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