Hunting for debris in the Milky Way with spectroscopic surveys - - PowerPoint PPT Presentation

Hunting for debris in the Milky Way with spectroscopic surveys

Gregory Ruchti

Lund Observatory in collaboration with: Justin Read, Oscar Agertz, Aldo Serenelli, Sofia Feltzing, and the Gaia-ESO Collaboration

• Our current cosmology requires the

merging and associated accretion of stars and dust to form large-scale structure.

• The halo is most sensitive to small

substructures ⟹ accreted halo stars

• The disc is more sensitive to

massive mergers that reach higher metallicity and suffer from dynamical friction and disc plane dragging ⟹ accreted disc stars

• Accreted disc stars probe late,

massive mergers and the dark matter disc.

Background | Accreted stars

Galactic Archaeology | the era of large surveys

Large-scale spectroscopic surveys deliver chemical composition and radial velocities for millions of stars in the Milky Way.

Present Future

• S. BRUNIER / ESO

Hunting for accreted stars | A chemodynamic template

Ruchti et al. 2014; see also, e.g., Tolstoy et al. (2009)

Ruchti et al. 2014

T y p e

• I

I T y p e

• I

a D w a r f MW-lowSFE

Hunting for accreted stars | A chemodynamic template

Ruchti et al. 2014, 2015; see also Read et al. 2008

in-situ stars accreted stars LMC-20o L M C

• 4
• LMC-60o

1 merger 2 mergers 3 mergers

Accreted stars inhabit low Jz/Jc and low [Mg/Fe], distinct from Galactic disc stars.

Hunting for accreted stars | A chemodynamic template

Ruchti et al. 2015

• Five year survey using ESO VLT to
• btain ~100,000 spectra in the

Milky Way (see Gilmore et al. 2012).

• Medium resolution (R~18,000) iDR3

GIRAFFE data, with S/N > 15

• Only Milky Way field stars

(e.g. no clusters, etc.).

• ~4800 stars with distances; proper

motions + radial velocities.

• Calculate Jz/Jc for each star

assuming a simple Galactic model.

Hunting for accreted stars | The Gaia-ESO Survey

−3 −2 −1 1 −0.4 −0.2 0.0 0.2 0.4 0.6 0.8 1.0

[Fe/H] [Mg/Fe]

Hunting for accreted stars | The Gaia-ESO Survey

Ruchti et al. 2015

Ruchti et al. 2015

0.2 0.0

• 0.2
• 0.4
• 0.6
• 0.8
• 1.0
• 1.2

0.0 0.1 0.2 0.3 0.4

[Fe/H] [Mg/Fe]

a c b e d f 3 2 1 4 7 6 5 8 9

!"#$ %&'( !")$ %'*( !"+$ %"*( !",$ %-( !".$ %/(

01 0+

!234 234 !"5$ %'(

01 0+

!234 234 !/5$ %/( !/.$ %'( !/,$ %/( !/+$ %6( !/)$ %"2( !/#$ %"&(

01 0+

!234 234 !75$ %/( !7.$ %/( !7,$ %7( !7+$ %*( !7)$ %"'( !7#$ %//(

01 0+

!234 234 !'5$ %7( !'.$ %/( !',$ %7( !'+$ %"4( !')$ %4'( !'#$ %/7(

01 0+

!234 234 !45$ %2( !4.$ %"( !4,$ %"2( !4+$ %76( !4)$ %'*( !4#$ %4(

01 0+

!234 234 !*5$ %/( !*.$ %4( !*,$ %"/( !*+$ %"6( !*)$ %"2(

01 0+

!234 234 !-5$ %4( !-.$ %"/( !-,$ %"2( !-+$ %4(

01 0+

!234 234 !&5$ %4( !&.$ %"'( !&,$ %'(

01 0+

!234 234 !65$ %&( !6.$ %7( !6,$ %"(

Ruchti et al. 2015

Ruchti et al. 2015

0.0 0.5 1.0 1.5 2.0

High − α : 211 Low − α : 38 a) Bin A: −0.8 < [Fe/H] < −0.2

Density

0.0 0.5 1.0 1.5

High − α : 88 Low − α : 34 b) Bin B: −1.3 < [Fe/H] < −0.8

Density

−1.0 −0.5 0.0 0.5 1.0 0.0 0.5 1.0 1.5

High − α : 106 Low − α : 21 c) Bin C: [Fe/H] < −1.3

Density Jz Jc

The Gaia-ESO Survey | Boosting Statistics & “Ages”

Jz/Jc Age (Gyr)

• 0.8 < [Fe/H] < -0.2

[Fe/H] < -1.3

• 1.3 < [Fe/H] < -0.8

Ruchti et al. 2015

3 4 5 6 7 8

• 2.5
• 2.0
• 1.5
• 1.0
• 0.5

〈[Fe/H]〉

MW dSph M31 dSph LG dIrr MW dSph M31 dSph MW dSph

DEIMOS spectroscopic stellar metallicities SDSS spectroscopic stellar metallicities (Gallazzi et al. 2005)

9 10 11 12 0.0

log (Z*/ZO

• )

log (M*/MO

• )

Kirby et al. 2013

⟨[Fe/H]⟩ ~ -1.0 ⇒ M✷ ~ 108.2 M⊙ ⟨[Fe/H]⟩ ~ -1.3 ⇒ M✷ ~ 107.4 M⊙ Both < MLMC ~ 108.9 M⊙ ⇒ low-mass mergers

The Gaia-ESO Survey | Only low-mass mergers

• Cosmological simulations from Agertz et
• al. (2015), with resolution of ~70pc.
• Separate the main galaxy and the

merging satellite galaxy at z~2.

• Stars with azimuthal velocities (Vᵩ) less

than zero (on retrograde orbits) originate in both the main galaxy as well as the last major merger.

• Stars from the last major merger:
• [Fe/H] < -0.8
• Vᵩ < 0 km/s
• Possible traces in the Gaia-ESO Survey

iDR4 data…

The Milky Way’s Last Major Merger | A New Template

Ruchti et al. 2016, in prep.; see also Agertz et al. 2015

−3.0 −2.5 −2.0 −1.5 −1.0 −0.5 0.0 0.5 [Fe/H] 0.0 0.5 1.0 1.5 2.0 f

vφ < 0 | Main | 5649 vφ < 0 | Merger | 1504 All | 111660

40 20 20 40 x/kpc 40 20 20 40 y/kpc 8 6 4 2 2 4 6 8 log10 M kpc3

Merger Main

z~2

• We built a chemo-dynamical template to identify accreted stars, especially an

accreted disc component — detritus from late, massive mergers

• Works for all large spectroscopic surveys of the MW.
• Follow-up most interesting candidates to chemically characterize

progenitors.

• Current evidence suggests the Milky Way had a quiescent merger history and

a correspondingly light dark matter disc.

• Using state-of-the-art simulations to hunt for signatures of the Milky Way’s last

major merger.

• Uncertainty in proper motions & distance limit our ability => Gaia!
• Stay tuned…

Hunting for debris in the Galaxy | Conclusions