8th sino german workshop kunming feb 23 28 2009
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8th Sino-German Workshop Kunming, Feb 23-28, 2009. Milky Way vs. M31: a Tale of Two Disks. Jinliang HOU In collaboration with : Ruixiang CHANG, Shiyin SHEN, Jun YIN, Jian FU et al. Center for Galaxy and Cosmology Shanghai Astronomical Observatory, CAS. Content.

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8th sino german workshop kunming feb 23 28 2009
8th Sino-German WorkshopKunming, Feb 23-28, 2009

Milky Way vs. M31: a Tale of Two Disks

Jinliang HOU

In collaboration with :

Ruixiang CHANG, Shiyin SHEN, Jun YIN, Jian FU et al.

Center for Galaxy and Cosmology

Shanghai Astronomical Observatory, CAS



  • MW vs. M31: observed properties
      • Halo
      • Disk
  • Two disks: chemical evolution model
  • Summary
Observed properties: MW vs. M31

Milky Way M31

785kpc from the Sun


Components in the Milky Way Galaxy

dark halo

stellar halo

thick disk

thin disk


We would like to understand how our Galaxy came to looklike this.

the milky way typical or not
The Milky Way, typical or not?
  • It is always regarded that the MWG is the typical spiral in the universe, especially at its mass range.
  • How about M31 galaxy, it is a spiral that is comparable with MWG in the Local Group, and now it is possible to have detailed observations.
differences in general
Differences : in general


  • M31: metal-rich for field populations
  • M31: more globular clusters ( ~ 3 times )
  • M31: more substructures


  • M31: 2 times larger than MW
  • M31: present day SFR ~ 1/10 of MW
  • M31: gas fraction ~ 1/2 of MW

Halo properties

Metal - Velocity

Tully-Fish Relation

SDSS: 1047 edge-on spirals

Hammer et al. 2007


Halo properties


X -- M33

Since M31 has a metal rich halo

Metallicity – luminosity relation

Mouhcine et al. 2005


Halo properties

Chapman et al. 2006


Stellar Halo Definition

Chapman et al (2006 ) kinematically defined stellar halo : metal-poor

X -- M33

Metallicity – luminosity relation ???

Black dot: simulation from Renda et al. 2005

halo globular clusters
Halo Globular Clusters

Number distribution

 Double peak


 M31: 700, metal rich

 MW : 200


Disk scale length

M31 distance: 785kpc

Band Observed scale length ( kpc )

M31 the Milky Way

U 7.7

B 6.6 4.0-5.0

V 6.0

R 5.5 2.3-2.8

I 5.7

K 4.8

L 6.1

Note: SDSS average rd = 4.8kpc (Pizagno et al. 2006)

disk profiles
Disk Profiles

Total gas fraction

M31: 1/2 of MW

Total disk SFR



Yin et al. 2009

o h gradient from young objects

Two gradients reported:

Steep: -0.07 dex / kpc

(Rudolph et al. 2006 )

Flat: -0.04 dex/kpc

(Deharveng et al. 2000

Dalfon and Cunha 2004)

[O/H]gradient from young objects

Scaled gradient



M31 :-0.094

-0.017 dex / kpc

scaled profiles
Scaled profiles










purpose of the chemical evolution study for the milky way and m31 disks
Purpose of the chemical evolution studyfor The Milky Way and M31 disks

Using the same model

  • Find common features
  • Find which properties are galaxy dependent
  • M31 and MWG, which one is typical ?
unified one component model
Unified One Component Model
  • Disk forms by gas infall from outer dark halo
  • Infall is inside-out
  • SFR:
    • modified KS Law (SFR prop to v/r)
why use modified ks law m ks law
Why use modified KS law (M-KS law)?

Strong correlation between the average gas mass surface density and SFR density for nearby disk and starburst galaxies (Kennicutt 1998)

two types of correlations ks law
Two types of correlations: KS law

The later form implies SFR depends on the angular frequency of the gas in the disk. This suggestion is based on the idea that stars are formed in the galactic disk when the ISM with angular frequency Omega is periodically compressed by the passage of the spiral pattern.

applications of ks law
Applications of KS law

When the Kennicutt law is applied in the detailed studies of galaxy formation and evolution, there are several formulism that often adopted by the modelers :


previous work using m ks law milky way disk
Previous work using M-KS law (Milky Way disk)

Boissier & Prantzos 1999; 2000

Boissier et al. 2001

Hou et al. 2000;2001;2005

Francois et al. 2004


Current properties of disk


This modified KS law is very successful in predicting the current properties of disk

  • Not much TESTED for the disk history

– less constrains available

  • Recently, observed abundance gradient from Open Clusters and Planetary Nebulae have made this possible
how about the history of mwd the evolution of abundance gradient along mwd
How about the history of MWD ?The evolution of abundance gradient along MWD


SF Law

Model A, B

Model C

adoption of sfr law for the chemical evolution model of spiral galaxies
Adoption of SFR Law for the chemical evolution model of spiral galaxies

M-KS law

  • For the average properties of a galaxies, KS law is OK and (r) = 0.25
  • For local properties, SFR could be local dependent, that is, (r) radial dependent, M-KS law is preferred
radial profiles as constrains
Radial Profiles as constrains
  • Gas profile
  • SFR profile
  • Abundance gradient
  • Metallicity distribution Functions in different positions
  • Do the similar chemical evolution models reproduce the global properties for the Milky Way and M31 disks ?


Yin et al. 2009

m31 gas and sfr in disk
M31 gas and SFR in disk
  • Observed of gas and SFR profiles are abnormal when compared with Kennicutt law.
  • Gas and SFR must be modified by some interaction




Two rings


Evidence of M31 disk interaction

Block et al. (Nature 2006)



age = 13Gyr

M31 disk

age = 5-7Gyr

Yin et al. 2009

3 summary ii m31 disk properties
3. Summary (II):M31 disk properties
  • Current star formation properties are atypical in the M31 disk.
    • Disk evolution could be affected by events
  • Has low current SFR in disk
    • Shorter time scale for the infall in disk
3 summary iii problems in two disks
3. Summary (III): Problems in two disks
  • Chemical evolution model cannot reproduce the outer profiles of gas surface density and SFR profiles at the same time for M31 disk
  • The observed abundance gradient along the Milky Way disk still not consistent
  • The evolution of gradients is very important.

Two tracers :

      • PN (Maciel et al. 2003, 2006, 2007)
      • Open Clusters (Chen et al. 2003; 2007; LAMOST Survey)