Galaxy Encounters Research Articles

The MeerKAT Fornax Survey

We present MeerKAT Fornax Survey H I observations of NGC 1427A, a blue irregular galaxy with a stellar mass of ∼2 × 109 M⊙ located near the centre of the Fornax galaxy cluster. Thanks to the excellent resolution (1–6 kpc spatially, 1.4 km s−1 in velocity) and H I column density sensitivity (∼4 × 1019 to ∼1018 cm−2 depending on resolution), our data deliver new insights on the long-debated interaction of this galaxy with the cluster environment. We confirm the presence of a broad, one-sided, starless H I tail stretching from the outer regions of the stellar body and pointing away from the cluster centre. We find the tail to have 50% more H I (4 × 108 M⊙) and to be 3 times longer (70 kpc) than in previous observations. In fact, we detect scattered H I clouds out to 300 kpc from the galaxy in the direction of the tail – possibly the most ancient remnant of the passage of NGC 1427A through the intracluster medium of Fornax. Both the velocity gradient along the H I tail and the peculiar kinematics of H I in the outer region of the stellar body are consistent with the effect of ram pressure given the line-of-sight motion of the galaxy within the cluster. However, several properties cannot be explained solely by ram pressure and suggest an ongoing tidal interaction. This includes: the close match between dense H I and stars within the disturbed stellar body; the abundant kinematically anomalous H I; and the inversion of the H I velocity gradient near the base of the H I tail. We rule out an interaction with the cluster tidal field, and conclude that NGC 1427A is the result of a high-speed galaxy encounter or of a merger started at least 300 Myr ago, where ram pressure shapes the distribution and kinematics of the H I in the perturbed outer stellar body and in the tidal tails.

Orbit decay in encounters between anisotropic spherical galaxies of equal mass

ABSTRACT We investigate the effect of radial anisotropy on the rate of orbit decay in parabolic encounters of identical spherical galaxies. Our galaxy models have Hernquist density profiles and Osipkov–Merritt velocity distributions. We find that radially anisotropic models merge in as little as half the time of their isotropic counterparts. Anisotropic models are more susceptible to tidal deformation; this accelerates the transfer of orbital angular momentum to internal degrees of freedom. Even during the initial approach, the anisotropic models become more distorted, and arrive at pericentre already having lost substantial amounts of angular momentum. Our results may have implications for estimates of merger rates and the persistence of tidal tails.

The magnetized and thermally unstable tails of jellyfish galaxies

ABSTRACT Jellyfish galaxies are promising laboratories for studying radiative cooling and magnetic fields in multiphase gas flows. Their long, dense tails are observed to be magnetized, and they extend up to 100 kpc into the intracluster medium (ICM), suggesting that their gas is thermally unstable so that the cold gas mass grows with time rather than being fully dissolved in the hot wind as a result of hydrodynamical interface instabilities. In this paper, we use the arepo code to perform magnetohydrodynamical windtunnel simulations of a jellyfish galaxy experiencing ram-pressure stripping by interacting with an ICM wind. The ICM density, temperature and velocity that the galaxy encounters are time-dependent and comparable to what a real jellyfish galaxy experiences while orbiting the ICM. In simulations with a turbulent magnetized wind, we reproduce observations, which show that the magnetic field is aligned with the jellyfish tails. During the galaxy infall into the cluster with a near edge-on geometry, the gas flow in the tail is fountain-like, implying preferential stripping of gas where the rotational velocity vectors add up with the ram pressure while fall-back occurs in the opposite case. Hence, the tail velocity shows a memory of the rotation pattern of the disc. At the time of the nearest cluster passage, ram-pressure stripping is so strong that the fountain flow is destroyed and instead the tail is dominated by the removal of gas. We show that gas in the tail is very fragmentative, which is a prediction of shattering due to radiative cooling.

Heavy Elements Unveil the Non-primordial Origin of the Giant H I Ring in Leo

Abstract The origin and fate of the most extended extragalactic neutral cloud known in the local universe, the Leo ring, is still debated 38 yr after its discovery. Its existence is alternatively attributed to leftover primordial gas with some low level of metal pollution versus enriched gas stripped during a galaxy−galaxy encounter. Taking advantage of Multi Unit Spectroscopic Explorer operating at the Very Large Telescope, we performed optical integral field spectroscopy of three H I clumps in the Leo ring where ultraviolet continuum emission has been found. We detected, for the first time, ionized hydrogen in the ring and identify four nebular regions powered by massive stars. These nebulae show several metal lines ([O III], [N II], [S II]) that allowed reliable measures of metallicities, found to be close to or above the solar value (0.8 ≤ Z/Z ⊙ ≤ 1.4). Given the faintness of the diffuse stellar counterparts, less than 3% of the observed heavy elements could have been produced locally in the main body of the ring and not much more than 15% in the H I clump toward M96. This inference, and the chemical homogeneity among the regions, convincingly demonstrates that the gas in the ring is not primordial, but has been pre-enriched in a galaxy disk, then later removed and shaped by tidal forces and it is forming a sparse population of stars.

Investigating Early-type Galaxy Evolution with a Multiwavelength Approach. III. Insights from SPH Simulations with Chemophotometric Implementation

We are exploring galaxy evolution in low density environments exploiting smooth particle hydrodynamic simulations including chemo-photometric implementation. From a large grid of simulations of galaxy encounters and mergers starting from triaxial halos of gas e dark matter, we single out the simulations matching the global properties of our targets. These simulations are used to give insights into their evolution. We focus on 11 early-type galaxies selected because of their nearly passive stage of evolution in the nuclear region. However, a variety of UV features are detected in more than half of these galaxies. We find no significant differences in the formation mechanisms between galaxies with or without UV features. Major and minor mergers are able to reproduce their peculiar UV morphologies, galaxy encounters are more suitable for 'normal' early-type galaxies. Their star formation rate self-quenches several Gyr later the merger/encounter occurred, via gas exhaustion and stellar feedback, moving the galaxy from blue to red colors, driving the galaxy transformation. The length of the quenching is mass dependent and lasts from 1 to 5 Gyr or more in the less massive systems. All our targets are gas rich at redshift 1. Three of them assembled at most 40% of their current stellar mass at z>1, and seven assembled more than 50% between redshift 0.5 and 1. Their stellar mass grows with 4% by crossing the Green Valley before reaching their current position on the NUV-r vs. Mr diagram.

MUSE sneaks a peek at extreme ram-pressure stripping events – IV. Hydrodynamic and gravitational interactions in the Blue Infalling Group

We report new wide-field (~ 4 x 4 square arcmin) MUSE observations of the Blue Infalling Group (BIG), a compact group of galaxies located at a projected distance of ~ 150 kpc from the X-Ray centre of the A1367 cluster at z = 0.021. Our MUSE observations map in detail the extended ionized gas, primarily traced by H$\alpha$ emission, in between the members of the group. The gas morphology and its kinematics appear consistent with a tidal origin due to galaxy encounters, as also supported by the disturbed kinematics visible in one of the group members and the presence of tidal dwarf systems. A diffuse tail extending in the direction opposite to the cluster centre is also detected, hinting at a global ram-pressure stripping of the intra-group material as BIG falls inside A1367. Based on the analysis of spatially-resolved emission line maps, we identify multiple ionization mechanisms for the diffuse gas filaments, including in situ photoionization from embedded HII regions and shocks. Combining spatially resolved kinematics and line ratios, we rule out the association of the most massive galaxy, CGCG097-120, with the group as this system appears to be decoupled from the intragroup medium and subject to strong ram pressure as it falls into A1367. Through our new analysis, we conclude that BIG is shaped by pre-processing produced by gravitational interactions in the local group environment combined with ram pressure stripping by the global cluster halo.

Property of young massive clusters in a galaxy–galaxy merger remnant

Abstract We investigate the properties of young massive clusters (YMCs) in a galaxy–galaxy merger remnant by analyzing the data obtained by a gas-rich major merger simulation in Matsui et al. (2012, ApJ, 746, 26). We found that the YMCs are distributed at a few kpc and at ∼10 kpc from the galactic center; in other words, there are two components of their distribution. The former are formed in filamentary and turbulent gas generated at a few kpc from the center as a result of galaxy encounters, and the latter are formed in tidal tails which are far from the center. The YMCs are much less concentrated than galaxy stars. The mass function of the YMCs is dN/dM∝M−2. Most YMCs are formed between the second encounter and the final coalescence phase of the galactic cores, and their formation rate is especially high at the final coalescence phase. Most of them consist of single stellar population in age, but YMCs with multi-stellar populations in age are also formed. The multiple populations are produced by the following process: a YMC captures dense gas, and another generation of stars form within the cluster. There are several YMCs formed in an isolated disk before the encounter of galaxies. These candidates contain stars with various ages by capturing dense gas and forming stars. YMCs in a merger remnant have various orbits, but a large fraction of candidates have circular orbits.

Disclosing the properties of low-redshift dual AGN through XMM-Newton and SDSS spectroscopy

We report on an optical (SDSS) and X-ray (XMM) study of an optically selected sample of four dual AGN systems at projected separations of 30--60~kpc. All sources are detected in the X-ray band (0.3-10 keV); seven objects are optically identified as Seyfert, while one source, optically classified as a LINER, is likely powered by accretion in virtue of its relatively high X-ray luminosity (1.2$\times10^{41}$ erg/s). Six of the eight objects are obscured in X-rays with N$_{\rm H} \geq$ 10$^{23}$ cm$^{-2}$; three of these, whose X-ray spectrum is dominated by a reflection component, are likely Compton-thick (N$_{\rm H} \geq$ 10$^{24}$ cm$^{-2}$). This finding is in agreement with the hypothesis that galaxy encounters are effective in driving gas inflow toward the nuclear region, thus increasing the obscuration. We compare the absorption properties in our dual AGN with those in larger samples observed in X-rays but selected in different ways (optical, IR and hard X-rays). We find that the obscured (N$_{\rm H} \geq$ 10$^{22}$ cm$^{-2}$) AGN fraction within the larger sample is 84$\pm$4 per cent (taking into account the 90 per cent error on the N$_{\rm H}$ measure) up to large pair separations ($\sim$100~kpc). This is statistically higher than the fraction of obscured AGN in isolated galaxies found in X-ray surveys.

Modelling the baryon cycle in low-mass galaxy encounters: the case of NGC 4490 and NGC 4485

Discoveries of low mass galaxy pairs and groups are increasing. Studies indicate that dwarf galaxy pairs are gas rich in the field and exhibit elevated star formation rates, suggestive of interactions. Lacking are dynamical models of observed dwarf galaxy pairs to disentangle the physical processes regulating their baryon cycles. We present new optical data and the first detailed theoretical model of an observed tidal encounter between two isolated low mass galaxies, NGC 4490 & NGC 4485. This system is an isolated analog of the Magellanic Clouds and is surrounded by a ~50 kpc extended HI envelope. We use hybrid $N$-body and test-particle simulations along with a visualization interface $Identikit$ to simultaneously reproduce the observed present-day morphology and kinematics. Our results demonstrate how repeated encounters between two dwarf galaxies can "park" baryons at very large distances, without the aid of environmental effects. Our best match to the data is an 8:1 mass ratio encounter where a one-armed spiral is induced in the NGC 4490-analog, which we postulate explains the nature of diffuse starlight presented in the new optical data. We predict that the pair will fully merge in ~370 Myr, but that the extended tidal features will continue to evolve and return to the merged remnant over ~5 Gyr. This pre-processing of baryons will affect the efficiency of gas stripping if such dwarf pairs are accreted by a massive host. In contrast, in isolated environments this study demonstrates how dwarf-dwarf interactions can create a long-lived supply of gas to the merger remnant.

High-redshift major mergers weakly enhance star formation

Galaxy mergers are believed to trigger strong starbursts. This is well assessed by observations in the local Universe. However, the efficiency of this mechanism has poorly been tested so far for high-redshift, actively star-forming, galaxies. We present a suite of pc-resolution hydrodynamical numerical simulations to compare the star formation process along a merging sequence of high- and low-redshift galaxies, by varying the gas mass fraction between the two models. We show that, for the same orbit, high-redshift gas-rich mergers are less efficient than low-redshift ones at producing starbursts; the star formation rate excess induced by the merger and its duration are both around 10 times lower than in the low gas fraction case. The mechanisms that account for the star formation triggering at low redshift – the increased compressive turbulence, gas fragmentation, and central gas inflows – are only mildly, if not at all, enhanced for high gas fraction galaxy encounters. Furthermore, we show that the strong stellar feedback from the initially high star formation rate in high-redshift galaxies does not prevent an increase of the star formation during the merger. Our results are consistent with the observed increase of the number of major mergers with increasing redshift being faster than the respective increase in the number of starburst galaxies.

Shocks and angular momentum flips: a different path to feeding the nuclear regions of merging galaxies

We study the dynamics of galaxy mergers, with emphasis on the gas feeding of nuclear regions, using a suite of hydrodynamical simulations of galaxy encounters. The high spatial and temporal resolution of the simulations allows us to not only recover the standard picture of tidal-torque induced inflows, but also to detail another, important feeding path produced by ram pressure. The induced shocks effectively decouple the dynamics of the gas from that of the stars, greatly enhancing the loss of gas angular momentum and leading to increased central inflows. The ram-pressure shocks also cause, in many cases, the entire galactic gas disc of the smaller galaxy to abruptly change its direction of rotation, causing a complete "flip" and, several $10^8$ yr later, a subsequent "counter-flip". This phenomenon results in the existence of long-lived decoupled gas-stellar and stellar-stellar discs, which could hint at a new explanation for the origin of some of the observed kinematically decoupled cores/counter-rotating discs. Lastly, we speculate, in the case of non-coplanar mergers, on the possible existence of a new class of remnant systems similar to some of the observed X-shaped radio galaxies.

Star formation triggered by galaxy interactions in modified gravity

Together with interstellar turbulence, gravitation is one key player in star formation. It acts both at galactic scales in the assembly of gas into dense clouds, and inside those structures for their collapse and the formation of pre-stellar cores. To understand to what extent the large scale dynamics govern the star formation activity of galaxies, we present hydrodynamical simulations in which we generalise the behaviour of gravity to make it differ from Newtonian dynamics in the low acceleration regime. We focus on the extreme cases of interacting galaxies, and compare the evolution of galaxy pairs in the dark matter paradigm to that in the Milgromian Dynamics (MOND) framework. Following up on the seminal work by Tiret & Combes, this paper documents the first simulations of galaxy encounters in MOND with a detailed Eulerian hydrodynamical treatment of baryonic physics, including star formation and stellar feedback. We show that similar morphologies of the interacting systems can be produced by both the dark matter and MOND formalisms, but require a much slower orbital velocity in the MOND case. Furthermore, we find that the star formation activity and history are significantly more extended in space and time in MOND interactions, in particular in the tidal debris. Such differences could be used as observational diagnostics and make interacting galaxies prime objects in the study of the nature of gravitation at galactic scales.

On the fragility of nuclear stellar discs against galaxy mergers: surviving photometric and kinematic signatures of nuclear discs

Nuclear stellar discs (NSDs) can help to constrain the assembly history of their host galaxies, as long as we can assume them to be fragile structures that are disrupted during merger events. In this work we investigate the fragility of NSDs by means of N-body simulations reproducing the last phases of a galaxy encounter, when the nuclear regions of the two galaxies merge. For this, we exposed a NSD set in the gravitational potential of the bulge and supermassive black hole of a primary galaxy to the impact of the supermassive black hole from a secondary galaxy. We explored merger events of different mass ratios, from major mergers with a 1:1 mass ratio to intermediate and minor interactions with 1:5 and 1:10 ratios, while considering various impact geometries. We analyse the end results of such mergers from different viewing angles and looked for possible photometric and kinematic signatures of the presence of a disc in the remnant surface density and velocity maps, while adopting detection limits from real observations. Our simulations show that indeed NSDs are fragile against major mergers, which leave little trace of NSDs both in images and velocity maps, while signatures of a disc can be found in the majority of the intermediate to minor-merger remnants and in particular when looking at their kinematics. These results show that NSDs could allow to distinguish between these two modes of galaxy assembly, which may indeed pertain to different kinds of galaxies or galactic environments.

Mapping galaxy encounters in numerical simulations: the spatial extent of induced star formation

We employ a suite of 75 simulations of galaxies in idealised major mergers (stellar mass ratio ~2.5:1), with a wide range of orbital parameters, to investigate the spatial extent of interaction-induced star formation. Although the total star formation in galaxy encounters is generally elevated relative to isolated galaxies, we find that this elevation is a combination of intense enhancements within the central kpc and moderately suppressed activity at large galacto-centric radii. The radial dependence of the star formation enhancement is stronger in the less massive galaxy than in the primary, and is also more pronounced in mergers of more closely aligned disc spin orientations. Conversely, these trends are almost entirely independent of the encounter's impact parameter and orbital eccentricity. Our predictions of the radial dependence of triggered star formation, and specifically the suppression of star formation beyond kph-scales, will be testable with the next generation of integral-field spectroscopic surveys.

Rings of star formation:Imprints of a close galaxy encounter

AbstractIn this talk, I report results from galaxy merger simulations, which suggest the existence of a ring of star formation produced by close galaxy encounters. This is a generic feature of all galaxy interactions, provided that the disc spins are sufficiently aligned. This signature can be used to identify close galaxy pairs that have actually suffered a close interaction.

Galaxies as simple dynamical systems: observational data disfavor dark matter and stochastic star formation

According to modern theory, galactic evolution is driven by the dynamics of dark matter and stochastic star formation, but galaxies are observed to be simple systems. The existence of dark matter particles is a key hypothesis in present-day cosmology and galactic dynamics. Given the large body of high-quality work within the standard model of cosmology (SMoC), the validity of this hypothesis is challenged significantly by two independent arguments: (i) The dual dwarf galaxy theorem must be true in any realistic cosmological model. But the data now available appear to falsify this postulate when the dark-matter-based model is compared with the observational data. A consistency test for this conclusion comes from the significantly anisotropic distributions of satellite galaxies (baryonic mass <108M⊙) that orbit in the same direction around their hosting galaxies in disk-like structures, which cannot be derived from dark matter models. (ii) The action of dynamical friction due to expansive and massive dark matter halos must be evident in the galaxy population. The evidence for dynamical friction is poor or even absent. Indendently of this, the long history of failures of the SMoC have reduced the likelihood that it describes the observed Universe to less than 10−4%. The implication for fundamental physics is that exotic dark matter particles do not exist and that consequently effective gravitational physics on the scales of galaxies and beyond ought to be non-Newtonian and (or) non-Einsteinian. An analysis of the kinematic data in galaxies shows them to be described elegantly in the weak-gravitational regime by scale-invariant dynamics, as discovered by Milgrom. The full classical regime of gravitation is effectively described by Milgromian dynamics. This leads to a natural emergence of the simple laws that galaxies are indeed observed to obey. Such success has not been forthcoming in dark-matter-based models. Observations of stellar populations in galaxies suggest that the galaxy-wide initial mass function varies with star formation rate and that stochastic descriptions of star formation are inconsistent with the data. This requires a reinterpretation of the stellar mass assembly in galaxies and thus of the accretion rates onto galaxies. A consequence of this understanding of galactic astrophysics is that most dwarf satellite galaxies are formed as tidal dwarf galaxies in galaxy–galaxy encounters, that they follow the mass–metallicity relation, that galactic mergers are rare, that galaxies immersed in external potentials are physically larger than isolated galaxies, and that star-forming galaxies form a main sequence. Eight predictions are offered that will allow the concepts raised here to be tested. A very conservative, cold- and warm-dark-matter-free cosmological model may be emerging from these considerations.

Perseus I and the NGC 3109 association in the context of the Local Group dwarf galaxy structures

The recently discovered dwarf galaxy Perseus I appears to be associated with the dominant plane of non-satellite galaxies in the Local Group (LG). We predict its velocity dispersion and those of the other isolated dSphs Cetus and Tucana to be 6.5, 8.2, and 5.5 km/s, respectively. The NGC 3109 association, including the recently discovered dwarf galaxy Leo P, aligns with the dwarf galaxy structures in the LG such that all known nearby non-satellite galaxies in the northern Galactic hemisphere lie in a common thin plane (rms height 53 kpc; diameter 1.2 Mpc). This plane has an orientation similar to the preferred orbital plane of the Milky Way (MW) satellites in the vast polar structure. Five of seven of these northern galaxies were identified as possible backsplash objects, even though only about one is expected from cosmological simulations. This may pose a problem, or instead the search for local backsplash galaxies might be identifying ancient tidal dwarf galaxies expelled in a past major galaxy encounter. The NGC 3109 association supports the notion that material preferentially falls towards the MW from the Galactic south and recedes towards the north, as if the MW were moving through a stream of dwarf galaxies.

Dwarf galaxy planes: the discovery of symmetric structures in the Local Group

Both major galaxies in the Local Group (LG) are surrounded by thin planes of mostly co-orbiting satellite galaxies, the vast polar structure (VPOS) around the Milky Way (MW) and the Great Plane of Andromeda (GPoA) around M31. We summarize the current knowledge concerning these structures and compare their relative orientations and properties in a common coordinate system. The existence of coherent satellite structures motivates an investigation of the distribution of the more distant non-satellite galaxies in the LG. This results in the discovery of two planes (diameters of 1-2 Mpc) which contain almost all nearby non-satellite galaxies. The two LG planes are surprisingly symmetric, inclined by only 20 deg relative to the galactic disc of M31, similarly thin (heights of ~60 kpc) and have near-to-identical offsets from the MW and M31. They are inclined relative to each other by 35 deg. Comparing the plane orientations with each other and with additional features indicates an intimate connection between the VPOS and the GPoA. They are both polar to the MW, have similar orbital directions and are inclined by ~45 deg relative to each other. The Magellanic Stream approximately aligns with the VPOS and the GPoA, but also shares its projected position and line-of-sight velocity trend with the dominating structure of non-satellite dwarf galaxies. The proper motion of M31 indicates a prograde orbit of the MW-M31 system, the VPOS and the GPoA. The alignment with other features such as the Supergalactic Plane and the over-density in hypervelocity stars are discussed as well. We end with a summary of proposed scenarios trying to explain the LG galaxy structures as either originating from cosmological structures or from tidal debris of a past galaxy encounter. There currently exists no full detailed model which satisfactorily explains the existence of the thin symmetric LG planes. (abridged)

The evolution of the AGN content in groups up toz~ 1

Determining the AGN content in structures of different mass/velocity dispersion and comparing them to higher mass/lower redshift analogs is important to understand how the AGN formation process is related to environmental properties. We use our well-tested cluster finding algorithm to identify structures in the GOODS North and South fields, exploiting the available spectroscopic redshifts and accurate photometric redshifts. We identify 9 structures in GOODS-south (presented in a previous paper) and 8 new structures in GOODS-north. We only consider structures where at least 2/3 of the members brighter than M_R=-20 have a spectroscopic redshift. For those group members that coincide with X-ray sources in the 4 and 2 Msec Chandra source catalogs respectively, we determine if the X-ray emission originates from AGN activity or it is related to the galaxies' star-formation activity. We find that the fraction of AGN with Log L_H > 42 erg/s in galaxies with M_R < -20 is on average 6.3+-1.3%, much higher than in lower redshift groups of similar mass and more than double the fraction found in massive clusters at a similarly high redshift. We then explore the spatial distribution of AGN in the structures and find that they preferentially populate the outer regions. The colors of AGN host galaxies in structures tend to be confined to the green valley, thus avoiding the blue cloud and, partially, also the red-sequence, contrary to what happens in the field. We finally compare our results to the predictions of two sets of semi analytic models to investigate the evolution of AGN and evaluate potential triggering and fueling mechanisms. The outcome of this comparison attests the importance of galaxy encounters, not necessarily leading to mergers, as an efficient AGN triggering mechanism. (abridged)

The interaction-driven starburst contribution to the cosmic star formation rate density

An increasing amount of observational evidence supports the notion that there are two modes of star formation: a quiescent mode in disk-like galaxies, and a starburst mode, which is generally interpreted as driven by merging. Using a semi-analytic model of galaxy formation, we derive the relative contribution to the cosmic star formation rate density of quiescently starforming and starburst galaxies, predicted under the assumption that starburst events are triggered by galaxy encounters (merging and fly-by kind) during their merging histories. We show that, within this framework, quiescently starforming galaxies dominate the cosmic star formation rate density at all redshifts. The contribution of the burst-dominated starforming galaxies increases with redshift, rising from <5% at low redshift (z<0.1) to ~20% at z>5. We estimated that the fraction of the final (z=0) galaxy stellar mass which is formed through the burst component of star formation is ~10% for 10^10 M_\odot<M_*<10^11.5 M_\odot. Starburst galaxies, selected according to their distance from the galaxy main sequence, account for ~10% of the star formation rate density in the redshift interval 1.5<z<2.5, i.e. at the cosmic peak of the star formation activity.