lecture #4 observational facts olivier le fèvre – lam cosmology summer school 2014
TRANSCRIPT
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SURVEYS: THE MASS ASSEMBLY AND STAR FORMATION HISTORY
Lecture #4
Observational factsOlivier Le Fèvre – LAM Cosmology Summer School 2014
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Putting it all together
Clear survey strategies Instrumentation and observing
procedures Selection function estimates
Let’s measure galaxy evolution !
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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 ?
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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 !
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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,…
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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
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A lot of “definitive” theories and simulations
Hopkins et al., 2006
White and Rees, 1978
White & Frenk, 1991
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Dekel, 2013
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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
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Main evolution indicators Luminosity function, luminosity
density Star formation rate density Stellar mass function Stellar mass density Merging Accretion …
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The luminosity function
From lecture #1
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The reference at z~0.1: SDSS
Blanton, 200110000 galaxies
Blanton, 2003150000 galaxies
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Galaxy types vs. color
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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
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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
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LF at z~1 from DEEP2 and VVDS
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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
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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
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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
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Quenching
Star formation is stopped
But what produces quenching ? Merging Mass-related
(feedback ?) Environment
Peng et al., 2010
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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
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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
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Comparing Luminosity density from UV and IR
Same shape: transformation is extinction E(B-V)
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Deriving dust extinction
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Star formation rate evolution: today
Cucciati et al., 2012• SFRD rise to z~2, then flat, then decreases• Considerable uncertainties at z>3
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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
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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
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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
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Mass function: evolution scenario
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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
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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
?
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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
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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 !
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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
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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
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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
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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
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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 ?
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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 !
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Circa 2002
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Hopkins et al., 2008