lecture #4 observational facts olivier le fèvre – lam cosmology summer school 2014
TRANSCRIPT
SURVEYS: THE MASS ASSEMBLY AND STAR FORMATION HISTORY
Lecture #4
Observational factsOlivier Le Fèvre – LAM Cosmology Summer School 2014
Putting it all together
Clear survey strategies Instrumentation and observing
procedures Selection function estimates
Let’s measure galaxy evolution !
Lecture plan
1. What are the main contenders to drive galaxy SFR and mass growth ?
2. The luminosity function and its evolution
3. The star formation history: luminosity density and SFRD
4. The mass function and the stellar mass density evolution
5. Mass assembly from merging6. A scenario for galaxy evolution ?
What may drive galaxy evolution ? A rich theory/simulation literature… Identify key physical processes When ? On which timescales ?
Beware: fashion of the day (e.g. from simulations) may fade quickly…
…Stick to facts !
Main physical processes driving evolution
Hierarchical assembly by merging Increases mass “catastrophically”
Gaz accretion Cold / Hot Fuels star formation Increases mass continuously along the cosmic web
Feedback: sends matter back to the IGM AGN (jets, …) Supernovae (explosion)
Star formation and stellar evolution Luminosity / color, lifetime Star formation quenching
Environnement, f(density) Quenching, Harassement, Stripping,…
Hierarchical merging• The basics: hierarchical
growth of structures• Merging of DM halos• Galaxies in DM halos
merge by dynamical friction
• Major mergers can produce spheroids from disks
• Merging increases star formation (but maybe short lived)
• Increases mass (minor, major)
• Merger Rate (1+z)m
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Stellar mass growth from star formation and evolution of stellar populations
In-situ gas at halo collapse transforms into stars
Accreted gas along lifetime transforms into stars
Stars evolve (HR diagram) Luminosity evolution Color evolution
Stellar population synthesis models: (Bruzual&Charlot, Maraston,…)
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Along the filaments of the cosmic web
Steady flow for some billion years can accumulate a lot of gas
Gas transforms into stars Produces important mass
growth From Press-Schechter
theory
Simulations
Dekel et al., 2009At z~2
Cold gas accretion
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Feedback Takes material out of a
galaxy back to DM halo May quench star formation ? AGN feedback
f=0.05 (thermal coupling efficiency)r=0.1 (radiative efficiency)
SNe feedback : instantaneous
SFR
feedback efficiencyVhot=485km/s and hot=3.2
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Example: combined effect of feedback and cooling on mass function
A lot of “definitive” theories and simulations
Hopkins et al., 2006
White and Rees, 1978
White & Frenk, 1991
Dekel, 2013
Cool simulations, but…need to measure galaxy
evolution !A short summary of previous lectures… With deep galaxy surveys
Imaging & Spectroscopy In large volumes
Minimize cosmic variance For large numbers
Statistical accuracy Measure properties at different epochs to
trace evolution Use these measurements to derive a physical
scenario
Main evolution indicators Luminosity function, luminosity
density Star formation rate density Stellar mass function Stellar mass density Merging Accretion …
The luminosity function
From lecture #1
The reference at z~0.1: SDSS
Blanton, 200110000 galaxies
Blanton, 2003150000 galaxies
Galaxy types vs. color
Evolution ! Canada-France Redshift Survey back in 1995
600 zspec
First evidence of evolution over ~7 Gyr
M* brightens by ~1 magnitude
Global LFLilly et al., 1995
Le Fèvre et al., 1995
1 mag
CFRS: LF evolution per type to z~1 The LF of red galaxies
evolves very little since z~1 Red early-type galaxies are
already in place at z~1 Consistent with passive
evolution (no new star formation)
Strong evolution of the LF for blue star-forming galaxies Luminosity or number
evolution ?
Little evolution
Strong evolution
LF at z~1 from DEEP2 and VVDS
A jump to z~2-4: UV LF from LBG samples
Using the LBG samples of Steidel et al. ~700 galaxies with
redshifts
Continued evolution in luminosity L*
Steeper faint end slope
From Reddy et al., 2008
Probing the LF to z~4 with the magnitude-selected VVDS
Steep slope for z>1
Continuous evolution in luminosity
Evolution in density before z~2
Cucciati et al. 2012
1 mag
2.5 mag
Downsizing
The most massive / luminous galaxies form first, followed by gradually lower mass galaxies
The most massive galaxies stop forming stars first, with lower mass galaxies becoming quiescent later
This is ‘anti-hierarchical’ !
SFR(z) vs. Halo mass
De Lucia et al., 2006
Quenching
Star formation is stopped
But what produces quenching ? Merging Mass-related
(feedback ?) Environment
Peng et al., 2010
The Star Formation Rate Evolution: the ‘Madau diagram’ back in 1996
Putting together several measurement: the strong evolution in
luminosity density observed by the CFRS from z~0 to z~1
Lower limits on SFRD from LBG samples at z~3
Lower limits on SFRD from HST LBG samples 2.7<z<4
A peak in SFRD at z~1-2 ?
From CFRS
From Steidel et al.
Let’s call it the “et al. diagram”…
From HSTHubble Deep Field
SFRD from the UV
Direct observation of UV photons produced by young stars
But absorbed by dust: need to estimate dust absorption
SFRD from the IR
UV photons produced by young stars are warming-up dust
Dust properties: calibration of UV photons to IR flux
Comparing Luminosity density from UV and IR
Same shape: transformation is extinction E(B-V)
Deriving dust extinction
Star formation rate evolution: today
Cucciati et al., 2012• SFRD rise to z~2, then flat, then decreases• Considerable uncertainties at z>3
Stellar mass function evolution
Get stellar mass of galaxies from SED fitting Uncertainties ~x2
(Initial Mass Function, Star formation history, number of photometric points on the SED, …)
Compute the number of galaxies at a given mass per unit volume
Stellar mass function evolution
Use double Schechter function Because of the different
shape of the MF for different galaxy types (next slide)
Massive galaxies are in place at z~1.5
Strong evolution of the low-mass slope
Evolution in number density
Redshift
MF evolution per type Star-forming
galaxies Strong evolution in
M* Strong evolution of
Quiescent galaxies Strong evolution in
M* to z~1.5, then no-evolution
Strong evolution in number density
Ilbert et al., 2013
Mass function: evolution scenario
The mass growth of galaxies: stellar mass density * evolution
Integrate the MF Global and per type
Smooth increase of the global *
z=1-3: the epoch of formation of quiescent/early-type galaxies Almost x100 from z~3 to
z~1
Galaxy mass assembly: Cold gas accretion or merging ?
Cold gas accretion: The main mode of gas/mass assembly ? « This stream-driven scenario for the formation of disks and spheroids is an alternative to the merger picture » (Dekel et al., 2010)
Merging major merging ? minor merging ? Occasional but large mass increase
Over time mergers can accumulate a lot of mass
Need to measure the GMRH since the formation of galaxies Mergers more/less frequent in the
past Integral mass accrued from mergers 38
?
Method 1, A priori: pairs of galaxies
Method 2, A posteriori: merger remnants, shapes
Both methods require a
timescale Timescale for the pair to merge
(vs. mass and separation) Timescale for features visibility
(vs. redshift, type of feature…)
At high redshifts z>1: pairs Faint tails/wisps lost to (1+z)4
surface brightness dimming
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Measuring the evolution of the galaxy merger rate
A wide range of measurements… Different selection functions
Different luminosity/mass Photometric pair samples
Pairs confused with star-forming regions
Background/foreground correction
Merger remnants Redshift dependant Subjective classifications
Different merger timescales
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Conselice et al., 2008
With Fmg~F0(1+z)m m=0 to 6 !
Merging rate from pair fraction
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Merging rate Pair count Numberdensity
Merger probabilityin Tmg
MergingTimescale
Tmg depends on separation rp and stellar mass
Kitzbichler & White 2008 computed timescales ~x2 larger than previously assumed ~1Gy vs. 500My
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z=0.35
z=0.63
z=0.93
Spectroscopy enables to identify real pairs
Both galaxies have a spectroscopic redshiftNo contamination issue
Galaxy Merger Rate History since z~1
Major merger rate depends on luminosity/mass Higher and faster
evolution for low mass mergers
Explains some of the discrepancy between different samples
Minor merger rate has slightly increased since z~1, while major merger rate has strongly decreased
Major mergers more important for the mass growth of ETGs (40%) than LTGs (20%)
Major mergers, de Ravel et al. 2009
Minor mergers, Lopez-SanJuan et al. 2010
m=4.7
m=1.5
Mergers at z~1.5 from MASSIV survey
80 galaxies selected from VVDS Observed with SINFONI: 3D velocity fields Straightforward classification: 1/3 galaxies are mergers
10kpc
Mergers at z~1.5
44Lopez-SanJuan, 2013
What about merging at early epochs ?Merging pairs at higher z from VUDS
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Merging pair at z~2.96
HST/ACS VIMOS spectra
Tasca et al, 2013
Galaxy Merger Rate History since z~3 from spectroscopic pairs Peak in major merger
rate at z~1.5-2 ? Integrate the merger
rate: >40% of the mass in galaxies has been assembled from merging with >1/10 mass ratio
Merging is an important contributor to mass growth
Other processes at play
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Cold gas accretion ?First evidence in 2013 ?
Building a galaxy evolution scenario ?
Several key processes have been identified, Direct: mergers, stellar evolution Indirect: accretion, feedback, environment
Properties have been quantified over >12Gyr Observationnal references exist to confront models
Semi-analytical models Take the DM halo evolution Plug-in the physical description of processes Get simulated galaxy populations
Semi-successful… some lethal failures Over-production of low-mass/low-z and under-production of
high-mass/high-z galaxies Reproducing low-z LF/MF AND high-z LF/MF
More to be done !
Circa 2002
Hopkins et al., 2008