Issue |
A&A
Volume 518, July-August 2010
Herschel: the first science highlights
|
|
---|---|---|
Article Number | A56 | |
Number of page(s) | 11 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/201014304 | |
Published online | 02 September 2010 |
The dilution peak, metallicity evolution, and dating of galaxy interactions and mergers
M. Montuori1 - P. Di Matteo2 - M. D. Lehnert2 - F. Combes3 - B. Semelin3
1 - SMC-ISC-CNR & Physics Department, Univ. Roma ``La Sapienza'', Pl. Aldo Moro 2, 00185 Rome, Italy
2 - Observatoire de Paris, section de Meudon, GEPI, 5 Place Jules Jannsen, 92195 Meudon, France
3 - Observatoire de Paris, LERMA, CNRS, UPMC, 61 Avenue de l'Observatoire, 75014 Paris, France
Received 22 February 2010 / Accepted 18 May 2010
Abstract
Strong inflows of gas from the outer disk to the inner kiloparsecs
are induced during the interaction of disk galaxies. This inflow of
relatively low-metallicity gas dilutes the metallicity
of the circumnuclear gas. This process is critical for the galaxy
evolution. We have investigated several aspects of the process as the
timing and duration of the dilution and its correlation with the
induced star formation. We analysed major (1:1) gas-rich
interactions and mergers, spanning a range of initial orbital
characteristics. Star formation and metal enrichment from SNe are
included
in our model. Our results show that the strongest trend is between the
star formation rate and the dilution of the metals in the nuclear
region; i.e., the more intense the central burst of star formation, the
more the gas is diluted. This trend comes from strong inflows of
relatively metal-poor gas from the outer regions of both disks, which
fuels the intense star formation and lowers the overall metallicity for
a time. The strong inflows happen on timescales of about 108 years
or less (i.e., on an internal dynamical time of the disk in the
simulations), and the most intense star formation and lowest gas phase
metallicities are seen generally after the first pericentre passage.
As the star formation proceeds and the merger advances, the
dilution reduces and enrichment becomes dominant - ultimately
increasing the metallicity of the circumnuclear gas to a level higher
than the initial metallicities of the merging galaxies. The
``fly-bys'' - pairs that interact but do not merge - also
cause some dilution. We even see some dilution early in the merger or
in the ``fly-bys'' and thus do not observe a strong trend between the
nuclear metallicities and
separation in our simulations until the merger is well advanced. We
also analyse the O and Fe enrichment of the ISM, and show that the
evolution of the
ratios, as well as the dilution of the central gas metallicity, can be used as a clock for dating the interaction.
Key words: galaxies: interactions - galaxies: formation - galaxies: evolution
1 Introduction
Understanding how galaxy interactions and mergers shape the ensemble population of galaxies in the local universe is crucial for our understanding of galaxy evolution. It has been recognised for several decades that mergers may play an important role in the evolution of galaxies, especially for the early types (e.g., Toomre 1977). While there have been many observations and theoretical studies of ``first-order'' effects in interactions and mergers (such as morphological and kinetical evolution and star formation), much less work has been done on second order effects such as the evolution of metallicity.
Recently, we have undertaken the study of the evolution of metallicity in major dry (i.e., gas poor) mergers, showing that they can lead to metallicity gradients in agreement with those of local ellipticals (Di Matteo et al. 2009). However, in gas-poor mergers, the evolution of the local galaxy metallicity is driven only by the mixing of stars during the coalescence process. Gas can alter the properties of the final remnant significantly, affecting both the final gas phase and stellar metal abundance and ratios. In particular, gas phase metallicity can play an important role in constraining our understanding of the complex interaction between gas flows and star formation in interactions and mergers and allow us to constrain the relative time scales of the different epochs (i.e., date the merger/interaction).
If the progenitor disks have strong gas metallicity gradients, as observed in many galaxies in the local universe (Dinerstein 1996; Shields 1990), one would expect that interaction-induced gas inflows will drive a noticeable amount of gas into the central regions from the outer disk and thus initially lower the circumnuclear gas phase metallicity. Outflows of gas ejected in tidal tails may contribute to modifying the metallicity profile of the final merger remnant. Subsequent star formation (and outflows) may increase (decrease) the gas phase circumnuclear metallicity. The exact timing of the inflows versus the initiation of intense star formation and subsequent metal enrichment may provide robust constraints on the underlying physical mechanisms that determine the rate and relative timing of gas flows and star formation within the models. Comparing the evolution of the metallicity and its relationship to both inflows and star formation will allow us to constrain the dissipation time scales, the star formation evolution, the time delay between the onset of inflows, sufficient to dilute the metallicity, etc.
In the past few years, a number of studies have investigated
the role played by interactions and environment in determining the gas
phase metallicities of galaxies. Donzelli & Pastoriza (2000)
show that merging galaxies have on average higher excitation in their
optical emission line spectra than interacting pairs, and attribute
this difference to lower gas metallicity in the mergers. Márquez et al. (2002) studied a sample of more than one hundred spiral galaxies, ranging from isolated
to interaction with strong morphological distortions, and show that the [NII]/H ratios, used as a metallicity indicator, indicate a clear trend from the metallicity to morphological type, with
earlier type spirals showing higher ratios, while they found no trend with the status of the interaction. Recently, Kewley et al. (2006)
have derived the luminosity-metallicity relation for a sample of local
galaxy pairs and compared it with that of nearby field galaxies. They
found that pairs with small projected separations (
)
have systematically lower metallicities than either isolated galaxies
or pairs with larger separations, for a given luminosity. They also
found a correlation between gas metallicity and burst strengths -
all galaxies in their interacting sample with strong central bursts
having close companions and metallicities lower than the comparable
field galaxies or pairs with wider separations. Rupke et al. (2008),
in a study of strong interactions with high star formation rates
(ultra-luminous infrared galaxies), found that the metal abundance in
these intense starbursts is a factor of two lower than that of galaxies
of comparable luminosity and mass. These results have been generally
explained by gas inflows induced by the interaction, which dilute the
pre-existing nuclear gas to produce a lower metallicity than that
observed for wider separated pairs or isolated systems. More recently, Peeples et al. (2008,2009)
have studied, respectively, a sample of low-mass, high-metallicity
and high-mass, low-metallicity outliers from the mass-metallicity
relation of star-forming galaxies selected from the Sloan Digital Sky
Survey (SDSS). They showed that
the low-mass, high-metallicity outliers are usually isolated galaxies,
with no evident companion or strong interactions. On the other hand,
the high-mass, low-metallicity outlierts typically consist of
systems that have high star formation rates and evidence for disturbed
morphologies.
However, Cooper et al. (2008)
found a strong metallicity-density relation for star forming galaxies
in the local universe, with the more metal-rich galaxies apparently
favouring regions of higher galaxy overdensity (see also Ellison et al. 2009).
They conclude that the discrepancy found with the other studies
(including those just discussed) is due to the fact that the number of
close pairs (
)
in the SDSS sample constitutes only a tiny fraction of the whole
sample (less than 1% of galaxies). In this case, close pairs with
low central metallicity could not contribute significantly to the
scatter found in the mass-metallicity relation. Of course it can
be difficult to discern trends in the gas phase metallicity during a
merger when studying large samples of galaxies in SDSS as other factors
may dominate the overall mass-metallicity relationship and its scatter.
A confirmation that the mass-metallicity relation is affected by
interactions only for close pairs showing signs of strong disturbances
has been found by Michel-Dansac et al. (2008),
who pointed
out that, in such pairs, the gas metallicity depends on the
mass ratio of the two interacting systems: less massive members are
systematically enriched, while a galaxy in interaction with a
comparable stellar mass companion shows a metallicity lower than that
of a galaxy in isolation.
So while mergers and interactions alone may not drive the overall
scatter in the mass-metallicity relationship, it may be a
contributing factor. All these studies suggest that the dilution and
enrichment of the ISM in the central regions of disk galaxies strongly
depend on the exact timing
of the different processes at play: gas inflows, interaction-driven
star formation, gas consumption, feedback and subsequent enrichment.
While a number of numerical studies have investigated the
response of the gaseous component of galaxies during tidal interactions
and the subsequent star formation such interactions induce (Di Matteo et al. 2008; Cox et al. 2008; Di Matteo et al. 2007; Kapferer et al. 2005; Mihos & Hernquist 1994a; Cox et al. 2006; Iono et al. 2004; Springel 2000),
little attention has been given to the detailed evolution of the metal
content during galaxy encounters. Using a galaxy pair catalogue from
cosmological simulations, Perez et al. (2006) have shown that the O/H abundance ratio in the central regions of close galaxy pairs (
)
shows a lower level of enrichment than the mean O/H abundance
ratio of a control sample, thus confirming the role played by gas
inflow in diluting the metal content of the nuclear regions. Recently, Rupke et al. (2010)
have analysed simulations of major galaxy mergers, studying the
dilution of the gas metallicity in the nuclear regions due to gas
inflow. They found a dilution of about 0.1-0.3 dex,
happening shortly after the first pericentre passage between the two
galaxies. However, their models do not include either star formation
prescriptions or metal enrichment, so that while it has been
possible to give predictions about the strength of the dilution,
nothing is still known about the role played by interaction-induced
star formation in the metal dilution and then subsequent enrichement.
Moreover, the exact timing of the dilution peak and its
correlation with the increase in the amplitude of the star formation
have not been studied yet in any detail.
2 Models and initial conditions
Here we study mergers and flybys involving two massive
Sbc galaxies, having a mass ratio 1:1. The Sbc galaxies
(hereafter called gSb) are composed of a spherical dark matter
halo and a spherical
bulge, represented by Plummer spheres (Binney & Tremaine 1987) with total masses
and
and core radii
and
respectively. The stellar disk is represented by a Miyamoto-Nagai density profile (Binney & Tremaine 1987) with mass
M*=40.6
,
vertical and radial scale lengths given respectively by
and
.
The galaxies
initially contain a gas mass
,
redistributed in a Miyamoto-Nagai disk with vertical and radial scale lengths given respectively by
and
.
For each pair of interacting galaxies, we performed
24 simulations, varying the galaxies orbital initial conditions
(initial orbital energy E and angular momentum L) and taking
into account both direct and retrograde orbits. The initial orbital
parameters (initial distance between the two galaxies, initial relative
velocity, specific angular momentum, orbital energy and spin) for the
different runs are fully described in Table 7 of Chilingarian et al. (2010), and we refer the reader to this paper for their complete description.
![]() |
Figure 1: The dynamics of the gas during the interaction and subsequent merger of two gSb galaxies for several orbits (shown with different colours). (First panel): relative distance of the two galaxy centres versus time for some simulated orbits. (Second panel): gas mass inside 2 kpc from one of the two galaxy centres versus time. (Third panel): evolution of the star formation rate (SFR) relative to the rate for the corresponding initial galaxies evolved in isolation. (Fourth panel): evolution of the central gas metallicity. The metallicity has been normalised to that of the initial galaxies used in the merger simulations evolved isolated. In the second, third, and fourth panel, squares and asterisks respresent respectively the time of the first pericentre passage and the time of final coalescence for the different orbits. |
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We chose a reference frame with its origin at the barycentre of the system and x-y plane
corresponding to the orbital plane. For each interacting pair, we
have kept the disk of one of the two galaxies in the orbital plane (
), and varied the inclination i2 of the companion disk, considering:
,
,
,
and
.
The orbital angular momentum can be parallel (direct orbit) or anti-parallel (retrograde orbit) to the z-axis of the reference frame.
All the simulations (96 in total) were run using the Tree-SPH code described in Semelin & Combes (2002). Each galaxy contains a total of
particles,
equally redistributed among gas, stars and dark matter. A Plummer
potential is used to soften gravity at small scales, with constant
softening lengths of
pc for all particles. The gas is modelled as isothermal, with a temperature
.
The assumption of an isothermal ISM is justified by the short cooling
times in the gaseous disk at the resolution of our models, such that
fluctuations in the gas temperature, those due to SN explosions
for example, would be quicky radiated away
(see the discussion in, e.g., Mihos & Hernquist 1996).
We refer the reader to Semelin & Combes (2002) for a detail description of and the tests performed to validate the code. Other tests have been presented in Di Matteo et al. (2008). The equations of motion are integrated using a leapfrog algorithm with a fixed time step of
.
![]() |
Figure 2: Same as Fig. 1, but for the simulations of ``flybys''. Note that simulations resulting in starbursts with higher intensities also have gas phase metallicities with higher dilution. Even in flybys, the star formation rate and dilution of the metallicity are related. |
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2.1 Star formation and metal enrichment
The numerical recipes used to implement star formation and metal enrichment have been fully described elsewhere (Chilingarian et al. 2010; Di Matteo et al. 2007),
thus here we only summarise those features that are most directly
relevant to our analysis. Star formation efficiency has been
parametrised as:
where




The effect of star formation on the surrounding interstellar medium
has been implemented as follows. For each star-forming hybrid
particle, we evaluate the fraction of stars formed with masses
,
adopting a Miller & Scalo (1979)
initial mass function (IMF), and we assume that stars above this mass
threshold instantaneously become supernovae. Each of the SN leave
behind a remnant of
and releases their remaining mass to the surrounding ISM. The mass released
also enriches the surrounding gas with metals. This is done assuming a yield
= 0.02, where
is the total mass of all reprocessed metals and M* the total mass in stars. For each gas particle, the return of mass and metals is applied to the ith neighbour gas particle, using a weight wi based on the smoothing kernel. The metallicity is initially distributed in a gradient of the form:
with R the particle distance from the galaxy centre and

Supernovae explosions also inject energy into the surrounding ISM. This is taken into account assuming that a fraction,
,
of the energy,
,
released by a SNe goes into kinetic energy giving a radial kick to the
velocities of neighbouring gas particles. The value of
has
been chosen such that the total amount of kinetic energy received by a
gas particle due to the contribution from all the surrounding
neighbours, is less than 1 km s-1,
thus preventing a rapid growth of the vertical thickness of the gaseous
disk. A more detailed description of the implementation of
feedback recipes used in our code is given in Chilingarian et al. (2010); Semelin & Combes (2002).
3 Results
3.1 Gas flows: where and when
It is known that close passages of galaxies can destabilise galaxy disks, producing asymmetries, such as bars, which are efficient mechanisms for driving gas into the circumnuclear region of a galaxy, producing a burst of star formation (Combes 2008; Iono et al. 2004). If, prior to the interaction, the gas is distributed in the disk with a negative metallicity gradient, with the outer regions being relatively more metal poor than the inner ones as observed in local galaxies (Dinerstein 1996; Shields 1990), then a portion of this low-metallicity gas will fall into the nuclear regions, diluting the pre-existing gas with gas of lower metallicity. This dilution will last until the star formation, which is enhanced by the gas inflow, releases reprocessed metals to enrich the gas.
This cycle (dilution and enrichment) is shown in Figs. 1 and 2
for some of our galaxy merger simulations. For the analysis of the
effect of mergers on the metallicity of galaxies, we have defined the
metallicity dilution (often referred to as simply dilution) as
,
where z is the metallicity of a galaxy in the pair during the merger or flyby and
is the corresponding value for the same galaxy evolved isolated. Both z and
are
measured in apertures of 1-2 kpc and thus represent the evolution
of the metallicity for the nuclear or circumnuclear regions of galaxies
(which also corresponds reasonably well with the aperture sizes used to
estimate the metallicities of galaxies; see discussion in
the Sect. 1).
We find that as soon as the two galaxies have undergone their
first pericentre passage, an intense inflow of gas, lasting for
300-400 Myr, takes place. A detailed discussion of the
mechanism
generating the gas inflows we observe in our simulations is given in Di Matteo et al. (2007).
They show that each close passage of the two interacting galaxies is
accompanied by the amplification of stellar asymmetries - stellar
bars. These stellar bars are responsible of removing angular momentum
from gas, transferring it to the stellar component, and in regulating
the subsequent inflow of gas in the inner kpc. We note that this result
has already been discussed in several previous studies using
simulations (see for example Mihos & Hernquist 1994a,b, Mihos & Hernquist 1996).
This mechanism does not require any stellar feedback to work,
that is, the gas inflows are a natural consequence of the dynamics
of the interaction and the redistribution of the angular momentum that
is induced during the progression of the merger (Rupke et al. 2010; Di Matteo et al. 2008). This intense inflows can deposit in few 108 ys into the central (
)
regions
of gas, originally located before the interaction at an average distance of
from the galaxy centre. The amount and origin of the inflowing gas
obviously depends on characteristics of the merger like orbital
parameters of the interaction and gas fraction (see Di Matteo et al. 2007,
for a discussion). In all cases, this inflow of gas fuels the
first increase in the star formation rate during the merging process.
A second, sometimes even stronger peak of SF takes place in the
final phases of the interaction when the two galaxies are close
to coalescence.
![]() |
Figure 3: The duration that the circumnuclear metallicity is below 0.8 (shown in red) and between 0.8 and 1.0 (shown in black) relative to an isolated simulated spiral with the same properties as the individual merging galaxies. We chose 0.8 as the threshold because this is the span of the most extreme metallicity offset observed in nearby mergers (e.g. Rupke et al. 2008; Kewley et al. 2006). |
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The time of strong enhancement in the SFR also coincides with the time
of high-dilution. Typically, we find that the dilution lasts for 2
109 years, with half of the sample substaining a
for less than 5
108 (Fig. 3).
In the
case of fly-bys, this dilution is still visible far after the
pericentre passage, when the separation between the two galaxies is
several hundreds of kpc. We emphasise that the gas redistribution
and subsequent dilution takes place on scales of several kiloparsecs
and is not due to the effects of stellar feedback which typically act
over much smaller physical scales. Moreover, significant star formation
is initiated only
after a substantial gas inflow has occurred. So feedback from
SNe explosion has little effect on the inflowing gas and certainly
is unable to prevent substantial inflows of gas initially.
A possible concurrent physical mechanism for redistributing
metals in the interstellar medium of the galaxy could be through gas
diffusion, which is not included in our code.
At scales of hundreds of parsecs (which corresponds roughly to our
numerical resolution), the diffusion law can take the form of a
heat equation, with a diffusion coefficient D=9.25
(see Martínez-Serrano et al. 2008). This means that the typical timescale for diffusing metals from the outer regions of the disk (R = 7 kpc
) is
.
This timescale is significantly longer than the average inflow
timescale which is of-order 300-400 Myr. Thus, we can safely
assert that the process of dilution of the central gas
metallicity studied here is driven by interaction-driven inflows rather
than metal diffusion.
Once the gas inflow and the corresponding burst of star
formation is over, the re-processed gas released by
SNe explosions starts to enrich the central metallicity,
so that the
ratio becomes greater than one (Figs. 1 and 2,
fourth panel). We checked that during the interaction, the contrast in
the gas density between the nuclear region and the outer disk are
sufficient to result in gas depletion times that are significantly
lower than the times typically required to re-enrich the ISM in metals.
This substantiates
our interpretation that the dilution in the metallicity of gas in the
nucleus is due to strong merger-driven gas inflows.
3.2 Relationship between the star formation and metallicity
We have seen that the dilution of the gas-phase metallicity takes place at approximately the same time as the strongest enhancement in the star formation. In general, we find that the peak of SF and the maximum dilution are roughly coeval, with a delay between the two of 108 yr at most. This correlation between the starburst phase and the strong dilution is clearly seen in the results of the simulations shown in Fig. 4. The maximum dilution is reached in the simulations at approximately the peak in the star formation rate. In this phase, the higher SFR, which is a sign of the strongest gas inflow are indeed associated with the strongest dilution as would be expected. We note that a strong correlation between metallicity and central burst strength has been observed by Kewley et al. (2006) in a sample of galaxy pairs. Specifically, they pointed out that all five galaxies in the pairs sample with strong central bursts have close companions and metallicities lower than the comparable field galaxies.
Before the peak in the star formation rate, galaxies tend to populate a region of low to moderate SFRs and dilutions (typically
;
see Fig. 4). During the burst phase and peak of star formation rate, the ejecta of the SNe starts to enrich the gas, and
indeed, once the burst is over, galaxies have low star formation rates and high-metallicities (Fig. 4).
The first two phases of the relation between star formation and
dilution are associated with before, during, and after the first
pericentre passage and then close to final coalescence of the merging
galaxies, the last phase in the metallicity evolution is
always associated with well after their first pericentre passage and is
near or after coalescence. This final state of the interaction is true
whether one considers mergers or simply flybys.
![]() |
Figure 4:
The star formation rate, |
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3.3 Relationship between separation and metallicity
While the correlation between SFR (at the peak of intensity) and
metallicity dilution is quite remarkable, we expect a larger scatter
when plotting the dilution as a function of the physical distance of
galaxies in the pairs, especially for those at the smallest
separations. This is
because the interacting pair can be in a number of different
evolutionary states at any given distance, except for perhaps the
smallest separations. For example, a galaxy at moderate
separations of several 10 s of kpc could be in an initial
encounter and before first pericentre passage when the circumnuclear
metallicity is largely unchanged. Or it could be just after the
first pericentre passage when its circumnuclear gas could be highly
diluted (Fig. 5). This causes a range of possible metallicity dilutions, from
to
(Fig. 5, right panel), with an average value of
.
At the smallest separations, say less than 10-20 kpc, but
still distinguishable systems, galaxies are likely to be either near
the pericentre passage or near final coalescence. This leads to both a
strong dilution and, on average, lower circumnuclear metallicity
(see Fig. 5).
At very small separations, less than about 4 kpc, galaxies
can be near first pericentre passage or near coalescence which in
either case have relatively large dilution and relatively
low-metallicity, or can have coalesced and be strong enriched by
supernovae ejecta and have relatively high-metallicity. We show this in
Fig. 5 where in the
smallest separation bin, if we include galaxy pairs with projected
separations of less than 4 kpc we find a net average increase in
the metallicity (i.e., z>1, with an average value of
)
while if we exclude such systems, we find a net decrease in the circumnuclear metallicity (
). This indicates the predominance of the metal
enriched mergers at small projected separations.
Kewley et al. (2006) have shown that galaxy pairs with small projected separations (
)
and strong bursts of star formation have metallicities lower than the
comparable field galaxies. Note that in this analysis we are
considering the physical (3D) distance
between the two galaxies, and not their projected separation.
Considering the effects of projection would likely contribute to
increasing the scatter in the relation between metallicity and
separation.
![]() |
Figure 5: (Left panel:) the physical 3-dimensional separation of the galaxy pairs and mergers versus their relative circumnuclear metallicity. The black line represents the average relation, for physical distances greater than 4 kpc and binned every 10 kpc. The blue line represents the average relation when in addition including mergers with physical separations below 4 kpc in the smallest bin (0-10 kpc). (Right panel:) evolution of two mergers (blue and green curves) and a fly-by (red curve) in the metallicity-distance plane. The simulations suggest that even at wide separations there are gas inflows which dilute the metallicities of the nuclear regions and that this dilution persists until the pairs are strongly interacting, overlapping, and near or after coalescence. During the final merging process, a wide range of metallicities can be observed but on average will show a much higher metallicity than if they had not interacted or merged. |
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3.4 O and Fe enrichment
![]() |
Figure 6:
Rate of oxygen and iron released in the ISM by SNe II and SNeIa explosions (in units of |
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Our simulations do not track the evolution of individual elements
directly in the code. However, to track their evolution in an
approximate way, we can use some simple arguments to give an estimate
of the amount of iron and elements, like oxygen, released in the ISM by type I and II SNe. At a given time, t, during the evolution of a galaxy, the rate of oxygen mass released in the ISM by SNe II can be estimated by:
![]() |
(3) |
where


![]() |
(4) |
The progenitors of SNe II have very short lifetimes, less than a few 10 Myr, so that we can assume that the SNe II rate follows closely the star formation rate of the galaxy, and so that the release of oxygen and iron by SNe II in the ISM is approximately instantaneous
![[*]](/icons/foot_motif.png)
The stars formed at time t contributes also in enriching the
ISM with Fe released through type Ia SNe, which are the
product of a thermonuclear detonation of a white dwarf accreting matter
from a companion star. While the progenitors of SNe II have very
short lifetimes, the time delay
between the birth of the progenitors of SNe Ia and their explosion
can be quite long, of the order of 1 Gyr or more depending on the
progenitors, the binary parameters, etc. (Strolger et al. 2010).
We will assume delay time between the birth of their progenitors and
the explosion is uniformly distributed between 1 and 4 Gyr
after the birth of the parent population, thus obtaining that, at any
time t, the amount of iron released by SNeIa is:
![]() |
(5) |
where







We note that if the coalescence of the two galaxies happens on a
short time scale (i.e., less than 1 Gyr after the first
pericentre passage), there is not enough time to enrich the ISM by
SNe Ia. This means that the stellar population forming during the
final burst forms from gas still
relatively enhanced in -elements (see Fig. 6,
second panel). If, in turn, the merger happens on longer time scales
(i.e., longer than 1 Gyr), the iron released by
SNe Ia explosions has time to lower the [
] ratio of the ISM, and the stellar population formed in the merging phase will be less enhanced in
elements (Fig. 6, fourth panel).
The O and Fe ejected into the ISM obviously affects the ratio between these two elements (Fig. 7).
Assuming that the initial galaxies have percentages of O and Fe similar
to the Sun, as star formation proceeds during the interaction, the
mass ratio of these two elements changes: in particular the oxygen
abundance increases, until the iron release from SNeIa explosions
inverts the trend. Note that the influence of the iron release from
SNe Ia on the populations of
new stars formed during the merger depends on the delay time between
the birth of the parent population and the onset of SNe Ia -
shorter delay times cause a more rapid decline in the O/Fe ratio.
We caution the reader that in all our discussion of the relative
abundance of -elements,
we are neglecting any possible contribution from SNe Ia from the
pre-existing old stellar population. This is of course an important
simplification of these models which we will investigate in future
studies.
![]() |
Figure 7:
The evolution of the oxygen over iron mass ratio for two of
our galaxy mergers, whose rate of mass release in the ISM is shown in Fig. 6. For each panel, three different curves are shown corresponding to three different delay times for onset of SNeIa, |
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4 Discussion
Overall, our results are not surprising. It has been known for the last few decades that mergers of gas-rich massive galaxies lead to transformations in the morphological type (Barnes 1992; Barnes & Hernquist 1991) and drive strong inflows of gas into the circumnuclear region (Barnes 2002; Di Matteo et al. 2007; Braine & Combes 1993; Barnes & Hernquist 1996; Casoli et al. 1991). These inflows are a result of strong tidal torques, dissipation, and strong asymmetries in the stellar distribution (e.g., bars; see Barnes 2002; Barnes & Hernquist 1996) leading to efficient star formation (Di Matteo et al. 2007). While the infall of gas in mergers has been discussed within the context of simulations and observations (Barnes 2002; Di Matteo et al. 2007; Braine & Combes 1993; Barnes & Hernquist 1996; Casoli et al. 1991), currently, there has been only limited quantitative attempts to address the relationship between infall, star formation, and the metallicity (e.g., Rupke et al. 2010; Perez et al. 2006). The trends we find in the simulations are fascinating and explain a number of important phenomenon that have been observed in interacting and merging galaxies. Metallicity changes during the merger event is a powerful way of testing the timing and strength of the gas inflows and ultimately in understanding the fate of post-merger/interacting galaxies.
Overall, we find broad agreement with the results of our
simulations for the evolution of the metallicity and what is observed.
We find for example that the net decrease in the metallicity is a few
tenths of a dex, which is comparable to what is observed in
merging and interacting galaxies
(Sol Alonso et al. 2010; Michel-Dansac et al. 2008; Rupke et al. 2008; Kewley et al. 2006).
In addition, as is likely observed, our simulations show that
the peak of the star formation is close in time to the minimum in the
circumnuclear metallicity (Rodrigues et al. 2008; Rupke et al. 2008).
The trends could be explained entirely by the strength of the gas
inflows of relatively metal-poor gas from the outer regions of the
disks which both fuel star formation and dilute the metal content of
the gas in and around the nucleus. The strength
of the inflows likely depends both on the initial gas content of the
disks, how it is distributed, the characteristics of the orbits through
which the galaxies merge, and the mass ratios of the galaxies. Here we
concentrated on the modelling a range of orbital parameters and
a 20
gas fractions. Subsequent papers will focus on varying the mass ratios, gas fraction, and gas distributions.
The simulation results suggest that the strongest star formation and
dilution of the initial nuclear metallicity occurs around pericentre
passage of the merger. This effect has been observed in that the star
formation at small separations exhibit a tail to high H
equivalent widths (e.g., Woods et al. 2006; Barton et al. 2000). At this time, we also expect the minimum in the metallicity to occur. This expect relationship has been emphasised in Kewley et al. (2006) based on a study
of pairs of galaxies. The sample of Kewley et al. (2006)
is particularly appropriate for this type of study as it is a robust
sample of pairs of interacting galaxies and show generally a modest
increase in their star formation rates and modest likely dilutions.
For example, their most intense starbursts, which show significant
offsets in metallicity, only have offsets of about 0.2-0.3 dex
compared to non-interacting galaxies with similar B-band absolute magnitudes. This is the amount
we find in our simulations.
We can explain the full range of circumnuclear metallicities observed in more violent mergers in the local (Rupke et al. 2008) or distant universe (Rodrigues et al. 2008). By violent mergers, we mean in this context galaxies which are luminous in the infrared and have star formation rates several to almost 100 times that of the Milky Way. These are the most appropriate for comparison because, as we have shown, as the merger advances, the star formation rate will decrease and the metallicity increases reaching values that are higher than the initial metallicities.
To show that our models can reproduce the data, we have made a
monte-carlo simulation of the metallicity distribution we might obtain
if we observed a set of galaxies near their maximum star formation rate
and allow for a range in their properties and the intrinsic scatter in
the mass-metallicity relationship. For such a comparison sample, we
would naturally draw on the most violent star-forming galaxies in the
local universe which are major mergers. This is the point in our
simulations, where we get the most significant influx of material into
the circumnuclear regions and near the peak of star formation.
At this stage, we can dilute the circumnuclear gas with relatively
metal poor gas from the outer disk decreasing the overall metallicity
by about a factor of 2 and relatively brief time (of-order an
internal dynamical time of about 108 yrs).
Thus, in our simple monte-carlo approach to the metallicity
estimates, we allow for a decrease of 0.3
0.1 dex. We make this allowance as we only have single metallicity gradient in all our models (-0.1 dex kpc-1). However, metallicity gradients in spiral galaxies range from about 0.0 to -0.2 dex kpc-1.
To allow for this variation, we have assigned a range in the
metallicity allowed of 0.1 dex. We do not know the nature of the
progenitors either. They could have a range of initial circumnuclear
metallicities and the mass metallicity relationship has a significant
scatter 0.15 dex (Tremonti et al. 2004).
Thus we allow for an additional source of scatter of 0.15 dex to
reflect the scatter in the mass-metallicity relation. Finally, there is
also
measurement uncertainties which are about 0.1 dex (Rupke et al. 2008).
We assume that each source of scatter is Gaussian and that each can be linearly combined. This assumption is appropriate since (at the metallicities observed here) the uncertainty in the metallicity is not correlated to the metallicity, our numerical simulations are completely independent of the data, and the scatter in the mass-metallicity relationship is intrinsic. One such monte-carlo simulation is shown in Fig. 8 but all reproduce the data from Rupke et al. (2008) reasonably well. Unfortunately, Rupke et al. (2008) did not actually estimate masses directly for their sample but used a random set of masses generated from a typical mass and scatter from the literature were then assigned. While we could have used their masses directly which would have made the agreement look more spectacular, the reality is that we have simply reproduced the scatter in the metallicity by making a set of simple assumptions. Thus this agreement between the data and model should not be taken literally but demonstrates that numerical simulations of the type run here can reproduce the spread in metallicity without making any assumptions beyond suggesting that the highest levels of star formation in a merger occurs near the peak in the strength of the gas flows something that every model over the last decades has shown and can be argued on robust physical grounds.
![]() |
Figure 8: Monte-carlo simulation of the metallicity distribution predicted by the models compared to the data from Rupke et al. (2008). The up-side down triangles are the simulated results (see text for details), the blue stars and red circle are galaxy values taken directly from Rupke et al. (2008) as described there. Unfortunately, Rupke et al. (2008) did not actually measure the mass of any of the individual galaxies but used a single mass allowing for some scatter (see that paper for details). We have approximately reproduced their mass distribution for this comparison. |
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In addition to investigating the general evolution of the metallicity,
we also adopted a crude model for the evolution of Oxygen and Iron
during mergers and interactions. We were motivated to make these
simplistic estimates by the fact that following the evolution of elements
like Oxygen can be used to ``date'' mergers. In analogy with
studies of the stellar populations of early type galaxies, bulges,
or in components of the Milky Way such as the thick disk and
bulge, [O/Fe] or [
/Fe] ratios
can be used as a chronometer telling us over what timescale the bulk of
the (luminosity-weighted) stellar population formed (e.g., Zoccali et al. 2006; Thomas et al. 2005).
Our hope was
that even a crude model such as the one we have adopted could provide
useful information about how observationally we might go about dating
mergers and interactions.
How this might work is that if the galaxy is in the early
phases of merging, we would expect to see both overall low-metallicity
and depending on the distribution of [/Fe]
in initial disks,
evidence of what radii the gas came from through its abundance ratios.
The bulge of our galaxies, at least in the stars, is enriched in
elements relative to the sun (Lecureur et al. 2007; Zoccali et al. 2008; Ryde et al. 2010).
Moreover, the disk of our Milky Way appears to have an enhancement
out to relatively large radii but this ratio depends on the radius (see
Acharova et al. 2010, and references
therein). Of course these measurements are for the stellar metal
abundances, not for the gas phase so the comparison is not completely
fair. It does however at least suggests a possible trend that may
be useful in determining the radii at which freshly deposited gas came
from. At any rate, we may therefore expect in the early stages,
a mild
enhancement
in the gas. As the merger proceeds, this enhancement will become
more pronounced, until the late stages where it will begin to lessen as
the type Ia SNe become more important. An interesting
question is how much of this enhanced material makes its way into each
generation of star formation. While in principle this can happen
quickly as the star formation lasts for several 100 Myr (several
crossing times) sufficient for the metals to mix with the ambient gas,
it is not clear if this actually happens.
If it does, then we would predict that a majority of the stars
formed would be
enhanced but may have metallicities lower than stars formed from gas
before the burst occurred. Only after about 1-2 Gyr or so would
both the metallicity increase above what an isolated galaxy would have
and would the
enhancement begin to lessen. Of course this exact time scale is uncertain for both the metallicity and
enhance because we do not have a firm understanding of the delay time for type Ia SNe.
Taken at face value, the results of our simulations would suggest that
we can make realistic bulges and early-type galaxies. It is well
known that the bulge of the Milky Way is both metal-rich
(typically about [Fe/H]
-0.3 to 0.3 with a mean of about 0) (Zoccali et al. 2008) and is
enhanced (Lecureur et al. 2007; Zoccali et al. 2008; Ryde et al. 2010).
Both of these results are consistent with our simulations modulo the
uncertainty in the delay times of type Ia SNe, and the mass
and characteristics of pre-existing bulges in the merging galaxies (see
Pipino et al. 2009). This general result appears to hold generally for bulges, namely they are metal-rich and
enhanced (Thomas et al. 2005) and normalizing for their mass differences, very similar to early type galaxies (Thomas et al. 2005). Interestingly, there is some evidence for a radial gradient in the average metallicity of the stars in the galactic bulge (Zoccali et al. 2008).
Depending on the initial conditions, this may be consistent with our
simulations. In the simulations it is likely there is a gradient in the
star formation with radius as the strongest gas flows and most intense
star formation is in the deepest part of the potential well. However,
a complete discussion of the metallicity,
enhancement,
and radial gradients in bulges and early-type galaxies is beyond the
scope of this paper, our simulation results are at least consistent
with these general properties of spheroids. We will address these
questions in more detail in a subsequent manuscript.
Finally, these results have important implications for our understanding of metallicity evolution of galaxies as observed at high-redshift. Since we have found that during the most intense phases of star formation, the gas phase metallicity can be reduced (diluted) by infalling material from the outer regions of the merging galaxies. Mergers and interactions are known to play an important role in initiating intense starbursts in the distant universe (Law et al. 2007,2009; Epinat et al. 2009; Nesvadba et al. 2008). In order to estimate gas phase metallicities of distant galaxies, they must have high surface brightnesses in their emission line gas (see Lehnert et al. 2009, and references therein ). It is now reasonably well established that the mass-metallicity relation appears to have evolved over the last 10 Gyr (Erb et al. 2006; Rodrigues et al. 2008; Mannucci et al. 2009; Queyrel et al. 2009; Savaglio et al. 2005; Halliday et al. 2008). However, combining the possible limitations in the observations due to surface brightness dimming and the importance of mergers in initiating intense (high surface brightness) starbursts, it may be the determinations of the evolution in metallicity may be strongly influenced by dilution. If this is so, we would predict that, if the importance of mergers increases with redshift (e.g., de Ravel et al. 2009), we should see both a decrease in the average metallicity and an increase in the scatter in the mass-metallicity relation with increasing redshift. While we have no modelled a full range of initial conditions, our results here suggest that this increasing scatter would be about several tenths of a dex which is less than a factor of 2 more than the scatter in the relation for local galaxies (Gallazzi et al. 2005).
5 Conclusions
In this work, we have studied the dilution and subsequent enrichment of the interstellar medium during interactions and mergers of disk galaxies of comparable mass. Our models, which include star formation and chemical enrichment, have made it possible to follow, for the first time, the evolution of the circumnuclear gas phase metallicity from the initial interaction until the coalescence phase, thus significantly improving the existing simulations and leading to insight between star formation, gas inflows and dilution, and metallicity evolution in mergers and interactions.
Our main results can be summarised as follows:
- The dilution of the gas initially in circumnuclear regions of the merger pairs starts just after the first pericentre passage, as the disk is destabilised and gas flows inwards. For mergers, usually a second dilution peak is observed in the final phase of coalescence. A significative dilution can take place also in fly-bys, just after their close passage, mimicking to some extent what we observe in our merger simulations. Thus even flybys may be important for the star formation and evolution of galaxies.
- On average, the amplitude of the metallicity dilution we find (0.2-0.3 dex) is in agreement with observations (Rupke et al. 2008; Kewley et al. 2006).
- The strongest correlation we found is between the maximum in the SFR and the strength of the circumnuclear dilution - pairs experiencing the strongest bursts also show the strongest circumnuclear dilution. This can be explained in terms of interaction-driven gas inflows from the outer disk regions into the galaxy centre.
- The dilution phase (defined as
) is very rapid, half of the sample sustains this high dilution phase for less than 5
108 years.
- The gas inflow causes concomitant enhanced star formation; such enhanced star formation releases a significant amount of enriched material in the ISM. This enrichment results in merger remnants which have higher final circumnuclear metallicities than the corresponding galaxies evolved in isolation. The same can happen for fly-bys which experience enhanced star formation rates.
- Type Ia and II SNe formed during the interaction release O
and Fe in the ISM, thus changing the average mass ratio of these two
elements. In particular, we note that, if the merger takes
place on short time scales (lower than the typical delay time of
SNe Ia, which unfortunately is not well constrained), the stellar
population formed during the last SF burst forms from an
-enriched ISM, while, for mergers with longer time scales, SNeIa can have the time to release a sufficient amount of iron in the gas. The resulting stellar population has thus lower values of
ratios.
- The circumnuclear metallicity, as well as the [
] ratios, can be used as an indicator of the timing of merger/interaction state, thus helping in disentangle projection effects, i.e., galaxies which appears close in projection, but that are not strongly interacting. The remnants of these mergers appear qualitatively consistent with what is known about the bulge of our galaxy, other nearby galaxies, and early type galaxies generally. Thus, in agreement with many authors, it appears when considering chemical evolution that mergers can reproduce early type systems.
- If mergers play a significant role in the galaxies that have had their metallicities determined at high-redshift, then it is possible that dilution is signficantly affecting these estimates.
P.D.M. is supported by a grant from the French Agence Nationale de la Recherche (ANR). These simulations are available as part of the GalMer simulation data base (http://galmer.obspm.fr). The authors wish to thank the referee for their constructive and helpful comments and A. Pipino for a critical reading of an early version of this manuscript and helpful suggestions.
Appendix A: Metallicity evolution in the isolated galaxy
![]() |
Figure A.1: Metallicity profiles of the gas in the gSb galaxy simulation which were evolved in isolation (i.e., did not experience a merger or interaction during the simulation). The profiles are given at different times, as indicated in the labels. |
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Since the goal of this paper is to investigate the role interactions and mergers play in the gas phase metallicity evolution, it is essential to study the evolution of the gas properties of simulated galaxies that did not experience a merger or interaction (i.e., an isolated galaxy). This is essential to distinguish between secular processes that drive the evolution of galaxies from the effects that occur during the interaction and merger process. For the isolated galaxy simulation, initial metallicity profile (given in Eq. (2)) evolves rapidly in the first 500 Myr of evolution, due to the compression of gas into density waves and inflow into the central regions (Fig. A.2). At this time, spiral arms and a bar are formed, as clearly shown in the maps of the stellar distribution (Fig. A.3). The galaxy models studied in this paper were not evolved in isolation before starting the interaction. However, as shown in Figs. A.1 and A.2, tidally driven gas inflows from the outer disk take place after the pericentre passage (with our initial choice of the orbital parameters, this corresponds to around 400 Myr from the beginning of the simulation) and in the merging phase (between 1 and 3 Gyr with the exact timing depending on the orbital characteristics). By this time - 400 Myr - the metallicity profile has already reached an equilibrium configuration (see Fig. A.1), and the galaxy has relaxed.
![]() |
Figure A.2:
From top-left to bottom-right: maps of the gas distribution of the model gSb galaxy evolved in isolation. Maps are shown from t=0 to t=3 Gyr in steps of 500 Myr. Each map is 40 kpc |
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![]() |
Figure A.3:
From top-left to bottom-right: maps of the stellar distribution
of the gSb galaxy model evolved in isolation. Maps are shown for
every 500 Myr over t=0 to t=3 Gyr. Each map is 40 kpc |
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Footnotes
- ... code
- We thank the referee for pointing out the importance of diffusion in this problem.
- ... = 7 kpc
- This is the average distance from the galaxy centre of the gas that strongly participates in the interaction-driven inflow.
- ... instantaneous
- Any possible time delay would result into a time-shift between the SFR peak and the oxygen and SNeII-Fe peak.
All Figures
![]() |
Figure 1: The dynamics of the gas during the interaction and subsequent merger of two gSb galaxies for several orbits (shown with different colours). (First panel): relative distance of the two galaxy centres versus time for some simulated orbits. (Second panel): gas mass inside 2 kpc from one of the two galaxy centres versus time. (Third panel): evolution of the star formation rate (SFR) relative to the rate for the corresponding initial galaxies evolved in isolation. (Fourth panel): evolution of the central gas metallicity. The metallicity has been normalised to that of the initial galaxies used in the merger simulations evolved isolated. In the second, third, and fourth panel, squares and asterisks respresent respectively the time of the first pericentre passage and the time of final coalescence for the different orbits. |
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In the text |
![]() |
Figure 2: Same as Fig. 1, but for the simulations of ``flybys''. Note that simulations resulting in starbursts with higher intensities also have gas phase metallicities with higher dilution. Even in flybys, the star formation rate and dilution of the metallicity are related. |
Open with DEXTER | |
In the text |
![]() |
Figure 3: The duration that the circumnuclear metallicity is below 0.8 (shown in red) and between 0.8 and 1.0 (shown in black) relative to an isolated simulated spiral with the same properties as the individual merging galaxies. We chose 0.8 as the threshold because this is the span of the most extreme metallicity offset observed in nearby mergers (e.g. Rupke et al. 2008; Kewley et al. 2006). |
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In the text |
![]() |
Figure 4:
The star formation rate, |
Open with DEXTER | |
In the text |
![]() |
Figure 5: (Left panel:) the physical 3-dimensional separation of the galaxy pairs and mergers versus their relative circumnuclear metallicity. The black line represents the average relation, for physical distances greater than 4 kpc and binned every 10 kpc. The blue line represents the average relation when in addition including mergers with physical separations below 4 kpc in the smallest bin (0-10 kpc). (Right panel:) evolution of two mergers (blue and green curves) and a fly-by (red curve) in the metallicity-distance plane. The simulations suggest that even at wide separations there are gas inflows which dilute the metallicities of the nuclear regions and that this dilution persists until the pairs are strongly interacting, overlapping, and near or after coalescence. During the final merging process, a wide range of metallicities can be observed but on average will show a much higher metallicity than if they had not interacted or merged. |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
Rate of oxygen and iron released in the ISM by SNe II and SNeIa explosions (in units of |
Open with DEXTER | |
In the text |
![]() |
Figure 7:
The evolution of the oxygen over iron mass ratio for two of
our galaxy mergers, whose rate of mass release in the ISM is shown in Fig. 6. For each panel, three different curves are shown corresponding to three different delay times for onset of SNeIa, |
Open with DEXTER | |
In the text |
![]() |
Figure 8: Monte-carlo simulation of the metallicity distribution predicted by the models compared to the data from Rupke et al. (2008). The up-side down triangles are the simulated results (see text for details), the blue stars and red circle are galaxy values taken directly from Rupke et al. (2008) as described there. Unfortunately, Rupke et al. (2008) did not actually measure the mass of any of the individual galaxies but used a single mass allowing for some scatter (see that paper for details). We have approximately reproduced their mass distribution for this comparison. |
Open with DEXTER | |
In the text |
![]() |
Figure A.1: Metallicity profiles of the gas in the gSb galaxy simulation which were evolved in isolation (i.e., did not experience a merger or interaction during the simulation). The profiles are given at different times, as indicated in the labels. |
Open with DEXTER | |
In the text |
![]() |
Figure A.2:
From top-left to bottom-right: maps of the gas distribution of the model gSb galaxy evolved in isolation. Maps are shown from t=0 to t=3 Gyr in steps of 500 Myr. Each map is 40 kpc |
Open with DEXTER | |
In the text |
![]() |
Figure A.3:
From top-left to bottom-right: maps of the stellar distribution
of the gSb galaxy model evolved in isolation. Maps are shown for
every 500 Myr over t=0 to t=3 Gyr. Each map is 40 kpc |
Open with DEXTER | |
In the text |
Copyright ESO 2010
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