Merger History Research Articles

Clusters in the disperse cosmic web

ABSTRACT Galaxy cluster mass haloes (‘clusters’) in a dark matter simulation are matched to nodes in several different cosmic webs found using the disperse cosmic web finder. The webs have different simulation smoothings and disperse parameter choices; for each, four methods are considered for matching disperse nodes to clusters. For most of the webs, disperse nodes outnumber clusters, but not every cluster has a disperse node match (and sometimes >1 cluster matches to the same disperse node). The clusters frequently lacking a matching disperse node have a different distribution of local shear trends and perhaps merger histories. It might be interesting to see in what other ways, e.g. observational properties, these clusters differ. For the webs with smoothing ≤ 2.5 Mpc h−1, and all but the most restrictive matching criterion, ∼3/4 of the clusters always have a disperse node counterpart. The nearest cluster to a given disperse node and vice versa, within twice the smoothing length, obey a cluster mass-disperse node density relation. Cluster pairs where both clusters match disperse nodes can also be assigned the filaments between those nodes, but as the web and matching methods are varied most such filaments do not remain. There is an enhancement of subhalo counts and halo mass between cluster pairs, averaging over cluster pairs assigned disperse filaments increases the enhancement. The approach here also lends itself to comparing nodes across many cosmic web constructions, using the fixed underlying cluster distribution to make a correspondence.

The Metallicity Distribution Function in Outer Halo Fields of Simulated Elliptical Galaxies Compared to Observations of NGC 5128

Stellar metallicity distribution functions (MDFs) have been measured for resolved stellar populations in the outer halos of many galaxies in nearby groups. Among them, the MDF of NGC 5128, the central giant elliptical in the Centaurus group, provides essential constraints for theories of massive galaxy formation and hierarchical assembly. To investigate the formation and chemical evolution history of the outer halo of giant elliptical galaxies, we examine the chemical properties of three zoom-in high-resolution cosmological hydrodynamical simulations of an NGC 5128–like giant elliptical galaxy and compare their outer halo MDFs to the observed one of NGC 5128. Even though the simulated galaxies have different merging histories and age distributions, all predicted MDFs are in good qualitative agreement with the observed one. The median metallicity of the simulated galaxies is, on average, [M/H] = −0.41 ± 0.06 compared to the observed value of [M/H] = −0.38 ± 0.02 for NGC 5128, and the dispersion in metallicity is ∼0.77 dex for both observed and simulated galaxies. We investigate the origin of the stars ending up in the outer halo field of simulated galaxies and show that most have an “accreted” origin, formed in other small galaxies and later accreted in mergers. Only ∼15% of the stars are formed in situ within the main progenitor of the galaxy and radially migrate outward. We show that the contribution of metal-rich in situ stars is subdominant in the outer halos of our simulated galaxies but can be prominent in the inner regions.

Signatures of the Many Supermassive Black Hole Mergers in a Cosmologically Forming Massive Early-type Galaxy

We model here the merger histories of the supermassive black hole (SMBH) population in the late stages of a cosmological simulation of a ∼ 2 × 1013 M ⊙ galaxy group. The gravitational dynamics around the several tens of SMBHs (M • > 7.5 × 107 M ⊙) hosted by the galaxies in the group is computed at high accuracy using regularized integration with the KETJU code. The 11 SMBHs that form binaries and a hierarchical triplet eventually merge after hardening through dynamical friction, stellar scattering, and gravitational wave (GW) emission. The binaries form at eccentricities of e ∼ 0.3–0.9, with one system evolving to a very high eccentricity of e = 0.998, and merge on timescales of a few tens to several hundred megayears. During the simulation, the merger-induced GW recoil kicks eject one SMBH remnant from the central host galaxy. This temporarily drives the galaxy off the M •–σ ⋆ relation; however, the galaxy returns to the relation due to subsequent galaxy mergers, which bring in new SMBHs. This showcases a possible mechanism contributing to the observed scatter of the M •–σ ⋆ relation. Finally, we show that pulsar timing arrays and LISA would be able to detect parts of the GW signals from the SMBH mergers that occur during the ∼4 Gyr time span simulated with KETJU.

The Fornax3D project: The environmental impact on gas metallicity gradients in Fornax cluster galaxies

The role played by environment in galaxy evolution is a topic of ongoing debate among astronomers. There has been little success in elucidating the degree to which environment can alter, re-shape, or drive galaxy evolution, that is, using either observations or simulations. However, our knowledge of the effect of environment on gas metallicity gradients remains limited. Here we present our analysis of the gas metallicity gradients for a sample of ten Fornax cluster galaxies observed with MUSE as part of the Fornax3D project. We used detailed maps of emission lines to determine precise values of gas metallicity and metallicity gradients. The integrated gas metallicity of our Fornax cluster galaxies shows slightly higher metallicities (∼0.045 dex) in comparison to a control sample. In addition, we find signs of a mass and metallicity segregation from the center to the outskirts of the cluster. By comparing our Fornax cluster metallicity gradients with a control sample we find a general median offset of ∼0.04 dex/Re, with eight of our galaxies showing flatter or more positive gradients. The intermediate infallers in our Fornax sample show more positive gradients with respect to the control sample. We find no systematic difference between the gradients of recent and intermediate infallers when considering the projected distance of each galaxy to the cluster center. To identify the origin of the observed offset in the metallicity gradients, we performed a similar analysis with data from the TNG50 simulation. We identify 12 subhalos in Fornax-like clusters and compared their metallicity gradients with a control sample of field subhalos. This exercise also shows a flattening in the metallicity gradients for galaxies in Fornax-like halos, with a median offset of ∼0.05 dex/Re. We also analyzed the merger history, Mach numbers (ℳ), and ram pressure stripping of our TNG50 sample. We conclude that the observed flattening in metallicity gradients is likely due to a combination of galaxies traveling at supersonic velocities (ℳ > 1), which are experiencing high ram pressure stripping and flybys.

Resilience of sloshing cold fronts against subsequent minor mergers

ABSTRACTMinor mergers are common in galaxy clusters. They have the potential to create sloshing cold fronts (SCFs) in the intracluster medium of the cluster. However, the resilience of SCFs to subsequent minor mergers is unknown. Here we investigate the extent to which SCFs established by an off-axis minor merger are disrupted by a subsequent minor merger. We perform a suite of 13 hydrodynamic + N-body simulations of idealized triple cluster mergers in which we vary the approach direction and impact parameter of the tertiary cluster. Except for ∼1 Gyr after the first core passage of the tertiary cluster, clear SCFs are present in all merger configurations. Subsequent head-on minor mergers reduce the number of SCFs significantly, while subsequent off-axis minor mergers only moderately reduce the number of SCFs. In particular, outer ($\gtrapprox 500\,$kpc) SCFs are resilient. The results of this work indicate that SCFs are easily formed in the course of a minor merger and are long-lived even if a further minor merger takes place. SCFs thus should be ubiquitous, but deriving the merger history of a given cluster based on its observed SCFs might be more complex than previously thought.

Constructing high-fidelity halo merger trees inabacussummit

ABSTRACTTracking the formation and evolution of dark matter haloes is a critical aspect of any analysis of cosmological N-body simulations. In particular, the mass assembly of a halo and its progenitors, encapsulated in the form of its merger tree, serves as a fundamental input for constructing semi-analytic models of galaxy formation and, more generally, for building mock catalogues that emulate galaxy surveys. We present an algorithm for constructing halo merger trees from abacussummit, the largest suite of cosmological N-body simulations performed to date consisting of nearly 60 trillion particles, and which has been designed to meet the Cosmological Simulation Requirements of the Dark Energy Spectroscopic Instrument (DESI) survey. Our method tracks the cores of haloes to determine associations between objects across multiple time slices, yielding lists of halo progenitors and descendants for the several tens of billions of haloes identified across the entire suite. We present an application of these merger trees as a means to enhance the fidelity of abacussummit halo catalogues by flagging and ‘merging’ haloes deemed to exhibit non-monotonic past merger histories. We show that this cleaning technique identifies portions of the halo population that have been deblended due to choices made by the halo finder, but which could have feasibly been part of larger aggregate systems. We demonstrate that by cleaning halo catalogues in this post-processing step, we remove potentially unphysical features in the default halo catalogues, leaving behind a more robust halo population that can be used to create highly accurate mock galaxy realizations from abacussummit.

Effects of neutrino masses and asymmetries on dark matter halo assembly

Massive cosmological neutrinos suppress the Large-Scale Structure (LSS) in the Universe by smoothing the cosmic over-densities, and hence structure formation is delayed relative to that in the standard Lambda-Cold Dark Matter (ΛCDM) model.We characterize the merger and mass accretion history of dark matter halos with the halo formation time a 1/2, tree entropy s and halo leaf function ℓ(X) and measure them using neutrino-involved N-body simulations. We show that a non-zero sum of neutrino masses Mν delays the a 1/2 for halos with virial mass between 1013 M ⊙ and 3 × 1013 M ⊙, whereas a non-zero neutrino asymmetry parameter η 2 has the opposite effect. While the mean tree entropy s̅ does not depend significantly on either Mν or η 2, the halo leaf function does. Furthermore, the dependencies of ℓ on Mν and η 2 have significant evolution in redshift z, with the relative contributions of Mν and η 2 showing a sigmoid-like transition as a function of z around z ≈ 0.6. Together with the matter power spectrum, these halo parameters allow us to break the parameter degeneracy between Mν and η 2 so that they can both be constrained in principle.

The present-day globular cluster kinematics of lenticular galaxies from the E-MOSAICS simulations and their relation to the galaxy assembly histories

ABSTRACT We study the present-day rotational velocity (Vrot) and velocity dispersion (σ) profiles of the globular cluster (GC) systems in a sample of 50 lenticular (S0) galaxies from the E-MOSAICS galaxy formation simulations. We find that $82{{\ \rm per\ cent}}$ of the galaxies have GCs that are rotating along the photometric major axis of the galaxy (aligned), while the remaining $18{{\ \rm per\ cent}}$ of the galaxies do not (misaligned). This is generally consistent with the observations from the SLUGGS survey. For the aligned galaxies, classified as peaked and outwardly decreasing ($49{{\ \rm per\ cent}}$), flat ($24{{\ \rm per\ cent}}$), and increasing ($27{{\ \rm per\ cent}}$) based on the Vrot/σ profiles out to large radii, we do not find any clear correlation between these present-day Vrot/σ profiles of the GCs and the past merger histories of the S0 galaxies, unlike in previous simulations of galaxy stars. For just over half of the misaligned galaxies, we find that the GC misalignment is the result of a major merger within the last $10\, \mathrm{Gyr}$ so that the ex-situ GCs are misaligned by an angle between 0° (co-rotation) and 180° (counter-rotation), with respect to the in situ GCs, depending on the orbital configuration of the merging galaxies. For the remaining misaligned galaxies, we suggest that the in situ metal-poor GCs, formed at early times, have undergone more frequent kinematic perturbations than the in situ metal-rich GCs. We also find that the GCs accreted early and the in situ GCs are predominantly located within 0.2 virial radii (R200) from the centre of galaxies in 3D phase-space diagrams.

Extremely massive disc galaxies in the nearby Universe form through gas-rich minor mergers

ABSTRACT 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* > 1011.4 M⊙) discs form primarily via minor mergers between spheroids and gas-rich satellites, which create new rotational stellar components and leave discs as remnants. Here, we use UV-optical and H i data of massive galaxies, from the Sloan Digital Sky Survey, Galaxy Evolution Explorer, Dark Energy Camera Legacy Survey (DECaLS), and Arecibo Legacy Fast ALFA surveys, to test these theoretical predictions. Observed massive discs account for ∼13 per cent of massive galaxies, in good agreement with theory (∼11 per cent). ∼64 per cent of the observed massive discs exhibit tidal features, which are likely to indicate recent minor mergers, in the deep DECaLS images (compared to ∼60 per cent in their simulated counterparts). The incidence of these features is at least four times higher than in low-mass discs, suggesting that, as predicted, minor mergers play a significant (and outsized) role in the formation of these systems. The empirical star formation rates agree well with theoretical predictions and, for a small galaxy sample with H i detections, the H i masses and fractions are consistent with the range predicted by the simulation. The good agreement between theory and observations indicates that extremely massive discs are indeed remnants of recent minor mergers between spheroids and gas-rich satellites.

The Milky Way Revealed by Variable Stars. I. Sample Selection of RR Lyrae Stars and Evidence for Merger History

Abstract In order to study the Milky Way, RR Lyrae (RRL) variable stars identified by Gaia, ASAS-SN, and ZTF sky survey projects have been analyzed as tracers in this work. Photometric and spectroscopic information of 3417 RRLs including proper motions, radial velocity, and metallcity are obtained from observational data of Gaia, LAMOST, GALAH, APOGEE, and RAVE. Precise distances of RRLs with typical uncertainties less than 3% are derived by using a recent comprehensive period–luminosity–metallicity relation. Our results from kinematical and chemical analysis provide important clues for the assembly history of the Milky Way, especially for the Gaia–Sausage ancient merger. The kinematical and chemical trends found in this work are consistent with those of recent simulations that indicated that the Gaia–Sausage merger had a dual origin in the Galactic thick disk and halo. As recent similar works have found, the halo RRL sample in this work contains a subset of radially biased orbits besides a more isotropic component. This higher orbital anisotropy component amounts to β ≃ 0.8, and it contributes between 42% and 83% of the halo RRLs at 4 < R( kpc) < 20.

How cosmological merger histories shape the diversity of stellar haloes

ABSTRACT We introduce and apply a new approach to probe the response of galactic stellar haloes to the interplay between cosmological merger histories and galaxy formation physics. We perform dark matter-only, zoomed simulations of two Milky Way-mass hosts and make targeted, controlled changes to their cosmological histories using the genetic modification technique. Populating each history’s stellar halo with a semi-empirical, particle tagging approach then enables a controlled study, with all instances converging to the same large-scale structure, dynamical and stellar mass at z = 0 as their reference. These related merger scenarios alone generate an extended spread in stellar halo mass fractions (1.5 dex) comparable to the observed population, with the largest scatter achieved by growing late (z ≤ 1) major mergers that spread out existing stars to create massive, in-situ dominated stellar haloes. Increasing a last major merger at z ∼ 2 brings more accreted stars into the inner regions, resulting in smaller scatter in the outskirts which are predominantly built by subsequent minor events. Exploiting the flexibility of our semi-empirical approach, we show that the diversity of stellar halo masses across scenarios is reduced by allowing shallower slopes in the stellar mass–halo mass relation for dwarf galaxies, while it remains conserved when central stars are born with hotter kinematics across cosmic time. The merger-dependent diversity of stellar haloes thus responds distinctly to assumptions in modelling the central and dwarf galaxies respectively, opening exciting prospects to constrain star formation and feedback at different galactic mass-scales with the coming generation of deep, photometric observatories.

Dwarf stellar haloes: a powerful probe of small-scale galaxy formation and the nature of dark matter

ABSTRACT We use N-body cosmological simulations and empirical galaxy models to study the merger history of dwarf-mass galaxies (with $M_{\rm halo}\sim 10^{10}\, \mathrm{M}_\odot$). Our input galaxy models describe the stellar mass–halo mass relation, and the galaxy occupation fraction. The number of major and minor mergers depends on the type of dark matter; in particular, minor mergers are greatly suppressed in warm dark matter models. In addition, the number of mergers that bring in stars is strongly dependent on the galaxy occupation model. For example, minor mergers are negligible for stellar halo growth in models with a high mass threshold for galaxy formation (i.e. $10^{9.3}\, \mathrm{M}_\odot$ at z = 0). Moreover, this threshold for galaxy formation can also determine the relative difference (if any) between the stellar haloes of satellite and field dwarfs. Using isolated simulations of dwarf–dwarf mergers, we show that the relative frequency of major and minor mergers predict very different stellar haloes: Typically, ‘intermediate’ dark matter merger ratios (∼1:5) maximize the growth of distant stellar haloes. We discuss the observability of dwarf stellar haloes and find that the surface brightness of these features are incredibly faint. However, when several dwarfs are stacked together, models that form particularly rich stellar haloes could be detectable. Finally, we show that stellar streams in the Galactic halo overlapping in phase space with known dwarf satellites are likely remnants of their stripped stellar haloes. The mere existence of dwarf stellar haloes can already put constraints on some small-scale models, and thus observational probes should be a high priority.

The galaxy population of the core of the Coma cluster

ABSTRACT In this paper, we present the structural properties and morphology of galaxies in the central region of the Coma Cluster brighter than 19.5 mag in the F814W band from the HST/ACS Coma Cluster Treasury Survey. Using mainly spectroscopic redshifts, we find 132 members from our sample of 219 galaxies. In our sample of 132 members, we find 51 non-dwarfs and 81 dwarfs, and amongst our 32 non-members, we find 4 dwarfs and 28 non-dwarfs. We do not have redshifts for the remaining 55 galaxies. We present bulge–disc decomposition of the sample using galfit and obtain parameters for our sample. Using visual inspection of residuals, we do a morphological classification of the galaxies. We studied the relation of morphological types with Bulge to Total Light Ratio (B/T), colour–magnitude relation (CMR), Sérsic index (n), Kormendy relation, and cross-correlations between these parameters for the bulges and galaxies. This work helps us understand important relations between various parameters like B/T, colour, and n as well as insights into the merger history of these galaxies in terms of their positions in the Kormendy Diagram and their Sérsic indices. We find that there are significantly more E/SO, SOs galaxies in the member population compared to non-members.

Merger Histories and Environments of Dwarf AGN in IllustrisTNG

Abstract The relationship between active galactic nuclei (AGN) activity and environment has been long discussed, but it is unclear if these relations extend into the dwarf galaxy mass regime—in part due to the limits in both observations and simulations. We aim to investigate if the merger histories and environments are significantly different between AGN and non-AGN dwarf galaxies in cosmological simulations, which may be indicative of the importance of these for AGN activity in dwarf galaxies, and whether these results are in line with observations. Using the IllustrisTNG flagship TNG100-1 run, 6771 dwarf galaxies are found with 3863 (∼57%) having some level of AGN activity. In order to quantify environment, two measures are used: (1) the distance to a galaxy’s 10th nearest neighbor at six redshifts and (2) the time since last merger for three different minimum merger mass ratios. A similar analysis is run on TNG50-1 and Illustris-1 to test for the robustness of the findings. Both measures yield significantly different distributions between AGN and non-AGN galaxies; more non-AGN than AGN galaxies have long term residence in dense environments, while recent (≤4 Gyr) minor mergers are more common for intermediate AGN activity. While no statements are made about the micro or macrophysics from these results, it is nevertheless indicative of a non-negligible role of mergers and environments.

ACACIA: a new method to produce on-the-fly merger trees in the ramses code

ABSTRACT The implementation of ACACIA, a new algorithm to generate dark matter halo merger trees with the Adaptive Mesh Refinement code RAMSES, is presented. The algorithm is fully parallel and based on the Message Passing Interface. As opposed to most available merger tree tools, it works on the fly during the course of the N-body simulation. It can track dark matter substructures individually using the index of the most bound particle in the clump. Once a halo (or a sub-halo) merges into another one, the algorithm still tracks it through the last identified most bound particle in the clump, allowing to check at later snapshots whether the merging event was definitive, or whether it was only temporary, with the clump only traversing another one. The same technique can be used to track orphan galaxies that are not assigned to a parent clump anymore because the clump dissolved due to numerical overmerging. We study in detail the impact of various parameters on the resulting halo catalogues and corresponding merger histories. We then compare the performance of our method using standard validation diagnostics, demonstrating that we reach a quality similar to the best available and commonly used merger tree tools. As a proof of concept, we use our merger tree algorithm together with a parametrized stellar-mass-to-halo-mass relation and generate a mock galaxy catalogue that shows good agreement with observational data.

Quenched, bulge-dominated, but dynamically cold galaxies in IllustrisTNG and their real-world counterparts

ABSTRACT Galaxy morphologies, kinematics, and stellar populations are thought to be linked to each other. However, both simulations and observations have pointed out mismatches therein. In this work, we study the nature and origin of the present-day quenched, bulge-dominated, but dynamically cold galaxies within a stellar mass range of $10.3\, \leqslant \, \log \, M_{\ast }/\mathrm{M_{\odot }}\, \leqslant \, 11.2$ in the IllustrisTNG-100 Simulation. We compare these galaxies with a population of normal star-forming dynamically cold disc galaxies and a population of normal quenched dynamically hot elliptical galaxies within the same mass range. The populations of the present-day quenched and bulge-dominated galaxies (both being dynamically cold and hot) used to have significantly higher star formation rates and flatter morphologies at redshift of z ∼ 2. They have experienced more frequent larger mass-ratio mergers below z ∼ 0.7 in comparison to their star-forming disc counterparts, which is responsible for the formation of their bulge-dominated morphologies. The dynamically cold populations (both being star forming and quenched) have experienced more frequent prograde and tangential mergers especially below z ∼ 1, in contrast to the dynamically hot ellipticals, which have had more retrograde and radial mergers. Such different merging histories can well explain the differences on the cold and hot dynamical status among these galaxies. We point out that the real-world counterparts of these dynamically cold and hot bulge-dominated quenched populations are the fast- and slow-rotating early-type galaxies, respectively, as seen in observations and hence reveal the different evolution paths of these two distinct populations of early-type galaxies.

Galaxy Core Formation by Supermassive Black Hole Binaries: The Importance of Realistic Initial Conditions and Galaxy Morphology

Abstract The binding energy liberated by the coalescence of supermassive black hole (SMBH) binaries during galaxy mergers is thought to be responsible for the low density cores often found in bright elliptical galaxies. We use high-resolution N-body and Monte Carlo techniques to perform single and multistage galaxy merger simulations and systematically study the dependence of the central galaxy properties on the binary mass ratio, the slope of the initial density cusps, and the number of mergers experienced. We study both the amount of depleted stellar mass (or mass deficit), M def, and the radial extent of the depleted region, r b. We find that r b ≃ r SOI and that M def varies in the range of 0.5–4M •, with r SOI the influence radius of the remnant SMBH and M • its mass. The coefficients in these relations depend weakly on the binary mass ratio and remain remarkably constant through subsequent mergers. We conclude that the core size and mass deficit do not scale linearly with the number of mergers, making it hard to infer merger histories from observations. On the other hand, we show that both M def and r b are sensitive to the morphology of the galaxy merger remnant, and that adopting spherical initial conditions, as done in early work, leads to misleading results. Our models reproduce the range of values for M def found in most observational work, but span nearly an order-of magnitude range around the true ejected stellar mass.

NGC 5746: Formation history of a massive disc-dominated galaxy

ABSTRACT The existence of massive galaxies lacking a classical bulge has often been proposed as a challenge to ΛCDM. However, recent simulations propose that a fraction of massive disc galaxies might have had very quiescent merger histories, and also that mergers do not necessarily build classical bulges. We test these ideas with deep MUSE observations of NGC 5746, a massive (∼1011 M⊙) edge-on disc galaxy with no classical bulge. We analyse its stellar kinematics and stellar populations, and infer that a massive and extended disc formed very early: 80 per cent of the galaxy’s stellar mass formed more than 10 Gyr ago. Most of the thick disc and the bar formed during that early phase. The bar drove gas towards the centre and triggered the formation of the nuclear disc followed by the growth of a boxy/peanut-shaped bulge. Around ∼8 Gyr ago, a ∼1:10 merger happened, possibly on a low-inclination orbit. The satellite did not cause significant vertical heating, did not contribute to the growth of a classical bulge, and did not destroy the bar and the nuclear disc. It was however an important event for the galaxy: by depositing its stars throughout the whole galaxy it contributed ∼30 per cent of accreted stars to the thick disc. NGC 5746 thus did not completely escape mergers, but the only relatively recent significant merger did not damage the galaxy and did not create a classical bulge. Future observations will reveal if this is representative of the formation histories of massive disc galaxies.

Formation of massive disc galaxies in the IllustrisTNG simulation

ABSTRACT We investigate the formation history of massive disc galaxies in hydrodynamical simulation – the IllustrisTNG, to study why massive disc galaxies survive through cosmic time. 83 galaxies in the simulation are selected with M*,z = 0 > 8 × 1010 M⊙ and kinematic bulge-to-total ratio less than 0.3. We find that 8.4 per cent of these massive disc galaxies have quiet merger histories and preserve disc morphology since formed. 54.2 per cent have a significant increase in bulge components in history, then become discs again till present time. The rest 37.3 per cent experience prominent mergers but survive to remain discy. While mergers and even major mergers do not always turn disc galaxies into ellipticals, we study the relations between various properties of mergers and the morphology of merger remnants. We find a strong dependence of remnant morphology on the orbit type of major mergers. Specifically, major mergers with a spiral-in falling orbit mostly lead to disc-dominant remnants, and major mergers of head-on galaxy–galaxy collision mostly form ellipticals. This dependence of remnant morphology on orbit type is much stronger than the dependence on cold gas fraction or orbital configuration of merger system as previously studied.

Age-dating Red Giant Stars Associated with Galactic Disk and Halo Substructures

Abstract The vast majority of Milky Way stellar halo stars were likely accreted from a small number (≲3) of relatively large dwarf galaxy accretion events. However, the timing of these events is poorly constrained and predominantly relies on indirect dynamical mixing arguments or imprecise age measurements of stars associated with debris structures. Here, we aim to infer robust stellar ages for stars associated with galactic substructures to more directly constrain the merger history of the Galaxy. By combining kinematic, asteroseismic, and spectroscopic data where available, we infer stellar ages for a sample of 10 red giant stars that were kinematically selected to be within the stellar halo, a subset of which are associated with the Gaia–Enceladus–Sausage halo substructure, and compare their ages to 3 red giant stars in the Galactic disk. Despite systematic differences in both absolute and relative ages determined here, age rankings of stars in this sample are robust. Passing the same observable inputs to multiple stellar age determination packages, we measure a weighted average age for the Gaia–Enceladus–Sausage stars in our sample of 8 ± 3 (stat.) ± 1 (sys.) Gyr. We also determine hierarchical ages using isochrones for the populations of Gaia–Enceladus–Sausage, in situ halo and disk stars, finding a Gaia–Enceladus–Sausage population age of 8.0 − 2.3 + 3.2 Gyr. Although we cannot distinguish hierarchical population ages of halo or disk structures with our limited data and sample of stars, this framework should allow a distinct characterization of Galactic substructures using larger stellar samples and additional data available in the near future.