giant branch) indicate that it is not a newly formed galaxy, as has been proposed for some other similar low-metallicity star-forming galaxies.

Abstract Copyright:

Journal keyword(s): Galaxies: Abundances - Galaxies: Dwarf - Galaxies: Evolution - Galaxies: Individual: Name: Leo A - Galaxies: Individual: Alphanumeric: GR 8 - Galaxies: Irregular

Nomenclature: Paragr. 2.1: [VSH2006] N=2. Fig. 2, Table 3: [VSH2006] Leo A +XXX+YYY N=5. Fig. 3, Table 4: [VSH2006] GR 8 +XXX+YYY N=8.

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