Purely dry mergers do not reproduce the
observed evolution of early-type galaxies
Alessandro Sonnenfeld1, Carlo Nipoti2, Tommaso Treu1,3
1
University of California Santa Barbara, 2Bologna University, 3University of California Los Angeles
Introduction
Early-type galaxies (ETGs) are believed to grow as a result of mergers, but the details of this process are still largely unknown. Do the mergers involve
significant amounts of gas (wet) or are they dissipationless (dry)?Theoretical studies aimed at matching the observed size evolution of quiescent galaxies
have focused on dry mergers, as the low star formation rates measured in these galaxies leaves little room for a significant occurrence of wet mergers. The
predicted and observed merger rates seem to be able to account for the late (z < 1.5) size evolution of quiescent galaxies. Can we then explain all
observations with dry mergers? What about the evolution of the density profile?
Observations: the slope of
the total density profile from
strong lensing and stellar
kinematics
What is the density profile of ETGs and how
does it evolve? Strong gravitational lensing is
an excellent tool for studying the mass content
of ETGs, as it allows for measurements of the
total mass of objects at cosmological distance
with a few percent accuracy. Stellar
kinematics information, i.e. velocity dispersion
measurements, provide an independent
constraint on the gravitational potential that
can be used in combination with strong
lensing to measure the slope of the total
density profile. In practice, this is done by
fitting a power-law density profile to the
lensing and velocity dispersion measurements:
Schematics of a density slope measurement in a strong gravitational lens system. Strong lensing gives
the total projected mass within a cylinder of radius Reinst, while stellar kinematics is mostly sensitive to
the mass enclosed within the sphere of the scale of the half-light radius. By fitting a power-law density
profile to these two independent probes of the gravitational potential we can measure the slope of
the total density profile.
We measured the density slope for a sample
of 83 lenses in the redshift range 0 < z < 0.8
(Sonnenfeld et al. 2013) and were able to
constrain the evolution of this quantity. We
found that ETGs evolve at approximately
constant density slope.
Sonnenfeld et al. (2013). Denstiy slope vs. redshift (left) and vs. Stellar mass density (right). The density
slope anti-correlates with the former and correlates with the latter. Since ETGs become less
concentrated with time (decreasing stellar mass density), the net result is that ETGs evolve while
keeping their density slope approximately constant.
Comparison with theory:
predicted dry merger evolution
of the density slope
Does the dry merger evolution scenario
reproduce the observation of a density slope
that stays roughly constant with time? We used
numerical simulations to answer this question. We
first looked at the effect of isolated dry mergers
on the density slope with high-resolution N-body
simulations, then folded the results into a model
for the evolution of the population of massive
ETGs between z=1 and z=0, where galaxies grow
as a result of dry mergers with rates suggested by
the Millennium simulation.
This purely dry merger model predicts a strong
decrease of the density slope with time, ruled out
by observations with a 3-sigma significance. A
purely dry merger scenario then, while
reproducing the observed size evolution of ETGs,
fails in matching density slope observations.
References:
Sonnenfeld et al.
(2014).
Top panel: change in
density slope for unit
increase in stellar mass
as a function of
merger mass ratio for
a series of binary dry
mergers with different
initial conditions. All
mergers tend to
decrease the density
slope. Minor mergers
(left side of the plot)
have a stronger
impact on the slope
than major mergers,
for the same
accreted mass.
Bottom panel:
evolution of the
average density slope
for a mock population
of massive ETGs (solid
line) and as observed
with strong lensing
and stellar kinematics
(shaded region). The
discrepancy between
model and
observation is larger
than 3-sigma.
Nipoti C., Treu, T., Leauthaud, A., et al. 2012, MNRAS, 422, 1714
Sonnenfeld, A., Treu, T., Gavazzi, R., et al. 2013, ApJ, 777, 98
Sonnenfeld, A., Nipoti, C., Treu, T. 2014, ApJ, 786, 89
Toy model: adding
dissipation
A more successful model might be one where
we allow for some dissipation. We test this
scenario with a toy model, where we assume
that a small fraction of the accreted baryons is
in the form of gas. This gas falls to the center of
the galaxy and forms stars, while the density
profile contracts adiabatically. Allowing for
dissipation results in a better agreement with
observations.
Sonnenfeld et al.
(2014). Evolution of the
average density slope
for a mock population
of massive ETGs where
10% of the accreted
baryonic mass is in the
form of gas (solid line)
and as observed with
strong lensing and
stellar kinematics
(shaded region).
Predictions for a few
cosmological
simulations are also
shown.