Ageabundance gradients in the Milky Ways thick disk - PowerPoint PPT Presentation

Title: Ageabundance gradients in the Milky Ways thick disk


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Age/abundance gradients in the Milky Ways thick
disk

• Heather Morrison, Paul Harding, Case
• Ed Montiel, U of Arizona
• Deokkeun An, Caltech
• Special thanks to Connie Rockosi, Tim Beers and
  Zeljko Ivezic for SEGUE work

Overcoming Great Barriers in Galactic
Archaeology, Palm Cove, 2009
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Origin of the thick disk

• (1) Separate population caused by a specific
  event such as accretion and heating of disk by
  small satellite (eg Kazantzidis et al 2008)
• OR
• (2) Continuous change from thin to thick disk, no
  dichotomy (Norris 1987, Ivezic et al 2008)

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(1) Large infalling satellite heats up the disk
Kazantzidis et al 2008
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Discrete thick disk age controversy
Nordstrom et al 2004
Edvardsson et al 1993
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(2) Continuous change

• Norris (1987) extended disk-- data could be fit
  as well with a continuous change from thin to
  thick disk, more metal-poor material
  kinematically hotter
• Ivezic et al (2008) used SDSS photometric
  metallicities and proper motions not possible to
  fit data with simple sum of two Gaussians (thick
  and thin disk).

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Ivezic et al 2008, Bond et al 2009
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Ivezic et al 2008, Bond et al 2009
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No trend of rotational velocity with metallicity
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Radial mixing in disks

• Resonant scattering of disk stars by spiral
  structure from circular orbits to circular
  orbits at a different radius
• Dynamics Sellwood and Binney 2002
• High resolution n-bodySPH Roskar et al 2008a,b
• Analytical chemical evolution including
  kinematics Schoenrich and Binney 2008

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Redistribution by (Transient) Spiral Arms
(slide from Rok Roskar)
Radius
Time
20 kpc
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Age-metallicity relations near Sun
Roskar et al 2008
Schoenrich and Binney 2008
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Mimicking a thick disk

• Radial migration can reproduce solar neighborhood
  thick disk observations without an extra
  component (Schoenrich and Binney 2008)
• Thick disk stars with higher alpha/Fe are
  born closer to GC and migrate out
• Data on thick disk stars in other places??

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Radial migration flattens Fe/H gradients
Roskar et al 2008
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Model predictions kindly provided by Ralph
Schoenrich
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SEGUE photometric metallicities no radial
abundance gradient in thick disk!
From Ivezic et al 2008 NB DR6 metallicity
calibration omitted metal-richest stars.
Re-calibration (Bond et al 2009) does not affect
abundance non-gradient in R
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SEGUE stripes across the plane
Yanny 107 others, 2009
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Check of Schlegel et alreddening
estimatesusing globular clusters with good
CMDsred ratty CMDcyan blt5line
EBVEBV(SFD)/1.07
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Good reddening spectra
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CMD reading
M71 fiducial green (old, thick disk) M92
fiducial blue (old,halo) (An et al 2008)
g018 turnoff star is 5 kpc away (z1.5), g017
turnoff star 3 kpc away (z.9)
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Avoiding the Monoceros
In outer disk stripes (lgt94) we see asymmetries
associated with Monoceros for g0gt19 in North, and
possibly for g0gt18 in South so stay brighter
than g18
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Turnoff color difference
Turnoff color in inner disk significantly (0.08
mag) redder than in outer disk. Not photometric
errors (.02 mag at most) see consistently at
different latitudes and different stripes
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Is it age or metallicity?

• Photometric metallicities of Ivezic et al would
  suggest age, since they see no abundance gradient
• Check with SEGUE spectra
• SEGUE spectroscopic targeting 101
• -- Metallicity bias for turnoff stars
  (preferentially metal weak)
• -- G dwarfs have no metallicity bias

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Red G dwarfs Blue metal poor turnoff
stars Stars plotted have distances 2-6 kpc
No clear sign of abundance gradient in stars with
disk kinematics! Zeljko was right!
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Age or abundance?

• SEGUE photometric metallicities show no radial
  abundance gradient in thick disk
• SEGUE spectroscopy confirms this (R6-14 kpc)
• Turnoff color change suggests significant age
  difference (4 Gyr) between thick disk stars at
  R6 kpc and R14 kpc

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Summary

• New theories of radial migration challenge our
  ideas on the origin of the thick disk
• The thick disk has little abundance gradient over
  R6-14 kpc, but a significant age gradient (4
  Gyr younger at 14 kpc)
• Difficult to organise an age gradient with a
  single accretion event 10 Gyr ago.

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Acknowledgements

• This material is based on work supported by the
  National Science Foundation under Grant AST
  0607518