Metallicity distribution functions, radial velocities, and alpha element abundances in three off-axis bulge fields

Johnson, Christian I. and Rich, R. Michael and Kobayashi, Chiaki and Kunder, Andrea and Pilachowski, Catherine A. and Koch, Andreas and Propris, R. De (2013) Metallicity distribution functions, radial velocities, and alpha element abundances in three off-axis bulge fields. The Astrophysical Journal, 765 (2): 157. ISSN 0004-637X

Abstract

We present radial velocities and chemical abundance ratios of [Fe/H], [O/Fe], [Si/Fe], and [Ca/Fe] for 264 red giant branch stars in three Galactic bulge off-axis fields located near (l, b) = (–5.5, –7), (–4, –9), and (+8.5, +9). The results are based on equivalent width and spectrum synthesis analyses of moderate resolution (R ≈ 18,000), high signal-to-noise ratio (S/N ~ 75-300 pixel-1) spectra obtained with the Hydra spectrographs on the Blanco 4 m and WIYN 3.5 m telescopes. The targets were selected from the blue side of the giant branch to avoid cool stars that would be strongly affected by CN and TiO; however, a comparison of the color-metallicity distribution in literature samples suggests that our selection of bluer targets should not present a significant bias against metal-rich stars. We find a full range in metallicity that spans [Fe/H] ≈–1.5 to +0.5, and that, in accordance with the previously observed minor-axis vertical metallicity gradient, the median [Fe/H] also declines with increasing Galactic latitude in off-axis fields. The off-axis vertical [Fe/H] gradient in the southern bulge is estimated to be ~0.4 dex kpc-1; however, comparison with the minor-axis data suggests that a strong radial gradient does not exist. The (+8.5, +9) field exhibits a higher than expected metallicity, with a median [Fe/H] = –0.23, that might be related to a stronger presence of the X-shaped bulge structure along that line-of-sight. This could also be the cause of an anomalous increase in the median radial velocity for intermediate metallicity stars in the (+8.5, +9) field. However, the overall radial velocity and dispersion for each field are in good agreement with recent surveys and bulge models. All fields exhibit an identical, strong decrease in velocity dispersion with increasing metallicity that is consistent with observations in similar minor-axis outer bulge fields. Additionally, the [O/Fe], [Si/Fe], and [Ca/Fe] versus [Fe/H] trends are identical among our three fields, and are in good agreement with past bulge studies. We find that stars with [Fe/H] lesssim –0.5 are α-enhanced, and that the [α/Fe] ratios decline at higher metallicity. At [Fe/H] lesssim 0, the α-element trends are indistinguishable from the halo and thick disk, and the variations in the behavior of individual α-elements are consistent with production in massive stars and a rapid bulge formation timescale.