The Metallicity Distribution Function of the Halo of the Milky Way

There is a long-standing debate over the origin of the metal-poor stellar populations of the Milky Way (MW) bulge, with the two leading scenarios being that these populations are either (i) part of a classical metal-poor spheroid or (ii) the same population as the chemically defined thick disc seen at the solar neighbourhood. Here we test whether the latter scenario can reproduce the observed chemical properties of the MW bulge. To do so we compare an N-body simulation of a composite (thin+thick) stellar disc – which evolves secularly to form a bar and a boxy/peanut (b/p) bulge – to data from APOGEE DR13. This model, in which the thick disc is massive and centrally concentrated, can reproduce the morphology of the metal-rich and metal-poor stellar populations in the bulge, as well as the mean metallicity and [α/Fe] maps as obtained from the APOGEE data. It also reproduces the trends, in both longitude and latitude, of the bulge metallicity distribution function (MDF). Additionally, we show that the model predicts small but measurable azimuthal metallicity variations in the inner disc due to the differential mapping of the thin and thick disc in the bar. We therefore see that the chemo-morphological relations of stellar populations in the MW bulge are naturally reproduced by mapping the thin and thick discs of the inner MW into a b/p.

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The Pristine Survey – VIII. The metallicity distribution function of the Milky Way halo down to the extremely metal-poor regime

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz3619 ◽

2020 ◽

Vol 492 (4) ◽

pp. 4986-5002 ◽

Cited By ~ 4

Author(s):

K Youakim ◽

E Starkenburg ◽

N F Martin ◽

G Matijevič ◽

D S Aguado ◽

...

Keyword(s):

Distribution Function ◽

Globular Clusters ◽

Milky Way ◽

Current Data ◽

Low Latitude ◽

Chemical Enrichment ◽

Merger Event ◽

Low Metallicity ◽

Metallicity Distribution ◽

Milky Way Halo

ABSTRACT The Pristine survey uses narrow-band photometry to derive precise metallicities down to the extremely metal-poor regime ($ \rm [Fe/H] \lt -3$), and currently consists of over 4 million FGK-type stars over a sky area of $\sim 2500\, \mathrm{deg}^2$. We focus our analysis on a subsample of ∼80 000 main-sequence turn-off stars with heliocentric distances between 6 and 20 kpc, which we take to be a representative sample of the inner halo. The resulting metallicity distribution function (MDF) has a peak at $ \rm [Fe/H] =-1.6$, and a slope of Δ(LogN)/$\Delta \rm [Fe/H] = 1.0 \pm 0.1$ in the metallicity range of $-3.4\; \lt\; \rm [Fe/H]\; \lt -2.5$. This agrees well with a simple closed-box chemical enrichment model in this range, but is shallower than previous spectroscopic MDFs presented in the literature, suggesting that there may be a larger proportion of metal-poor stars in the inner halo than previously reported. We identify the Monoceros/TriAnd/ACS/EBS/A13 structure in metallicity space in a low-latitude field in the anticentre direction, and also discuss the possibility that the inner halo is dominated by a single, large merger event, but cannot strongly support or refute this idea with the current data. Finally, based on the MDF of field stars, we estimate the number of expected metal-poor globular clusters in the Milky Way halo to be 5.4 for $ \rm [Fe/H]\; \lt\; -2.5$ and 1.5 for $ \rm [Fe/H]\; \lt\; -3$, suggesting that the lack of low-metallicity globular clusters in the Milky Way is not due simply to statistical undersampling.

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Metallicity distribution functions using Gaia-DR1 data

Proceedings of the International Astronomical Union ◽

10.1017/s174392131700597x ◽

2017 ◽

Vol 12 (S330) ◽

pp. 172-175

Author(s):

Georges Kordopatis ◽

Keyword(s):

Distribution Function ◽

Milky Way ◽

Distribution Functions ◽

Gaia Dr1 ◽

The Galaxy ◽

Vertical Changes ◽

Metallicity Distribution

AbstractThe metallicity distribution function (MDF) of the stellar components of the Milky Way hold valuable information regarding the processes that have taken place in the evolution of our Galaxy. In this proceeding, we investigate updates concerning the MDF now that the Tycho-Gaia Astrometric Solution (TGAS) catalogue has been released and that trigonometric distances are available. In particular, vertical changes and skewness of the MDF are investigated, together with the properties of the metal-rich stars in the sample, at different positions in the Galaxy.

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The effects of the selection function on metallicity trends in spectroscopic surveys of the Milky Way

Proceedings of the International Astronomical Union ◽

10.1017/s1743921317008365 ◽

2017 ◽

Vol 13 (S334) ◽

pp. 345-346

Author(s):

G. Nandakumar ◽

M. Schultheis ◽

M. Hayden ◽

A. Rojas-Arriagada ◽

G. Kordopatis ◽

...

Keyword(s):

Distribution Function ◽

Stellar Population ◽

Milky Way ◽

Selection Function ◽

Population Synthesis ◽

Stellar Population Synthesis ◽

Metallicity Distribution

AbstractWe investigate here the effect of the selection function on the metallicity distribution function (MDF) as well as on the vertical metallicity gradient by studying similar lines-of-sight using four different spectroscopic surveys (APOGEE, LAMOST, RAVE and Gaia-ESO) which have different targeting strategies and therefore different selection functions. We create mock fields for each survey using two stellar population synthesis models, GALAXIA and TRILEGAL. The effects of the selection function are studied in detail by applying the selection function to the two models and comparing the MDF as well as vertical metallicity gradients of the selected sources with that of the underlying sample. We find a negligible selection function effect on the MDF as well as on the vertical metallicity gradients for APOGEE, RAVE and LAMOST, and estimate a mean vertical metallicity gradient of -0.241±0.028 dex kpc−1.

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The Metallicity Distribution of the Milky Way Bulge

Publications of the Astronomical Society of Australia ◽

10.1017/pasa.2015.51 ◽

2016 ◽

Vol 33 ◽

Cited By ~ 18

Author(s):

M. Ness ◽

K. Freeman

Keyword(s):

Distribution Function ◽

Milky Way ◽

Vertical Gradient ◽

The Galaxy ◽

Stellar Halo ◽

Different Populations ◽

Formation And Evolution ◽

The Mean ◽

Inner Region ◽

Metallicity Distribution

AbstractThe Galactic bulge of the Milky Way is made up of stars with a broad range of metallicity, –3.0 < [Fe/H] < 1 dex. The mean of the metallicity distribution function decreases as a function of height z from the plane and, more weakly, with galactic radius RGC. The most metal-rich stars in the inner Galaxy are concentrated to the plane and the more metal-poor stars are found predominantly further from the plane, with an overall vertical gradient in the mean of the metallicity distribution function of about − 0.45 dex kpc−1. This vertical gradient is believed to reflect the changing contribution with height of different populations in the innermost region of the Galaxy. The more metal-rich stars of the bulge are part of the boxy/peanut structure and comprise stars in orbits which trace out the underlying X-shape. There is still a lack of consensus on the origin of the metal-poor stars ([Fe/H] < −0.5) in the region of the bulge. Some studies attribute the more metal-poor stars of the bulge to the thick disk and stellar halo that are present in the inner region, and other studies propose that the metal-poor stars are a distinct ‘old spheroid’ bulge population. Understanding the origin of the populations that make up the metallicity distribution function of the bulge, and identifying if there is a unique bulge population which has formed separately from the disk and halo, has important consequences for identifying the relevant processes in the formation and evolution of the Milky Way.

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The mass of the Milky Way out to 100 kpc using halo stars

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3984 ◽

2020 ◽

Author(s):

Alis J Deason ◽

Denis Erkal ◽

Vasily Belokurov ◽

Azadeh Fattahi ◽

Facundo A Gómez ◽

...

Keyword(s):

Distribution Function ◽

Mass Concentration ◽

Milky Way ◽

Function Analysis ◽

Large Magellanic Cloud ◽

Systematic Bias ◽

Cosmological Simulations ◽

Halo Stars ◽

Systematic Effects ◽

Mass Estimate

Abstract We use a distribution function analysis to estimate the mass of the Milky Way out to 100 kpc using a large sample of halo stars. These stars are compiled from the literature, and the vast majority ($\sim \! 98\%$) have 6D phase-space information. We pay particular attention to systematic effects, such as the dynamical influence of the Large Magellanic Cloud (LMC), and the effect of unrelaxed substructure. The LMC biases the (pre-LMC infall) halo mass estimates towards higher values, while realistic stellar halos from cosmological simulations tend to underestimate the true halo mass. After applying our method to the Milky Way data we find a mass within 100 kpc of M( < 100kpc) = 6.07 ± 0.29(stat.) ± 1.21(sys.) × 1011M⊙. For this estimate, we have approximately corrected for the reflex motion induced by the LMC using the Erkal et al. model, which assumes a rigid potential for the LMC and MW. Furthermore, stars that likely belong to the Sagittarius stream are removed, and we include a 5% systematic bias, and a 20% systematic uncertainty based on our tests with cosmological simulations. Assuming the mass-concentration relation for Navarro-Frenk-White haloes, our mass estimate favours a total (pre-LMC infall) Milky Way mass of M200c = 1.01 ± 0.24 × 1012M⊙, or (post-LMC infall) mass of M200c = 1.16 ± 0.24 × 1012 M⊙ when a 1.5 × 1011M⊙ mass of a rigid LMC is included.

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CHEMICAL CARTOGRAPHY WITH APOGEE: METALLICITY DISTRIBUTION FUNCTIONS AND THE CHEMICAL STRUCTURE OF THE MILKY WAY DISK

The Astrophysical Journal ◽

10.1088/0004-637x/808/2/132 ◽

2015 ◽

Vol 808 (2) ◽

pp. 132 ◽

Cited By ~ 276

Author(s):

Michael R. Hayden ◽

Jo Bovy ◽

Jon A. Holtzman ◽

David L. Nidever ◽

Jonathan C. Bird ◽

...

Keyword(s):

Chemical Structure ◽

Milky Way ◽

Distribution Functions ◽

Metallicity Distribution

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Effects of the selection function on metallicity trends in spectroscopic surveys of the Milky Way

Astronomy and Astrophysics ◽

10.1051/0004-6361/201731099 ◽

2017 ◽

Vol 606 ◽

pp. A97 ◽

Cited By ~ 15

Author(s):

G. Nandakumar ◽

M. Schultheis ◽

M. Hayden ◽

A. Rojas-Arriagada ◽

G. Kordopatis ◽

...

Keyword(s):

Stellar Population ◽

Milky Way ◽

Target Selection ◽

Selection Function ◽

Population Synthesis ◽

Sample Distribution ◽

Stellar Population Synthesis ◽

Source Sample ◽

Correction Terms ◽

Metallicity Distribution

Context. Large spectroscopic Galactic surveys imply a selection function in the way they performed their target selection. Aims. We investigate here the effect of the selection function on the metallicity distribution function (MDF) and on the vertical metallicity gradient by studying similar lines of sight using four different spectroscopic surveys (APOGEE, LAMOST, RAVE, and Gaia-ESO), which have different targeting strategies and therefore different selection functions. Methods. We use common fields between the spectroscopic surveys of APOGEE, LAMOST, RAVE (ALR) and APOGEE, RAVE, Gaia-ESO (AGR) and use two stellar population synthesis models, GALAXIA and TRILEGAL, to create mock fields for each survey. We apply the selection function in the form of colour and magnitude cuts of the respective survey to the mock fields to replicate the observed source sample. We make a basic comparison between the models to check which best reproduces the observed sample distribution. We carry out a quantitative comparison between the synthetic MDF from the mock catalogues using both models to understand the effect of the selection function on the MDF and on the vertical metallicity gradient. Results. Using both models, we find a negligible effect of the selection function on the MDF for APOGEE, LAMOST, and RAVE. We find a negligible selection function effect on the vertical metallicity gradients as well, though GALAXIA and TRILEGAL have steeper and shallower slopes, respectively, than the observed gradient. After applying correction terms on the metallicities of RAVE and LAMOST with respect to our reference APOGEE sample, our observed vertical metallicity gradients between the four surveys are consistent within 1σ. We also find consistent gradient for the combined sample of all surveys in ALR and AGR. We estimated a mean vertical metallicity gradient of − 0.241 ± 0.028 dex kpc-1. There is a significant scatter in the estimated gradients in the literature, but our estimates are within their ranges. Conclusions. We have shown that there is a negligible selection function effect on the MDF and the vertical metallicity gradients for APOGEE, RAVE, and LAMOST using two stellar population synthesis models. Therefore, it is indeed possible to combine common fields of different surveys in studies using MDF and metallicity gradients provided their metallicities are brought to the same scale.

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Chemodynamical Evolution of the Milky Way

Symposium - International Astronomical Union ◽

10.1017/s0074180900207559 ◽

2003 ◽

Vol 208 ◽

pp. 419-420 ◽

Cited By ~ 1

Author(s):

Chiaki Kobayashi ◽

Naohito Nakasato ◽

Ken'ichi Nomoto

Keyword(s):

Milky Way ◽

Metallicity Distribution

We simulate the chemodynamical evolution of the Milky Way using our GRAPE-SPH code, and reproduce the age-metallicity relation, the [O/Fe]-[Fe/H] relation, and the metallicity distribution.

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The Metallicity Distribution Function of Halo Dwarfs and Globular Clusters

Symposium - International Astronomical Union ◽

10.1017/s0074180900042893 ◽

1988 ◽

Vol 126 ◽

pp. 517-518

Author(s):

J. B. Laird ◽

M. P. Rupin ◽

B. W. Carney ◽

D. W. Latham ◽

R. L. Kurucz

Keyword(s):

Distribution Function ◽

Simple Model ◽

Chemical Evolution ◽

Globular Clusters ◽

Dwarf Stars ◽

Metallicity Distribution

Metallicities have been determined for a chemically unbiased sample of field halo dwarf stars. Their metallicity distribution function is similar to the predictions of a simple model of chemical evolution, but somewhat different from that of globular clusters.

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