The Stellar History of The Galaxy Rosemary Wyse Valencia, June 27, 2006 Bernard’s 5th PhD student, from Cambridge period

The Fossil Record   Stars of mass like the Sun live for the age of the Universe – studying low-mass old stars allows us to do Cosmology locally.   There are copious numbers of stars nearby that formed at redshifts > 2 (ages/lookback times of > 10 Gyr)   Complementary approach to direct study at high redshift.   Stars retain memory of initial/early conditions – age, chemical abundances, orbital angular momentum (modulo resonances, torques)

Exciting times to be studying resolved stellar populations: Large, high-resolution simulations of structure formation are allowing predictions of Galaxy formation in a cosmological context Large observational surveys of stars in Local Group galaxies are now possible using wide- field imagers and multi-object spectroscopy High-redshift surveys are now quantifying the stellar populations and morphologies of galaxies at high look-back times

Clues from the Fossil Record   Star formation history   Chemical evolution   Merging history: for which systems have we derived SFH? Match models? CDM?   Stellar Initial mass function   Is the Milky Way typical?   Is the Local Group typical?

The Local Group The motions, spatial distributions and chemical elemental compositions can be measured (with varying accuracies!) for individual stars in galaxies throughout the Local Group The Milky Way, M31, M33, gas-rich and gas-poor satellites Analyse to test models e.g.  CDM

Formation of a disk galaxy in  CDM Abadi et al 2003 Stars are colour-coded by age: red = old, blue = young Face-onEdge-on

Consequences of mergers:   Orbital energy goes into internal degrees of freedom of the merging systems   Low density outer regions of smaller systems tidally removed   Thin disks are heated: gas cools, stars do not   Angular momentum is redistributed – outer parts gain and inner parts lose   Gas and stars driven to the center (bar helps)   Disk formed subsequently has short scale- length : corollary, need angular momentum conservation to form extended disks as observed (Fall & Efstathiou 1980)

Predictions for disk galaxies:   Extended disks form late, after most merging complete, or redshift ~ unity (~8Gyr ago) (mass-dependent, M  )   Hundreds of satellite dark haloes   Stellar halo formed from disrupted satellites   Minor mergers (< 20% mass of disk ) heat thin disk, create thick disk and add gas to bulge   More significant mergers transform disk galaxy to SO or even elliptical   (Re-) accrete gas to re-form disk   Perhaps accrete stars too into disks

Left : z=10, small haloes dominate. Red indicates possible site of star formation at this time (very dense regions). Right: Present time, many of the small haloes have merged into the model Milky Way halo; oldest stars found throughout the Milky Way (most in bulge) and in satellites CDM simulation of the Local group Moore et al Mpc box 300kpc box

Stellar Components of the Milky Way Galaxy:   Thin disk: large-scale structure is exponential with scale-length of ~3kpc and scaleheight of older stars of ~300pc. Mass ~ 6 x M    Thick disk: exponential scale-length ~3kpc, scale- height of ~ 1kpc, local normalisation ~5%   Central bulge: exponential scale-length ~500pc, mildly triaxial, scale-height ~300pc, mass ~10 10 M    Stellar halo: power-law density profile beyond solar circle, total mass ~ 10 9 M 

The Thin Disk: SFH   Best studied at the solar neighborhood   Star formation history locally is consistent with early onset, with oldest stars ~2-3 Gyr younger than metal-poor globulars (e.g. Hipparcos data analyses of Binney et al 2000 & Sandage et al 2003; Nordstrom et al 2004), or ~11Gyr   Evidence for ‘bursts’ of amplitude 2—3, perhaps superposed on slow decline (e.g. Gilmore et al 2000; Rocha-Pinto et al 2000); spiral arm passages?

Old stars in local thin disk formed at redshift z > 1.5   = 0.7,  M =0.3   = 0,  M = 0.3 Ages of oldest stars from Binney et al 2000

The Thin Disk: old stars   Scale length of old stars is ~ kpc (e.g. Siegel et al 2002) thus if the old stars were formed in the disk, star formation was initiated at ~ 3 scalelengths at z > 1.5   Then the formation of extended disks was not delayed until after a redshift of unity, as has been proposed in CDM-models with feedback (e.g. Weil et al 1998; Thacker & Couchman 2001)   M31 also shows extended disk in older stars (Ferguson & Johnson 2001; Guhathakurta 2004).   Problem for CDM models…(?)

  Or is the old thin disk stellar debris from accreted satellites? cf. Abadi et al 2003   Ongoing (e.g. RAVE, SDSS2/SEGUE) spectroscopic surveys will detect substructure in the thin disk, and constrain the merger history M. Williams poster

Tides: Satellite Snacks K.V. Johnston

Ongoing snacking….. Sgr dSph as known in 1997 Wyse, Gilmore & Franx 1997

2Mass revealed streams from Sagittarius dwarf around the sky (Majewski et al 2003)

Field of Streams SDSS data, 19< r< 22, g-r < 0.4 colour-coded by magnitude/distance, blue (~10kpc), green, red (~30kpc) Belokurov et al (2006) disk accretion?

Belokurov et al 06

Thin disk IMF:  Salpeter slope, or slightly steeper, for massive stars  Slope flattens around 0.5 M , perhaps peaks  Only low-significance evidence for variations, especially when take binarism, variable extinction and mass-segregation in clusters into account and observe wide area e.g. Kroupa for central Arches cluster dynamical evolution can cause sufficient mass segregation to explain observations (Stolte et al 2002; Kroupa 2004)

The Thick Disk  Defined 20 years ago (Gilmore & Reid 1983) through star counts  Local normalisation ~5%, scaleheight ~1kpc, factor ~ 3 thicker than thin disk, same scalelength ~3kpc; mass ~10--20% of thin disk, i.e. ~10 10 M   Well-established now as a distinct component, not tail of stellar halo or of thin disk, by kinematics, metallicity and age distributions.  Similar structures seen in external disk galaxies Mould 2004, Yoachim & Dalcanton 2005

The Thick Disk: OLD Gilmore, Wyse & JB Jones 1995 Few stars are bluer than the old turnoff at a given metallicity, indicated by x or *. Consistent with old age, ~ same as 47 Tuc, ~ 12 Gyr (open circle) Scatter plot of Iron abundance vs B-V for F/G stars 1—2 kpc above the Galactic Plane

The Thick Disk:   Different pattern of elemental abundances than in thin disk: different star formation histories   Same ‘type II plateau’ value implying invariant massive star IMF.   Downturn implies > 1Gyr age spread Bensby et al 2004 Thick (filled) and thin disk (open) stars show distinct trends ~

Elemental Abundances   Type II supernovae have progenitors > 8 M  and explode on timescales ~ 10 7 yr, less than typical duration of star formation   Main site of  -elements, e.g. O, Mg, Ti, Ca, Si   Low mass stars enriched by only Type II SNe show enhanced ratio of  -elements to iron, with value dependent on mass distribution of SNe progenitors – if well-mixed system, see IMF-average   Type Ia SNe produce very significant iron, on longer timescales, few x 10 8 – 10 9 yr (binaries)

Gibson 1998 Progenitor mass Ejecta Type II Supernova yields Salpeter IMF gives [  /Fe] ~ 0.4

Schematic [O/Fe] vs [Fe/H] Wyse & Gilmore 1993 Slow enrichment SFR, winds.. Fast IMF biased to most massive stars Self-enriched star forming region. Assume good mixing so IMF-average yields Type II only Plus Type Ia

LMC stars show sub-solar ratios of [  /Fe], consistent with expectations from extended star formation. Smith et al 2003Gilmore & Wyse 1991 Hiatus then burst Continuous star formation gas

Formation of Thick Disk  High stellar velocity dispersions (  W ~ 40 km /s and  tot ~ 80 km/s) argue against normal disk heating mechanisms e.g. GMC, spiral arms, as they saturate at  W ~ 20 km/s of old disk  Old age plus continual star formation in the thin disk argues against exceptional heating of thin disk (e.g. by massive halo black holes, Lacey & Ostriker 1985 ) unless only very early  Merger-induced heating of thin disk, by accretion of fairly massive and dense satellite?  Lack of vertical gradients difficult for slow dissipational settling (e.g. Burkert et al 2002)

Merger-heating is re-expression of out-of-equilibrium heating of Jones & Wyse?

The Thick Disk: merger-heating   If merger origin through heated thin disk, last significant (> 20% mass ratio to disk, robust dense satellite) dissipationless merger happened a long time ago, (~12 Gyr or z~ 2) And disk in place then. Velazquez & White 1999 Thick disk will be mix of satellite debris plus heated disk – seen? Gilmore, RW & Norris 02

Berlind, priv comm  CDM, 1000 realisations of MW-mass halo, now M  Halo of the mass of the Milky Way will typically have experienced 1—2 mergers with mass ratio of > 0.2 satellite halo: total halo in the past 10Gyr. Do not reach regime for thick disk : many more.

Shredded satellite will contribute to ‘thick disk’ Huang & Carlberg 1997

The local thick disk is quite metal-rich; if accreted debris dominates, need large system to be this enriched long ago when thick-disk stars formed.

The Thick Disk: OLD – but how old?   Reliable ages very important since dates last significant merger to heat disk: typically in  CDM expect several 10-20% to TOTAL mass mergers after z=2: need higher-resolution simulations for the 20% to disk mass mergers that can form thick disks   In situ sample selection also important since can have contamination of local ‘thick disk’ by local thin disk stars ejected by e.g. binary supernova

The Central Bulge:   Age of the dominant population constrained by HST and ISO Color-Magnitude Diagrams : for projected Galactocentric distances of > 300pc, typical age is OLD, ≥10 Gyr; closer in, see younger stars (disk?) van Loon et al 03   Mean metallicity ~ –0.2 dex (e.g. McWilliam & Rich 1994; Ibata & Gilmore 1995) :   ~ solar metallicity, low gas fraction at z ~ 2, like red galaxies!   Enhanced alpha elemental abundance ratios (Fulbright McWilliam & Rich 06; Cunha et al 06) some decline as [Fe/H] increases: fixed massive IMF   Low-mass IMF same as metal-poor globulars (Zoccali et al 2000) – same as in Ursa Minor dSph (Wyse et al 2002) and in local disk

The Central Bulge: old Van Loon et al 2003 BW=0.9,-4 Age distributions determined from ISO color-magnitude data. Old age also from HST CMDs e.g. Zoccali et al 2003 l,b=0,1

Low-Mass MF in Bulge: Zoccali et al 2000

UMi dSph I-band LF M92 M15 50% completeness I=27.2 M 814 =+8.1, M  0.3 M  I-band luminosity functions are indistinguishable. STIS/LP data and V-band data similar limits, agree. Piotto et al 97; Shifted and renormalised   Wyse et al 2002 NGC7078

The Central Bulge: Formation   During mergers, expect disk stars and gas to be added to the bulge (cf. Kauffmann 1996)   Also expect gas inflows driven by the bar (Gerhardt 2001)   Bulge is dominated by old, metal-rich stars, with high [  /Fe], not favoring recent mergers, or recent disk instability to form a bar/pseudo-bulge   All point to intense burst of star formation in situ a long time ago, SFR ~ 10 M  /yr   Early merger – related to thick disk? – or simply low angular momentum gas?

Bulge—Stellar halo connection? Wyse & Gilmore 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 cf Hartwick 1979 Bulge, halo Thick, thin disks

The Stellar Halo :   Stellar halo traced by high-velocity stars locally -- ~ 30% of total mass of ~ 2 x 10 9 M  -- is rather uniform in properties: old and metal-poor, enhanced elemental abundances indicating short duration of star formation, in low-mass star- forming regions, with ‘normal’ IMF.  (  Unlike most stars in satellite galaxies now (cf. Tolstoy et al 2003)   Accretion from stellar satellites not important for last ~8Gyr for local halo (cf. Unavane et al 1996) – no more than 10% from typical satellite since then, biased to metal-rich stars.

Unavane, Wyse & Gilmore 1996 Scatter plot of [Fe/H] vs B-V for local high-velocity halo stars (Carney): again few stars bluer (younger) than old turnoffs (5Gyr, 10Gyr, 15Gyr Yale) Stellar halo is OLD

Carina dSph Leo I dSph Hernandez, Gilmore & Valls-Gabaud 2000 Intermediate-age population dominates in typical dSph satellite galaxies – Ursa Minor atypical, has dominant old population, and narrow metallicity spread (also normal IMF Wyse et al 2002) Caveat: assume fixed metallicity, but intermediate-age secure

 Field stellar (inner) halo cannot have formed from dSph that were accreted after the formation of the dSph dominant intermediate-age population – this limits accretion to have occurred > 8Gyr ago.  Perhaps more stringent limits come from the different elemental abundances, since timescale for Type Ia SNe only a few Gyr, but need detailed chemical evolution models.  Halo can be formed from any system that formed stars early on, for only brief period, and did not self enrich significantly.

Tolstoy et al 2003 Large open colored symbols are stars in dwarf Spheroidals, black symbols are Galactic stars: the stars in typical satellite galaxies tend to have lower values of [  /Fe] at a given [Fe/H], Consistent with fixed IMF and extended SFH.

How well-mixed was the stellar halo?  There is a remarkable lack of scatter in the elemental abundance ratios of [  /Fe] for metal-poor local halo stars, implying enrichment by a well-sampled massive-star IMF and good mixing – how was this achieved?  Few star-forming progenitors?  In CDM form halo only from the ~10 most massive, earliest collapsing satellites (Bullock & Johnston 05)

Cayrel et al Cosmic scatter in elemental abundances of metal poor halo stars is extremely low, 0.05 dex – fully sampled IMF of massive stars? Invariant IMF [  /Fe] from Type II Supernovae depends on progenitor mass ‘Type II plateau’

 Outer halo may be younger: globular clusters indicate perhaps half around 8–10 Gyr, including Sgr dSph clusters, rather than 10-12Gyr  Accreted as dwarf galaxies plus globulars? Structural and HB morphologies similar to those in Fornax dSph, Sgr dSph, LMC (Mackay & Gilmore 2004)  Halo stars with low [  /Fe] may be accreted, or may just have formed in denser more-bound blobs. Those known have high-energy, radial orbits. Outer Stellar Halo

Outer Stellar Halo:   The outer halo, with dynamical timescales of > 1Gyr, is best place to find structure. Several streams found, in both coordinate space and kinematics   Most due to the Sagittarius Dwarf e.g. Ibata et al 2001; Majewski et al 2003   V ery fast-moving field! Several (~ 5) candidate new dSph and streams announced this year (spot them in the Field of Streams…)   mass function crucial for ‘satellite problem’

Concluding remarks All stellar components of the Milky Way contain very old stars (but where are first stars?) Little evidence for variations in stellar IMF, over wide range of metallicity, age, local density… Small-scale problems with  CDM persist, but things are evolving rapidly and the next few years will really see model predictions and observations able to confront one another

Complementary high-redshift data:   Analysis of sizes of disks as a function of luminosity and of stellar mass, based on rest- frame optical imaging for galaxies out to z ~ 3, (Trujillo et al 2005) concluded little evolution in sizes, significantly less than predicted by semi- analytic CDM models e.g. Mo, Mao & White 1999   Consistent with simple gaseous infall and star- formation in fixed potential, SFR highest in central regions: ‘stellar disks form from early- on in large halos’ (Trujillo et al 2005)

 Most stars form in unbound systems, not clusters; indeed present cluster population (1% of halo) occupies ‘survivor’ phase space (cf. Fall & Rees 1977)  Short duration of star formation in individual regions leads to enhanced [  /Fe]  Low angular momentum -- Gas loss leads to reduced mean metallicity cf true yield, flows to central regions… Field Inner Stellar Halo as Dissolved Star-Forming Regions

Dwarf Spheroidals   Low luminosity, low surface-brightness satellite galaxies, L ~ 10 6 L ,  V ~ 24 mag/    Extremely gas-poor   Apparently dark-matter dominated  ~ 10km/s, 10 < M/L < 300   Metal-poor, mean stellar metallicity < –1.5 dex   Extended star-formation histories typical, from earliest epochs   Important tests for CDM models: mass function, ρ(r), luminosity function, tidal effects…. ~ ~

Cosmological context   (  )CDM predicts dwarf galaxies are the first to form stars: building block of bigger systems?   How do survivors differ?   Need to hide most dwarf dark haloes (Klypin et al 1999; Moore et al 1999)   Radiative feedback perhaps as important as SNe in truncating star formation particularly at lowest potential wells (Efstathiou 1998; Bullock et al 2000)   Can we understand their star formation history?   Dark halo scaling properties define dSph (Dekel & Silk 1986; Dekel & Woo 2003; Kormendy & Freeman 2004)

Merger History: Constrain by characterizing the stellar populations in components predicted to be predominantly formed in mergers: stellar halo, thick disk, bulge For the Milky Way, dominantly OLD, seems to have been rather quiescent since z ~ 2; atypical in CDM? M31 more violent history? (A. Ferguson et al 2002; Brown et al 03) M33 more quiet – is there a thick disk, bulge, halo?

LMC as template thick disk progenitor? Cole et al 2000 Hill et al 2000 Metallicity distribution of inner LMC disk agrees, and total stellar mass about right, but stars are intermediate- age -- took many more Gyr to self-enrich to this level.

Globular clusters also can give rise to streams; streams not necessarily a signature of accretion Odenkirchen et al 2003; Pal 5 R gc =18.5kpc