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HUBBLE TYPES
and
STAR_FORMATION
HISTORIES
Roughly speaking, the Hubble
sequence is also a sequence in
star formation histories.
Sandage 1986
STAR FORMATION HISTORY
OF NON-STARFORMING GALAXIES
1. Star formation from fossil records:
why useful and what for
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 faint
Old populations are cool
SPECTROPHOTOMETRIC MODELS
Simply adding up the light of all stars:
a Single Stellar Population (SSP)
Monochromatic luminosity emitted by a star
with mass m, metallicity Z and age T
stellar IMF
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
Z = 1.24
(Kodama & Bower 2004, Toft et al. 2004, Strazzullo et al. 2006)
Blakeslee et al. 2003
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 Picture
Homogeneity 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)
Kelson et al. 2001
UV scatter
Fundamental plane
-0.4
-0.2
0.0
0.2 0.4
V-I
(Rest frame U-B)
Kuntschner & Davies (1998 MNRAS, 295, L33)
Spectral signatures
V-I
I814
Tight color-luminosity relations: stars are old
z  0.5
z0
U-V
(U-V)0
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
Unfortunately, degeneracy between age and
metallicity -
Young populations are bright
Old populations are faint
Young populations are hot
Old populations are cool
Metal poor populations are hot Metal rich populations are cool
Integrated Colors
Young populations are BLUE
Metal poor populations are BLUE
From Colors
AGE, Z
Old populations are RED
Metal rich populations are RED
but degenerate…
Spectral Indices
Broad Band Colors are affected by the AGE-METALLICITY DEGENERACY
Spectral indices have been introduced to overcome this problem
Kuntschner & Davies 1997
Lick Indices
Measurement:
EW, e.g.:
MAG, e.g.:
Fe52  (1 
Fl
) 
Fc
Mg 2  2.5 Log
Fl
FC
2

F 
EW    1  l d
Fc 
1 
 1
Mag  2.5 Log 
 
Fl 
 F d 
1 c

2
SSP Lick Indices:
an example
Metallic line strenghts increase with
both AGE and Metallicity
Hβ gets weaker as age increases
and as Metallicity increases
Use combination of metallic and
Balmer line strengths to solve the
AGE-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 index
is 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-starforming 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
NS0/NE
EVOLUTION OF S0s
0
0.6
Redshift
Dressler et al. 1997
Fasano et al. 2000
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)
The age of ellipticals
Ellipticals in clusters
terminated their SFH at
high redshift
In contrast, a significant
fraction of the S0
galaxies finished forming
stars more recently
Fornax cluster -- Kuntschner & Davies 1998
(also Coma cluster Poggianti et al. 2001b, Abell 2218 Smail et al. 2001)
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
Differences Es vs S0s **not** visible at the
brightest magnitudes
Poggianti et al. 2001
DOWNSIZING EFFECT
MB
>-15.6
-17.3
<-18.6
empty circles:
lum. wei. age > 9 Gyr
crosses:
3 < age < 9 Gyr
filled circles:
age < 3 Gyr
z=1-1.5
z=0.25
Poggianti et al. 2001a
THE BUILD-UP OF THE RED
COLOR-MAGNITUDE
SEQUENCE
ESO Distant Cluster Survey,
De Lucia et al. 2004
The build-up of the CM sequence
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
See Tran et al. 2003 & Poggianti et al. 2004 for
downsinzing of the post-starburst cluster
population
Results: larger galaxies older than smaller ones
VIRGO
Caldwell et al. (2003)
Yamada etal.
(2005a)
σ
Cowie et al. 1996
Kauffmann etal. (2003)
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
z > 1-1.5
Epoch of latest star formation
0.25 < z < 1
in galaxies in the Coma cluster
z < 0.25
3%
22%
18%
58%
20%
IN NUMBER OF GALAXIES
79%
IN TOTAL STELLAR MASS
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?
Color-photometric z’s in COMBO-17
Red luminosity density
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)
Gemini Deep Deep Survey
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?
R-K
(Can reconcile these
contradictory observations
if mass assembly is itself
mass-dependent)
Redshift
Glazebrook et al Nature 430, 181 (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?)
McCarthy et al Ap J 614, L9 (2004)
“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
“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?
“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”
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)
ONE STAR
Martins et al. 2005
Martins et al. 2005
ONE SSP
Gonzalez-Delgado et al. 2005
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 poststarburst 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 --
Downsizing-effect: evolution of the k+a population in clusters
The maximum velocitydispersion (=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
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 environmentallyrelated 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