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STScI, March 30, 2005. The Stellar IMF at High Redshift. Long Ago and far Away: The Fossil Record in an External Galaxy. Rosemary Wyse. The Fossil Record. Stars of mass like the Sun live for the age of the Universe – studying low-mass old stars allows us to do Cosmology locally. - PowerPoint PPT Presentation
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The Stellar IMF at High Redshift
Long Ago and far Away:
The Fossil Record in an External Galaxy
Rosemary Wyse
STScI, March 30, 2005
The Fossil Record
Stars of mass like the Sun live for the age of the Universe – studying low-mass old stars allows us to do Cosmology locally.
Complementary approach to direct study at high redshift.
Stars retain some memory of initial conditions – age, chemical abundances (modulo mass transfer), orbital angular momentum (modulo resonances, torques)
Clues from the Fossil Record
Resolved stars – Local
Group galaxies Star formation history Chemical evolution Merging history: for
which system have we derived SFH? Match CDM?
Stellar Initial mass function Today’s talk
Left : z=10, small haloes dominate. Red indicates possible site of star formation at this time (very dense regions). Right: Present time, many of the small haloes have merged into the model Milky Way halo; oldest stars found throughout the Milky Way and in satellites galaxies.
CDM simulation of the Local group Moore et al. 2001
6Mpc box 300kpc box
The IMF Long Ago & Far Away: Faint Stars in the UMi dSph Galaxy
Dwarf spheroidal in Ursa Minor is apparently dark-matter dominated, of very low surface brightness, and total luminosity equal to a globular cluster
Stars are all old, and metal-poor Fossil record of long-lived, low-mass stars
yields luminosity function via star counts High-mass IMF leaves signature in elemental
abundances of stars they enriched
A Typical Satellite Galaxy:
Leo I Dwarf spheroidal
Umi dSph is ~3 mag lower central surface brightnessand a factor of ten lower luminosity
The Ursa Minor Dwarf Spheroidal Distance ~ 70kpc Total luminosity ~ 3 x 105 LV, ~ that of a
globular cluster Central surface brightness ~ 25.5 V-mag/sq Stellar velocity dispersion ~ 10km/s Mass-to-V-band light ratio 10–70M /L,
significantly above a globular cluster (~ 2) Most stars are very old, as old as halo globulars Stars are metal-poor, [Fe/H] ~ –2 dex
Mateo 98; Kleyna et al 98, 01, 04; Bellazzini et al 02; Carrera et al 02; Palma et al 03; Gomez-Flechoso et al 03; Winnick 03
Stars in UMi dSph are OLD
Hernandez et al 00
Most stars formed at early times , 12Gyr ago, or redshifts > 2
Most stars in Umi dSph formed at redshift z ~ 2
= 0.7, M =0.3 = 0, M = 0.3
Faint Stellar Luminosity Function The dominant stellar population in the UMi dSph is
old and metal-poor, similar to a halo globular cluster, a system with no dark matter
Most robust result is direct comparison between luminosity function of stars in UMi dSph and in globular clusters of same age and metallicity, observed in same bandpasses, same telescope/
detector. Same stellar populations, equivalent to comparison of mass functions.
Deep Imaging with Hubble Space Telescope
Deep images of a field close to the centre of the Ursa Minor dwarf spheroidal (extant WFPC2 data); STIS as primary (optical LP filter), WFPC2 (V606 & I814) and NICMOS (NIC2/H) in parallel. ~30,000s total.
Off-field at 2—3 tidal radii, same exposure Globular clusters M15 ([Fe/H] ~ –2 dex) and
47 Tuc ([Fe/H] ~ –0.7 dex) with extant WFPC2 V & I, new data in STIS/LP and NIC2/H.
Wyse, Gilmore, Houdashelt, Feltzing, Hebb, Gallagher & Smecker-Hane 2002
V-band WFPC2 Umi dSph centerCrowding not an issue even at faint magnitudes (hindsight..)
WFPC2 V, V-I CMD plus selection for LFs
V606
All
Very few unresolved objects in off field meet and sharpness criteria. Broad colour range, little contamination of Umi dSph main sequence.
UMi dSph V-band LFM92
M15
50% completenessV=28.35 M606 =+9.1
V-band luminosity functions of UMi dSph and ofglobular clusters are indistinguishable.
Piotto et al 97;Shifted and renormalised
UMi dSph I-band LFM92
M15
50% completenessI=27.2 M814 =+8.1
I-band luminosity functions are indistinguishable. STIS/LP data provide independent check – agree.
Piotto et al 97;Shifted and renormalised
UMi dSph STIS LP LFSingle-band data, used offset field to correct (lower panel).
M15, shifted & scaled.
Again, indistinguishableluminosity functions
STIS LP transformed to I-band:
Histogram is derived from STIS LP data, using M15 data, open points are the directly observed I-band data.
…….and Statistics Various statistical tests were employed to quantify
the agreement of the various datasets: Linear, least-square fits to Log N vs Mag over
ranges of magnitude and various bin choices gave agreement to better than 2
K-S tests on unbinned data for a variety of magnitude ranges; rather sensitive to systematics such as relative distance moduli, but again general agreement at better than 5% significance level
-square tests on binned data, again range of bin centers and magnitude ranges; agree better than 5% significance
NICMOS DataThe NICMOS images are extremely sparse and not very deep (NB this program took several years to complete due to successive STIS failures and NIC2 was the best camera at the time initiated).
No new information on stars with normal main sequence colours
Excludes a hypothetical population of extremely red stars just below WFPC2 and STIS limits
Possible Complications Has mass segregation affected the LF in the
comparison globular clusters? – not significantly at these magnitudes at these radii, as estimated through modeling data in annuli
Is the binary population the same in UMi and in the comparison globular clusters? – see evidence for normal binary sequence in UMi, plus blue stragglers, so probably OK.
Reddening, relative distance moduli? – large bins, as adopted, lessen sensitivity to errors in these
Is Umi dSph relaxed? – several fields, same results
Invariant mass function Indistinguishable luminosity functions in two
systems of same (narrow) age and metallicity distributions
Same underlying stellar mass functions, despite very different in most other ways e.g. different galaxy, different dark matter content, different mean stellar densities…. Stellar density in UMi probably significantly
lower now than when stars formed, but likely still much less than a globular cluster
Same low-mass IMF in Galactic Bulge
Zoccali et al 2002
High metallicity, -0.3 dex; old, ~ 12Gyr
M15
Mass Functions If adopt Baraffe et al models, our 50%
completeness limits all correspond to ~0.3M
and a power-law slope somewhat flatter than Salpeter over the range 0.3M–0.8M:
consistent with local disk (Kroupa 03) But M-L transformation not well-defined for K/M dwarfs, especially as function of age and metallicity
Find eclipsing low-mass binary systems in open clusters of known age and metallicity (Hebb, Wyse & Gilmore 2004; 2005..)
Photometric Monitoring of Open Clusters
Selected six nearby open clusters, old enough to have low-mass stars on the main sequence
Age and metallicity from brighter member stars Age range 0.2–4Gyr, metallicity –0.2 dex to solar Monitored 1 degree FOV, well beyond nominal
cluster radii Survey designed to detect eclipse events in low-
mass systems, primary 0.3M – 0.5M , with periods of hours to days
Expected detection efficiency (percentage) for low-mass systems with adopted survey parameters,from Monte Carlo simulations (two different mass ratio distributions assumed, solid and dotted lines)
Six observing runs 2002-03, KPNO 4m + INT
For example, DSS image of NGC 6633 plus INT pointings
Montage of differential light curves
Useful to check forsystematics
Standard deviation in lightcurves vs I-band mag for all non-blended objects in M67 field (KPNO)
Detection of Eclipse Signal
Simple rms of light curve fails to distinguish Observing strategy used pairs of observations;
Stetson J-index designed for such programs. Measures the residuals of pairs, but also in fact no great improvement in finding (simulated) eclipses
Box-fitting algorithm (Tingley 03; Kovacs et al 02) designed to detect periodic signals which alternate between two discrete levels best
Output by box-fitting algorithm for a set of simulated lightcurves with sampling rate and rms values chosen to match real data: open circles no eclipse, stars with eclipse added. Algorithm recovers correct eclipse period for I < 20, where rms = signal
Signal Detection Efficiency vs I-mag
M35
Phase
Eclipse candidates found!
Spectroscopic & high-frequency photometric follow-up planned.
V
JSolid: single M3 ~0.4M
Dashed: binaries Empirical SEDs from Leggett (92)
NGC1647
Solid: M2V 0.5M
Unequal mass binary
Empirical SEDs from Leggett (92)
K ITiO
TiO
TiO
Fe I
Na I
TiO
CaH
CaH
H
~~
~~
Julian Date - 2450000
~~
Hel
ioce
n tri
cR
a dia
l Vel
o cit
y(k
m/s
)
TiO
TiOTiO
M2V from TiO
Radial velocity
Period of model radial velocity curves taken from light curves.
NGC 1647 candidate
km/s
~0.2M ~0.5M
Data also ideal for :
Studies of mass segregation and dynamical evolution in clusters
Calibrate metallicity scale of K/M dwarfs ‘G-dwarf problem’ with K/M dwarfs, true
unevolved stars… Field Galactic structure….lines of sight
include the outer disk…
Massive Star IMF at High Redshift Type II supernovae have progenitors > 8 M
and explode on timescales ~ 107 yr, less than dynamical timescale of typical dwarf galaxy, and less than duration of star formation
Low mass stars enriched by only Type II SNe show enhanced ratio of -elements to iron, with value dependent on mass distribution of SNe progenitors – if well-mixed system, see IMF-average
Type Ia SNe produce very significant iron, on longer timescales, few x 108 – 109 yr
Gibson 1998
Progenitor mass
Eje
cta
Type II Supernova yields
Schematic [O/Fe] vs [Fe/H]Wyse & Gilmore 1993
Slow enrichmentFast
IMF biased to most massive stars
Self-enriched star forming region.Assume good mixing so IMF-average yields
Type II only
Plus Type Ia
Schematic of expected pattern of [O/Fe] in stars for system with continuous star formation. The plateau reflects enrichment by a massive-star-IMF-average and value will vary with IMF; slope =1.2 [O/Fe]=0.3The turn-down is due to input of iron from Type Ia SNe which starts at some delay, 1—2 Gyr (?), after birth of progenitor binary system. The [Fe/H] reached by this time depends on SFH.
Gilmore & Wyse 1991
Cayrel et al. 2004
Cosmic scatter in elemental abundances of metal poor halo stars is extremely low, 0.05 dex – fully sampled IMF of massive stars?
Invariant IMF!
Nucleosynthesis implies slope similar to Salpeter, same as localdisk, gives [/Fe]
Edvardsson et al 1993; Nissen 2004
Different symbols for different stellar populations: Filled circles are thick disk (kinematically), open circles are thin disk: both consistent same IMF
Local F/G dwarfs
LMC stars show sub-solar ratios of [/Fe], consistent with expectations from extended star formation.
Smith et al 2003 Gilmore & Wyse 1991
Hiatus then burst
Continuous star formation
gas
Tolstoy et al 2003
Large open colored symbols are stars in dSph galaxies, black symbols are Galactic stars: the stars in typical dSph tend to have lower values of [/Fe] at a given [Fe/H], consistent with fixed IMF and extended SFH, plus perhaps α-enhanced winds
Massive Star IMF: Elemental Abundances
Shetrone et al 01
Open symbols, UMi dSphFilled symbols, M92, M3
UMi and globular stars show Type II plateau, with value as expected for approx Salpeter IMF.
UMi perhaps downturn to higher [Fe/H]; some iron from Type I supernovae, expected if star formation duration 1–2 Gyr?
UMi distribution
~
Same Massive IMF in Bulge
McWilliam & Rich 04 Oxygen???
Conclusions and Future Work The low-mass stellar IMF is remarkably
invariant, over a range of metallicities, age, star-formation rates, surface densities, dark matter content, formation epoch…..most of the parameters that might have thought important – not Jeans mass fragmentation?
High mass IMF also apparently invariant, with close to Salpeter slope
Calibrate M/L for low-mass stars Understand how dSph evolve (ongoing
VLT/Flames/UVES project)
Evolution of Dwarf Spheroidals Expectation of simplest theory is that initial burst
of star formation drives a powerful wind that quenches subsequent star formation (e.g. Larson 1974; Dekel & Silk 1987; Wyse & Silk 1986)
Does not agree with spread in ages of dSph stars, or with modest derived rates of star formation
CDM models with star formation suppressed by reionization have similar problem with age range
Re-accretion of gas?? (Silk, Wyse & Shields 1987) Trend with distance, but why each dSph so different? Dark haloes accrete too and would light up..
Carina dSph metallicity distribution
Data Koch et al 2004 (inc. Wyse)Left, stochastic modelRight, model from Lafranchi & Matteucci 2003
Stochastic Enrichment model
Independent star-forming regions, each with identical ‘enrichment events’ occuring at a fixed mean rate
Enrichment is then a Poisson process For constant overall star formation rate,
metallicity distribution of long-lived stars given by integral over events
Model parameter is the mean number of enrichment events per region – changes shape. Adopted value of 2 here.
Searle 1977
Open Questions and Future Work What was the star formation history of the disk of the Milky Way? Was the merger history of the Milky Way mostly quiecsent since z ~ 2 (gas, fluffy old satellites..) Same questions for all galaxies! How important are flows of gas in evolution of different galaxies? Out and/or in? How did the dSph evolve? What are the parameters of their dark matter haloes? Need self-consistent model matching star- formation input to CMD, gas flows to dark halo etc.
More Open Questions and Future Work For MWG, ideally need elemental abundances, physical parameters (including age) distances and 3D kinematics for stars many kpc from the Sun – GAIA + GWFMOS (2010….) Large telescopes will extend such work beyond Local Group – need more than 30m! What is predicted in different models? – need to put star formation into models and detailed output such as elemental abundances and radial and temporal dependencies of merging, flows….
More Future Work
Need simulations to focus on accretion into the baryonic galaxy in more detail – radial dependences of mass ratios, densities, epochs
Star formation! Population III stars – where are they and
what was their IMF?
Concluding remarks:
There is a wealth of information in There is a wealth of information in resolved stellar populations, to constrain resolved stellar populations, to constrain how galaxies form and evolve.how galaxies form and evolve.
We are only beginning to understand this We are only beginning to understand this for the Milky Way –for the Milky Way –
is the Milky Way unusual????is the Milky Way unusual???? Lots of PhD theses…….Lots of PhD theses…….
Tolstoy et al 2003
Large open colored symbols are stars in dwarf Spheroidals, black symbols are Galactic stars: the stars in typical satellite galaxies tend to have lower values of [/Fe] at a given [Fe/H].
Unavane, Wyse & Gilmore 1996
Scatter plot of [Fe/H] vs B-V for local high-velocity halo stars (Carney): again few stars bluer (younger) than old turnoffs (5Gyr, 10Gyr, 15Gyr Yale)
Stellar halo is OLD
Carina dSph Carina dSph Leo I dSphLeo I dSph
Hernandez, Gilmore & Valls-Gabaud 2000Hernandez, Gilmore & Valls-Gabaud 2000
Intermediate-age population dominates in typicaldSph satellite galaxies – Ursa Minor atypical, has dominant old population (also normal IMF Wyse et al 2002)
LMC stars show sub-solar ratios of [/Fe], consistent with expectations from extended star formation.
Smith et al 2003
gas
Bensby et al thick disk
Very different pattern from LMC stars