THE CHEMICAL EVOLUTION OF THE MILKY WAY DISK

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1 THE CHEMICAL EVOLUTION OF THE MILKY WAY DISK 1. The simple picture of disk evolution: independent ring evolution, successes and failures 2. The dynamical picture: stars (and gas) moving around 3. A model for the MW disk

2 The Milky Way : not a simple system Local disk (8 kpc from GC): Composition of nearby young stars and gas is ~ solar (But: older stars are more metal poor, by a factor up to ten) Inner Galactic disk: metal rich young stars and gas (up to 3 times solar) Outer Galactic disk: metal poor young stars and gas (down to 1/3 solar) Galactic halo: very metal poor old stars (from 1/10 down to 1/10000 solar) But: relative abundances between metals always about the same

3 Solar Neighborhood Constraints: Local column densities of gas, stars, SFR as well as Age Metallicity (uncertain) O/Fe vs Fe/H Metallicity distribution (requires slow infall) O/Fe vs Fe/H (requires SNIa for late Fe) Age - Metallicity Metallicity distribution

4 Dame 1993 Nakanishi and Sofue 2003 Olling and Merrifield 2001 Pohl et al The Milky Way disk: (1) Gas profiles BULGE data in all panels: Ferriere et al Dame 1993 Nakanishi and Sofue 2006 Olling and Merrifield 2001 Kalberla and Dedes DISK HII data: Cordes and Lazio 2002 HI: relatively flat, H2: peaks in inner disk Bulge

5 The Milky Way disk: (2) Star and SFR profiles Exponential stellar profile Σ(R) = Σ 0 exp(-r/s R ), S R ~ 2.5 kpc Gas Mass ~ M Star Mass ~ M Inner disk more processed than outer disk (gas fraction profile) Disks Thin Thick Scalelength (kpc) Scaleheight (kpc) Local star surface density(m /pc 2 ) 3 < ~30 ~10

6 Radial dependence of SFR Ohnisi (1975), Wyse and Silk (1989): SFR induced by spiral density waves, hitting the ISM with frequency Ω Ω P [ Ω=V(R)/R and Ω P = frequency of spiral pattern ] For V(R) const and Ω P Ω SFR V(R)/R R -1 Alternatively : SFR GAS N STAR M Dopita and Ryder N+M 2.5 N=5/3 and M =1/3 or N=1 and M=0.5 Shi et al N=1 and M=0.35

7 The Milky Way disk: Evolution /Gyr 4 Inside-Out formation and radially varying SFR efficiency required to reproduce observed SFR, gas and colour profiles (Scalelengths: R B 4 kpc, R K 2.5 kpc) (Boissier and NP1999)

8 Observation of abundance profiles at high redshift in lensed galaxies The abundance profiles appear to be much steeper than typical profiles of local disks SDSSJ ( the Clone ) Z~2 SP1149 Z~1.5 Jones et al Yuan et al Yuan et al. 2011

9 HISTORY OF MILKY WAY DISK IN THE PAST 12 GYR M STARS increased by 3-4 ; M GAS remained ~constant ; Z GAS increased by 2-3 De Lucia and Helmi 2008 N-body + SPH Stars Gas Boissier and Prantzos 1999 Semi-analytical Boissier and Prantzos model 1999 Zgas/Zsun Simple models constitute a good approximation of more complex (realistic?)ones

10 Inadequacy of the simple, independent-ring models Casagrande et al GCS Survey of Solar neighborhood (Nordstrom et al. 2004, Holmberg et al. 2009) -Little metallicity evolution in the past 10 Gyr - Large scatter in Fe/H at all ages (also Edvardssson et al. 1993) Old stars of both high and low metallicities Inadequacy of the simple (independent ring) model (Edvardsson et al. 1993, Haywood 2006)

11 Thick Thin Data: Reddy et al The issue of the thick disk Old (>9 Gyr) stars, away from the plane having higher O/Fe ratio than thin disk stars of the same metallicity Thick Thin

12 THE CHEMICAL EVOLUTION OF THE MILKY WAY DISK 1. The simple picture of disk evolution: independent ring evolution, successes and failures 2. The dynamical picture: stars (and gas) moving around 3. A model for the MW disk

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14 Infall Fountain Gas flows Radial inflow Radial mixing Minor merger Disk heating Star motions

15 Stellar orbits change through interactions with inhomogeneities of gravitational potential (molecular clouds, spiral arms, bar) Resonant interactions at corotation may induce radial mixing of stars far beyond what is expected from simple epicyclic motion

16 EFFECT OF BAR AND SF Friedli, Benz, Kennicutt 1994 Stars No bar No SF Gas Bar No SF No Bar SF Bar SF

17 M 101 non barred Strong gradient Martin and Roy (1994) The value of the abundance gradient is crucial regarding the action of a galactic bar A bar drives (metal-poor) gas from the corotation region inwards, and this radial inflow tends to reduce abundance gradients Dors and Copetti (2005) NGC 1365 barred Weak gradient

18 EFFECT OF TRANSIENT SPIRALS (Sellwood and Binney 2002) Stars just inside corotation swap places with those just outside it; Radial migration (churning) without heating radially the disk It can «naturally» explain (assuming a disk metallicity gradient) the observed dispersion in local age-metallicity relation Casagrande et al Sellwood and Binney 2002

19 Impact of radial migration on local disk (Roskar et al. 2008, N-body+SPH) Fraction of stars now present In Solar neighborhood, but born in other places of the disk Only 50% born in situ! This impacts the Age-metallicity relation Making it flatter and more dispersed And the local Metallicity distribution Making it broader What if the Sun has migrated from the inner (higher than local metallicity) MW disk?

20 Bars (and asymmetries in the gravitational potential in general) affect chemical evolution by: 1) Moving around a «passive agent» of chemical evolution : the gas 2) Moving around a tracer of chemical evolution : low-mass long lived stars (~1 M ~10 Gyr) 3) Moving around another «passive agent» of chemical evolution : Low mass long lived stars (several Gyr) which dilute metallicity by ejecting lately metal poor gas (impact on D) 4) Moving around some active agents of chemical evolution : long lived nucleosynthesis sources SNIa: Fe-peak elements M stars (1-3 Gyr): s-process elements

21 Probabilistic treatment of radial migration (Sellwood and Binney 2002)) Blurring (epicycles) Churning (radial migration) û C = ë(r)ü N + ì(r) Epicyclic frequency Radial velocity dispersion Coefficients (r,t) and β (r,t) extracted from the numerical simulation of KPA2013 at t=2.5 Gyr (bar similar in size to the one of the Milky Way)

22 N-body+SPH simulations of a disk galaxy (Kubryk, NP, Athanassoula MNRAS 2013) DM halo + baryonic disk + SFR + chemistry (1 «metal» + IRA), no infall GAS 1 Gyr 5 Gyr 10 Gyr STARS «Early type galaxy» with a strong bar, formed after the first 2 Gyr

23 STARS GAS

24 Modelling radial migration as a diffusion process with coefficients depending on time and radius (see also Brunetti et al. 2011) σ(r0,t) = a(r0) t N + b(r0)

25 Evolution of global quantities for Milky Way Bulge (<2 kpc) and Disk (>2 kpc)

26 Comparison to average («stacked») evolution of van Dokkum et al. (2013) with 3D-HST and CANDELS data up to z~2.5

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28 In the MW also, the SFR follows better the molecular gas SFR / H 2 / f 2 Î GAS H2 f 2 = HI+H2 H R mol = 2 HI Blitz and Rossolowski 2006 :

29 Radial migration affects a large fraction of the disk In the solar neighborhood it brings stars mostly from inner regions,, (on average, 1.5 kpc inwards) mostly older than the locally formed, ones (by 1.5 Gyr)

30 Solar neighborhood 1. Older stars come from inner regions 2. The local age-metallicity relation flattens ; dispersion in Fe/H increases with age except for the oldest stars (thick disk) Data: Bensby et al Very little dispersion in O/Fe (best «chronometer»?

31 Solar Neighborhood Radial Migration 1. Modifies the apparent local SFR Casagrande et al Bensby et al Dispersion 2. Creates dispersion in the age-metallicity relation Casagrande et al Bensby et al Total Epicycles 3. much more than the epicyclic motion

32 Solar Neighborhood Radial Migration 1. Increases the average stellar age by ~1 Gyr In situ In situ + epicycles In situ + Epicycles + RadMigr 2. and brings locally stars from ~1. kpc inwards (on average) 3. The most metal-rich local stars come from several kpc inwards and are ~4 Gyr old

33 Solar Neighborhood : stars with different ages and from different regions at all metallicities Obs (CGS ) Casagrande et al. 2011

34 Data: Adibekyan et al Data: Bensby et al Assuming that the thick disk is the old disk (>9 Gyr) we recover the [a/fe] vs Fe/H behaviour and the metallicity distributions of both the thick and thin disks

35 The yields of Nomoto et al have some problems in the region P to V

36 Evolution of thin (<9 Gyr) and thick (>9 Gyr) disks with yields NORMALISED to solar for AVERAGE LOCAL (8 kpc) STAR 4.5 Gyr old Data: Adibekyan et al Data: Bensby et al. 2014

37 O Abundance profiles Fe

38 Abundance profiles in Cepheids

39 In situ After Radial Migration Radial migration flattens the past stellar profiles (Roskar et al. 2008) bringing old and metal poor stars in the outer disk

40 SUMMARY 1 Radial profiles available for several variables (stars, gas HI and H2, SFR, colours, chemical abundances) Observations suggest an inside-out formation of the stellar disk (redder inside) and a larger SFR efficiency in inner regions Implications for SFR (radial or H2 dependence, H2 dependence) and/or infall prescriptions Simple models (semi-analytical, independent-ring) of MW and other disks provide a satisfactory first idea of disk evolution (global quantities and radial profiles of stars, gas HI and H2, SFR, colours, metals, prediction of abundance gradients flatenning with time, as observed) as well as the solar neighborhood evolution of the MW However, they fail to reproduce several key local features (dispersion in age-metallicity, old and metal-rich stars, thick vs thin O/Fe vs Fe/H) Other ingredients required

41 SUMMARY 2 Impact of star migration on chemical observables of the disk LOCALLY: - Increases dispersion in age-metallicity relation - Broadens metallicity distribution - Modifies apparent SF histories(more in outer disk than in inner disk) GALAXYWIDE: - Flattens abundance profiles of X/H - Modifies profiles of X/Y with X and Y produced by different sources: short-lived (0) vs long-lived (Fe or s-elements) - Erases past abundance profiles of stars - May produce a thick disk BUT these observables are ALSO affected to various extents by other factors e.g. infall [ dm/dt(r,t) and Z(R,t) ], galactic fountains/outflows, mergers, etc. It will not be easy to disentangle those effects even after systematic observations (e.g. GAIA for the MW)