Transcript Document

The Local Group in Cosmological
Context
Rosemary Wyse
Johns Hopkins University
Subaru/NOAJ Symposium, Nov 2011
Exciting times to study local galaxies
 Large observational surveys of stars in Local
Group galaxies are feasible using wide-field
imagers and multi-object spectroscopy,
complemented by space-based imaging and
spectroscopy, then Gaia and full phase space
information
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Important role for Subaru
 There are copious numbers of stars nearby that
have ages >~ 10 Gyr : formed at redshifts > 2
Exciting times to study local galaxies
 High-redshift surveys are now quantifying the stellar
populations and morphologies of galaxies at high
look-back times
 Large, high-resolution simulations of structure
formation are allowing predictions of Galaxy
formation in a cosmological context, ΛCDM now,
Warm Dark Matter soon
The Fossil Record: Galactic Archaeology
 Complementary approach to direct study of
galaxies at high redshift
 Snapshots of different galaxies vs evolution of same
galaxy
 Derive metallicity and elemental abundance
distributions, and age distributions….separately
 break degeneracies of integrated light
 Stellar IMF

Kinematics of stars to dynamics, dark matter
ΛCDM cosmology extremely successful on large scales.
Galaxies are the scales on which one must see the
nature of dark matter & astrophysics
Ostriker & Steinhardt 03
Galaxy mass function
depends on DM type
Inner DM mass density depends
on the type(s) of DM
‘MW’ Dark Halo in ΛCDM:
(Far Too) Much Substructure
GHALO (Stadel et al 09)
ΛCDM: Hierarchical clustering
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Merging very important in evolution of Milky
Way mass systems, to late times
Merging builds up bulges (both stars and gas)
and heats thin stellar disks, can destroy (reformed later), add gas and stars (all ages)
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Angular momentum transport causes compact
disks
Low-density/low mass systems disrupted early
can form stellar halo
Thick stellar disks and massive bulges:
Edge-on projected present-day stellar luminosity distributions
from a suite of SPH simulations of Milky Way-mass galaxies in
ΛCDM (Scannipieco et al. 2011; see also House et al 2011)
 Mergers also re-arrange thin disk radially
 Migration, maintaining circular orbits, plus heating
Radial Migration
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Transient gravitational perturbations can cause
stars and gas in circular orbits at corotation to
migrate radially in disk (Sellwood & Binney 2002)
Additional effects from bar/spiral interaction
(Minchev et al. 2011)
Proposed as mechanism to form thick disk without
any mergers (e.g. Schönrich & Binney 2009)
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Efficiency for stars on non-circular orbits?
How to produce/maintain very narrow iron abundance
distribution at given radius?
How to form a disk galaxy like the MW?
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Generic disk galaxy in ΛCDM has large bulge-disk ratio
and active merging history
Select atypical Galaxy-mass halo with no significant
(1:10) merging since redshift of three, re-simulate at
high-resolution using SPH (Guedes et al 2011; Eris)
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Suppress early star formation through high gas density
threshold
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But late-type (Sb/Sbc) disk galaxies are not rare!
Few old stars in disk at two scale-lengths now
Strong feedback in central regions to remove low angular
momentum gas
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Potential well assembled (no merging) so needs to be very strong
Stellar Halo
 In ΛCDM, stellar halo
forms from stars of
disrupted subhaloes
 Satellite galaxies from
surviving subhaloes
Johnston et al 08
Dark-matter halos in ΛCDM have
‘cusped’ density profiles
Diemand et al 08
Continually varying
power-law (Einasto profile)
ραr
-1
in inner regions
Main halo
Sub-halos
Lower limits
here
Test best in systems
with least contribution
to mass from baryons :
dwarf spheroidal
galaxies
The Milky Way
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as templates
Stellar halo, bulge, thick disk and even some part
of (old?) thin disk predicted to be created through
mergers
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and M31
Should see signatures in stellar populations
Stars retain memory of conditions when formed
Coordinate space structure
 Kinematic (sub)structure
 Chemical signatures: self-enrichment and massive-star
IMF
 Age distributions date merger
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 Satellite galaxies and streams from deep, uniform
imaging, followed by spectroscopy
Satellite stellar content:
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Orders of magnitude discrepancy in number
compared to dark subhaloes in ΛCDM
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Suppress star formation
Cannot simultaneously fit LMC and fainter
 Stellar populations, spatial distribution also
remain problems
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Semi-analytic models (left, Koposov et al 2008;
right, Rashkov et al 2011) cannot fit luminosity
function for all luminosities – problem with LMC
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Cannot appeal to variable stellar IMF
LMC also very blue—on first orbit?
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Why stream so long? (Nidever et al 2010)
Main sequence luminosity functions of UMi dSph
and of globular clusters are indistinguishable.
 normal low-mass IMF at 10-12Gyr lookback
HST star counts
Wyse et al 2002
M92 
M15 
0.3M
Massive-star
IMF constrained
by elemental
abundances –
also normal
dSphs vs. MWG abundances
(from A. Koch, 2009 + updates)
Gilmore et al; Norris et al 10 BooI
Simon et al 10 Leo IV
Frebel et al 10 Scl
Boo I
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Scl
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Leo IV
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Shetrone et al. (2001, 2003): 5 dSphs
Sadakane et al. (2004): Ursa Minor
Monaco et al. (2005): Sagittarius
Koch et al. (2006, 2007): Carina
Letarte (2006): Fornax
Koch et al. (2008): Hercules
Shetrone et al. (2008): Leo II
Frebel et al. (2009): Coma Ber, Ursa Major
Aoki et al. (2009): Sextans
Hill et al. (in prep): Sculptor
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Same ‘plateau’ in [α/Fe] in all systems at lowest
metallicities
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Type II enrichment only: massive-star IMF invariant, and
well-sampled – good mixing required
Stellar halo could form from any system(s) in which starformation is short-lived, and inefficient so that mean
metallicity kept low, ISM well-mixed
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Star clusters, galaxies, transient structures…
Complementary, independent age information that bulk
of halo stars are OLD further constrains progenitors (e.g.
Unavane, Wyse & Gilmore 1996)
Spectroscopy of luminous dSph  kinematics 
Velocity dispersion profile  mass
Gilmore et al 07; see also Walker et al 07; Walker et al 09; 11
1
ρ (M/pc3)
0.1
Isotropic Jeans analysis:
Very dark-matter dominated;
all dSph similar, favour cores,
not CDM cusp. Range of
Full DF modelling underway (low) star-formation histories,
hard to re-arrange all by
larger stellar samples
feedback. WDM instead?
Same from gas-rich dwarfs)
0.1
R (kpc)
1
Thick Disk
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Clear identification by vertical star-counts – two
exponentials fit and one does not
Stars predominantly old, 10-12Gyr
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Thick disk globular clusters are as old as the stellar
halo globulars
Turn-off age also same as stellar halo
 Old age of thick disk unusual in CDM; requires
only ancient mergers to heat thin stellar disk
 95% of 1012M have merger with 5x1010M (=Mdisk )
in last 10Gyr (Stewart et al 08)
How best to define?
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If early (minor) merger heated a pre-existing
thin disk to create thick disk, and that thin disk
had (expected) lower star formation rates in
outer parts, thick disk in outer parts could well
have lower [α/Fe] than in inner parts/solar
neighborhood – shortest delay time for Type Ia
less than 1Gyr, perhaps 100Myr
 could be dangerous to identify`thick disk’ by
invariant elemental abundances, far from Sun in
Galactocentric radius
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Early thick disks will be compressed and
heated by accretion/re-formation of thin disk
(Ostriker 1990; Elmegreen & Elmegreen 2006)
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Adiabatic growth would lead to
ΔH/H ~ - ΔMgas/Mdisk
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Δσ2/σ2 ~ -2 ΔH/H
Clumpy turbulent disks at redshift ~ 2 may
form bulges
Old stars everywhere…ages 10-12Gyr
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Bulge beyond 300pc is old, narrow age range
Thick disk is old (within several kpc of sun),
narrow age range
Local thin disk contains old stars
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See well-formed disks at redshift 2, look-back 10Gyr
Formed in situ?
Satellite galaxies all contain old stars, as old as
could be detected
 not natural in ΛCDM  WDM?
Concluding remarks
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Baryon physics critical to understanding
dark matter, particularly on galaxy scales
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Resolved stellar populations unique role
Many aspects of Local Group galaxies
pose challenges for current paradigm
`More high quality data for carefully
selected samples are needed, plus wellmotivated robust models’