OPCIONES PRIMERA APLICACIÓN

The new numerical model for the Antennae galaxies presented in this Chapter improves on previous models in several key aspects. We find an excellent morphological and kine- matical match to the observed large-scale morphology and Hi velocity fields (Hibbard et al. 2001). In addition, our model produces a fair morphological and kinematical representation of the observed central region. A strong off-center starburst naturally develops in our fiducial simulation - in good qualitative and quantitative agreement with the observed extra-nuclear star-forming sites (Mirabel et al. 1998; Wang et al. 2004b; Brandl et al. 2009; Klaas et al. 2010). This is a direct consequence of our im-

5.6 Conclusions ₇₃

Figure 5.7: Left panel: Synthetic CO map of the inner regions of the Antennae from the fiducial simulation with qEQS= 0.01, where the integrated CO masses are obtained in a two-step

process. H2 surface densities are obtained from the SFR surface densities of the SPH particles

using an “inverse” Bigiel et al. (2008) relation. Then, we apply the same CO-to-H2 conversion

factor as the observations to map theH2 masses back to CO.Right panel: CO integrated intensity

map, adapted from Figure 1 inWilson et al.(2000). The two galactic nuclei and five super-giant molecular clouds in the overlap region are indicated. The filled black oval in the upper left corner gives the size of the synthesized beam in the observations.

proved merger orbit. All previous studies using traditional orbits failed to reproduce the overlap starburst (see e.g. Karl et al. 2008). Additionally, we are naturally guided to choose a less vigorous stellar feedback in our adopted star formation model in order to reproduce the observed properties related to the extra-nuclear starburst in the An- tennae.

The exact timing after the second encounter shortly before the final merger ensures that the galaxies are close enough for the efficient tidally-induced formation of the overlap region. The formation of the extra-nuclear starburst is likely to be supported by compressive tidal forces, which can dominate the overlap region in Antennae-like galaxy mergers during close encounters as discussed by Renaud et al. (2008, 2009). In particular, in this simulation, and other simulations in our parameter study with similar central properties, we only find prominent star formation in the overlap region for a very short period of time after the second encounter (see also Figure 4.5). In addition, we find that the observed high-surface density region in the Antennae is not just a chance overlap of the two disks projected along our line-of-sight. Rather it is caused by the actual proximity of the two galaxies after the second close encounter. Thus, a central conclusion of our study is that the strong localized, off-center star- bursts observed in the overlap region is a short-lived transient phase in the merging process associated with the recent second encounter, which has a very short lifetime

(_≈ 20 Myr) if compared to the full merger process (_≈ 650 Myr from first encounter to final merger). This serves as a plausible explanation for the fact that such features are scarcely observed in interacting galaxies (Xu et al. 2000). However, the observed puzzling gas concentration between the two nuclei of NGC 6240 (Tacconi et al. 1999;

Engel et al. 2010) and the off-center star formation found in other interacting systems (see Section 2.2) might be of a similar origin suggesting that the Antennae overlap region, although rare, is not a unique feature.

Most importantly, our improved model can serve as a solid basis and testbed for further theoretical studies of the enigmatic interacting NGC 4038/39 system, such as the studies presented in detail in the following Chapters. Apart from these, in a first application using this new orbital configuration, we have been able to qualitatively and quantitatively reproduce the magnetic field morphology of the Antennae galaxies (Kotarba et al. 2010) as observed by Chyży & Beck (2004). Thereby, the magnetic field strength was found to saturate at a level corresponding to equipartition between turbulent and magnetic pressure, independent of the initial field strength. High ampli- fications of the magnetic fields during the interaction suggest that galaxy mergers may be efficient drivers for the cosmic evolution of magnetic fields (see also Drzazga et al. 2011 and Geng et al. 2011 for further discussion).

Finally, accurate modeling of other nearby interacting systems would be desirable to provide further insights into the merger dynamics and timing of observed merging sys- tems. The Antennae galaxies are traditionally in the first place of the classical Toomre sequence which orders galaxies according to their apparent merger stage (Toomre 1977; see also Laine et al. 2003). The “Playing Mice” galaxies (NGC 4676) usually take the second place in the sequence behind the Antennae. They have been successfully mod- eled with a time of best match between their first and second pericenter (Barnes 2004). According to our proposed model, however, the Antennae galaxies would be in a later merger phase than the Mice, already past the second pericenter. As a consequence, the Antennae would lose their first place in the sequence to the Mice galaxies, requiring a revision of the classical Toomre sequence.

Chapter

6

Disruption of Star Clusters in

the Antennae Galaxies

In this Chapter, we reexamine the age distribution of star clusters in the Anten- nae in the context ofN-body+hydrodynamical simulations of these interacting galaxies. All of the simulations that account for the observed morphology and other properties of the Antennae have star formation rates that vary relatively slowly with time, by factors of only 1.3₋2.5 in the past 108 _{yr. In contrast,}

the observed age distribution of the clusters declines approximately as a power law, dN/dτ _∝ τγ _{with γ = −1.0, for ages 10}6 _{yr . τ . 10}9 _{yr. These two}

facts can only be reconciled if the clusters are disrupted progressively for at least _∼108 _{yr and possibly ∼10}9 _{yr. When we combine the simulated forma-}

tion rates with a power-law model, fsurv ∝τδ, for the fraction of clusters that

survive to each age τ, we match the observed age distribution with exponents in the range ₋0.9.δ .₋0.6 (with a slightly differentδ for each simulation). The similarity betweenδ andγ indicates that dN/dτ is shaped mainly by the disruption of clusters rather than by the variations in their formation rate. Thus, the situation in the interacting Antennae resembles that in relatively quiescent galaxies such as the Milky Way and the Magellanic Clouds. Parts of this Chapter have been published in Karl, Fall, & Naab 2011, ApJ, 734, 11.

6.1

Why Study Star Clusters in the Interacting An-

tennae Galaxies?

Interacting galaxies in the nearby universe are laboratories for direct studies of several physical processes that were important in the formation and early evolution of galax- ies. From such studies, we hope to learn, for example, how interactions and mergers affect the cycle in which baryonic matter is converted from diffuse interstellar gas into

dense molecular clouds, then into star clusters, and eventually, by disruption, into a relatively smooth stellar field. It is clear that interactions and mergers boost the rate of star and cluster formation. But do they also change the rate at which clusters are disrupted? This is the question we address in this Chapter.

The most intensively studied interacting galaxies are the Antennae (NGC 4038/39), at a distanceD_∼20 Mpc (Schweizer et al. 2008, see also Section2.2). They consist of two normal disk galaxies that began to collide a few_×108_{yrago. The stellar population}

and ISM of the Antennae have been observed over an enormous range of wavelengths, from X-ray to radio (see, e.g. Zhang et al. 2001; Hibbard et al. 2001; Kassin et al. 2003; Zezas et al. 2006; Brandl et al. 2009; Klaas et al. 2010, and Chapter 2). The star clusters have been the focus of numerous studies based on observations with the HST, culminating in well-determined luminosity, mass, age, and space distributions (see Whitmore et al. 2010, Section 2.2, and references therein).

The Antennae have also been the focus of several dynamical simulations, first by

Toomre & Toomre (1972) and then by Barnes(1988). As laid out in Chapter 3, these pioneering studies demonstrated that gravity alone can account for the gross features of the observed morphology and kinematics of the stellar components of the merger. Subsequent simulations have included an interstellar medium and star formation, with the additional goal of matching the observed space distribution of young stars in the Antennae (Mihos et al. 1993; Teyssier et al. 2010; Karl et al. 2010, hereafter MBR93, TCB10, and K10, respectively; see also Chapter 5). There have also been two recent attempts to match the observed age distribution of the clusters, with different assump- tions about their disruption histories (Bastian et al. 2009, K10).

The purpose of this Chapter is to investigate the issues raised by the observed age distribution of the clusters in the Antennae. We can show that there is nothing spe- cial about the disruption history of clusters in the interacting Antennae galaxies; it is similar to that in quiescent (non-interacting) galaxies. The Chapter is organized as follows. In Section 6.2, we review the evidence for a quasi-universal age distribution of star clusters in different galaxies, and in Section 6.3, we assemble the star formation histories from all available N-body+hydrodynamical simulations of the Antennae. We then combine these and compare the results with observations in Section6.4. We sum- marize our conclusions and their implications in Section 6.5.

Before proceeding, we offer a few remarks about nomenclature. We use the term “cluster” for any concentrated aggregate of stars with a density much higher than that of the surrounding stellar field, whether or not it also contains gas and whether or not it is gravitationally bound. This is the standard definition in the star formation community (see, e.g. Lada & Lada 2003; McKee & Ostriker 2007). Some authors use the term “cluster” only for gas-free or gravitationally bound objects. Such definitions are not appropriate in the present context for two reasons: (1) A key element in our