Near-field Cosmology from the Andromeda galaxy and subgroup

Scott C. Chapman IoA, University of Cambridge With: R.Ibata, M.Irwin, G.Lewis, A.Ferguson, N.Tanvir, N.Martin, A.McConnachie, J. Penarrubia, M. Collins, D. Trethewey

Outline

 The M31 outer disk  The M31 outer halo: The “first” galaxy  (later:) Dwarf galaxy satellites of M31 (and the Milky Way)   Is And-XII a “true” fossil?

Minimum DM halo mass?

Context: Hierarchical Galaxy Formation

- Little galaxies merge to make big galaxies … - How/when are the galaxy components assembled?

Big Bang … Cosmic Microwave Background … … Galaxy Formation and Evolution … Fossil Records today!

Local galaxies

(MW, Andromeda) are ideal laboratories to study archeology.

Bullock et al. (2005) Model/Approach:

1. Construct accretion histories for Milky-Way type halos using semi analytic “merger tree”. 2. For each accreted system, model its previous star formation history based on expected mass growth history 3. Embed stars in the center of accreted dark matter halo.

4. Follow evolution within the (growing) host halo

Bullock+2005, Font+2006 Predictions: 1.) Substructure in halo.

2.) Chemically distinct outer halo. Observational Requirements: 1.) Spatial coverage.

2.) Radial velocities.

3.) Chemical distribution.

4.) Ages ??

Bullock+,Font+,Johnston+ model is our best current prediction for MW/M31. (Changing with Aquarius --

Springel et al. 2008

)

Observational Tests:

Local Galaxy Archeology (Milky Way, Andromeda, satellites)

Dissecting the history of a galaxy by

digging

up its stars 1 by 1: QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

Near-Field cosmology -where are missing satellites -are DM profiles universal? (cuspy NFW?) -DM: extent, nature, spatial distribution -how were MW and M31 constructed: typical disks?

-role of accretion in formation of halo, disk, bulge?

-stars maintain birth statistical pattern -chemical evolution proceeds in 1 direction

Imaging and Spectroscopic study of Andromeda -- M31

M31 (M33) Fossil Record of Galaxy Formation:

Using the Keck 10m / DEIMOS spectrograph: … dissect components & piece together the evolutionary history (Ibata+ 04,05,06; Chapman+ 04,05,06; McConnachie+04,06) Classical (Palomar) view of M31 Modern (wide-field CCD) view of M31: a train wreck! (Irwin+05)

6 degrees (12 full moons) 100 kpc

Fine structure of simulated galaxies

Abadi, Navarro, Steinmetz & Eke 2003

age in Gyr

Building the Spheroid (Bullock+06)

QuickTime™ and a Cinepak decompressor are needed to see this picture.

M31 K inematics a nd M etallicity E xperiment

Ibata et al. (2004,2005); Chapman et al. (2005,2006,2008)

Imaging Surveys: INT2.5m widefield survey of M31 CFHT/MegaCam halo survey of

entire

M31 halo out to 150kpc Initiated w/ Keck/DEIMOS: Sept 2002 Status Nov, 2008 Number of Nights : 21 (70% usable) Number of Fields : 75 Limiting

I

-mag < 22

Candidate M31 Spectra: ~14,000 (and ~6000 Milky Way foreground stars)

Data products: ~5-10km/s velocity determination, (Calcium Triplet, cross-correlation) [Fe/H] measurement by EWs of CaT Fainter lines of other species …

Surprise!

the “messy halo” of stars surrounding M31 is actually a giant rotating disk! (not a train wreck halo) (Ibata et al. 2005)

DEIMOS spectra

Technique: Sort stars by kinematics

Velocity distribution of all stars 1hour Keck exposures Apply disk model (flat rotation curve to >70kpc) A new ``extended disk'' galaxy component

disk halo  Discovered that all structures participate in giant rotating disk

Separation of outer disk/halo in velocity Outer disk of stars rotates like the inner disk disk ~15% of light of inner disk >40% of angular momentum!

Irregular morphology, lots of substructure … transitory?

(Ibata+2005, Chapman+2006)

Star velocities in giant disk

40kpc Distance (Major Axis) 40kpc

Separation of outer disk/halo in velocity disk halo

What’s left?

A primeval “Halo” of stars!

-

few heavy metals (formed early)

-

An relic of M31’s beginning



v = 125 km/s Monotonic decrease in



v(R) No rotation. Detectable “spikey” substructure?

(Chapman, et al. 2006; 2009 in prep)

Mass of M31 from Halo stars

First assume simple rotating isothermal halo: not rotating, and 125km/s  v Then ignore rotation and allow  v Monotonic decline in  v to decrease linearly with projected R.

with radius … better fit.

Mass of M31 from Halo stars

Monotonic decline in sigma_v with radius - ignore stream spikes … (Trethewey+08) Fit to an NFW dark matter halo (assuming the stars are a reasonable tracer of the halo: And taking Klypin et al. 2002 limit for concentration (caveats) M_virial > 9e11 Msun, 99% confidence.

Consistent with other estimates of M31’s DM halo mass (satellites - Evans&Wilkinson, giant stream - Ibata,Chapman et al. 2004)

Compare HALO and DISK chemistry: R~70kpc Extended Disk - Metallicities

Average spectrum at each Keck position All fields have similar average metallicities [Fe/H] ~ -1.0 [sigma=0.4] - More metal rich than MW halo.

Average “extended disk” star from 15kpc - 70kpc probing similar global star population!

R=10-70kpc Stellar

Halo

:

Metallicities Average spectrum at each Keck position fields have average metallicities [Fe/H] = -1.4 [



=0.2] Stars selected like those in MW halo (non-rotating), have similar metallicity and velocity distribution.

NFW model fit => 10^12 Msun Solves “puzzle” of metal-rich halo in M31!

Metal Poor halo:

no abundance gradient

Halo radial Fe/H constant: detect the metal-poor halo component from 50kpc right up to 17kpc … as opposed to minor axis where velocities of all components overlap in the M31 systemic velocity range.

Giant stream

consistent with Koch et al. 2007

Koch et al. (2007):

M31 minor axis from 10-120kpc

combined M.Rich & S.Chapman Keck-DEIMOS data

Abundance transition at 20kpc: metal-rich to metal-poor … inconsistent with previous sparsely sampled minor axis study ( Kalirai et al. 2006 ).

M33

(1/10 the mass of M31 and MW)

kinematically selected halo

( McConnachie+06, Trethewey+09)

Keck/DEIMOS M31 halo study … on edge of disk/halo transition from Ibata et al. (2007) Keck spectra find: 1) Metal poor halo Fe/H = -1.5

2) Extended disk 3) Unknown “stream”

“Mouhcine plot”

VERY HARD to see metal-poor primordial halos in more distant galaxies without kinematics!

L vs Fe/H correlated in spiral galaxy halos? (Mouhcine+05) Kinematically selected Halos in M31 (Chapman+06) M33 (McConnachie+06) MW (Chiba&Beers+00,01) … all Fe/H ~ -1.5

Are we comparing apples with apples in distant (10Mpc) spiral galaxies?

Dots are Renda+05 model

Conclusions

  In halos of big (L*) Spiral galaxies (M31), extended rotating components may be common => difficult to interpret more distant galaxies without kinematics Beginning to understand the primeval halo of M31 (and the MW …), versus later accretions.

 More work required to understand substructure and mass function of first accretions  Halo stars in front of M31, outer edge of the MW halo  Growing discoveries of dSph galaxies (and their characterizations) are an excellent testbed of galaxy evolution and cosmology.

Halo stars in front of M31

CDM Has a Missing Satellite Problem

CDM predicts large numbers of subhalos (~100-1000 for a Milky Way-sized galaxy) Milky Way only has 23 known satellites M31 has 25 satellites What happened to the rest of them?

V.Springel et al. 2001

CDM Has a Missing Satellite Problem

CDM predicts large numbers of subhalos (~100-1000 for a Milky Way-sized galaxy) V.Springel et al. 2001 Many never form stars

What is a dwarf Fossil*?

*defined by Ricotti & Gnedin (2005)

Survivors

(M > 10 9 M  ) * star formation started after reionization * mostly dIrr, some dE

LMC M32 Polluted fossils

(M ~ 10 6 - 10 8/9 M  ) * significant star formation after reionization * tidal effects from host cause additional bursts * dSph and dE

Pegasus True fossils

(M ~ 10 6 - 10 8/9 M  ) * < 30% of stars formed after reionization * never accreted gas from the IGM * dSph

Cetus (Whiting et al. 1999)

R02a,b predictions.

Known survivors Known polluted fossils Known true fossils New ultra-faint dwarfs

Fossil Properties

~ Scatter in Z due to: - pollution from nearby halos - multiple bursts of star formation ( ie. Stinson et al (2007) ) ~ Fossil properties at z = 0 are simply related to their properties at reionization.

QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture.

Ricotti & Gnedin (2005), Bovill et al. (2007)

Formation and Evolution of dwarf galaxies

 Environment of dwarfs severely affects their properties.

Most dwarfs have been orbiting around our Local environment for most of the age of the Universe (>10 Billion years) McConnachie & Irwin 2005 QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

CDM predicts late accreting DM halos

  

But we’ve never seen one …

have any of them formed stars?

Dwarf Galaxies still bringing in primeval material?

Late accretions:

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

4% today

 

Objects that accrete late do so from larger average distances than those that accrete early. late accreting objects

interesting both observationally and theoretically   

spent the majority of their lives in different environments, far from the disruptive tidal forces of larger galaxies, a direct prediction of the theoretical CDM model for structure formation.

Ludlow et al. 2009

Dwarf Galaxies still bringing in primeval material?

DISCOVERY of AndXII, a faint Dwarf galaxy building up the Local Group environment:

falling in for the first time!

 Direct observational evidence for the hierarchical formation of the Local Group.

 Insights into processes responsible for the dynamical evolution of dwarfs?

(Chapman et al. 2007) QuickTime™ and a TIFF (Uncompre ssed ) decomp resso r are need ed to se e th is p icture.

“Mateo” Plot

   Simplest possible model: equilibrium, spherical, isotropic systems where M follows L (c.f.

Strigari et al. 2007) Do all dwarfs live in similar halos?

Is there a minimum mass for dwarfs?

And13 And12 And11 And15 And16 

Globular star clusters, no DM

 Kinematics in dwarfs: MW: Martin+07; Simon+07 M31: Chapman+05,07; Collins+08; Letarte+08 And more to be studied/discovered … And17,18,19,20,21 (Irwin+08, McConnachie+08)

What Does This Problem Tell Us?

 Two basic sets of possible solutions:  Modifications to CDM  What modifications? Power spectrum, DM particle mass/decay/interaction cross section?

 Astrophysics prevents stars from forming in most low-mass halos  Reionization, feedback, winds …

Angular Momentum (

J

) Catastrophe

Sizes of galactic disks linked to

J

of parent DM halos (Fall & Efstathiou 1980) • distribution of halo spin parameters ( N-body simulations, e.g. Bullock+ 2001) • baryons and dark matter initially share the same distribution of specific angular • momentum,

j

, within the halos (e.g. van den Bosch etal. 2002)

j

is conserved as the baryons contract to form the disk (e.g. Mestel 1963). Disk sizes with these assumptions, roughly comparable to those observed.

But, Hydrodynamics shows this process is invalid.

=> significant fraction of

J

of the baryons *is* transferred to DM, … disk sizes 10x too small! (Navarro & Steinmetz 2000) SOLUTION? Feedback … remove incoming dwarf galaxy low-j baryons (Maller&Dekel 2003)

Forming the Edisk

How to further increase angular moomentum by 50% ???

QuickTime™ and a BMP decompressor are needed to see this picture.

Accretion origin to extended disk?

(Penarrubia+06) BUT: Requires specialized conditions; large in-plane accretion(s); … would be consistent with observations

Evolving Fossils to z = 0

 Fossil properties at z = 0 are simply related to their properties at reionization.

 Properties of the new Sloan and M31 dwarfs agree well with predictions for primordial galaxies