Transcript The cosmic history of star formation and mass assembly: a

How do galaxies accrete their mass?
Quiescent and star-forming
massive galaxies at high z
Paola Santini
Osservatorio Astronomico di Roma
Post-doc
Roman Young Researchers Meeting 2009
July 21st Università “Tor Vergata”
Before starting……
REDSHIFT
observed  emitted
z
emitted
emit
obs
Increase in the wavelength of e-m
radiation received by a detector compared
with the wavelength emitted by the source.
Cosmological redshift is due to the
expansion of the Universe: the distance
between the emitting source and the
observer increases while the light is
propagating.
high z Universe = distant Universe = young Universe
Recession velocity
increases with distance
(Hubble’s law)
c is finite
The global picture of galaxy formation
Hierarchical formation: massive
galaxies were assembled recently
from mergers of smaller subunits
(smaller DM halos collapse earlier)
“ab initio” MODELS OF
GALAXY FORMATION
Kauffman et al. 93 ; Cole et al. 94; Somerville & Primack 00;
Cole et al. 00; Menci et al. 02; Wu, Fabian, Nulsen 00, etc…
Dynamical evolution of dark
matter condensations
supported by observations: e.g.
ongoing star formation at low z,
paucity of high mass galaxies at z >1
The different models differ in the description of the baryonic processes,
especially the two main processes driving galaxy evolution:
Gas uniformly distributed
in the halo, star formation
on a disk
• star formation
conversion of gas into stars
Gas distributed on filaments,
disk fragments, star formation
on blobs
Dekel+09
• suppression of star formation
most efficient: AGN feedback
introduced to reproduce passive galaxies at high z
Radio mode
Quasar mode
z ≥ 2 : major phase in the assembly of massive galaxies
Redshift
1) What drives the evolution
of stellar mass at z ~ 2? (SF
inside galaxies? Mergers?)
2) Quenching mechanisms?
3) Are these processes
reproduced by the models?
M>7
1010
Mo
Fontana+06
 Searching observables which
directly reflect these two
processes
The GOODS-MUSIC sample
Great Observatories Origins Deep Survey-MUltiwevelength Southern Infra-red Catalog
(Grazian+06, Santini+09, http://lbc.oa-roma.inaf.it/goods)
U35 U38 (MPG/ESO-WFI)
U VIMOS (VLT)
B V i z (HST-ACS)
J H Ks (VLT-ISAAC)
3.6 4.5 5.8 8.0 µm (Spitzer-IRAC)
Photometry from 0.3 to 24 µm (15 bands)
~ 143 arcmin2 CDF-South
~ 15000 objects z, Ks and 4.5 µm selected
~ 1800 spectroscopic z + well calibrated zphot
24 µm (Spitzer-MIPS)
Multiwavelength surveys
U
B
V
R
I
J
K
360nm
420nm
520nm
650nm
800nm
1250nm
2200nm
Multicolour surveys allow to
estimate photometric redshifts
and physical properties of each
object of the catalog from the
overall spectral shape
SED fitting
Photometric z
M(stars)
SFR
Dust
Z
SFR estimate
Future: Herschel
PACS
55  210 mm
Absorbed UV
light
250, 350,
500 mm
Dust emission
Stellar
emission
PAH features
MIPS
24 μm filter
M82 (ISO)
SPIRE
Quiescent – star-forming selection
Dusty star-forming
0.9 µm
2.2 µm
4.5 µm
24 µm
Passively evolving
Empirical UV-to-midIR SEDs (Polletta+07)
0.9 µm
2.2 µm
4.5 µm
24 µm
Quiescent galaxies
The very quiescent tail: Red&Dead galaxies
24 µm undetected
galaxies
Combined IR
emission + SED
fitting analysis
SFR/M
SFR/M < 10-11 yr-1
“RED&DEAD”
galaxies
SFR/M
Fontana+09
The cosmic evolution of Red&Dead galaxies
Galaxies with very low levels of SFR
Time
M>7 1010 Mo
Fontana+09
K07: Kitzbichler&White07
(Millennium Simulation)
M06: Menci+06
F07: MORGANA
(Monaco+07)
N06: Nagamine+06
• 20% of massive galaxies is already in a
very quiescent phase at z~2-3
• Sensitive observable to constrain models:
quenching mechanisms
Star-forming galaxies
The Specific
SFR — stellar
mass relation
What drives
the evolution
of stellar
SFR/M
Kitzbichler&White07
mass
at z ~ 2 ?
(Millennium Simulation)
Total accreted mass = <SFR>active x <∆t active phase>
“Duty cycle” argument:
65% of M > 7 1010 Mo galaxies is actively
SF-ing at 1.5<z<2.5, with <SFR>~300Mo/yr
assuming that the active fraction is
proportional to burst duration, the stellar
mass acquired in this epoch is >1011Mo
At z ~ 2 massive galaxies are rapidly forming.
The SFR directly observed in massive galaxies
is enough to produce the bulk of the observed
stellar mass density.
Intense star formation processes within massive
galaxies prevail over merging events at z ~ 2.
Santini+09
Comparison with theoretical predictions
Menci+06
Models qualitatively
reproduce the observations
but underestimate the SFR
in massive galaxies
MORGANA (Monaco+07)
Kitzbichler&White07
(Millennium Simulation)
Santini+09
(See also Daddi+07, Davé+08, Fontanot+09)
Summary & conclusions
1) The epoch z>~2 is a major phase in the assembly of massive galaxies
2) “Red&Dead” galaxies exist up to z=3 and most likely above: need for
efficient feedback/quenching of SF mechanisms at high z
3) At z~2, more than 50% of massive galaxies are experiencing a major
peak in their SFRH: during this process they accrete a substantial fraction
of their mass (see also Daddi+07)
4) Theoretical models fail in predicting simultaneously the SFR (typically
under-predicted) and the quenching of SF
5) Need for a different/new physics?