Galactic Structure STScI May 2003 Clues to the Mergingand Star Formation Histories  Clues to the Merging and Star Formation Histories How typical is the Galaxy?  How typical is the Galaxy? Quiescent merging since z ~ 2 – accreted only low-mass, diffuse stellar systems and/or gas Deciphering the Milky Way Galaxy

Galactic Fossils Stars of mass like the Sun live for the age of the Universe – studying low-mass old stars in our Galaxy allows us to do Cosmology locally. Stars of mass like the Sun live for the age of the Universe – studying low-mass old stars in our Galaxy allows us to do Cosmology locally. Complementary approach to direct study at high redshift. Complementary approach to direct study at high redshift. Retain some memory of initial conditions -- Chemical abundances, orbital angular momentum (modulo resonances, torques) Retain some memory of initial conditions -- Chemical abundances, orbital angular momentum (modulo resonances, torques)

Large Scale Structure Thin stellar disk : extended, old disk, high angular momentum, stars of all ages Thin stellar disk : extended, old disk, high angular momentum, stars of all ages Thick disk : old, plausibly created by heating a thin stellar disk a long time ago – heated stellar system will not cool. Thick disk : old, plausibly created by heating a thin stellar disk a long time ago – heated stellar system will not cool. Central bulge : centrally concentrated, dominant population is old and metal rich Central bulge : centrally concentrated, dominant population is old and metal rich Stellar halo : fairly uniform population, old and metal-poor Stellar halo : fairly uniform population, old and metal-poor

The Thin Disk: LSS Best studied at the solar neighborhood Best studied at the solar neighborhood Star formation history locally is consistent with early onset, with oldest stars ~1-2 Gyr younger than metal-poor globulars (e.g. Hipparcos data analysis of Binney et al 2000; 11 Gyr age). Star formation history locally is consistent with early onset, with oldest stars ~1-2 Gyr younger than metal-poor globulars (e.g. Hipparcos data analysis of Binney et al 2000; 11 Gyr age). Evidence for `bursts’ of amplitude 2—3, perhaps superposed on slow decline (e.g. Gilmore et al 2000; Rocha-Pinto et al 2000) Evidence for `bursts’ of amplitude 2—3, perhaps superposed on slow decline (e.g. Gilmore et al 2000; Rocha-Pinto et al 2000)

Hernandez et al 2000 Local recent star formation rate varied, by factors of several, with period ~0.5Gyr. Overall SFR is ~200 M o /Myr/kpc 2, slowly declining with time.  Local recent star formation rate varied, by factors of several, with period ~0.5Gyr. Overall SFR is ~200 M o /Myr/kpc 2, slowly declining with time. Hipparcos data Volume-limited

The Thin Disk: LSS  Scale length of old stars is ~ 2—4 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 ~ 2  Then the formation of extended disks was not delayed until after a redshift of unity, as has been proposed in some 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 talk).

The Thick Disk: LSS Best studied locally : old age, intermediate metallicity, most stars have enhanced alpha-elements, some evidence for lower values at higher metallicities Best studied locally : old age, intermediate metallicity, most stars have enhanced alpha-elements, some evidence for lower values at higher metallicities Fuhrmann 2003

The Thick Disk: LSS Gilmore, Wyse & 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 local thick disk is quite metal-rich; if accreted need large system to be this enriched long ago.

The Thick Disk: LSS If merger origin through heated thin disk, last significant (> 20% mass ratio to disk, robust dense satellite) dissipationless merger happened a long time ago, 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) (~12 Gyr or z~ 2) And disk in place And disk in place then. then. Velazquez & White 1999

The Central Bulge: LSS 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 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 Mean metallicity ~ -0.3 dex (e.g. McWilliam & Rich 1994; Ibata & Gilmore 1995) Mean metallicity ~ -0.3 dex (e.g. McWilliam & Rich 1994; Ibata & Gilmore 1995) Enhanced alpha elemental abundances (McWilliam & Rich 1994) Enhanced alpha elemental abundances (McWilliam & Rich 1994)

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

The Central Bulge:LSS During mergers, expect disk stars and gas to be added to the bulge (cf. Kauffmann 1996) 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) Also expect gas inflows driven by the bar (Gerhardt 2001) Bulge is dominated by old, metal-rich stars, not favoring recent mergers, or recent disk instability Bulge is dominated by old, metal-rich stars, not favoring recent mergers, or recent disk instability All point to intense burst of star formation in situ a long time ago, SFR ~ 10 M o /yr All point to intense burst of star formation in situ a long time ago, SFR ~ 10 M o /yr

The Stellar Halo:LSS Stellar halo traced by high-velocity stars locally -- ~ 30% of total mass of ~ 2 x 10 9 M o -- is rather uniform in properties: old and metal-poor, enhanced elemental abundances indicating short duration of star formation, in low-mass star- forming regions Stellar halo traced by high-velocity stars locally -- ~ 30% of total mass of ~ 2 x 10 9 M o -- is rather uniform in properties: old and metal-poor, enhanced elemental abundances indicating short duration of star formation, in low-mass star- forming regions Unlike most stars in satellite galaxies now (cf. Tolstoy et al 2003) 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. 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.

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].

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)

Large Scale Structure: Merging Bulk properties of halo, thick disk and bulge are consistent with little merging and accretion of stars for at least 10Gyr. Bulk properties of halo, thick disk and bulge are consistent with little merging and accretion of stars for at least 10Gyr. Can compare with merging trees of N-body simulations. Can compare with merging trees of N-body simulations. But haloes But haloes not galaxies not galaxies Helmi et al 2003, after Lacey & Cole

Merging History Virgo GIF  CDM simulations (White et al 1999) have only 7% of final halos with mass similar to the Milky Way (2 x M o ) have not merged with another halo of at least 20% by mass since a redshift of 2 – parameters chosen to match the age and mass ratio estimated needed to form the thick disk (modulo density, gas etc) Virgo GIF  CDM simulations (White et al 1999) have only 7% of final halos with mass similar to the Milky Way (2 x M o ) have not merged with another halo of at least 20% by mass since a redshift of 2 – parameters chosen to match the age and mass ratio estimated needed to form the thick disk (modulo density, gas etc) If reduce to 10% mass ratio then none (0/26) have no such merger since z=2 If reduce to 10% mass ratio then none (0/26) have no such merger since z=2 If set last merger to z=1, increase to 35% and 4% If set last merger to z=1, increase to 35% and 4% Are there signatures of the minor mergers? Are there signatures of the minor mergers?

Small Scale Structure Thin disk : moving groups, scatter in age- metallicity relationship, spiral arms, central bar, outer ring (?) Thin disk : moving groups, scatter in age- metallicity relationship, spiral arms, central bar, outer ring (?) Thick disk : some complexity in all properties? Many ways of heating stars Thick disk : some complexity in all properties? Many ways of heating stars Bulge : asymmetries – bar-related? Bulge : asymmetries – bar-related? Stellar halo : Outer halo contains many streams – all due to Sagittarius dwarf? Stellar halo : Outer halo contains many streams – all due to Sagittarius dwarf?

Thin Disk : Small Scale Structure Scatter in the age-metallicity relationship for stars and the differences between solar and local ISM chemical abundances can be explained by combination of radial mixing (e.g. Sellwood & Binney 2002) and infall of metal-poor gas (e.g. Geiss et al 2002). Gas may be from either companion galaxies or the general IGM. Scatter in the age-metallicity relationship for stars and the differences between solar and local ISM chemical abundances can be explained by combination of radial mixing (e.g. Sellwood & Binney 2002) and infall of metal-poor gas (e.g. Geiss et al 2002). Gas may be from either companion galaxies or the general IGM.

Thin Disk: SSS The ‘ring’ seen in star counts (Newberg et al 2002; Ibata et al 2003) at Galactocentric distances of ~15kpc may be associated with the outer disk The ‘ring’ seen in star counts (Newberg et al 2002; Ibata et al 2003) at Galactocentric distances of ~15kpc may be associated with the outer disk Hierarchical clustering Hierarchical clustering predicts merger remnants predicts merger remnants in the thin disk in the thin disk (e.g. Abadi et al 2002) (e.g. Abadi et al 2002) Large kinematic/ Large kinematic/ metallicity surveys e.g. metallicity surveys e.g. RAVE should find many RAVE should find many

Thick Disk: SSS If the thick disk is formed by a merger heating a pre-existing thin disk, expect some remnant of the satellite that was responsible. Indeed hierarchical clustering If the thick disk is formed by a merger heating a pre-existing thin disk, expect some remnant of the satellite that was responsible. Indeed hierarchical clustering models predict a models predict a complex mix. complex mix. e.g Abadi et al 2002 e.g Abadi et al 2002

Thick Disk: SSS Structure in the thick disk, or steep gradient in kinematics and metallicity, apparent in large sample of faint (V=18-19) F/G stars compared to local samples Structure in the thick disk, or steep gradient in kinematics and metallicity, apparent in large sample of faint (V=18-19) F/G stars compared to local samples But even local samples yield varying results But even local samples yield varying results

Gilmore et al 2002  Solid lines are data for stars in rotation fields, line-of-sight velocity probes azimuthal streaming. line-of-sight velocity probes azimuthal streaming. Dashed histogram is model; top panel `standard’ Dashed histogram is model; top panel `standard’ thick disk, lower panel increased lag to 100km/s. thick disk, lower panel increased lag to 100km/s.

Norris et al  Canonical thick disk has mean metallicity –0.6dex  These stars are more metal-poor; structure in df?  Connected to ‘ring’? Satellite debris?

Thick Disk: SSS However, parameter values for even ‘canonical’ thick disk still vary from study to study e.g. scale height, normalisation and rotational lag behind the Sun However, parameter values for even ‘canonical’ thick disk still vary from study to study e.g. scale height, normalisation and rotational lag behind the Sun Furhmann 2003 Local thick disk in this sample has lag of ~80km/s, like the distant F/G stars……..‘usual’ value is ~35km/s

Central Bulge : SSS Bar-like asymmetry seen, but complicated by dust extinction – not so obvious in ISO maps (van Loon et al 2003) Bar-like asymmetry seen, but complicated by dust extinction – not so obvious in ISO maps (van Loon et al 2003) Current star formation in very central bulge/inner disk is high enough rate to form entire bulge, solar masses, if sustained for a Hubble time, but did not – why? Current star formation in very central bulge/inner disk is high enough rate to form entire bulge, solar masses, if sustained for a Hubble time, but did not – why? Central Black Hole : what role in governing star formation and evolution? (Ghez talk) Central Black Hole : what role in governing star formation and evolution? (Ghez talk)

Stellar Halo: SSS The outer halo, with dynamical timescales of > 1Gyr, is best place to find structure. Several streams found, in both coordinate space and kinematics (Majewski talk). The outer halo, with dynamical timescales of > 1Gyr, is best place to find structure. Several streams found, in both coordinate space and kinematics (Majewski talk). Are all due to the Sagittarius Dwarf? Most definitely are e.g. Ibata et al 2001; Majewski et al 2003 Are all due to the Sagittarius Dwarf? Most definitely are e.g. Ibata et al 2001; Majewski et al 2003 Streams are rare in the inner halo (contains most of the stellar mass!). Kinematics suggest one stream but mass uncertain (Helmi et al 1999; Chiba & Beers 2000). Streams are rare in the inner halo (contains most of the stellar mass!). Kinematics suggest one stream but mass uncertain (Helmi et al 1999; Chiba & Beers 2000).

Stellar Halo: SSS No structure seen in coordinate space of inner halo – 2pt correlation function flat for stars brighter than V=19 (e.g. Gilmore et al 1985; Lemon et al 2003). No structure seen in coordinate space of inner halo – 2pt correlation function flat for stars brighter than V=19 (e.g. Gilmore et al 1985; Lemon et al 2003). Smoothness of local halo implies either few streams or so many as to mimic a continuum, with only a few stars in each stream e.g. Gould (2003) Smoothness of local halo implies either few streams or so many as to mimic a continuum, with only a few stars in each stream e.g. Gould (2003)

Conclusions The Milky Way has merged with, is merging with, and will merge with, significant companion galaxies, contributing stars and gas – and dark matter The Milky Way has merged with, is merging with, and will merge with, significant companion galaxies, contributing stars and gas – and dark matter Most recent accretion probably predominantly gaseous – quiescent merger history, atypical in CDM models Most recent accretion probably predominantly gaseous – quiescent merger history, atypical in CDM models Large spectroscopic surveys will tightly constrain existence and origin of stellar substructure Large spectroscopic surveys will tightly constrain existence and origin of stellar substructure What about the rest of the Local Group? What about the rest of the Local Group?