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
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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 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.
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 define a thick disk:
(1) geometrically (or morphologically), based on decomposi-
tion of vertical density profiles (Gilmore & Reid 1983;
Yoachim & Dalcanton 2006; Jurić et al. 2008; Comerón
et al. 2011),oratafixed 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 definitions can be applied in the Milky
Way, only a geometric definition 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 defi
ned thick disk has a
large scale length (∼3.5–4 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 defined thick disk
and the geometrically defined 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 thin–thick disk decomposition
is still possible even in the outer disk, and that such
geometrically defined thick disks are very extended. This is
because most mono-age populations flare in their outer regions,
with the flaring radius increasing for younger populations (such
a flaring 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 defined
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 first 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 defined 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 flux 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 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.
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 confirm 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 defined thick disk, while having overall a flat
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 first 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] profiles are shown in the bottom
panels of Figure 1. At the solar radius, we find 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 first 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 find radial age gradients
at all heights above the midplane. At ∼1–2 kpc above the disk,
stellar ages go from ∼8–9 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 significantly 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 figure), which would
correspond to a 31% age error. The chemically defined thick
disk is thus remarkably homogeneous in terms of age and
seems to form a uniform population (but see Liu & van de Ven
2012, finding two families of stars in terms of orbital
eccentricity within the α-rich population, which suggests that
the chemically defined thick disk might be more complex than
suggested here).
We thus find that the geometrically defined 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-defined 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 find 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 defined 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 (defined 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 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.
Using both a different set of stellar ages and a different type
of stars, we thus confirm that the geometrically defined thick
Figure 1. Radial profiles 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 find
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 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. 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 defined 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 defined 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 defined thick
disk (i.e., stars far from the midplane ), while the bottom panel shows stars in
the chemically defined 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 defined thick disk changes
significantly 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 confirms
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.
![Figure 1. Radial profiles 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 find a radial gradient of age and [α/M] .](/figures/figure-1-radial-profiles-of-age-left-and-a-m-right-for-rc-3vza068q.webp)