Evolution of the Milky Waya look at just some of the bigger issues we don’t know
and future prospects for progress: Gaia
Gerry GilmoreIoA Cambridge
Stellar populations: 4.5 `types’• Kinematically cold, high-J (angular momentum), wide age-
range, narrow abundance range - dG-`problem’ late gas accretion? -- thin disk POP I• Old, high-J, discrete? intermediate PopII, thick disk I.5• Hot, low-J, old?, metal-rich, related to SMBH? – pseudo+bulge• Hot, low-J(???), old(??) metal-poor(?), late accretion(?),
probably itself 2-component – halo classical POPII• The first early stars POPIII? +POPIV +....
• CONCLUSION: We know little about unbiased samples• BUT each type shows surprisingly little element scatter • focus has been on extremes
The complexity we have available to study
The Galactic disks• disk (late type) galaxies are (too?) common• Most, perhaps all, are double disks • – thin disk plus thick disk• Most disks are big – too big? • Most disks are old – too old?• Most/all are metal-rich – too metal-rich?• All have complex secular dynamical evolution
g-dwarf `problem’
Big challenge:understand the DFof metal-poor disk stars
What created the elements at -1.5dexand high angularmomentum?
Where are the ancestors?Where are the minor mergerconsequences?
Solar cylinder metallicity DF
What came before the thin disk? The thick disk? The halo? Both? Neither?
Both the Thin and the Thick Disk look very homogeneous locally: hard to understand! Do few components imply few
origins?
Chemical evolution modelsallow very large element ratioscatter – not seenThis model from Wyse + gg
Element ratio data from Fuhrmann
Burnett, Binney etal 2011: RAVE data
Vertical metallicity DF with Z-height
Looks like a two-component thin+thickseparate population model fits best.
A very old model! (Wyse&GG 1995)
Consistent with thick disk formationending prior to thin disk, and gas retained.
Z>500pc
Z<500pc
Age Age-metallicity DF, from RAVE
The thin disk shows very smallchemical evolution over 8Gyr;the thick disk has a narrow age
Similar to Ohta results from Subaru/FMOS
Tight element ratios efficient ISM mixing
SFR ISM fountains
Burnett etal 2011andBinney, nipoti, fraternali MN 2009 397 1804
Thick disk merges into inner halo??
galactic field stars all see a mass-average yield, which is spatially well mixed.
This continuity tells us nothing about MWG merger/assembly duilding blocks – except they perhaps did not exist.
Derived ratios of several key α-elements to iron, for 215 red giants
Blue = HaloRed = Thick Disk Black = Thin DiskOrange = Thick/HaloGreen = Thin/Thick
Ruchti et al 2010, 2011. RAVE sample follow-up
MgI/FeI
SiI/FeI
CaI/FeI
TiI/FeI
TiII/FeII
Bulge—halo —disk connection? Which first?
Wyse & gg 1992
Bulge angular momentum distribution consistent with dissipational collapse of gaseous ejecta from stellar halo star-forming regions -- mass ratios also agree with low metallicity of stellar halo Hartwick 1979: impossible with significant late halo accretion or secular growth
Bulge, halo
Thick, thin disks
The first zero [Fe]stars should be in the bulge: are they?
SMBH connection??
Renzini 2008
Current data show the power of chemistry to measure `history’NB- ongoing star formation in very inner disk – Arches, etc – creating a pseudo-bulge?
Standard IMF (#1)Well-mixed (#2)Fast recycling (#3)
VISTA VHS data,b=40deg
Much focus on inner (pseudo-) bulge, not much on the very extendedBut it exists! Recall Ibata/Gilmore survey
Most of the stars here are halo, or disc(s)
Plateau universal IMFPlateau efficient mixingSharp break narrow timeSmall scatter good mixing..........
Lots and lots of physicsHALO
EMP stars are criticalprobes of the stellar IMFin the early UniverseTheir use to probe galaxy formation models is not clear-- lots of recent models.Current accretion – Sgr, SMCis making a young metal-richPop-II halo
Roederer 2009:Red=inner halo orbitsBlue=outer halo orbits
Stellar metallicity distributionsappear sequential:Halo thick disk thin disk
Halo MDF consistent with monolithic formation in situ,with gas loss into early disk.[ELS]
But this is not modern LCDMexpectation.
Coincidence or information?
We do not know the [Fe/H]DFs of any stellar ppulation
We do not know the [Fe/H]DFs of any stellar ppulation
The new “field of streams”double Sgr stream, much complexity
how does this relate to the “typical” halo?
Koposov, Belokurov, et al 2011
RAVE meeting Coonabarabran 1525 June 2011
• Gaia astrometric mission due for launch 2013– parallaxes and proper motions
for ~1 billion stars to mG<20 mag
– spectra for radial velocities and metallicities for 150 million stars
– Variability alerts from 2014– Full source data from 2016– Precise astrometry later
The coming revolution: GaiaGaia needs spectroscopy
16
Gaia Focal Plane
Star motion in 10 s4.4s per CCD
Total field: - active area: 0.75 deg2
- CCDs: 14 + 62 + 14 + 12
- 4500 x 1966 pixels (TDI)
- pixel size = 10 µm x 30 µm
= 59 mas x 177 mas
Astrometric Field CCDs
Blue Photometer CCDs
Sky Mapper CCDs
104.26cm
Red Photometer CCDs
Radial-Velocity Spectrometer CCDs
Basic Angle Monitor
Wave Front Sensor
Basic Angle Monitor
Wave Front Sensor
Sky mapper: - detects all objects to 20 mag
- rejects cosmic-ray events
- FoV discriminationAstrometry: - total detection noise: ~6 e-
Photometry: - spectro-photometer - blue and red CCDsSpectroscopy: - high-resolution spectra - red CCDs
42.3
5cm
Figure courtesy Alex Short
Science Alertsaims: •detect unexpected and rapid changes in the flux, spectrum or position•or appearance of new objects•trigger ground-based follow-up•provide targets to the community to be studied at peculiar states methods:•run in near-real-time: between couple of hours and 24h after observation•use photometric, spectroscopic and astrometric Gaia data•cross-match against existing information •Gaia spatial resolution makes for a real challenge – crowded fields•Motivation•Collaboration interests in special cases, esp rare objects
Advantage of spatial resolution: Gaia detects all sources with FWHM <0.65arcsecSo all the AGN, compact galaxies,.multi-lensed QSOs, kiler asteroids,...
R136 at 0.1”, 0.5”, 1.0”
Gaia is so many orders better than no win every parameter it is hard to appreciate,
Photometry selection – complete to V=20, drop to 50% at G=20.5Even in crowded fields – this is R136 (HST data) as seen by one Gaia CCDGreen box = Gaia will measure that source.
Co-PIsGerry Gilmore
(IoA, Cambridge)&
Sofia Randich(INAF-Arcetri)
plus......
The Gaia-ESO Spectroscopic Survey
• VLT-FLAMES public survey of all stellar populations of the MWG: Halo; Bulge; Thick & Thin disks; open clusters and associations
•240+60 nights (30n/semester) over 5 (4+1) years; start 1/ 2012 (P88), end 9/2016 (P97)+; visitor mode
• will yield: >105 Giraffe spectra (R~20,000); • > 104 UVES spectra (R~47,000)
[Mg,Ca,Ti, Si, Cr, Mn, Ni]
Gaia-ESO survey overview
Summary: we don’t know what “stellar population” means: dispersions in element ratios are always very small
• Big picture: pure disk formation problem• Disks are old – thick disks very common – Angular momentum disks link? • Vertical chemistry DF seems separate – also element ratios – why? • dG problem – chemistry DF narrow in Fe and element ratios - limits gradients and secular
effects??• Chemical evolution – none seen over 8Gyr: winds etc move gas round? • continuity to thick disk and halo? – constant IMF, efficient mixing, minimal or no local self-
enrichment. Short-lived star formation formed thick disk. • Q: how much scatter in elements in OCs at same age, radius? • Massive GC age/Na-O problems! MDF => not primordial building blocks• Secular models very uncertain – don’t know history of disk spiral/bar structure. Comparing
SFR and cluster survival numbers might help.• Merger models ditto – tough to test.• Turbulent disk formation – leads to thick disks but bulges – not dominant • Clusters – OCs only in thin disk – just age? Red GCs? But then inner halo/bulge link – opposite
to angular momentum relation?• .................. • Gaia will be here soon, to help – and challenge• Gaia-ESO spectroscopic survey an example of those to come.