Presentation on theme: "Evolution of the Milky Way a look at just some of the bigger issues we don’t know and future prospects for progress: Gaia Gerry Gilmore IoA Cambridge."— Presentation transcript:
Evolution of the Milky Way a look at just some of the bigger issues we don’t know and future prospects for progress: Gaia Gerry Gilmore IoA 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 DF of metal-poor disk stars What created the elements at -1.5dex and high angular momentum? Where are the ancestors? Where are the minor merger consequences? 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 models allow very large element ratio scatter – not seen This 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+thick separate population model fits best. A very old model! (Wyse&GG 1995) Consistent with thick disk formation ending prior to thin disk, and gas retained.
Z>500pc Z<500pc Age Age-metallicity DF, from RAVE The thin disk shows very small chemical 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 2011 and Binney, 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 = Halo Red = Thick Disk Black = Thin Disk Orange = Thick/Halo Green = 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 extended But it exists! Recall Ibata/Gilmore survey Most of the stars here are halo, or disc(s)
Plateau universal IMF Plateau efficient mixing Sharp break narrow time Small scatter good mixing.......... Lots and lots of physics HALO EMP stars are critical probes of the stellar IMF in the early Universe Their use to probe galaxy formation models is not clear -- lots of recent models. Current accretion – Sgr, SMC is making a young metal-rich Pop-II halo Roederer 2009: Red=inner halo orbits Blue=outer halo orbits
Stellar metallicity distributions appear 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 LCDM expectation. 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
25 June 2011RAVE meeting Coonabarabran15 Gaia astrometric mission due for launch 2013 – parallaxes and proper motions for ~1 billion stars to m G <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: Gaia Gaia needs spectroscopy
16 Gaia Focal Plane Star motion in 10 s 4.4s per CCD Total field: - active area: 0.75 deg 2 - 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 Monit or Wave Front Sensor Basic Angle Monit or Wave Front Sensor Sky mapper: - detects all objects to 20 mag - rejects cosmic-ray events - FoV discrimination Astrometry: - total detection noise: ~6 e - Photometry: - spectro-photometer - blue and red CCDs Spectroscopy: - high-resolution spectra - red CCDs 42.35cm Figure courtesy Alex Short
Science Alerts aims: 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 <0.65arcsec So 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=20.5 Even in crowded fields – this is R136 (HST data) as seen by one Gaia CCD Green box = Gaia will measure that source.
Co-PIs Gerry 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: >10 5 Giraffe spectra (R~20,000); > 10 4 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.