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

doi:10.3847/0004-637X/831/2/139

Martig, M, Minchev, I, Ness, M, Fouesneau, M and Rix, H-W

http://researchonline.ljmu.ac.uk/id/eprint/7280/

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Citation (please note it is advisable to refer to the publisher’s version if you

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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

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

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.

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

Summary (2 min read)

1. INTRODUCTION

2. DATA AND ANALYSIS

3. STRUCTURE OF THE GEOMETRICALLY DEFINED THICK DISK

4.1. Robustness of Our Results

4.2. The Milky Way Compared to Nearby Galaxies

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

Figures (4)

Citations

References

Related Papers (5)