Chemodynamics of the Milky Way

Abstract

Context. The Apache Point Observatory Galactic Evolution Experiment (APOGEE) features the first multi-object high-resolution fiber spectrograph in the near-infrared ever built, thus making the survey unique in its capabilities: APOGEE is able to peer through the dust that obscures stars in the Galactic disc and bulge in the optical wavelength range. Here we explore the APOGEE data included as part of the Sloan Digital Sky Survey's 10th data release (SDSS DR10). Aims. The goal of this paper is to a) investigate the chemo-kinematic properties of the Milky Way disc by exploring the first year of APOGEE data; and b) to compare our results to smaller optical high-resolution samples in the literature, as well as results from lower resolution surveys such as the Geneva-Copenhagen Survey (GCS) and the RAdial Velocity Experiment (RAVE). Methods. We select a high-quality (HQ) sample in terms of chemistry (amounting to around 20 000 stars) and, after computing distances and orbital parameters for this sample, we employ a number of useful subsets to formulate constraints on Galactic chemical and chemodynamical evolution processes in the solar neighbourhood and beyond (e.g., metallicity distributions -MDFs, [α/Fe] vs. [Fe/H] diagrams, and abundance gradients). Results. Our red giant sample spans distances as large as 10 kpc from the Sun. Given our chemical quality requirements, most of the stars are located between 1 and 6 kpc from the Sun, increasing by at least a factor of eight the studied volume with respect to the most recent chemodynamical studies based on the two largest samples obtained from RAVE and the Sloan Extension for Galactic Understanding and Exploration (SEGUE). We find remarkable agreement between the MDF of the recently published local (d < 100 pc) high-resolution high-S/N HARPS sample and our local HQ sample (d < 1 kpc). The local MDF peaks slightly below solar metallicity, and exhibits an extended tail towards [Fe/H] = −1, whereas a sharper cutoff is seen at larger metallicities (the APOGEE sample shows a slight overabundance of stars with metallicities larger than +0.3 with respect to the HARPS sample). Both samples also compare extremely well in an [α/Fe] vs. [Fe/H] diagram. The APOGEE data also confirm the existence of a gap in the abundance diagram. When expanding our sample to cover three different Galactocentric distance bins (inner disc, solar vicinity and outer disc), we find the high-[α/Fe] stars to be rare towards the outer zones (implying a shorter scale-length of the thick disc with respect to the thin disc), as previously suggested in the literature. Finally, we measure the gradients in [Fe/H] and [α/Fe], and their respective MDFs, over a range of 6 < R < 11 kpc in Galactocentric distance, and a 0 < z < 3 kpc range of distance from the Galactic plane. We find a good agreement with the gradients traced by the GCS and RAVE dwarf samples. For stars with 1.5 < z < 3 kpc (not present in the previous samples), we find a positive metallicity gradient and a negative gradient in [α/Fe].

Constraining the Milky Way assembly history with Galactic Archaeology

“…For this particular simulation the mean age decreases from ∼ 10.5 to ∼ 6 Gyr in four disc scale-lengths. Such an age drop of mean stellar age at high distances, |z|, from the disc midplane explains the inversion in [α/Fe] gradients with increasing mean z found by Anders et al (2014) in APOGEE data.…”

“…This model needs to be improved to account also for the gap in the [α/Fe]-[Fe/H] relation seen for a long time in high-resolution spectroscopic data in the solar vicinity (e.g., Adibekyan et al 2013;Bensby et al 2013;Fuhrmann 2004) and more recently found in a range of Galactic radii using APOGEE data (Anders et al 2014;Nidever et al 2014). This separation into low-and high-[α/Fe] sequences (sometime referred to as the thin and thick discs) 8 is most likely the result of a gap in the SFH at high redshift, as first suggested in the Two-Infall Model by Chiappini et al (1997).…”

Spectro-photometric distances to stars: A general purpose Bayesian approach

“…Furthermore, the alternative distance estimates may be compared to provide a means to evaluate the sensitivity of the method to the particular combination of spectroscopic and photometric parameters adopted. Often, the data set in a given sample only includes Anders et al 2014). Another important issue is extinction: the photometric data, m, and colours, must be corrected for extinction and redenning before comparing with the models.…”

“…Recent Sloan Digital Sky Survey III (SDSS-III, Eisenstein et al 2011) spectroscopic surveys, such as the Sloan Extension for Galactic Understanding and Exploration (SEGUE, Yanny et al 2009) and the Apache Point Observatory Galactic Evolution Experiment (APOGEE, Allende Prieto et al 2008a) have produced many more observational constraints, including stellar atmospheric parameters, kinematical and chemical data, which can be used to estimate reliable distances to tens of thousands of stars sampled from the SDSS and 2MASS (Skrutskie et al 2006) photometric data. These surveys have already had a clear impact on our understanding of the Galaxy (Carollo et al 2007(Carollo et al , 2010(Carollo et al , 2012Lee et al 2011a;Schlesinger et al 2012;Cheng et al 2012;Bovy et al 2012b,a;Anders et al 2014;Hayden et al 2015). The recently completed RAdial Velocity Experiment (RAVE) survey (Steinmetz et al 2006) has also provided atmospheric parameters, radial velocities and chemical abundances for six individual elements for more than 400 000 stars (Kordopatis et al 2013).…”

“…We are motivated by the analyses of SDSS-III SEGUE and APOGEE data led by the Brazilian Participation Group (BPG) and the Leibniz-Institut für Astrophysik Potsdam (AIP), which are presented in Anders et al (2014) and Brauer (2015). These papers use APOGEE giants and SEGUE G-dwarfs, respectively, to improve chemodynamical constraints to the Galactic components, especially the discs (see discussions in Minchev et al 2014b;Chiappini et al 2015).…”