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Early phases of galaxy
evolution
Gianfranco De Zotti (INAF-OAPd)
Andrea Lapi, Alessandro Bressan, Luigi
Danese, Mattia Negrello
What is the main driver of
galaxy formation?
• The spectacular success of hierarchical-CDM at explaining
the large-scale structure (> 1Mpc) has led to the widespread
opinion that the formation of visible galaxies parallels that
of dark matter halos: small objects form first and merge
together to make larger ones
• Most of the star-formation, and of the black hole accretion,
in the early universe is triggered by galaxy interactions and
mergers, and therefore occurs in bursts
Mg bFe5015
Mg2Mg1
Fe5270
Fe5335
Fe5406
Hb
Narrow Band Indices Lick-IDS System
A5V 0.5- 1 Gyr
G0V 4.5 Gyr
K0III red giant
M0III RGB
M.S. Clemens, A. Bressan,
B. Nikolic, Rampazzo 09
∑
Age, Z
[a/Fe]Model
s
14000 ETGs 0.01 < z < 0.1
Lick Indices: Hb Hdf
Mg1 Mg2 Mgb
Fe4383 Fe4531 Fe5270 Fe5335
• Galaxies with higher mass
are older, more metal rich and
a-enhanced.
The star formation was
rapid (< 1Gyr) in ETGs with
high mass
The star formation was
slow (several Gyr) at low
masses
• Environment affects only the
mean age of ETGs, at a given s:
ETGs in high density form first
• Environment has no effect on
metallicity and a-nhancement.
After collapse, formation
and evolution are driven by the
potential well
Galaxy Formation
all
r5 is the distance to the fifth-nearest
neighbour. 1/r5 < 0.5: low-density
environment; 1/r5 > 1.5: high-density
environment;
Galaxy Formation
all
Evolution of
DM Halos:
smaller halos
form first
Feedback reverses
formation
timescales:
in smaller galaxies
star formation
proceeds slowly
lower [a/Fe]
indicates
longer duration
Problems with merger-driven galaxy evolution - 1
• Correlations tight enough to allow little room for random
processes and for sensitivity to environment have been
known for a long time (colour-luminosity; fundamental
plane relations; dynamical mass - luminosity) and have been
recently confirmed with very large samples. More recently:
luminosity–size (Nair et al. 2010), SFR-mass at z~2
(Maraston et al. 2010; Pannella et al. 2009)
• Insensitivity to environment
• Tight contraints on increase of stellar mass of massive
ETGs since z=1.5-2 (Pérez-González et al. 2008;
Marchesini et al. 2009; Mancone et al. 2010; Fan et al.
2010).
Problems with merger-driven galaxy evolution - 2
• Integral-field near-IR spectroscopy has shown that many of
these galaxies have ordered, rotating velocity fields with no
kinematic evidence for ongoing merging (Genzel et al.
2006; Förster-Schreiber et al. 2009; Carilli et al. 2010). A
complex morphology is not necessarily a symptom of
merging .
• At z ∼ 2.3, massive quiescent galaxies are typically 5 times
more compact, and two orders of magnitude more dense
than local ellipticals of the same mass (Cimatti et al. 2008;
van Dokkum et al. 2008). The size evolution by mergers is
tightly constrained by limits on mass increase (Fan et al.
2010).
Problems with merger-driven galaxy evolution - 3
• Large (~50-60%, Driver et al. 2008) stellar mass fraction in
fragile disks. The remarkably thin disc of the Milky Way
formed ≈ 8.8±1.7 Gyr ago (del Peloso et al. 2005),
corresponding to a formation redshift of ≈1.3(+0.9,-0.45)
and exhibits a quiescent history. The kinematic observations
(Shen et al. 2010) show no sign of a significant merger-
made bulge. Another example is the giant edge-on galaxy
NGC 4565 (Kormendy & Barentine 2010). Giant, pure disk
galaxies seem to be common in environments far from rich
clusters of galaxies (Kormendy et al. 2010). How did these
galaxies grow so large with no observational sign that they
suffered a major merger in the last 9 – 10 Gyr?
Problems with merger-driven galaxy evolution - 4
• The distribution of galaxy luminosities is very different from
the distribution of DM halo masses: many fewer faint
galaxies relative to bright galaxies than low mass to high
mass DM halos and exponential decline at the high end of
the luminosity function: the efficiency of galaxy formation
must depend strongly upon halo mass (see e.g. Shankar et al.
2006; Behroozi et al. 2010).
It is now generally agreed that baryonic processes must come
into play to suppress the formation of stars in lowest and
highest mass dark matter halos (probably energy/momentum
input from supernovae explosions at low masses, AGN
feedback at high masses). But is the role of baryon processes
even more important?
Are baryon processes the main driver?
The above difficulties have been addressed and overcome,
under some conditions, by models in the framework of the
merging scenario. In some cases, however, the solutions
rely on assumptions not very clearly justified and are
somewhat contrived.
Difficulties are more easily overcome if baryon processes,
rather than merging, are the main drivers of galaxy
evolution (Granato et al. 2001, 2004; Lapi et al. 2006), still
in the general framework of hierachical clustering in a
ΛCDM universe.
Key ingredients - 1
• In the build-up of a galactic halo we can schematically
identify two phases (Zhao et al. 2003):
– a fast accretion phase (timescale « H(z)-1) in which the potential
well is created by major mergers, and that can define the formation
epoch of the galaxy
– a slow accretion (minor merger) phase in which mass is added in
the outskirts of the halo, affecting only weakly the central region
where the stellar component resides.
In general this second phase affects only marginally the
stellar component, although occasionally major mergers
also occur, but they involve a minority of large galaxies.
Halo mass vs velocity evolution
(Zhao et al. 2003)
Halos form, gas is shock heated to virial T
Gas cools, collapse and forms stars directly, in small
halos SNae quench SF, in big ones nothing prevents a
huge burst of SF (~1000 Msun/yr over 0.5 Gyr), SMGs
phase
(almost) passive evolution of stellar population follows.
ERO phase with dormant SMBH
SF promotes the growth of a SMBH, powering high z
QSO. QSO activity expels the ISM, terminating SF and
its own growth. QSO phase
Co-evolution scheme by Granato et al. (2004)
HOT GAS
COLD GAS
RESERVOIR
(low J)
STARS IGM
SMBH-QSO
SNae feedback
QSO feedback
Radiative
cooling
Radiation
drag
(SFR)
Viscous
accretion
Collapse
Physical processes
Stellar
evolution
Arrows correspond to a set of differential
equations describing the various processes,
solved numerically. Approximate analytical
solutions given by Mao et al. (2007).
Baryon-driven evolution:
implications and predictions - 1
• Mbh – σ correlation (Silk & Rees 1998)
• Downsizing: SN explosions quench star-formation (and
may unbind the gas) in low-mass halos, but may enhance it
(through shocks) in massive halos (Granato et al 2001;
2004)
• Critical mass (Mhalo ~ 3×1011 Msun corresponding to M* ~
1.2×1010 Msun ) above which the AGN feedback overcomes
Snae as the main agent controlling the SFR (Granato et al.
2004; Shankar et al. 2006). A “halo mass floor” Mmin ≈
1011 Msun was introduced on essentially empirical grounds,
by Bouché et al. (2010)
• M* - Mhalo relationship (Shankar et al. 2006)
Baryon-driven evolution:
implications and predictions - 2
• Ωstar « Ωb : the total mass density in stars is
Ωstar=0.0027±0.0005 (Fukugita & Peebles 2004), i.e.
5.8%±1.1% of the total baryonic mass density of the
Universe Ωb = 0.0456 ± 0.0016 (Komatsu et al. 2010); the
ISM adds little.Therefore, only a small fraction of all
baryons are kept in galaxies.
• Very high (thousands Msun/yr) SFRs for very massive high-
z galaxies, without invoking non-standard (top-heavy)
IMFs. SCUBA counts and redshift distributions easily
accounted for.
• SPT & Herschel counts, including strongly lensed sources
(Negrello et al. 2007).
1.4 mm cumulative SPT counts (Vieira et al. 2010)
Models: short dashed Lagache et
al. (2004) ; long dashed line,
Negrello et al. (2007); dot-dashed
line, Pearson & Khan (2009)
Top panel: all dust-dominated
sources included.
Bottom panel: counts calculated
excluding sources that have IRAS-
FSC counterparts (within 1') and
models calculated excluding
populations that should be
detectable in the IRAS 60 μm band
above the typical FSC limit of 0.2 Jy.
Negrello et al. is the only physical
(forward) model. While predicted
counts are similar, redshift
distributions are radically different:
Negrello et al. predicts 2 peaks, at
z~0.02 and at z~3 (see below).
Herschel/SPIRE 500µm counts
Negrello et al. (2010); counts
by Clements et al. (2010)
The model curves are
from Negrello et al.
(2007). The model
predicted 0.4 deg-2
strongly lensed sources
and 0.55 deg-2 low-z
star-forming galaxies,
i.e. 5.8 and 6.6,
respectively, in the 14.4
deg2 H-ATLAS SDP
field. The predicted
surface density of
strongly lensed gal-
axies is sensitive to the
redshift distribution.
Herschel/SPIRE 500µm redshift distributions
.
The H-ATLAS SDP catalog contains 10 sources (plus an Galactic dust
cloud) with 500 μm flux density above 100mJy: 4 galaxies with
spectroscopic redshifts in the range 0.01− 0.05, 1 blazar and 5 strongly
lensed galaxies in the redshift range 1.6 − 3.1. The figures show the
corresponding predictions by Negrello et al. (2007).
500µm
Normal late-type
Starburst
Lapi et al. 06
Merger-driven QSO activity (Hopkins et al. 2008)
Light curve with multiple peaks and SFR well correlated with LQSO
Star-formation vs AGN activity
Star forming luminosity vs. AGN luminosity for the GOODS-N AGN
and a local reference sample of extremely hard X-ray selected BAT
AGN. The dashed line is the relation implied by Netzer et al. (2009).
Shao et al. (2010)
Baryon-driven evolution:
implications and predictions - 3
We expect a correlation
between SFR and AGN
luminosity at high z
because both are correlated
with the halo mass, but not
linear (because the AGN
accretion rate is mostly
Eddington limited) and
with a large dispersion.
Bonfield et al. (2010) find
LFIR ~ LQSO0.32 (with some
uncertain dependence on
z) .
Hatziminaoglou et al. (2010) find LFIR ~ LQSO0.35 for objects at z > 2,
with a large dispersion (see figure)
Hatziminaoglou et al. (2010)
Baryon-driven evolution:
implications and predictions - 4
• During most of the active star-formation phase, the AGN
mass is well below the final value. The median X-ray
luminosity of FIR/SM bright galaxies is therefore predicted
to be relatively low. For the z~2 ULIRGS with 24 μm fluxes
of 0.14–0.5 mJy, studied by Fadda et al. (2010), the model
yields a median accretion rate of ~0.01 Msun/yr (Granato et
al. 2006) corresponding to a bolometric luminosity of ~ 6 ×
1043 erg/s and to a 2 ̶ 10 keV X-ray luminosity of ~ 3 ×
1042 erg/s, in very good agreement with the estimate (1± 6)×
1042 erg/s) by Fadda et al.
Evolution of the luminosity function - 1
Two main factors come into play:
• The UV luminosity is proportional to the SFR which
increases with increasing z since it is controlled by
tcond=max[tcool,tdyn], which is shorter at high z, when
densities are higher. For the mass range of interest: SFR
MH0.8(1+z)1.5 (Mao et al. 2007).
• The higher dust extinction for more massive objects damps
down the contribution to the LF of the most massive halos,
whose density is decreasing fast with increasing z. In the
mass range which dominates the contribution to the LF, the
increase of the SFR with redshift compensates for the
decrease of the comoving density at fixed mass.
A very low evolution of the LF is predicted.
Extinction evolution with galactic age at
z=6
MH = 1010
Msun
1011 Msun
1012 Msun
1013 Msun
SFR vs mass and z
Pannella et al. (2009)Z=2.1
Z=1.6
Dunne et al. (2009)
Rodighiero et al. (2010)
Rodighiero et al. (2010)Mao et al. (2007):
SFR ~ M0.8 (1+z)1.5 for z >1.5
and Mh > 3 × 1011 Msun
Evolution of the cumulative LAE LF
Lines:
model by
Mao et al.
(2007)
“Why the Lyα luminosity function should conspire to be unchanging over 3< z <6 when
there is such a significant change in the LBG population is particularly intriguing” (Ellis,
2008).
Evolving LBG luminosity function
Model by Mao et al. (2007) compared with recent data