Merger Event Research Articles

Galaxy Evolution in The Post-Merger Regime I – Most merger-induced in-situ stellar mass growth happens post-coalescence

Abstract Galaxy mergers can enhance star formation rates throughout the merger sequence, with this effect peaking around the time of coalescence. However, owing to a lack of information about their time of coalescence, post-mergers could only previously be studied as a single, time-averaged population. We use timescale predictions of post-coalescence galaxies in the UNIONS survey, based on the Multi-Model Merger Identifier deep learning framework (Mummi) that predicts the time elapsed since the last merging event. For the first time, we capture a complete timeline of star formation enhancements due to galaxy mergers by combining these post-merger predictions with data from pre-coalescence galaxy pairs in SDSS. Using a sample of 564 galaxies with M* ≥ 1010M⊙ at 0.005 < z < 0.3 we demonstrate that: 1) galaxy mergers enhance star formation by, on average, up to a factor of two; 2) this enhancement peaks within 500 Myr of coalescence; 3) enhancements continue for up to 1 Gyr after coalescence; and 4) merger-induced star formation significantly contributes to galaxy mass assembly, with galaxies increasing their final stellar masses by, 10 % to 20 % per merging event, producing on average $\log (M_*/M_\odot ) = {9.56_{-0.19}^{+0.13}}$ more mass than non-interacting star-forming galaxies solely due to the excess star formation.

Deciphering Galactic Halos: A Detailed Review of Star Formation in NGC 5128 (Cen A)

NGC 5128 (Centaurus A), the closest giant elliptical galaxy outside the Local Group to the Milky Way, is one of the brightest extragalactic radio sources. It is distinguished by a prominent dust lane and powerful jets, driven by a supermassive black hole at its core. Using previously identified long-period variable (LPV) stars from the literature, this study aims to reconstruct the star formation history (SFH) of two distinct regions in the halo of NGC 5128. These regions reveal remarkably similar SFHs, despite being located about 28 kpc apart on opposite sides of the galaxy’s center. In Field 1, star formation rates (SFRs) show notable increases at approximately 800 Myr and 3.8 Gyr ago. Field 2 exhibits similar peaks at these times, along with an additional rise around 6.3 Gyr ago. The increase in SFR around 800 Myr ago is consistent with earlier research suggesting a merger event. Since no LPV catalog exists for the central region of NGC 5128, we focused our investigation on its outer regions, which has provided new insights into the complex evolutionary history of this cornerstone galaxy. The SFH traced by LPVs supports a scenario in which multiple events of nuclear activity have triggered episodic, jet-induced star formation.

The ALMA-CRISTAL Survey. Spatial extent of [CII] line emission in star-forming galaxies at z=4-6

We investigate the spatial extent and structure of the line emission in a sample of 34 galaxies at $z=4-6$ from the Resolved ISM in STar-forming galaxies with ALMA (CRISTAL) Survey. By modeling the distribution of the line emission in the interferometric visibility data directly, we derive the effective radius of line emission assuming an exponential profile. These measurements comprise not only isolated galaxies but also interacting systems that were identified thanks to the high spatial resolution of the data. The line radius ranges from 0.5 to 3.5 kpc with an average value of $ e CII kpc. We compare the sizes with the sizes of rest-frame ultraviolet (UV) and far-infrared (FIR) continua, which were measured from the HST F160W images and ALMA Band-7 continuum images, respectively. We confirm that the line emission is more spatially extended than the continuum emission, with average size ratios of $ e CII /R_ e UV and $ e CII /R_ e FIR although about half of the FIR-detected sample shows a comparable spatial extent between the line and the FIR continuum emission e CII e FIR $). The residual visibility data of the best-fit model do not show statistical evidence of flux excess, indicating that the line emission in star-forming galaxies can be characterized by an extended exponential disk profile. Overall, our results suggest that the spatial extent of line emission can primarily be explained by photodissociation regions associated with star formation activity, while the contribution from diffuse neutral medium (atomic gas) and the effects of past merger events may further expand the line distributions, causing their variations among our sample. Finally, we report the negative correlation between the surface density ($ CII $) and the Lyalpha equivalent width (EW$_ Ly alpha $), and a possible negative correlation between $R_ e CII /R_ e UV $ and $ EW Ly alpha $, which may be in line with the scenario that atomic gas component largely contributes to the extended line emission. Future three-dimensional analysis of Lyalpha and Halpha lines will shed light on the association of the extended line emission with atomic gas and outflows.

UGMRT Sub-GHz View of the Sausage Cluster Diffuse Radio Sources

CIZA J2242.8+5301, or the Sausage cluster, is well-studied over a range of frequencies. Since its first discovery, a lot of interesting features and unique characteristics have been uncovered. In this work, we report some more new morphological features using the uGMRT band-3 and band-4 data. In the north relic, we observe variation in the spectral index profiles across the relic width from the east to west, which may indicate a decrease in the downstream cooling rate in that direction. We reconfirm the presence of an additional ∼930 kpc relic in the north. We classify the filamentary source in the downstream region to be a narrow-angle tail (NAT) radio galaxy. The bright arc in the east relic shows a substructure in the spectral index profile, which may indicate the presence of finer filaments. We further report the presence of a double-strand structure in the east relic similar to the “Toothbrush” relic. We categorize the bright “L”-shaped structure in the southern relic to be a NAT radio galaxy, as well as trace the actual ∼1.1 Mpc relic component. We reconfirm the existence of the faint southern extent, measuring the relic length to be ∼1.8 Mpc. Furthermore, we suggest the southern relic to be a union of individual component relics rather than a single giant filamentary relic. Lastly, based on the morphological symmetry between northern and southern relics, we suggest a schematic shock structure associated with the merger event in an attempt to explain their formation scenario.

Dynamical friction and the evolution of black holes in cosmological simulations: A new implementation in OpenGadget3

Aims. We introduce a novel sub-resolution prescription to correct for the unresolved dynamical friction (DF) onto black holes (BHs) in cosmological simulations, to describe BH dynamics accurately, and to overcome spurious motions induced by numerical effects. Methods. We implemented a sub-resolution prescription for the unresolved DF onto BHs in the OpenGadget3 code. We carried out cosmological simulations of a volume of (16 comoving Mpc)3 and zoomed-in simulations of a galaxy group and of a galaxy cluster. We assessed the advantages of our new technique in comparison to commonly adopted methods for hampering spurious BH displacements, namely repositioning onto a local minimum of the gravitational potential and ad hoc boosting of the BH particle dynamical mass. We inspected variations in BH demography in terms of offset from the centres of the host sub-halos, the wandering population of BHs, BH–BH merger rates, and the occupation fraction of sub-halos. We also analysed the impact of the different prescriptions on individual BH interaction events in detail. Results. The newly introduced DF correction enhances the centring of BHs on host halos, the effects of which are at least comparable with those of alternative techniques. Also, the correction becomes gradually more effective as the redshift decreases. Simulations with this correction predict half as many merger events with respect to the repositioning prescription, with the advantage of being less prone to leaving substructures without any central BH. Simulations featuring our DF prescription produce a smaller (by up to ~50% with respect to repositioning) population of wandering BHs and final BH masses that are in good agreement with observations. Regarding individual BH–BH interactions, our DF model captures the gradual inspiraling of orbits before the merger occurs. By contrast, the repositioning scheme, in its most classical renditions, describes extremely fast mergers, while the dynamical mass misrepresents the dynamics of the black holes, introducing numerical scattering between the orbiting BHs. Conclusions. The novel DF correction improves the accuracy if tracking BHs within their hosts galaxies and the pathway to BH- BH mergers. This opens up new possibilities for better modeling the evolution of BH populations in cosmological simulations across different times and different environments.

Orbital dynamics in galactic potentials under mass transfer

Context. Time-dependent potentials are common in galactic systems that undergo significant evolution, interactions, or encounters with other galaxies, or when there are dynamic processes such as star formation and merging events. Recent studies show that an ensemble approach along with the so-called snapshot framework in the theory of dynamical systems provide a powerful tool to analyze the time-dependent dynamics. Aims. In this work, we aim to explore and quantify the phase space structure and dynamical complexity in time-dependent galactic potentials consisting of multiple components. Methods. We applied the classical method of Poincaré surface of sections to analyze the phase space structure in a chaotic Hamiltonian system subjected to parameter drift. This, however, makes sense only when the evolution of a large ensemble of initial conditions is followed. Numerical simulations explore the phase space structure of such ensembles while the system undergoes a continuous parameter change. The pair-wise average distance of ensemble members allowed us to define a generalized Lyapunov exponent, which might also be time-dependent, to describe the system stability. Results. We provide a comprehensive dynamical analysis of the system under circumstances where linear mass transfer occurs between the disk and bulge components of the model.

Large-scale and very-large-scale motions in a moderate-Reynolds-number turbulent boundary layer over a flat plate towed in a tank

An experimental work is conducted to investigate large-scale motions (LSMs) and very-large-scale motions (VLSMs) of a turbulent boundary layer generated by a flat plate towed in a tank. Reynolds number based on friction velocity and boundary layer thickness covers a range of 1200 ≤Reτ≤ 3100. The streamwise–wall-normal plane and the streamwise–spanwise plane are separately measured using time-resolved stereoscopic particle image velocimetry (TR-SPIV) and time-resolved planar PIV (TR-PIV), respectively. The large-scale coherent structures in the two orthogonal planes are identified by the clustering method. Based on spatiotemporal evolutions of instantaneous flow fields, it is observed that low- and high-speed coherent structures collide and lean against each other, generating a strong shear layer between them. The low-speed coherent structures move away from the wall, while the high-speed coherent structures travel toward the wall near the interaction regions during the shearing, which may lead to merging and splitting of the coherent structures. The coherent structures grow in streamwise scales by self-stretching and coalescence with adjacent coherent structures. The properties of the LSMs and VLSMs, including scales, inclination angles, meandering behaviors, and contributions of high- or low-speed VLSMs to skin friction, are quantitatively examined. Furthermore, the dynamic characteristics of the LSMs and VLSMs are classified into merging, splitting, growing, and shortening, with occurrence frequencies independent of Reynolds number. Effects of shearing on the merging and splitting events are discussed. Results from this work suggest that the primary formation mechanism of the VLSMs is the self-growing of LSMs vigorously stretching along the streamwise direction.

The Decline and Fall of ROME. V. A Preliminary Search for Star-disrupted Planet Interactions and Coronal Activity at 5 GHz among White Dwarfs within 25 pc

The recent discovery of planetesimals orbiting white dwarfs has renewed interest in the final chapters of the evolution of planetary systems. Although observational and theoretical studies have examined the dynamical evolution of these systems, studies of their magnetic star–planet interactions, as powered by unipolar induction, have thus far only been assessed theoretically. This fifth paper of the Radio Observations of Magnetized Exoplanets series presents the results of a targeted minisurvey of nine white dwarfs within 25 pc without known stellar mass companions in search of radio emissions generated by magnetic interactions between white dwarfs and their planetary remnants. This ∼5 GHz Arecibo radio telescope survey achieved mJy-level sensitivity over <1 s integration times. Although no exoplanet-induced stellar radio flares were detected, this is the first survey to search for magnetic star–planet interactions between white dwarfs and planetary companions, cores, or disrupted planetesimals. It is also the most extensive and sensitive radio survey for intrinsic coronal emissions from apparently isolated white dwarfs. The study of radio emissions from white dwarf systems may present a new means to detect and measure DC and DQ magnetic fields, search for white dwarf coronae, characterize the density and spatial distribution of white dwarf magnetospheric plasma, characterize the dynamical and electrical properties of planetary cores, and offer new constraints on the modeling of double-degenerate merger events.

Mass Function of Stellar Black Holes as Revealed by the LIGO–Virgo–KAGRA Observations

Ninety gravitational-wave events have been detected by the LIGO–Virgo–KAGRA network and are released in the Gravitational-Wave Transient Catalog. Among these events, 83 cases are definitely binary black hole mergers, since the masses of all the objects involved significantly exceed the upper limit of neutron stars. The black holes in these merger events naturally form two interesting samples, a premerger sample that includes all the black holes before the mergers and a postmerger sample that consists of the black holes generated during the merging processes. The former represents black holes that once existed in the Universe, while the latter represents newly born black holes. Here we present a statistical analysis of these two samples. The nonparametric τ statistic method is adopted to correct for the observational selection effect. The Lynden-Bell C − method is further applied to derive the mass distribution and density function of black holes. It is found that the mass distribution can be expressed as a broken power-law function. More interestingly, the power-law index in the high-mass region is comparable for the two samples. The number density of black holes is found to depend on redshift as ρ(z) ∝ z −2.06—z −2.12 based on the two samples. The implications of these findings on the origin of black holes are discussed.

BD+44°493: Chemo-dynamical Analysis and Constraints on Companion Planetary Masses from WIYN/NEID Spectroscopy* *The WIYN Observatory is a joint facility of the University of Wisconsin Madison, Indiana University, NSF NOIRLab, the Pennsylvania State University, Purdue University, and Princeton University. Based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. Based on observations made

In this work, we present high-resolution (R ∼ 100,000), high signal-to-noise ratio (S/N ∼ 800) spectroscopic observations for the well-known, bright, extremely metal-poor, carbon-enhanced star BD+44°493. We determined chemical abundances and upper limits for 17 elements from WIYN/NEID data, complemented with 11 abundances redetermined from Subaru and Hubble data, using the new, more accurate, stellar atmospheric parameters calculated in this work. Our analysis suggests that BD+44°493 is a low-mass (0.83 M ⊙), old (12.1–13.2 Gyr) second-generation star likely formed from a gas cloud enriched by a single metal-free 20.5 M ⊙ Population III star in the early Universe. With a disk-like orbit, BD+44°493 does not appear to be associated with any major merger event in the early history of the Milky Way. From the precision radial-velocity NEID measurements (median absolute deviation = 16 m s−1), we were able to constrain companion planetary masses around BD+44°493 and rule out the presence of planets as small as msini=2 M J out to periods of 100 days. This study opens a new avenue of exploration for the intersection between stellar archaeology and exoplanet science using NEID.

Global Assessment of Mesoscale Eddies with TOEddies: Comparison Between Multiple Datasets and Colocation with In Situ Measurements

The present study introduces a comprehensive, open-access atlas of mesoscale eddies in the global ocean, as identified and tracked by the TOEddies algorithm implemented on a global scale. Unlike existing atlases, TOEddies detects eddies directly from absolute dynamic topography (ADT) without spatial filtering, preserving the natural spatial variability and enabling precise, high-resolution tracking of eddy dynamics. This dataset provides daily information on eddy characteristics, such as size, intensity, and polarity, over a 30-year period (1993–2023), capturing complex eddy interactions, including splitting and merging events that often produce networks of interconnected eddies. This unique approach challenges the traditional single-trajectory perspective, offering a nuanced view of eddy life cycles as dynamically linked trajectories. In addition to traditional metrics, TOEddies identifies both the eddy core (characterized by maximum azimuthal velocity) and the outer boundary, offering a detailed representation of eddy structure and enabling precise comparisons with in situ data. To demonstrate its value, we present a statistical overview of eddy characteristics and spatial distributions, including generation, disappearance, and merging/splitting events, alongside a comparative analysis with existing global eddy datasets. Among the multi-year observations, TOEddies captures coherent, long-lived eddies with lifetimes exceeding 1.5 years, while highlighting significant differences in the dynamic properties and spatial patterns across datasets. Furthermore, this study integrates TOEddies with 23 years of colocalized Argo profile data (2000–2023), allowing for a novel examination of eddy-induced subsurface variability and the role of mesoscale eddies in the transport of global ocean heat and biogeochemical properties. This atlas aims to be a valuable resource for the oceanographic community, providing an open dataset that can support diverse applications in ocean dynamics, climate research, and marine resource management.

Estimating false alarm rates of sub-dominant quasi-normal modes in GW190521

Abstract A major aim of gravitational wave astronomy is to test observationally the Kerr nature of black holes. The strongest such test, with minimal additional assumptions, is provided by observations of multiple ringdown modes, also known as black hole spectroscopy. For the gravitational wave merger event GW190521, we have previously claimed the detection of two ringdown modes emitted by the remnant black hole. In this paper we provide further evidence for the detection of multiple ringdown modes from this event. We analyse the recovery of simulated gravitational wave signals designed to replicate the ringdown properties of GW190521. We quantify how often our detection statistic reports strong evidence for a sub-dominant ( ℓ , m , n ) = ( 3 , 3 , 0 ) ringdown mode, even when no such mode is present in the simulated signal. We find this only occurs with a probability ∼0.02, which is consistent with a Bayes factor of 56 ± 1 (1σ uncertainty) found for GW190521. We also quantify our agnostic analysis of GW190521, in which no relationship is assumed between ringdown modes, and find that only 1 in 250 simulated signals without a ( 3 , 3 , 0 ) mode yields a result as significant as GW190521. Conversely, we verify that when simulated signals do have an observable ( 3 , 3 , 0 ) mode they consistently yield a strong evidence and significant agnostic results. We also find that constraints on deviations from the ( 3 , 3 , 0 ) mode on GW190521-like signals with a ( 3 , 3 , 0 ) mode are consistent with what was obtained from our previous analysis of GW190521. Our results support our previous conclusion that the gravitational wave signal from GW190521 contains an observable sub-dominant ( ℓ , m , n ) = ( 3 , 3 , 0 ) mode.

Evidence for large scale compressible turbulence in the ism of CSWA13, a star-Forming Lensed Galaxy at z = 1.87 with outflowing wind

Recently, Keerthi Vasan G. et al. (2024) presented spatially resolved observations of a wind outflow in CSWA13, a gravitationally lensed Star-Forming galaxy at z = 1.87. The gravitational lensing allowed for a substantially improved spatial and kinematic resolution of the wind and of the nebular gas. In this paper we take advantage of the resolved data to test for the existence of turbulence and to study its nature. We obtained the autocorrelation functions of the two velocity fields, and derived the spatial structure functions of the residual nebular and wind velocities along the major axis of the galaxy. The structure functions, of both velocity fields, reveal the existence of an underlying k −2 power spectrum scaling. This scaling suggests the existence of supersonic compressible turbulence. The autocorre- lation functions exhibit correlations over scales comparable to the total extent of the velocity fields. Thus, the turbulence is a large scale one. The turbulent timescale corresponding to the largest scale is about 200 Myr, an order of magnitude larger than the estimated age of the wind and of the young stars. This implies that the turbulence in the ISM formed well before the wind and the young stars. Given the large spatial scale of the turbulence, it is plausible that the source of the turbulence is a large scale one e.g. a merger or tidal event that triggered the formation of molecular clouds, in the cores of which, the young stars formed. A steepening of the structure functions on the smaller scales provides an estimate for the effective depth along the line of sight of the turbulent layer. The latter turns out to be ∼ 2kpc

Crimson Behemoth: A massive clumpy structure hosting a dusty AGN at z=4.91

Abstract The current paradigm for the co-evolution of galaxies and their supermassive black holes postulates that dust-obscured active galactic nuclei (AGNs) represent a transitional phase towards a more luminous and unobscured state. However, our understanding of dusty AGNs and their host galaxies at early cosmic times is inadequate due to observational limitations. Here, we present JWST observations of CID-931, an X-ray-detected AGN at a spectroscopic redshift of $z_{\rm spec}=4.91$. Multiband NIRCam imaging from the COSMOS-Web program reveals an unresolved red core, similar to JWST-discovered dusty AGNs. Strikingly, the red core is surrounded by at least eight massive star-forming clumps spread over ${1{^{\prime \prime}_{.}}6} \approx 10\,\,{\rm kpc}$, each of which has a stellar mass of $10^9$–$10^{10}\, M_{\odot }$ and a radius of $\sim$0.1–1 kpc. The whole system amounts to $10^{11}\, M_{\odot }$ in stellar mass, higher than typical star-forming galaxies at the same epoch. In this system, gas inflows and/or complex merger events may trigger clump formation and AGN activity, thus leading to the rapid formation of a massive galaxy hosting a supermassive black hole. Future follow-up observations will provide new insights into the evolution of the galaxy–black hole relationship during such transitional phases in the early universe.

From spherical stars to disk-like structures: 3D common-envelope evolution of massive binaries beyond inspiral

Self-consistent three-dimensional modeling of the entire common-envelope phase of gravitational wave progenitor systems until full envelope ejection is challenged by the vast range of spatial and temporal scales involved in the problem. Previous attempts were either terminated shortly after the rapid spiral-in with significant amounts of gravitationally bound material left in the system or they omitted this plunge-in phase and modeled the system afterward. We investigated the common-envelope interactions of a 10 M⊙ red supergiant primary star with a black hole and a neutron star companion, respectively, until full envelope ejection (≳97% of the envelope mass). In contrast to the expectation from e.g. population synthesis models, we find that the dynamical plunge-in of the systems determines (to leading order) the orbital separations of the core binary system, while the envelope ejection by recombination acts only at later stages of the evolution and fails to harden the core binaries down to orbital frequencies where they qualify as progenitors of gravitational-wave-emitting double-compact object mergers. Diverging from the conventional picture of an expanding common envelope that is ejected more or less spherically, our simulations show a new mechanism: The rapid plunge-in of the companion transforms the spherical morphology of the giant primary star into a disk-like structure. During this process, magnetic fields are amplified, and the subsequent transport of material through the disk around the core binary system drives a fast jet-like outflow in the polar directions. While most of the envelope material is lost through a recombination-driven wind from the outer edge of the disk, about 7% of the envelope leaves the system via the magnetically driven outflows. We further explored the potential evolutionary pathways of the post-common-envelope systems in light of the expected remaining lifetime of the primary core (2.97 M⊙) until core collapse (6 × 104 yr), most likely forming a neutron star. We find that the interaction of the core binary system with the circumbinary disk substantially increases the likelihood of giving rise to a double-neutron star merger (55%) or a neutron star black hole (5%) merger event.

A comparative high-resolution spectroscopic analysis of in situ and accreted globular clusters

Globular clusters (GCs) are extremely intriguing systems that help in reconstructing the assembly of the Milky Way via the characterisation of their chemo-chrono-dynamical properties. In this study, we use high-resolution spectroscopic archival data from UVES and UVES-FLAMES at the VLT to compare the chemistry of GCs dynamically tagged as either Galactic (NGC 6218, NGC 6522, and NGC 6626) or accreted from distinct merger events (NGC 362 and NGC 1261 from Gaia-Sausage-Enceladus, and Ruprecht 106 from the Helmi Streams) in the metallicity regime where abundance patterns of field stars with different origin effectively separate (−1.3 ≤ [Fe/H] ≤ −1.0 dex). We find remarkable similarities in the abundances of the two Gaia-Sausage-Enceladus GCs across all chemical elements. They both display depletion in the α-elements (Mg, Si and Ca) and statistically significant differences in Zn and Eu compared to in situ GCs. Additionally, we confirm that Ruprecht 106 exhibits a completely different chemical makeup from the other target clusters, being underabundant in all chemical elements. This demonstrates that when high precision is achieved, the abundances of certain chemical elements can not only efficiently separate in situ from accreted GCs, but can also distinguish among GCs born in different progenitor galaxies. In the end, we investigate the possible origin of the chemical peculiarity of Ruprecht 106. Given that its abundances do not match the chemical patterns of the field stars associated with its most likely parent galaxy (i.e. the Helmi Streams), being depleted in the abundances of α-elements in particular, we believe Ruprecht 106 to originate from a less massive galaxy compared to the progenitor of the Helmi Streams.

Simulating Short Gamma-Ray Burst Jets in Realistic Late Binary Neutron Star Merger Environments

The electromagnetic emission and the afterglow observations of the binary neutron star merger event GW170817A confirmed the association of the merger with a short gamma-ray burst (GRB) harboring a narrow (5°–10°) and powerful (1049–1050 erg) jet. Using the 1 s long neutrino-radiation general relativistic MHD simulation of coalescing neutron stars of K. Kiuchi et al., and following the semi-analytical estimates of M. Pais et al., we inject a narrow, powerful, unmagnetized jet into the post-merger phase. We explore different opening angles, luminosities, central engine durations, and times after the merger. We explore early (0.1 s following the merger) and late (1 s) jet launches; the latter is consistent with the time delay of ≈1.74 s observed between GW170817 and GRB 170817A. We demonstrate that the semi-analytical estimates correctly predict the jets’ breakout and collimation conditions. When comparing our synthetic afterglow light curves to the observed radio data of GW170807, we find a good agreement for a 3 × 1049 erg jet launched late with an opening angle in the range ≃5°–7°.

VEGAS-SSS: Tracing globular cluster populations in the interacting NGC 3640 galaxy group

Context. Globular clusters (GCs) are among the oldest stellar systems in the universe. As such, GC populations are valuable fossil tracers of galaxy formation and interaction history. This paper is part of the VEGAS-SSS series, which focuses on studying the properties of small stellar systems (SSSs) in and around bright galaxies. Aims. We used the multiband wide-field images obtained with the VST to study the properties of the GC population in an interacting pair of galaxies. Methods. We derived ugri photometry over 1.5 × 1.5 sq. degrees centered on the galaxy group composed of two elliptical galaxies: NGC 3640 and its fainter companion NGC 3641. We studied the GC system properties from both the ugri and gri matched catalogs. GC candidates were identified based on a combination of photometric properties (colors and magnitudes) and morphometric criteria (concentration index, elongation, FWHM, etc.), using sources with well-defined classifications from spectroscopic or imaging data available in the literature and numerical simulations as references. The selection criteria were also applied to empty fields to determine a statistical background correction for the number of identified GC candidates. Results. The 2D density maps of GCs appear to align with the diffuse light patches resulting from the merging events of the galaxies. The highest density peak of GCs is observed to be on NGC 3641 rather than NGC 3640, despite the latter being the more massive galaxy. The azimuthal averaged radial density profiles in both galaxies reveal that the GC population extends beyond the galaxy light profile and this indicates the likely presence of an intra-group GC component. A color bimodality in (u − r) and (g − i) is observed for NGC 3641, whereas NGC 3640 shows a broad unimodal distribution. Analysis of the GC luminosity function indicates that both galaxies are roughly located at the same distance (∼27 Mpc). We provide an estimate of the total number of GCs, and determine the specific frequency for NGC 3640, SN = 2.0 ± 0.6, which aligns with expectations, while for NGC 3641 we find a large SN = 4.5 ± 1.6.

Hydrodynamic 3D Simulation of Roche Lobe Overflow in High-mass X-Ray Binaries

Abstract While binary merger events have been an active area of study in both simulations and observational work, the formation channels by which a high-mass star extends from Roche lobe overflow (RLO) in a decaying orbit of a black-hole (BH) companion to a binary black-hole (BBH) system merits further investigation. Variable length-scales must be employed to accurately represent the dynamical fluid transfer and morphological development of the primary star as it conforms to a diminishing Roche lobe under the runaway influence of the proximal BH. We have simulated and evolved binary mass flow under these conditions to better identify the key transitional processes from RLO to BBHs. We demonstrate a new methodology to model RLO systems to unprecedented resolution simultaneously across the envelope, donor wind, tidal stream, and accretion disk regimes without reliance upon previously universal symmetry, mass flux, and angular momentum flux assumptions. We have applied this method to the semidetached high-mass X-ray binary M33 X-7 in order to provide a direct comparison to recent observations of an RLO candidate system at two overflow states of overfilling factors f = 1.01 and f = 1.1. We found extreme overflow (f = 1.1) to be entirely conservative in both mass and angular momentum transport, forming a conical L1 tidal stream of density and deflected angle comparable to existing predictions. This case lies within the unstable mass transfer (MT) regime as recently proposed of M33 X-7. The f = 1.01 case differed in stream geometry, accretion disk size, and efficiency, demonstrating nonconservative stable MT through a ballistic uniform-width stream. The nonconservative and stable nature of the f = 1.01 case MT also suggests that existing assumptions of semidetached binaries undergoing RLO may mischaracterize their role and distribution as progenitors of BBHs and common envelopes.

An optically dark merging system at z ∼ 6 detected by JWST

Context. Near- to mid-infrared observations (from Spitzer and JWST) have revealed a hidden population of galaxies at redshift z = 3 − 6 called optically dark objects, which are believed to be massive and dusty star-formers. They contribute substantially to the cosmic star-formation rate (SFR) density at z ∼ 4 − 5 (up to 30 − 40%). Aims. While optically dark sources are widely recognized as a significant component of the stellar mass function, the history of their stellar mass assembly (and the evolution of their interstellar medium) remains unexplored. However, they are thought to be the progenitors of the more massive early-type galaxies found in present-day groups and clusters. It is thus important to examine the possible connection between dark sources and merging events in order to understand the environment in which they live. Methods. Here, we report our search for close companions in a sample of 19 optically-dark objects identified in the SMACS0723 JWST deep field. They were selected in the NIRCam F444W band and undetected below 2 μm. We restricted our analysis to the reddest (i.e., F277W–F444W &gt; 1.3) and brightest (F444W &lt; 26 mag) objects. Results. We identified KLAMA, an optically dark source showing a very close companion (angular distance &lt; 0.5″). The spatially resolved SED fitting procedure indicates that all components lying within 1.5″ of who is it the dark source are indeed at z ∼ 5.7. Tidal features (leading to a whale-shaped morphology) corroborate the hypothesis that KLAMA is the most massive (log(M⋆/M⊙) &gt; 10.3) and dusty (AV ∼ 3 at the core) system of an ongoing merger with a mass ratio of ∼10. Thus, around ten similar merger events would be required to double the stellar mass of KLAMA. Merging systems with properties similar to KLAMA are identified in the SERRA simulations, allowing us to reconstruct their stellar-mass assembly history and predict their molecular gas properties (in particular, the [CII] emission for the simulated system). Conclusions. The discovery of mergers within dark galaxies at the end of the Epoch of Reionization highlights the importance of conducting a statistical search for additional candidates in deep NIRCam fields. Such research will aid in our understanding of the significance of merging processes during the obscured phase of stellar-mass accumulation.