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Journal ArticleDOI

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
TL;DR: In this paper, the authors show that the geometrically defined thick disk in the Milky Way has a strong radial age gradient, from old in their central regions to younger in their outskirts.
Abstract: In 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.

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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Journal ArticleDOI
TL;DR: In this paper, the authors used positions, abundances and ages for 31 244 red giant branch stars from the Sloan Digital Sky Survey (SDSS)-APOGEE survey, spanning 3 < Rgc < 15 kpc, to dissect the disc into mono-age and mono-[Fe/H] populations at low and high [α/Fe].
Abstract: The measurement of the structure of stellar populations in the Milky Way disc places fundamental constraints on models of galaxy formation and evolution. Previously, the disc’s structure has been studied in terms of populations defined geometrically and/or chemically, but a decomposition based on stellar ages provides a more direct connection to the history of the disc, and stronger constraint on theory. Here, we use positions, abundances and ages for 31 244 red giant branch stars from the Sloan Digital Sky Survey (SDSS)-APOGEE survey, spanning 3 < Rgc < 15 kpc, to dissect the disc into mono-age and mono-[Fe/H] populations at low and high [α/Fe]. For each population, with age < 2 Gyr and [Fe/H] < 0.1 dex, we measure the structure and surface-mass density contribution. We find that low [α/Fe] mono-age populations are fit well by a broken exponential, which increases to a peak radius and decreases thereafter. We show that this profile becomes broader with age, interpreted here as a new signal of disc heating and radial migration. High [α/Fe] populations are well fit as single exponentials within the radial range considered, with an average scalelength of 1.9 ± 0.1 kpc. We find that the relative contribution of high to low [α/Fe] populations at R0 is f� = 18 per cent ± 5 per cent; high [α/Fe] contributes most of the mass at old ages, and low [α/Fe] at young ages. The low and high [α/Fe] populations overlap in age at intermediate [Fe/H], although both contribute mass at R0 across the full range of [Fe/H]. The mass-weighted scaleheight hZ distribution is a smoothly declining exponential function. High [α/Fe] populations are thicker than low [α/Fe], and the average hZ increases steadily with age, between 200 and 600 pc.

146 citations

Journal ArticleDOI
TL;DR: In this article, the authors studied 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.
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.

141 citations

Journal ArticleDOI
TL;DR: In this article, a semi-empirical, largely model-independent approach for estimating Galactic birth radii, r_birth, for Milky Way disk stars is presented, which relies on the justifiable assumption that a negative radial metallicity gradient in the interstellar medium (ISM) existed for most of the disk lifetime.
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.

123 citations

Journal ArticleDOI
TL;DR: In this paper, the authors used six very high resolution cosmological zoom simulations of Milky Way-sized haloes to study the prevalence and formation of chemically distinct disc components, and they found that their simulations developed a clearly bimodal distribution in the $[\rm \alpha/Fe]$ -- $[ \rm Fe/H]$ plane.
Abstract: The stellar disk of the Milky Way shows complex spatial and abundance structure that is central to understanding the key physical mechanisms responsible for shaping our Galaxy. In this study, we use six very high resolution cosmological zoom simulations of Milky Way-sized haloes to study the prevalence and formation of chemically distinct disc components. We find that our simulations develop a clearly bimodal distribution in the $[\rm \alpha/Fe]$ -- $[\rm Fe/H]$ plane. We find two main pathways to creating this dichotomy which operate in different regions of the galaxies: a) an early ($z>1$) and intense high-$\rm[\alpha/Fe]$ star formation phase in the inner region ($R\lesssim 5$ kpc) induced by gas-rich mergers, followed by more quiescent low-$\rm[\alpha/Fe]$ star formation; and b) an early phase of high-$\rm[\alpha/Fe]$ star formation in the outer disc followed by a shrinking of the gas disc owing to a temporarily lowered gas accretion rate, after which disc growth resumes. In process b), a double-peaked star formation history around the time and radius of disc shrinking accentuates the dichotomy. If the early star formation phase is prolonged (rather than short and intense), chemical evolution proceeds as per process a) in the inner region, but the dichotomy is less clear. In the outer region, the dichotomy is only evident if the first intense phase of star formation covers a large enough radial range before disc shrinking occurs; otherwise, the outer disc consists of only low-$\rm[\alpha/Fe]$ sequence stars. We discuss the implication that both processes occurred in the Milky Way.

105 citations

References
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Journal ArticleDOI
TL;DR: The existence of a vertical age gradient in the Milky Way disc has been indirectly known for long. as mentioned in this paper measured it directly for the first time with seismic ages, using red giants observed by Kepler and found that low-mass stars dominate at increasing Galactic heights, whereas closer to the Galactic plane they exhibit a wide range of ages and metallicities.
Abstract: The existence of a vertical age gradient in the Milky Way disc has been indirectly known for long. Here, we measure it directly for the first time with seismic ages, using red giants observed by Kepler. We use Stromgren photometry to gauge the selection function of asteroseismic targets, and derive colour and magnitude limits where giants with measured oscillations are representative of the underlying population in the field. Limits in the 2MASS system are also derived. We lay out a method to assess and correct for target selection effects independent of Galaxy models. We find that low-mass, i. e. old red giants dominate at increasing Galactic heights, whereas closer to the Galactic plane they exhibit a wide range of ages and metallicities. Parametrizing this as a vertical gradient returns approximately 4 Gyr kpc(-1) for the disc we probe, although with a large dispersion of ages at all heights. The ages of stars show a smooth distribution over the last similar or equal to 10 Gyr, consistent with a mostly quiescent evolution for the Milky Way disc since a redshift of about 2. We also find a flat age-metallicity relation for disc stars. Finally, we show how to use secondary clump stars to estimate the present-day intrinsic metallicity spread, and suggest using their number count as a new proxy for tracing the ageing of the disc. This work highlights the power of asteroseismology for Galactic studies; however, we also emphasize the need for better constraints on stellar mass-loss, which is a major source of systematic age uncertainties in red giant stars. (Less)

156 citations

Journal ArticleDOI
TL;DR: In this article, the authors examined the α-element abundance ratio, [α/Fe], of 5620 stars, observed by the Sloan Extension for Galactic Understanding and Exploration survey in the region 6.kpc
Abstract: We examine the α-element abundance ratio, [α/Fe], of 5620 stars, observed by the Sloan Extension for Galactic Understanding and Exploration survey in the region 6 kpc

154 citations

Journal ArticleDOI
TL;DR: In this article, the authors examined the abundance ratio of 5620 stars observed by the Sloan Extension for Galactic Understanding and Exploration survey in the region 6 kpc < R < 16 kpc, 0.15kpc < |Z| < 1.5kpc, as a function of Galactocentric radius R and distance from the Galactic plane |Z |.
Abstract: We examine the \alpha-element abundance ratio, [\alpha/Fe], of 5620 stars, observed by the Sloan Extension for Galactic Understanding and Exploration survey in the region 6 kpc < R < 16 kpc, 0.15 kpc < |Z| < 1.5 kpc, as a function of Galactocentric radius R and distance from the Galactic plane |Z|. Our results show that the high-\alpha\ thick disk population has a short scale length (L_thick ~ 1.8 kpc) compared to the low-\alpha population, which is typically associated with the thin disk. We find that the fraction of high-\alpha\ stars in the inner disk increases at large |Z|, and that high-\alpha\ stars lag in rotation compared to low-\alpha\ stars. In contrast, the fraction of high-\alpha\ stars in the outer disk is low at all |Z|, and high- and low-\alpha\ stars have similar rotational velocities up to 1.5 kpc from the plane. We interpret these results to indicate that different processes were responsible for the high-\alpha\ populations in the inner and outer disk. The high-\alpha\ population in the inner disk has a short scale length and large scale height, consistent with a scenario in which the thick disk forms during an early gas-rich accretion phase. Stars far from the plane in the outer disk may have reached their current locations through heating by minor mergers. The lack of high-\alpha\ stars at large R and |Z| also places strict constraints on the strength of radial migration via transient spiral structure.

139 citations

Journal ArticleDOI
TL;DR: In this article, the vertical distribution of the resolved stellar populations in six low-mass (Vmax = 67-131 km s-1), edge-on, spiral galaxies observed with the Hubble Space Telescope Advanced Camera for Surveys was analyzed.
Abstract: We analyze the vertical distribution of the resolved stellar populations in six low-mass (Vmax = 67–131 km s-1), edge-on, spiral galaxies observed with the Hubble Space Telescope Advanced Camera for Surveys. In each galaxy we find evidence for an extraplanar stellar component extending up to 15 scale heights (3.5 kpc) above the plane, with a scale height typically twice that of two-dimensional fits to Ks-band Two Micron All Sky Survey images. We analyze the vertical distribution as a function of stellar age by tracking changes in the color-magnitude diagram. The young stellar component (108 yr) is found to have a scale height larger than the young component in the Milky Way, suggesting that stars in these low-mass galaxies form in a thicker disk. We also find that the scale height of a stellar population increases with age, with young main-sequence stars, intermediate-age asymptotic giant branch stars, and old red giant branch (RGB) stars having successively larger scale heights in each galaxy. This systematic trend indicates that disk heating must play some role in producing the extraplanar stars. We constrain the rate of disk heating using the observed trend between scale height and stellar age and find that the observed heating rates are dramatically smaller than in the Milky Way. The color distributions of the RGB stars well above the midplane indicate that the extended stellar components we see are moderately metal-poor, with peak metallicities around [Fe/H] = -1 and with little or no metallicity gradient with height. The lack of metallicity gradient can be explained if a majority of extraplanar RGB stars were formed at early times and are not dominated by a younger heated population. Our observations suggest that, like the Milky Way, low-mass disk galaxies also have multiple stellar components. In its structure, mean metallicity, and old age, the RGB component in these galaxies seems analogous to the Milky Way thick disk. However, without additional kinematic and abundance measurements, this association is only circumstantial, particularly in light of the clear existence of some disk heating at intermediate ages. Finally, we find that the vertical dust distribution has a scale height somewhat larger than that of the main-sequence stars.

131 citations

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