A RADIAL AGE GRADIENT IN THE GEOMETRICALLY THICK DISK OF THE MILKY WAY

By M. Martig, I. Minchev, M. Ness, Morgan Fouesneau, Hans-Walter Rix. 2016

59 citations recorded by the source corpus.

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.

Subfields: Astronomy and Astrophysics, Instrumentation

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

1. I. Minchev
2. Jo Bovy
3. Hans-Walter Rix
4. M. Martig
5. C. Chiappini
6. S. Majewski
7. J. Holtzman
8. M. Shetrone

In Instrumentation

1. I. Minchev
2. M. Martig
3. C. Chiappini
4. Jo Bovy
5. Hans-Walter Rix
6. S. Majewski
7. David W. Hogg
8. D. Nidever

Citations in the corpus

• Yule-Simpson’s paradox in Galactic Archaeology (2019, methodology): “This model of thick disk formation predicts a negative radial age gradient in geometrically-defined thick disks, which was found to be indeed the case for the Milky Way using APOGEE data (Martig et al. 2016).”
• THE RELATIONSHIP BETWEEN MONO-ABUNDANCE AND MONO-AGE STELLAR POPULATIONS IN THE MILKY WAY DISK (2016, methodology): “An initial central component is formed during an early epoch of violent merger activity, where gas-rich mergers supply the initial reservoir of gas at high redshift and merger activity decreases with redshift, similarly to what is expected for the Milky Way.”
• The age–metallicity structure of the Milky Way disc using APOGEE (2017, methodology): “It is also important to account for all other cuts made by Martig et al. (2016b) on the stellar parameters in the APOGEE catalogue, outlined in full at the beginning of their Section 6.2.”
• Red Clump Stars from LAMOST II: the outer disc of the Milky Way (2017, background): “Note that Martig et al. (2016) claimed the median age of the RC stars located at 0 < |Z| < 0.5 kpc decreases from about 8 Gyr at R ∼ 5 kpc to 4 Gyr at ∼ 8 kpc.”
• Mapping the Milky Way with LAMOST– III. Complicated spatial structure in the outer disc (2018, background): “…populations show larger scale heights and are treated as the thicker component, while the low-α abundance populations display smaller scale heights and are considered as the thinner component (for more details about different definitions of the thick disc, please refer to Martig et al. 2016).”
• Estimating stellar birth radii and the time evolution of Milky Way’s ISM metallicity gradient (2018, methodology): “This model of thick disk formation predicts a negative radial age gradient in geometrically-defined thick disks, which was found to be indeed the case for the Milky Way using APOGEE data (Martig et al. 2016).”