arXiv subjects

Ewa L. Lokas

Publications and source records attributed to Ewa L. Lokas.

At least 19 recordsLinked to original sources

arXiv2026

Cluster-centric trends in bar size and pattern speed: the case of Abell 2199

We investigate how the environment of a dynamically unrelaxed galaxy cluster influences the structure and dynamics of stellar bars. In particular, we examine cluster-centric variations in normalised bar size and bar pattern speed in Abell 2199. Our analysis is based on 578 spectroscopically confirmed members of Abell 2199, including a master sample of 325 galaxies with homogeneous stellar mass and star formation rate measurements. We identify 39 barred galaxies and measure their structural properties using isophotal ellipse fitting and three-component (bulge+disc+bar) photometric decompositions. For 22 barred galaxies with MaNGA integral-field spectroscopy, we estimate bar pattern speeds using the Tremaine-Weinberg method, obtaining robust measurements for 12 galaxies. Stellar population age and projected specific angular momentum are analysed using $D4000_{R_{\mathrm{e}}}$ and $λ_{R_{\mathrm{e}}}$ from the MaNGA Pipe3D catalogue. Abell2199 exhibits star formation-density and morphology-density relations despite its non-relaxed dynamical state. Early-type spiral (ETS) barred galaxies show systematic cluster-centric variations in normalised bar size, with relatively larger bars towards the cluster centre and smaller bars at intermediate radii. A corresponding variation in bar pattern speed with cluster-centric distance is also observed. These trends motivate a division at $\sim$0.5$R_{\mathrm{vir}}$, within which morphology-dependent environmental signatures become clearer, as barred galaxies in the inner region tend to host older stellar populations and lower projected angular momentum than those in the outskirts, with ETS+Bar galaxies retaining higher angular momentum compared to S0+Bar galaxies at comparable radii.

astro-ph.GA↗

arXiv2026

Diverse lifestyles of bar-like galaxies and their coevolution with the brightest galaxy in the most massive cluster of TNG50

Clusters can provide propitious environments for bar formation in galaxies. This work studies the formation and evolution of 15 bar-like galaxies in the most massive cluster of the TNG50 simulation from the IllustrisTNG suite. The selection includes galaxies from the last simulation output from well-resolved subhalos with a strongly prolate stellar component. Eleven galaxies form or strongly enhance their bars during a pericenter passage around one or more progenitors of the brightest cluster galaxy (BCG). Two form their bars early as a result of minor mergers, one via an interaction with another massive galaxy, and one via disk instability. The bar formation times differ considerably, ranging between 3-11 Gyr. The lengths of the bars also differ, ranging between 2-6 kpc, and do not correlate with the amount of tidal forcing experienced. All galaxies have at least one pericenter passage around a BCG progenitor, but the number of interactions varies strongly and is reflected in the different amount of mass stripping the galaxies experience. Most bar formation events take place before the BCG is fully formed. In three cases, they occur just before different progenitors of the BCG merge. For six bar-like galaxies, the merger events leading to the final formation of the BCG cause significant changes of their orbits. Their diverse evolutionary histories illustrate the different paths to bar formation in clusters and emphasize the complex nature of the process, which includes coevolution with BCG progenitors.

astro-ph.GA↗

arXiv2025

High-redshift Merger-induced Bar-like Galaxies in IllustrisTNG

Recent discoveries made with JWST observations include a significant number of barred galaxies at high redshift. Their origin remains unclear and their presence seems difficult to reproduce in cosmological simulations of galaxy formation and evolution. In this Letter I present four examples of high-redshift bars selected from a sample of bar-like galaxies studied previously using IllustrisTNG simulations. All the galaxies formed their bars at redshifts z > 3 via mergers with smaller satellites, although one had its bar formed even earlier, at z > 5. The bars were born long, with lengths on the order of 3 kpc, and grew in time. Three of the four galaxies were later accreted by clusters and underwent multiple interactions with their respective brightest cluster galaxies. Their bar strength was to some extent affected by these interactions but all the galaxies preserved their bar-like shape until the present time. By the end of the evolution, all the galaxies lost their gas and stopped forming stars, they retained essentially no disk component and were no longer rotationally supported. The examples demonstrate that high-z bars do not evolve into present-day barred disk galaxies similar to the Milky Way but rather into S0s or ellipticals typically found in galaxy clusters.

astro-ph.GA↗

arXiv2025

Tidally induced bar-like galaxies in simulated clusters

One of the scenarios for bar formation in galaxies involves their interaction with a more massive companion. The stellar component is then transformed from a disk into a bar-like prolate spheroid. I investigated a subsample of 77 bar-like galaxies tidally induced in the cluster environment, selected among the previously studied sample of bar-like galaxies from the IllustrisTNG simulations. I present six clear, convincing examples of bar-like galaxies formed after an interaction with a progenitor of a massive brightest cluster galaxy (BCG) and describe the properties of their bars. For the whole sample, the time of bar formation is strongly correlated with and typically slightly greater than the time of the pericenter passage. All galaxies are strongly stripped of dark matter and gas, and their rotation is similarly diminished. A larger pericenter distance typically requires a higher host mass in order to transform the galaxy, but the interactions show no preference for prograde configurations. The final strength of the bars does not correlate with the amount of tidal stripping experienced because of the variety of initial properties of the progenitors and the subsequent evolution over the next pericenter passages. In spite of difficulties in the interpretation of some cases involving mergers and multiple interactions, the results confirm in the cosmological context the reality of tidal bar formation in cluster environments previously studied using controlled simulations.

astro-ph.GA↗

arXiv2025

The oldest tidally induced bar-like galaxy in the IllustrisTNG cluster

New JWST observations have revealed the presence of a significant number of high-redshift barred galaxies. The origin of these bars remains unclear, and their properties appear difficult to reconcile with the results of cosmological simulations of galaxy formation. I present an example of a tidally induced bar-like galaxy formed at z = 2.9 in the TNG100 suite of the IllustrisTNG simulations. The galaxy was identified among the sample of bar-like galaxies studied before and has the earliest bar formation time among the tidally induced subsample of those objects. Its disk transformed into a bar as a result of a close interaction with a massive progenitor of a brightest cluster galaxy (BCG). It remained on a tight orbit around the host and survived until the present, losing most of its initial mass and becoming red but preserving its prolate shape. Even before the interaction, at z = 3.5, the galaxy experienced a few mergers, which elongated its shape. This temporary distortion also made it look like a bar with spiral extensions of up to 6 kpc. The long-lived bar formed later was about 3 kpc long and grew over the next few gigayears. This example demonstrates that high-z bars should not be sought among the progenitors of present-day simulated barred galaxies but rather among the tidally interacting early population of galaxies in forming groups and clusters. Some of these galaxies may have survived as ellipticals, and some may have merged with their BCGs.

astro-ph.GA↗

arXiv2025

On the nature of buckling instability in galactic bars

Many strong simulated galactic bars experience buckling instability, which manifests itself as a vertical distortion out of the disk plane, and later dissipates. Using a simulation of an isolated Milky Way-like galaxy, I demonstrate that the phenomenon can be divided into two distinct phases. In the first one, the distortion grows and its pattern speed remains equal to the pattern speed of the bar, so that the distortion remains stationary in the reference frame of the bar. The growth can be described with the mechanism of a driven harmonic oscillator with time-dependent force, which decreases the vertical frequencies of the stars. At the end of this phase, most bar-supporting orbits have banana-like shapes with a resonant vertical-to-horizontal frequency ratio close to two. The increase of amplitudes of vertical oscillations leads to the decrease of the amplitudes of horizontal oscillations and the shrinking of the bar. The mass redistribution causes the harmonic oscillators to respond adiabatically and increase the horizontal frequencies. In the following second phase of buckling, the pattern speed of the distortion increases - reaching one third of the circular frequency - but it decreases with radius. The distortion propagates as a kinematic bending wave and winds up, leaving behind a pronounced boxy/peanut shape. The increased horizontal frequencies cause the weakening of the bar and the transformation of banana-like orbits into pretzel-like ones, except in the outer part of the bar, where the banana-like orbits and the distortion survive. The results strongly suggest that the buckling of galactic bars is not related to the fire-hose instability, but it can be fully explained by the mechanism of vertical resonance creating the distortion that later winds up.

astro-ph.GA↗

arXiv2024

A Sagittarius-like simulated dwarf spheroidal galaxy from TNG50

The Sagittarius dwarf spheroidal (Sgr dSph) galaxy provides one of the most convincing examples of tidal interaction between satellite galaxies and the Milky Way (MW). The main body of the dwarf was recently demonstrated to have an elongated, prolate, bar-like shape and to possess some internal rotation. Whether these features are temporary results of the strong tidal interaction at the recent pericenter passage or are due to a disky progenitor is a matter of debate. I present an analog of Sgr selected among bar-like galaxies from the TNG50 simulation of the IllustrisTNG project. The simulated dwarf is initially a disky galaxy with mass exceeding $10^{11}$ M$_\odot$ and evolves around a MW-like host on a tight orbit with seven pericenter passages and a period of about 1 Gyr. At the second pericenter passage, the disk transforms into a bar and the bar-like shape of the stellar component is preserved until the end of the evolution. The morphological transformation is accompanied by strong mass loss, leaving a dwarf with a final mass of below $10^{9}$ M$_\odot$. The gas is lost completely and the star formation ceases at the third pericenter passage. At the last pericenters, the dwarf possesses a bar-like shape, a little remnant rotation, and the metallicity gradient, which are consistent with observations. The more concentrated metal-rich stellar population rotates faster and has a lower velocity dispersion than the more extended metal-poor one. The metallicity distribution evolves so that the most metal-poor stars are stripped first, which explains the metallicity gradient detected in the Sgr stream. This study demonstrates that a dSph galaxy with properties akin to the Sgr dwarf can form from a disky progenitor with a mass of above $10^{11}$ M$_\odot$ by tidal evolution around the MW in the cosmological context.

astro-ph.GA↗

arXiv2023

Preprocessing in small groups: Three simulated galaxies interacting prior to cluster infall

The formation of galaxy clusters is a complicated process that probably involves the accretion of galaxies in groups, as observed in nearby clusters, such as Virgo and Fornax. The members of the groups undergo "preprocessing" prior to cluster infall, which affects their stellar populations and morphology. In this paper I present an extreme example of such an accretion event selected from the IllustrisTNG100 simulation. The group, composed of three full-sized disky galaxies and a number of smaller satellites, is accreted early, with the first pericenter around the cluster at redshift z=1.3. Before the infall, the three galaxies interact strongly in pairs within the group, which produces tidally induced bars in the two more massive ones. The interactions also lead to mass exchange and trigger some star formation activity resulting in temporary rejuvenation of their stellar populations. After infall, they all undergo seven pericenter passages around the cluster, experiencing strong mass loss in the dark matter and gas components, as well as reddening of the stellar populations. Their tidally induced bars are, however, preserved and even enhanced probably due to the loss of gas via ram-pressure stripping in the intracluster medium. The study demonstrates that group accretion can happen very early in cluster formation and proposes another scenario for the formation of tidally induced bars.

astro-ph.GA↗

arXiv2023

Merging galaxy clusters in IllustrisTNG

Mergers between galaxy clusters are an important stage in the formation of the large-scale structure of the Universe. Some of the mergers show a spectacular bow shock that formed as a result of recent passage of a smaller cluster through a bigger one, the classic example of this being the so-called bullet cluster. In this paper, I describe ten examples of interacting clusters identified among 200 of the most massive objects, with total masses above $1.4 \times 10^{14}$ M$_{\odot}$, from the IllustrisTNG300 simulation by searching for prominent bow shocks in their temperature maps. Despite different mass ratios of the two merging clusters, the events are remarkably similar in many respects. In all cases, the companion cluster passed close to the main one only once, between 0.9 and 0.3 Gyr ago, with the pericenter distance of 100-530 kpc and a velocity of up to 3400 km s$^{-1}$. The subcluster, typically an order of magnitude smaller in mass than the main cluster before the interaction, loses most of its dark matter and gas in the process. The displacement between the collisionless part of the remnant and the bow shock is such that the remnant typically lags behind the shock or coincides with it, with a single exception of the merger occurring with the largest velocity. Usually about 1% of the gas cells in the merging clusters are shocked, and the median Mach numbers of these gas cells are around two. Due to the relatively small size of the simulation box, no close analog of the bullet cluster was found, but I identified one case that is similar in terms of mass, velocity, and displacement. The presented cases bear more resemblance to less extreme observed interacting clusters such as A520 and Coma.

astro-ph.GA↗

arXiv2022

Formation of a barred galaxy in a major merger: The role of AGN feedback

Among the many processes involved in galaxy evolution, those of bar formation, quenching, and feedback from an active galactic nucleus (AGN) seem to be connected, however, the nature of these relations remains unclear. In this work, we aim to elucidate them by studying the formation of a barred galaxy in a major merger of two disks in the IllustrisTNG simulations. This merger involves a coalescence of two supermassive black holes and a sudden switch to the kinetic mode of AGN feedback implemented in the simulations, which leads to the removal of the gas from the inner part of the galaxy, followed by quenching of star formation and the formation of the bar. This causal relation between AGN feedback and bar formation explains a number of correlations observed in the data, such as the higher frequency of bars among red spirals and the presence of central gas holes in barred galaxies. In such a picture, the bars do not feed the black holes, so their presence does not increase the AGN strength, and they do not cause quenching. However, bars do form in regions characterized by a low gas fraction resulting from AGN feedback. This scenario is probably applicable to many barred galaxies, not only those formed in major mergers.

astro-ph.GA↗

arXiv2022

The origin and properties of red spirals: Insights from cosmological simulations

A significant fraction of spiral galaxies are red, gas-poor, and have low star formation rates (SFRs). We study these unusual objects using the IllustrisTNG100 simulation. Among 1912 well-resolved disk galaxies selected from the last simulation output, we identify 377 red objects and describe their properties and origins using a few representative examples. The simulated red spirals turn out to be typically very gas-poor, have very low SFRs, are more metal-rich, and have larger stellar masses than the remaining disks. Only about 13% of red spirals suffered strong mass loss and thus could have resulted from environmental quenching by ram pressure and tidal stripping of the gas, or similar processes. The majority of red disks were probably quenched by feedback from the active galactic nucleus (AGN). This conclusion is supported by the higher black hole masses and lower accretion rates of red disks, as well as the larger total AGN feedback energies injected into the surrounding gas in the kinetic feedback mode implemented in the IllustrisTNG simulations. The timescales of the gas loss correlate with the black hole growth for the AGN-quenched galaxies and with the dark-matter loss for the environmentally quenched ones. The red spirals are more likely to possess bars, and their bars are stronger than in the remaining disks, which is probably the effect of gas loss rather than the reason for quenching.

astro-ph.GA↗

arXiv2022

Lopsided galactic disks in IllustrisTNG

A significant fraction of nearby late-type galaxies are lopsided. We study the asymmetry of the stellar component in a sample of well-resolved disky galaxies selected from the last snapshot of the Illustris TNG100 simulation based on their flatness and rotational support. Among 1912 disks, we identify 161 objects with significant asymmetry in terms of the m=1 Fourier mode of the stellar component within (1-2) stellar half-mass radii and describe their properties using three representative examples. The profiles of the m=1 mode typically increase with radius, and the corresponding phase is constant in the asymmetric region, signifying a global distortion. Following the evolution of the lopsided disks over time, we find that their history is rather uneventful and the occurrence of the asymmetry is fairly recent. Only about 1/3 of the lopsided disks experienced any strong interaction recently that could have led to the distortion of their shape: 24% were affected by a more massive object and 9% underwent a gas-rich merger. Still, a majority of lopsided disks show a significant increase in their recent star formation rate. The most frequent mechanism for the formation of lopsided disks thus seems to be asymmetric star formation probably related to gas accretion, although the distortions in the gas and stars are not strongly correlated. This picture is supported by the finding that the lopsided population on average contains more gas, has higher star formation rate, lower metallicity and bluer color than the remaining disks. These correlations are similar to those seen in real galaxies, even though the fraction of simulated lopsided disks (8%) is much lower than in observations (30%). The observed correlation between the presence of the asymmetry and a bar is not reproduced either. These discrepancies may be due to overquenching or insufficient resolution of IllustrisTNG simulations.

astro-ph.GA↗

arXiv2022

Multiple stellar populations in Schwarzschild modeling and the application to the Fornax dwarf

Dwarf spheroidal (dSph) galaxies are believed to be strongly dark matter dominated and thus are considered perfect objects to study dark matter distribution and test theories of structure formation. They possess resolved, multiple stellar populations that offer new possibilities for modeling. A promising tool for the dynamical modeling of these objects is the Schwarzschild orbit superposition method. In this work we extend our previous implementation of the scheme to include more than one population of stars and a more general form of the mass-to-light ratio function. We tested the improved approach on a nearly spherical, gas-free galaxy formed in the cosmological context from the Illustris simulation. We modeled the binned velocity moments for stars split into two populations by metallicity and demonstrate that in spite of larger sampling errors the increased number of constraints leads to significantly tighter confidence regions on the recovered density and velocity anisotropy profiles. We then applied the method to the Fornax dSph galaxy with stars similarly divided into two populations. In comparison with our earlier work, we find the anisotropy parameter to be slightly increasing, rather than decreasing, with radius and more strongly constrained. We are also able to infer anisotropy for each stellar population separately and find them to be significantly different.

astro-ph.GA↗

arXiv2021

Lopsided galactic bars

Most of the observed and simulated galactic bars are symmetric in the face-on view. However, there are indeed cases of bars that are off-center with respect to the disk or have an asymmetric shape. The only well-known example showing both these features is the Large Magellanic Cloud. We report on the identification of several lopsided galactic bars in the Illustris TNG100 simulation found among a sample of elongated bar-like galaxies studied in the past. The bars show a clear asymmetry in the face-on view, which is in the shape of a footprint. We measured the evolution of the different parameters of the bars' shape and asymmetry as a function of time and find that the asymmetry is preserved for a few Gyr. It can grow together with a bar or appear later, after bar formation. We considered two scenarios leading to the formation of lopsided bars using controlled simulations. In the first, a Milky Way-like galaxy interacts with a massive companion placed on a radial orbit in the plane of the disk and perpendicular to the orientation of the bar at the time of the first passage. In the second, the galaxy initially has an off-center disk and the growth of the bar and its asymmetry is more similar to the one found in IllustrisTNG galaxies, where it is also preceded by the presence of an asymmetric disk. It is thus possible that lopsided bars are born in lopsided disks, although in some cases, the time difference between the occurrence of the asymmetry in the two components is quite large.

astro-ph.GA↗

arXiv2021

Bar-like galaxies in IllustrisTNG

We study a sample of bar-like galaxies in the Illustris TNG100 simulation, in which almost the whole stellar component is in the form of a prolate spheroid. The sample is different from the late-type barred galaxies studied before. In addition to the requirement of a high enough stellar mass and resolution, the 277 galaxies were selected based on the single condition of a low enough ratio of the intermediate to long axis of the stellar component. We followed the mass and shape evolution of the galaxies as well as their interactions with other objects and divided them into three classes based on the origin of the bar and the subsequent history. In galaxies of class A (comprising 28% of the sample), the bar was induced by an interaction with a larger object, most often a cluster or group central galaxy, and the galaxies were heavily stripped of dark matter and gas. In classes B and C (27% and 45% of the sample, respectively) the bars were induced by a merger or a passing satellite, or they were formed by disk instability. Class B galaxies were then partially stripped of mass, while those of class C evolved without strong interactions, thus retaining their dark matter and gas in the outskirts. We illustrate the properties of the different classes with three representative examples of individual galaxies. In spite of the different evolutionary histories, the bars are remarkably similar in strength, length, and formation times. The gas fraction in the baryonic component within two stellar half-mass radii at the time of bar formation is always below 0.4 and usually very low, which confirms in the cosmological context the validity of this threshold, which has previously been identified in controlled simulations. Observational counterparts of these objects can be found among early-type fast rotators, S0 galaxies, or red spirals with bars.

astro-ph.GA↗

arXiv2020

An interesting case of the formation and evolution of a barred galaxy in the cosmological context

Elongated, bar-like galaxies without a significant disk component, with little rotation support and no gas, often form as a result of tidal interactions with a galaxy cluster, as was recently demonstrated using the IllustrisTNG-100 simulation. Galaxies that exhibit similar properties are, however, also found to be infalling into the cluster for the first time. We use the same simulation to study in detail the history of such a galaxy over cosmic time in order to determine its origin. The bar appears to be triggered at t=6.8 Gyr by the combined effect of the last significant merger with a subhalo and the first passage of another dwarf satellite, both ten times less massive than the galaxy. The satellites deposit all their gas in the galaxy, contributing to its third and last star-formation episode, which perturbs the disk and may also contribute to the formation of the bar. The galaxy then starts to lose its gas and dark matter due to its passage near a group of more massive galaxies. The strongest interaction involves a galaxy 22 times more massive, leaving the barred galaxy with no gas and half of its maximum dark matter mass. During this time, the bar grows steadily, seemingly unaffected by the interactions, although they may have aided its growth by stripping the gas. The studied galaxy, together with two other similar objects briefly discussed in this letter, suggest the existence of a new class of early-type barred galaxies and thereby demonstrate the importance of interactions in galaxy formation and evolution.

astro-ph.GA↗

arXiv2020

Tidal evolution of galaxies in the most massive cluster of IllustrisTNG-100

We study the tidal evolution of galaxies in the most massive cluster of the IllustrisTNG-100 simulation. For the purpose of this work we select 112 galaxies with the largest stellar masses at present and follow their properties in time. Using their orbital history we divide the sample into unevolved (infalling), weakly evolved (with one pericenter passage) and strongly evolved (with multiple pericenters). The samples are clearly separated by the value of the integrated tidal force from the cluster the galaxies experienced during their entire evolution and their properties depend strongly on this quantity. As a result of tidal stripping, the galaxies of the weakly evolved sample lost between 10 and 80% of their dark mass and less than 10% of stars, while those in the strongly evolved one more than 70% of dark and between 10 and 55% of stellar mass, and are significantly less or even not dark matter dominated. While 33% of the infalling galaxies do not contain any gas, this fraction increases to 67% for the weakly evolved and to 100% for the strongly evolved sample. The strongly evolved galaxies lose their gas earlier and faster (within 2-6 Gyr) but the process can take up to 4 Gyr from the first pericenter passage. These galaxies are redder and more metal rich, and at redshift z=0.5 the population of galaxies in the cluster becomes predominantly red. As a result of tidal stirring, the morphology of the galaxies evolves from oblate to prolate and their rotation is diminished thus the morphology-density relation is reproduced in the simulated cluster. The strongly evolved sample contains at least six convincing examples of tidally induced bars and six more galaxies that had their bars enhanced by their interaction with the cluster.

astro-ph.GA↗

arXiv2020

Tidally induced warps of spiral galaxies in IllustrisTNG

Warps are common features in both stellar and gaseous disks of nearby spiral galaxies with the latter usually easier to detect. Several theories have been proposed in the literature to explain their formation and prevalence, including tidal interactions with external galaxies. Observational correlations also suggest the importance of tides for warp formation. Here, we use the TNG100 run from the magnetohydrodynamical cosmological simulation suite IllustrisTNG to investigate the connection between interactions and the formation of gas warps. We find that in the sample of well-resolved gas-rich spiral galaxies ($10^{10}\lesssim\mathrm{M_{*}/M_{\odot}}\lesssim10^{11}$ at $z=0$) from the simulation TNG100-1, about $16\%$ possess the characteristic S-shaped warp. Around one third of these objects have their vertical morphology induced by interactions with other galaxies. Half of these interactions end with the perturber absorbed by the host. Warps formed in interactions are more asymmetrical than the remaining sample, however after the interaction the asymmetry decreases with time. We find that warps induced by interactions survive on average for $<1$ Gyr. The angle between the orbital angular momentum of the perturber and the angular momentum of the host's disk that most likely leads to warp formation is around 45 degrees. While our main goal is to investigate tidally induced warps, we find that during interactions in addition to tides, new gas that is accreted from infalling satellites also can contribute to warp formation.

astro-ph.GA↗