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 CosmologyShanghai Astronomical Observatory, CAS
1. MW vs. M31: observed properties
Halo
Disk
2. Two disks: chemical evolution model
3. Summary
Content
1. Observed properties: MW vs. M31
Milky Way M31
785kpc from the Sun
M31 and MWG have similar mass and morphology
Components in the Milky Way Galaxy
dark halo
stellar halo
thin disk
thick disk
bulge
We would like to understand how our Galaxy came to look like this.
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
Halo:
M31: metal-rich for field populations M31: more globular clusters ( ~ 3 times ) M31: more substructures
Disk:
M31: 2 times larger than MW M31: present day SFR ~ 1/10 of MW M31: gas fraction ~ 1/2 of MW
Hammer et al. 2007
Halo properties
Metal - Velocity
Tully-Fish Relation
SDSS: 1047 edge-on spirals
Mouhcine et al. 2005
Halo properties
Metallicity – luminosity relation
X
X -- M33
Since M31 has a metal rich halo
Chapman et al. 2006
Halo properties
Metallicity – luminosity relation ???
X
X -- M33
Stellar Halo Definition
Chapman et al (2006 ) kinematically defined stellar halo : metal-poor
Black dot: simulation from Renda et al. 2005
Halo Globular Clusters
Number distribution
Double peak
Number:
M31: 700, metal rich MW : 200
Disk scale lengthDisk scale length
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)
M31 distance: 785kpc
Disk Profiles
Yin et al. 2009
MW
M31
Total gas fraction
M31: 1/2 of MW
Total disk SFR
[O/H] gradient from young objects
- 0.017 dex / kpc
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)Scaled gradient
MWD: - 0.161 - 0.093
M31 : - 0.094
Scaled profiles
MW
M31
MW
M31
Gas SFR
Gasfraction
Gradient
Observed disk properties: summary
MWD M31 / MWD
Rd (kpc) ~ 2.3 2.4
Mass(1010Ms) ~ 3.7 2.0
Vflat (km/s) 220 1.0
[X/H] (scaled) ~ - 0.16 / - 0.09 ~ 0.5 / 1.0
Total fGas 0.19 1/2
SFR (Ms/yr) ~ 1-2 1/10
2. Two disks: chemical evolution comparison
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
1. Disk forms by gas infall from outer dark halo
2. Infall is inside-out
3. SFR:
modified KS Law (SFR prop to v/r)
M31 disk MW disk
Mtot (Ms) 7 1010 3.5 1010
rd (kpc) ( R band) 5.5 2.3
Vflat(km/s) 220 226
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
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
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 :
SFR
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
Etc……
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
Infall
SF Law Model A, B
Model C
Fu et al. 2009
Adoption of SFR Law for the chemical evolution model of spiral galaxies
• 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
M-KS law
Radial Profiles as constrains
• Gas profile • SFR profile• Abundance gradient• Metallicity distribution Functions in different posi
tions
Do the similar chemical evolution models
reproduce the global properties for the Milky
Way and M31 disks ?
SFR
Yin et al. 2009
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
Block et al. (Nature 2006)
Observed
Simulation
M32 Two rings structure
Evidence of M31 disk interaction
MDFs
MWD
age = 13Gyr
M31 disk
age = 5-7Gyr
Yin et al. 2009
3. Summary (I): Comparing two disks
MWD M31 disk
Infall &Timescale
Quiet
~ 7Gyr
Interaction
~ 7Gyr
SFR Local dependent
Modulated by events
Age Old Young ?
[X/H] Gradient Steep/flat ? Flat
3. Summary (II) : M31 disk properties
1. Current star formation properties are atypical in the M31 disk.
Disk evolution could be affected by events
2. Has low current SFR in disk
Shorter time scale for the infall in disk
3. Summary (III) : Problems in two
disks 1. Chemical evolution model cannot reproduce th
e outer profiles of gas surface density and SFR profiles at the same time for M31 disk
2. The observed abundance gradient along the Milky Way disk still not consistent
3. The evolution of gradients is very important.
Two tracers : 1. PN (Maciel et al. 2003, 2006, 2007)
2. Open Clusters (Chen et al. 2003; 2007; LAMOST Survey)
Thanks
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