Tidal Dwarf Galaxies and missing baryons Frederic Bournaud CEA Saclay, DSM/IRFU/SAP, F-91191 Gif-Sur-Yvette Cedex, France Abstract Tidal dwarf galaxies form during the interaction, collision or merger of massive spiral galaxies. They can resemble “normal” dwarf galaxies in terms of mass, size, and become dwarf satellites orbiting around their massive progenitor. They nevertheless keep some signatures from their origin, making them interesting targets for cosmological studies. In particular, they should be free from dark matter from a spheroidal halo. Flat rotation curves and high dynamical masses may then indicate the presence of an unseen component, and constrain the properties of the “missing baryons”, known to exist but not directly observed. The number of dwarf galaxies in the Universe is another cosmological problem for which it is important to ascertain if tidal dwarf galaxies formed frequently at high redshift, when the merger rate was high, and many of them survived until today. In this article, I use “dark matter” to refer to the non-baryonic matter, mostly located in large dark halos – i.e., CDM in the standard paradigm. I use “missing baryons” or “dark baryons” to refer to the baryons known to exist but hardly observed at redshift zero, and are a baryonic dark component that is additional to “dark matter”. 1. Introduction: the formation of Tidal Dwarf Galaxies A Tidal Dwarf Galaxy (TDG) is, per definition, a massive, gravitationally bound object of gas and stars, formed during a merger or distant tidal interaction between massive spiral galaxies, and is as massive as a dwarf galaxy [1] (Figure 1). It should also be relatively long-lived, so that it survives after the interaction, either orbiting around its massive progenitor or expelled to large distances. This requires a lifetime of at least 1Gyr, and a transient structure during a galaxy interaction would not deserve to be considered as a real TDG. The formation of TDGs in mergers has been postulated for decades [31], including potential candidates in the Antennae galaxies (NGC4038/39) [32], and became an increasingly active research topic after the study of these tidal dwarf candidates by Mirabel et al. [33]. Tidal tails are a common feature in galaxy interactions. There are also some tidal bridges and collisional rings that come about from the same processes and have similar properties, even though some details differ. The tidal tails are those long filaments seen around many interacting galaxies. They are made-up of material expelled from the disk of a parent spiral galaxy [2]. This material is expelled partly under the effect of tidal forces exerted by the other interacting galaxy, as the name suggests, but in fact for a large part by gravitational torques as the name does not suggest. The perturbing galaxy exerts non-radial forces in the disturbed disk, so that some gas loses angular momentum, flows towards the center where it can fuel a starburst [3], and some other gas gains angular momentum and flies away in long tidal tails. At least two mechanisms can lead to the formation of massive substructures in tidal tails (Figures 1 and 2). First, gravitational instabilities can develop, as in any gas-rich medium. Having the material expelled from the disk eases gravitational collapse, as the stabilizing effect of rotation in the parent disk disappears, or weakens. The process is then a standard Jeans instability: when a region collects enough gas to overcome internal pressure support, it collapses into a self-bound object. The same process is believed to drive the formation of molecular clouds in spiral galaxy. The difference is that the interaction stirs and heats the gas, and increases its velocity dispersion [4]. As a result, the typical Jeans mass is high, enabling relatively massive objects to form. These star-forming gas clumps form at the Jeans mass, with a regular spacing, the Jeans wavelength, all along the long tidal tails. They resemble “beads on a string” [5]. Numerical simulations model their formation accurately, provided that the resolution is high (to revolve the instability length) and gas content is accounted for with some hydrodynamic model [6]. This mechanism can result in relatively numerous TDGs, maybe ten per major merger, but these are not very massive, at most a few 7 10 MO. The same process can form a large number of less massive structures, which are super star clusters rather than dwarf galaxies, potentially evolving into globular clusters. Figure 1. NGC7252 (left) is a recent merger of two spiral galaxies into a partially-relaxed central spheroidal galaxy. Two massive TDGs are found near the tip of the two long tidal tails (blue = HI, pink = H! -- image courtesy of Pierre-Alain Duc). AM 1353-272 (right) does not have prominent, massive TDGs at the tip of tidal tails, but has instead may lower-mass objects all along its tail [37]. The bright spot on the northern tail is a foreground star. Some tidal dwarfs are much more massive, a few 108 or 109 solar masses, and typically form as single objects at the tip of tidal tails [7]. They cannot form just by local instabilities in a tidal tail. These massive TDGs result from the displacement of a large region of the outer disk of the progenitor spiral galaxy into the outer regions of the tidal tail, where material piles up and remains or becomes self-bound [8,9]. As the interaction stirs the gas, the increased turbulence will provide the required pressure support to avoid fragmentation into many lower-mass objects. The shape of cold dark matter haloes, that are much more extended than the visible part of galaxies, make this process more efficient [10]. The pile-up of material in massive TDGs often occurs at the tip of tidal tails, but can also occur in different situations like gas captured and swung around the companion [34,5] or gas bridges linking two interacting galaxies [40,35]. A few tidal dwarfs of moderate mass, from the first mechanism, are found frequently in observed interacting galaxies. Massive TDGs in the very outer regions are more rare, at most 2-3 per merger, but some mergers do not have any at all. Observations suggest that the presence of one type of TDG reduces the number of TDGs of the other type [11]. This is likely because the formation of a massive, tip-of-tail TDG collects a large fraction of the gas, and less remains available for the formation of numerous low-mass TDGs along the tail, by the first mechanism [11]. Sections 2 and 3 will review the internal mass content of TDGs and how this can constrain the nature of missing baryons. Different constraints, arising from statistics on TDG formation and survival, will be discussed in Section 4. Figure 2. Simulation of the formation of long-lived TDGs, from the Bournaud & Duc sample [14]. The two colliding spiral galaxies are initially seen face-on. Yellow-red to blue colors code old stars versus young stars; time is in Myr. Massive TDGs form at the tip of the tails, and some lower-mass ones also form in particular in the Northern tail. A multi-grid technique increases the resolution on the most massive TDG to 10 parsec. It is resolved with a small internal spiral structure, and survives several Gyr around a massive red elliptical galaxy. 2. Why should Tidal Dwarf Galaxies be free of dark matter? A common property of all TDGs, be they formed by a local instability, or by the gathering of a large portion of the progenitor spiral galaxy at the tip of a tidal tail, is that they are made-up only from material that comes from the disk of the parent spiral galaxy. Indeed, only the material initially in rotating disks, with velocity dispersions much lower than the rotation velocity, is strongly affected by tidal forces and gravity torques and forms tidal tails – and subsequently tidal dwarfs. Spheroids dominated by random velocity dispersions instead of rotation will barely develop a weak egg-shaped distortion during the interaction, but no long and dense tidal tail; this applies to the bulge of a spiral galaxy, a whole elliptical galaxy, but also to the dark matter halo of any galaxy. Dark matter forms a spheroidal halo around galaxies, in both the standard Cold Dark Matter theories [12] and in other models (e.g., Warm Dark Matter [13]). All simulations show this dark matter cannot participate in the formation of TDGs [6,9,14]. Once a TDG has formed, its escape velocity is low, at most a few tens of km s-1 for the biggest ones. The TDG will be embedded in the large halo of the parent spiral galaxy (since halos are much more extended than stellar and gaseous disks), so some dark matter particles will cross the TDG, but without being held by the gravitational well of the TDG. Indeed, the randomly-oriented velocities of dark matter particles in the halo of a spiral galaxy like the Milky Way are around 200 km s-1, so the vast majority of these particles escape the tidal dwarf. At any instant, some dark matter particles from the halo will incidentally be at the position of the TDG, but this makes up only a negligible fraction of the mass of any TDG, at most a few percent. TDGs should then be free of dark matter. This means that the dynamical mass measured from their rotation velocity and size, should not exceed their visible mass in stars and gas – in contrast with “normal” dwarf galaxies and spiral galaxies. Finding a dynamical mass in significant excess would mean that TDGs contain an unseen component.