Ken thin disk

Slide1 : The Galactic Thin Disk


Slide2 : Based on Annual Reviews article 2002 with Joss Bland-Hawthorn 'The New Galaxy: Signatures of its Formation' Soon we will have vast amounts of data on the motions and chemical properties of millions to billions of stars in the Milky Way. What can we learn in this new era about the formation of our Galaxy


Slide3 : What does our Galaxy look like ? Near infrared image from COBE/DIRBE - dust is transparent in near-IR NGC 891: our Galaxy probably looks much like this in visible light


Slide4 : We would like to understand how our Galaxy came to look like this The Milky Way is a disk galaxy: the disk is the primary stellar component Some galaxies have much larger bulges Also see a small central bulge


Slide5 : Our galactic bulge is really a small bar seen edge-on NGC 1300 is a strongly barred galaxy Bars are believed to come from instabilities of the disk ; they are not alway so obvious ...


Slide6 : M83 in blue light (L) and K light (near-IR) (R) The bar is much more obvious in the near-IR


Slide7 : Overview of our Galaxy dark halo stellar halo thin disk thick disk bulge


Slide8 : How did the Galaxy come to be like this ? To study the formation of galaxies observationally, we have a choice ... we can observe distant galaxies at high redshift - we see the galaxies directly as they were long ago, at various stages of their formation and evolution but not much detail can be measured about their chemical properties and motions of their stars


Slide9 : d Hubble Deep Field


Slide10 : or we can recognise that the main structures of the Galaxy formed long ago at high redshift. We can study the motions and chemical properties of stars in our Galaxy at a level of detail that is impossible for other galaxies, and probe into the formation epoch of the Galaxy. This is near-field cosmology. the halo formed at z andgt; 4 the disk formed at z ~ 2


Slide11 : The ages of the oldest stars in the Galaxy are similar to the lookback time for the most distant galaxies observed in the HDF. Both give clues to the sequence of events that led to the formation of galaxies like the Milky Way


Slide12 : The luminosity function of white dwarfs in the nearby disk Legget et al 1998 age = 9 Gyr


Slide13 : The thin disk is the defining stellar component of disk galaxies. It is the end product of the dissipation of most of the baryons, and contains almost all of the baryonic angular momentum Understanding its formation is an important goal of galaxy formation theory. The thin disk


Slide14 : star formation history in galactic thin disk : roughly uniform, with episodic star bursts for ages andlt; 10 Gyr, but lower for ages andgt; 10 Gyr Rocha-Pinto et al (2000)


Slide15 : Solar neighborhood kinematics: Several mechanisms for heating disk stars: transient spiral arms, GMC scattering (eg Fuchs et al 2001), large-scale bending modes of anisotropic disk (*Sotnikova 2003), accretion events, star cluster dissolution (Kroupa 2001) Expect heating mechanisms to saturate after a few Gyr: stochastic heating : heated stars spend less time near galactic plane bending modes : heating decreases as vertical heating reduces the anisotropy What do observations show ?


Slide16 : Freeman 1991; Edvardsson et al 1993; Quillen andamp; Garnett 2000 Velocity dispersions of nearby F stars old disk thick disk Disk heating saturates at 2-3 Gyr appears at age ~ 10 Gyr


Slide17 : exponential in R and z : scaleheight ~ 300 pc, scalelength 2-4 kpc (!) velocity dispersion decreases from ~ 100 km/s near the center (similar to bulge) to ~ 15 km/s at 18 kpc Lewis andamp; KCF 1989 2 1.5 1 R (kpc) log (velocity dispersion) Structure of the thin disk


Slide18 : Moving Stellar Groups These are stars in the solar neighborhood with common motions and chemical properties : some are surviving fossils of star forming events in the disk. HR 1614 group (Feltzing 2000). Thin disk group, age ~ 2 Gyr, [Fe/H] ~ 0.2 Arcturus group (Eggen 1971). Old thick disk group, velocity V = -116 km/s relative to LSR, [Fe/H] ~ -0.6, These are nice examples of substructures surviving in the galactic disk. Gayandhi da Silva is working on the chemical homogeneity of these groups for her thesis at RSAA. These moving groups in the disk will become very interesting with RAVE and GAIA


Slide19 : Some moving groups are probably associated with local resonant kinematic disturbances by the inner bar : OLR is near solar radius (Hipparcos data) : Dehnen (1999), Fux (2001), Feast (2002) Sirius and Hyades streams - mainly earlier-type stars Hercules disturb- ance from OLR mainly later-type stars Dehnen 1999


Slide20 : Chemical properties of the nearby disk The age-abundance relation Edvardsson et al 1993 old disk thick disk young disk


Slide21 : Chemical properties of the nearby disk : [X/Fe] Edvardsson et al 1993 thin disk thin + thick (see also Prochaska et al 2000; Bensby et al 2003; Yong et al 2003)  s


Slide22 : Chemical properties of the nearby disk : [a´/Fe] = [(Ca, Si)/H] thin disk thin + thick Edvardsson et al 1993 (Rm is mean orbital radius)


Slide23 : Abundance gradient in the old disk Abundance gradient for the old open clusters (age andgt; Hyades) Friel 1995


Slide24 : slides from valencia


Slide25 : Disk heating Stellar velocity dispersion of galactic disk long believed to increase with age, but the facts are unclear. One view is that dispersion ~ t0.2-0.5 via some diffusive process. Other view is that heating occurs for the first ~ 2 Gyr, then saturates. Requires some kind of disk heating - the mechanism is uncertain. Heating needs to produce the observed ratio of z / R ~ 0.5. Current belief is that disk heats by a combination of transient spiral wave heating plus scattering by GMCs.


Slide26 : exponential in R and z : scaleheight ~ 300 pc, scalelength ~ 4 kpc velocity dispersion decreases from ~ 100 km/s at small R (similar to bulge) to ~ 15 km/s at 18 kpc. Heating more effective at small R. Lewis andamp; KCF 1989 2 1.5 1 R (kpc) log (velocity dispersion) Velocity dispersion of the thin disk


Slide27 : Solar neighborhood kinematics: Several mechanisms for heating disk stars: transient spiral arms, GMC scattering (eg Fuchs et al 2001), large-scale bending modes of anisotropic disk (Sotnikova 2003), accretion events, star cluster dissolution (Kroupa 2001) Expect heating mechanisms to saturate after a few Gyr: stochastic heating : heated stars spend less time near galactic plane bending modes : heating decreases as vertical heating reduces the anisotropy


Slide28 : Simulations of in-plane heating by transient spiral waves (De Simone et al 2004) for spirals of different pitch angles. Rapid heating for first 2 Gyr followed by steady slower heating. This does not include vertical heating of stars which will lead to saturation of heating as z increases What do observations show ?


Slide29 : The most direct data on the age-velocity relation come from Edvardsson et al (1993) who measured accurate individual velocities and ages for ~ 200 nearby stars. Edvardsson et al data indicate heating for the first ~ 2 Gyr, with no significant subsequent heating. Disk heating in the solar neighborhood appears to saturate when z ~ 20 km/s.


Slide30 : Freeman 1991; Edvardsson et al 1993; Quillen andamp; Garnett 2000 Velocity dispersions of nearby F stars old disk thick disk Disk heating saturates at 2-3 Gyr appears at age ~ 10 Gyr


Slide31 : Galactic halo shows kinematical substructure - believed to be the remains of accreted objects that built up the halo The galactic disk also shows kinematical substructure in the solar neighborhood: usually called moving stellar groups • Some are associated with dynamical resonances (bar) (Hercules group: Dehnen) • Some are debris of star-forming aggregates in the disk. (HR1614: Feltzing andamp; Holmberg) • Others may be debris of infalling objects, as seen in CDM simulations: eg Abadi et al 2003 (Arcturus, Navarro et al)


Slide32 : We can extend the approach of reconstruction to the disk of the Galaxy. Understanding disk formation is more important than understanding the halo, because most of the galactic baryons are in the disk. The moving groups are potential examples of substructures surviving in the galactic disk. If true, then these moving groups in the disk will become very interesting with RAVE and GAIA


Slide33 : Eggen identified many stellar moving groups Are they • dispersed debris of old star forming aggregates (interesting for reconstructing the history of the galactic disk) or • dynamically induced by the bar (eg Dehnen 1999) or spiral structure (eg de Simone et al 2004) - both can cause lumpy structure to develop in the velocity distribution of disk stars


Slide34 : Some moving groups may be associated with local resonant kinematic disturbances by the inner bar : OLR is near solar radius (Hipparcos data) : Dehnen (1999), Fux (2001), Feast (2002) Sirius and Hyades streams - mainly earlier-type stars Hercules disturb- ance from OLR mainly later-type stars Dehnen 1999 (U,V are relative to the LSR)


Slide35 : We might expect that groups originating from a common star formation episode have common ages and chemical properties. Not so easy to test : groups defined by their (U,V) distributions will be superimposed in (U,V) plane on distribution of stars that do not belong to the group.


Slide36 : eg Famaey et al (2005) looked at the ages of stars in the Hyades and Hercules groups and argued for an age spread 3000 Hipparcos stars  / andlt; 0.20, isochrone log(age) = 8.3 to 9.7 (Looks consistent with small age spread for group superimposed on background of non-group stars) Hyades Hercules MV V-I V-I


Slide37 : Some evidence now that at least some of the groups are dynamical in origin. Need to test more convincingly whether group members have common age and common chemical properties, as expected if they have a common origin in a star-forming event. Not much done on this problem yet. Show some old unpublished work on the chemical properties of the Hyades and Sirius groups, and new work on the age and chemical properties of Eggen's HR1614 group. Both indicate that the group stars do have common properties.


Slide38 : The Hyades and Sirius moving groups (Wilson, Freeman, Kalnajs 1988) These are fairly young groups (ages andlt; 1 Gyr). If they are dispersing aggregates passing through the solar neighborhood, then we expect the stars to lie on Lindblad dispersion orbits and therefore to lie on well-defined tracks in the (longitude - radial velocity) plane.


Slide39 : Hyades and Sirius group - dispersion orbits • Sun


Slide40 : Wilson measured accurate radial velocities and [Fe/H] values for several hundred stars within 500 pc of the sun, to see if the expected (Vrad - l) relation could be seen in any intervals of [Fe/H].


Slide41 : Hyades andamp; Sirius groups: expected dispersion orbit loci are seen only in narrow abundance range (similar to abundance of Hyades cluster). Suggests that they are chemically well defined, as expected for dispersing star forming event. -0.1 andgt; [Fe/H] andgt; -0.2 [Fe/H] andlt; -0.25


Slide42 : We need to put these stars on a color-magnitude diagram to see if they are coeval. Now look at the HR1614 group (age ~ 2 Gyr, [Fe/H] = +0.2). Studied by Feltzing andamp; Holmberg (2000) who argued for its reality. De Silva (2006) measured very precise chemical abundances for many elements in HR1614 stars, and finds a very small spread in abundances and in ages.


Slide43 : HR1614 group: [Fe/H] abundances relative to the star HR1614 (Na, Al, Mg, Si, Ca, Mn, Ni, Zr, Ba, Ce, Nd, Eu are similarly homogenous) De Silva 2006


Slide44 : HR1614 moving group stars: the (U,V) plane Epicyclic theory predicts constant V. (Eggen, Woolley) The small tilt is expected because epicyclic theory is not valid for these larger V-values. De Silva 2006


Slide45 : HR1614 moving group stars: the color-magnitude diagram. The stars lie close to the 2 Gyr isochrone. Two of the 4 chemical outliers are binaries. One other lies well outside the group's UV distribution. Expect that ~ 2 non-group stars from the background will have [Fe/H] and UV consistent with the group De Silva 2006


Slide46 : Summary: Moving Groups The younger Hyades andamp; Sirius moving groups and the older HR1614 moving group appear to be consistent with origin as star-forming event, rather than having a dynamical origin (eg from dynamical effects of spiral structure) This is good: it indicates that some moving groups can survive dynamically for at least ~ 2 Gyr against the effects of disk heating, so we can use them to reconstruct the galactic disk. Need to investigate the ages and chemical properties of more extreme groups like the Arcturus group (Mary Williams) with andlt;Vandgt; ~ -100 km/s : thick disk star formation site or infalling galaxy ? Also need to investigate the Dehnen-Fux Hercules group to see if it is really dynamical in origin (from effects of bar): does it include a wide range of stellar abundances and ages as expected ?


Slide47 : slides from lorentz


Slide48 : We would like to extend the approach of reconstruction to the disk and thick disk of the Galaxy. Understanding disk formation is more important than understanding the halo, because most of the galactic baryons are in the disk. Disk Reconstruction


Slide49 : NGC 4762 - a disk galaxy with a bright thick disk (Tsikoudi 1980) Most spirals (including our Galaxy) have a second thicker disk component . In some galaxies, it is easily seen The thin disk The thick disk


Slide50 : Our Galaxy has a significant thick disk • its surface brightness is about 10% of the thin disk’s. • it rotates almost as rapidly as the thin disk • its stars are older than 12 Gyr, and are • significantly more metal poor than the thin disk (-0.5 andgt; [Fe/H] andgt; -2.2) and • alpha-enriched


Slide51 : The galactic thick disk: -enriched kinematically selected


Slide52 : Because of its rapid rotation, the Galactic thick disk may have formed from heating of the early stellar disk by accretion events or minor mergers In some other galaxies, the thick disk rotates more slowly and may have come from an early rapid phase of gas accretion or from merger debris (Brook et al 2004, Yoachim andamp; Dalcanton 2004)


Slide53 : eg CDM simulations of formation of an early-type disk galaxy (Abadi et al 2003) show that not all disk stars form in the disk. Many of the oldest stars in the disk are debris from accreted satellites which are dragged down into disk plane by dynamical friction and end up in the thin and thick disk. (This effect is now thought to be less important, because feedback greatly reduces the number of accreted objects that finish up in the disk)


Slide54 : Like the halo, the galactic disk also shows kinematical substructure : usually called moving stellar groups. Not all of these moving groups are fossils • Some are associated with dynamical resonances (bar) (Hercules group: Dehnen, OLR, new high velocity group: Fuchs, 4:1) • Some are debris of star-forming aggregates in the disk. (HR1614: Feltzing andamp; Holmberg, de Silva et al) • Others may be debris of infalling objects, as seen in CDM simulations: eg Arcturus group : Navarro et al 2004, Mary Williams 2006.


Slide55 : Dehnen 1999 (U,V are relative to the LSR)


Slide56 : Quick overview of the star formation history and chemical evolution of the solar neighborhood • see a roughly constant star formation rate over the last 10 Gyr (eg Rocha-Pinto et al 2000), and • no apparent mean chemical evolution in the old disk for stellar ages between 2 and 10 Gyr (Edvardsson et al 1993 ... ) As background to reconstruction of the disk ...


Slide57 : star formation history in galactic thin disk : roughly uniform, with episodic star bursts for ages andlt; 10 Gyr, but lower for ages andgt; 10 Gyr Rocha-Pinto et al (2000)


Slide58 : old disk thick disk No significant chemical evolution in the old disk for ages 2-10 Gyr


Slide59 : Solar neighborhood stellar kinematics: Several mechanisms for heating disk stars: transient spiral arms, GMC scattering (eg Jenkins andamp; Binney 1990), large-scale bending modes of anisotropic disk (Sotnikova 2003), accretion events (eg Quinn andamp; Goodman 1986), star cluster dissolution (Kroupa 2001) Stellar velocity dispersion of galactic disk long believed to increase with age. This would limit disk reconstruction from kinematic substructure, eg via moving groups. So it is really important to understand this disk heating Heating needs to produce the observed anisotropy in the local velocity dispersion of disk stars: z / R ~ 0.5.


Slide60 : scaleheight ~ 300 pc, scalelength ~ 4 kpc velocity dispersion decreases from ~ 100 km/s at small R (similar to bulge) to ~ 15 km/s at 18 kpc. Heating more effective at small R. Lewis andamp; KCF 1989 2 1.5 1 R (kpc) log (R velocity dispersion) Radial variation of the disk velocity dispersion


Slide61 : Expect heating mechanisms to saturate : stochastic heating : heated stars spend less time near galactic plane bending modes : heating decreases as vertical heating reduces the anisotropy What is the observed form of the heating with time ? The facts are not yet clear ... • One view is that stellar velocity dispersion  ~ t 0.2-0.5 eg Wielen 1977, Dehnen andamp; Binney 1998, Binney et al 2000.


Slide62 : Wielen 1977 stellar age velocity dispersion (km/s) W is in the vertical (z) direction total (McCormick dwarfs, CaII emission ages) 2 = (a + b t)1/2 W = 0.4total


Slide63 : Proper motion velocity dispersion S vs B-V for Hipparcos dwarfs. This does not directly give the (velocity dispersion) - age relation. With assumptions about IMF and star formation history, it indicates that S ~ t 0.33 Binney et al 2000


Slide64 : The most direct data on the age-velocity relation come from Edvardsson et al (1993) who measured accurate individual velocities and ages for ~ 200 nearby subgiants. Edvardsson et al data indicate heating for the first ~ 2 Gyr, with no significant subsequent heating. Disk heating in the solar neighborhood appears to saturate when z ~ 20 km/s. • Another view is that heating occurs for the first ~ 2 Gyr, then saturates.


Slide65 : Freeman 1991; Edvardsson et al 1993; Quillen andamp; Garnett 2000 Velocity dispersions of nearby F stars old disk Disk heating saturates at 2-3 Gyr appears at age ~ 10 Gyr thick disk


Slide66 : Can disks preserve useful fossil information ? A lot of dynamical information is lost in the dissipation that led to disk formation and the subsequent heating by spiral arm and giant molecular clouds Churning by transient spiral waves (large changes of stellar angular momentum with little increase in random motion: Sellwood andamp; Binney 2000) leads to further loss of dynamical information in the disk. Some kinematical substructures, like the Hercules moving group, are probably associated with the galactic bar resonances, and do not represent a fossil structure.


Slide67 : However ... we are not restricted to dynamical techniques. Much fossil information is locked up in the detailed distribution of chemical elements in stars. The thick disk is particularly interesting ... because it is thick, it may also retain some fossil dynamical information


Slide68 : We would like to reconstruct the ancient star-forming aggregates of the thick disk: phase mixing has dispersed them azimuthally right around the Galaxy Structurally invisible - may see them in velocity space (disk moving groups) and integral space (as for halo) and in their chemical properties. May be able to detect evolution of the cluster mass function, the star formation rate, and epochs of satellite infall and star-bursts during the formation of the thick disk.


Slide69 : Use the detailed chemical abundances of thick disk stars ([Fe/H], [/Fe], r- and s- process elements) to tag them to common ancient star-forming aggregates with similar abundance patterns (eg Freeman andamp; Bland-Hawthorn ARAA 2002) The detailed abundance pattern reflects the chemical evolution of the gas from which the aggregate formed. Chemical Tagging Different supernovae provide different yields (depending on mass, metallicity, detonation details, ejected mass ...) leading to scatter in detailed abundances, especially at lower metallicities (enrichment by only a few SN)


Slide70 : We seek signatures or fossils from the epoch of Galaxy formation, to give us insight about the processes that took place as the Galaxy formed. Aim to reconstruct the star-forming aggregates that built up the disk and halo of the Galaxy Some of these dispersed aggregates can still be recognized kinematically as stellar moving groups. For others, the dynamical information was lost through disk heating processes, but they are still recognizable by their chemical signatures (chemical tagging).


Slide71 : Chemical tagging is not just assigning stars chemically to a particular population (thin disk, thick disk, halo) That would be the analog of kinematically assigning stars to a population, which can only give probabilities of population membership. Chemical tagging is intended to assign stars chemically to substructure which is no longer detectable kinematically


Slide72 : For chemical tagging to work, need a few conditions: • stars form in large aggregates - believed to be true • aggregates are chemically homogenous aggregates have unique chemical signatures defined by several (andgt; 10) elements which do not vary in lockstep from one aggregate to another • Need sufficient spread in abundances from aggregate to aggregate so that chemical signatures can be distinguished with accuracy achievable (~0.05 dex differentially) Last two are the goals of Gayandhi De Silva's thesis: how chemically homogeneous are open star clusters and moving groups ?


Slide73 : Burris et al (2000) some heavy element production varies in lockstep from site to site some elements do correlate ... eg Eu,Dy - both r-process


Slide74 :


Slide75 : De Silva Acquired high S/N, high resolution spectra (VLT/UVES, APO, AAT, Keck) for 8 open clusters and 2 moving groups. Look at two open clusters and one moving group ... age [Fe/H] N Hyades 650 Myr 0.13 48 Collinder 261 8 Gyr 0.03 13 HR1614 2 Gyr 0.19 25 Measured differential abundances for samples of stars in restricted range of Te and log g for each system. Measuring errors in the differential abundances are 0.025 to 0.050 dex, depending on the element, including all known sources of error.


Slide76 : Hyades (Keck) Paulson et al (2003) found [Fe/H] spread andlt; 0.04 dex. Similar limits for Na, , Fe-peak elements • Hyades members


Slide77 : Hyades n-capture elements: de Silva (2006) intrinsic rms spread andlt; 0.02 to 0.05 (90%)


Slide78 : (UVES) De Silva et al (2006) Intrinsic rms andlt; 0.02 to 0.05 at 90% confidence Coll 261 • Coll 261 members


Slide79 : Coll 261


Slide80 : We might expect that groups originating from a common star formation episode have common ages and chemical properties. Not so easy to test : groups defined by their (U,V) distributions will be superimposed in (U,V) plane on distribution of stars that do not belong to the group. Chemical Homogeneity of stellar moving groups Hercules


Slide81 : Need to put candidate group stars on a color - absolute magnitude diagram to see if they are coeval (or at least have a coeval subset superimposed on a background of non-group members). The Hercules group (Bensby et al 2006)


Slide82 : Hercules group is believed to be dynamical: associated with OLR. No evident component homogeneous in age and abundance. Group appears to include thick and thin disk stars. Bensby et al 2006 Hercules group stars


Slide83 : Hercules group: includes thick and thin disk stars Bensby et al 2006


Slide84 : Now look at the HR1614 group (age ~ 2 Gyr, [Fe/H] = +0.2). Studied by Feltzing andamp; Holmberg (2000) who argued for its reality. De Silva (2006) measured precise chemical abundances for many elements in HR1614 stars (AAT: UCLES), and finds a very small spread in abundances and in ages. Group is very well defined kinematically despite its age.


Slide85 : HR1614 group: [Fe/H] abundances relative to the star HR1614 (Na, Al, Mg, Si, Ca, Mn, Ni, Zr, Ba, Ce, Nd, Eu are similarly homogenous) De Silva 2006


Slide86 : De Silva 2006 HR1614 group stars


Slide87 : HR1614 moving group stars: the (U,V) plane Epicyclic theory predicts constant V. (Eggen, Woolley) The small tilt is expected because epicyclic theory is not valid for these larger V-values. De Silva 2006


Slide88 : HR1614 moving group stars: the color-magnitude diagram. The stars lie close to the 2 Gyr isochrone. Two of the 4 chemical outliers are binaries. One other lies well outside the group's UV distribution. Expect that ~ 2 non-group stars from the background will have [Fe/H] and UV values consistent with the group's. De Silva 2006 These are all nearby stars with accurate Hipparcos parallax and motions


Slide89 : Cluster abundance patterns Hyades Coll 261 HR1614 Zr Ba


Slide90 : Conclusions The two open clusters (Hyades and Coll 261) are chemically homogeneous at the level of 0.02 to 0.05 dex in alpha, Fe peak and n-capture elements. This is promising for chemical tagging. Clusters have their individual abundance patterns: also promising for chemical tagging. The HR1614 moving group is coeval and chemically homogeneous. It appear to be consistent with origin as star-forming event, rather than having a dynamical origin (eg from spiral structure perturbations) This is good: it indicates that some moving groups can survive dynamically for at least ~ 2 Gyr against the effects of disk heating, so we can use them to reconstruct the galactic disk. RAVE will identify many new moving groups and new group members. eg the thick disk Arcturus group (Mary Williams) with andlt;Vandgt; ~ -100 km/s. Is it the debris of a thick disk star formation site or an infalling galaxy ?


Slide91 : Gemini is considering a multi-fiber high resolution (R= 40,000) spectrometer with 1000 fibers (WFMOS) which would be very well suited to a large chemical tagging survey WFMOS also has about 3000 fibers feeding low resolution spectrographs for parallel observations


Slide92 : The achievable level of abundance precision (~ 0.05 dex) provides many independent cells in abundance space for chemical tagging Need simulations to determine the optimal wavelength and resolution requirements for chemical tagging with single-order echelle spectra (eg Gemini WFMOS) - iron-peak, , r and s-process elements required.


Slide93 : A model chemical tagging survey: Gemini/Subaru WFMOS assume 1000 high resolution fibers in a 1 square deg field At V = 17, the typical stellar density at |b| ~ 30o is about 1000 stars per square degree, matching the instrument: no color cut Fractional contribution from galactic components Dwarf Giant Thin disk 0.80 0.005 Thick disk 0.10 0.05 Halo 0.01 0.02 Disk dwarfs are seen out to distances of about 3 kpc Disk giants 40 Halo giants 60


Slide94 : 20 thick disk dwarfs from each of about 500 star formation sites 30 halo giants from each of about 100 star formation sites Also get a large number of thin disk dwarfs to map the kinematical and chemical transition between thin and thick disk. Chemical tagging may also be possible for the thin disk stars, using elements which show some scatter in their [X/Fe]-[Fe/H] correlations: eg K, S, Sc, Sr, Y , Ba, Ce, Nd, Eu ( Reddy et al 2003). May be able to detect about 20 stars in each of about 5000 sites * A smaller survey means less stars from a similar number of sites Simulations (Bland-Hawthorn andamp; Freeman 2004) show that a random sample of 106 stars with V andlt; 17 would allow detection of about ...


Slide95 : A high resolution chemical tagging program (Gemini WFMOS) (1000 stars per field) x (1000 fields) Integrations ~ 4 hours : two fields per night 500 clear nights using 1000 fibers: the other 3000 fibers are available for low resolution parallel programs All aspects of such programs need proper modelling: • numbers of stars, • region of the Galaxy, and which hemisphere • magnitude and color range • optimal resolution, wavelength interval, S/N ... This is just an order of magnitude indication of the likely scope of a chemical tagging program to identify ancient galactic substructure


Slide96 : WFMOS and GAIA GAIA (~ 2015) will provide precision astrometry for about 109 stars For V = 17,  = 25 as,  = 20 as yr -1 (10% distance errors at 4 kpc, 4 km s -1 velocity errors at 40 kpc) accurate transverse velocities for all stars in the WFMOS sample, and accurate distances for all of the older main sequence stars and subgiants.


Slide97 : and light elements: less scatter but some internal dispersion eg Mg, Ti, Al not rigidly coupled to Si, Ca Fe peak: less scatter but some internal dispersion (eg Cr, Mn) For a large program of multi-object high resolution spectroscopy using just a single echelle order, what is the optimal region of the spectrum ?


Slide98 : Clusters vs the nearby field stars Hyades Coll 261 HR1614


Slide99 :


Slide100 : Hyades de Silva 2005 rms 0.03 in [X/Fe] rms 0.04 in [Fe/H] Paulson et al 2003


Slide101 : HR1614 moving group Abundances


Slide102 : HR1614 Isochrones


Slide103 : HR1614 Kinematics


Slide104 : Out of this process come galactic disks with a high level of regularity in their structure and scaling laws We need to understand the reasons for this regularity


Slide105 : Disks have a roughly exponential light distribution in R and z I(R,z) = Io exp (-R/hR) exp (-z/hz) out to R = (3 to 5) hR, then often truncated truncation quantified first by van der Kruit andamp; Searle (1981, 1982)


Slide106 : Out of this process come galactic disks with a high level of regularity in their structure and scaling laws We need to understand the reasons for this regularity


Slide107 : Disks have a roughly exponential light distribution in R and z I(R,z) = Io exp (-R/hR) exp (-z/hz) out to R = (3 to 5) hR, then often truncated truncation quantified first by van der Kruit andamp; Searle (1981, 1982)


Slide108 : Reason for the form of the exponential radial light distribution is not well understood : extreme options are collapse of a torqued gas cloud within dark halo with the right internal angular momentum distribution M(j), conserving M(j) -andgt; exponential gas disk, in place before star formation gas in disk is radially redistributed by viscous torques: tends to an exponential disk if star formation timescale ≈ viscous timescale


Slide109 : The vertical structure of disks is directly associated with their star formation history and dynamical history: scattering, accretion, heating, warping … these processes generate a vertical scale height hz for the old thin disk that is usually about 200-300 pc 10


Slide110 : K-band vertical luminosity profiles de Grijs et al 1997


Slide111 : Radial gradient of disk scaleheight de Grijs andamp; Peletier 1997 Earlier-type galaxies show a radial gradient in the scaleheight : hz increases with radius Sc Sb


Slide112 : The outer regions of disks


Slide113 : NGC 4565


Slide114 : What is the origin of this disk truncation - common and seen more easily in edge-on galaxies than in face-on galaxies Kregel et al (2001) find Rmax /hR = 3.6 ± 0.6 for 34 edge-on disk galaxies


Slide115 : M33 Surface Brightness Profile: i-band surface photometry out to R ~ 35' profile extended to R ~ 60' using star counts Disk Truncation cf. van der Kruit's (1982) disk edges: ~3-5 scalelengths, then abrupt truncation (also Pohlen et al 2002) Ferguson et al 2003 sharp decrease in surface brightness beyond 5 scalelengths..


Slide116 : Corbelli et al 1989 M33 HI distribution Outer contour 2 x 1019 cm-2 star count limit


Slide117 : Interpretations of the truncation radius ? the radius associated with the maximum angular momentum of the disk baryons in the proto-galaxy - unlikely - many disks have HI out far beyond the truncation radius.


Slide118 : ? the radius where the gas density goes below the critical value for star formation (Kennicutt 1989) - star formation regulated by disk stability - likely. ? the radius to which the disk has grown today - unlikely The outer disk IS younger but still typically many Gyr old ( eg Bell andamp; de Jong 2000, Ferguson et al 2003). In some galaxies (eg M83, Milky Way), star formation continues in the outer disk but there is an underlying old component


Slide119 : Stellar Content of the Outer Disk of M33 looks like an intermediate/old, fairly metal-poor ([Fe/H]~ -1.2) population dominating the outer disk of M33 Ferguson et al 2003 20


Slide120 : The outer disk of NGC 300 Bland-Hawthorn, Vlajic, Freeman, Draine, astroph/503488 Similar to M33 In Scl group, distance 2.1 Mpc Deep Gemini GMOS images: 0'.6 arcsec seeing 2.2 hours per field stellar photometry complete to r = 27 mag


Slide121 : r band, 2 GMOS imaging fields


Slide122 : • r-band star counts NGC 300: deep r'-band counts from Gemini GMOS : exponential disk goes for at least 10 scale lengths without truncation


Slide123 : NGC 300: HI (Puche et al 1990) outer Gemini field


Slide124 : Pohlen's (2005) classification of surface brightness profile morphologies: disk profiles show three classes of shape


Slide125 : Truncation of outer disks is not understood yet: remains an interesting problem


Slide126 : The outer disk of the Galaxy The galactic disk shows an abundance gradient, as in M31 (eg galactic cepheids: Luck et al 2006 - young stars)


Slide127 : Not a simple axisymmetric gradient : Luck et al (2006) cepheids


Slide128 : Yong andamp; Carney 2005; Carney andamp; Yong 2005: high resolution spectra of open clusters and stars in the outer disk The abundance gradient for the open clusters (ages 1 to 5 Gyr) bottoms out, at RG = 12 kpc (RG = 15 kpc in M31), and at an abundance of [Fe/H] = -0.5 (as in M31). Outer disk is -enhanced, with [/Fe] = + 0.2 (also Eu-enhanced): indicates fairly rapid star formation history in the outer disk, unlike the solar neighborhood. Suggests that outer disk stars formed from reservoir of gas that had a different star formation history from the solar neighborhood. Star formation may be triggered by a merger event in the outer disk.


Slide129 : Yong andamp; Carney 2005 Galactic open clusters • abundance gradient bottoms out at RG = 12 kpc, [Fe/H] = -0.5 for clusters with ages = 1 to 5 Gyr (no age-abundance relation) • outer (older) disk is -enhanced


Slide130 : Carney andamp; Yong 2005 + cepheids, other symbols are open clusters in the Galaxy. Clusters have ages 1-5 Gyr, cepheids are younger The abundance gradient and [/Fe]-gradient in the disk has flattened with time, tending towards solar values.


Slide131 : Summary of outer disks The disks of some spirals (NGC 300, M31) extend out beyond 10 scale lengths. The outer disks of M31, M33 and the Milky Way include a component that is at least several Gyr old. The abundance gradients in the outer disks of M31 and the Galaxy bottom out at [Fe/H] = - 0.5 The older stars of the outer galactic disk are -enhanced, indicating that they formed rapidly. This -enhancement is less for the younger stars of the outer disk Tidal effects as in M83 may contribute to the very extended gas disks: some have a low level of ongoing star formation.