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

Background | Accreted stars • 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.

Galactic Archaeology | the era of large surveys Present Future S. BRUNIER / ESO Large-scale spectroscopic surveys deliver chemical composition and radial velocities for millions of stars in the Milky Way.

Hunting for accreted stars | A chemodynamic template Ruchti et al. 2014; see also, e.g., Tolstoy et al. (2009)

Hunting for accreted stars | A chemodynamic template T y p e - I I D w a r f T y p e - I a MW-lowSFE Ruchti et al. 2014

Hunting for accreted stars | A chemodynamic template LMC-20 o o LMC-60 o L M C - 4 0 in-situ stars 1 merger accreted stars 2 mergers 3 mergers Accreted stars inhabit low J z /J c and low [Mg/Fe], distinct from Galactic disc stars. Ruchti et al. 2014, 2015; see also Read et al. 2008

Hunting for accreted stars | The Gaia-ESO Survey • Five year survey using ESO VLT to obtain ~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 J z /J c for each star assuming a simple Galactic model. Ruchti et al. 2015

Hunting for accreted stars | The Gaia-ESO Survey 1.0 0.8 0.6 0.4 [Mg/Fe] 0.2 0.0 − 0.2 − 0.4 − 3 − 2 − 1 0 1 [Fe/H] Ruchti et al. 2015

Ruchti et al. 2015

1 2 3 4 5 [Mg/Fe] !"#$ !/#$ !7#$ !'#$ !4#$ %&'( %"&( %//( %/7( %4( a 6 0.4 !")$ !/)$ !7)$ !')$ !4)$ !*)$ %'*( %"2( %"'( %4'( %'*( %"2( b 7 0.3 !"+$ !/+$ !7+$ !'+$ !4+$ !*+$ !-+$ %"*( %6( %*( %"4( %76( %"6( %4( c 8 9 0.2 !",$ !/,$ !7,$ !',$ !4,$ !*,$ !-,$ !&,$ !6,$ %-( %/( %7( %7( %"2( %"/( %"2( %'( %"( d 0.1 !".$ !/.$ !7.$ !'.$ !4.$ !*.$ !-.$ !&.$ !6.$ %/( %'( %/( %/( %"( %4( %"/( %"'( %7( e 0.0 !"5$ !/5$ !75$ !'5$ !45$ !*5$ !-5$ !&5$ !65$ %'( %/( %/( %7( %2( %/( %4( %4( %&( f ! 234 234 ! 234 234 ! 234 234 ! 234 234 ! 234 234 ! 234 234 ! 234 234 ! 234 234 ! 234 234 0 1 0 + 0 1 0 + 0 1 0 + 0 1 0 + 0 1 0 + 0 1 0 + 0 1 0 + 0 1 0 + 0 1 0 + [Fe/H] -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 Ruchti et al. 2015

The Gaia-ESO Survey | Boosting Statistics & “Ages” 2.0 a) Bin A: − 0.8 < [Fe/H] < − 0.2 -0.8 < [Fe/H] < -0.2 1.5 High − α : 211 Low − α : 38 Density 1.0 0.5 0.0 -1.3 < [Fe/H] < -0.8 b) Bin B: − 1.3 < [Fe/H] < − 0.8 1.5 High − α : 88 Low − α : 34 Density 1.0 0.5 0.0 c) Bin C: [Fe/H] < − 1.3 [Fe/H] < -1.3 1.5 High − α : 106 Low − α : 21 Density 1.0 0.5 0.0 − 1.0 − 0.5 0.0 0.5 1.0 J z J c Age (Gyr) J z /J c Ruchti et al. 2015

The Gaia-ESO Survey | Only low-mass mergers • ) log ( Z * / Z O 0.0 -0.5 ⟨ [Fe/H] ⟩ ~ -1.3 MW dSph MW dSph MW dSph M31 dSph M31 dSph SDSS spectroscopic LG dIrr ⇒ M ✷ ~ 10 7.4 M ⊙ stellar metallicities -1.0 (Gallazzi et al. 2005) 9 10 11 12 〈 [Fe/H] 〉 -1.5 Kirby et al. 2013 -2.0 ⟨ [Fe/H] ⟩ ~ -1.0 ⇒ M ✷ ~ 10 8.2 M ⊙ -2.5 DEIMOS spectroscopic stellar metallicities 3 4 5 6 7 8 log ( M * / M O • ) Both < M LMC ~ 10 8.9 M ⊙ ⇒ low-mass mergers Ruchti et al. 2015

The Milky Way’s Last Major Merger | A New Template 8 40 Merger 6 4 20 • Cosmological simulations from Agertz et log 10 M � kpc � 3 2 al. (2015), with resolution of ~70pc. Main y/ kpc 0 0 � 2 • Separate the main galaxy and the � 20 � 4 merging satellite galaxy at z~2. � 6 z~2 � 40 � 8 • Stars with azimuthal velocities (V ᵩ ) less � 40 � 20 0 20 40 x/ kpc than zero (on retrograde orbits) originate 2 . 0 in both the main galaxy as well as the v φ < 0 | Main | 5649 v φ < 0 | Merger | 1504 last major merger. All | 111660 1 . 5 • Stars from the last major merger: • [Fe/H] < -0.8 1 . 0 f • V ᵩ < 0 km/s • Possible traces in the Gaia-ESO Survey 0 . 5 iDR4 data… 0 . 0 − 3 . 0 − 2 . 5 − 2 . 0 − 1 . 5 − 1 . 0 − 0 . 5 0 . 0 0 . 5 [Fe / H] Ruchti et al. 2016, in prep.; see also Agertz et al. 2015

Hunting for debris in the Galaxy | Conclusions • 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…