Review Article Tidal Dwarf Galaxies and Missing Baryons
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Hindawi Publishing Corporation Advances in Astronomy Volume 2010, Article ID 735284, 7 pages doi:10.1155/2010/735284 Review Article Tidal Dwarf Galaxies and Missing Baryons Frederic Bournaud CEA Saclay, DSM/IRFU/SAP, F 91191 Gif-Sur-Yvette Cedex, France Correspondence should be addressed to Frederic Bournaud, [email protected] Received 23 June 2009; Accepted 4 August 2009 Academic Editor: Elias Brinks Copyright © 2010 Frederic Bournaud. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 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 paper, “dark matter” is used to refer to the nonbaryonic matter, mostly located in large dark halos, that is, CDM in the standard paradigm, and “missing baryons” or “dark baryons” is used 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 under the effect of tidal forces exerted by the other interacting Tidal Dwarf Galaxies galaxy, as the name suggests, but in fact for a large part by gravitational torques as the name does not suggest. The Tidal dwarf galaxy (TDG) is, per definition, a massive, perturbing galaxy exerts nonradial forces in the disturbed gravitationally bound object of gas and stars, formed during disk, so that some gas loses angular momentum, flows a merger or distant tidal interaction between massive spiral toward the center where it can fuel a starburst [7], and some galaxies, and is as massive as a dwarf galaxy [1](Figure 1). other gas gains angular momentum and flies away in long It should also be relatively long-lived, so that it survives after tidal tails. the interaction, either orbiting around its massive progenitor At least two mechanisms can lead to the formation of or expelled to large distances. This requires a lifetime of at massive substructures in tidal tails (Figures 1 and 2). First, least 1 gigayear, and a transient structure during a galaxy gravitational instabilities can develop, as in any gas-rich interaction would not deserve to be considered as a real TDG. medium. Having the material expelled from the disk eases The formation of TDGs in mergers has been postulated for gravitational collapse, as the stabilizing effect of rotation in decades [2], including potential candidates in the Antennae the parent disk disappears, or weakens. The process is then galaxies (NGC4038/39) [3], and became an increasingly a standard Jeans instability: when a region collects enough active research topic after the study of these tidal dwarf gas to overcome internal pressure support, it collapses into a candidates by Mirabel et al. [4]. self-bound object. The same process is believed to drive the Tidal tails are a common feature in galaxy interactions. formation of molecular clouds in spiral galaxy. The difference There are also some tidal bridges and collisional rings that is that the interaction stirs and heats the gas, and increases come about from the same processes and have similar its velocity dispersion [8]. As a result, the typical Jeans mass properties, even though some details differ. The tidal tails are is high, enabling relatively massive objects to form. These those long filaments seen around many interacting galaxies. star-forming gas clumps form at the Jeans mass, with a They are made up of material expelled from the disk of regular spacing, the Jeans wavelength, all along the long a parent spiral galaxy [6]. This material is expelled partly tidal tails. They resemble “beads on a string” [9]. Numerical 2 Advances in Astronomy (a) (b) Figure 1: NGC7252 (a) 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 = Hα – image courtesy of Pierre-Alain Duc). AM 1353-272 (b) does not have prominent, massive TDGs at the tip of tidal tails, but has instead may lower-mass objects all along its tail [5]. The bright spot on the northern tail is a foreground star. simulations model their formation accurately, provided that T = 0 T = 300 the resolution is high (to revolve the instability length) and gas content is accounted for with some hydrodynamic model [10]. This mechanism can result in relatively numerous TDGs, maybe ten per major merger, but these are not very 7 massive, at most a few 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. 50 kpc Some tidal dwarfs are much more massive, a few 108 9 or 10 solar masses, and typically form as single objects at = the tip of tidal tails [12]. They cannot form just by local T 1000 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 [13, 14]. 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, which are much more extended than the visible part of galaxies, making this process Figure 2: Simulation of the formation of long-lived TDGs, from more efficient [15]. The pile-up of material in massive TDGs theBournaud&Ducsample[11]. The two colliding spiral galaxies often occurs at the tip of tidal tails, and can also occur in ff are initially seen face-on. Yellow-red to blue colors code old stars di erent situations like gas captured and swung around the versus young stars; time is in megayear. Massive TDGs form at the companion [9, 16] or gas bridges linking two interacting tip of the tails, and some lower-mass ones also form in particular in galaxies [17, 18]. the Northern tail. A multi-grid technique increases the resolution A few tidal dwarfs of moderate mass, from the first on the most massive TDG to 10 parsec. It is resolved with a small mechanism, are found frequently in observed interacting internal spiral structure, and survives several gigayears around a galaxies. Massive TDGs in the very outer regions are more massiveredellipticalgalaxy. rare,atmost2–3permerger,butsomemergersdonothave any at all. Observations suggest that the presence of one type of TDG reduces the number of TDGs of the other type [19]. This is likely because the formation of a massive, tip-of-tail Sections 2 and 3 will review the internal mass content TDG collects a large fraction of the gas, and less remains of TDGs and how this can constrain the nature of missing available for the formation of numerous low-mass TDGs baryons. Different constraints, arising from statistics on TDG along the tail, by the first mechanism [19]. formation and survival, will be discussed in Section 4. Advances in Astronomy 3 Dark matter forms a spheroidal halo around galaxies, in both the standard Cold Dark Matter theories [20] and in other models (e.g., Warm Dark Matter [21]). All simulations show that this dark matter cannot participate in the formation of TDGs [10, 11, 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 −1 (a) spiral galaxy like the Milky Way are around 200 km s ,so the vast majority of these particles escape the tidal dwarf. At 100 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 80 few percent. TDGs should then be free of dark matter. This means that 60 the dynamical mass measured from their rotation velocity and size, should not exceed their visible mass in stars and 40 gas—in contrast with “normal” dwarf galaxies and spiral Velocity (km/s) galaxies. Finding a dynamical mass in significant excess 20 would mean that TDGs contain an unseen component. This could be dark matter only if (part of the) dark matter is in a rotating disk component in their progenitor spiral galaxies, 0 012345just like stars and gas. Otherwise, this would require an unseen baryonic component in spiral galaxies, not just in Radius (kpc) the form of a hot gas halo—this one does not participate in Observed the formation of TDGs—but in the form of very cold gas Allowed by visible mass in the rotating disk.