arXiv subjects

Brett H. Andrews

Publications and source records attributed to Brett H. Andrews.

At least 55 records · Page 3Linked to original sources

arXiv2018

The Fourteenth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the extended Baryon Oscillation Spectroscopic Survey and from the second phase of the Apache Point Observatory Galactic Evolution Experiment

The fourth generation of the Sloan Digital Sky Survey (SDSS-IV) has been in operation since July 2014. This paper describes the second data release from this phase, and the fourteenth from SDSS overall (making this, Data Release Fourteen or DR14). This release makes public data taken by SDSS-IV in its first two years of operation (July 2014-2016). Like all previous SDSS releases, DR14 is cumulative, including the most recent reductions and calibrations of all data taken by SDSS since the first phase began operations in 2000. New in DR14 is the first public release of data from the extended Baryon Oscillation Spectroscopic Survey (eBOSS); the first data from the second phase of the Apache Point Observatory (APO) Galactic Evolution Experiment (APOGEE-2), including stellar parameter estimates from an innovative data driven machine learning algorithm known as "The Cannon"; and almost twice as many data cubes from the Mapping Nearby Galaxies at APO (MaNGA) survey as were in the previous release (N = 2812 in total). This paper describes the location and format of the publicly available data from SDSS-IV surveys. We provide references to the important technical papers describing how these data have been taken (both targeting and observation details) and processed for scientific use. The SDSS website (www.sdss.org) has been updated for this release, and provides links to data downloads, as well as tutorials and examples of data use. SDSS-IV is planning to continue to collect astronomical data until 2020, and will be followed by SDSS-V.

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arXiv2018

SDSS-IV MaNGA: The Spatially Resolved Stellar Initial Mass Function in $\sim$400 Early-Type Galaxies

MaNGA provides the opportunity to make precise spatially resolved measurements of the IMF slope in galaxies owing to its unique combination of spatial resolution, wavelength coverage and sample size. We derive radial gradients in age, element abundances and IMF slope analysing optical and near-infrared absorption features from stacked spectra out to the half-light radius of 366 early-type galaxies with masses $9.9 - 10.8\;\log M/M_{\odot}$. We find flat gradients in age and [$α$/Fe] ratio, as well as negative gradients in metallicity, consistent with the literature. We further derive significant negative gradients in the [Na/Fe] ratio with galaxy centres being well enhanced in Na abundance by up to 0.5 dex. Finally, we find a gradient in IMF slope with a bottom-heavy IMF in the centre (typical mass excess factor of 1.5) and a Milky Way-type IMF at the half-light radius. This pattern is mass-dependent with the lowest mass galaxies in our sample featuring only a shallow gradient around a Milky Way IMF. Our results imply the local IMF-$σ$ relation within galaxies to be even steeper than the global relation and hint towards the local metallicity being the dominating factor behind the IMF variations. We also employ different stellar population models in our analysis and show that a radial IMF gradient is found independently of the stellar population model used. A similar analysis of the Wing-Ford band provides inconsistent results and further evidence of the difficulty in measuring and modelling this particular feature.

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arXiv2018

SDSS-IV MaNGA: The Spectroscopic Discovery of Strongly Lensed Galaxies

We present a catalogue of 38 spectroscopically detected strong galaxy-galaxy gravitational lens candidates identified in the Sloan Digital Sky Survey IV (SDSS-IV). We were able to simulate narrow-band images for 8 of them demonstrating evidence of multiple images. Two of our systems are compound lens candidates, each with 2 background source-planes. One of these compound systems shows clear lensing features in the narrow-band image. Our sample is based on 2812 galaxies observed by the Mapping Nearby Galaxies at APO (MaNGA) integral field unit (IFU). This Spectroscopic Identification of Lensing Objects (SILO) survey extends the methodology of the Sloan Lens ACS Survey (SLACS) and BOSS Emission-Line Survey (BELLS) to lower redshift and multiple IFU spectra. We searched ~ 1.5 million spectra, of which 3065 contained multiple high signal-to-noise background emission-lines or a resolved [OII] doublet, that are included in this catalogue. Upon manual inspection, we discovered regions with multiple spectra containing background emission-lines at the same redshift, providing evidence of a common source-plane geometry which was not possible in previous SLACS and BELLS discovery programs. We estimate more than half of our candidates have an Einstein radius > 1.7", which is significantly greater than seen in SLACS and BELLS. These larger Einstein radii produce more extended images of the background galaxy increasing the probability that a background emission-line will enter one of the IFU spectroscopic fibres, making detection more likely.

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arXiv2018

SDSS-IV MaNGA: Evidence of the importance of AGN feedback in low-mass galaxies

We present new evidence for AGN feedback in a subset of 69 quenched low-mass galaxies ($M_{\star} \lesssim 5\times10^{9}$ M$_{\odot}$, $M_{\rm{r}} > -19$) selected from the first two years of the SDSS-IV MaNGA survey. The majority (85 per cent) of these quenched galaxies appear to reside in a group environment. We find 6 galaxies in our sample that appear to have an active AGN that is preventing on-going star-formation; this is the first time such a feedback mechanism has been observed in this mass range. Interestingly, five of these six galaxies have an ionised gas component that is kinematically offset from their stellar component, suggesting the gas is either recently accreted or outflowing. We hypothesise these six galaxies are low-mass equivalents to the "red geysers" observed in more massive galaxies. Of the other 63 galaxies in the sample, we find 8 do appear for have some low-level, residual star formation, or emission from hot, evolved stars. The remaining galaxies in our sample have no detectable ionised gas emission throughout their structures, consistent with them being quenched. This work shows the potential for understanding the detailed physical properties of dwarf galaxies through spatially resolved spectroscopy.

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arXiv2017

SDSS-IV MaNGA: Identification of active galactic nuclei in optical integral field unit surveys

In this paper, we investigate 2727 galaxies observed by MaNGA as of June 2016 to develop spatially resolved techniques for identifying signatures of active galactic nuclei (AGN). We identify 303 AGN candidates. The additional spatial dimension imposes challenges in identifying AGN due to contamination from diffuse ionized gas, extra-planar gas and photoionization by hot stars. We show that the combination of spatially-resolved line diagnostic diagrams and additional cuts on H$α$ surface brighness and H$α$ equivalent width can distinguish between AGN-like signatures and high-metallicity galaxies with LINER-like spectra. Low mass galaxies with high specific star formation rates are particularly difficult to diagnose and routinely show diagnostic line ratios outside of the standard star-formation locus. We develop a new diagnostic -- the distance from the standard diagnostic line in the line-ratios space -- to evaluate the significance of the deviation from the star-formation locus. We find 173 galaxies that would not have been selected as AGN candidates based on single-fibre spectral measurements but exhibit photoionization signatures suggestive of AGN activity in the MaNGA resolved observations, underscoring the power of large integral field unit (IFU) surveys. A complete census of these new AGN candidates is necessary to understand their nature and probe the complex co-evolution of supermassive black holes and their hosts.

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arXiv2017

The Thirteenth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-IV Survey MApping Nearby Galaxies at Apache Point Observatory

The fourth generation of the Sloan Digital Sky Survey (SDSS-IV) began observations in July 2014. It pursues three core programs: APOGEE-2, MaNGA, and eBOSS. In addition, eBOSS contains two major subprograms: TDSS and SPIDERS. This paper describes the first data release from SDSS-IV, Data Release 13 (DR13), which contains new data, reanalysis of existing data sets and, like all SDSS data releases, is inclusive of previously released data. DR13 makes publicly available 1390 spatially resolved integral field unit observations of nearby galaxies from MaNGA, the first data released from this survey. It includes new observations from eBOSS, completing SEQUELS. In addition to targeting galaxies and quasars, SEQUELS also targeted variability-selected objects from TDSS and X-ray selected objects from SPIDERS. DR13 includes new reductions of the SDSS-III BOSS data, improving the spectrophotometric calibration and redshift classification. DR13 releases new reductions of the APOGEE-1 data from SDSS-III, with abundances of elements not previously included and improved stellar parameters for dwarf stars and cooler stars. For the SDSS imaging data, DR13 provides new, more robust and precise photometric calibrations. Several value-added catalogs are being released in tandem with DR13, in particular target catalogs relevant for eBOSS, TDSS, and SPIDERS, and an updated red-clump catalog for APOGEE. This paper describes the location and format of the data now publicly available, as well as providing references to the important technical papers that describe the targeting, observing, and data reduction. The SDSS website, http://www.sdss.org, provides links to the data, tutorials and examples of data access, and extensive documentation of the reduction and analysis procedures. DR13 is the first of a scheduled set that will contain new data and analyses from the planned ~6-year operations of SDSS-IV.

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arXiv2017

Timing the Evolution of the Galactic Disk with NGC 6791: An Open Cluster with Peculiar High-$α$ Chemistry as seen by APOGEE

We utilize elemental-abundance information for Galactic red giant stars in five open clusters (NGC 7789, NGC 6819, M67, NGC 188, and NGC 6791) from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) DR13 dataset to age-date the chemical evolution of the high- and low-$α$ element sequences of the Milky Way. Key to this time-stamping is the cluster NGC 6791, whose stellar members have mean abundances that place it in the high-$α$, high-[Fe/H] region of the [$α$/Fe]-[Fe/H] plane. Based on the cluster's age ($\sim 8$ Gyr), Galactocentric radius, and height above the Galactic plane, as well as comparable chemistry reported for APOGEE stars in Baade's Window, we suggest that the two most likely origins for NGC 6791 are as an original part of the thick-disk, or as a former member of the Galactic bulge. Moreover, because NGC 6791 lies at the \textit{high metallicity end} ([Fe/H] $\sim 0.4$) of the high-$α$ sequence, the age of NGC 6791 places a limit on the \textit{youngest age} of stars in the high-metallicity, high-$α$ sequence for the cluster's parent population (i.e., either the bulge or the disk). In a similar way, we can also use the age and chemistry of NGC 188 to set a limit of $\sim 7$ Gyr on the \textit{oldest age} of the low-$α$ sequence of the Milky Way. Therefore, NGC 6791 and NGC 188 are potentially a pair of star clusters that bracket both the timing and the duration of an important transition point in the chemical history of the Milky Way.

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arXiv2016

Inflow, Outflow, Yields, and Stellar Population Mixing in Chemical Evolution Models

Chemical evolution models are powerful tools for interpreting stellar abundance surveys and understanding galaxy evolution. However, their predictions depend heavily on the treatment of inflow, outflow, star formation efficiency (SFE), the stellar initial mass function, the Type Ia supernova delay time distribution, stellar yields, and stellar population mixing. Using flexCE, a flexible one-zone chemical evolution code, we investigate the effects of and trade-offs between parameters. Two critical parameters are SFE and the outflow mass-loading parameter, which shift the knee in [O/Fe]-[Fe/H] and the equilibrium abundances that the simulations asymptotically approach, respectively. One-zone models with simple star formation histories follow narrow tracks in [O/Fe]-[Fe/H] unlike the observed bimodality (separate high-alpha and low-alpha sequences) in this plane. A mix of one-zone models with inflow timescale and outflow mass-loading parameter variations, motivated by the inside-out galaxy formation scenario with radial mixing, reproduces the two sequences better than a one-zone model with two infall epochs. We present [X/Fe]-[Fe/H] tracks for 20 elements assuming three different supernova yield models and find some significant discrepancies with solar neighborhood observations, especially for elements with strongly metallicity-dependent yields. We apply principal component abundance analysis (PCAA) to the simulations and existing data to reveal the main correlations amongst abundances and quantify their contributions to variation in abundance space. For the stellar population mixing scenario, the abundances of alpha-elements and elements with metallicity-dependent yields dominate the first and second principal components, respectively, and collectively explain 99% of the variance in the model. flexCE is a python package available at https://github.com/bretthandrews/flexCE.

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arXiv2016

The Data Reduction Pipeline for the SDSS-IV MaNGA IFU Galaxy Survey

Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) is an optical fiber-bundle integral-field unit (IFU) spectroscopic survey that is one of three core programs in the fourth-generation Sloan Digital Sky Survey (SDSS-IV). With a spectral coverage of 3622 - 10,354 Angstroms and an average footprint of ~ 500 arcsec^2 per IFU the scientific data products derived from MaNGA will permit exploration of the internal structure of a statistically large sample of 10,000 low redshift galaxies in unprecedented detail. Comprising 174 individually pluggable science and calibration IFUs with a near-constant data stream, MaNGA is expected to obtain ~ 100 million raw-frame spectra and ~ 10 million reduced galaxy spectra over the six-year lifetime of the survey. In this contribution, we describe the MaNGA Data Reduction Pipeline (DRP) algorithms and centralized metadata framework that produces sky-subtracted, spectrophotometrically calibrated spectra and rectified 3-D data cubes that combine individual dithered observations. For the 1390 galaxy data cubes released in Summer 2016 as part of SDSS-IV Data Release 13 (DR13), we demonstrate that the MaNGA data have nearly Poisson-limited sky subtraction shortward of ~ 8500 Angstroms and reach a typical 10-sigma limiting continuum surface brightness mu = 23.5 AB/arcsec^2 in a five arcsec diameter aperture in the g band. The wavelength calibration of the MaNGA data is accurate to 5 km/s rms, with a median spatial resolution of 2.54 arcsec FWHM (1.8 kpc at the median redshift of 0.037) and a median spectral resolution of sigma = 72 km/s.

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arXiv2016

Equilibrium and Sudden Events in Chemical Evolution

We present new analytic solutions for one-zone (fully mixed) chemical evolution models and explore their implications. In contrast to existing analytic models, we incorporate a realistic delay time distribution for Type Ia supernovae (SNIa) and can therefore track the separate evolution of $α$-elements produced by core collapse supernovae (CCSNe) and iron peak elements synthesized in both CCSNe and SNIa. In generic cases, $α$ and iron abundances evolve to an equilibrium at which element production is balanced by metal consumption and gas dilution, instead of continuing to increase over time. The equilibrium absolute abundances depend principally on supernova yields and the outflow mass loading parameter $η$, while the equilibrium abundance ratio [$α$/Fe] depends mainly on yields and secondarily on star formation history. A stellar population can be metal-poor either because it has not yet evolved to equilibrium or because high outflow efficiency makes the equilibrium abundance itself low. Systems with ongoing gas accretion develop metallicity distribution functions (MDFs) that are sharply peaked, while "gas starved" systems with rapidly declining star formation have broadly peaked MDFs. A burst of star formation that consumes a significant fraction of a system's available gas can temporarily boost [$α$/Fe] by 0.1-0.3 dex, a possible origin for rare, $α$-enhanced stars with intermediate age or high metallicity. Other sudden transitions in system properties can produce surprising behavior, including backward evolution of a stellar population from high metallicity to low metallicity. An Appendix provides a user's guide for calculating enrichment histories, [$α$/Fe] tracks, and MDFs for a wide variety of scenarios, including flexible forms of star formation history.

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arXiv2016

A Recalibration of Strong Line Oxygen Abundance Diagnostics via the Direct Method and Implications for the High Redshift Universe

We use direct method oxygen abundances in combination with strong optical emission lines, stellar masses ($M_{\star}$), and star formation rates (SFRs) to recalibrate the N2, O3N2, and N2O2 oxygen abundance diagnostics. We stack spectra of $\sim$200,000 star-forming galaxies from the Sloan Digital Sky Survey in bins of $M_{\star}$ and SFR offset from the star forming main sequence to measure the weak emission lines needed to apply the direct method. All three new calibrations are reliable to within $\pm 0.10$ dex from $\log(M_{\star}/M_{\odot}) \sim 7.5 - 10.5$ and up to at least $200~M_{\odot}$ yr$^{-1}$ in SFR. The N2O2 diagnostic is the least subject to systematic biases. We apply the diagnostics to galaxies in the local universe and investigate the $M_{\star}$-$Z$-${\rm SFR}$ relation. The N2 and O3N2 diagnostics suggest the SFR dependence of the $M_{\star}$-$Z$-${\rm SFR}$ relation varies with both $M_{\star}$ and $Δ\log(SSFR)$, whereas the N2O2 diagnostic suggests a nearly constant dependence on SFR. We apply our calibrations to a sample of high redshift galaxies from the literature, and find them to be metal poor relative to local galaxies with similar $M_{\star}$ and SFR. The calibrations do reproduce direct method abundances of the local analogs. We conclude that the $M_{\star}$-$Z$-${\rm SFR}$ relation evolves with redshift.

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arXiv2015

The Eleventh and Twelfth Data Releases of the Sloan Digital Sky Survey: Final Data from SDSS-III

The third generation of the Sloan Digital Sky Survey (SDSS-III) took data from 2008 to 2014 using the original SDSS wide-field imager, the original and an upgraded multi-object fiber-fed optical spectrograph, a new near-infrared high-resolution spectrograph, and a novel optical interferometer. All the data from SDSS-III are now made public. In particular, this paper describes Data Release 11 (DR11) including all data acquired through 2013 July, and Data Release 12 (DR12) adding data acquired through 2014 July (including all data included in previous data releases), marking the end of SDSS-III observing. Relative to our previous public release (DR10), DR12 adds one million new spectra of galaxies and quasars from the Baryon Oscillation Spectroscopic Survey (BOSS) over an additional 3000 sq. deg of sky, more than triples the number of H-band spectra of stars as part of the Apache Point Observatory (APO) Galactic Evolution Experiment (APOGEE), and includes repeated accurate radial velocity measurements of 5500 stars from the Multi-Object APO Radial Velocity Exoplanet Large-area Survey (MARVELS). The APOGEE outputs now include measured abundances of 15 different elements for each star. In total, SDSS-III added 2350 sq. deg of ugriz imaging; 155,520 spectra of 138,099 stars as part of the Sloan Exploration of Galactic Understanding and Evolution 2 (SEGUE-2) survey; 2,497,484 BOSS spectra of 1,372,737 galaxies, 294,512 quasars, and 247,216 stars over 9376 sq. deg; 618,080 APOGEE spectra of 156,593 stars; and 197,040 MARVELS spectra of 5,513 stars. Since its first light in 1998, SDSS has imaged over 1/3 of the Celestial sphere in five bands and obtained over five million astronomical spectra.

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arXiv2015

Chemical Cartography with APOGEE: Metallicity Distribution Functions and the Chemical Structure of the Milky Way Disk

Using a sample of 69,919 red giants from the SDSS-III/APOGEE Data Release 12, we measure the distribution of stars in the [$α$/Fe] vs. [Fe/H] plane and the metallicity distribution functions (MDF) across an unprecedented volume of the Milky Way disk, with radius $3 11$ kpc. The peak of the midplane MDF shifts to lower metallicity at larger $R$, reflecting the Galactic metallicity gradient. Most strikingly, the shape of the midplane MDF changes systematically with radius, with a negatively skewed distribution at $3 1$ kpc or [$α$/Fe]$>0.18$, the MDF shows little dependence on $R$. The positive skewness of the outer disk MDF may be a signature of radial migration; we show that blurring of stellar populations by orbital eccentricities is not enough to explain the reversal of MDF shape but a simple model of radial migration can do so.

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arXiv2014

Tracing chemical evolution over the extent of the Milky Way's Disk with APOGEE Red Clump Stars

We employ the first two years of data from the near-infrared, high-resolution SDSS-III/APOGEE spectroscopic survey to investigate the distribution of metallicity and alpha-element abundances of stars over a large part of the Milky Way disk. Using a sample of ~10,000 kinematically-unbiased red-clump stars with ~5% distance accuracy as tracers, the [alpha/Fe] vs. [Fe/H] distribution of this sample exhibits a bimodality in [alpha/Fe] at intermediate metallicities, -0.9<[Fe/H]<-0.2, but at higher metallicities ([Fe/H]=+0.2) the two sequences smoothly merge. We investigate the effects of the APOGEE selection function and volume filling fraction and find that these have little qualitative impact on the alpha-element abundance patterns. The described abundance pattern is found throughout the range 5<R<11 kpc and 0<|Z|<2 kpc across the Galaxy. The [alpha/Fe] trend of the high-alpha sequence is surprisingly constant throughout the Galaxy, with little variation from region to region (~10%). Using simple galactic chemical evolution models we derive an average star formation efficiency (SFE) in the high-alpha sequence of ~4.5E-10 1/yr, which is quite close to the nearly-constant value found in molecular-gas-dominated regions of nearby spirals. This result suggests that the early evolution of the Milky Way disk was characterized by stars that shared a similar star formation history and were formed in a well-mixed, turbulent, and molecular-dominated ISM with a gas consumption timescale (1/SFE) of ~2 Gyr. Finally, while the two alpha-element sequences in the inner Galaxy can be explained by a single chemical evolutionary track this cannot hold in the outer Galaxy, requiring instead a mix of two or more populations with distinct enrichment histories.

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arXiv2014

The Tenth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-III Apache Point Observatory Galactic Evolution Experiment

The Sloan Digital Sky Survey (SDSS) has been in operation since 2000 April. This paper presents the tenth public data release (DR10) from its current incarnation, SDSS-III. This data release includes the first spectroscopic data from the Apache Point Observatory Galaxy Evolution Experiment (APOGEE), along with spectroscopic data from the Baryon Oscillation Spectroscopic Survey (BOSS) taken through 2012 July. The APOGEE instrument is a near-infrared R~22,500 300-fiber spectrograph covering 1.514--1.696 microns. The APOGEE survey is studying the chemical abundances and radial velocities of roughly 100,000 red giant star candidates in the bulge, bar, disk, and halo of the Milky Way. DR10 includes 178,397 spectra of 57,454 stars, each typically observed three or more times, from APOGEE. Derived quantities from these spectra (radial velocities, effective temperatures, surface gravities, and metallicities) are also included.DR10 also roughly doubles the number of BOSS spectra over those included in the ninth data release. DR10 includes a total of 1,507,954 BOSS spectra, comprising 927,844 galaxy spectra; 182,009 quasar spectra; and 159,327 stellar spectra, selected over 6373.2 square degrees.

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arXiv2013

Exploring the chemical link between local ellipticals and their high-redshift progenitors

We present Keck/MOSFIRE K-band spectroscopy of the first mass-selected sample of galaxies at $z\sim2.3$. Targets are selected from the 3D-HST Treasury survey. The six detected galaxies have a mean [NII]$λ$6584/H$α$ ratio of $0.27\pm0.01$, with a small standard deviation of 0.05. This mean value is similar to that of UV-selected galaxies of the same mass. The mean gas-phase oxygen abundance inferred from the [NII]/H$α$ ratios depends on the calibration method, and ranges from 12+log(O/H)$_{gas}=8.57$ for the {Pettini} & {Pagel} (2004) calibration to 12+log(O/H)$_{gas}= 8.87$ for the {Maiolino} {et~al.} (2008) calibration. Measurements of the stellar oxygen abundance in nearby quiescent galaxies with the same number density indicate 12+log(O/H)$_{stars}= 8.95$, similar to the gas-phase abundances of the $z\sim2.3$ galaxies if the {Maiolino} {et~al.} (2008) calibration is used. This suggests that these high-redshift star forming galaxies may be progenitors of today's massive early-type galaxies. The main uncertainties are the absolute calibration of the gas-phase oxygen abundance and the incompleteness of the $z\sim2.3$ sample: the galaxies with detected H$α$ tend to be larger and have higher star formation rates than the galaxies without detected H$α$, and we may still be missing the most dust-obscured progenitors.

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arXiv2013

The Mass-Metallicity Relation with the Direct Method on Stacked Spectra of SDSS Galaxies

The relation between galaxy stellar mass and gas-phase metallicity is a sensitive diagnostic of the main processes that drive galaxy evolution, namely cosmological gas inflow, metal production in stars, and gas outflow via galactic winds. We employed the direct method to measure the metallicities of ~200,000 star-forming galaxies from the SDSS that were stacked in bins of (1) stellar mass and (2) both stellar mass and star formation rate (SFR) to significantly enhance the signal-to-noise ratio of the weak [O III] 4363 and [O II] 7320, 7330 auroral lines required to apply the direct method. These metallicity measurements span three decades in stellar mass from log(Mstar/Msun) = 7.4-10.5, which allows the direct method mass-metallicity relation to simultaneously capture the high-mass turnover and extend a full decade lower in mass than previous studies that employed more uncertain strong line methods. The direct method mass-metallicity relation rises steeply at low mass (O/H ~ Mstar^{1/2}) until it turns over at log(Mstar/Msun) = 8.9 and asymptotes to 12 + log(O/H) = 8.8 at high mass. The direct method mass-metallicity relation has a steeper slope, a lower turnover mass, and a factor of two to three greater dependence on SFR than strong line mass-metallicity relations. Furthermore, the SFR-dependence appears monotonic with stellar mass, unlike strong line mass-metallicity relations. We also measure the N/O abundance ratio, an important tracer of star formation history, and find the clear signature of primary and secondary nitrogen enrichment. N/O correlates tightly with oxygen abundance, and even more so with stellar mass.

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arXiv2012

Principal Component Abundance Analysis of Microlensed Bulge Dwarf and Subgiant Stars

Elemental abundance patterns can provide vital clues to the formation and enrichment history of a stellar population. Here we present an investigation of the Galactic bulge, where we apply principal component abundance analysis (PCAA)---a principal component decomposition of relative abundances [X/Fe]---to a sample of 35 microlensed bulge dwarf and subgiant stars, characterizing their distribution in the 12-dimensional space defined by their measured elemental abundances. The first principal component PC1, which suffices to describe the abundance patterns of most stars in the sample, shows a strong contribution from alpha-elements, reflecting the relative contributions of Type II and Type Ia supernovae. The second principal component PC2 is characterized by a Na--Ni correlation, the likely product of metallicity-dependent Type II supernova yields. The distribution in PC1 is bimodal, showing that the bimodality previously found in the [Fe/H] values of these stars is robustly and independently recovered by looking at only their relative abundance patterns. The two metal-rich stars that are alpha-enhanced have outlier values of PC2 and PC3, respectively, further evidence that they have distinctive enrichment histories. Applying PCAA to a sample of local thin and thick disk dwarfs yields a nearly identical PC1; in PC1, the metal-rich and metal-poor bulge dwarfs track kinematically selected thin and thick disk dwarfs, respectively, suggesting broadly similar alpha-enrichment histories. However, the disk PC2 is dominated by a Y--Ba correlation, likely indicating a greater contribution of s-process enrichment from long-lived asymptotic giant branch stars that is absent from the bulge PC2 because of its rapid formation.

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