Lectures on Early-type galaxies PART II (M. Bernardi)

Lectures on Early-type galaxiesLectures on Early-type galaxiesPART IIPART II (M. Bernardi)(M. Bernardi)

Plan for today: Galaxy formation models Stellar Populations

Age/Metallicity/-enhancement Lick Indices and Colors

Correlations with L, and environmentComparison between Models and Observations

Environment and Evolution in the SDSS Constraints on galaxy formation models

Initial fluctuations are seeds of structure

Growth is hierarchical;smaller dark matter ‘halos’ merge to form larger ones

Gas cools within ‘halos’ Galaxies

Gastrophysics of galaxy formation

Hierarchical models predict the spatial distribution of galaxies (successfully)

Also describe galaxy formation and evolution

CDM: hierarchical gravitational clustering: The most massive galaxies are the last to be assembled, though their stars may be oldest

Age of stellar population may be different from that of host dark matter halo

Measure ages of stellar populations to constrain galaxy formation models

The optical portion of the galaxy spectrum is due to the light of stellar photospheres

K giant star

Typical elliptical galaxy

Linear combination of models galaxy properties (fluxes, colors, and spectra of galaxies)

1) Stellar library (observables)2) Stellar evolution codes (age/metal) + 1) Star Formation Rate 2) Metal enrichment law 3) Initial Mass Function

INGREDIENTS FOR STELLAR POPULATION MODELS

MODEL

1) Star Formation Rate (t) Instantaneous burst: (t) ~ (t) (usually called “single stellar population” model SSP) Exponential declining: (t) ~ -1 exp(-t/) Single burst of length : (t) ~ -1 for t ≤ tfort Constant: (t) = const

where is the e-folding timescale

INGREDIENTS FOR STELLAR POPULATION MODELS (Isochrone Synthesis)

Spectral energy distribution at time t:

1) Star Formation Rate (t)2) Metal enrichment law t S[t’,(t-t’)] is the power radiated per unit wavelength per unit initial mass

by a “single stellar population” (SSP) of age t’ and metallicity (t-t’) S[t’,(t-t’)] is the sum of the spectra of stars defining the isochrone of a

SSP of age t’ and metallicity (t-t’) It is computed by interpolating the isochrone at age t’ from the tracks in

the HR diagram

INGREDIENTS FOR STELLAR POPULATION MODELS (Isochrone Synthesis)

Spectral energy distribution at time t:

1) Star Formation Rate (t)2) Metal enrichment law t 3) Initial Mass Function (m) defined such that (m)dm is the number of

stars born with masses between m and m+dm

INGREDIENTS FOR STELLAR POPULATION MODELS (Isochrone Synthesis)

Spectral energy distribution at time t:

mc = 0.08 M

= 0.69

Age (Gyrs)

Evolution of the spectrum of a “single stellar population” (SSP) model

Colors and M/L vs Age

for a solar metallicity model

Comparison model/data

--- model spectrum--- observed spectrum

metallicity changes increase of heavy elements due to SN explosions

Problem: Age-Metallicity degeneracy

Stars weak in heavy elements are bluer than metal-rich stars (line blanketing effects and higher opacities)

Galaxy models must account for

Different Age – Same Metallicity

Easy to separate young and old populations of the same metallicity

Same Age – Different Metallicity

Easy to separate coeval populations of different metallicity

Age – Metallicity degeneracyHard to separate populations which have a combination of age and metallicity

Degeneracy: (∂ lnt/∂ lnZ) ~ -3/2

BUT…

Although the continuum spectrum is similar, the absorption lines are stronger for higher metallicity

SO…

How to disentangle age from metallicity? Absorption lines (e.g. Lick indices)

H Mgb FeAverage pseudo-continuum flux level:

Fp = F d/(1 –2)

EW = 1FIFCd

where FC represents the straight line

connecting the midpoints of the blue and red pseudo-continuum levels

1

1

Lick Indices

The central velocity dispersion appears to play a stronger role in determining the stellar population

Correlation Mg- tight over large range in galaxy size and all types of hot stellar systems

■ Giant ellipticals (GE) (M < -20.5 mag)▲Ellipticals of intermediate L (IE) (-20.5 < M < -18.5 mag)● Compact galaxies (CE)♦ Bright dwarf galaxies (BDW) (M > -18.5 mag)▪ Faint dwarf galaxies (FDW)x Bulges of S0/Sa (B)

■▲♦●▪ galaxies with anisotropic kinematics □∆◊○ galaxies rotationally flattened

Bender et al. 1996

SDSS

Galaxies with larger are older and/or more metal rich Stellar population evolves

--- 0.05 < z < 0.07 --- 0.07 < z < 0.09 --- 0.09 < z < 0.12 --- 0.12 < z < 0.15 --- 0.15 < z < 0.20

Vice-versa galaxies with larger have weaker Balmer absorption lines

Strong evolution

hi –z(younger population)

low –z(older population)

No correlation between Fe and L --- only with Differential evolution? more massive galaxies evolve differently (slower?) than less massive ones?

How to disentangle age from metallicity? Absorption lines (e.g. Lick indices) Stellar population modelsLick Indices vs Age

metallicity

age

Stellar population models

How to disentangle age from metallicity? Absorption lines (e.g. Lick indices)

Additional complication [/Fe] enhancement

The [/Fe] enhancement problemSN, which produce most of the metals, are of two types:

Large are-enhanced

--- z < 0.07 --- 0.07 < z < 0.09 --- 0.09 < z < 0.12 --- 0.12 < z < 0.15

Additional complication [/Fe] enhancement

-elements: Ne, Mg, Si, S, A, Ca(so-named because formed by adding 2,3,…-particles, i.e. 4He nuclei, to 16O)

Formation time and timescale

SNae Type II from massive stars/short lives

Top-heavy IMF or short formation timescales at high redshift

Stellar Population Synthesis Models

Some recent models

Corrected for -enhancement ☺[/Fe] > [/Fe]

Age

Metallicity

do not match well all the observed parameters !! !!

But ……

Problems??

H ~ 1.5Ǻ

Big correction in D4000!

D4000 ~ 0.3!

Problems with models

Can we learn something just from the observed absorption lines?

Testing predictions of galaxy formation models …

Early-type galaxies in the field should be younger than those in clusters

Metallicity should not depend on environment The stars in more massive galaxies are coeval or

younger than those in less massive galaxies

Environment ….

SDSS C4 Cluster Catalog (Miller et al. 2005)

L > 3L*

Lcl > 1.75 x 1011 h-2 L ~ 10L*

From ~ 25,000 early-types at z < 0.14

4500 in low density regions3500 in high density regions

Cluster galaxies 0.1 mag fainterthan field galaxies

Cluster galaxies older than field by ~ 1Gyr?

BCGs more homogeneous

--- Cluster--- Field --- BCG

The Fundamental PlaneThe virial theorem:

Three observables + M/L M/L ~ L0.14

FP is combination with minimum scatter

oldyoung

Bernardi et al. 1998

No differences in the Mg2- relation

If Mg2 is a indicator of the age of the stellar population

Stars in field andcluster early-typegalaxies formed mostly at high redshift

Mg2- shows no differences because:

Galaxies in the field are younger but have higher

metallicity

Kuntschner et al. 2002

….. Evolution Z ~ 0.05

Z~ 0.17 t ~ 1.3Gyr

D4000 increases with time; H, H decreases

Evolution as a clock

Over small lookback times, metallicity cannot have changed significantly; hence observed evolution is due entirely to age differences, not metallicity!

Comparison of environmental differences with evolution measurement allows one to quantify effect of age difference between environments; so calibrate mean metallicity difference too!

Some implications:

early-type galaxies in the field should be younger

than those in clusters

Observed differences cluster-field small (~ 1 Gyr)

Color-Magnitude- relation

Age – Metallicity from Color-Magnitude

Models from Bruzual & Charlot (2003)

12

4

Age

[Z/H]=0.6

[Z/H]=0

9

1

[Z/H]=0.6

[Z/H]=0

12

2

Age

[Z/H]=0

[Z/H]=0.6

1

9 Age

Age

Bernardi et al. (2004b)

L ↑ Age↑ [Z/H] ↑

L ↑ Age↑ [Z/H] ↓

Kodama et al. (1998)

Slope of C-Mindependent of redshift out to z~1

C-M due toMass-[Z/H] not Mass-Age

C-M due to Mass-[Z/H] residuals from C-M due to Age

In contrast to published semi-analytic galaxy formation models

Bernardi et al. (2004b)

Age

Age of stellar population increases with galaxy mass: Massive galaxies are older

At fixed L/Mass: 1) more massive galaxies are older 2) fainter galaxies are older 3) galaxies with smaller R are older 4) higher galaxies are older

Color-Magnitude

Color-Magnitude is a consequence of Color- & L-

The Most Massive Galaxies: Double Trouble? 105 objects with ( > 350 km/s) Single/Massive?

Galaxy formation models assume < 250 km/s BHs (2 x 109 M)

Superposition? interaction ratesdust contentbinary lenses

● Single/Massive Double ڤ◊ BCG

Sheth et al. 2003

Expect 1/300 objects to be a superposition

Bernardi et al. 2005c

‘Double’ from spectrum and image

‘Double’ from spectrum, not image

‘Single’ ?

● Single/Massive Double ڤ◊ BCG

Doubles are outliers

BCGs are bluer thanmain sample at fixed

Dry Mergers?

HST images: with ACS-HRC

SDSS

HST = 407 ± 27 km/s

SDSS J151741.7-004217.6

3”

1’

SDSS J204712.0-054336.7

= 404 ± 32 km/sHST

SDSS

1’

3’

HST: ACS-HRC

6 single 4 multiple

= 369 ± 22 = 383 ± 27 = 385 ± 34 = 385 ± 24

= 395 ± 27 = 402 ± 35 = 404 ± 32 = 407 ± 27

= 408 ± 39 = 413 ± 35

Single galaxies with ~ 400 km/s

Some semi-analytic modelsuse a cut at Vc = 350 km/s

(i.e. = 350/√2 ~ 250 km/s)

Cut should be at higher Vc??