Star Formation in High Redshift
Galaxies
Mauro Giavalisco
Space Telescope Science Institute
and the GOODS Team
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Finding high-redshift galaxies:
color selection
1.
Color selection is very efficient in
finding galaxies with specific spectral
types in a pre-assigned redshift range
2.
Wide variety of methods
available, targeting a range of
redshifts, galaxies’ SEDs:
•
Lyman and Balmer break
(Steidel, Adelberger, MG)
•
DRG (Franx, Labbe et al.)
•
BzK (Daddi et al.)
•
Photo-z (Mobasher et al)
B435
V606
i775
z850
Unattenuated Spectrum
Spectrum Attenuated
by IGM
z~4
Here, the case of
“Lyman-break galaxies”
GOODS yielded the deepest and
largest quality samples of LBGs
at z~4 to ~6 (7?)
B435 V606
z850
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Vanzella et al. 2006
Color selection at z>3
B-band dropouts: 3.5<z<4.5
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Color selection at z>3
V-band dropouts: 4.5<z<5.5
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Color selection at z>3
i-band dropouts: 5.5<z<6.5
Color selection at z>3
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z-band dropouts: 6.5<z<7.5
The Redshift Distribution
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LBGs at z>3 are targets of the
ongoing GOODS spectroscopic
time with the ESO VLT and
Keck
#183
#27
Vanzella et al. 2006, 2005, 2006 in prep.
Stern et al. 2006 in prep.
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Variety of spectral “types”
Very similar to the z~3 galaxies
Emission of Lya observed together
with weak interstellar absorption
lines
Stronger absorption lines are present
when Lya is obsered in absorption
Effect of geometry of ISM?
Vanzella et al., in prep.
z~4 spectroscopy
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z~3 spectroscopy
Popesso et al., Vanzella et al. in prep.
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Popesso et al, in prep.
z~4 spectroscopy
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Exploring the geometry
of the ISM
No obvious correlation of spectral
“types” with UV color or ellipticity of
the galaxies
Whatever causes the absorption does
not know about the geometry of the
UV-luminous galaxy
Outer ISM phase surrounding the
UV-emitting regions whose spatial
geometry DOES NOT correlate?
Abs.
Em.
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At z~5 and 6 selection effects make
“emission” galaxies easier to confirm
spectroscopically
Vanzella et al. in prep.
z~5 spectroscopy
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Composite spectrum of
i-band dropouts
The spectral properties of
“observed” LBGs at z~6 are very
similar to some LBGs observed
at z~3.
At z~6 it is very hard to obtain
spectra of those LBGs with no Lya.
Selection effect!
Vanzella et al., Giavalisco et al 2006, in prep.
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LBG luminosity function
Relatively mild evolution of the UV luminosity function at 2.5<z<5.5
Giavalisco et al. 2006 in prep.
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The history of the cosmic
star formation activity:
This plot spans 94% of the
cosmic time!
We find that at z~6 the cosmic
star formation activity was
nearly as vigorous as it was at
its peak, between z~2 and z~3.
a=-1.6 assumed
Giavalisco et al. 2004
Giavalisco et al. 2006, in prep.
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Derive from far-UV continuum
luminosity
Dust obscuration correction:
Calzetti starburst obscuration law
Some rates are low, like z~0 spirals;
other are prodigiously high
But, does “corrected UV” trace
SF well?
Quite likely in these systems
(Kennicutt et al., Calzetti et al
2006; also Dickinson’s talk)
Star formation rates
z~4 B-band dropouts
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The morphology of the
LBGs
•Smaller
•Regulars,
•Irregulars,
•Merging,
•Spheroids?
•Disks?
•No Hubble Seq.
•No l-dependence
Rest-UV light
Rest-optical light
Morphology does not depend much on wavelength: young systems
Giavalisco et al. 1994, 1996, 1998
Steidel, Giavalisco, Dickinson & Adelberger 1996;
Lowenthal et al. 1997; Dickinson 1998; Giavalisco 1998;
Papovich, Giavalisco, Dickinson, Conselice & Ferguson 2003
Papovich, Dickinson, Giavalisco, Conselice & Ferguson 2004
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Galaxies get smaller at
high redshift…
First measures at these redshifts
Testing key tenets of the theory
Galaxies appear to grow hierarchically
R~H(z)-2/3
Standard ruler
Ferguson et al. 2004
R~H(z)-1
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Surface Brightness
Profile Analysis:
- 2-D modelling using a single Sérsic function:
Exponential disks: n = 1
R1/4 spheroids : n = 4
GALFIT
• allows convolution by the point spread function
• better handle on flux in the galaxy wings where S/N
drops at low surface brightness levels
• Measurement biases minimized
Quality control: low chi2, small errors on parameters, mfit = mauto±0.5
[Ravindranath et al. 2006]
B-dropout with n > 3.0 (spheroid-like)
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B-dropout with n~ 0.8 (disk-like)
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B-dropout with n ≥ 5 (centrally concentrated)
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
3"

 3"
100 x 100 pixels
B-dropout with n<0.5
(mergers, multiple cores)
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Profile Distribution of LBGs and z=1.2
starbursts (all M<0.5MUV*)
LBGs at z > 2.5:
~ 40% exponential disks
~ 30% spheroid-like
~ 30% mergers, multiple cores
Star - forming galaxies at
z = 1.2:
~ 26% exponential disks
~16% spheroid-like
~ 58% mergers, irregulars?
Similar conclusions from non-parametric study based on GINI, M20 and CAS coefficients
Lotz, Madau, Giavalisco, Primack & Ferguson 2005
Probing the Intrinsic Shapes
Through Ellipticity Distribution
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Observed peak in the
 = (1- b/a) , and
skewed distribution

Not only spheroids and
circular disks seen at
random orientations
Intrinsically elongated
galaxies
Peak  is lower at
lower z
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Ellipticity
distribution for
different LBG
profile types…….
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Possible explanations for the
excess of “Elongated”
morphologies among LBGs !
 Rotation-dominated disks? Edge-on
projections and selection effects
 Star forming clumps along gas-rich
filaments of cold gas infall in DM halos
 High-z bars at early epochs of galaxy
formation?
Star-formation in filaments of cold gas in DM
halos?
Ravindranath et al. 2006
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35 kpc (180 comoving)
Hydro Simulation: ~Massive M=3x1011
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Kravtsov et al.
Dekel &
Birnboim 06
z=4
M=3x1011
Tvir=1.2x106
Rvir=34 kpc
virial shock
virial shock
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Cold, dense filaments and clumps (50%)
riding on dark-matter filaments and sub-halos
Birnboim,
Zinger,
Dekel,
Kravtsov
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Observing the first gas-rich bars among
LBGs at z > 2.5?
Classic bar morphology in
the first few billion years!
Ravindranath et al. 2006
Bar in DGs encompasses the whole
galaxy; ~2-3 kpc scalelength
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More bar signatures among LBGs
at z > 2.5
Spiral arms from bar ends?
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More possible bars among
LBGs at z > 2.5
Star formation at bar ends?
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The mass of LBGs:
spatial clustering
• Galaxies at high redshifts have “strong” spatial clustering,
i.e. they are more clustered than the z~0 halos “de-evolved
back” at their redshift.
– High-redshift galaxies are biased, I.e. they occupy only the most
massive portion of the mass spectrum.
– Today, the bias of the mix is b~1.
• Idea is to test key tenets of the gravitational instability
paradigm
– evolution of galaxy clustering contains information on how the
mass spectrum gets populated with galaxies as the cosmic time
goes on.
– Clustering of star-forming galaxies at a given redshift contains
information on relationship between mass and star formation
activity
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Giavalisco et al. 1998
The mass of LBGs:
spatial clustering
r0=3.3+/- 0.3 Mpc h-1
g = -1.8 +/- 0.15
Steidel et al. 2003
Adelberger et al. 1998
Strong clustering, massive halos
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g=1.55
r0 =3.6 Mpc h-1
Porciani & Giavalisco 2002
Adelberger et al. 2004
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Clustering strength depends on UV
luminosity:
mass drives LUV (SFR)
GOODS Ground
Lee et al. 2006
Adelberger et al. (2004)
Giavalisco & Dickinson (2001)
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Clustering segregation is
detected in the GOODS
ACS sample at z~4
Lee et al. 2006
See also Ouchi et al.
2004, 2006
Clustering segregation at z~4
and 5
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Halos and Galaxies at z~3-5:
Evidence of Evolution?
Clustering scaling in good
agreement with hierarchical
theory
Implied halo mass:
>5x1010 MO(faint samples)
>1012 MO (bright samples)
1-σ scatter between mass
and SFR ~smaller that 100%
LBG halos at z ~ 5 are less
Massive.
Specific star formation
higher at higher redshift.
Up-sizing!
Giavalisco & Dickinson 2001
Porciani & Giavalisco 2002
Adelberger et al. 2004; Lee et al. 2006
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Implications
• Halo mass, I.e. local gravity, is a key parameter
to control star fomation
• Relationship between mass and star formation is
tight
• Possible to reconstruct the LUV(MH) distribution
function (e.g. CLF)
Giavalisco & Dickinson 2002;
Lee et al. 2006 in prep.
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ACS depth made
possible to observe
structure within the
halo.
Break observed at
~10 arcsec
Note: 10 arcsec
at z~4 is about
~350 kpc, about the
size of the virial
radius for M~1012
Mo .
Lee et al. 2006;
see also Ouchi et
al. 2006
Halo sub-structure at z~4
HOD at z~5
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Lee et al. 2006
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The Halo Occupation
Distribution at z~4
<Ng>=(M/M1)a
M>Mmin
Major improvement
from COSMOS
(Lee et al. PhD Thesis)
Lee et al. 2006
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Halo substructure:
we observe an excess of faint
galaxies around bright ones.
massive halos contain more
than one LBG
“Bright Centers”:
z_850<24.0
“Faint centers”: 24.0< z_850 <24.7
“Satellites”:
z_850 >25.0
Substructure is observed
with good S/N at faint
luminosity L<L*/2
Lee et al. 2006
Halos and Galaxies at z~4
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Inside the halo at z~4: are we
seeing dwarf galaxies?
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Inside the halo at z~4: are we
seeing dwarf galaxies?
Conclusions
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• With large samples of high-z galaxies it is possible to test key ideas on
star formation and galaxy evolution
• LBGs at z>4 have mix of spectroscopic properties
– Tracing geometry of ISM
• Relatively high SFR; mild evolution of the UV lum. density at high z
• Mix of UV morphology
– Spheroid and disk-like systems observed
– Higher fraction of irregular systems at z~1.5 than at z>3
– Intrinsic excess of elongated systems that disappear at lower redshifts
• Evidence of cold accretion in filaments?
• Large-scale bars?
• Size evolution consistent with hierarchical growth
• Detected halo sub-structure at z~4 (thanks to ACS sensitivity)
– Proving key prediction of theory
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Color selection at z~2
Distant Red Galaxies (DRGs):
J-K>2.3
• UV-IR SEDs span range of Hubble sequence or dusty galaxies, (Forster-Schreiber et al.)
• 50% detected with F(24mm)>60 mJy. SEDs consistent with either AGN or starbursts.
• 24mm-detected DRGs are typically ULIRGs (L IR >1012 Lo)
F(24mm) & z -> LIR using
Chary & Elbaz 2001 templates
X-ray
detected
GTO 24mm
50% completeness
Papovich et al. 2005
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DRGs at z~2
Galaxies selected from near-IR
photometry [(J-K)>2.3]
Most would NOT be selected
by LBG criteria (UV selection)
However, overlap with LBG not quantified
And certainly significant (see Adelberger
Et al. 2004).
They appear in general more evolved, I.e.
more massive (larger clustering), with larger
stellar mass, more metal rich, and more dust
obscured) than LBGs. Occurrence of AGN
also seems higher.
At z~3 these galaxies have about
50% of the volume density of LBGs
(highly uncertaint). However; they
possibly contribute about up to 100%
of the LBG stellar mass density, because
they have higher M/L ratios
Van Dokkum et al. 2004
IRX-b for Distant Red Galaxies
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UV spectral slope b
measured from ACS colors.
DRGs typically have redder b
than LBGs: <A1600> = 3.1
mag
LIR for DRGs typically
exceeds expectation from
LUV and b by factors of 10100x
DRG IR excess larger than
that for less luminous
(typically more UV-bright)
HDFN 24mm sources.
SFR~10 to 1000 Mo/yr
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Stellar population
modeling
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Stellar population
modeling
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Stellar masses & properties
of GOODS-S DRGs
Typical DRG stellar masses ~few x 1011 Mo,
(cf. FIRES work).
GOODS-S sample is roughly complete at
>1011 Mo for 2 < z < 3
2-component models frequently (but not
always) give better fits to the photometry.
Masses increase, but not as much as for
blue, lower-mass HDFN LBGs.
Loosely dividing by reddening:
Heavily obscured: EB-V > 0.35:
• < z > = 1.7
• LIR ~ expected from LUV, b
Lightly obscured: EB-V < 0.35:
• < z > = 2.5
• LIR >> expected from LUV, b (for 24mmdetected objects)
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Specific star formation
rates (SSFRs)
Low-z comparison samples from
COMBO-17: z ~ 0.4 and z ~ 0.7
• Stellar masses estimated from
COMBO-17 photometry
• SFRs from GTO MIPS 24mm data
z < 1: galaxies with M > 1011 Mo
tend to be forming stars at low
SSFRs.
z > 1: Galaxies over a broad range
of masses tend to span a broad
range of SSFRs, with many DRGs
forming stars prodigeously.
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“Downsizing” of star formation
in massive galaxies
Treating COMBO-17 and
GOODS DRG samples as
representative for M > 1011Mo:
z~2.3 DRGs forming stars with
SSFR > cosmic average
z < 1 massive galaxies forming
stars more slowly than the
global average
Further evidence that 1.5 < z <
3 was a key era for the rapid
growth of stellar mass in the
most massive galaxies.
Global average from
co-moving rSFR(z)
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Color selection at z~2:
BzK galaxies
BzK selection: 1.4<z<2.5
BzK selection well suited for 24mm
MIPS studies:
• Selected range 1.4 < z < 2.5 places
strong mid-IR features in 24mm band
• Color selection includes objects
with red UV continuum, e.g., from
extinction
• K-band selection suitable for
relatively massive galaxies
(Daddi et al. 2005)
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BzK samples in GOODSN&S
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24mm detection of BzK
galaxies
245 BzKs with K < 20.6
169 BzKs with K < 20
At present, spectroscopic redshifts
available for only a few; Keck
LRIS+DEIMOS runs ongoing.
36/169 detected in hard X-rays
(mostly AGN; not considered for
now)
109/133 (82%) for non-Xray BzKs
detected at 24mm
(undetected fraction consistent
with expected number of “passive”
BzKs)
Median <f24> = 110 mJy
Fainter K-band --> fainter 24mm
Multi-wavelength measures of SFR
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On average, multi-wavelength SFR tracers agree reasonably well
with expectations from low-z correlations, templates & analogs.
MIPS: <f(24mm)>=125 mJy, <z>=1.9,
and CE01 templates:
<LIR> = 1.7e12 Lo, <SFR> ~ 300 Mo/yr
UV continuum + reddening:
<SFR> ~ 220 Mo/yr
Radio: stacked VLA data <f(20cm)> = 17
mJy
<LIR> = 2e12 Lo, <SFR> ~ 340 Mo/yr
Sub-mm: stacked <f(850mm)> = 1.0 mJy
(5s) <LIR> = 1.0e12 Lo, <SFR> ~ 170
Mo/yr
X-ray: stacked 8.5s soft-band detection,
no significant hard-band. Far below
expected AGN level.
<SFR> = 100 - 500 Mo/yr (Persic 2004,
Ranalli 2003 conversions)
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UV vs. IR SFRs: BzKselected galaxies at z ~ 2
B-band samples ~1500A UV
continuum at z~2; B-z
measures UV continuum slope.
f(24mm) / f(B) correlates strongly
with B-z color, as expected if UV
continuum slope results from
dust reddening. Log scatter is a
factor of ~3 (including effects of
the broad BzK z-range).
Brighter/more luminous mid-IR
sources (LIR > 1012 Lo) tend to
exceed expected IRX-b, while
less luminous sources match or
fall below it (possibly including
“passive” BzKs.
Measure of mass in progress.
Star formation at z~1.5 – 2.5
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• Typical BzK and DRG galaxies appear to be both massive (~1011 Mo)
and rapidly star forming (LIR ~1012 Lo, ~ 200 Mo/yr), with space
density ~1000x larger than present-day ULIRGs
• 10-20% may be AGN; X-ray stacking favors star formation for the
majority.
• ~ 80% MIPS detection rate for BzKs implies that most massive
galaxies at 1.4 < z < 2.5 are forming stars prodigiously:
– Implies high duty cycle for SF
– Substantial mass build-up over this redshift range
• BzKs should form r* >~ 5x107 Mo/Mpc3 over ~2 Gyr, comparable to local stellar
mass density in galaxies with M* > 2x1011 Mo
• Specific star formation rate (SFR/M*) for massive (>1011 Mo) galaxies
at 2 < z < 3 is much higher than at z < 1 and than cosmic average ->
downsizing.
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•
•
•
•
VIMOS LBGs
U
B
V 25 MR (Rwfi<24.5) i