Sandage 1986

Roughly speaking, the Hubble sequence is also a sequence in star formation histories.

HUBBLE TYPES

and

STAR_FORMATION HISTORIES

1. Star formation from fossil records:

why useful and what for

STAR FORMATION HISTORY

OF NON-STARFORMING GALAXIES

Indicators of ongoing star-formation activity - Timescales

Emission lines < 3 x 107 yrs

UV-continuum emission it depends…

FIR emission < a few 10^7 (but…)

Radio emission as FIR (?)

(Could be higher: relativistic electrons have lifetimes ≤ 10^8 yr)

THE IMPORTANCE OF FOSSIL RECORDS:

Goal: understanding galaxy formation and evolution

Going to higher-z is not always the solution….problem of connecting progenitors and descendants

The evolutionary history of a galaxy is written in its stars, therefore in the light they emit – the light of all stars together form the galaxy integrated spectrum

A method to reconstruct the past star-formation history

All these techniques are based on one single fact:

that STARS OF DIFFERENT AGES HAVE DIFFERENT SPECTRAL ENERGY DISTRIBUTIONS (in shape and lines)

Young populations are bright Young populations are hot

Old populations are faintOld populations are cool

SPECTROPHOTOMETRIC MODELS

Simply adding up the light of all stars:

a Single Stellar Population (SSP) stellar IMF

Monochromatic luminosity emitted by a star with mass m, metallicity Z and age T

SPECTROPHOTOMETRIC MODELS

Simply adding up the light of all stars:

a galaxy (composite spectrum)

The oldest galaxies at any redshift Color-Magnitude sequence: zero-point, slope and scatter

passive evolution of stellar populations formed at z>2-3.

Slope is primarily driven by mass-metallicity relation.

Morphologically (HST)-selected Es and S0s (Bower et al. 1992, Aragon-Salamanca et al. 1993, Rakos et al. 1995, Stanford et al. 1995, 1996, 1997, 1998, Schade et al. 1996, 1997, Ellis et al. 1997, Lopez-Cruz 1997, Kodama et al. 1998, Barger et al. 1998, van Dokkum et al.1998, 1999, 2000, 2001, Gladders et al. 1998, de Propris et al. 1999, Terlevich et al. 1999, 2001, Vazdekis et al. 2001, Andreon 2003, Merluzzi et al. 2003; Rosati et al. 1999, Lubin et al. 2000, Stanford et al. 1998, 2002, Kajisawa et al. 2000, van Dokkum et al. 2000, Blakeslee et al. 2003)

Fundamental Plane, Mass-to-Light

ratios and Mg-sigma relation (van Dokkum & Franx 1996, Kelson et al. 1997,

2000, 2001, van Dokkum et al. 1998, Bender et al. 1996,

1998, Ziegler & Bender 1997, Ziegler et al. 2001, Holden

et al. 2004)

Bright-end of K-band (mass)

luminosity function (Kodama & Bower 2004, Toft et al. 2004, Strazzullo et al. 2006)

Blakeslee et al. 2003

Z = 1.24

The big problem for hierarchical models like CDM: For the biggest galaxies, the halos continue to merge until late times, z~1 or even z~0.5. This is why a picture in which ellipticals were made by merging spirals at late times seemed the “perfect fit.”

However, the stars of elliptical galaxies (and all big spheroids = bulges) really are old, and they are enhanced in alpha-elements compared to spirals. The stars in spheroids seem to be uniformly old, very few, or none of them, are young.

Dressler

Passive Galaxies: The Classical PicturePassive Galaxies: The Classical PictureHomogeneity of Cluster E/S0 U-V Colors

z 0.0 z 0.5 (HST)

Virgo & Coma: (U-V)o< 0.05 (Bower, Lucey & Ellis 1990, Bower et al 1998)

Morphs: <z> = 0.5 sample: (U-V)o< 0.07 (Ellis et al 1997)

Ellis et al. 1997 (ApJ 483,582)

V-I

I814

V-I

0.0 0.2 0.4-0.2-0.4

(Rest frame U-B) Kuntschner & Davies (1998 MNRAS, 295, L33)

Kelson et al. 2001

UV scatter

Fundamental plane

Spectral signatures

Universal relation for Es and S0s (Sandage & Visvanathan 1978)

Scatter dominated by observational errors (Bower et al 1990, Bower et al 1998)

(U-V) is sensitive probe of decline rate of MS component (Buzzoni 1989)

uniform star formation history:

synchronisation of recent activity or old stellar population zF > 3

Tight color-luminosity relations: stars are old

z 0 z 0.5

U-V(U-V)0

Unfortunately, degeneracy between age and metallicity -

Young populations are bright Young populations are hotMetal poor populations are hot

Old populations are faintOld populations are coolMetal rich populations are cool

Integrated Colors

Young populations are BLUEMetal poor populations are BLUE

Old populations are REDMetal rich populations are RED

From Colors AGE, ZAGE, Z but degenerate…

Spectral IndicesBroad Band Colors are affected by the AGE-METALLICITY DEGENERACYSpectral indices have been introduced to overcome this problem

Kuntschner & Davies 1997

Lick Indices

C

l

F

FLogMg 5.22

Measurement:

)1(52c

l

F

FFeEW, e.g.:

MAG, e.g.:

dF

FEW

c

l

2

1

1

dF

FLogMag

c

l2

1

15.2

SSP Lick Indices:an example

Metallic line strenghts increase withboth AGE and Metallicity

Hβ gets weaker as age increases and as Metallicity increases

Use combination of metallic andBalmer line strengths to solve theAGE-METALLICITY degeneracy

A Balmer line versus a metallicity indicator….

In this way, obtaining luminosity-weighted ages and metallicity (~epoch of most recent star formation)

Alpha elements overabundance in Es

Worthey, Faber & Gonzales 1992:

At given Fe index, the data Mg indexis stronger than the model predictions

Interpreted as a supersolar Mg/Fe ratio

Among various possibilities: Short Formation timescales for Es

Solution:

Find a combination of indices that does not depend on overabundance (eg Thomas et al., Tantalo et al., et al.)

RELATED ISSUES AND PROBLEMS:

In practice, galaxies are not SSPs !!! (again, degeneracies…)

Dust normally considered negligible in non-star-forming galaxies

Emission can get in the way: filling of Balmer lines

Slit effects

Never trust absolute ages, only relative ones

You get what you’ve put in: model limits

There is not a “best method” in an absolute sense. It depends on resolution and S/N of the data

Fasano et al. 2000

NS

0/N

E

0 0.6

Redshift

Dressler et al. 1997

EVOLUTION OF S0s

Stellar populations as a function of galaxy morphology

Reality of E-S0 differences “confirmed” from spectroscopy and colors (Kuntschner & Davies 1998 in Fornax, Terlevich et al. 1999 in Coma, Smail et al. 2001 in A2218, Poggianti et al. 2001 in Coma, Thomas 2002 PhD Leiden ENACS) but not all studies find differences (Ellis et al.1997, Jorgensen 1999, Lewis et al. 2001, Ziegler et al. 2001)

Fornax cluster -- Kuntschner & Davies 1998

(also Coma cluster Poggianti et al. 2001b, Abell 2218 Smail et al. 2001)

Ellipticals in clusters terminated their SFH at high redshift

In contrast, a significant fraction of the S0 galaxies finished forming stars more recently

The age of ellipticals

Poggianti et al. 2001b

Jones, Smail & Couch 2001

Stellar populations as a function of galaxy morphology

Reality of E-S0 differences “confirmed” from spectroscopy

and colors (Kuntschner & Davies 1998 in Fornax, Terlevich et al. 1999 in Coma, Smail et al. 2001 in A2218, Poggianti et al. 2001 in Coma, Thomas 2002 PhD Leiden ENACS) but not all studies find differences (Ellis et al.1997, Jorgensen 1999, Lewis et al. 2001, Ziegler et al. 2001) :

due to delay between evolution of SF and morphology? (Poggianti et al. 1999) – or to the different luminosity distribution of samples? (P. et al. 2001)

Trends with galaxy mass/luminosity

Poggianti et al. 2001a

Poggianti et al. 2001

Differences Es vs S0s **not** visible at the brightest magnitudes

Poggianti et al. 2001a

empty circles: lum. wei. age > 9 Gyr z=1-1.5

crosses: 3 < age < 9 Gyr

filled circles: age < 3 Gyr z=0.25

MB >-15.6 -17.3 <-18.6DOWNSIZING EFFECT

THE BUILD-UP OF THE RED COLOR-MAGNITUDE SEQUENCE

ESO Distant Cluster Survey,

De Lucia et al. 2004

De Lucia et al. 2004 ApJL

A deficiency of red galaxies at faint magnitudes compared to Coma

-- A synchronous formation of stars in all red sequence galaxies is ruled out

-- Most luminous galaxies are the first ones to conclude their SF activity - The more luminous, the older their stellar populations,and the higher the redshift of their last SF

Downsizing effect: the star formation histories of galaxies are anti-hierarchical

The build-up of the CM sequence

See Tran et al. 2003 & Poggianti et al. 2004 for downsinzing of the post-starburst cluster population

Results: larger galaxies older than smaller ones

Kauffmann etal. (2003)

Caldwell et al. (2003)

Yamada etal. (2005a)

VIRGO

σσ

Cowie et al. 1996

Downsizing-effect

Going to lower redshifts, the maximum luminosity/mass of galaxies with significant SF activity progressively decreases.

Active star formation in low mass galaxies seems to be (on average) more protracted than in massive galaxies.

IN ALL ENVIRONMENTS.

The more luminous/massive, the older their stellar populations, the higher the redshift of their last SF activity

More massive galaxies on average older, more metal-rich, higher alpha/iron

58%20%

22%

IN NUMBER OF GALAXIES

79%

18%

3%

IN TOTAL STELLAR MASS

z > 1-1.5

0.25 < z < 1

z < 0.25

Epoch of latest star formation

in galaxies in the Coma cluster

Stars in cluster ellipticals are old, there seems little doubt, and they also appear to be assembled into mature galaxies very early. But, cluster ellipticals are rarer than those in lower-density environments -- are these “field” ellipticals really all that different (stellar age, assembly age) from cluster E’s?

Declining Red Sequence to z=1: Agreement with CDM?Declining Red Sequence to z=1: Agreement with CDM?

COMBO-17 data suggests 3 decline in `red sequence’ luminosity density to z=1: consistent with hierarchical predictions (Bell et al ApJ 608, 752 2004)

Color-photometric z’s in COMBO-17 Red luminosity density

Gemini Deep Deep SurveyGemini Deep Deep Survey

Glazebrook et al Nature 430, 181 (2004)

By contrast, the Gemini DD Survey find an abundance of high mass old objects at redshifts z>1 - in seeming contradiction with the COMBO-17 results to z~1?

(Can reconcile these contradictory observations if mass assembly is itself mass-dependent)

R-K

Redshift

McCarthy et al Ap J 614, L9 (2004)

Gemini Deep Deep Survey: Spectroscopic Age-dating

• 20 red galaxies z~1.5, age 1.2 - 2.3 Gyr, zF=2.4 - 3.3

• Progenitors have SFRs ~ 300-500 M yr-1 (sub-mm gals?)

“Evolved Galaxies at z>1.5 from the Gemini Deep Deep Survey: The Formation Epoch of Massive Stellar Systems”, McCarthy et al. (GDDS), 2004 ApJ, 614, L9

“Spectra of evolved GDDS galaxies with z > 1.3. The SDSS LRG composite has been overlaid on each spectrum, and an offset has been applied to each, in steps of 10^-18 ergs cm-2 Å-1”

“Both composite spectra show strong Mg II 2800, Mg I 2852 absorption and broad spectral features due primarily to Fe II absorption. Overlaid in red is a single-burst Bruzual & Charlot spectral synthesis model with an age of 2 Gyr, solar abundances, and a Salpeter IMF cutoff at 120 Msun”

“Conservative age estimates for 20 galaxies with z > 1.3…give a median age of 1.2 Gyr and zf = 2.4. One-quarter of the galaxies have inferred zf > 4. Models restricted to [Fe/H] ~0 give median ages and zf of 2.3 Gyr and 3.3, respectively. These galaxies are among the most massive and contribute 50% of the stellar mass density at 1 < z < 2. …Our results point toward early and rapid formation for a significant fraction of present-day massive galaxies.”

Can feedback with hierarchical CDM explain?

How much has the red sequence grown since z = 1?

Faber et al. (Deep2) -- a factor of 4 growth since z=1, and this includes the brightest (most massive) red galaxies -- need a lot of merging and quenching to accomplish that!

Borch et al. (Combo-17) 2006 A&A, 453, 869. “We find that the total stellar mass density of the universe has roughly doubled since z ~ 1…Intriguingly, the integrated stellar mass of blue galaxies with young stars has not significantly changed since z ~ 1…instead, the growth of the total stellar mass density is dominated by the growth of the total mass in the largely passive galaxies on the red sequence.”

Cimatti et al. 2006 A&A 453, L29 -- same conclusion, a factor of two growth in the red sequence, and no growth for the massive systems.

Brown et al. 2006 astro-ph 0609584 -- NOAO and Spitzer IRAC survey: “…the stellar mass contained within the red galaxy population has roughly doubled over the past 8Gyr. This is consistent with starforming galaxies being transformed into <L* red galaxies by a decline in their star formation rates.” Only passive evolution for >4L* galaxies since z=1. “ While red galaxy mergers have been observed, such mergers do not produce rapid growth of 4L* red galaxy stellar masses between z=1 and the present day.”

Brown et al. and Cimatti et al. emphasize that, if only a factor of two in mass is added to the red sequence since z~1, and it is mainly in lower luminosity (< 1011Msun) galaxies, then simple “running down” of star formation in disk galaxies, turning them red, can account for the growth.

A key point to be resolved, and one that may be telling as to how much the hierarchical picture is in trouble.

Post-starburst galaxies:

a class apart

EW(Hdelta) (4102 A) > 3 A and no line detected in emission

Balmer lines in absorption are best indicators of “recent” SF

(some more than others)

Martins et al. 2005

ONE STAR

Martins et al. 2005

Gonzalez-Delgado et al. 2005

ONE SSP

EW(Hdelta) of an SSP

Gonzalez-Delgado et al. 2005

Jacopo Fritz 2005

EW(Hdelta) (4102 A) > 3 A and no line detected in emission

SOME OBSERVED SPECTRA

“...these spectra are consistent with an old population mixed with an equal blue luminosity of A stars, which indicates a large burst of star formation 10^9 years before the light left the galaxy.”

Dressler & Gunn 1983

An abrupt interruption of the SF activity gives rise to a post-starburst spectrum for ~1-1.5 Gyr after the SF is terminated

EW(Hdelta) of an SSP

NOMENCLATURE: A ZOO!!

E+A galaxies

k+a galaxies

HDS(Hdelta strong) galaxies

Post-starburst/Post-starforming galaxies (PSB/PSF)

Balmer-strong galaxies

ADVANTAGES:

It is a signature that DOES NOT depend on metallicity and abundance ratios

Easy to identify them….

DISADVANTAGES:

…but difficult to understand the details - Strenght, age and duration of burst highly unconstrained – only lower limits can be placed in most cases

ADVANTAGES:

It is a signature that DOES NOT depend on metallicity and abundance ratios

Easy to identify them….

DISADVANTAGES:

…but difficult to understand the details - Strenght, age and duration of burst highly unconstrained – only lower limits can be placed in most cases

Emission lines < 5 x 107 yrs

FIR emission < a few 10^7 (but..)

Radio emission as FIR (but…) etc.

Spectra w/out emission lines

k+a spectra 5 x 107 to 1.5 Gyr

abs line indices > 1.5 Gyr

k+a galaxies Dressler & Gunn 1983, Couch & Sharples 1987, Henry & Lavery 1987, Newberry et al. 1990,

Fabricant et al. 1991,1994, Dressler & Gunn 1992, Charlot & Silk 1994, Jablonka & Alloin 1994, Belloni et al. 1995, 1996, Abraham et al. 1996, Barger et al. 1996, Leonardi & Rose 1996, Poggianti & Barbaro 1996, Fisher et al. 1998, Morris et al. 1998, Couch et al. 1998, Balogh et al. 1999, Dressler et al. 1999, Poggianti et al. 1999, Bartholomew et al. 2001, Ellingson et al. 2001, Bekki et al. 2001, Shioya et al. 2001,2002, Tran et al. 2003, Goto 2003, Poggianti et al. 2004, Balogh et al. 2005, Yamauchi & Goto 2005

• ~10 Myr to 1.5 Gyr after SF stopped

• strongest cases need a starburst

• weak cases also simply truncated SF

• combination EW-color helps for evol.stage

• a slow declining SF does not work

Recently, Hdelta versus D4 study of SLOAN galaxies (Kauffmann et al. 2003)

ABSORPTION-LINE SPECTRA: the smoking guns

When first spectra of galaxies in distant clusters, surprise surprise...

Spectra with strong Balmer lines in absorption and no emission (E+A/k+a galaxies) – post-starburst/post-starforming galaxies (Dressler & Gunn 1982,1983, Couch & Sharples 1987, Henry & Lavery 1987, Fabricant et al. 1991,1994, Dressler & Gunn 1992, Barger et al. 1996, Belloni et al. 1995, 1996, Abraham et al. 1996, Fisher et al. 1998, Morris et al. 1998, Couch et al. 1998)

Larger % in clusters (10-20%) than in field at similar z’s (Dressler et al. 1999, Poggianti et al. 1999, Tran et al. 2003,2004 – as opposed to Balogh et al. 1999)

-- SF truncation in clusters --

The maximum velocity-dispersion (=mass) of k+a galaxies in clusters decreases towards lower redshifts (Tran et al. 2003)

At z=0, in the Coma cluster, observing late star-forming faint galaxies becoming “dwarf ellipticals”

About 10% of the dwarf cluster population in the Coma cluster

(see also Caldwell et al.’s works, De Propris et al.)

Poggianti et al. 2004

Downsizing-effect: evolution of the k+a population in clusters

POST-STARBURST GALAXIES

Any process that halts the SF on a short timescale will produce a k+a spectrum (running out of fuel after a SB? Having the ISM removed due to the dense environment?)

Probably different origin in clusters of galaxies and in the field

Complete picture is still lacking, but current scenario:

post-SB galaxies in clusters: an important fraction of the luminous galaxy populations at intermediate redshift (15-20%) – low mass galaxies in the nearby universe – an environmentally-related phenomenon, occurring in dense regions

post-SB galaxies in the field: locally, a very small fraction of the luminous galaxies (0.1%), probably related to mergers – at high-z, recent studies start to find luminous k+a galaxies