milky way components: thin and thick disk · 2013. 12. 17. · mean rotational velocity ~100 km/s...

04.12.2013 Grebel: The Milky Way! 0! 04.12.2013! Grebel: The Milky Way! 1!

Contents

1.  Thin and thick disk 2.  Bulge and bar

3.  Stellar halo

4.  Outlook

04.12.2013! Grebel: The Milky Way! 2! 04.12.2013! Grebel: The Milky Way! 3!

Milky Way Components: Thin and Thick Disk

Buser 2000

8 kpc

04.12.2013! Grebel: The Milky Way! 4!Cignoni et al. 2006

Star formation rate in Solar neighborhood showed maximum activity ~ 3 Gyr ago. Varies on time scales of ~ 6 Gyr. (Estimates based on Hipparcos parallaxes, photometry, synthetic CMDs).

2 – 3 x increased star formation rate ~3 Gyr ago: Due to accretion event?

However, no evidence of starbursts.

Unknown: global SFH of disk; detailed age- metallicity-velocity relation.

Star Formation History of the Solar Neighborhood

04.12.2013! Grebel: The Milky Way! 5!

Moving Groups

  Most of the local kinematic groups still detected at ~ 1 kpc distance.   Large-scale features.

  Some: Age & metallicity spread; shifts in velocity plane at larger dist.   Consistent with effects caused by spiral arms or (bar) resonances.

  Others: Consistent with dissolved star clusters/star-forming aggregates.

  Yet others: Possible debris of infalling/accreted objects.   Chemical tagging + kinematics + ages to uncover nature.

Antoja et al. 2012, MNRAS 426, L1

Velocity struc- tures at scales of 11–22 km/s in cylindrical velocities. Red: 3σ con- fidence limit. Yellow: posi- tions of local groups (< 200 pc, |Z| < 300 pc).

Solar circle!(–8.1 < X < –7.5 kpc)!

Inside solar circle!(–7.5 < X < –6.9 kpc)!

Outside solar circle!(–8.7 < X < –8.1 kpc)!

04.12.2013! Grebel: The Milky Way! 6!Luck & Lambert 2011

Clear abundance gradient as function of Galactocentric radius (e.g., in Cepheids: −0.06 dex / kpc)

Radial Abundance Gradient (1)


Older open clusters vs. Cepheids (30 – 300 Myr): No strong dependence on cluster age, but somewhat steeper for clusters with ages ≥ 2.5 Gyr than for Cepheids inside-out disk formation? (Cluster) gradient flattens at [Fe/H] ≈ −0.4 dex for RGC > 15 kpc.

Radial Abundance Gradient (2)

Yong et al. 2012


Slow decline in mean metallicity with age (lower solid line). Slow increase in metallicity dispersion with age (upper solid line). Large metallicity spread of 0.8 dex for large age range of 3 to 10 Gyr caused by radial migration? Or selection effect due to uncertain ages? Mixing would move stars from one near-circular orbit to another, e.g., stars from inner or outer disk with different mean abundance to Solar region.

Age-Metallicity Relation (Solar Neighborhood)

Haywood 2008

!


Age – Velocity Dispersion Relation

Velocity dispersion of disk stars increases with increasing age. Some studies suggest: Most of the heating during first 2 Gyr.

Age [Gyr]!

σ [

km/s

]!

Casagrande et al. 2011

Still many uncertainties due to poorly known ages.


Che

n, H

ou, &

Wan

g 20

03

Open Star Clusters (1) Increasing scale height with increasing age

Open star clusters


Che

n, H

ou, &

Wan

g 20

03

Larger scale height for lower [Fe/H]

Open Star Clusters (2)


Thin and Thick Disk

Higher [α/Fe] subsample ( thick disk): Larger scale height, lower [Fe/H], rotational velocity lag, larger σ for all 3 velocity components, higher ages. Differences in [α/Fe] reflect different star formation time scales:   High [α/Fe]: Enrichment by SNe II, short time scales.   Low [α/Fe]: Enrichment also by SNe Ia, longer time scales.

Thin/thick disk bimodality best seen in [α/Fe]. Two simple components sufficient, consistent with double-exponential fit. But more complex structure is not excluded, nor is a continuous distribution.

Ivezić et al. 2012

[α/Fe] [Fe/H] Thin disk 0.1 −0.2 Thick disk 0.35 −0.6


Thick and Thin Disk

  Thick disks ubiquitous   in disk galaxies.

  In larger disk galaxies   like Milky Way: thick disk contains 10 – 15% of baryons.   In smaller disk galaxies: up to ~ 50%.

  Galactic disks contain substantial fraction of a disk galaxy’s baryonic   matter and angular momentum. Most evolutionary activity in disks. Milky Way’s thick disk:   Vertical velocity dispersion 40 km/s.   Scale height ~ 1000 pc (thin disk: ~ 300 pc)   Surface brightness of thick disk: ~ 10% of thin disk’s.   Stars mainly older than 10 Gyr (10 – 12 Gyr).   Mean metallicity −0.5 – −1.0 dex (much more metal-poor than thin disk)   No obvious vertical abundance gradient.   Enhanced in [α/Fe], suggesting rapid formation (within 1 Gyr).

Freeman

Fuhrmann et al. 2011


Thick Disk Formation

  Thin disks: site of gas, dust, star formation, small scale height,   high stellar density, small velocity dispersion, ordered rotation.   Thick disks: Older, more metal-poor stars, lower stellar density,   larger scale height & velocity dispersion; ordered but slower rotation.

  Violent origin?   Heating of early thin disk by minor merger   Accretion from disrupted satellite

  Secular origin?   Gas-rich merger, subsequent star formation   Dissipation of star clusters or SF clumps   Radial migration

Eccentricity distribution of thick disk stars may permit one   to distinguish scenarios (Sales et al. 2009).   Mild preference for early, gas-rich merger.   Age of Galactic thick disk suggests rather quiescent merger history.

Whatever the origin, thick disks appear to be ubiquitous in disk galaxies. Mechanism(s?) should be universally valid.

Ivezić et al. 2012 04.12.2013! Grebel: The Milky Way! 15!

Milky Way Components: The Bulge and Bar

Buser 2000

8 kpc


2MAS

S

Infrared: Left-right asymmetry

�Peanut� shape of the bulge: Triaxial bulge or a bar, whose near side is left and seen under an inclination angle of 20° - 25° . Side closer to us appears bigger. Bar extends 3–4 kpc from center.

Flat, boxy bulge: ~ 20% of Milky Way’s luminosity. Bulge stars ~ circular orbits, large random velocities; mean rotational velocity ~100 km/s (thin disk: 220 km/s).

The Bar and the Bulge


The Bar and the Bulge (2)

Split red clump seen in bulge: [Fe/H] > –0.5.

  Bimodal distribution of stars along line of   sight; associated with bar/boxy bulge.

  Generic feature of boxy/peanut-shaped   bulges that grew through bar instability.

Metal-rich ([Fe/H] > –0.5) bulge stars:

  Two components; both probably due to   instability-driven bar/bulge formation from thin disk.

  [Fe/H] ≈ −0.25: wider extent in z; thicker. From early, less metal-rich   thin disk. Main bulge component.   [Fe/H] ≈ +0.15: kinematically colder; closer to plane. From colder, more   metal-rich stars of early thin disk; triggering event somewhat later?

  Clear spatial and kinematic separation: no significant merger since time   when bulge-forming instabilities occurred. Internal evolution dominates. Gonzalez et al. 2011; Robin et al. 2012; Ness et al. 2012, 2013; Uttenthaler et al. 2012.

Split red clump in CMD. Saito et al. 2011


Inner Galaxy cont’d.   More metal-poor components do not   contribute to the boxy/peanut bulge.   Seem to match thick disk, metal-   weak thick disk (−1.2 dex), and halo   (−1.7 dex).   Ages: 10 – 12 Gyr. α-enhanced.   Decrease in [α/Fe] with radius Chemical evolution in inner Galaxy   faster than in disk at larger radii.

Bulge CMD. Solid red line:

solar metallicity isochrone, 10 Gyr. Sahu et al. 2006

Ness et al. 2013 Metallicity distribution function

Ness et al. 2013


Milky Way Components: The Stellar Halo

Buser 2000

8 kpc


Rotational velocity or VLSR vs. heavy element content; [Fe/H]

Thin disk stars: rapid rotation,

higher metallicity

Halo stars:!slow rotation, low metallicity

Striking kinematical difference between disk and halo.!

Reddy et al. 2006

Milky Way Component Kinematics

Thick disk stars


Orbital eccentricity vs. [Fe/H]

Thin disk stars: Low-eccentricity orbits,

more metal-rich

Halo stars:!Highly eccentric orbits,

metal-poor

Striking kinematical difference between disk and halo.!

Milky Way Component Kinematics

Reddy et al. 2006

Ecc

entri

city!

Thick disk stars

04.12.2013! Grebel: The Milky Way! 22!Zolotov et al. 2009

McCarthy et al. 2012

“Dual Origin of Stellar Halos” (Simulations)

  Stellar halos composed in part of   accreted stars, in part of stars   formed in situ.

  Origin of gas for in-situ formation:   Brought into primary galaxy in   smooth cold gas flows (similar to   disks). Only small percentage   from gas stripped from accreted   subhalos.

  Most in-situ stars had formed by z ~ 3.

  Stars form initially mainly in central “protodisk”, but due to dynamical   heating in subsequent mass accretion events stars acquire more   extended halo-like orbits by z ~ 2. Retain net prograde rotation w.r.t.   disk and flattened distribution.


Satellite Accretion (Simulations)

  Models suggest that a wide variety of   satellite accretion histories exists,   from smooth growth to discrete events.   Satellite accretion occurs mainly between   1 < z < 3.   Halos from “from inside out”.   ≤ 5 luminous satellites are the main   contributors to stellar halos.

  The most significant contributors have   masses in the range of the most luminous   dSphs (For, Sgr) or the SMC.

  Majority of mass (50 – 80%) from several   massive (108 – 109 M) satellites that   merged > 9 Gyr ago (inner halo).

Font et al. 2006 De Lucia & Helmi 2008; Cooper et al. 2010


Carollo et al. 2010; Ivezić et al. 2012

Carollo et al. 2007, 2010, 2012; Beers et al. 2012

Galactic Stellar Halo (Observations)

Inner halo:

  Out to RGC = 15 – 20 kpc.   Flattened density distribution.   〈[Fe/H]〉 = –1.6.   No rotation.

Outer halo:   Beyond 15 – 20 kpc.   More spherical density distribution.   Net retrograde rotation.   〈[Fe/H]〉 = –2.2.   Twice the CEMP fraction as inner halo.   Fine and coarse substructure stronger.   GC structural properties like in dwarfs.   Small system accretion origin proposed.

04.12.2013! Grebel: The Milky Way! 25!Bell et al. 2008

Halo substructure traced by SDSS main-sequence stars.

Galactic Stellar Halo (obs.)

Abundant Substructure: Sagittarius, Virgo stellar stream, Virgo overdensity, Monoceros, Tri-Andromeda, Hercules-Aquila cloud, Pisces overdensity, Orphan stream, Aquarius stream...

  Features differ in age and metallicity (e.g., many/no blue HB stars).   Suggested origin: Debris from disrupted satellites.   Stellar population constraints: No evidence for accretion of young/very metal-   rich stars from massive satellites; lower-mass satellite progenitors preferred.

04.12.2013 Grebel: The Milky Way! 26!

Lower [α / Fe] at a given [Fe/H] in dSphs than in Galactic halo:

  Low SFRs (little contribution from massive SNe II (α)), or   Loss of metals and SN ejecta by galactic winds, or   Larger contribution from SNe Ia (Fe enhanced over α); inefficient enrichment.

Shetrone et al. 2001

Koch et al. 2010

Constraints from element abundances


“Elements of Cold Halo Substructure”   ECHOS vs. dSphs: ECHOS plausibly associated with progenitor like Scl or Leo I.

  ECHOS are more metal-poor than thick disk stars and more metal-poor and α-enhanced than typical thin disk. Schlaufman et al. 2011


Milky Way : Outlook

Buser 2000

8 kpc


Total mass of the Milky Way:   ~ 1 - 2·1012 M!

Stellar mass   of the bulge:   2·1010 M!

  of the disk:   5·1010 M!

  of the halo:   1·109 M!

Relation between Metallicity and Age in the Milky Way

Buser 2000

8 kpc

Freeman & Bland-Hawthorn 2002

bulge�


Summary:

Bulge:   Complex composite structure.   Likely secular origin (pseudobulge); transformed old thin disk(s). Thick disk:   Old (10–12 Gyr). Higher velocity disp. & scale height than thin disk.   Origin: early gas-rich merger? Importance of radial migration? Thin disk:   Wide age range. Star formation rate shows long-term fluctuations. Why? Stellar Halo:   Inner/outer halo dichotomy. Part in-situ; part accreted.   Role, magnitude, time of in-situ star formation vs. accretion? Overall fairly quiescent merger history of Milky Way. Particularly age determinations remain a difficulty asteroseismology? In addition to ground-based surveys, major advances expected from Gaia.

milky way components: thin and thick disk · 2013. 12. 17. · mean rotational velocity ~100 km/s...

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