On the likelihoods of finding very metal-poor (and old) stars in the Milky Way’s disc, bulge, and halo (original) (raw)
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OBSERVATIONAL PROPERTIES OF THE METAL-POOR THICK DISK OF THE MILKY WAY AND INSIGHTS INTO ITS ORIGINS
The Astrophysical Journal, 2011
We have undertaken the study of the elemental abundances and kinematic properties of a metalpoor sample of candidate thick-disk stars selected from the RAVE spectroscopic survey of bright stars to differentiate among the present scenarios of the formation of the thick disk. In this paper, we report on a sample of 214 red giant branch, 31 red clump/horizontal branch, and 74 main-sequence/sub-giant branch metal-poor stars, which serves to augment our previous sample of only giant stars. We find that the thick disk [α/Fe] ratios are enhanced, and have little variation (< 0.1 dex), in agreement with our previous study. The augmented sample further allows, for the first time, investigation of the gradients in the metal-poor thick disk. For stars with [Fe/H] < −1.2, the thick disk shows very small gradients, < 0.03 ± 0.02 dex kpc −1 , in α-enhancement, while we find a +0.01 ± 0.04 dex kpc −1 radial gradient and a −0.09 ± 0.05 dex kpc −1 vertical gradient in iron abundance. In addition, we show that the peak of the distribution of orbital eccentricities for our sample agrees better with models in which the stars that comprise the thick disk were formed primarily in the Galaxy, with direct accretion of stars contributing little. Our results thus disfavor direct accretion of stars from dwarf galaxies into the thick disk as a major contributor to the thick disk population, but cannot discriminate between alternative models for the thick disk, such as those that invoke high-redshift (gas-rich) mergers, heating of a pre-existing thin stellar disk by a minor merger, or efficient radial migration of stars.
The Astrophysical Journal, 2014
A new set of very high signal-to-noise (S/N > 100/1), medium-resolution (R ∼ 3000) optical spectra have been obtained for 302 of the candidate "weak-metal" stars selected by Bidelman & MacConnell. We use these data to calibrate the recently developed generalization of the Sloan Extension for Galactic Exploration and Understanding and Exploration (SEGUE) Stellar Parameter Pipeline, and obtain estimates of the atmospheric parameters (T eff , log g, and [Fe/H]) for these non-Sloan Digital Sky Survey/SEGUE data; we also obtain estimates of [C/Fe]. The new abundance measurements are shown to be consistent with available high-resolution spectroscopic determinations, and represent a substantial improvement over the accuracies obtained from the previous photometric estimates reported in Paper I of this series. The apparent offset in the photometric abundances of the giants in this sample noted by several authors is confirmed by our new spectroscopy; no such effect is found for the dwarfs. The presence of a metal-weak thick-disk (MWTD) population is clearly supported by these new abundance data. Some 25% of the stars with metallicities −1.8 < [Fe/H] −0.8 exhibit orbital eccentricities e < 0.4, yet are clearly separated from members of the inner-halo population with similar metallicities by their location in a Lindblad energy versus angular momentum diagram. A comparison is made with recent results for a similar-size sample of Radial Velocity Experiment stars from Ruchti et al. We conclude, based on both of these samples, that the MWTD is real, and must be accounted for in discussions of the formation and evolution of the disk system of the Milky Way.
The stellar metallicity distribution of disc galaxies and bulges in cosmological simulations
Monthly Notices of the Royal Astronomical Society, 2012
By means of high-resolution cosmological hydrodynamical simulations of Milky Waylike disc galaxies, we conduct an analysis of the associated stellar metallicity distribution functions (MDFs). After undertaking a kinematic decomposition of each simulation into spheroid and disc sub-components, we compare the predicted MDFs to those observed in the solar neighbourhood and the Galactic bulge. The effects of the star formation density threshold are visible in the star formation histories, which show a modulation in their behaviour driven by the threshold. The derived MDFs show median metallicities lower by 0.2−0.3 dex than the MDF observed locally in the disc and in the Galactic bulge. Possible reasons for this apparent discrepancy include the use of low stellar yields and/or centrally-concentrated star formation. The dispersions are larger than the one of the observed MDF; this could be due to simulated discs being kinematically hotter relative to the Milky Way. The fraction of low metallicity stars is largely overestimated, visible from the more negatively skewed MDF with respect to the observational sample. For our fiducial Milky Way analog, we study the metallicity distribution of the stars born in situ relative to those formed via accretion (from disrupted satellites), and demonstrate that this low-metallicity tail to the MDF is populated primarily by accreted stars. Enhanced supernova and stellar radiation energy feedback to the surrounding interstellar media of these pre-disrupted satellites is suggested as an important regulator of the MDF skewness.
Very Metal-Poor Stars in the Outer Galactic Bulge Found by the Apogee Survey
The astrophysical journal, 2013
Despite its importance for understanding the nature of early stellar generations and for constraining Galactic bulge formation models, at present little is known about the metal-poor stellar content of the central Milky Way. This is a consequence of the great distances involved and intervening dust obscuration, which challenge optical studies. However, the Apache Point Observatory Galactic Evolution Experiment (APOGEE), a wide-area, multifiber, highresolution spectroscopic survey within Sloan Digital Sky Survey III, is exploring the chemistry of all Galactic stellar populations at infrared wavelengths, with particular emphasis on the disk and the bulge. An automated spectral analysis of data on 2403 giant stars in 12 fields in the bulge obtained during APOGEE commissioning yielded five stars with low metallicity ([Fe/H] −1.7), including two that are very metal-poor [Fe/H] ∼ −2.1 by bulge standards. Luminosity-based distance estimates place the 5 stars within the outer bulge, where 1246 of the other analyzed stars may reside. A manual reanalysis of the spectra verifies the low metallicities, and finds these stars to be enhanced in the α-elements O, Mg, and Si without significant α-pattern differences with other local halo or metal-weak thick-disk stars of similar metallicity, or even with other more metal-rich bulge stars. While neither the kinematics nor chemistry of these stars can yet definitively determine which, if any, are truly bulge members, rather than denizens of other populations co-located with the bulge, the newly identified stars reveal that the chemistry of metal-poor stars in the central Galaxy resembles that of metal-weak thick-disk stars at similar metallicity.
The Relationship between Age, Metallicity, and Abundances for Disk Stars in a Simulated Milky Way
The Astrophysical Journal
Observations of the Milky Way’s low-α disk show that several element abundances correlate with age at fixed metallicity, with unique slopes and small scatters around the age–[X/Fe] relations. In this study, we turn to simulations to explore the age–[X/Fe] relations for the elements C, N, O, Mg, Si, S, and Ca that are traced in a FIRE-2 cosmological zoom-in simulation of a Milky Way–like galaxy, m12i, and understand what physical conditions give rise to the observed age–[X/Fe] trends. We first explore the distributions of mono-age populations in their birth and current locations, [Fe/H], and [X/Fe], and find evidence for inside-out radial growth for stars with ages <7 Gyr. We then examine the age–[X/Fe] relations across m12i’s disk and find that the direction of the trends agrees with observations, apart from C, O, and Ca, with remarkably small intrinsic scatters, σ int (0.01 − 0.04 dex). This σ int measured in the simulations is also metallicity dependent, with σ int ≈ 0.025 dex ...
The EMBLA survey – metal-poor stars in the Galactic bulge
Monthly Notices of the Royal Astronomical Society, 2016
Cosmological models predict the oldest stars in the Galaxy should be found closest to the centre of the potential well, in the bulge. The Extremely Metal-poor BuLge stars with AAOmega survey (EMBLA) successfully searched for these old, metal-poor stars by making use of the distinctive SkyMapper photometric filters to discover candidate metal-poor stars in the bulge. Their metal-poor nature was then confirmed using the AAOmega spectrograph on the Anglo-Australian Telescope. Here we present an abundance analysis of 10 bulge stars with −2.8 < [Fe/H] < −1.7 from MIKE/Magellan observations, in total determining the abundances of 22 elements. Combining these results with our previous high-resolution data taken as part of the Gaia-ESO Survey, we have started to put together a picture of the chemical and kinematic nature of the most metal-poor stars in the bulge. The currently available kinematic data are consistent with the stars belonging to the bulge, although more accurate measurements are needed to constrain the stars' orbits. The chemistry of these bulge stars deviates from that found in halo stars of the same metallicity. Two notable differences are the absence of carbonenhanced metal-poor bulge stars, and the α element abundances exhibit a large intrinsic scatter and include stars which are underabundant in these typically enhanced elements.
Metallicities and ages of stellar populations at a high Galactic latitude field
Monthly Notices of the Royal Astronomical Society, 2009
We present an analysis of U BV RI data from the Selected Area SA 141. By applying recalibrated methods of measuring ultraviolet excess (UVX), we approximate abundances and absolute magnitudes for 368 stars over 1.3 square degrees out to distances over 10 kpc. With the density distribution constrained from our previous photometric parallax investigations and with sufficient accounting for the metallicity bias in the UVX method, we are able to compare the vertical abundance distribution to those measured in previous studies. We find that the abundance distribution has an underlying uniform component consistent with previous spectroscopic results that posit a monometallic thick disk and halo with abundances of [F e/H] = −0.8 and −1.4, respectively. However, there are a number of outlying data points that may indicate contamination by more metal-rich halo streams. The absence of vertical abundance gradients in the Galactic stellar populations and the possible presence of interloping halo streams would be consistent with expectations from merger models of Galaxy formation. We find that our UVX method has limited sensitivity in exploring the metallicity distribution of the distant Galactic halo, owing to the poor constraint on the U BV properties of very metal-poor stars. The derivation of metallicities from broadband U BV photometry remains fundamentally sound for the exploration of the halo but is in need of both improved calibration and superior data.
The Astrophysical Journal, 2019
The stellar populations in the inner kiloparsecs of the Milky Way (MW) show complex kinematical and chemical structures. The origin and evolution of these structures are still under debate. Here we study the central region of a fully cosmological hydrodynamical simulation of a disk galaxy that reproduces key properties of the inner kiloparsecs of the MW: it has a boxy morphology and shows an overall rotation and dispersion profile in agreement with observations. We use a clustering algorithm on stellar kinematics to identify a number of discrete kinematic components: a high-and low-spin disk, a stellar halo, and two bulge components, one fast-rotating and one slowrotating. We focus on the two bulge components and show that the slow-rotating one is spherically symmetric while the fast-rotating component shows a boxy/peanut morphology. Although the two bulge components are kinematically discrete populations at present day, they are both mostly formed over similar timescales, from disk material. We find that stellar particles with lower initial birth angular momentum (most likely thick-disk stars) end up in the slow-rotating low-spin bulge, while stars with higher birth angular momentum (most likely thin-disk stars) are found in the high-spin bulge. This has the important consequence that a bulge population with a spheroidal morphology does not necessarily indicate a merger origin. In fact, we do find that only ∼2.3% of the stars in the bulge components are ex situ stars brought in by accreted dwarf galaxies early on. We identify these ex situ stars as the oldest and most metal-poor stars on highly radial orbits with large vertical excursions from the disk.
Evidence for a metal-poor population in the inner Galactic bulge
Astronomy and Astrophysics, 2015
The inner Galactic bulge has, until recently, been avoided in chemical evolution studies because of extreme extinction and stellar crowding. Large, near-IR spectroscopic surveys, such as the Apache Point Observatory Galactic Evolution Experiment (APOGEE), for the first time allow the measurement of metallicities in the inner region of our Galaxy. We study metallicities of 33 K/M giants situated in the Galactic center region from observations obtained with the APOGEE survey. We selected K/M giants with reliable stellar parameters from the APOGEE/ASPCAP pipeline. Distances, interstellar extinction values, and radial velocities were checked to confirm that these stars are indeed situated in the inner Galactic bulge. We find a metal-rich population centered at [M/H] = +0.4 dex, in agreement with earlier studies of other bulge regions, but we also discovered a peak at low metallicity around [M/H] = −1.0 dex. This finding suggests the presence of a metal-poor population, which has not previously been detected in the central region. Our results indicate a dominant metal-rich population with a metal-poor component that is enhanced in the α-elements. This metal-poor population may be associated with the classical bulge and a fast formation scenario.
Metallicity gradients of disc stars for a cosmologically simulated galaxy
Monthly Notices of the Royal Astronomical Society, 2011
We analyse for the first time the radial abundance gradients of the disc stars of a disc galaxy simulated with our three dimensional, fully cosmological chemodynamical galaxy evolution code GCD+. We study how [Fe/H], [N/O], [O/Fe], [Mg/Fe] and [Si/Fe] vary with galactocentric radius. For the young stars of the disc, we found a negative slope for [Fe/H] and [N/O] but a positive [O/Fe], [Mg/Fe] and [Si/Fe] slope with radius.