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

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 Halo properties Metal - Velocity Tully-Fish Relation SDSS: 1047 edge-on spirals

Mouhcine et al Halo properties Metallicity – luminosity relation X X -- M33 Since M31 has a metal rich halo

Chapman et al 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 length Band Observed scale length ( kpc ) M31 the Milky Way U7.7 B V6.0 R I5.7 K4.8 L6.1 Note: SDSS average r d = 4.8kpc (Pizagno et al. 2006) M31 distance: 785kpc

Disk Profiles Yin et al MW M31 Total gas fraction M31: 1/2 of MW Total disk SFR

[O/H] gradient from young objects － dex / kpc Two gradients reported: Steep: － 0.07 dex / kpc (Rudolph et al ) Flat: － 0.04 dex/kpc (Deharveng et al Dalfon and Cunha 2004) Scaled gradient MWD: － － M31 : － 0.094

Scaled profiles MW M31 MW M31 Gas SFR Gas fraction Gradient

Observed disk properties: summary MWDM31 / MWD Rd (kpc)~ Mass(10 10 Ms)~ V flat (km/s) [X/H] (scaled)~ / ~ 0.5 / 1.0 Total f Gas 0.191/2 SFR (Ms/yr)~ 1-21/10

2.Two disks: chemical evolution comparison

Purpose of the chemical evolution study for 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 diskMW disk M tot (Ms) 7   r d (kpc) ( R band) V flat (km/s)220226

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 Hou et al. 2000;2001;2005 Francois et al 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 positions  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 MWDM31 disk Infall & Timescale Quiet ~ 7Gyr Interaction ~ 7Gyr SFRLocal dependent Modulated by events AgeOldYoung ? [X/H] GradientSteep/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 the 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