Implications of elemental abundances in dwarf spheroidal galaxies

National Astronomical Observatory, Mitaka-shi, Tokyo 181-8588, Japan e-mail: taku.tsujimoto@nao.ac.jp

Received: 30 August 2005
Accepted: 17 September 2005

Abstract

Unusual elemental abundance patterns observed for stars belonging to nearby dwarf spheroidal (dSph) galaxies are discussed. Analysis of the [ α/H] vs. [Fe/H] diagrams where α represents Mg or an average of α-elements reveals that Fe from type Ia supernovae (SNe Ia) does not contribute to the stellar abundances in the dSph galaxies where the member stars exhibit low α/Fe ratios except for the most massive dSph galaxy, Sagitarrius. The more massive dwarf (irregular) galaxy, the Large Magellanic Cloud, also has an SNe Ia signature in the stellar abundances. These findings suggest that whether SNe Ia contribute to chemical evolution in dwarf galaxies is likely to depend on the mass scale of galaxies. Unusual Mg abundances in some dSph stars are also found to be the origin of the large scatter in the [Mg/Fe] ratios and are responsible for a seemingly decreasing [Mg/Fe] feature with increasing [Fe/H]. In addition, the lack of massive stars in the dSph galaxies does not satisfactorily account for the low-α signature. Considering the assembly of deficient elements (O, Mg, Si, Ca, Ti and Zn), all of which are synthesized in pre-SN massive stars and in SN explosions, the low-α signature appears to reflect the heavy-element yields of massive stars with less rotation compared to solar neighborhood stars.