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