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 starburst 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.

3 The interface regions

3. 1 Observations: star formation in bridges and tails Not all the star-forming regions in mergers are concentrated in the central regions. Extended star forming regions are also observed, in particular in the so-called "interface" regions of overlapping disks, in the bridges linking still well-separated galaxies, or along the tidal tails emanating from each of them. These star-forming regions are generally revealed by their blue optical color, their associated HII regions and the presence of Giant Molecular Clouds. Like in the nuclear regions, they can also be dust enshrouded - see for instance the prominent dusty condensations in the

430 Figure 3: ISOCAM mid-infraredcontours superimposed on an HST image of the Antennae (Adapted from Mirabel et al ., 1998) interface region of the Antennae shown in Fig. 3 (Mirabel et al., 1998)24. In some tidal tails, the Ha luminosity of individual giant HII complexes (GHCs) may even exceed that of the Giant HII Regions of normal disks (Weilbacher et al., 2003) 31 • Star formation may locally be efficient enough to produce Super Star Clusters and even Globular Clusters.

3.2 Simulations: rules fo r the extended star fo rmation

Simple, density dependent models for star formation are actually not able to properly repro duce the extended SF observed in interacting systems. Pinpointing the problem, Barnes(2004)1 recently proposed to implement a new rule for star formation that takes into account the me chanical heating due to shocks and hence direct cloud-cloud collisions. Such a model appears to be more realistic. The formation of star-forming gravitationally bound objects along tidal tails - the pos sible progenitors of Super Star Clusters -, is also observed in numeral simulations (Barnes & Hernquist, 1996)2. They have initially typical masses of a few 107, up to 108 M0(see also Fig. 4). 431 Myr yr 0 800 IL 720 l4yr

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432 120 320 Myr 12a 440 Myr 120 470 Myr

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Figure 5: Formation of Tidal Dwarf Galaxies in full N-body simulations of a collision between massive spirals surrounded by an extended dark matter halo. The system is seen face-on. The gas is displayed with a logarithm intensity sc

4 The tip of tidal tails

4.1 Observations: Tidal Dwarf Galaxies Star formation may extend even further, up to the very tip of long tidal tails, more than 100 kpc from thenucleus of the parent galaxy. There, large accumulations of tidal material were reported in several interacting systems (see a fewof them in Fig. 2). As these objects have apparent masses and sizes comparable to dwarf galaxies, they are referredto Tidal Dwarf Galaxies (TDGs) (e.g. Due et al., 2000jl. They contain large quantities of gas in atomic, molecular and ionized form and have luminous masses of typically 109 M8 (See the �ontribution by J. Braine in this volume and Braine et al., 20015). Because they are most oftenobserved at or near the tip of long optical tidal tails, the very existence of such massive objects hasbeen challenged. Indeed an apparent accumulation of tidal material could be artificial. In 3-D space, tidal tails are curved. Seen edge on, they appear as linear structures and may show at their tip fake mass concentrations caused by material projected along the line of sight (Hibbard, 2004; Mihos, 2004)13•20. Kinematical studies of tidal tails help in identifying such projection effects (Bournaud et al., 2004)3 . The spatial and velocity coincidence of the different phasesof the tidal gas towards a TDG candidate adds circumstantial evidence as to its reality (Braine et al., 2001) 5. The Star Formation Rate so far measured in TDGs vary from 0.01 M8/yr to 0.5 M8/yr. TD Gs seem to form stars with an efficiency (defined as the ratio between the SFR and the molecular gas content) comparable to that measured in spiral disks (Braine et al., 2001)5, i.e. less than in typical dwarf irregular galaxies. Note however that the H2 massin classical dwarfs, and hence the star formation efficiency, is difficultto estimate because of their low metallicity and thus weak CO emission. Having a prominent gas reservoir, TDGs can sustain star formation for more than 1 Gyr, i.e. ten times longer than the infrared luminous nuclear starburst. Therefore, in advanced mergers, the only on-going activity may actually take place at the tip of tidal tails.

433 4. 2 Simulations: forming proto-TDGs in extended dark matter haloes If the formation of bound intermediate-mass clumps along tidal tails has been reproduced in numerical simulations for more than a decade, the genesis of massive proto-TDGs at the tip of the tails has just recently been achieved by Bournaud et al. (2003)4. Their simulations (see Fig. 5) include extended dark matter haloes (at least ten time the stellar radii), which turned out to be a key ingredient. Indeed, within extended haloes, the tidal field can efficiently carry away from the parent disk a large fraction of the gas, while maintaining its surface density to a high value. This creates a density contrast near the tip of the tail. Otherwise, outside a truncated dark matter halo, the tidal material is diluted along the tail. Later-on, self-gravity takes over; the gas clouds collapse and start forming stars. Thus, such TDGs were fundamentally formed following a kinematical process, according to a top-down scenario (Due et al., 2004)7. Their origin hence differs fromthe less massive Super Star Clusters that are also present around mergers, but were formed from growing local instabilities.

5 Up to the intergalactic medium

5. 1 Intergalactic HII regions In some specific environments, such as compact groups of galaxies or clusters, em1ss10n line regions, characterized by a very low underlying old stellar content, and sometimes by their compact aspect (the so-called EL-Dots) were detected even further away from their parent galaxy, in what could already be considered as the intergalactic medium (see for instance the spectacular Stephan's Quintet shown in Fig. 6) Their optical spectra are typical of star-forming HII regions (e.g. Gerhard et al., 2002; Cortese et al., 2004; Ryan-Weber et al., 2004; Mendes de Oliveira et al., 2004) 11•6•25•19. Each individual region has a modest star-formation rate, usually of the order of 0.001-0.005 M0/yr. Their rather high oxygen abundances indicate that they are formed of pre-enriched gas most probably stripped from spiral disks. However, contrary to the objects in stellar tidal tails, the umbilical cord linking them to their parent galaxies is much more difficult to observe, at least through the optical window. Given the distance to their progenitors and the time scale for the gas removal (at least 100 Myr), star-formation has started in situ in the IGM.

5.2 Enriching the intergalactic medium The stripping of the gaseous material fueling the intergalactic star-forming regions could either be due to tidal interactions, as discussed above and/or to ram-pressure if the parent galaxy is moving through a dense medium. Because this mode of star-formation occurs outside any pre-formed local galactic potential (although in the outskirts of the dark matter halo around the parent galaxy), it should contribute to the IGM enrichment in a particularly efficient way. Indeed the supernovae ej ecta are directly transfered into the IGM.

6 Back to the galaxies

6. 1 Observations: shells and rings around mergers Advanced mergers, such as NGC 7252, and old mergers (among them, some " disturbed" elliptical galaxies) are surrounded by stellar rings, loops and shells. These remnants of a past collision are probably tidal material falling back onto the galaxies. Their presence on optical images illustrates the fact that the most likely fate of the tidal material sent into the IGM is to simply return to their progenitors. Gravity and dynamical friction cause their orbital decay. Thus, the merger that temporarily lost part of its gas will be supplied again and able to form stars long

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Figure 6: Star forming regions in the intragroup medium of the Stephan's Quintet. Top: Ha contours (courtesy of Jorge Iglesias-Paramo) superimposed on an optical CFHT image of the group. The location of several apparently isolated intergalactic HII regions is delineated by the ellipse. They actually lie within an extended HI structure not shown on the figure. Bottom: close-up around the SQ-B area showing the Plateau de Bure CO(l--0) contours superimposed on an HST image (adapted from Lisenfeld et al., 2004). In this region located near the tip of an optical tidal tail, large quantities of molecular gas, equivalent to the total H2 content of the Milky Way, were found (Lisenfeld et al., 2002). The PdB CO emission closely matches the morphology of an optical dust lane and of an associated, partly obscured, HU-region. SQ-B was also detected at mid-infrared wavelengths (Xu et al., 2003). Its inferred integrated SFR is as high as 0.5 M0/yr. Its properties resemble more those of Tidal Dwarf Galaxies than the intergalactic star-forming regions located further to the North, where no CO signal was detected.

435 Table 1: Modes of star-formation in and around mergers

Mode Location Onset Time Duration Triggering Mecha nism

Nuclear starburst Central regions Between the pe- Less than Condensation of riapse and final 100 Myr gas transported via merger bars

Extended Star For- Bridges, tails, inter- Around the peri- Several 100 ISM-ISM shocks mati on face region apse Myr

Star Formation in Tip of tidal tails After the periapse Several 100 Kinematical ongm TD Gs Myr + in situ collapse of massive gas clouds

Intergalactic Star the IGM/ICM ?? 10 Myr ? Tidal or ram- Formation In pressure stripping

Delayed Star For- Outer regions Soon after peripase up to several Return of tidal ma- mation Gyr terial after the central starburst has ceased. The tidal interaction is hence responsible for a delayed SF episode.

6.2 Simulations: the return of tidal material Using a numerical model of NGC 7252, Hibbard & Mihos (1995)14 computed how long it takes for tidal material to fall back. They showed that this rate decreases with time but, depending on its initial location along the tidal tail, the return may take several Gyrs for material at the tip of the tail or only a few Myr for material at the base of the tail.

Conclusions 7 Table 1 summarizes the different modes of star formation taking place in and around mergers and discussed in this review. Some of them may actually be weak or absent in a given interacting system. Depending on the initial physical conditions in the parent galaxies (in particular the amount, location and kinematics of the gas reservoirs), the geometrical parameters of the collision (in particular, whether the collision is prograde or retrograde) and the large-scale environment (whether the collision occurs in a void, in a group or a cluster), one mode of star-formation or another may be favored. One should, in any case,keep in mind that although the particularly active but short-lived nuclear starbursts have received a lot of attention, star formation in major mergers may be, spatially, extended, and, as a result, last much longer than usually believed.

Acknowledgments

I wish to thank all my colleagues working on extended star-formation in interacting systems and on Tidal Dwarf Galaxies, in particular Philippe Amrarn, Frederic Bournaud, Jonathan Braine,

436 Elias Brinks, Vassilis Charmandaris, Stephane Leon, Ute Lisenfeld, Frederic Masset and Peter Weilbacher.

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