Galactic Structure

STScI May 2003

Clues to the MergingClues to the Merging and Star Formation and Star Formation HistoriesHistories

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 GalaxyDeciphering the Milky Way Galaxy

Galactic Fossils

Stars of mass like the Sun live for the age of Stars of mass like the Sun live for the age of the Universe – studying low-mass old stars in the Universe – studying low-mass old stars in our Galaxy allows us to do Cosmology our Galaxy allows us to do Cosmology locally. locally.

Complementary approach to direct study at Complementary approach to direct study at high redshift. high redshift.

Retain some memory of initial conditions -- Retain some memory of initial conditions -- Chemical abundances , orbital angular Chemical abundances , orbital angular momentum (modulo resonances, torques) momentum (modulo resonances, torques)

Large Scale Structure Thin stellar disk : extended, old disk, high Thin stellar disk : extended, old disk, high

angular momentum, stars of all agesangular momentum, stars of all ages Thick disk : old, plausibly created by heating a Thick disk : old, plausibly created by heating a

thin stellar disk a long time ago – heated thin stellar disk a long time ago – heated stellar system will not cool.stellar system will not cool.

Central bulge : centrally concentrated, Central bulge : centrally concentrated, dominant population is old and metal richdominant population is old and metal rich

Stellar halo : fairly uniform population, old and Stellar halo : fairly uniform population, old and metal-poormetal-poor

The Thin Disk: LSS Best studied at the solar neighborhoodBest studied at the solar neighborhood Star formation history locally is consistent with Star formation history locally is consistent with

early onset, with oldest stars ~1-2 Gyr early onset, with oldest stars ~1-2 Gyr younger than metal-poor globulars younger than metal-poor globulars (e.g. Hipparcos (e.g. Hipparcos data analysis of Binney et al 2000; 11 Gyr age). data analysis of Binney et al 2000; 11 Gyr age).

Evidence for `bursts’ of amplitude 2—3, Evidence for `bursts’ of amplitude 2—3, perhaps superposed on slow decline perhaps superposed on slow decline (e.g. Gilmore et (e.g. Gilmore et al 2000; Rocha-Pinto et al 2000)al 2000; Rocha-Pinto et al 2000)

Hernandez et al 2000

Local recent star formation rate varied, by factors Local recent star formation rate varied, by factors of several, with period ~0.5Gyr . Overall SFR is of several, with period ~0.5Gyr . Overall SFR is ~200 M~200 Moo/Myr/kpc/Myr/kpc22, slowly declining with time. , slowly declining with time.

Hipparcos dataHipparcos dataVolume-limitedVolume-limited

The Thin Disk: LSS Scale length of old stars is ~ 2—4 kpc Scale length of old stars is ~ 2—4 kpc (e.g. Siegel (e.g. Siegel

et al 2002) et al 2002) thus if the old stars were formed in the thus if the old stars were formed in the disk, star formation was initiated at ~ 3 disk, star formation was initiated at ~ 3 scalelengths at z ~ 2scalelengths at z ~ 2

Then the formation of extended disks was Then the formation of extended disks was notnot delayed until after a redshift of unity, as has delayed until after a redshift of unity, as has been proposed in some CDM-modelsbeen proposed in some CDM-models with with feedbackfeedback (e.g. Weil et al 1998; Thacker & Couchman 2001) (e.g. Weil et al 1998; Thacker & Couchman 2001)

M31 also shows extended disk in older stars M31 also shows extended disk in older stars (Ferguson & Johnson 2001; Guhathakurta talk).(Ferguson & Johnson 2001; Guhathakurta talk).

The Thick Disk: LSS Best studied locally : old age, intermediate Best studied locally : old age, intermediate

metallicity, most stars have enhanced metallicity, most stars have enhanced alpha-elements, some evidence for lower alpha-elements, some evidence for lower values at higher metallicities 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 Few stars are bluer than the old turnoff at a given metallicity, indicated by x or *. Consistent with old metallicity, indicated by x or *. Consistent with old age, ~ same as 47 Tuc, ~ 12 Gyr (open circle)age, ~ same as 47 Tuc, ~ 12 Gyr (open circle)

Scatter plot of Scatter plot of Iron abundance Iron abundance vs B-V for F/G vs B-V for F/G stars 1—2 kpc stars 1—2 kpc above theabove theGalactic PlaneGalactic Plane

The local thick diskThe local thick diskis quite metal-rich; is quite metal-rich; if accreted need if accreted need large system to be large system to be this enriched longthis enriched longago.ago.

The Thick Disk: LSS If merger origin through heated thin disk, last If merger origin through heated thin disk, last

significant (> 20% mass ratio to disk, robust dense significant (> 20% mass ratio to disk, robust dense satellite) dissipationless merger happened a long satellite) dissipationless merger happened a long time ago,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 Age of the dominant population constrained

by HST and ISO Color-Magnitude by HST and ISO Color-Magnitude Diagrams : for projected Galactocentric Diagrams : for projected Galactocentric distances of > 300pc, typical age is OLD, distances of > 300pc, typical age is OLD, ~ 10 Gyr, closer in, see younger stars ~ 10 Gyr, closer in, see younger stars

Mean metallicity ~ -0.3 dex Mean metallicity ~ -0.3 dex (e.g. McWilliam & (e.g. McWilliam & Rich 1994; Ibata & Gilmore 1995)Rich 1994; Ibata & Gilmore 1995)

Enhanced alpha elemental abundances Enhanced alpha elemental abundances (McWilliam & Rich 1994)(McWilliam & Rich 1994)

The Central Bulge: LSS

Van Loonet al 2003

BW=0.9,-4

Age distributions determined from ISO Age distributions determined from ISO color-magnitude data. color-magnitude data. Old age alsoOld age also from HST CMDs from HST CMDs e.g. Zoccali et al 2003e.g. Zoccali et al 2003

l,b=0,1

The Central Bulge:LSS During mergers, expect disk stars and gas to During mergers, expect disk stars and gas to

be added to the bulge (cf. Kauffmann 1996)be added to the bulge (cf. Kauffmann 1996) Also expect gas inflows driven by the bar Also expect gas inflows driven by the bar

(Gerhardt 2001) (Gerhardt 2001) Bulge is dominated by old, metal-rich stars, Bulge is dominated by old, metal-rich stars,

not favoring recent mergers, or recent disk not favoring recent mergers, or recent disk instabilityinstability

All point to intense burst of star formation in All point to intense burst of star formation in situ a long time ago, SFR ~ 10 Msitu a long time ago, SFR ~ 10 Moo/yr/yr

The Stellar Halo:LSS Stellar halo traced by high-velocity stars locally Stellar halo traced by high-velocity stars locally

-- ~ 30% of total mass of ~ 2 x 10-- ~ 30% of total mass of ~ 2 x 1099 M Moo-- is -- is rather uniform in properties: old and metal-rather uniform in properties: old and metal-poor, enhanced elemental abundances indicating poor, enhanced elemental abundances indicating short duration of star formation, in low-mass short duration of star formation, in low-mass star-forming regionsstar-forming regions

Unlike most stars in satellite galaxies now Unlike most stars in satellite galaxies now (cf. (cf. Tolstoy et al 2003)Tolstoy et al 2003)

Accretion from stellar satellites not important Accretion from stellar satellites not important for last ~8Gyr for local halo for last ~8Gyr for local halo (cf. Unavane et al 1996) (cf. Unavane et al 1996) – – no more than 10% from typical satellite since no more than 10% from typical satellite since then, biased to metal-rich stars. then, biased to metal-rich stars.

Tolstoy et al 2003

Large open colored symbols are stars in dwarf Large open colored symbols are stars in dwarf Spheroidals, black symbols are Galactic stars: Spheroidals, black symbols are Galactic stars: the stars in typical satellite galaxies tend to have the stars in typical satellite galaxies tend to have lower values of [lower values of [/Fe] at a given [Fe/H]./Fe] at a given [Fe/H].

Unavane, Wyse Unavane, Wyse & Gilmore 1996& Gilmore 1996

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

Large Scale Structure: Merging Bulk properties of halo, thick disk and Bulk properties of halo, thick disk and

bulge are consistent with little merging and bulge are consistent with little merging and accretion of stars for at least 10Gyr. accretion of stars for at least 10Gyr.

Can compare with merging trees of N-body Can compare with merging trees of N-body simulations.simulations.

But haloes But haloes not galaxies not galaxies

Helmi et al 2003, after Lacey & Cole

Merging History Virgo GIF Virgo GIF CDM simulations (White et al 1999) CDM simulations (White et al 1999)

have only have only 7%7% of final halos with mass similar to of final halos with mass similar to the Milky Way (2 x 10the Milky Way (2 x 1012 12 MMoo) have not merged ) have not merged with another halo of at least 20% by mass since a with another halo of at least 20% by mass since a redshift of 2 – parameters chosen to match the redshift of 2 – parameters chosen to match the age and mass ratio estimated needed to form the age and mass ratio estimated needed to form the thick disk (modulo density, gas etc)thick disk (modulo density, gas etc)

If reduce to 10% mass ratio then none (0/26) have If reduce to 10% mass ratio then none (0/26) have no such merger since z=2no 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-Thin disk : moving groups, scatter in age-

metallicity relationship, spiral arms, central metallicity relationship, spiral arms, central bar, outer ring (?)bar, outer ring (?)

Thick disk : some complexity in all Thick disk : some complexity in all properties? Many ways of heating starsproperties? Many ways of heating stars

Bulge : asymmetries – bar-related? Bulge : asymmetries – bar-related? Stellar halo : Outer halo contains many Stellar halo : Outer halo contains many

streams – all due to Sagittarius dwarf? streams – all due to Sagittarius dwarf?

Thin Disk : Small Scale Structure

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

Thin Disk: SSS The ‘ring’ seen in star counts The ‘ring’ seen in star counts (Newberg et(Newberg et al 2002; Ibata et al al 2002; Ibata et al

2003)2003) at Galactocentric distances of ~15kpc at Galactocentric distances of ~15kpc maymay be be associated with the outer disk associated with the outer disk

Hierarchical clustering Hierarchical clustering predicts merger remnants predicts merger remnants in the thin diskin 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 If the thick disk is formed by a merger heating a

pre-existing thin disk, expect some remnant of pre-existing thin disk, expect some remnant of the satellite that was responsible. Indeed the satellite that was responsible. Indeed hierarchical clustering hierarchical clustering

models predict a models predict a complex mix. complex mix. e.g Abadi et al 2002e.g Abadi et al 2002

Thick Disk: SSS Structure in the thick disk, or steep gradient Structure in the thick disk, or steep gradient

in kinematics and metallicity, apparent in in kinematics and metallicity, apparent in large sample of faint (V=18-19) F/G stars large sample of faint (V=18-19) F/G stars compared to local samplescompared 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, 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 –0.6dexCanonical thick disk has mean metallicity –0.6dexThese stars are more metal-poor; structure in df?These stars are more metal-poor; structure in df?Connected to ‘ring’? Satellite debris? Connected to ‘ring’? Satellite debris?

Thick Disk: SSS However, parameter values for even However, parameter values for even

‘canonical’ thick disk still vary from study ‘canonical’ thick disk still vary from study to study e.g. scale height, normalisation and to study e.g. scale height, normalisation and rotational lag behind the Sun rotational lag behind the Sun Furhmann 2003

Local thick diskLocal thick diskin this sample has in this sample has lag of ~80km/s, lag of ~80km/s, like the distant F/Glike the distant F/Gstars……..‘usual’ stars……..‘usual’ value is ~35km/svalue is ~35km/s

Central Bulge : SSS Bar-like asymmetry seen, but complicated by Bar-like asymmetry seen, but complicated by

dust extinction – not so obvious in ISO maps dust extinction – not so obvious in ISO maps (van Loon et al 2003)(van Loon et al 2003)

Current star formation in very central Current star formation in very central bulge/inner disk is high enough rate to form bulge/inner disk is high enough rate to form entire bulge, 10entire bulge, 101010 solar masses, if sustained for a solar masses, if sustained for a Hubble time, but did not – why?Hubble time, but did not – why?

Central Black Hole : what role in governing star Central Black Hole : what role in governing star formation and evolution? (Ghez talk)formation and evolution? (Ghez talk)

Stellar Halo: SSS The outer halo, with dynamical timescales The outer halo, with dynamical timescales

of > 1Gyr, is best place to find structure. of > 1Gyr, is best place to find structure. Several streams found, in both coordinate Several streams found, in both coordinate space and kinematics (Majewski talk). space and kinematics (Majewski talk).

Are all due to the Sagittarius Dwarf? Most Are all due to the Sagittarius Dwarf? Most definitely are definitely are e.g.e.g. Ibata et al 2001; Majewski et al Ibata et al 2001; Majewski et al 20032003

Streams are rare in the inner halo (contains Streams are rare in the inner halo (contains most of the stellar mass!). Kinematics most of the stellar mass!). Kinematics suggest one stream but mass uncertain suggest one stream but mass uncertain (Helmi (Helmi et al 1999; Chiba & Beers 2000).et al 1999; Chiba & Beers 2000).

Stellar Halo: SSS No structure seen in coordinate space of No structure seen in coordinate space of

inner halo – 2pt correlation function flat for inner halo – 2pt correlation function flat for stars brighter than V=19 stars brighter than V=19 (e.g. Gilmore et al (e.g. Gilmore et al 1985; Lemon et al 2003). 1985; Lemon et al 2003).

Smoothness of local halo implies either few Smoothness of local halo implies either few streams or so many as to mimic a streams or so many as to mimic a continuum, with only a few stars in each continuum, with only a few stars in each stream stream e.g. Gould (2003)e.g. Gould (2003)

Conclusions The Milky Way has merged with, is merging The Milky Way has merged with, is merging

with, and will merge with, significant with, and will merge with, significant companion galaxies, contributing stars and gas – companion galaxies, contributing stars and gas – and dark matterand dark matter

Most recent accretion probably predominantly Most recent accretion probably predominantly gaseous – quiescent merger history, atypical in gaseous – quiescent merger history, atypical in CDM modelsCDM models

Large spectroscopic surveys will tightly Large spectroscopic surveys will tightly constrain existence and origin of stellar constrain existence and origin of stellar substructuresubstructure

What about the rest of the Local Group?What about the rest of the Local Group?