The Apache Point Observatory Galactic Evolution Experiment (APOGEE) provides spectroscopic information of regions of the inner Milky Way, which are inaccessible to optical surveys. We present the ...first large study of the metallicity distribution of the innermost Galactic regions based on high-quality measurements for 7545 red giant stars within 4.5 kpc of the Galactic center, with the goal to shed light on the structure and origin of the Galactic bulge. Stellar metallicities are found, through multiple Gaussian decompositions, to be distributed in several components, which is indicative of the presence of various stellar populations such as the bar or the thin and the thick disks. Super-solar (Fe/H = +0.32) and solar (Fe/H = +0.00) metallicity components, tentatively associated with the thin disk and the Galactic bar, respectively, seem to be major contributors near the midplane. A solar-metallicity component extends outwards in the midplane but is not observed in the innermost regions. The central regions (within 3 kpc of the Galactic center) reveal, on the other hand, the presence of a significant metal-poor population (Fe/H = −0.46), tentatively associated with the thick disk, which becomes the dominant component far from the midplane ( kpc). Varying contributions from these different components produce a transition region at +0.5 kpc , characterized by a significant vertical metallicity gradient.
We derive age constraints for 1639 red giants in the APOKASC sample for which seismic parameters from Kepler, as well as effective temperatures, metallicities and α/Fe values from APOGEE DR12 (Apache ...Point Observatory Galactic Evolution Experiment Data Release 12) are available. We investigate the relation between age and chemical abundances for these stars, using a simple and robust approach to obtain ages. We first derive stellar masses using standard seismic scaling relations, then determine the maximum possible age for each star as function of its mass and metallicity, independently of its evolutionary stage. While the overall trend between maximum age and chemical abundances is a declining fraction of young stars with increasing α/Fe, at least 14 out of 241 stars with α/Fe >0.13 are younger than 6 Gyr. Five stars with α/Fe ≥0.2 have ages below 4 Gyr. We examine the effect of modifications in the standard seismic scaling relations, as well as the effect of very low helium fractions, but these changes are not enough to make these stars as old as usually expected for α-rich stars (i.e. ages greater than 8–9 Gyr). Such unusual α-rich young stars have also been detected by other surveys, but defy simple explanations in a galaxy evolution context.
ABSTRACT
We report evidence from APOGEE for the presence of a new metal-poor stellar structure located within ∼4 kpc of the Galactic Centre. Characterized by a chemical composition resembling those ...of low-mass satellites of the Milky Way, this new inner Galaxy structure (IGS) seems to be chemically and dynamically detached from more metal-rich populations in the inner Galaxy. We conjecture that this structure is associated with an accretion event that likely occurred in the early life of the Milky Way. Comparing the mean elemental abundances of this structure with predictions from cosmological numerical simulations, we estimate that the progenitor system had a stellar mass of ∼5 × 108 M⊙, or approximately twice the mass of the recently discovered Gaia-Enceladus/Sausage system. We find that the accreted:in situ ratio within our metal-poor (Fe/H < –0.8) bulge sample is somewhere between 1:3 and 1:2, confirming predictions of cosmological numerical simulations by various groups.
The Apache Point Observatory Galactic Evolution Experiment (APOGEE), one of the programs in the Sloan Digital Sky Survey III (SDSS-III), has now completed its systematic, homogeneous spectroscopic ...survey sampling all major populations of the Milky Way. After a three-year observing campaign on the Sloan 2.5 m Telescope, APOGEE has collected a half million high-resolution (R ∼ 22,500), high signal-to-noise ratio (>100), infrared (1.51-1.70 m) spectra for 146,000 stars, with time series information via repeat visits to most of these stars. This paper describes the motivations for the survey and its overall design-hardware, field placement, target selection, operations-and gives an overview of these aspects as well as the data reduction, analysis, and products. An index is also given to the complement of technical papers that describe various critical survey components in detail. Finally, we discuss the achieved survey performance and illustrate the variety of potential uses of the data products by way of a number of science demonstrations, which span from time series analysis of stellar spectral variations and radial velocity variations from stellar companions, to spatial maps of kinematics, metallicity, and abundance patterns across the Galaxy and as a function of age, to new views of the interstellar medium, the chemistry of star clusters, and the discovery of rare stellar species. As part of SDSS-III Data Release 12 and later releases, all of the APOGEE data products are publicly available.
We present long-slit spectroscopy of the elliptical galaxy M32, obtained with the 8 m Subaru telescope at Mauna Kea, the 1.5 m Tillinghast telescope at the F. L. Whipple Observatory, and the 4 m ...Mayall telescope at the Kitt Peak National Observatory. The spectra cover the Lick index red spectral region, as well as higher order Balmer lines in the blue. Spectra have been taken with the slit offset from the nucleus to avoid scattered-light contamination from the bright nucleus of M32. An analysis of numerous absorption features, particularly involving the Hg and Hb Balmer lines, reveals that systematic radial trends are evident in the integrated spectrum of M32. Population synthesis models indicate a radial change in both the age and chemical composition of the light-weighted mean stellar population in M32, from the nucleus out to 33'', i.e., an approximately 1.0 effective radius, Re. Specifically, the light-weighted mean stellar population at 1Re is older by ~3 Gyr and more metal-poor by about -0.25 dex in Fe/H than the central value of ~4 Gyr and Fe/H ~ 0.0. We show that this apparent population trend cannot be attributed to a varying contribution from either hot stars or emission-line contamination. The increase in age and decrease in metal abundance with radius are sufficiently well matched to explain the flat radial color profiles previously observed in M32. In addition, the ratio of Mg to Fe abundance, Mg/Fe, increases from about -0.25 in the nucleus to about -0.08 at 1Re. Finally, we find spuriously pronounced line-strength gradients in the Mayall data that are an artifact of scattered light from the bright nucleus. Scattered-light issues may explain the lack of consistency among previously published studies of radial line strength gradients in M32.
We present population synthesis models designed to represent the star formation histories of L* red sequence galaxies (RSGs). Earlier work has shown that single-burst stellar populations (SSPs) are ...unable to match Balmer line strengths simultaneously at high and low redshift. We therefore consider alternative star formation histories in which RSGs contain intermediate-aged stars even at late epochs. The models are compared to Balmer Hd sub(F) absorption strengths, U - B color data, and the number density of red sequence galaxies from z = 1 to z = 0. We find that quenched models, models of constant star formation histories truncated at regularly spaced intervals, average to an RSG population that matches the data well, showing slow evolution in color and Balmer line strength and a rise in number density by a factor of a few after z = 1. The data are best fit by a turn-on of quenching at redshifts z = 1.5-2.
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 plus or minus 5.47 km s super( -1), and a mean Fe/H of -0.15 plus or minus 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.
ABSTRACT The SDSS-III/Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey operated from 2011-2014 using the APOGEE spectrograph, which collects high-resolution (R ∼ 22,500), ...near-IR (1.51-1.70 m) spectra with a multiplexing (300 fiber-fed objects) capability. We describe the survey data products that are publicly available, which include catalogs with radial velocity, stellar parameters, and 15 elemental abundances for over 150,000 stars, as well as the more than 500,000 spectra from which these quantities are derived. Calibration relations for the stellar parameters ( , , M/H, /M) and abundances (C, N, O, Na, Mg, Al, Si, S, K, Ca, Ti, V, Mn, Fe, Ni) are presented and discussed. The internal scatter of the abundances within clusters indicates that abundance precision is generally between 0.05 and 0.09 dex across a broad temperature range; it is smaller for some elemental abundances within more limited ranges and at high signal-to-noise ratio. We assess the accuracy of the abundances using comparison of mean cluster metallicities with literature values, APOGEE observations of the solar spectrum and of Arcturus, comparison of individual star abundances with other measurements, and consideration of the locus of derived parameters and abundances of the entire sample, and find that it is challenging to determine the absolute abundance scale; external accuracy may be good to 0.1-0.2 dex. Uncertainties may be larger at cooler temperatures ( ). Access to the public data release and data products is described, and some guidance for using the data products is provided.
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