Ken stellar halo

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Information about Ken stellar halo

Published on November 15, 2007

Author: FunSchool


Slide1:  Saas-Fee Lectures 2007 The Origin of the Galaxy and the Local Group Ken Freeman, RSAA, ANU I will talk about stellar properties and dark matter in some of the Local Group galaxies Slide2:  Our Galaxy Note the dominant disk and the small boxy bulge Slide3:  Overview of our Galaxy dark halo stellar halo thin disk thick disk bulge Slide4:  The thin disk is metal-rich and covers a wide age range The other stellar components are all relatively old (note similarity of [Fe/H] range for thick disk and globular clusters) Slide5:  Total mass ~ 2 x 1012 M_sun : ( 5 x 1011 M_sun out to 50 kpc) Wilkinson & Evans (1999), Sakamoto et al (2003) Stellar mass in bulge ~ 1 x 1010 M_sun disk 6 x 1010 M_sun halo 1 x 109 M_sun Ages of components: globular clusters ~ 12 Gyr; some outer clusters 1-2 Gyr younger thick disk : > 10 Gyr thin disk : star formation started about 10 Gyr ago from white dwarfs (eg Legget et al 1998) 8 Gyr ago from old subgiants (Sandage et al 2003) star formation in the disk has continued at a more or less constant rate to the present time Slide6:  Start with the stellar halo in our Galaxy, and then the globular clusters in our Galaxy and other Local Group members. Then • dark matter in the galaxies of the Local Group • the Galactic thin and thick disks • the Galactic bulge, and then M33 LMC M31 Slide7:  Some historical landmarks in studies of the stellar halo • weak-lined stars near the sun have high space motions (Spitzer & Schwarzschild 1950, Roman 1950) • Eggen, Lynden-Bell & Sandage (1962) propose rapid collapse of the Galaxy, to produce the high orbital eccentricities seen among the halo stars, followed by slower chemical evolution during the dissipative formation of the disk. This was the first attempt to build an observationally-based scenario for the formation of disk galaxies. Although it is probably not correct, ELS was very influential in getting people thinking seriously about how our Galaxy formed. The Galactic Stellar Halo Slide8:  Chiba & Beers 2000 An argument against the ELS picture: the expected continuous relation between eccentricity and abundance is not present among the metal-poor stars Slide9:  ELS Cambridge 1986 Slide10:  • Searle & Zinn (1978) propose that the halo was built from infalling fragments which had some period of chemical evolution before being accreted by the Galaxy. This was based on the lack on radial abundance gradient in the halo globular cluster system. At this time, current ideas on hierarchical galaxy formation were not yet fully formed. This is still the working picture for the formation of the galactic halo. • The 1978 Yale Conference, at which the idea that galaxy mergers were important became standard wisdom (Toomre, Searle & Zinn) 10 Slide11:  The stellar halo and globular clusters include the oldest objects in our Galaxy. We now believe that much (maybe all) of the stellar halo was accreted in the form of small galaxies which had a short evolutionary life of their own before being accreted. Some (maybe all) of the globular clusters also came from outside. Before going on with the stellar halo, I want to discuss the main processes that act during accretion of small galaxies by larger galaxies. These processes are important for understanding the formation of the stellar halo,and also for the evolution of the dark matter distribution. Apologies if this is already familiar. Slide13:  Merging stimulates star formation and disrupts the galaxies. This is NGC 4038/ 9 - note the long tidal arms . The end product of the merger is often an elliptical galaxy. Disk galaxies interact tidally and merge. Slide14:  r(t) A B v Slide20:  The system's energy is now more positive (ie less bound), although the kinetic energy has in the end decreased as a result of the encounter plus virialization. The system typically swells, becomes less dense, and loses some mass to escaping stars. 20 Slide25:  Toomre & Toomre (1972): retrograde encounter Slide26:  Toomre & Toomre prograde encounter Slide30:  Accretion of small satellites 30 Slide31:  Decay of a prograde satellite orbit Slide32:  More on this later: now back to the stellar halo Slide33:  The metal-poor stellar halo abundance range [Fe/H] = -1 to -5 overlaps with the metal-poor tail of the thin disk Density distribution r ~ r -3.5, extends out to ~100 kpc mass of stellar halo ~ 1 x 109 M (total stellar mass of the Galaxy is about 6 x 1010 M) Slide34:  Chemical Properties of the metal-poor Halo a-enhancement (eg Mg Ca Si) associated with the short duration of star formation and enrichment Scatter in abundance of heavier (neutron capture) elements relative to Fe at low [Fe/H], associated with a small number of discrete enrichment events insights into the nature of the earliest SN from detailed chemical abundances of very metal poor halo stars Slide35:  Scatter in heavy element ratios at lower [Fe/H] Wallerstein et al 1997  elements have less scatter; Mg,Ti not rigidly coupled to Si,Ca light s heavy s r process Slide36:  Carney Laird Latham (1996) survey of proper motion stars : orbital eccentricity vs [m/H] - modern version of ELS disk stars: low-e orbits highly eccentric halo stars The CLL survey illustrates the striking kinematical difference between disk and halo Slide37:  Carney Laird Latham survey: rotational velocity relative to the sun vs [m/H] (V = -220 corresponds to zero angular momentum) rapidly rotating disk & thick disk slowly rotating halo Slide38:  Carney Lair Latham survey : metallicity distribution of halo stars and globular clusters : halo stars now known down to [Fe/H] < -5 GCs halo stars Slide39:  Not all of the metal-poor stars ([Fe/H] < -1) are in the halo. A large fraction belong to the thick disk. More on the thick disk later: although most of the stars in the old thick disk have abundances of [Fe/H] = - 0.7 to -1.0, the thick disk has a long metal weak tail extending down to [Fe/H] ~ -2. About 25% of the stars with [Fe/H] = -1.5 near the sun belong to the thick disk 40 Slide40:  Kinematics of halo For [Fe/H] < -1.7, slow rotation (30 - 50 km/s); pressure-supported s = (140,105,95) km/s For [Fe/H] > -1.7 rotation increases, probably due to contribution of the metal weak tail of the thick disk Chiba & Beers 2000 Even for stars with [Fe/H} ~ -2, the rotation is faster near the galactic plane Slide41:  Rotation of halo decreases with height above the galactic plane: decreasing contribution from the thick disk Chiba & Beers 2000 Slide42:  Little correlation of orbital eccentricity e with [Fe/H] (Chiba & Beers 2000) See the thick disk with its metal-poor tail at low e, and a clump of high-e stars at [Fe/H] ~ -1.7 Could come from infalling gas with low angular momentum, or from early infalling satellites - more later Slide43:  Fraction F of (metal-weak thick disk) of the (total metal-weak population) near the sun increases with [Fe/H] What is this MWTD ? Did it form during collapse of disk ? Remnant of very early thin disk heated by early merger ? Debris of accreted dwarf galaxies ? Some thick disk stars in the solar neighborhood have [Fe/H] abundances as low as the most metal-poor globular clusters. (Chiba & Beers 2000) Slide44:  eg decay of prograde satellite orbit around disk galaxy (Walker et al 1996) - dragged down into the disk plane by dynamical friction (against disk and halo) on timescale ~ 1 Gyr Would expect some accreted debris to settle to the disk Rotation of debris would decrease as z increases, as observed got to here Slide45:  More on the metal-weak thick disk Discovered by Bessell et al (1985) - not everyone agreed, but now it is a fact of life. RR Lyrae stars are good tracer of old populations (ages> 1010 yr). They are easy to recognise, easy to estimate their chemical abundances and distances, and they nicely show the metal-weak thick disk Maintz & de Boer (2005) : sample of 217 nearby RR Lyrae stars: 163 are in halo-like orbits (half retrograde). The other 54 lie in a rapidly rotating disklike distribution with a vertical scaleheight of ~ 1.3 kpc, much like the thick disk Slide46:  Maintz & deBoer 2005 Toomre diagram for nearby RR Lyrae stars disk  is radial velocity component,  is azimuthal, W is vertical Slide47:  Maintz & deBoer 2005 Nearby RR Lyrae stars - metal weak thick disk [Fe/H] Zmax(kpc) ecc ecc disk prograde retrograde 0 0 -2 15 Slide48:  Related study by Morrison (1998) of 130 RR Lyrae stars found a similar metal weak disk population. It has a mean lag of about -47 km/s relative to the sun (ie rotation of about 185 km/s) compared with the LSR (220 km/s), while the stellar halo has rotation of about 13 km/s (ie hardly rotating at all - just pressure-supported) We see again two very different kinds of kinematics here: the rapidly rotating disk (including some metal-poor stars), and the slowly rotating halo - not much in between Slide49:  Halo Streams Long orbital timescales  survival of identifiable debris eg Sgr tidal stream, discovered near the bulge, extends right around the Galaxy (probably a few times around) Ibata et al 1995 Majewski et al 2003 2MASS M giants Slide50:  A large fraction of the halo stars in the meridional plane could be associated with Sgr debris Spaghetti collaboration : Dohm-Palmer et al 2003 colored points are different wraps of simulated orbit of Sgr (Helmi) black points are halo giants from the spaghetti project 50 Slide51:  These tidal streams from the currently disrupting Sgr dwarf are interesting, but the ancient streams from small objects accreted long ago into the halo could be even more interesting. The long orbital periods allow these ancient streams to survive, so the metal-poor halo is the best place to attempt reconstruction of these accretion events. They are too faint to see in configuration space - may see them in phase space, eg (RG , VG ), or in integral space ie the space of integrals of the motion for stellar orbits, like energy and angular momentum (E , Lz ) Slide52:  Kinematics of nearby metal-poor stars: see halo substructure appearing as two streams in vz , detached in velocity space from the rest of the sample - mean metallicity ~ -1.5. This cannot be seen in space, only in velocity Helmi 99 Tidal Streams in the Galactic Halo (simulation of accretion of 100 satellite galaxies):  Tidal Streams in the Galactic Halo (simulation of accretion of 100 satellite galaxies) x (kpc) y (kpc) RGC (kpc) RVGC (km s-1) (Spaghetti: Harding) Slide54:  Input - different colors represent different satellites Output after 12 Gyr stars within 6 kpc of the sun - convolved with GAIA errors Helmi & de Zeeuw Accretion in integral space (E,Lz) Slide55:  Accretion is important for building the stellar halo, but not clear yet how much of the halo comes from discrete accreted objects (debris of star formation at high z) versus star formation during the baryonic collapse of the Galaxy Recent simulations of pure dissipative collapse (eg Samland et al 2003) suggest that the halo may have formed mainly through a lumpy collapse, with only ~ 10% of its stars coming from accreted satellites In any case, we may be able to trace the debris of these lumps and accreted satellites from their phase space structure. Slide56:  Helmi & de Zeeuw adopted a time-independent gravitational field for their simulation Recent cosmological simulations (eg Gill et al, Gao & White) indicate that the dark halo has doubled its mass since z =1 Gill et al showed that satellite debris retains its identity in the (E, Lz) plane, although its average (E, Lz) does change Dynamically reconstructing at least some of the objects that formed at high z and then became part of the galactic halo seems feasible: GAIA will contribute greatly. More on substructure: the omega Cen debris ... Slide57:  Omega Centauri Growing evidence from its chemical properties and very bound retrograde orbit that w Cen is the nucleus of a satellite of mass ~ 108 M which was dragged in to the Galaxy and then tidally disrupted This may have been the event that thickened the thick disk (Bekki & Freeman 2003) Finding the debris of the w Cen satellite is an interesting archaeological goal : several groups are working on this Slide58:  Meza et al (2006): evolution of disrupting satellite in VR - R plane Curves show a few loci of constant (E, Lz) Slide59:  Meza et al (2006): the Chiba-Beers metal poor stars - histogram of Vrot: see a few substructure peaks including possible retrograde omega Cen galaxy debris Arcturus peak is a thick disk feature: maybe debris of an accreted galaxy Histogram of Vrot for metal-poor stars Slide60:  Meza et al 2006 Gratton's (2003) sample of metal-poor stars with well-measured chemical abundances see the omega Cen and Arcturus features again Histogram of Jz 60 Slide61:  The red points are potential omega Cen debris candidates. Less -enriched than other halo stars : implies a longer history of chemical evolution, as observed in omega Cen itself Element abundances [/Fe] vs [Fe/H] (Meza et al 2006) Slide62:  While on the subject of streams .... also see substructures in the disk, maybe associated with accretion of small satellites Slide63:  Juric et al 2006 SDSS counts in two color intervals - red stars fit model well, blue stars show high latitude substructure r-i = 0.35-0.40 r-i = 1.10-1.20 Slide64:  Also the Monoceros/Canis Majoris feature in the outer half of the Galaxy, near plane : probably remains of an accreted dwarf but may just be a feature in the galactic disk. Not easy to visualize - look at current best-fit simulation of accretion of dwarf (Martin et al 2005) Slide65:  simulation of Can Maj accretion event, seen from N pole. Cold system,  ~ 11 km/s S Martin et al 2005 + Slide66:  Where are the first stars now ? Diemand et al 2005, Moore et al 2006, Gibson et al 2006 Material in the early rare peaks of the hierarchy is now very centrally concentrated to the present-day galaxy. First stars are in orbits of fairly high eccentricity, rather similar to observed eccentricity distribution for metal-poor stars in the galactic halo Slide67:  Distribution in present galaxy of debris from peaks selected at z > 12 (Moore et al 2006). Dashed cuve shows slope for metal-poor halo. Slide68:  [Fe/H] e Orbital eccentricity distributions first stars (Kawata et al 2006) 1200 metal-poor stars near the sun (Chiba & Beers 2000) Slide70:  Now show a numerical simulation of galaxy formation. The simulation summarizes our current view of how a disk galaxy like the Milky Way came together from dark matter and baryons MOVIE • much dynamical and chemical evolution • halo formation starts at high z • dissipative formation of the disk Slide71:  Simulation of galaxy formation • cool gas • warm gas • hot gas Slide72:  • z ~ 13 : star formation begins - drives gas out of the protogalactic mini-halos. Surviving stars will become part of the stellar halo - the oldest stars in the Galaxy • z ~ 3 : galaxy is partly assembled - surrounded by hot gas which is cooling out to form the disk • z ~ 2 : large lumps are falling in - now have a well defined rotating galaxy. Movie synopsis Slide73:  The large observed scatter in [X/Fe] for metal-poor stars suggests that the neutron capture elements in metal-poor stars are products of only a few nucleosynthesis events - confirmed by simulations Slide74:  White Dwarfs in the Halo Discovery of high proper motion WDs which could contribute some fraction of the dark halo density (Oppenheimer et al 2001) Much discussion - emerging view that these WDs are probably thick disk objects - no real consensus yet the number of true halo WDs appears consistent with the stellar halo (eg D. Carollo 2003, Salim et al 2003, Mendez 2002, Torres et al 2002) Slide75:  Reid et al 2001 • Oppenheimer WDs 2s contour for halo 2s contour for disk nearby M-dwarfs Many of these WDs are probably associated with the thick disk

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