the chemical history of the galactic bulge and disk

The chemical history of the Galactic bulge and disk

Thomas Bensby

Wednesday, December 7, 11

The Milky Way

Credit: R. Hurt (SSC), JPL-Caltech, NASA


http://ssc.spitzer.caltech.edu/

http://ssc.spitzer.caltech.edu/

http://www.jpl.nasa.gov/

http://www.jpl.nasa.gov/

http://www.nasa.gov/home/index.html

http://www.nasa.gov/home/index.html

Schematic view of the Milky Way

HALO

8 kpc

HALO

SUN

BULGE THICK DISK

THIN DISKGALACTIC CENTER


The Milky Way


How does a spiral galaxy such as the Milky Way form and evolve?

Where does its stellar populations come from?

Disentangling and characterizing the different stellar populations in a multi-dimensional space of

elemental abundances -- kinematics -- ages


Detailed elemental abundancesThe atmospheres of solar type stars retain the chemical composition

of the gas cloud they once formed from.

High-res spectroscopy

Kinematics

Spectroscopy

Ages

Essentially only for dwarf and turnoff stars.Spectroscopy or photometry


Part I

The nearby Galactic diskCollaborators:

Sofia Feltzing (Lund)Sally Oey (University of Michigan, USA)

Bensby et al. (2003, A&A, 410, 527), Feltzing et al. (2003, A&A, 397, L1),Bensby et al., (2004, A&A, 421, 969), Bensby et al. (2005,A&A, 433, 185),

Bensby & Feltzing (2006, MNRAS, 367, 1181), Bensby et al. (2007, ApJ, 655, L89),Bensby et al. (2007, ApJ, 633, L13)


NOT on La Palma16 nights

Magellan on Las Campanas14 nights

ESO 2.2 on La Silla20 hours

VLT on Paranal, 72 h

ESO 1.52 on La Silla2 nights

More collaborators:


Discovery of thick disks

(Burstein 1979, ApJ, 234, 829)


Discovery of the Galactic thick disk

(Gilmore & Reid, 1983, MNRAS, 202, 102)

Thin disk:scale height: 250 pcnormalisation: 98%

Thick disk:scale height: 1400 pc

normalisation: 2%


The Galactic thick disk - rotation

• No priory law that says that vertical star counts in spiral galaxies should fit single power laws

• Finding by Gilmore & Reid necessary but not sufficient to define thick disk as unique entity

• Rotational lag and velocity dispersion

• lags the LSR by ~40-50 km/s

• higher velocity dispersions

(Navarro et al., 2004, ApJ, 641, L43)

LSRThickdisk


The Galactic thick disk - metallicity

• Average metallicity: [Fe/H]=-0.7

• Intermediate to the thin disk and the halo

(Gilmore, Wyse & Jones, 1995, ApJ, 109,1095)


The Galactic thick disk - age and abundances

• Elemental abundance ratios

• Enhanced in alpha-elements compared to thin disk

• Old ages >10 Gyr

(Fuhrmann, 2008, MNRAS, 384, 173)

Sizes of circles scale with the ages of the stars


• Fuhrmann’s study is 85% volume complete for all mid-F type to early K-type stars down to Mv=6.0, north of dec=-15o, within a radius d<25pc from the Sun

• Two types of stars:

• Old stars with high [Mg/Fe] ratios

• Young stars with low [Mg/Fe] ratios

(Fuhrmann, 2008, MNRAS, 384, 173)

The Galactic thick disk - age and abundances


• Bensby et al. (2003, 2004, 2005)

(Bensby et al., 2004, A&A, 415, 155)

Kinematically selected samples


• Bensby et al. (2003, 2004, 2005)

• Reddy et al. (2003, 2006)(Reddy et al., 2006, MNRAS, 367, 1629)

Kinematically selected samples


Many more studies, most agree that the thick disk is:

• kinematically hot

• old

• metal-poor

• alpha-enhanced

Thick disk properties


Geneva-Copenhagen SurveyNordström et al., (2004), New parameters by Casagrande et al. (2011)

Text

Rotation of the thin diskRotation of the thick disk



The metal-rich thick diskHow metal-rich?How old? How young?



The metal-rich thick diskHow metal-rich?How old? How young?

The metal-poor thin diskHow metal-poor?How old?


A new stellar sample

• High-resolution spectra with high signal-to-noise (S/N>250) for 703 kinematically selected F and G dwarf stars in the solar neighbourhood

MIKE @ Magellan R=65000 (502 stars)

UVES @ VLT R=80000 (31 stars)UVES @ VLT R=110000 (27 stars)

SOFIN @ NOT R=80000 (52 stars)

FEROS @ ESO 1.5m R=42000 (62 stars)

FEROS @ ESO 2.2m R=42000 (29 stars)

(Bensby et al., to be submitted)


Titanium

(Bensby et al., to be submitted)


Chemical evolution

Massive stars: short-lived, produce a lot of α-elements

Low-mass stars:long-lived, produce none

or very littleof the α-elements


Ages

Text

Age errors < 3 Gyr

Red: stars with thick disk kinematics

AMR

Blue: stars with thin disk kinematics

No AMR


Summary - the nearby disk

Two distinct disk populations in the solar neighbourhoodin terms of kinematicsin terms of average metallicitiesin terms of abundance trendsin terms of stellar ages

Thin disk younger than 7-8 Gyr

Thick disk older than 8-9 Gyr

AMR in the thick disk, no AMR in thin disk

Thin disk have [Fe/H]>-0.7 (no thin disk stars below)


Part II

The inner and outer Galactic disk

Collaborators:Alan Aves-Brito (ANU, Australia)

Jorge Melendez (Brazil)Sally Oey (Univ. of Michigan, USA)

David Yong (ANU, Australia)

Bensby et al. (2010, A&A, 516, L13)Bensby et al. (2011, ApJ, 735, L46)


Magellan @ Las Campanas, Chile (6 nights)

More collaborators:


Stellar sample

29

44 inner disk K giants

20 outer disk K giants

Observed with MIKE@Magellan

R~55000

S/N>100

Analysed in the same way (and by the same person!) as the disk and bulge giants in Alves-Brito et al. (2010)

Truly diferential analysis between the bulge, the inner disk, the thin and thick disk in the SN, and the outer disk


Results: kinematics

Both the inner and outer disk samples contain stars

with cold and hot kinematics. If things are

similar as in the solar neighbourhood we should probe both the inner and the outer disks at these

locations (if they are there!)


Results: abundances

Thin and thick disk red giants

Alves-Brito et al. (2010)


Results: abundances

Inner disk red giants

Bensby, Alves-Brito, Oey, Yong, & Melendez, 2010,

A&A, 516, L13




Results: abundances

Outer disk red giants


ApJ, 735, L46



Inner disk red giants


A&A, 516, L13


Results: abundances

Where is the outer thick disk?(at majority of the stars are located at a distance from the galacticplane where the thick disk should be the dominating population)


Scale-lengths of the thin and thick disks

Red lines show the distance from the plane where the number densities of thin and thick disk stars are equal

Lthick = 3.6 kpcLthin = 2.6 kpc

Hthick = 0.9 kpcHthin = 0.3 kpc

ρthick = 0.12ρthin = 0.88

(Juric et al. 2008SDSS data)


A short thick disk scale-length



Hthick = 0.9 kpcHthin = 0.3 kpc


A short thick disk scale-length with varying scale-height



Hthick, and Hthin

vary linearly with RG/L


Lack of α-enhanced stars in outer disk

Local sample of 702 F and G dwarf stars

α-enhanced starsmainly come from

inner disk

Rmean is the mean distance from the Galactic centre of the orbits


Inner and outer disk summary

Inner disk: thin and thick disks present

Outer disk: not many alpha-enhanced stars: where is the outer thick disk?

a matter of scale-lengths?

(or is the outer disk chemically very different from thethick disk in the solar neighbourhood and the

thick disk in the inner disk?)


Part III

Microlensed dwarfs in the Bulge

Collaborators:Martin Asplund (ANU, Australia)

Daniel Aden (Lund)Judy Cohen (Caltech, USA)

Sofia Feltzing (Lund)Andy Gould (Ohio State univ, USA)

Jennifer Johnson (Ohio State univ, USA)Sara Lucatello (INAF, Italy)

Jorge Melendez (Brazil)Ian Thompson (Carnegie Observatories, Pasadena, USA)

Bensby et al. (2009, A&A, 499, 737), Bensby et al. (2009, ApJ, 699, L174),Bensby et al. (2010, A&A, 512, A41), Bensby et al. (2010, A&A,521, L57),

Bensby et al. (2011, A&A, 533, A134)


VLT @ Cerro Paranal, Chile (51 hours)

Keck on Hawaii, USA(7 hours)

Magellan @ Las Campanas, Chile(16 hours)

More collaborators:


Dwarf stars in the Galactic bulge

Faint: V~19-20Requires ~50 hour exposure with VLT/UVES

to get a decent high-resolution spectrum (R~42000, S/N>50)


Microlensing


Microlensing

OGLE and MOA monitors the bulge every night


Microlensing

Suddenly a star starts to brighten.....

JD=Julian days


Microlensing

Text

Brightness can increase by factors of several

hundred. Enough to get good spectrum in 2 hours

with UVES

Current sample consistsof 38 dwarf stars in the

Bulge.


Main results


A bulge - thick disk connection? metallicity distribution

Metal-poor bulge dwarfs have average

metallicity of [Fe/H]=-0.6

α-enhanced red giants in the inner disk has average

metallicity of [Fe/H]=-0.6

Average metallicity of the thick disk is [Fe/H]=-0.6(e.g. Carollo et al. 2010)


A bulge - thick disk connection? Abundances

Green: thin disk giantsBlue: thick disk giantsRed: Inner disk giantsAsterisks: bulge giants

Bensby et al. (2010)

Black: 38 microlensed bulge dwarfs

Red: local thick disk dwarfs

Blue: local thin disk dwarfs


A bulge - thick disk connection? Ages

The metal-poor microlensed bulge dwarfs are almost all old

as is the thick disk(e.g. Fuhrmann 1998,2004, 2008;

Bensby et al. 2004, 2007)


Bulge summary

Bulge MDF bimodal => multiple stellar populations?

Properties of the metal-poor bulge is very similar to the thick disk => bulge-thick disk connection?

Young stars in the metal-rich bulge : not seen in photometric surveys, currently a hot topic!


Part IV

Further ongoing investigations of the Galactic disk


Magellan @ Las Campanas, Chile (10 nights)

Collaborators:Sally Oey (Univ of Michigan)

Simone Daflon (Observatorio Nacional, Brazil)Katia Cunha (NOAO, Tucson, USA)

O and B stars in the Galactic disk


O and B stars in the Galactic disk

High-resolution and hign S/N spectra for

400 nearby O and B stars&

100 O and B stars in the outer disk

(out to 10-20 kpc from the GC)

Abundance analysis underway....


AAOmega@Anglo-Australian Telescope (8 nights)

Collaborators:Blair Conn (Max Planck, Heidelberg)Alessandro Ederoclite (IAC, Spain)

Hand-Walter Rix (Max Planck, Heidelberg)Nicolas Martin (Max Planck, Heidelberg)Noelia Noel (Max Planck, Heidelberg)

Richard Lane (Concepcion, Chile)Geraint Lewis (Sydney, Australia)

Rodrigo Ibata (Strasbourg, France)Mike Irwin (Michigan State, USA)Tim Beers (Michigan State, USA)

Young Sun Lee (Michigan State, USA)

EMPhaSSIS

ESO-Max Planck Photometric

and Spectroscopic

Survey in the South


EMPhaSSIS

Low-resolution spectra (R~8000) of 5000 dwarf stars in the outer disk (10-20 kpc from the GC).

Aims at identifying and characterizingthe stars of the outer disk and

the Monoceros stream/ring

Abundance analysis underway......


Gaia-ESO Survey

300 nights (over 5 years)with VLT/FLAMES

starting Dec 31, 2011

150 000 stars FLAMES/GIRAFFE (R~20000)

10 000 starsFLAMES/UVES (R~45000)

in all major componentsof the Milky Way

(thin disk, thick disk, bulge, halo, & clusters)


Gaia-ESO Survey


What will this tell us?

The properties of the thin and thick diskstwo disks or only one?

The properties of the Bulgeold?young?multiple populations?

Connections beteen stellar populationsthin and thick disks -- continuous or distinct?thick disk -- halo?metal-poor bulge -- thick disk?metal-rich bulge -- thin disk?

Where do all the structure in the halo and disk come from?infalling satellites?

Together with theoretical models this will add pieces to the puzzle of the formation of the Milky Way and spiral galaxies in general


the chemical history of the galactic bulge and disk

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