We present radial velocities and chemical abundance ratios of [ Fe/H ] , [ O/Fe ] , [ Si/Fe ] , and [ Ca/Fe ] for 264 red giant branch ( RGB ) 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 \approx 18,000 ) , high signal–to–noise ratio ( S/N \sim 75–300 pixel ^ { -1 } ) spectra obtained with the Hydra spectrographs on the Blanco 4m and WIYN 3.5m 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 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 ] \approx –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 \sim 0.4 dex kpc ^ { -1 } ; however , comparison with the minor–axis data suggests 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 \alpha –enhanced , and that the [ \alpha /Fe ] ratios decline at higher metallicity .
At [ Fe/H ] \lesssim 0 , the \alpha –element trends are indistinguishable from the halo and thick disk , and the variations in the behavior of individual \alpha –elements are consistent with production in massive stars and a rapid bulge formation timescale .