4) How are galaxies formed?

Recent observations of galaxy mergers inside galaxy cluster environments, such as NGC 5291 in the vicinity of Abell 3574, report high star formation rates in the ejected tidal tails, which point towards currently developing tidal dwarf galaxies. This prompts the intriguing question whether these newly formed stellar structures could get stripped from the galaxy potential by the cluster and thus populate it with dwarf galaxies. Aims. We verify whether environmental stripping of tidal dwarf galaxies from galaxy mergers inside galaxy cluster environments is a possible evolutionary channel to populate a galaxy cluster with low-mass and low surface brightness galaxies. Methods. We performed three high-resolution hydrodynamical simulations of mergers between spiral galaxies in a cluster environment, implementing a stellar mass ratio of 2:1 with M200 = 9.5 × 1011 M for the more massive galaxy. Between the three different simulations, we varied the initial orbit of the infalling galaxies with respect to the cluster center. Results. We demonstrate that cluster environments are capable of stripping tidal dwarf galaxies from the host potential independently of the infall orbit of the merging galaxy pair, without instantly destroying the tidal dwarfs. Starting to evolve separately from their progenitor, these newly formed dwarf galaxies reach total masses of Mtot ≈ 107−9 M within the limits of our resolution. In the three tested orbit scenarios, we find three, seven, and eight tidal dwarf galaxies per merger, respectively, which survive longer than 1 Gyr after the merger event. Exposed to ram pressure, these gas dominated dwarf galaxies exhibit high star formation rates while also losing gas to the environment. Experiencing a strong headwind due to their motion through the intracluster medium, they quickly lose momentum and start spiraling towards the cluster center, reaching distances on the order of 1 Mpc from their progenitor. About 4 Gyr after the merger event, we still find three and four intact dwarf galaxies in two of the tested scenarios, respectively. The other stripped tidal dwarf galaxies either evaporate in the hostile cluster environment due to their low initial mass, or are disrupted as soon as they reach the cluster center. Conclusions. The dwarf production rate due to galaxy mergers is elevated when the interaction with a cluster environment is taken into account. Comparing their contribution to the observed galaxy mass function in clusters, our results indicate that ∼30% of dwarf galaxies in clusters could have been formed by stripping from galaxy mergers. Key words. galaxies: clusters: intracluster medium – galaxies: dwarf – galaxies: formation – galaxies: interactions – galaxies: starburst

Observations and simulations suggest that dwarf-sized galaxies contain a significant amount of mass in the galaxy mass function of the local Universe (e.g., Sabatini et al. 2003; Schaye et al. 2015). Nevertheless, the variety of formation mechanisms and their respective contribution is still poorly understood. In particular, the presence of dark matter-deficient galaxies reported by observations (e.g., van Dokkum et al. 2018, 2019; Mancera Piña et al. 2019; Guo et al. 2020; Hammer et al. 2020) raises the need to search for different evolutionary scenarios other than the typical halo collapse model. It becomes crucial to consider the impact of the local environment and to study how external forces – namely tidal interaction and ram pressure – could impact existing galaxies and thereby invoke the formation of different kinds of galaxies, as observed objects are not always isolated. In that context, there have been several mech- ? Movie associated to Fig. 3 is available at https:// www.aanda.org anisms proposed, such as tidal stripping of the dwarf’s dark matter component by a massive companion or a cluster (e.g., Ogiya 2018; Jing et al. 2019; Niemiec et al. 2019; Jackson et al. 2021; Moreno et al. 2022) and separation between the baryonic and non-interacting dark matter component during high-velocity collisions of several dwarf galaxies (e.g., Silk 2019; Shin et al. 2020; Lee et al. 2021; Otaki & Mori 2023). Another pathway to produce dark matter-deficient objects discussed in the literature is the possibility of long-lived tidal dwarfs forming in the gaseous tails ejected by galaxy mergers (e.g., Mirabel et al. 1992; Bournaud & Combes 2003; Bournaud et al. 2004, 2008; Bournaud & Duc 2006; Kroupa 2012). In such a scenario, structure formation would be triggered either by gas collapse due to Jeans instability or by local potential wells of ejected old stars. In both cases, this induces further gas accretion, triggering star formation and ultimately the birth of a new gravitationally bound and kinematically decoupled object (Duc 2012). It is crucial to provide sufficiently large gas reservoirs to spark the formation of such star-forming pockets, suggesting that at least one of the merging galaxies needsto have a high gas mass fraction (Wetzstein et al. 2007). Due to the inherent deficiency of dark matter at the tidal formation sites, such dwarf galaxies are expected to be dark matter-poor (Barnes & Hernquist 1992). Numerical studies of tidal dwarfs forming both in isolated mergers (Bournaud & Duc 2006), as well as in cosmological simulations (Ploeckinger et al. 2018; Haslbauer et al. 2019) are able to produce dark matter-deficient tidal dwarf galaxies with a stellar population dominated by young stars. While isolated simulations predict that the produced tidal dwarfs generally will fall back into the merging galaxy pair, their development inside cosmological environments is still unclear, as the coarse temporal spacing between snapshots has not yet allowed a detailed investigation of the dwarfs’ evolution. Although observations find tidal dwarfs with the aforementioned properties, for example those forming in the vicinity of NGC 5291 (Duc & Mirabel 1998; Rakhi et al. 2023), they also report cases of dwarf galaxies inside galaxy clusters with significant dark matter content or mixed stellar age component (e.g., Kaviraj et al. 2012; Gannon et al. 2020; Román et al. 2021; Gray et al. 2023). However, it needs to be pointed out that unambiguously determining the tidal origin of dwarf galaxies in observations is difficult, especially once the optical bridges between progenitor and dwarf disappear. Afterwards, the only established observational fingerprint to identify isolated tidal dwarf galaxies are unusually high metallicities since their young stellar component forms from ejected, pre-enriched gas (Duc 2012). The presence of dwarf galaxies in galaxy clusters with particularly high metallicities for their luminosity (Rakos et al. 2000; Duc et al. 2001; Poggianti et al. 2001; Iglesias-Páramo et al. 2003) could signify a partly tidal origin, although different scenarios could also explain such peculiarities (Conselice et al. 2003b). Simulating the formation and evolution of tidal dwarf galaxies is challenging as well, since the necessity to properly model gas fragmentation and succeeding star formation requires high resolution (Teyssier et al. 2010). In case of an included environment, for example the hot gaseous atmosphere of a galaxy cluster, it is therefore necessary to resolve gas components of structures spanning many orders of magnitude in mass, driving up the numerical cost of the simulation. Nonetheless it becomes indispensable, as studies of single galaxies exposed to ram pressure already suggest a significant environmental impact by boosting the star formation activity, leading to so-called jellyfish galaxies (e.g., Kapferer et al. 2009; Tonnesen & Bryan 2021; Lee et al. 2022). The aforementioned system of NGC 5291 in the western outskirts of cluster Abell 3574 is a convincing exemplary case to highlight the scientific importance of studying galaxy mergers inside cluster environments. It is an ongoing major merger between two galaxies – the apparent early-type galaxy NGC 5291 and its distorted companion referred to as “the Seashell” (Longmore et al. 1979). The complex is surrounded by an extremely extended HI structure which is thought to be the gas-rich tidal arms stripped from the galaxies during the merger event. It features distinct star-forming knots towards the north and south of the system, which were first revealed by deep optical and spectroscopic studies (Pedersen et al. 1978; Longmore et al. 1979) and later confirmed by observations in ultraviolet (e.g., Boquien et al. 2007; Fensch et al. 2019; Rakhi et al. 2023). Such a constellation, combined with high metallicities and the absence of an old stellar population, suggests that these star-forming regions might be tidal dwarf galaxies (Malphrus et al. 1997; Duc & Mirabel 1998; Bournaud et al. 2008). As this merging system is situated within the outskirts of the galaxy cluster Abell 3574, the cluster is suspected to have already impacted the ongoing merger, therefore clearly demonstrating the importance of studying how environmental effects are influencing the structures in order to be able to reconstruct and predict both their past and future behavior. In this paper, we aim to answer the following questions: Is a cluster able to strip tidal dwarf galaxies from their progenitor? If so, how long would the stripped tidal dwarf galaxies survive while being exposed to the hostile conditions in a cluster? What general influence do these effects have on the probability of forming tidal dwarf galaxies? For this purpose, we perform hydrodynamical simulations of ongoing galaxy mergers in a cluster environment with high mass resolution and unprecedented time resolution, varying the infall direction between three different angles from a radial orbit to an initial infall angle of 45◦ . In Sect. 2 we describe the simulation parameters and numerical implementation details. We provide a general proof of concept for environmentally supported tidal dwarf galaxy formation in Sect. 3. The detailed results are presented in Sect. 4, where we analyze and discuss the emerging tidal dwarf population, as well as its temporal evolution. Furthermore, we use our results to provide an estimate of the tidal dwarf fraction among the total dwarf galaxy population inside galaxy clusters in Sect. 5. Our conclusions are summarized in Sect. 6.

The simulations are performed using the Tree-SPH (smoothed particle hydrodynamics) code Gadget-3, which adopts the gravity force tree algorithm (Appel 1985; Barnes & Hut 1986) from its publically available predecessor Gadget-2 (Springel 2005), but employs an improved SPH implementation (e.g., Beck et al. 2016). An SPH code decomposes the domain into “particles”, representative of the local mass distribution. The evolution of the system is then obtained by solving the hydrodynamic equations in Lagrangian form for each particle. Physical properties at each point can be retrieved by summing over the contributions of a specified number of neighboring particles, weighted by a radially symmetric smoothing kernel. In our simulations, we adopt smoothing over 64 neighboring particles with a Wendland C2 kernel (Wendland 1995; Dehnen & Aly 2012). The code includes the star formation and supernova feedback model by Springel & Hernquist (2003), which treats the interstellar gas as a two-phase system with cold, star-forming gas embedded in an ambient hot medium. When the density of a gas particle exceeds a given threshold, it is converted into a stellar particle, representative of a stellar isochrone population according to an assumed initial mass function (e.g., Salpeter 1955). We incorporate radiative cooling of optically thin gas in ionization equilibrium with an ultraviolet background (Katz et al. 1996). To follow the positions of the two galaxies, we place a massive tracer particle at each of the galaxy centers, representing a supermassive black hole. The initial conditions for the simulations are created in three steps: initializing the late-type galaxies and combining them into a merger configuration (Sect. 2.1), setting up the galaxy cluster (Sect. 2.2), and combining the galaxy merger and the cluster into the final simulation setup (Sect. 2.3).