Browsing by Author "Oravetz, Daniel"

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    Do galaxy global relationships emerge from local ones? The SDSS IV MaNGA surface mass density-metallicity relation
    (OXFORD UNIV PRESS, 2016) Barrera Ballesteros, Jorge K.; Heckman, Timothy M.; Zhu, Guangtun B.; Zakamska, Nadia L.; Sanchez, Sebastian F.; Law, David; Wake, David; Green, Jenny E.; Bizyaev, Dmitry; Oravetz, Daniel; Simmons, Audrey; Malanushenko, Elena; Pan, Kaike; Roman Lopes, Alexandre; Lane, Richard R.
    We present the stellar surface mass density versus gas metallicity (Sigma(*)-Z) relation for more than 500 000 spatially resolved star-forming resolution elements (spaxels) from a sample of 653 disc galaxies included in the SDSS IV MaNGA survey. We find a tight relation between these local properties, with higher metallicities as the surface density increases. This relation extends over three orders of magnitude in the surface mass density and a factor of 4 in metallicity. We show that this local relationship can simultaneously reproduce two well-known properties of disc galaxies: their global mass-metallicity relationship and their radial metallicity gradients. We also find that the Sigma(*)-Z relation is largely independent of the galaxy's total stellar mass and specific star formation rate (sSFR), except at low stellar mass and high sSFR. These results suggest that in the present-day universe local properties play a key role in determining the gas-phase metallicity in typical disc galaxies.
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    Two groups of red giants with distinct chemical abundances in the bulge globular cluster NGC 6553 through the eyes of APOGEE
    (OXFORD UNIV PRESS, 2017) Tang, Baitian; Cohen, Roger E.; Geisler, Doug; Schiavon, Ricardo P.; Majewski, Steven R.; Villanova, Sandro; Carrera, Ricardo; Zamora, Olga; Garcia Hernandez, D. A.; Shetrone, Matthew; Frinchaboy, Peter; Meza, Andres; Fernandez Trincado, J. G.; Munoz, Ricardo R.; Lin, Chien Cheng; Lane, Richard R.; Nitschelm, Christian; Pan, Kaike; Bizyaev, Dmitry; Oravetz, Daniel; Simmons, Audrey
    Multiple populations revealed in globular clusters (GCs) are important windows to the formation and evolution of these stellar systems. The metal-rich GCs in the Galactic bulge are an indispensable part of this picture, but the high optical extinction in this region has prevented extensive research. In this work, we use the high-resolution near-infrared (NIR) spectroscopic data from Apache Point Observatory Galactic Evolution Experiment (APOGEE) to study the chemical abundances of NGC 6553, which is one of the most metal-rich bulge GCs. We identify 10 red giants as cluster members using their positions, radial velocities, iron abundances, and NIR photometry. Our sample stars show a mean radial velocity of -0.14 +/- 5.47 km s(-1), land a mean [Fe/H] of -0.15 +/- 0.05. We clearly separate two populations of stars in C and N in this GC for the first time. NGC 6553 is the most metal-rich GC where the multiple stellar population phenomenon is found until now. Substantial chemical variations are also found in Na, O, and Al. However, the two populations show similar Si, Ca, and iron-peak element abundances. Therefore, we infer that the CNO, NeNa, and MgAl cycles have been activated, but the MgAl cycle is too weak to show its effect on Mg. Type Ia and Type II supernovae do not seem to have significantly polluted the second generation stars. Comparing with other GC studies, NGC 6553 shows similar chemical variations as other relatively metal-rich GCs. We also confront current GC formation theories with our results, and suggest possible avenues for improvement in the models.