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A Radial Age Gradient in the Geometrically Thick Disk of the Milky Way

03 Nov 2016-The Astrophysical Journal (American Astronomical Society; IOP Publishing)-Vol. 831, Iss: 2, pp 139

AbstractIn the Milky Way, the thick disk can be defined using individual stellar abundances, kinematics, or age, or geometrically, as stars high above the midplane. In nearby galaxies, where only a geometric definition can be used, thick disks appear to have large radial scale lengths, and their red colors suggest that they are uniformly old. The Milky Way's geometrically thick disk is also radially extended, but it is far from chemically uniform: α-enhanced stars are confined within the inner Galaxy. In simulated galaxies, where old stars are centrally concentrated, geometrically thick disks are radially extended, too. Younger stellar populations flare in the simulated disks' outer regions, bringing those stars high above the midplane. The resulting geometrically thick disks therefore show a radial age gradient, from old in their central regions to younger in their outskirts. Based on our age estimates for a large sample of giant stars in the APOGEE survey, we can now test this scenario for the Milky Way. We find that the geometrically defined thick disk in the Milky Way has indeed a strong radial age gradient: the median age for red clump stars goes from ~9 Gyr in the inner disk to 5 Gyr in the outer disk. We propose that at least some nearby galaxies could also have thick disks that are not uniformly old, and that geometrically thick disks might be complex structures resulting from different formation mechanisms in their inner and outer parts.

Topics: Thick disk (71%), Galaxy (55%), Milky Way (55%), Disc (51%), Stars (51%)

Summary (2 min read)

1. INTRODUCTION

  • These geometrically thick disks are extended (they form a red envelope all around the thin disks) and have scale lengths comparable to those of thin disks (Yoachim & Dalcanton 2006; Pohlen et al. 2007; Comerón et al. 2012).
  • Such a radial age gradient has also been seen independently in simulations by Rahimi et al. (2014) and Miranda et al. (2016).

2. DATA AND ANALYSIS

  • The authors use a sample of red giants selected from the APOGEE Data Release 12 (Holtzman et al. 2015).
  • In addition to these parameters, the authors have recently determined ages for ∼52,000 of the APOGEE red giants using two independent methods (M16; N16).
  • From cross-validation, the authors established that this model predicts masses with an rms error of 12% (42% for ages).
  • The authors restrict ourselves to regions of the parameter space covered by their training set.

3. STRUCTURE OF THE GEOMETRICALLY DEFINED THICK DISK

  • At the solar radius, the mean metallicity of stars decreases as a function of height above the midplane, while the mean [α/M] increases (e.g., Gilmore & Wyse 1985; Ivezić et al.
  • The geometrically defined thick disk (typically, stars farther than 1 kpc from the midplane) is thus locally made of stars that are metal-poor, α-rich, and old.
  • To estimate the uncertainty on the median in each bin, the authors draw 1000 bootstrap realizations of the sample, compute the median age or [α/M] for each realization, and then show in Figure 1 the range containing the 16th to 84th percentiles of all these medians.
  • Outside of the solar neighborhood, a first interesting result is that at any given radius, the median age of RC stars increases with height above the disk.
  • As already discussed, this radial age gradient is accompanied by a radial [α/M] gradient .

4.1. Robustness of Our Results

  • The current implementation of their age determination technique does not allow for a measure of the age uncertainties on a star-by-star basis, which prevents us from performing a proper study of how their results are affected by age uncertainties.
  • The authors find that the radial age gradients are still present if they create mock data samples by convolving their ages with a 40%-wide Gaussian error.
  • The age gradients are also found for RGB stars, although the gradients are shallower and the shape of the radial trends is slightly different: this reflects the different age distribution of RC versus RGB stars, but also the ∼3 times larger distance uncertainties for RGB stars compared to RC stars.
  • The authors emphasize again that the median age they find for RGB and RC stars is in no way representative of the age of the underlying total stellar population and as such cannot be directly compared to simulations.
  • The main obstacle is not so much the survey selection function (as discussed in Hayden et al.

4.2. The Milky Way Compared to Nearby Galaxies

  • The authors results show that the geometrically defined thick disk in the Milky Way has a strong radial age gradient.
  • A few Hubble Space Telescope (HST) studies have measured the properties of resolved stars in nearby edge-on disk galaxies and found older stars at large scale heights, but they do not probe the radial structure of the thick disk (Mould 2005; Seth et al. 2005; Tikhonov & Galazutdinova 2005).
  • These colors are very insensitive to age for populations older than ∼5 Gyr.
  • The authors test this using the PARSEC isochrones (Chen et al. 2014) combined with a Chabrier initial mass function (IMF).
  • This means that current broadband observations cannot exclude younger ages for the outer parts of thick disks and that deeper spectroscopic observations would be needed to probe the age structure of thick disks.

4.3. Final Words: Implications for Thick-disk Formation Scenarios

  • This suggests that complex age structures in thick disks might be a common feature of disk galaxy evolution.
  • The authors thank the referee for thoughtful comments that have improved the presentation of their results.
  • The research has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP 7) ERC Grant Agreement no.
  • Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science.

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Martig, M, Minchev, I, Ness, M, Fouesneau, M and Rix, H-W
A Radial Age Gradient in the Geometrically Thick Disk of the Milky Way
http://researchonline.ljmu.ac.uk/id/eprint/7280/
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Citation (please note it is advisable to refer to the publishers version if you
intend to cite from this work)
Martig, M, Minchev, I, Ness, M, Fouesneau, M and Rix, H-W (2016) A Radial
Age Gradient in the Geometrically Thick Disk of the Milky Way.
Astrophysical Journal, 831 (2). ISSN 0004-637X
LJMU Research Online

A RADIAL AGE GRADIENT IN THE GEOMETRICALLY
THICK DISK OF THE MILKY WAY
Marie Martig
1
, Ivan Minchev
2
, Melissa Ness
1
, Morgan Fouesneau
1
, and Hans-Walter Rix
1
1
Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany; marie.martig@gmail.com
2
Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany
Received 2016 May 13; revised 2016 August 12; accepted 2016 September 2; published 2016 November 3
ABSTRACT
In the Milky Way, the thick disk can be dened using individual stellar abundances, kinematics, or age, or
geometrically, as stars high above the midplane. In nearby galaxies, where only a geometric denition can be used,
thick disks appear to have large radial scale lengths, and their red colors suggest that they are uniformly old. The
Milky Ways geometrically thick disk is also radially extended, but it is far from chemically uniform: α-enhanced
stars are conned within the inner Galaxy. In simulated galaxies, where old stars are centrally concentrated,
geometrically thick disks are radially extended, too. Younger stellar populations are in the simulated disks outer
regions, bringing those stars high above the midplane. The resulting geometrically thick disks therefore show a
radial age gradient, from old in their central regions to younger in their outskirts. Based on our age estimates for a
large sample of giant stars in the APOGEE survey, we can now test this scenario for the Milky Way. We nd that
the geometrically dened thick disk in the Milky Way has indeed a strong radial age gradient: the median age for
red clump stars goes from 9 Gyr in the inner disk to 5 Gyr in the outer disk. We propose that at least some nearby
galaxies could also have thick disks that are not uniformly old, and that geometrically thick disks might be complex
structures resulting from different formation mechanisms in their inner and outer parts.
Key words: Galaxy: abundances Galaxy: disk Galaxy: structure
1. INTRODUCTION
Thick disks have now been known to exist for more than 30
yr, both in nearby galaxies ( Burstein 1979; Tsikoudi 1979) and
in the Milky Way (Gilmore & Reid 1983). However, there are
different ways to dene a thick disk:
(1) geometrically (or morphologically), based on decomposi-
tion of vertical density proles (Gilmore & Reid 1983;
Yoachim & Dalcanton 2006; Jurić et al. 2008; Comerón
et al. 2011),orataxed height above the disk midplane
(Yoachim & Dalcanton 2008a; Rejkuba et al. 2009;
Cheng et al. 2012a);
(2) kinematically (Morrison et al.
1990; Majewski 1992;
Bensby et al. 2003; Reddy et al. 2003; Adibekyan
et al. 2012; Haywood et al. 2013);
(3) chemically, as the α -rich sequence in the [α/Fe] versus
[Fe/H] plane (Fuhrmann 1998; Navarro et al. 2011;
Adibekyan et al. 2012; Bovy et al. 2012);
(4) as the old part of the disk (Haywood et al. 2013; Bensby
et al. 2014; Xiang et al. 2015).
While all of these denitions can be applied in the Milky
Way, only a geometric denition can be used for external
galaxies. These geometrically thick disks are extended (they
form a red envelope all around the thin disks) and have scale
lengths comparable to those of thin disks (Yoachim &
Dalcanton 2006; Pohlen et al. 2007; Comerón et al. 2012).
Their red colors (and the absence of radial color gradients) have
led to the tentative conclusion that they are made of uniformly
old stellar populations (Dalcanton & Bernstein 2002; Rejkuba
et al. 2009). However, the degeneracy between age and
metallicity measured from broadband photometry complicates
further exploration of the age and chemical structure of thick
disks in nearby galaxies.
In the Milky Way, the geometrically de
ned thick disk has a
large scale length (3.54 kpc, Ojha 2001; Jurić et al. 2008;
Jayaraman et al. 2013), in agreement with measurements for
nearby galaxies. However, in the Milky Way this geometrically
thick disk does not correspond to a uniform physical
component in terms of chemical properties. Indeed, the α-rich
thick disk is centrally concentrated with a short scale length of
about 2 kpc (Bensby et al. 2011; Bovy et al. 2012, 2016; Cheng
et al. 2012b), and very few α-rich stars are found in the outer
disk of the Milky Way (Nidever et al. 2014; Hayden
et al. 2015). This means that the chemically dened thick disk
and the geometrically dened thick disk have totally different
structures. While this discrepancy has been mentionned by
several authors (e.g., Bovy et al. 2012; Jayaraman et al. 2013),
the reasons for the discrepancy itself have received less
attention.
In Minchev et al. (2015) we used numerical simulations to
propose an explanation. We showed that in simulated disks the
oldest stellar populations are indeed concentrated within the
inner disk, while younger stellar populations have larger scale
lengths and smaller scale heights (see also Martig et al. 2014).
However, we also showed that a thinthick disk decomposition
is still possible even in the outer disk, and that such
geometrically dened thick disks are very extended. This is
because most mono-age populations are in their outer regions,
with the aring radius increasing for younger populations (such
a aring was recently found by Bovy et al. 2016 for α-poor
stars in the Milky Way). As a consequence, while the very
center of the galaxy is dominated by old stars, the more
extended parts of the thick disk are made of progressively
younger stellar populations, so that a geometrically dened
thick disk would have a radial age gradient going from old stars
in the center to young stars in the outskirts of the galaxy. Such
a radial age gradient has also been seen independently in
simulations by Rahimi et al. (2014) and Miranda et al. (2016).
However, we lack a direct observational test of this
theoretical picture using actual stellar ages instead of
The Astrophysical Journal, 831:139 (6pp), 2016 November 10 doi:10.3847/0004-637X/831/2/139
© 2016. The American Astronomical Society. All rights reserved.
1

abundance proxies. Two studies (Martig et al. 2016,
hereafter M16; Ness et al. 2016b, hereafter N16) have recently
(and for the rst time) determined ages for stars over a large
volume of the Galaxy within the Apache Point Observatory
Galactic Evolution Experiment (APOGEE) survey (Majewski
et al. 2015). In this paper we use these two sets of stellar ages to
show that in the Milky Way the geometrically dened thick
disk indeed shows a radial age gradient, as predicted by the
simulations.
In Section 2, we present the APOGEE data and the
techniques used to derive ages. We then present in Section 3
our results on the age structure of the disk of the Milky Way. In
Section 4, we discuss the robustness of our results, compare the
Milky Way to nearby galaxies, and conclude the paper with a
discussion of the implications of our results for thick-disk
formation scenarios.
2. DATA AND ANALYSIS
We use a sample of red giants selected from the APOGEE
Data Release 12 (Holtzman et al. 2015). APOGEE is a high-
resolution (R=22,500) spectroscopic survey in the H band
using the 2.5 m Sloan Digital Sky Survey (SDSS) telescope.
The spectra are treated by the APOGEE Stellar Parameter and
Chemical Abundances Pipeline (García Pérez et al. 2015),
providing stellar parameters (
T
eff
,
glog
, [M/H], [ α/M],
[C/M], and [N/M]), as well as 15 element abundances for
over 150,000 stars. In addition to these parameters, we have
recently determined ages for 52,000 of the APOGEE red
giants using two independent methods (M16; N16).
Both studies use as a training set a sample of 1500 stars
from the APOKASC survey (Pinsonneault et al. 2014), which
combines spectroscopic information from APOGEE and
asteroseismic information from the Kepler Asteroseismic
Science Consortium. This unique combination allows for a
good determination of stellar masses and by extension, of
stellar ages (using stellar evolution models).
Using the APOKASC sample, M16 determined an empirical
relation between the mass (and thus age) of red giants and their
surface properties. In M16, we built a model predicting mass
and age as a function of [M/H], [C/M], [ N/M], [C+N /M],
glog
, and
T
eff
. From cross-validation, we established that this
model predicts masses with an rms error of 12% (42% for
ages). We then applied this model to 52,286 giants in the rest of
APOGEE DR12 for which no seismic data (and hence no
precise mass and age information) are available. We restrict
ourselves to regions of the parameter space covered by our
training set.
By contrast, N16 determined stellar ages directly from the
spectra using The Cannon (Ness et al. 2015). From the training
set, The Cannon builds a model that maps stellar parameters to
the ux as a function of wavelength. N16 have shown that they
can extract age information from the APOGEE spectra with an
accuracy of 40%, similarly to M16.
In this paper, we mostly focus our analysis on a sample of
14,685 red clump (RC) stars for which distances are
determined with a precision of 5%10% by Bovy et al.
(2014). We also use the larger red giant branch (RGB) sample,
with distances computed by Ness et al. (2016a) with a precision
of 30%.
3. STRUCTURE OF THE GEOMETRICALLY
DEFINED THICK DISK
At the solar radius, the mean metallicity of stars decreases as
a function of height above the midplane, while the mean [α/M]
increases (e.g., Gilmore & Wyse 1985; Ivezić et al. 2008; Bovy
et al. 2012; Schlesinger et al. 2012) and the mean age increases
(Casagrande et al. 2016). The geometrically dened thick disk
(typically, stars farther than 1 kpc from the midplane) is thus
locally made of stars that are metal-poor, α-rich, and old.
Such a simple picture does not hold over the whole extent of
the Milky Way. The chemical abundance structure of the disk
of the Milky Way has already been studied extensively, notably
by Nidever et al. (2014) using the APOGEE RC sample and by
Hayden et al. (2015) using all the DR12 red giants. These
studies conrm that at all galactocentric radii metallicity
decreases and [α/M] increases with height above the midplane,
but a striking feature is the nearly total disappearance of α-rich
stars in the outer disk (beyond 11 kpc). This means that the
geometrically dened thick disk, while having overall a at
radial metallicity gradient (see also Cheng et al. 2012a), has a
strong radial [α/M] gradient and is mostly α-poor in its outer
regions. This is already a clear indication of the complex nature
of the thick disk, and we show here for the rst time that this
complexity is also found in terms of age structure.
We split the 14,685 RC stars into four bins corresponding to
distance from the midplane, from 0 to 2 kpc (see the top panel
of Figure 1 for an indication of the spatial location of the slices
superimposed on a DSS image of NGC 891). For each slice, we
compute the median age (using the M16 method) and [α/M] as
a function of radius in 1 kpc-wide radial bins, only showing in
Figure 1 the bins with more than 20 stars. To estimate the
uncertainty on the median in each bin, we draw 1000 bootstrap
realizations of the sample, compute the median age or [α/M]
for each realization, and then show in Figure 1 the range
containing the 16th to 84th percentiles of all these medians.
We caution that we are here studying the age distribution of
RC stars, which differs from the age distribution of the total
underlying stellar population (see Figure 15 of Bovy et al.
2014). This would need to be taken into account to directly
compare our results to simulations. However, in this paper, we
just aim at establishing the existence of a radial age gradient in
the geometric thick disk, and not at providing accurate
absolute ages.
The radial age and [α/
M] proles are shown in the bottom
panels of Figure 1. At the solar radius, we nd a median age for
RC stars of 4 Gyr within the midplane, increasing to 7.5 Gyr
for
∣∣<<z1.5 2 kpc
. This is roughly consistent with a vertical
age gradient of 4±2 Gyr kpc
1
measured for giant stars at the
solar radius by Casagrande et al. (2016).
Outside of the solar neighborhood, a rst interesting result is
that at any given radius, the median age of RC stars increases
with height above the disk. The vertical age gradients are
shallower in the outer disk, where stellar populations look more
uniform as a function of height. We also nd radial age gradients
at all heights above the midplane. At 12 kpc above the disk,
stellar ages go from 89 Gyr in the inner disk to 5 Gyr in the
outer disk. As already discussed, this radial age gradient is
accompanied by a radial [α/M] gradient (right panel in Figure 1).
The top panel in Figure 2 shows how the age distribution of
stars in the geometric thick disk changes with galactocentric
radius. The shaded regions represent the 1σ range obtained
from 1000 bootstrap realizations of our sample. The age
2
The Astrophysical Journal, 831:139 (6pp), 2016 November 10 Martig et al.

distributions at all radii are signicantly different, with younger
ages toward the outer disk. The fraction of stars younger than
6 Gyr goes from 15% in the inner disk to 70% in the outer disk.
These radial age variations are very similar to what we found in
our simulations (Figure 2 in Minchev et al. 2015), although a
direct comparison needs to take into account the data selection
function and the age distribution of RC stars.
The age gradient is not a consequence of a change in the
ages of α-rich stars. The bottom panel in Figure 2 shows that
the age distribution of α-rich stars is independent of radius
(except maybe at large radii, but this is based on only a very
small number of stars). This age distribution is roughly
consistent with a Gaussian centered on 8 Gyr, with a standard
deviation of 2.5 Gyr (black line in this gure), which would
correspond to a 31% age error. The chemically dened thick
disk is thus remarkably homogeneous in terms of age and
seems to form a uniform population (but see Liu & van de Ven
2012, nding two families of stars in terms of orbital
eccentricity within the α-rich population, which suggests that
the chemically dened thick disk might be more complex than
suggested here).
We thus nd that the geometrically dened thick disk
changes in terms of age as a function of radius, and that this age
change can be traced to the radial decrease in the fraction of α-
rich stars away from the disk midplane. Therefore, the
geometrically and the age-dened thick disks in the Milky
Way have fundamentally different structures.
4. DISCUSSION
4.1. Robustness of Our Results
The current implementation of our age determination
technique does not allow for a measure of the age uncertainties
on a star-by-star basis, which prevents us from performing a
proper study of how our results are affected by age
uncertainties. However, as described in M16, we used a
leave-one-out cross-validation algorithm to estimate that the
rms age error for our training set is 40%. We nd that the
radial age gradients are still present if we create mock data
samples by convolving our ages with a 40%-wide Gaussian
error.
We also test whether our results on the age gradient depend
on the method used to determine stellar ages. We repeat our
analysis of RC stars using ages obtained by N16 via The
Cannon (see top panel of Figure 3). With the N16 ages, the age
gradient in the geometrically dened thick disk is still present
it is even steeper, with older ages for thick-disk stars in the
inner disk. There is, however, a good general agreement
between the two age determination techniques, which is
reassuring.
Finally, we check that our results are not an artifact related to
the use of RC stars. This could arise either from the age values
themselves (less robust for RC stars because ages are affected
by mass loss during the RGB phase) or from the fact that RC
ages are a biased sampling of the underlying total stellar
population. We show in the bottom panel of Figure 3 the age
gradients for RGB stars (dened as giants in APOGEE DR12
but not in the RC catalog). We use ages determined by N16 and
distances from Ness et al. (2015). The age gradients are also
found for RGB stars, although the gradients are shallower and
the shape of the radial trends is slightly different: this reects
the different age distribution of RC versus RGB stars, but also
the 3 times larger distance uncertainties for RGB stars
compared to RC stars.
Using both a different set of stellar ages and a different type
of stars, we thus conrm that the geometrically dened thick
Figure 1. Radial proles of age (left) and [α/M] (right) for RC stars at different heights from the midplane of the Milky Way in the APOGEE survey. The solid lines
correspond to the median values in each bin, while the shaded areas represent the uncertainty on these medians (the range from the 16th to 84th percentiles, based on
1000 bootstrap samples). The top image is a DSS image of NGC 891 and illustrates the physical location of our different z slices. At all heights above the disk, we nd
a radial gradient of age and [α/M] .
3
The Astrophysical Journal, 831:139 (6pp), 2016 November 10 Martig et al.

disk is younger in its outer regions. We emphasize again that
the median age we nd for RGB and RC stars is in no way
representative of the age of the underlying total stellar
population and as such cannot be directly compared to
simulations. The main obstacle is not so much the survey
selection function (as discussed in Hayden et al. 2015, the
survey selection function does not depend strongly on
metallicity and the sample of giants observed is representative
of the underlying population of giants), but rather the complex
age distribution of RGB and RC stars. The age distribution of
RC and RGB stars tends to be biased toward younger ages, but
the strength of the effect depends on the stellar evolutionary
phase and the local star formation history (Girardi &
Salaris 2001; Bovy et al. 2014; Hayden et al. 2015 ). Correcting
for this age bias would require some complex modeling, which
is beyond the scope of this paper. We note, however, that the
age bias does not affect our main result, i.e., the existence of an
age difference between the inner and outer disk.
4.2. The Milky Way Compared to Nearby Galaxies
Our results show that the geometrically dened thick disk in
the Milky Way has a strong radial age gradient. This reconciles
measurements of a short scale length for the α-rich disk with
measurements of a large scale length for the geometrically thick
disk. Large scale lengths are also measured for (geometrically)
thick disks in external galaxies, but we do not know yet
whether these large scale lengths have the same origin as in the
Milky Way (in which case the disks would have a radial age
gradient), or whether these external geometrically thick disks
are uniformly old components. Given the variety of formation
histories for disk galaxies (e.g., Martig et al. 2012), it is likely
that both types of thick disks exist. The most direct way to test
this would be to identify which nearby galaxies also have an
age gradient in their geometrically dened thick disk. However,
measuring ages for thick disks outside of the Milky Way is
extremely challenging.
Figure 2. Cumulative age distributions for RC stars at different galactocentric
radii. The shaded areas represent for each distribution the 1σ range from 1000
bootstrap samples. The top panel shows stars in the geometrically dened thick
disk (i.e., stars far from the midplane ), while the bottom panel shows stars in
the chemically dened thick disk (i.e., α-rich stars). The α-rich population is
quite uniform as a function of galactocentric radius, with an age distribution
consistent with a 2.5 Gyr-wide Gaussian centered on 8 Gyr (black line).By
contrast, the age distribution of the geometrically dened thick disk changes
signicantly as a function of radius, with younger stars in the outer regions.
Figure 3. Radial age gradients using stellar ages computed by N16 using The
Cannon, for RC stars (top panel) and RGB stars (bottom panel). This conrms
the presence of strong radial age gradients at all heights above the midplane.
The errors on distances are larger for RGB stars, so that the radial gradients are
shallower than for RC stars.
4
The Astrophysical Journal, 831:139 (6pp), 2016 November 10 Martig et al.

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128 citations


Journal ArticleDOI
Abstract: We study the structure, age and metallicity gradients, and dynamical evolution using a cosmological zoom-in simulation of a Milky Way-mass galaxy from the Feedback in Realistic Environments project. In the simulation, stars older than 6 Gyr were formed in a chaotic, bursty mode and have the largest vertical scaleheights (1.5–2.5 kpc) by z = 0, while stars younger than 6 Gyr were formed in a relatively calm, stable disc. The vertical scaleheight increases with stellar age at all radii, because (1) stars that formed earlier were thicker ‘at birth’, and (2) stars were kinematically heated to an even thicker distribution after formation. Stars of the same age are thicker in the outer disc than in the inner disc (flaring). These lead to positive vertical age gradients and negative radial age gradients. The radial metallicity gradient is negative at the mid-plane, flattens at larger disc height |Z|, and turns positive above |Z| ∼ 1.5 kpc. The vertical metallicity gradient is negative at all radii, but is steeper at smaller radii. These trends broadly agree with observations in the Milky Way and can be naturally understood from the age gradients. The vertical stellar density profile can be well described by two components, with scaleheights 200–500 pc and 1–1.5 kpc, respectively. The thick component is a mix of stars older than 4 Gyr, which formed through a combination of several mechanisms. Our results also demonstrate that it is possible to form a thin disc in cosmological simulations even with a strong stellar feedback.

123 citations


Journal ArticleDOI
Abstract: We present a semi-empirical, largely model-independent approach for estimating Galactic birth radii, r_birth, for Milky Way disk stars. The technique relies on the justifiable assumption that a negative radial metallicity gradient in the interstellar medium (ISM) existed for most of the disk lifetime. Stars are projected back to their birth positions according to the observationally derived age and [Fe/H] with no kinematical information required. Applying our approach to the AMBRE:HARPS and HARPS-GTO local samples, we show that we can constrain the ISM metallicity evolution with Galactic radius and cosmic time, [Fe/H]_ISM(r, t), by requiring a physically meaningful r_birth distribution. We find that the data are consistent with an ISM radial metallicity gradient that flattens with time from ~-0.15 dex/kpc at the beginning of disk formation, to its measured present-day value (-0.07 dex/kpc). We present several chemo-kinematical relations in terms of mono-r_birth populations. One remarkable result is that the kinematically hottest stars would have been born locally or in the outer disk, consistent with thick disk formation from the nested flares of mono-age populations and predictions from cosmological simulations. This phenomenon can be also seen in the observed age-velocity dispersion relation, in that its upper boundary is dominated by stars born at larger radii. We also find that the flatness of the local age-metallicity relation (AMR) is the result of the superposition of the AMRs of mono-r_birth populations, each with a well-defined negative slope. The solar birth radius is estimated to be 7.3+-0.6 kpc, for a current Galactocentric radius of 8 kpc.

87 citations


Journal ArticleDOI
Abstract: The overlap between the spectroscopic Galactic Archaeology with HERMES (GALAH) survey & $Gaia$ provides a high-dimensional chemodynamical space of unprecedented size. We present a first analysis of a subset of this overlap, of 7066 dwarf, turn-off, & sub-giant stars. [...] We investigate correlations between chemical compositions, ages, & kinematics for this sample. Stellar parameters & elemental abundances are derived from the GALAH spectra with the spectral synthesis code SME. [...] We report Li, C, O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, as well as Ba & we note that we employ non-LTE calculations for Li, O, Al, & Fe. We show that the use of astrometric & photometric data improves the accuracy of the derived spectroscopic parameters, especially $\log g$. [...] we recover the result that stars of the high-$\alpha$ sequence are typically older than stars in the low-$\alpha$ sequence, the latter spanning $-0.7 8$ Gyr have lower angular momenta $L_z$ than the Sun, which implies that they are on eccentric orbits & originate from the inner disk. Contrary to some previous smaller scale studies we find a continuous evolution in the high-$\alpha$-sequence up to super-solar [Fe/H] rather than a gap, which has been interpreted as a separate "high-$\alpha$ metal-rich" population. Stars in our sample that are younger than 10 Gyr, are mainly found on the low $\alpha$-sequence & show a gradient in $L_z$ from low [Fe/H] ($L_z>L_{z,\odot}$) towards higher [Fe/H] ($L_z

83 citations


References
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Journal ArticleDOI
Abstract: Using the photometric parallax method we estimate the distances to ~48 million stars detected by the Sloan Digital Sky Survey (SDSS) and map their three-dimensional number density distribution in the Galaxy. The currently available data sample the distance range from 100 pc to 20 kpc and cover 6500 deg2 of sky, mostly at high Galactic latitudes (|b| > 25). These stellar number density maps allow an investigation of the Galactic structure with no a priori assumptions about the functional form of its components. The data show strong evidence for a Galaxy consisting of an oblate halo, a disk component, and a number of localized overdensities. The number density distribution of stars as traced by M dwarfs in the solar neighborhood (D < 2 kpc) is well fit by two exponential disks (the thin and thick disk) with scale heights and lengths, bias corrected for an assumed 35% binary fraction, of H1 = 300 pc and L1 = 2600 pc, and H2 = 900 pc and L2 = 3600 pc, and local thick-to-thin disk density normalization ρthick(R☉)/ρthin(R☉) = 12% . We use the stars near main-sequence turnoff to measure the shape of the Galactic halo. We find a strong preference for oblate halo models, with best-fit axis ratio c/a = 0.64, ρH ∝ r−2.8 power-law profile, and the local halo-to-thin disk normalization of 0.5%. Based on a series of Monte Carlo simulations, we estimate the errors of derived model parameters not to be larger than ~20% for the disk scales and ~10% for the density normalization, with largest contributions to error coming from the uncertainty in calibration of the photometric parallax relation and poorly constrained binary fraction. While generally consistent with the above model, the measured density distribution shows a number of statistically significant localized deviations. In addition to known features, such as the Monoceros stream, we detect two overdensities in the thick disk region at cylindrical galactocentric radii and heights (R,Z) ~ (6.5,1.5) kpc and (R,Z) ~ (9.5,0.8) kpc and a remarkable density enhancement in the halo covering over 1000 deg2 of sky toward the constellation of Virgo, at distances of ~6-20 kpc. Compared to counts in a region symmetric with respect to the l = 0° line and with the same Galactic latitude, the Virgo overdensity is responsible for a factor of 2 number density excess and may be a nearby tidal stream or a low-surface brightness dwarf galaxy merging with the Milky Way. The u − g color distribution of stars associated with it implies metallicity lower than that of thick disk stars and consistent with the halo metallicity distribution. After removal of the resolved overdensities, the remaining data are consistent with a smooth density distribution; we detect no evidence of further unresolved clumpy substructure at scales ranging from ~50 pc in the disk to ~1-2 kpc in the halo.

1,293 citations


Journal ArticleDOI
Abstract: Abridged: We estimate the distances to ~48 million stars detected by the Sloan Digital Sky Survey and map their 3D number density distribution in 100 1000deg^2 of sky towards the constellation of Virgo, at distances of ~6-20 kpc. Compared to a region symmetric with respect to the l=0 line, the Virgo overdensity is responsible for a factor of 2 number density excess and may be a nearby tidal stream or a low-surface brightness dwarf galaxy merging with the Milky Way. After removal of the resolved overdensities, the remaining data are consistent with a smooth density distribution; we detect no evidence of further unresolved clumpy substructure at scales ranging from ~50pc in the disk, to ~1 - 2 kpc in the halo.

1,182 citations


Journal ArticleDOI
Abstract: Aims. The aim of this paper is to explore and map the age and abundance structure of the stars in the nearby Galactic disk. Methods. We have conducted a high-resolution spectroscopic study of 714 F and G dwarf and subgiant stars in the Solar neighbourhood. The star sample has been kinematically selected to trace the Galactic thin and thick disks to their extremes, the metal-rich stellar halo, sub-structures in velocity space such as the Hercules stream and the Arcturus moving group, as well as stars that cannot (kinematically) be associated with either the thin disk or the thick disk. The determination of stellar parameters and elemental abundances is based on a standard analysis using equivalent widths and one-dimensional, plane-parallel model atmospheres calculated under the assumption of local thermodynamical equilibrium (LTE). The spectra have high resolution (R = 40 000-110 000) and high signal-to-noise)S/V = 150-300) and were obtained with the FEROS spectrograph on the ESO 1.5 in and 2.2 in telescopes, the SOFIN and PIES spectrographs on the Nordic Optical Telescope, the LIVES spectrograph on the E50 Very Large Telescope, the HARPS spectrograph on the ESO 3.6 m telescope, and the MIKE spectrograph on the Magellan Clay telescope. The abundances from individual Fe I lines were were corrected for non-LTE effects in every step of the analysis. Results. We present stellar parameters, stellar ages, kinematical parameters, orbital parameters, and detailed elemental abundances for 0, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y. and Ba for 714 nearby 12 and G dwarf stars. Our data show that there is an old and a-enhanced disk population, and a younger and less a-enhanced disk population. While they overlap greatly in metallicity between 0.7 < [Fe/HI] less than or similar to +0.1, they show a bimodal distribution in [alpha/Fe]. This bimodality becomes even clearer if stars where stellar parameters and abundances show larger uncertainties (T-eff less than or similar to 5400 K) are discarded, showing that it is important to constrain the data set to a narrow range in the stellar parameters if small differences between stellar populations are to be revealed. In addition, we find that the a-enhanced population has orbital parameters placing the stellar birthplaces in the inner Galactic disk while the loss-alpha stars mainly come from the outer Galactic disk, fully consistent with the recent claims of a short scale-length for the alpha-enhanced Galactic thick disk. We have also investigated the properties of the Hercules stream and the Arcturus moving group and find that neither of them presents chemical or age signatures that could suggest that they are disrupted clusters or extragalactic accretion remnants from ancient merger events. Instead, they are most likely dynamical features originating within the Galaxy. We have also discovered that a standard 1D. LTE analysis, utilising ionisation and excitation balance of Fe I and Fen lines produces a flat lower main sequence. As the exact cause for this effect is unclear we chose to apply an empirical correction. Turn-off stars and more evolved stars appear to be unaffected. (Less)

917 citations



Journal ArticleDOI
Abstract: Photospheric abundances are presented for 27 elements from carbon to europium in 181 F and G dwarfs from a differential local thermodynamic equilibrium (LTE) analysis of high-resolution and high signal-to-noise ratio spectra. Stellar effective temperatures (T eff) were adopted from an infrared flux method calibration of Stromgren photometry. Stellar surface gravities (g) were calculated from Hipparcos parallaxes and stellar evolutionary tracks. Adopted T eff and g values are in good agreement with spectroscopic estimates. Stellar ages were determined from evolutionary tracks. Stellar space motions (U , V , W ) and a Galactic potential were used to estimate Galactic orbital parameters. These show that the vast majority of the stars belong to the Galactic thin disc. Relative abundances expressed as (X/Fe) generally confirm previously published results. We give results for C, N, O, Na, Mg, Al, Si, S, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Sr, Y, Zr, Ba, Ce, Nd and Eu. The α elements - O, Mg, Si, Ca and Ti - show (α/Fe) to increase slightly with decreasing (Fe/H). Heavy elements with dominant contributions at solar metallicity from the s-process show (s/Fe) to decrease slightly with decreasing (Fe/H). Scatter in (X/Fe) at a fixed (Fe/H) is entirely attributable to the small measurement errors, after excluding the few thick disc stars and the s-process-enriched CH subgiants. Tight limits are set on 'cosmic' scatter. If a weak trend with (Fe/H) is taken into account, the composition of a thin disc star expressed as (X/Fe) is independent of the star's age and birthplace for elements contributed in different proportions by massive stars (Type II supernovae), exploding white dwarfs (Type Ia supernovae) and asymptotic red giant branch stars. By combining our sample with various published studies, comparisons between thin and thick disc stars are made. In this composite sample, thick disc stars are primarily identified by their V LSR in the range −40 to −100 km s −1 . These are very old stars with origins in the inner Galaxy and metallicities (Fe/H) −0.4. At the same (Fe/H), the sampled thin disc stars have V LSR ∼ 0k m s −1 , and are generally younger with a birthplace at about the Sun's Galactocentric distance. In the range −0.35 (Fe/H) −0.70, well represented by present thin and thick disc samples, (X/Fe) of the thick disc stars is greater than that of thin disc stars for Mg, Al, Si, Ca, Ti and Eu. (X/Fe) is very similar for the thin and thick disc for - notably - Na and iron-group elements. Barium ((Ba/Fe)) may be underabundant in thick relative to thin disc stars. These results extend previous ideas about composition differences between the thin and thick disc.

809 citations


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