Minor mergers and their impact on the kinematics of old and young stellar populations in disk galaxies

Abstract

By means of N-body simulations we investigate the impact of minor mergers on
the angular momentum and dynamical properties of the merger remnant. Our
simulations cover a range of initial orbital characteristics and gas-to-stellar
mass fractions (from 0 to 20%), and include star formation and supernova
feedback. We confirm and extend previous results by showing that the specific
angular momentum of the stellar component always decreases independently of the
orbital parameters or morphology of the satellite, and that the decrease in the
rotation velocity of the primary galaxy is accompanied by a change in the
anisotropy of the orbits. However, the decrease affects only the old stellar
population, and not the new population formed from gas during the merging
process. This means that the merging process induces an increasing difference
in the rotational support of the old and young stellar components, with the old
one lagging with respect to the new. Even if our models are not intended
specifically to reproduce the Milky Way and its accretion history, we find
that, under certain conditions, the modeled rotational lag found is compatible
with that observed in the Milky Way disk, thus indicating that minor mergers
can be a viable way to produce it. The lag can increase with the vertical
distance from the disk midplane, but only if the satellite is accreted along a
direct orbit, and in all cases the main contribution to the lag comes from
stars originally in the primary disk rather than from stars in the satellite
galaxy. We also discuss the possibility of creating counter-rotating stars in
the remnant disk, their fraction as a function of the vertical distance from
the galaxy midplane, and the cumulative effect of multiple mergers on their
creation.

Full text

Page 1

arXiv:1110.0769v1 [astro-ph.CO] 4 Oct 2011
Astronomy & Astrophysics manuscript no. ms
October 5, 2011
c ? ESO 2011
Minor mergers and their impact on the kinematics of old and young
stellar populations in disk galaxies
Yan Qu1, Paola Di Matteo1, Matthew D. Lehnert1, Wim van Driel1, and Chanda J. Jog2
1GEPI, Observatoire de Paris, CNRS, Universit´ e Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
e-mail: yan.qu@obspm.fr
2Department of Physics, Indian Institute of Science, Bangalore 560012, India
Accepted, Received
ABSTRACT
By means of N-body simulations we investigate the impact of minor mergers on the angular momentum and dynamical properties of
the merger remnant. Our simulations cover a range of initial orbital characteristics and gas-to-stellar mass fractions (from 0 to 20%),
and include star formation and supernova feedback. We confirm and extend previous results by showing that the specific angular
momentum of the stellar component always decreases independently of the orbital parameters or morphology of the satellite, and that
the decrease in the rotation velocity of the primary galaxy is accompanied by a change in the anisotropy of the orbits. However, the
decrease affects only the old stellar population, and not the new population formed from gas during the merging process. This means
that the merging process induces an increasing difference in the rotational support of the old and young stellar components, with the
old one lagging with respect to the new. Even if our models are not intended specifically to reproduce the Milky Way and its accretion
history, we find that, under certain conditions, the modeled rotational lag found is compatible with that observed in the Milky Way
disk, thus indicating that minor mergers can be a viable way to produce it. The lag can increase with the vertical distance from the
disk midplane, but only if the satelliteis accreted along a direct orbit, and in all cases the main contribution to the lag comes from stars
originally in the primary disk rather than from stars in the satellite galaxy. We also discuss the possibility of creating counter-rotating
stars in the remnant disk, their fraction as a function of the vertical distance from the galaxy midplane, and the cumulative effect of
multiple mergers on their creation.
Key words. galaxies: interaction – galaxies: formation – galaxies: evolution – galaxies: kinematics and dynamics
1. Introduction
Rotationally supported disks account for only a small fraction
of the mass in the local Universe, but they contain most of
the angular momentum (hereafter AM). The way disks acquire
and redistribute their AM represents one of the most challeng-
ing problems for models of galaxy formation and evolution.
According to the current cosmological paradigm, baryons and
dark halos in galaxies acquire their spin through tidal torques
exerted by adjacent structures at early times. This AM is then
redistributed among the different galaxy components through a
number of internal and external processes as the galaxy evolves.
Amongthe internal processes, bars, lopsidedness, spiral patterns
and other coherent structures are efficient in redistributing AM
in galaxies, as many studies have shown (e.g., Athanassoula
2005; Debattista et al. 2006; Minchev et al. 2011). These stel-
lar asymmetries can be stimulated or strengthened by external
processes, such as accretion of a few M⊙yr−1of gas from cos-
mological filaments (see Bournaud et al. 2005a,b) or tidal inter-
actionsandmergers(Jog & Maybhate2006; Mapelli et al. 2008;
Reichard et al. 2009). In particular, during an interaction orbital
AM is converted into internal rotation, in an outside-in manner:
the components which first interact are the most extended ones,
whilethemoretightlyboundcomponentsexperiencestrongtidal
effects only in the final phases of the merging process (Barnes
1992; Di Matteo et al. 2008b, 2009).
Many studies have shown that major mergers have a
catastrophic impact on the ordered motion of the pre-
existing galaxies. If the progenitors have disks, they are
usually destroyed by the strong energy and AM redistri-
bution taking place during the interaction (Toomre 1977;
Bendo & Barnes 2000; Naab & Burkert 2003; Bournaud et al.
2005c; Jesseit et al. 2009), unless peculiar orbital configurations
are chosen (Puerari & Pfenniger 2001; Crocker et al. 2009). The
fraction of gas present in the progenitor disks can also influ-
ence the morphology and kinematics of the final remnant (e.g.,
Hopkins et al. 2009), but this depends as well on the gas physics
implemented in the models (e.g., Bournaud et al. 2011). In the
case of pressure-supported progenitors, in turn, the tidal torques
exerted by the companion can be strong enough to produce
high rotational support (v/σ > 1) at large radii, even in merger
remnants having an elliptical-like morphology (Di Matteo et al.
2009). More attention has been given to the study of the im-
pact of AM redistribution in major mergers than in minor merg-
ers (with mass ratios ≤0.1). This despite the fact that minor
mergers are expected to be much more common than major
mergers (Fakhouri & Ma 2008) and that many traces of on-
going or past interactions are visible both in the Milky Way
(see Klement 2010, for a recent review), our neighbor galaxy
Andromeda (Ibata et al. 2001; McConnachie et al. 2009) and
other galaxies in the local Universe (Martinez-Delgado et al.
2010). Di Matteo et al. (2010) pointed out that the AM redistri-
butionduringminormergersalsohaveanimpactonthekinemat-
ics of stellar disks and may explain the distribution of the orbital
eccentricities of stars in the solar neighborhood. Understanding
how AM is redistributed during such episodes is fundamental
to the understanding of how disks can be maintained, how their
kinematics can be affected, and what the signatures are of these

Page 2

2Qu et al.: Minor mergers and their impact on the kinematics of old and young stellar populations in disk galaxies
Table 1. Parameters for the initial models of the bulge, halo and
stellar and gaseous disks.
gS0
10.
50.
40.
–
2.
10.
4.
0.5
–
–
gSa
10.
50.
40.
0.1
2.
10.
4.
0.5
5.
0.2
gSb
5.
75.
20.
0.2
1.
12.
5.
0.5
6.
0.2
dE0
7.
3.
0
–
1.3
2.2
–
–
–
–
dS0
1
5
4
–
0.6
3.2
1.3
0.16
–
–
dSa
1
5
4
0.1
0.6
3.2
1.3
0.16
1.6
0.06
dSb
0.5
7.5
2
0.2
0.3
3.8
1.6
0.16
1.9
0.06
MB[2.3 × 109M⊙]
MH[2.3 × 109M⊙]
M∗[2.3 × 109M⊙]
Mgas/M∗
rB[kpc]
rH[kpc]
a∗[kpc]
h∗[kpc]
agas[kpc]
hgas[kpc]
processes on the dynamical properties of different stellar popu-
lations.
Thispaperisthesecondofaseries wherewestudy,bymeans
of numerical simulations, the impact of minor mergers on AM
redistribution in galaxies. In Qu et al. (2010) (hereafter Paper I),
we investigated the impact of dissipationless minor mergers on
diskgalaxies,showingin particularthatthe initiallynon-rotating
dark matter halo of the primary galaxy always gains AM and
that the specific AM of the stellar component always decreases.
We also showed that this decrease in AM is accompanied by a
change in stellar velocity anisotropy as the stellar orbits become
less tangentially dominated as the merger advances. In this pa-
per, we aim to advance this analysis by studying simulations of
dissipativeminormergers,exploringa rangeofgas fractionsand
morphological parameters for the primary galaxy and the satel-
lite. Star formation and feedback from supernovae explosions
are included in the models, and we are able to trace the AM re-
distribution of all the galaxy components: dark matter, gas, old
stars (i.e., those already in the galaxies before the interaction
starts) and new stars (i.e., those formed from the gas during the
interaction).Inparticular,weaimtounderstandifdissipativemi-
normergersstill slow downthestellar disk oftheprimarygalaxy
and if stellar populations of different ages show a different AM
content and different dynamical properties in the final (i.e., post-
merger) disk.
The paper is organized as follows: the numerical code, the
initial galaxy models and orbital conditions adopted for the runs
are described in § 2. Section 3 presents the main results, in par-
ticular how the AM content of gas and the old and new stellar
populationsis affectedbyasingle andbytwo consecutive(§3.1)
minor mergers, and how the rotational lag induced by this redis-
tribution affects the old stellar component (§ 3.2). The contribu-
tion of accreted stars to the rotational lag, and the possibility to
distinguish them on the basis of their AM content, are discussed
in § 3.3. Section 3.4 discusses the effect of single and two con-
secutive retrograde mergers on the fraction of counter-rotating
stars. Finally, § 4 presents a discussion of the results found and
the main conclusions.
2. Models
All 121 simulations described in this paper are part of the
GalMer project1and were fully described in Chilingarian et al.
(2010). Here we recall the main characteristics of the adopted
galaxy models and orbital parameters, as well as those of the
numerical code employed to run the simulations.
1http://galmer.obspm.fr
In this paper, we study the interaction and successive merger
of a satellite galaxy with a ten times more massive primary disk
galaxy. Both the primary and the satellite consist of a spheri-
cal non-rotating dark halo and a central bulge, both modeled by
Plummer spheres of masses MHand MB, respectively, and core
radii rHand rB, a stellar and an optional gas disk, represented by
Myamoto-Nagai density profiles of masses M∗and Mgas, disk
scale lengths a∗and agas, and scale heights h∗and hgas, respec-
tively. Both the primary and the satellite galaxies span a range
of morphologies, with a variety of bulge to disk ratios, and gas-
to-stellar mass fractions (from fgas= 0 to fgas= 20%). Moving
from early to late type systems, we refer to these models respec-
tively with the nomenclature gS0, gSa and gSb for the primary
galaxy, and dS0, dSa and dSb for the satellite. For the satellites,
we also consider a simple spheroid dominatedmodel, consisting
only of a dark halo and a stellar bulge, without any disk compo-
nent – we refer to this model as dE0. All the parameters of the
galaxies describedabove are given in Table 1, and the numberof
particles adopted to describe the different galaxy components in
Table 2.
As we have done in Qu et al. (2010), all galaxy models were
first evolvedin isolationfor1 Gyrbeforetheinteractionstarts, to
let the initial system reach a stable configuration. Once relaxed,
the two galaxies were placed at an initial distance of 100 kpc,
with a variety of relative velocities, to simulate different orbits
(see Chilingarian et al. 2010, Table 9, for the orbital initial con-
ditions). To study the effect of multiple mergers, we also ran
some simulations in which the primary galaxy accretes consec-
utively two or three satellites over a period of 3-5 Gyr. We also
compared the dynamical properties of disks after minor mergers
to those heated by internal processes, such as scattering by mas-
sive clumps formed in an initially unstable disk (Bournaud et al.
2009). For this, we analyzed a simulation of a gas-rich, unstable
disk galaxy from Di Matteo et al. (2008a). It uses a total number
of 120,000 particles, equally distributed among gas, stars and
dark matter, and has the same parameters as the gSb model (see
Table 1) except for the gas mass fraction,which is initially much
higher, 50%.
All the simulations were run using the Tree-SPH code de-
scribed in Semelin & Combes (2002). Gas is treated as isother-
mal, at a fixed temperature of T=104K. Prescriptions for star
formation and feedback from supernovaeexplosions are also in-
cluded. The rate of star formation is governed locally by a vol-
ume density Schmidt law
˙
Mgas∝ ραMgaswith α=1.5, which is
implemented in the code by means of a hybrid particle approach
(see Mihos & Hernquist 1994; Chilingarian et al. 2010 for de-
tails). At eachtime step, a hybridparticleis characterizedby two
mass values: the first is its total mass Miwhich stays constant
during the simulation and is used to calculate the gravitational
force, and the second is given by the gas content of the particle,
Mi,gas, which changes with time according to the Schmidt law
and is used to evaluate hydrodynamical quantities. At each time
step the mass of the new stars, i.e. those formed since the be-
ginning of the simulation, is given by Mi−Mgas. If Mi,gasdrops
below 5% of the initial value, the “hybrid” particle is considered
to be totally convertedinto a star-like particleand its small resid-
ual amount of gas spread out over its (hybrid) neighbors. Note
that in the hybrid particle scheme, new stars follow the gas kine-
matics until the moment when the hybrid particle they reside in
is completelyconvertedintostars.Thiscanrepresentalimitation
of our approach, in the sense that it does not allow us to prop-
erly follow the heating with time of the newly formed stars by
secular processes. Finally, for the evaluation of the gravitational
forces a softening length ǫ=200 pc is employed. The equations

Page 3

Qu et al.: Minor mergers and their impact on the kinematics of old and young stellar populations in disk galaxies3
Table 2. Particle numbers for primary disk galaxies and satel-
lites.
gS0
–
gSa
80,000
240,000
160,000
gSb
Ngas
Nstar
NDM
160,000
160,000
160,000
320,000
160,000
dE0
–
32,000
16,000
dS0
–
32,000
16,000
dSa
8,000
24,000
16,000
dSb
16,000
16,000
16,000
Ngas
Nstar
NDM
ofmotionare integratedusinga leap-frogalgorithm,with a fixed
time step of 0.5 Myr. With these choices, the relative error in the
conservation of the total energy is close to 10−6per time step.
The nomenclature adopted for our simulations consists of a
string of 13 characters: the first three (gS0, gSa or gSb) for the
primary galaxy and the following three for the satellite galaxy
(dE0, dS0, dSa or dSb), followed by the encounter identifica-
tion string (see Chilingarian et al. 2010, Table 9) and the orien-
tationofthe disk oftheprimarygalaxywith respectto the orbital
plane (33 or 60 degrees). Multiple mergers are indicated by two
more characters– tn, where n is the numberof satellites accreted
by the primary galaxy. For example, the string gSadSat202dir33
refers to a gSa galaxy accreting two dwarf dSa galaxies, whose
initial orbital parameters are those corresponding to the string
02dir in Table 9 of Chilingarian et al. (2010).
3. Results
In Paper I, we showed that during dissipationless minor mergers
the orbital AM is redistributed into internal AM of the interact-
ing galaxies in an outside-in manner: the initially non-rotating
dark halos of primary and satellite start to acquire AM first,
just after the first pericenter passage between the two galaxies,
while the strongest changes in the primarystellar disk take place
mostly in the final phase of the collision, when the satellite is
close to merging. All our simulations also showed that the stel-
lar disk loses part of its initial AM. The aim of this section is to
investigate if these results are also valid for dissipative mergers,
if differences can be found in the rotational support of the dif-
ferent stellar populations in the primary disk once the merger is
completed and what their main kinematic characteristics are.
3.1. Gas, old stars, new stars and the evolution of their
angular momenta
As discussed in § 2, our model galaxies contain both “old stars”,
which were already present before the start of the interaction,
and “new stars” formed from gas during the interaction. We will
followthenomenclatureofoldandnewstarsthroughoutthetext.
An example of the spatial distribution and morphologicalevolu-
tion of these two stellar components, as well as the dissipative
gas component, is shown in Fig.1. Before the first close passage
of the satellite, which occurs at 0.5 Gyr, gas and new stars are
distributed in a morphologicallythin and dynamically cold disk,
and old stars in a thicker and hotter component and in a central
bulge. At 0.65 Gyr, the primary disk shows signs of a tidal per-
turbation: some gas and new stars are clearly displaced from the
midplane of the primary,and the old stellar disk starts to thicken
even further. Note also that the satellite morphology has already
beenstronglymodifiedat this time. Signs of tidal perturbationof
the primarygalaxyarevisible all alongthe mergingsequenceaf-
terthefirst closepassageofthesatellite.At 2.6Gyr,ormorethan
1 Gyr after the merger ended, its morphology has clearly been
profoundly modified: a thick disk made of old stars has been
formed (see Qu et al. 2011, for a description of its morphologi-
cal properties)whereas gas andnew stars are still distributedin a
thin disk componentwith some perturbations at large radii, such
as a warp and a tidal tail.
During the interaction, the effects of dynamical friction and
tidal forces redistribute the AM in the system: both the primary
and the satellite galaxy acquire part of the orbital AM, and, as
already found in Paper I, the most extended regions experience
thesechangesfirst. Inparticular,theimpactofaminormergeron
the AM content of the primary galaxy is shown in Fig. 2, where
the evolution with time of the specific AM of old stars, gas, new
stars anddark matteris shown fordifferentregionsof the galaxy.
The figure shows clearly that:
– the outermost regions of the initially non-rotating dark halo
are the first to acquire part of the orbital AM, already after
the first pericenter passage, at t = 0.5 Gyr, while inside 3Rd
(the initial disk scale length Rdis 4.8, 4.8 and 5.2 kpc for S0,
Sa and Sb type galaxies, respectively) tend to increase their
AM only during the final phases of the merger;
– the AM of the baryonic components is also affected by the
interaction. The old stars experience a slowing down of their
rotation at all radii, especially outside 0.5Rd. A less pro-
nounced decrease of the AM is visible in the gas (and new
stars) between Rdand 3Rd. For example, in the merger case
shown in Fig. 2 the fractional decrease in the specific AM
of old stars is ∆l/l = 0.19 at 2-3Rdwhereas ∆l/l = 0.09 for
gas (and new stars) in the same region. While the outermost
regions gain AM in the innermost ones the specific AM re-
mains unchanged.
The slowing down with time of the AM content of old stars
at a given radius is not only due to disk heating, but also an
effect of the merger process (as has been previously shown by
Quillen et al. 2009). To demonstrate this effect, in Fig. 3 we
show the total AM of old stars, gas and new stars (for dissi-
pative mergers), and dark matter of the primary galaxy for three
simulations with different gas fractions in the primary disk (0,
0.1 and 0.2). In all cases, one can see a substantial decrease of
the total AM of old stars (∆L/L = 0.13 for a gSbdSb direct
merger, 0.2 for a gSadSa merger and 0.36 for a dissipationless
merger). Also the gas (and new stars) tend to lose a fraction of
their AM. At the same time, the dark halo of the primary galaxy
acquires part of the AM, which is already apparent after the first
passage of the satellite, around t = 0.5 Gyr in the simulations.
These substantial changes in AM content of the different com-
ponents are clearly an effect of the minor merger itself. This can
be seen when comparing the evolution of the merging galaxies
with that of the corresponding galaxies which were evolved in
isolation (right panels of Fig. 3). Secular processes, which dom-
inate the evolution of the isolated galaxy simulations, cause less
pronouncedchangesin theAM, withthe decreasein ∆L/L ofthe
old stellar disk beingonlyabout 20-25%of what is estimated for
the minor merger simulations. This demonstrates that disk heat-
ing due to secular processes, which increases the radial stellar
velocity dispersions, cannot be the only mechanism responsible
for the slowing down of the disks observed in the minor merger
simulations.
Figs. 2 and 3 suggest that it should be possible to find a dif-
ference in the AM content of old and new stars in the final, post-

Page 4

4Qu et al.: Minor mergers and their impact on the kinematics of old and young stellar populations in disk galaxies
Fig.1. From left to right: evolution with time of gas (upper panels) and stars (middle and lower panels) during a minor merger
between a gSb and a dSb galaxy on a direct orbit (gSbdSb01dir33). The stellar component is separated into new stars (formed
during the interaction, middle row of panels), and old stars (those already present in the galaxies before the interaction, lower
panels). In this projection the disk of the primary galaxy is viewed edge-on.
≤ Rd/2
Rd/2< r ≤ Rd
Rd< r ≤ 2Rd
2Rd< r ≤ 3Rd
3Rd< r ≤ 4Rd
Fig.2. Evolution with time of the specific AM, l, of a gSb pri-
mary galaxy during the minor merger whose evolution is shown
in Fig.1. The specific AM of old stars (left panel), gas and new
stars (middle panel) and dark matter (right panel) is shown, for
five different radial zones in the galaxy, see the legend. Rdindi-
cates the disk scale length. The specific AM is in units of 100
kpc km s−1.
mergerdisk, as is indeed the case, see Fig. 4: comparingthe spe-
cific AM as a functionof radius forgas and new stars (left panel)
and old stars (middle panel), one can see that while the AM of
the new stellar component is unchanged after the merger, that of
the old stars decreases at all radii (∆l/l = 0.23 at Rd). This is due
to the fact that, during the merging process, old stars are heated
as shown by the increasing radial and vertical velocity disper-
sion (Fig. 5), which then leads to a slowing down of the stellar
disk, as we havearguedin paperI. However,gas can dissipate its
energy,andthus can preservemainlytangentialmotion andkeep
its AM unaffected, whereas stars newly formed from this gas re-
tain the AM that was acquired by the gas. This means that at a
givenradiusthe oldandthe newstellar populationshavea differ-
ent specific AM and thusdifferentamountsof rotationalsupport.
As discussed in Paper I for dissipationless mergers, the decrease
in the rotation speed of the old stellar disk of the primary galaxy
is accompanied by a change in the distribution of the types of
old stars
gas+new stars
halo
Fig.3. Left panels: Evolution with time of the total AM of gas +
newstars (dashedline),oldstars(dottedline)andthedarkmatter
halo (dashed-dottedline) in the regionat r<20 kpc from the cen-
ter of the primary galaxy during a minor merger with a satellite
galaxy on a direct orbit. From top to bottom, the gas mass frac-
tion of both primary galaxy and merging satellite are 20%, 10%
and 0, respectively. The AM is in units of 2.2×1011M⊙kpc km
s−1. Right panels: The evolution of the AM of the corresponding
components (gas + new stars, old stars and dark matter halo) of
isolated galaxies with the same initial structure as the primary
galaxies in the minor merger simulations.

Page 5

Qu et al.: Minor mergers and their impact on the kinematics of old and young stellar populations in disk galaxies5
Fig.4. From left to right: Specific AM as a function of radius
normalizedby the disk scale length, r/Rd, of the primarygalaxy,
for gas and new stars (left panel) and old stars (middle panel)
during the minor merger simulation whose evolution is shown
in Fig.1. In each panel the black line shows the initial AM and
the red line shows the final AM. The specific AM is in units of
100 kpc km s−1. Right panel: The anisotropy parameter β as a
functionof r/Rdfor old stars in the same merger.Both the initial
(black line) and final (red line) β are shown.
Fig.5. Left panel: Ratio of tangential and radial velocity dis-
persion as a function of scaled radius r/Rdfor old stars before
(black line) and after (red line) the minormergershown in Fig.1.
Right panel: Same as the left panel but for the ratio of vertical
and radial velocity dispersion.
stellar orbits: the radial componentof the velocity dispersionbe-
comes increasingly important during the merger, thus increasing
the anisotropy parameter, β = 1−
value.Thisis alsothecase fordissipativeminormergers(Fig.4).
Independent of the amount of gas present in the primary disk or
in the satellite galaxy, minor mergers always result in a slowing
down of the old stellar disk. The fractional decrease in the spe-
cific AM at Rdis ∆l/l = 0.23 for a gSb model, ∆l/l = 0.17 for a
gSa and ∆l/l = 0.29 for a gS0.
Note that the slowing down of the old stellar disk during
the merger is also visible in the time evolution of the AM of
those stars that were initially (at t=0) present in a given radial
bin, rather than in that of stars currently present at a given ra-
dius (Fig. 6). At any given initial radius, rini, the variation of the
AM is much stronger when the stellar disk undergoes a minor
merger than when it evolves in isolation. In the minor merger
simulations, the average value of ∆l/l decreases with time, espe-
cially for stars whose initial radii exceed Rd. The largest changes
take place in the outer disk where the effect of the perturbations
due to the satellite is the greatest (as already has been shown by
Quillen et al. 2009; Bird et al. 2011).
σt2
2σr2, from its initially negative
3.1.1. The impact of multiple mergers
If a single minor merger causes the slowing down of the old
stellar component of a galaxy disk, what would be the effect of
a subsequent minor merger on it? To answer this question, we
Fig.6. Upper panels: Relative change of the specific AM with
respect to the initial value, ∆lint/lint= (lint(t)−lint(t = 0))/lint(t =
0), of old stars of a gSb galaxy as a function of initial radius
rini(scaled by Rd) during a minor merger with a dSb galaxy on
a direct orbit (gSbdSb01dir33). Also shown is the average of
∆lint/lintas a function of rini/Rdin each epoch (overlaid solid
line). The verticalbar on the left shows the colorscale indicating
the number of old stars at certain ∆lint/lintvalues. Lower panels:
The distribution of ∆lint/lint of old stars of an isolated galaxy
with the same initial structure as the primarygalaxy in the minor
merger case.
ran ten simulations in which the primary galaxy consecutively
accretes two ten times less massive satellites over a period of 3-
5 Gyr. The second minor merger produces a further decrease in
the specific AM of the old stellar population (Fig. 7). After the
first merger, the specific AM decreases at all radii, accompanied
by an increase of the β parameter. In the second merger event,
the AM content of the old disk stellar component is further de-
creased and this is accompanied by an increase in the velocity
anisotropy parameter β at all radii in the disk. Furthermore, we
ran one simulation in which the primary galaxy consecutively
accretes three ten times less massive satellites over a period of
5 Gyr. After having accreted two satellites the stellar disk of the
primary galaxy still reacted dynamically to the third satellite ac-
cretion – its specific AM decreased further, its beta parameter
increased and the slowing down of the old stellar disk did not
saturate but increased (see Fig. 7). While old stars slow down
with time, whereas stars formed during the repeated mergers
tend to show little change in their AM (Fig. 8), the difference
in AM content between stars born at different times increases
during repeated mergers with low mass satellite galaxies. This
is only partially due to the hybrid method used to implement
star formation in our simulations, in which the new stars are
continuously formed from dissipative gas which keeps the AM
mostly unchanged. Even though secular disk heating processes
also have an effect on heating newly formed stars, this effect is
significantly lower than that induced by minor mergers, as al-
ready shown in Fig. 3.

Page 6

6Qu et al.: Minor mergers and their impact on the kinematics of old and young stellar populations in disk galaxies
Fig.7. Left panel: Specific AM as a function of scaled radius
r/Rdfor the old stars of a gSa galaxy undergoingthree consecu-
tiveminormergers.Thespecific AMis shownafterthe first (dot-
ted line), the second (dashed-dotted line), and the third (dashed
line) merger, and the initial AM is shown for comparison (solid
line).Thespecific AMis in unitsof100kpckms−1. Rightpanel:
Anisotropy parameter, β, as a function of scaled radius, for the
old stars of the gSa galaxy whose specific AM is shown in the
left panel.
Fig.8. Same as Fig. 7 but for newly formed stars in the gSa
galaxy.
3.2. Rotational lag
As the slowing down is only affecting the old stellar population,
we expect to see an increasing difference between the AM con-
tent of the old and new stellar components as mergers proceed.
This is indeed the case, as shown in Figs. 9 and 10, where the
distribution of the specific AM, l, and radial and tangential ve-
locities (vrand vt, respectively) of stars between radii of 2 and
3Rdare shown after a single minor merger and after two consec-
utive mergers. We chose this radial region as it should be rep-
resentative of the solar neighborhood. To compare the effects of
consecutive mergers to those of secular evolution, we show in
Fig. 11 the same distributions for three galaxy simulations that
do not undergoa mergerbut are evolvedin isolation: for gSa and
gSb model galaxies, and for a gas-rich, unstable clumpy disk. In
this plot we show the newly formed stars as well as the entire
old star population and the contribution of old stars at heights
| z |≤1 kpc, in order to distinguish old stars distributed in a thin
disk from the total. We emphasize that this is a very simple way
to distinguish between a thin, dynamically cold stellar compo-
nent and a thick and hotter stellar component, which is based on
the analysis of the vertical properties of thick disks formed in
minor mergers (Qu et al. 2011).
The results of this study show that:
– After a single merger the thin stellar disk (i.e., at | z |≤1 kpc)
consists of two different stellar components, a young and an
old population, the latter with a lower angular momentum
than the former. In the region between 2 and 3Rd, for a gS-
bdSb direct merger, where both galaxies initially have a gas
fraction of 20%, the old stars in the thin disk have an average
specific AM which is about 10% lower than the new stars;
– If the whole old stellar population is considered, its aver-
age specific AM is about 7% lower than that of old stars at
| z |≤1 kpc, indicating that the stars in the thick old disk
(| z |≥1 kpc) contribute to decrease the specific AM and are
thus less rotationally supported than the old stars in the thin
disk;
– This difference in specific AM content of the stellar popu-
lations is reflected in different tangential velocities of their
stars in the disk, with the tangential support of stellar orbits
decreasing from the thin new disk, through the thin old disk,
to the thick old disk;
– For the same gas fraction in the progenitor disks there is
a difference between direct and retrograde mergers, in the
sense that the specific AM content of old stars is lower if the
satellite orbit is retrograde. This lower AM content can be
explained by the presence of counter-rotating material (neg-
ative l ) which is found in all retrogradeencounters analyzed
(see § 3.4 for a detailed discussion);
– If one defines the rotational lag of old stars as vlag = v∗−
vnewstars, where v∗is the tangential velocity of stars in the
thin or thick old disk, and vnewstarsis that of new stars, one
can see that it depends on the amount of gas present in the
progenitor disks: for old thin and old thick disk stars, the
higher the gas fraction in the progenitor disk, the lower the
rotational lag in the remnant.
We refer the readerto Table 3 for a completesummary of the
results found in Figs. 9, 10 and 11.
The difference between the specific AM content of old and
new stars becomesstill more pronouncedafter a second, consec-
utive minor merger,leading to a furtherincrease in the rotational
lag of both the old thin and thick disk stars, as shown in Fig. 10.
Given that minor mergers have a cumulative impact on heating
and slowing down stellar disks (see also Fig. 7), two mergers of
mass ratio 1:10 are expected to result in a difference between
the specific AM content of old and new stars that is comparable
with that of one merger of mass ratio 1:5. In other words, more
massive satellites can have a larger impact on slowing down the
stellar disk and produce a larger rotational lag between the old
and new stellar populationsthan less massive satellites (Qu et al.
2010). The substantial difference between the specific AM con-
tent of the thin and thick disk stellar populations cannot be due
to secular evolution processes alone, which produce a consid-
erably smaller variation in both l and vtin 3 Gyr of evolution
(Fig. 11). Note also that secular evolution produces a narrower
distribution of l, vrand vt, thus suggesting that also mixing and
radial migration due to secular processes may be less effective
than in minor mergers. Whereas scattering of stars by massive
clumps can induce a rotational lag (in old thin and thick disk
stars), the overall impact of scattering by mass concentrations is
(moderately) lower than that produced by a direct 1:10 merger
on a galaxy with an initial gas fraction of 20%.
3.3. Accreted and heated disk stars
3.3.1. Contribution to the rotational lag
In the previous section we saw that old stars in the thin and thick
disk of a minor mergerremnantlag with respect to the new stars,
and that this lag is higher for stars further from the galaxy mid-
plane (| z |≥1 kpc). But what is this rotational lag due to – is
it mostly associated to stars originally in the satellite or in the
primary disk, and how does it depend on the orbital parameters?
To answer these questions,we analyzed the tangential veloc-
ity vtof old stars at radii between 2 and 3Rdin four different

Page 7

Qu et al.: Minor mergers and their impact on the kinematics of old and young stellar populations in disk galaxies7
Fig.9. Histograms of the specific AM, l (in units of 100 kpc km s−1), and the radial (vr) and tangential (vt) velocities (in units of
100 km s−1) of stars between radii of 2-3Rdin a number of minor merger remnants. The top three rows show direct minor mergers,
with disk gas mass fractions increasing from 0 to 0.2. Indicated in each panel are the entire old stellar population (gray), new stars
(blue) and old stars with | z |≤1 kpc (red). The vertical dashed lines show the average values of l, vrand vtfor each of these three
components. The three lowest rows are similar, but for retrograde mergers.

Page 8

8Qu et al.: Minor mergers and their impact on the kinematics of old and young stellar populations in disk galaxies
Fig.10. Same as Fig. 9, but for two consecutive minor mergers.
Fig.11. Same as Fig. 9, but for galaxies that were evolved in isolation for 3 Gyr: a gSa (upper panels) with a gas mass fraction
fgas= 0.1 and a gSb (middle panels) with fgas= 0.2, as well as a galaxy with a much higher gas fraction of 0.5 which went through
an unstable clumpy phase (bottom panels). In the latter case the parameters are also shown after 3 Gyr, when the clumps have
dissolved through tidal effects or spiraled into the galaxy center through dynamical friction and interaction.
regions above the disk midplane: | z |≤1 kpc, 1<| z |≤3 kpc,
3<| z |≤5 kpc and 5<| z |≤10 kpc. Results from some represen-
tative mergers are shown in Fig. 12, where for each encounter
we distinguished the primary stars from those originally in the
satellite. This figure shows some interesting trends. First of all,
there is a clear differencebetweendirect and retrogrademergers.
While in both cases the average vtof old stars is lower than that
of new stars (Table 3), the behavior of vtas a function of height
z is significantly different. Direct mergers produce disks with a
tangential velocity vtwhich decreases with height, meaning that

Page 9

Qu et al.: Minor mergers and their impact on the kinematics of old and young stellar populations in disk galaxies9
Fig.12. Distribution of tangential velocities vt(in units of 100 km s−1) of old stars for six minor mergers. For each encounter a
pair of panels is shown, with stars originally in the primary disk (left) and those accreted from the satellite (right). In each panel
the histograms correspond to different heights above and below the galaxy midplane: | z |≤1 kpc (black), 1<| z |≤3 kpc (blue),
3<| z |≤5 kpc (green) and 5<| z |≤10 kpc (orange). The average values of vtin the different regions is also indicated.

Page 10

10Qu et al.: Minor mergers and their impact on the kinematics of old and young stellar populations in disk galaxies
Table 3. Kinematics of stars in the post-mergerthin and thick old thick disks and of the new stars in disk galaxiesheated by different
physical processes. For old stars in the thin and the thick disks the rotational lag is defined with respect to the tangential velocity vt
of the new stars.
old stars | z |≤1 kpc
mass
[stellar mass]
0.38
0.28
0.34
0.23
0.4
0.24
0.22
0.25
0.79
0.91
0.53
old stars | z |>1 kpc
mass fraction
[stellar mass]
0.62
0.72
0.65
0.77
0.58
0.75
0.77
0.74
0.19
0.07
0.49
new stars
heating mechanism
fgas
vt
lag
vt
lagmass fraction
[stellar mass]
–
–
0.01
0.001
0.015
0.004
0.01
0.01
0.015
0.019
0.016
vt
[km s−1]
214.
203.
208.
190.
195.
183.
182.
162.
230.
214.
203.
[km s−1]
n.d.
n.d.
42.
60.
24.
41.
68.
52.
24.
16.
17.
[km s−1]
181.
176.
175.
179.
170.
172.
155.
144.
210.
200.
183.
[km s−1]
n.d.
n.d.
75.
71.
49.
52.
95.
70.
44.
30.
37.
[km s−1]
–
–
250.
250.
219.
224.
250.
214.
254.
230.
220.
1:10 merger (direct)
1:10 merger (retrograde)
1:10 merger (direct)
1:10 merger (retrograde)
1:10 merger (direct)
1:10 merger (retrograde)
2x(1:10) merger
2x(1:10) merger
secular
secular
clumpy
0.
0.
0.1
0.1
0.2
0.2
0.1
0.2
0.1
0.2
0.5
Fig.13. Same as Fig. 12 but for an isolated, very gas-rich galaxy
(fgas = 0.5), after an unstable, clumpy phase. Note that in this
case the fourth and highest region is missing, due to a lack of
stars above/below 5 kpc from the galaxy midplane.
the rotational lag increases with z, whereas in retrograde merg-
ers vtis constant with height up to z∼5 kpc, and decreases only
at greater heights. The satellite stars, which in each region con-
stitute only a small percentage of the total stellar content, do not
contributesignificantlyto the lag. Moreover,if onecomparesthe
tangential velocities of satellite and primary stars for direct or-
bits, one can find a variety of behaviors in the regions analyzed:
in some cases the tangential velocities of satellite stars are al-
ways smaller than those of stars from the primary (e.g. for orbit
“gS0dS001dir33”),while in other cases the values are compara-
ble, and satellite stars show even higher velocities than primary
stars in the outer regions (e.g. orbit “gSadSa01dir33”).In all the
cases, however, the strongest variations in vtas a function of z
are associated with stars originally in the primary rather than
from the satellite. For retrograde orbits, the tangential velocities
of satellite stars show only small variations with increasing z,
as is the case for primary stars. We note also that a stellar thick
disk formed in an unstable clumpy galaxy is characterized by
tangential velocities whose variation with z is very similar to
that produced in a direct encounter with fgas = 0.2 (compare,
e.g. Fig. 13 with orbit “gSbdSb01dir33”in Fig. 12).
3.3.2. How to distinguish satellite stars
Villalobos & Helmi (2009) recently proposed an interesting
method to use the vertical AM to discriminate between stars
formed in the primary thin disk and then heated by a minor
merger from those originally in the satellite and then accreted.
Their simulations showed (see their Fig.6) that satellite stars in
the height zone | z |≤1 kpc have a typical AM content which
is constant with radius, while stars from the primary disk tend
to have an AM l ∝ r. Their simulations are gas-free and in-
volve satellites more massive than the ones studied in this pa-
per. Because of these differences we decided to reinvestigate
the validity of their method, exploring if the result is similar for
larger mass ratios like ours (1:10) and for other satellite and pri-
mary galaxy morphologies. Our results for the specific AM of
stars originally in the primary disk and in the satellite galaxy are
shown in Figs. 14 and 15. We divided the stars into two regions,
one at | z |≤1 kpc and another at | z |≥1 kpc. The general result
confirms that the difference between the specific AM of satellite
stars and primary disk stars is significant in the outer disk re-
gions: typically the difference is 20% at radii of 1.5-2Rd. But at
these radii the fraction of satellite stars with | z |≤1 kpc is only a
fewpercentofthe totalnumberofstars. This meansthatit canbe
extremely difficult to detect these stars in an observational sam-
ple. We find, however, that the fraction of satellite stars is five to
ten times larger at greater heights: at | z |≥1 kpc, outside a radius
of 1.5-2Rdthe specific AM content of accreted stars can be sig-
nificantly different from that of primary stars, and their fraction
sufficiently higher (around 10%) to make the observational de-
tectionofthis signaturemorelikely.Notehoweverthatgenerally
the satellite stars with the lowest AM content are found very far
fromthe galaxymidplane(| z |≥5kpc).We want to pointout that
there are also cases in which there is no discernible difference in
the specific AM of satellite and primary disk stars at any radius,
even at the largest radii plotted in Figs. 14 and 15). This is in
agreement with what was found in Fig. 12, where primary and
satellite stars show remarkably similar tangential velocities.
Obviously, the specific AM becomes a better discriminant
if satellite orbits counter-rotate with respect to the primary disk
– a clear sign of an external origin of stars. We find counter-
rotation in all merger remnants which formed from satellites on
retrograde orbits (some examples are shown in Fig. 15). We will
discuss the main properties of counter-rotating stars further in
the next section.
3.4. Counter-rotation
Counter-rotating stellar disks are observed in external galaxies
(see Yoachim & Dalcanton 2005) and counter-rotation is also

Page 11

Qu et al.: Minor mergers and their impact on the kinematics of old and young stellar populations in disk galaxies11
Fig.14. Specific AM, l, as a functionof scaled radius for old stars in three direct mergerremnants. For each encountera pair of plots
is shown, where stars are selected on their heights above (or below) the galaxy midplane: | z |≤1 kpc (upper panels) and | z |>1 kpc
(lower panels). In each panel stars originally in the primary galaxy are shown as black points, stars originally in the satellite as blue
points, and satellite stars with | z |>5 kpc as red points. The specific AM is in units of 100 kpc km s−1.
Fig.15. Same as Fig. 14, but for three retrograde mergers.
foundin the MilkyWay halo (see Carollo et al. 2007). Thepres-
ence of stars (in the thick disk or in the halo) rotating in a direc-
tionwhichis reverseofthegalaxymainspincannotbeexplained
with secular evolution processes, and is a strong evidence for an
external origin.
It is thus of great interest to investigate the characteristics
of counter-rotating stars in our merger remnants – specifically,
their fraction and vertical distribution with radius. Examples are
shown in Figs. 16 and 17 for a number of dissipationless and
dissipative retrograde mergers. In these plots four different re-
gions are selected: (1) at | z |≤1 kpc, (2) at 1≤| z |≤3 kpc,
(3) at 3≤| z |≤5 kpc and (4) at 5≤| z |≤10 kpc. It is clear that
counter-rotatingstarsarefoundatallvaluesofz,fromthegalaxy
midplane to 10 kpc above it, and that the probability to find
counter-rotatingstars increases with height: the fraction Nct/Ntot
of counter-rotating stars to the total number of stars in the re-
gion is ≤5% up to | z |≤3 kpc, typically around 10% in the
3 kpc≤| z |≤5 kpc and it can reach 30% in the region highest

Page 12

12Qu et al.: Minor mergers and their impact on the kinematics of old and young stellar populations in disk galaxies
Fig.16. Fraction of counter-rotating stars (shown in blue) for four dissipationless retrograde mergers. Stars are divided into four
regions, according to their height above (or below) the galaxy midplane: (1) at | z |≤1 kpc, (2) at 1 kpc≤| z |≤3 kpc, (3) at
3 kpc≤| z |≤5 kpc and (4) at 5 kpc≤| z |≤10 kpc. For each region the fraction of satellite stars to the total number of stars is also
shown (gray).
Fig.17. Same as Fig.16, but for four dissipative retrograde mergers.
Fig.18. Same as Fig.16, but for a gS0 galaxy after a single ret-
rograde merger (left panel) and after two consecutive retrograde
mergers (right panel) with a dE0 satellite.
above the disk. This trend is also due to the fact that the fraction
of satellite stars Nsto the total number of stars increases with z.
Inotherwords,it is the overallincreasein thefractionofsatellite
stars with z which determines this trend, not simply a decrease
in the numberof disk stars as a functionof heightabovethe disk.
Furthermore the fraction of counter-rotating stars depends also
on the initial orbital inclination of the satellite: for a satellite on
an orbit inclined by 60 degrees with respect to the primary disk,
this fraction is lower at all z (Villalobos & Helmi 2008).
If counter-rotating stars are a natural outcome of low orbital
inclination in single minor mergers with satellites on retrograde
orbits, would a second retrograde merger then increase the frac-
tion of counter-rotating stars even further at all heights? In our
models (Fig. 18) the effect of a second retrograde merger with
the same mass ratio is to increase the fractionof counter-rotating
stars by a factor of about two, at all heights. We can conclude
from this that repeated retrograde minor mergers have a cumu-
lative effect on the number of counter-rotating stars in the thick
disk and inner halo.
3.5. Kinematics of the remnant disk
The AM evolution of the baryonic component during merging
processes discussed in the § 3.1 is naturally reflected in an evo-
lution of the line-of-sight velocity vlos and the vlos/σv ratio.
Minor mergers always produce a decrease in vlos and an in-
crease in the velocity dispersion σv of the stellar component
at all radii (Fig. 19). The remnant stellar disks usually have a
smaller vlos/σvratio than the initial values before interaction.
Consecutive satellite accretions have cumulative effects on de-
creasing the vlosand increasing the σv, and thus deceasing the
vlos/σvratio. Nevertheless, smaller variations in both vlosand
σvcan be found in minor mergers with gas, compared to gas-
free merger cases, due to the fact that dissipative gas can help
preserve disk rotation by forming new rotating disk stars during
mergers. The decreases in vlosand the vlos/σvratio indicate that
stellar disks become hotter and rotate more slowly after minor
merger events, which is consistent with the AM loss and the in-
crease of the anisotropy parameter β (Figs. 4 and 7).
4. Discussion and Conclusions
By means of N-body/SPH simulations, we studied the kinemat-
ics of stars in galaxyremnantsof 1:10 mass ratio minor mergers.
The simulated interactions span a range in gas mass fractions,
from 0 to 0.2 in the primary and satellite galaxies, and a range of
orbital parameters.
As shown in Qu et al. (2010), minor mergers result in a re-
distribution of orbital into internal angular momentum, which
affects all galaxy components. In particular, old stars, i.e. those
already in place before the interaction, always lose angular mo-
mentumduringthemergingprocess.Thedecreaseofthespecific
AM of old stars is accompanied by a redistribution of stellar
orbits, as traced by the anisotropy parameter β, which become
increasingly radial. In minor mergers with gas in the disk of the
primarygalaxywe find a similar trend of old stars losingangular
momentum and as a result their orbits becoming more radially
dominated. However, when a new stellar component forms from
gaspresentintheprimarydiskduringthemerger,its AMcontent
is significantly different: the orbits tend to be more tangentially
dominated, thus providing a higher rotational support. This dif-
ferent behavior results in a final stellar disk with two different
stellar populations with significantly different AM content. In
particular, old stars always show a rotational lag with respect to
the young stellar component. If one separates all stars into thin

Page 13

Qu et al.: Minor mergers and their impact on the kinematics of old and young stellar populations in disk galaxies 13
disk stars (at heights | z |≤1 kpc from the galaxy midplane) and
thick disk stars (at | z |>1 kpc), three different components can
be found, with different dynamical properties: (1) young stars
in the thin disk, which are rotationally supported and show the
highest values of vt, (2) old thin disk stars lagging with respect
to the new stars and (3) old thick disk stars lagging with respect
to both thin disk components. For a minor 1:10 merger, with a
satellite accreted on a direct orbit and with an initial primary
disk gas fraction of 0.2, the old stars in the thin disk have a ro-
tational lag of about 20 km s−1, while the old stars in the thick
disk havea velocityabout50 kms−1lower thanthe youngstellar
component, both lag values being compatible with the estimates
for the Milky Way (see Gilmore et al. 2002). Multiple mergers
can further reduce the tangential velocity of the old stellar com-
ponents, while leaving that of the new stars mostly unchanged,
thus resulting in a further increase in rotational lag with every
successive accretion episode. As the two populations, old stars
and new stars, have different tangential velocities, in a plot of vt
as function of age we expect to find a discontinuity at the time
when the merger occurs. Of course, the newly formed stars will
evolve in time, and if no other merger takes place they will be
slowly heated by secular effects. However, as we have shown in
Fig.3,secularprocessesaremuchless effectiveinalteringstellar
kinematics than minor mergers. Therefore, in our opinion, a dis-
continuity in the age-vtplane (or the age-β parameter) between
the old and new stellar populationsshould still be visible, even if
secular processes, and asymmetric drift in particular, contribute
to the slow heatingof the new stellar populations.Unfortunately,
the hybrid particle method we adopted to implement star forma-
tion in the simulations does not allow us to follow properly the
heating of the newly formed stars with time. We do think, how-
ever, that this discontinuity in the age-vtbehavior may be a dis-
tinctive feature of minor mergers compared to secular processes
in galaxy disks, which deserves to be modeled accurately in the
near future.
Minor mergers can quantitatively reproduce the increase in
the rotational lag with height above and below the galaxy mid-
plane that was found for the Milky Way (Chiba & Beers 2000;
Girard et al. 2006), but only if the satellite is accreted along a di-
rect orbit; for retrogradeorbits no trend of the rotationallag with
z is found, up to distances of 5 kpc from the midplane, above
which it even decreases slightly. Together with the increasing
trendof stellar eccentricities with height(Di Matteo et al. 2010),
thisis anotherpieceofevidencesuggestingthatiftheMilkyWay
thick disk has formed through heating of a pre-existing thin disk
by minor merger(s),the orbit of the satellite(s) should have been
prograde.
Recently, Bournaud et al. (2009) proposed a scenario in
which stellar thick disks could have formed in gas-rich galaxies
at high redshifts, through scattering of stars by massive clumps.
Comparing the kinematics of minor merger remnants with those
of thick disks formedin clumpy disks we find that, under certain
conditions, both scenarios can reproduce a rotational lag of the
old thin and old thick disk stars that are compatible with obser-
vations for the Milky Way. In particular, disks heated by mas-
sive clumps show a rotational lag which increases with height
z, as found for minor mergers on direct orbits. Even so, the
two processes leave distinctly different signatures in the verti-
cal surface profiles of the remnant disks, as shown in Qu et al.
(2011). Obviously, disks heated by massive clumps cannot lead
to the counter-rotating thick disks observed in external galaxies
(Yoachim & Dalcanton 2008). For this to occur, an external ac-
cretion of material (gas or stars) is necessary.
Fig.19.Line-of-sightvelocity,v, velocitydispersion,σv, andthe
v/σvratio as a function of scaled radius in remnant stellar disks
after both a single and two consecutive dissipative minor merg-
ers. The same data are also shown for the initial stellar disks,
before merging. The gas mass fraction of the merging galaxies
increases from the left to the right, from 0 for the S0 case, to 0.2
for the Sbs.
Counter-rotating material is found in all our simulations
where the satellite is accreted along a retrograde orbit. The
amount of counter-rotating stars (all associated to stars initially
in the satellite galaxy) increases with z as a result of the increas-
ing contributionofsatellite stars to the wholestellar content,and
at z = 3 − 5 kpc the fraction of counter-rotating stars is between
10% and 15%. We showed that the effect of multiple mergers
on the generation of a counter-rotating disk is cumulative, in the
sense that the fraction of counter-rotating stars doubles after the
accretionofasecond,comparativelymassive,satelliteonaretro-
grade orbit. It is thus possible that some of the galaxies observed
to have a high fraction of counter-rotating material in the thick
disk(Yoachim & Dalcanton2005)couldhaveaccreteda number
of satellites onretrogradeorbits.The fractionof counter-rotating
stars we find in our minor merger simulations is quantitatively
consistent with the amount of counter-rotation observed in the
thick disks of nearby galaxies (Yoachim & Dalcanton 2005).
Finally, we discussed the possibility of discriminating stars
accreted from the satellite galaxy from those formed in the pri-
mary disk on the basis of their AM content, as recently proposed
by Villalobos & Helmi (2009). We find that in general the AM
content is a better discriminant for stars located at vertical dis-
tances from the galaxy midplane greater than 1 kpc rather than
at smaller heights, as proposed by Villalobos & Helmi (2009).
Moreover, as the fraction of satellite stars increases with z in
our models, their detection at high z should also be more likely.
However,we want to pointout that it may notalways be possible
to use the AM content to distinguish between stars originally in
the primary and those accreted from the satellite because there
are cases where no difference in their AM content is seen at any
radius.
Overall, the qualitative and some quantitative agreement
with observations of the thick disk of the MW and other nearby
galaxies suggest that minor merger remains a viable mechanism
for forming the thick disk. With future observations, it may be
possible to search for other signs of the impact of minor merg-
ers on disk galaxies, such as determining the relative angular
momentum of stars as a function of age in nearby disk galax-
ies. Minor merger models, such as those discussed here, make
specific predictions which can be tested observationally(e.g., by
GAIA).

Page 14

14Qu et al.: Minor mergers and their impact on the kinematics of old and young stellar populations in disk galaxies
Acknowledgments
YQ and PDM are supported by a grant from the French Agence
Nationale de la Recherche (ANR). We are grateful to Benoˆ ıt
Semelin and Franc ¸oise Combes for developing the code used in
this paper and for their permission to use it. These simulations
willbemadeavailableas partoftheGalMersimulationdatabase
(http://galmer.obspm.fr). We wish to thank Gary Mamon for his
extensive and useful comments and the anonymous referee for a
careful, in-depth and constructive report which helped improve
the paper considerably.
References
Athanassoula, E. 2005, NYASA, 1045, 168
Barnes, J. E. 1992, ApJ, 393, 484
Bendo, G. J., & Barnes, J. E. 2000, MNRAS, 316, 315
Bird, J. C., Kazantzidis, S., & Weinberg, D. H. 2011, arXiv1104.0933B
Bournaud, F., Chapon, D., Delaye, L., et al. 2011, ApJ, 730, 4
Bournaud, F., Combes, F., Jog, C. J., & Puerari, I. 2005a, A&A, 438, 507
Bournaud, F., Combes, F., & Semelin, B. 2005b, MNRAS, 364, 18
Bournaud, F., Elmegreen, B. G., & Martig, M. 2009, ApJ, 707L, 1
Bournaud, F., Jog, C. J., & Combes, F. 2005c, A&A, 437, 69
Carollo, D., Beers, T. C., Lee, Y. S., et al. 2007, Nature, 450, 1020
Chiba, M., & Beers, T. C. 2000, AJ, 119, 2843
Chilingarian, I., Di Matteo, P., Combes, F., Melchior, A.-L., & Semelin, B. 2010,
A&A, 518, 61
Crocker, A. F., Jeong, H., Komugi, S., et al. 2009, MNRAS, 393, 1255
Debattista, V. P., Mayer, L., Carollo, C. M., et al. 2006, ApJ
Di Matteo, P., Bournaud, F., Martig, M., et al. 2008a, A&A, 492, 31
Di Matteo, P., Combes, F., Melchior, A.-L., & Semelin, B. 2008b, A&A, 477,
437
Di Matteo, P., Jog, C. J., Lehnert, M. D., Combes, F., & Semelin, B. 2009, A&A,
501, 9
Di Matteo, P., Lehnert, M. D., Qu, Y., & van Driel, W. 2010, A&A, 525, L3
Fakhouri, O., & Ma, C.-P. 2008, MNRAS, 386, 577
Gilmore, G., Wyse, R. F. G., & Norris, J. E. 2002, ApJ, 574, 39
Girard, T. M., Korchagin, V. I., Casetti-Dinescu, D. I., et al. 2006, AJ, 132, 1768
Hopkins, P. F., Cox, T. J., Younger, J. D., & Hernquist, L. 2009, ApJ, 691, 1168
Ibata, R., Irwin, M., Lewis, G., Ferguson, A. M. N., & Tanvir, N. 2001, Natur.
Jesseit, R., Cappellari, M., Naab, T., Emsellem, E., & Burkert, A. 2009,
MNRAS, 397, 1202
Jog, C. J., & Maybhate, A. 2006, MNRAS, 370, 891
Klement, R. J. 2010, A&ARv, 18, 567
Mapelli, M., Moore, B., & Bland-Hawthorn, J. 2008, MNRAS, 388, 697
Martinez-Delgado, D., Gabany, R. J., Crawford, K., et al. 2010, AJ, 140, 962
McConnachie, A. W., Irwin, M. J., Ibata, R. A., et al. 2009, Nature, 461, 66
Mihos, J. C., & Hernquist, L. 1994, ApJ, 427, 112
Minchev, I., Famaey, B., Combes, F., et al. 2011, A&A, 527, 147
Naab, T., & Burkert, A. 2003, ApJ, 597, 893
Puerari, I., & Pfenniger, D. 2001, Ap&SS, 276, 909
Qu, Y., Di Matteo, P., Lehnert, M., & van Driel, W. 2011, A&A, 530, 10
Qu, Y., Di Matteo, P., Lehnert, M., van Driel, W., & Jog, C. J. 2010, A&A, 515,
11
Quillen, A. C., Minchev, I., Bland-Hawthorn, J., & Haywood, M. 2009,
MNRAS, 397, 1599
Reichard, T. A., Heckman, T. M., Rudnick, G., et al. 2009, ApJ, 691, 1005
Semelin, B., & Combes, F. 2002, A&A, 388, 826
Toomre, A. 1977, Evolution of Galaxies and Stellar Populations, Proceedings
of a Conference at Yale University, May 19-21, 1977. Edited by Beatrice M.
Tinsley and Richard B. Larson. New Haven: Yale University Observatory
Villalobos, A., & Helmi, A. 2008, MNRAS, 391, 1806
Villalobos, A., & Helmi, A. 2009, MNRAS, 399, 166
Yoachim, P., & Dalcanton, J. 2005, ApJ, 624, 701
Yoachim, P., & Dalcanton, J. J. 2008, ApJ, 682, 1004