Mergers play an important role in galaxy evolution. In particular, major mergers are able to have a transformative effect on galaxy morphology. In this paper, we investigate the role of magnetic fields in gas-rich major mergers. To this end, we run a series of high-resolution magnetohydrodynamic (MHD) zoom-in simulations with the moving-mesh code Arepo and compare the outcome with hydrodynamic simulations run from the same initial conditions. This is the first time that the effect of magnetic fields in major mergers has been investigated in a cosmologically-consistent manner. In contrast to previous non-cosmological simulations, we find that the inclusion of magnetic fields has a substantial impact on the production of the merger remnant. Whilst magnetic fields do not strongly affect global properties, such as the star formation history, they are able to significantly influence structural properties. Indeed, MHD simulations consistently form remnants with extended discs and well-developed spiral structure, whilst hydrodynamic simulations form more compact remnants that display distinctive ring morphology. We support this work with a resolution study and show that whilst global properties are broadly converged across resolution and physics models, morphological differences only develop given sufficient resolution. We argue that this is due to the more efficient excitement of a small-scale dynamo in higher resolution simulations, resulting in a more strongly amplified field that is better able to influence gas dynamics.
We show how wet major mergers can create disc galaxies in a cosmological context, using the Illustris simulation. We select a sample of 38 disc galaxies having experienced a major merger in their history with no subsequent significant minor merger, and study how they transform into discs after the merger. In agreement with what was previously found in controlled simulations of such mergers, we find that their disc is built gradually from young stars formed after the merger in the disc region, while the old stars born before the merger form an ellipsoidal component. Focusing on one fiducial cas…
Understanding the processes that trigger morphological transformation is central to understanding how and why the Universe transitions from being disc-dominated at early epochs to having the morphological mix that is observed today. We use Horizon-AGN, a cosmological hydrodynamical simulation, to perform a comprehensive study of the processes that drive morphological change in massive (M > 10^10 MSun) galaxies over cosmic time. We show that (1) essentially all the morphological evolution in galaxies that are spheroids at z=0 is driven by mergers with mass ratios greater than 1:10, (2) major me…
Galaxy pairs with separations of only a few kpc represent important stages in the merger-driven growth of supermassive black holes (SMBHs). However, such mergers are difficult to identify observationally due to the correspondingly small angular scales. In Paper I we presented a method of finding candidate kpc-scale galaxy mergers that is leveraged on the selection of X-ray sources spatially offset from the centers of host galaxies. In this paper we analyze new Hubble Space Telescope (HST) WFC3 imaging for six of these sources to search for signatures of galaxy mergers. The HST imaging reveals …
[Abridged] Typical disc galaxies forming in a LambdaCDM cosmology encounter a violent environment, where they often experience mergers with massive satellites. The fact that disc galaxies are ubiquitous in the local Universe suggests that a quiescent history is not necessary for their formation. Modern cosmological simulations can now obtain relatively realistic populations of disc galaxies, but it still remains to be clarified how discs manage to survive massive mergers. Here we use a suite of high-resolution hydrodynamical simulations set in a LambdaCDM cosmology to elucidate the fate of dis…
In our hierarchical structure-formation paradigm, the observed morphological evolution of massive galaxies -- from rotationally-supported discs to dispersion-dominated spheroids -- is largely explained via galaxy merging. However, since mergers are likely to destroy discs, and the most massive galaxies have the richest merger histories, it is surprising that any discs exist at all at the highest stellar masses. Recent theoretical work by our group has used a cosmological, hydrodynamical simulation to suggest that extremely massive (M* > 10^11.4 MSun) discs form primarily via minor mergers betw…
Emission lines with a double-peak (DP) shape, detected in the centre of galaxies, have been extensively used in the past to identify peculiar kinematics such as dual active galactic nuclei, outflows or mergers. From a large DP galaxy sample, a connection to minor merger galaxies with ongoing star formation was suggested. To gain a better understanding of different mechanisms creating a DP signature, we here explore synthetic SDSS spectroscopic observations computed from disc models and simulations. We show how a DP signature is connected to the central part of the rotation curve of galaxies, w…
We employ our Bayesian Machine Learning framework BINGO (Bayesian INference for Galactic archaeOlogy) to obtain high-quality stellar age estimates for 68,360 red giant and red clump stars present in the 17th data release of the Sloan Digital Sky Survey, the APOGEE-2 high-resolution spectroscopic survey. By examining the denoised age-metallicity relationship of the Galactic disc stars, we identify a drop in metallicity with an increase in [Mg/Fe] at an early epoch, followed by a chemical enrichment episode with increasing [Fe/H] and decreasing [Mg/Fe]. This result is congruent with the chemical…
Interpreting the scaling relations measured by recent large kinematic surveys of $z < 1$ galaxies has remained hampered by large observational scatter. We show that the observed ISM and morpho-dynamical properties along the average $z \sim 0.6$ major merger describe a very self-consistent picture in which star formation is enhanced during first passage and fusion as a result of gravitational perturbations due to the interaction, while the gas velocity dispersion is simultaneously enhanced through shocks that convert kinematic energy associated with bulk orbital motions into turbulence at small…
The Merger-Driven Origin of the Vast Extended Stellar Disc Around the Andromeda Galaxy
National and Kapodistrian University of Athens（雅典国立卡波蒂斯坦大学）；European Southern Observatory（欧洲南方天文台）；Observatoire de Paris（巴黎天文台）；PSL Research University（PSL研究大学）；CNRS（法国国家科学研究中心）；Max-Planck-Institut für extraterrestrische Physik (MPE)（马克斯·普朗克地外物理研究所）；University Observatory Munich (USM)（慕尼黑大学天文台）
The survey of planetary nebulae in Andromeda (M31) VI. Kinematics of M31 inner-halo substructures and comparison with major-merger simulation predictions
Major mergers with small galaxies - the discovery of a Magellanic-type galaxy at z=0.12
我们报告的偶然发现的恒星形成星系的红移z=0.116的形态特征，表明正在进行的合并。这个物体展示了两个明显分离的组件，颜色明显不同，加上可能的潮汐流。蓝色部分的后续光谱显示出低恒星形成活动为0.09 M $_{\odot}$ /年，高金属丰度为12+log（O/H）=8.6。基于质量-恒星形成率和质量-金属丰度关系的比较，以及光谱能量分布的拟合，我们获得了3 × 10 $^9$ M $_{\odot}$的恒星质量，这使得该天体可与大麦云（LMC）相媲美。因此，我们的发现提供了进一步的证据，证明一个大的合并已经在小的、矮星系般的尺度上发生了作用。
We report the discovery of ancient massive merger events in the early-type galaxies NGC 1380 and NGC 1427, members of the Fornax galaxy cluster. Both galaxies have been observed by the MUSE IFU instrument on the VLT, as part of the Fornax3D project. By fitting recently-developed population-orbital superposition models to the observed surface brightness as well as stellar kinematic, age, and metallicity maps, we obtain the stellar orbits, age and metallicity distributions of each galaxy. We then decompose each galaxy into multiple orbital-based components, including a dynamically hot inner stel…
Minor merger of galaxies are common during the evolutionary phase of galaxies. Here, we investigate the dynamical impact of a minor merger (mass ratio 1:10) event on the final fate of a stellar bar in the merger remnant. To achieve that, we choose a set of minor merger models from the publicly available GalMer library of galaxy merger simulations. The models differ in terms of their orbital energy, orientation of the orbital spin vector, and morphology of the satellite galaxy (discy/spheroidal). We demonstrate that the central stellar bar, initially present in the host galaxy, undergoes a tran…
Understanding how rotationally-supported discs transform into dispersion-dominated spheroids is central to our comprehension of galaxy evolution. Morphological transformation is largely merger-driven. While major mergers can efficiently create spheroids, recent work has highlighted the significant role of other processes, like minor mergers, in driving morphological change. Given their rich merger histories, spheroids typically exhibit large fractions of `ex-situ' stellar mass, i.e. mass that is accreted, via mergers, from external objects. This is particularly true for the most massive galaxi…
Context: In a series of papers, we study the major merger of two disk galaxies in order to establish whether or not such a merger can produce a disc galaxy. Aims: Our aim here is to describe in detail the technical aspects of our numerical experiments. Methods: We discuss the initial conditions of our major merger, which consist of two protogalaxies on a collision orbit. We show that such merger simulations can produce a non-realistic central mass concentration, and we propose simple, parametric, AGN-like feedback as a solution to this problem. Our AGN-like feedback algorithm is very simple: a…
Signatures of minor mergers in the Milky Way disc I: The SEGUE stellar sample
现在我们知道，微小的合并能够在其宿主星系盘的相空间分布中产生结构。为了在银河系中寻找这样的印记，我们分析了SEGUE F/G矮星和Schuster等人（2006）的恒星样本。我们在这两个完全独立的恒星样本中发现了类似的特征，与银河系小型合并事件的预测一致。接下来，我们将同样的分析应用于人马座矮星系和银河系之间相互作用的高分辨率，理想化的N体模拟。恒星粒子样本在主盘小空间区域内的能量分布揭示了结构随位置的强烈变化。我们发现，与人马座暗物质晕祖先$\lessapprox 10^{11}$ M $_{\odot}$的质量模型的观测结果很好地匹配。因此，我们表明，这种分析可以用来提供前所未有的严格限制人马座的轨道参数，以及放置一个较低的限制其质量。
Using numerical hydrodynamics code, we perform various idealized galaxy merger simulations to study the star formation (SF) of two merging disk galaxies. Our simulations include gas accretion onto supermassive black holes and active galactic nucleus (AGN) feedback. By comparing AGN simulations with those without AGNs, we attempt to understand when the AGN feedback effect is significant. With ~70 simulations, we investigated the SF with the AGN effect in mergers with variety of mass ratios, inclinations, orbits, galaxy structures and morphologies. Using these merger simulations with AGN feedbac…
Major mergers are popularly considered too destructive to produce the relaxed regular structures and the morphological inner components (ICs) usually observed in lenticular (S0) galaxies. We aim to test if major mergers can produce remnants with realistic S0 morphologies. We have selected a sample of relaxed discy remnants resulting from the dissipative merger simulations of the GalMer database and derived their properties mimicking the typical conditions of current observational data. We compare their global morphologies, visual components, and merger relics in mock photometric images with th…
We use the GALFORM semi-analytical model of galaxy formation and the Planck-Millennium simulation to investigate the origins of stellar mass in galaxies and their spheroids. We compare the importance of mergers and disc instabilities, as well as the starbursts that they trigger. We find that the fraction of galaxy stellar mass formed \textit{ex situ} ($f_\mathrm{ex}$) increases sharply from $M_*=10^{11}$ M$_\odot$ upwards, reaching $80\%$ at $M_*=10^{11.3}$ M$_\odot$. For low-mass galaxies we find larger \textit{\textit{ex situ}} contributions at $z=0$ than in other models ($7$-$12\%$), with a…
Galaxy mergers are a fundamental part of galaxy evolution. To study the resulting mass distributions of different kinds of galaxy mergers, we present a simulation suite of 36 high-resolution isolated merger simulations, exploring a wide range of parameter space in terms of mass ratios (mu = 1:5, 1:10, 1:50, 1:100) and orbital parameters. We find that mini mergers deposit a higher fraction of their mass in the outer halo compared to minor mergers, while their contribution to the central mass distribution is highly dependent on the orbital impact parameter: for larger pericentric distances we fi…
In this work we present two new $\sim10^9$ particle self-consistent simulations of the merger of a Sagittarius-like dwarf galaxy with a Milky Way-like disc galaxy. One model is a violent merger creating a thick disc, and a Gaia-Enceladus/Sausage like remnant. The other is a highly stable disc which we use to illustrate how the improved phase space resolution allows us to better examine the formation and evolution of structures that have been observed in small, local volumes in the Milky Way, such as the $z-v_z$ phase spiral and clustering in the $v_{\mathrm{R}}-v_ϕ$ plane when compared to prev…
The correspondence between galaxy major mergers and starburst activity is well-established observationally and in simulations of low redshift galaxies. However, the evolution of the properties of interactions and of the galaxies involved suggests that the starburst response of galaxies to merger events could vary across cosmic time. Using the VINTERGATAN cosmological zoom-in simulation of a Milky Way-like galaxy, we show here that starbursts, i.e. episodes of fast star formation, are connected with the onset of tidal compression, itself induced by mergers. However, this compression becomes str…
The majority of gravitational wave (GW) events detected so far by LIGO/Virgo originate from binary black hole (BBH) mergers. Among the different binary evolution paths, the merger of BBHs in accretion discs of active galactic nuclei (AGNs) is a possible source of GW detections. We consider an idealised analytical model of the orbital evolution of BBHs embedded in an AGN accretion disc. In this framework, the disc-binary interaction increases the orbital eccentricity and decreases the orbital separation, driving the BBH into a regime where GW emission eventually leads to coalescence. We compute…
The relative roles of mergers and star formation in regulating galaxy growth are still a matter of intense debate. We here present our DECODE, a new Discrete statistical sEmi-empiriCal mODEl specifically designed to predict rapidly and efficiently, in a full cosmological context, galaxy assembly and merger histories for any given input stellar mass-halo mass (SMHM) relation. DECODE generates object-by-object dark matter merger trees (hence discrete) from accurate subhalo mass and infall redshift probability functions (hence statistical) for all subhaloes, including those residing within other …
The Environmental Switch in Black Hole Feeding: Bar-Driven vs. Merger-Driven Growth in IllustrisTNG50
Indian Institute of Science, Education and Research (IISER) Tirupati（印度科学教育与研究学院提鲁帕蒂分校）；Department of Space, Platenary and Astronomical Science and Engineering, Indian Institute of Technology (IIT), Kanpur（坎普尔印度理工学院空间、行星与天文学科学与工程系）
We present the results of a self-consistent $N$-body simulation following the evolution of a primordial population of thick disc globular clusters (GCs). We study how the internal properties of such clusters evolve under the action of mutual interactions, while they orbit a Milky Way-like galaxy. For the first time, through analytical and numerical considerations, we find that physical encounters between disc GCs are a crucial factor that contributed to the shape of the current properties of the Galactic GC system. Close passages or motion on similar orbits may indeed have a significant impact…
We propose a new formation channel for intermediate mass black hole (IMBH) binaries via globular cluster collisions in the Galactic disc. Using numerical simulations, we show that the IMBHs form a tight binary that enters the gravitational waves (GWs) emission dominated regime driven by stellar interactions, and ultimately merge in $\lesssim 0.5$ Gyr. These events are clearly audible to LISA and can be associated with electromagnetic emission during the last evolutionary stages. During their orbital evolution, the IMBHs produce runaway stars comparable with GAIA and LAMOST observations.
We analyse a set of cosmological magneto-hydrodynamic simulations of the formation of Milky Way-mass galaxies identified to have a prominent radially anisotropic stellar halo component similar to the so-called "Gaia Sausage" found in the Gaia data. We examine the effects of the progenitor of the Sausage (the Gaia-Enceladus-Sausage, GES) on the formation of major galactic components analogous to the Galactic thick disc and inner stellar halo. We find that the GES merger is likely to have been gas-rich and contribute 10-50$\%$ of gas to a merger-induced centrally concentrated starburst that resu…
