THE MILKY WAY HAS NO DISTINCT THICK DISK

By Jo Bovy, Hans-Walter Rix, David W. Hogg. 2011

217 citations recorded by the source corpus.

Different stellar sub-populations of the Milky Way's stellar disk are known to have different vertical scale heights, their thickness increasing with age. Using SEGUE spectroscopic survey data, we have recently shown that mono-abundance sub-populations, defined in the [α/Fe]–[Fe/H] space, are well described by single-exponential spatial-density profiles in both the radial and the vertical direction; therefore, any star of a given abundance is clearly associated with a sub-population of scale height hz. Here, we work out how to determine the stellar surface-mass density contributions at the solar radius R0 of each such sub-population, accounting for the survey selection function, and for the fraction of the stellar population mass that is reflected in the spectroscopic target stars given populations of different abundances and their presumed age distributions. Taken together, this enables us to derive , the surface-mass contributions of stellar populations with scale height hz. Surprisingly, we find no hint of a thin–thick disk bi-modality in this mass-weighted scale-height distribution, but a smoothly decreasing function, approximately , from hz ≈ 200 pc to hz ≈ 1 kpc. As hz is ultimately the structurally defining property of a thin or thick disk, this shows clearly that the Milky Way has a continuous and monotonic distribution of disk thicknesses: there is no “thick disk” sensibly characterized as a distinct component. We discuss how our result is consistent with evidence for seeming bi-modality in purely geometric disk decompositions or chemical abundances analyses. We constrain the total visible stellar surface–mass density at the solar radius to be pc−2.

Subfields: Astronomy and Astrophysics, Education, Instrumentation

View Paper: https://www.semanticscholar.org/paper/b524e739ed75c53f17f284c054946eb8bfbd76cf

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Citations in the corpus

PLATO as it is: a legacy mission for Galactic archaeology (2017, unknown): “Is there a smooth transition from an 𝛼 -rich to an 𝛼 -poor disk (Bovy et al. 2012)?”
THE METALLICITY DISTRIBUTION FUNCTIONS OF SEGUE G AND K DWARFS: CONSTRAINTS FOR DISK CHEMICAL EVOLUTION AND FORMATION (2011, background): “However, Norris & Ryan (1991) and recent work by Bovy et al. (2012a, 2012b) and Liu & van de Ven (2012), focusing on [α/Fe] versus [Fe/H] and kinematics, questioned whether or not the two components were actually separable from one another, proposing that the Milky Way disk has a “thicker disk…”
A new determination of the local dark matter density from the kinematics of K dwarfs (2012, background): “Unfortunately, current estimates of the stellar and gaseous inventory in the Solar neighbourhood are too uncertain to confirm or rule out our favoured Σs (e.g. Bovy et al. 2012; Ferrière 2001).”
THE APOGEE RED-CLUMP CATALOG: PRECISE DISTANCES, VELOCITIES, AND HIGH-RESOLUTION ELEMENTAL ABUNDANCES OVER A LARGE AREA OF THE MILKY WAY'S DISK (2014, methodology): “…a scale ∆R across an azimuthal range ∆φ near the Sun would disappear in approximately few×Tφ×(∆φ/360 ◦)× ( R0/max(∆R, σRg ) ) , where Tφ is the rotational period (≈ 250Myr) and σRg is the spread in guiding-star radii (≈ 0.1R near R0 or about 1 kpc for the intermediate-age disk; Bovy et al. 2012a).”
CHEMICAL CARTOGRAPHY WITH APOGEE: LARGE-SCALE MEAN METALLICITY MAPS OF THE MILKY WAY DISK (2013, methodology): “The nature of the vertical structure continues to be debated, with some studies suggesting that the thin and thick-disks are distinct populations, while others report that there is just a single component with a continuous distribution of properties (e.g., Ivezic et al. 2008; Bovy et al. 2012a,b,c).”
The age structure of stellar populations in the solar vicinity Clues of a two-phase formation history of the Milky Way disk (2013, methodology): “…Dib et al. (2006), we have evaluated the galactic surface area as Area = π(3rd)2, with rd being the thick disk scale length, i.e. rd = 1.8 kpc (Bovy et al. 2012b). gradient in the Fe enrichment with time indicates that star formation proceeded at a sustained rate in the inner parts, but the…”
INSIDE OUT AND UPSIDE DOWN: TRACING THE ASSEMBLY OF A SIMULATED DISK GALAXY USING MONO-AGE STELLAR POPULATIONS (2013, methodology): “The observed mono-abundance populations show trends between vertical kinematics and age proxy: more alpha-rich, iron-poor populations have larger vertical velocity dispersions than theiralphapoor, iron-rich counterparts (Bovy et al. 2012c).”
STARS, GAS, AND DARK MATTER IN THE SOLAR NEIGHBORHOOD (2015, methodology): “This is marginally consistent with the result of Bovy et al. (2012b), who found M30 1 pc 2 - .”
Chemodynamical evolution of the Milky Way disk - I. The solar vicinity (2012, methodology): “Our model predicts scale-heights of up to ∼ 1.9 kpc for the oldest, most metal-poor samples; this should be compared to a maximum of∼ 1 kpc inferred from the Bovy et al. mass-corrected SEGUE G-dwarf data.”
Dissecting simulated disc galaxies - I. The structure of mono-age populations (2014, methodology): “In particular, g92 is unique in having a very flat scaleheight profile: each population can truly be described by a single scale-height at least out to R ∼ 20 kpc (as seen in the Milky Way by Bovy et al. 2012b, even though they explore a much more limited radial range).”
Chemodynamics of the Milky Way - I. The first year of APOGEE data (2013, methodology): “…al. 2013b), or – Due to yet another selection effect related to the inhomogeneous coverage of the Galactic disc(s) by finite-sightline observations (Bovy et al. 2012c), which is present in all the currently available large-scale Galactic survey data (APOGEE, 22 Rg ≈ Rmed is the orbital »guiding…”
Vertical disc heating in Milky Way-sized galaxies in a cosmological context (2015, background): “…inferred from star counts in the solar neighbourhood is well fit by a superposition of two exponentials, leading to the popular interpretation that the stellar disc is made of two separate components of fundamentally different origin, although this scenario is contested (e.g., Bovy et al. 2012).”
The structure of the Milky Way's bar outside the bulge (2015, background): “…alone would be weak evidence for the presence of two distinct components: As demonstrated by the controversial thin/thick disk discussion in the solar neighbourhood it is possible for a continuum stellar distribution to mimic a double exponential in vertical profile (e.g. Bovy et al. 2012).”
The GALAH survey: scientific motivation (2015, background): “The origin of the thick disc is a matter of ongoing debate (e.g. Abadi et al. 2003; Brook et al. 2005; Villalobos & Helmi 2008; Loebman et al. 2011; Bovy et al. 2012; Minchev et al. 2012), with suggestions that it was accreted, heated from the thin disc by interactions with satellite galaxies,…”
Chemodynamical evolution of the Milky Way disk II: Variations with Galactic radius and height above the disk plane (2014, methodology): “The overall decrease of high-[Mg/Fe] stars in the outer disk is consistent with the results of recent observational studies (e.g., Bensby et al. 2011; Anders et al. 2014; Bovy et al. 2012b), which have shown that the chemically defined Milky Way thick disk is more centrally concentrated than the thi”