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Galactic Collisions Are Rather Rare in the Local Universe, at Least Those Involving Two Massive Galaxies

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Galactic Collisions Are Rather Rare in the Local Universe, at Least Those Involving Two Massive Galaxies MODES OF STAR FORMATION IN AND AROUND INTERACTING SYSTEMS P.-A. DUC CNRS FRE 2591 and CEA -Saclay, Service d'astrophysique 91191 Gif sur Yvette cedex, France Interacting systems and mergers are particularly well known for the exceptionally strong star­ burst that some of them - the so-called ultraluminous infrared galaxies - host in their nuclear region. The massive gas clouds that have accumulated there condense and form stars with a rate of several hundreds of solar masses per year. At such a rate the gas reservoir is exhausted within typically 100 Myr. For long, the dazzling nuclear starbursts have diverted our attention from the fact that star formation in interacting systems may be spatially extended. Other modes of star formation that are much quieter but last longer actually occur in interacting systems: in the interface region between the colliding galaxies, along tidal bridges and tails, at the tip of long tidal tails, in the so-called Tidal Dwarf Galaxies, or even in the intergalactic medium surrounding the parent galaxies, at more than 100 kpc from their nucleii. I present in this review the variety of SF modes encountered in and around interacting systems and a few numerical models that might account for them. Keywords: Galaxies: interactions; Galaxies: starburst; Galaxies: ISM 1 Introduction Galactic collisions are rather rare in the Local Universe, at least those involving two massive galaxies. Less than a few percent of spirals are currently involved in a tidal interaction. Why then bother studying the different modes of star formation in colliding galaxies ? First of all, major collisions where much more frequentin the past. The HST optical surveys have shown that the number of disturbed galaxies raises to several 103 at a redshift of 0.7-1 (e.g. Griffithset al., 1994; Le Fevre et al., 2000f2·15. Beside, infrared surveys indicate that a large proportion of the stars in the Universe have probably formed during a luminous infrared phase (see contribution by D. Elbaz in this volume). The fraction of interacting galaxies among the Luminous Infrared Galaxies (LIRGs) is high (Flores et al., 1999)9. Therefore, a significant, if not the majority, of 427 Figure 1: Simulated sequence of a galaxy collision, corresponding to the interacting system Arp 245 (NGC 2992/3). The stellar component is represented by the grayscale, and the gaseous component by the contours. The real system (inset to the right) is well matched at T=l.1 or about 100 Myr after the periapse (Adapted from Due et al., 2000) . stars in the Universe could have been formed, triggered by a large-scale event, such as a major galaxy collision. Second, interacting systems are rather easy to model with numerical simulations. The shape of their long tidal tails, and their kinematics, provide strong constraints on the orbital parameters of the collision (i.e., the impact parameter, relative velocities, position of the orbital planes) , and on the initial conditions in the colliding galaxies (i.e., relative distributions of the stellar and gaseous components, rotational velocity, etc ...) . For instance, a fast encounter at more than 500 km s-1 will have less impact on the morphology of the target galaxy than a slow encounter for which tidal perturbances have more time to develop. A prograde encounter (the rotation of the target galaxy and its orbital trajectory have the same direction) will produce longer and sharper tails than a retrograde encounter. Once best guesses for the initial. conditions are determined, one may run a numerical model and, comparing it with the real system at different stages of the collision, estimate its dynamical age (see Fig. 1). Thus the simulations set the clock. The ability of dating some particular events associated with the collision is particularly useful and lacks in isolated galaxies. The typical time scale unit for galaxy interactions is 100 Myr. Mergers typically occur 500 Myr after the initial encounter. Long tidal tails survive for about 1 Gyr, e while morphological disturbances can be seen for a f w Gyr (See the comprehensive review on mergers by Struck, 1999)3°. Finally, interacting systems appear asmultiple -usage laboratories. Indeed, within one single system, one may encounter a unique variety of physical conditions that would not be accessible in an isolated spiral galaxy, starting with the most extreme conditions in the nucleii of merging galaxies. Indeed, the galaxies known to form, in the nearby Universe, stars at the highest rates (several hundreds of solar masses per year) are all advanced mergers showing a compact infrared 428 -4" NGC 5291 Figure 2: Composite image of six interacting systems showing the old stellar component (optical image), the atomic hydrogen (VLA HI map) and in light grey the regions of current star formation (Fabry-Perot Ha image) . The nuclear starbursts are indicated by the circle, the star·forming regions along the tails by a rectangle and in Tidal Dwarf Galaxies by a polygon (HI courtesy of J. Dickey, P.-A. Due, J. Hibbard, B. Malphrus and B. Smith) ermss1on excess. On the other hand, conditions in the external regions of mergers are milder, while the tip of the tidal tails may show an unexpectedly high level of activity. In the following, we systematically explore the various conditions exhibited by mergers from the most central regions (less than 100 pc) to the most external ones (at radial distances greater than 100 kpc). Note that I do not address here the triggering of star formation by minor mergers, i.e. when the respective masses of the target and bullet galaxies differ than more than a factor of 3-5. Fig. 2 shows a composite image of several interacting systems observed at different wave­ lengths. The star-forming regions in various environments are highlighted. 2 The central regions 2.1 Observations: the nuclear starburst Soon after the discovery of the Ultraluminous Infrared Galaxies (ULIRGs) by IRAS, optical and near-infrared ground-based observations revealed that almost all of them were advanced mergers (Sanders et al., 1988f8. IRAS did not have the spatial resolution to pinpoint the precise location of the emitting region. It was even difficult to tell which of the two merging galaxies showed the infrared excess at 60 and 100 µm. However, studies at other wavelengths revealed that the IR emission was very compact and centered on the merging nucleii. The very 429 good correlation between the radio and far infrared on one hand, and between the far infrared and the mid-infrared on the other ensured that the emission came from the nuclear region at all these wavelengths. For instance ground-based mid-infrared imaging in the N-band indicates that the 10 µm emission is less extended than 500 pc (Soifer et al., 2000)29. The 20 cm emission of ULIRGs is also very compact. It has been debated for almost 20 years whether an AGN or a nuclear star burst is responsible for the heating of the dust and subsequent IR emission (Sanders & Mirabel, 1996)26. Most likely, both contribute. Converted into a star formation rate, the IR emission corresponds, ff powered by star-formation, to values as high as several hundreds of solar masses per year. Whether stars in these extreme conditions formwith a different efficiency than in more quiescent star-forming regions is debated (see the contribution by U. Fritze in this volume). Large quantities of highly concentrated gas are available in the central dust enshrouded regions for fueling the nuclear starburst or the AGN (Sanders et al., 1991)27 . It is most often observed in molecular phase, as if the atomic hydrogen had been compressed locally and transformed into H2 (Mirabel & Sanders, 1989)23. Given the high SFRs, this gas will be rapidly consumed, typically with a time scale of only 100 Myr, i.e. much less than the time scale of the merging process. At which stage of the collision did the IR phase occur ? How the gas originally distributed all over the disks could end up in the inner 1 kpc ? Numerical simulations may once again help in answering these questions. 2.2 Simulations: transporting gas via bars Feeling a dynamical friction, the gas which was initially in rotation in the disk of the merging galaxies looses its angular momentum, before being funneled into the central regions via stellar bars. Such a simple scenario is qualitatively very well reproduced by numerical simulations (Barnes & Hernquist, 1996)2. However, one single bar may only drive material in the inner kpc. A supplementary mechanism is required to push the gas further inside. The formation of a "bar within a bar", i.e. a nuclear bar (Friedli & Martinet, 1993; Maciejewski et al., 2002)10•18 may help. When does the nuclear starburst start ? How long does it last ? This can be studied in numerical simulations that implement simple rules for the star formation, such as a Schmidt law - the SFR is proportional to the gas density. Whereas no significant increase of the star­ forming activity is observed before the first encounter between the colliding galaxies at the periapse, the SFR will either peak before or after the final merging depending on the orbital parameters and initial conditions in the merging galaxies. For instance, in a prograde/prograde encounter involving galaxies with massive stellar bulges, the starburst occurs later, after the merging occurred (Mihos et al., 1992; Mihos & Hernquist, 1994)22•21, and the overall production of stars is less. Indeed, the strength of the bars may depend on these initial conditions. Also, as we will see later, the ability for a merging system to form or not long tidal tails is primordial. With long tails (favored by a prograde merger and a small bulge) a significant fraction of the gas can escape, leaving less gas for the central regions.
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