Galaxy Interactions Research Articles

What is Important? Morphological Asymmetries are Useful Predictors of Star Formation Rates of Star-forming Galaxies in SDSS Stripe 82

Abstract The morphology and structure of galaxies reflect their star formation and assembly histories. We use the framework of mutual information (MI) to quantify the interdependence among several structural variables and to rank them according to their relevance for predicting the specific star formation rate (SSFR) by comparing the MI of the predictor variables with the SSFR and penalizing variables that are redundant. We apply this framework to study ∼3700 face-on star-forming galaxies (SFGs) with varying degrees of bulge dominance and central concentration and with stellar mass M ⋆ ≈ 109 M ⊙−5 × 1011 M ⊙ at redshift z = 0.02–0.12. We use the Sloan Digital Sky Survey (SDSS) Stripe 82 deep i-band imaging data, which improve measurements of asymmetry and bulge dominance indicators. We find that star-forming galaxies are a multiparameter family. In addition to M ⋆, asymmetry emerges as the most powerful predictor of SSFR residuals of SFGs, followed by bulge prominence/concentration. Star-forming galaxies with higher asymmetry and stronger bulges have higher SSFR at a given M ⋆. The asymmetry reflects both irregular spiral arms and lopsidedness in seemingly isolated SFGs and structural perturbations by galaxy interactions or mergers.

An IFU View of the Active Galactic Nuclei in MaNGA Galaxy Pairs

Abstract The role of active galactic nuclei (AGNs) during galaxy interactions and how they influence the star formation in the system are still under debate. We use a sample of 1156 galaxies in galaxy pairs or mergers (hereafter “pairs”) from the MaNGA survey. This pair sample is selected by the velocity offset, projected separation, and morphology, and is further classified into four cases along the merger sequence based on morphological signatures. We then identify a total of 61 (5.5%) AGNs in pairs based on the emission-line diagnostics. No evolution of the AGN fraction is found, either along the merger sequence or compared to isolated galaxies (5.0%). We observe a higher fraction of passive galaxies in galaxy pairs, especially in the pre-merging cases, and associate the higher fraction to their environmental dependence. The isolated AGN and AGNs in pairs show similar distributions in their global stellar mass, star-formation rate (SFR), and central [O iii] surface brightness. AGNs in pairs show radial profiles of increasing specific SFR and declining Dn4000 from center to outskirts, and no significant difference from the isolated AGNs. This is clearly different from star-forming galaxies (SFGs) in our pair sample, which show enhanced central star formation, as reported before. AGNs in pairs have lower Balmer decrements at outer regions, possibly indicating less dust attenuation. Our findings suggest that AGNs are likely follow an inside-out quenching and the merger impact on the star formation in AGNs is less prominent than in SFGs.

Star-forming Dwarf Galaxies in Filamentary Structures around the Virgo Cluster: Probing Chemical Pre-processing in Filament Environments

Abstract It has been proposed that the filament environment is closely connected to the pre-processing of galaxies, where their properties may have been changed by environmental effects in the filament before they fell into the galaxy cluster. We present the chemical properties of star-forming dwarf galaxies (SFDGs) in five filamentary structures (Virgo III, Leo Minor, Leo II A, Leo II B, and Canes Venatici) around the Virgo cluster using the Sloan Digital Sky Survey optical spectroscopic data and Galaxy Evolution Explorer ultraviolet photometric data. We investigate the relationship between stellar mass, gas-phase metallicity, and specific star formation rate (sSFR) of the SFDGs in the Virgo filaments in comparison to those in the Virgo cluster and field. We find that, at a given stellar mass, SFDGs in the Virgo filaments show lower metallicity and higher sSFR than those in the Virgo cluster on average. We observe that SFDGs in the Virgo III filament show enhanced metallicities and suppressed star formation activities comparable to those in the Virgo cluster, whereas SFDGs in the other four filaments exhibit similar properties to the field counterparts. Moreover, about half of the galaxies in the Virgo III filament are found to be morphologically transitional dwarf galaxies that are supposed to be on the way to transforming into quiescent dwarf early-type galaxies. Based on the analysis of the galaxy perturbation parameter, we propose that the local environment represented by the galaxy interactions might be responsible for the contrasting features in chemical pre-processing found in the Virgo filaments.

Do AGN triggering mechanisms vary with radio power? – II. The importance of mergers as a function of radio power and optical luminosity

ABSTRACT Investigation of the triggering mechanisms of radio active galactic nuclei (radio AGN) is important for improving our general understanding of galaxy evolution. In the first paper in this series, detailed morphological analysis of high-excitation radio galaxies (HERGs) with intermediate radio powers suggested that the importance of triggering via galaxy mergers and interactions increases strongly with AGN radio power and weakly with optical emission-line luminosity. Here, we use an online classification interface to expand our morphological analysis to a much larger sample of 155 active galaxies (3CR radio galaxies, radio-intermediate HERGs, and Type 2 quasars) that covers a broad range in both 1.4 GHz radio power and [O iii] λ5007 emission-line luminosity. All active galaxy samples are found to exhibit excesses in their rates of morphological disturbance relative to 378 stellar-mass- and redshift-matched non-active control galaxies classified randomly and blindly alongside them. These excesses are highest for the 3CR HERGs (4.7σ) and Type 2 quasar hosts (3.9σ), supporting the idea that galaxy mergers provide the dominant triggering mechanism for these subgroups. When the full active galaxy sample is considered, there is clear evidence to suggest that the enhancement in the rate of disturbance relative to the controls increases strongly with [O iii] λ5007 emission-line luminosity but not with 1.4 GHz radio power. Evidence that the dominant AGN host types change from early-type galaxies at high radio powers to late-type galaxies at low radio powers is also found, suggesting that triggering by secular, disc-based processes holds more importance for lower-power radio AGN.

From large-scale environment to CGM angular momentum to star-forming activities – I. Star-forming galaxies

ABSTRACT The connection between halo gas acquisition through the circumgalactic medium and galaxy star formation (SF) has long been studied. In this series of two papers, we put this interplay within the context of the galaxy environment on large scales (several hundreds of kpc), which, to a certain degree, maps out various paths for galaxy interactions. We use the IllustrisTNG-100 simulation to demonstrate that the large-scale environment modulates the circumgalactic gas angular momentum, resulting in either enhanced (Paper I) or suppressed (Paper II) SF inside a galaxy. In this paper (Paper I), we show that the large-scale environment around a star-forming galaxy is often responsible for triggering new episodes of SF. Such an episodic SF pattern is well synced with a pulsating motion of the circumgalactic gas, which, on the one hand, receives angular momentum modulations from the large-scale environment, yielding in-spiralling gas to fuel the star-forming reservoir, while, on the other hand, is affected by the feedback activities from the galaxy centre. As a result, a present-day star-forming galaxy may have gone through several cycles of star-forming and quiescent phases during its evolutionary history, with the circumgalactic gas carrying out a synchronized cadence of ‘breathing in and out’ motions out to ∼100 kpc.

Active Galactic Nuclei Abundance in Cosmic Voids

Abstract The abundance of active galactic nuclei (AGN) in cosmic voids is relatively unexplored in the literature, but can potentially provide new constraints on the environmental dependence of AGN activity and the AGN-host coevolution. We investigated AGN fractions in one of the largest samples of optically selected cosmic voids from Sloan Digital Sky Survey Data Release 12 for redshift range 0.2–0.7 for moderately bright and bright AGN. We separated inner and outer void regions based on the void size, given by its effective void radius. We classified galaxies at a distance <0.6 R eff as inner void members and galaxies in the interval 0.6 < R/R eff < 1.3 as outer void galaxies. We found higher average fractions in the inner voids (4.9 ± 0.7)% than for their outer counterparts (3.1 ± 0.1)% at z > 0.42, which clearly indicates an environmental dependence. This conclusion was confirmed upon further separating the data in narrower void-centric distance bins and measured a significant decrease in AGN activity from inner to outer voids for z > 0.42. At low redshifts (z < 0.42), we find very weak dependence on the environment for the inner and outer regions for two out of three bins. We argue that the higher fraction in low-density regions close to void centers relative to their outer counterparts observed in the two higher-redshift bins suggests that more efficient galaxy interactions may occur at a one-to-one level in voids that may be suppressed in denser environments due to higher velocity dispersions. It could also indicate less prominent ram pressure stripping in voids or some intrinsic host or void environment properties.

Asymmetric H i 21 cm Lines of Fast Radio Burst Hosts: Connection with Galaxy Interaction

Abstract Fast radio bursts (FRBs) are enigmatic transients with very short radio emission. Their nature is still widely debated. I provide the first analysis of atomic gas properties of a small sample of FRB hosts to constrain their nature. H i observations exist for NGC 3252, the host of FRB 181030A, M81, the host of FRB 200120E, and the Milky Way, the host of FRB 200428. I report three observables: (i) all three FRB hosts are interacting galaxies; (ii) the H i spectra of both FRB hosts with such data available are highly asymmetric, several standard deviations above the general population of galaxies; (iii) two FRB hosts have normal atomic gas properties and one is strongly deficient in atomic gas. This indicates that nearby and repeating FRBs are connected with a recent enhancement of star formation due to interaction. This supports fast FRB channels, for example, a massive star with a short delay time so that interaction signatures giving rise to the birth of the progenitor are still visible. Long gamma-ray burst (GRB) and broad-lined type Ic supernova (SN) hosts exhibit much more symmetric spectra, even though they were claimed to experience gas inflow from the intergalactic medium. The difference can be explained by the interactions experienced by FRB hosts being more disruptive than these gas inflows, or by the mass effect, with GRB/SN hosts at lower masses having less organized gas motions, so with H i lines closer to a symmetrical Gaussian. This also suggests that the emission mechanisms of FRBs and GRBs are different.

Exploring and interrogating astrophysical data in virtual reality

Scientists across all disciplines increasingly rely on machine learning algorithms to analyse and sort datasets of ever increasing volume and complexity. Although trends and outliers are easily extracted, careful and close inspection will still be necessary to explore and disentangle detailed behaviour, as well as identify systematics and false positives. We must therefore incorporate new technologies to facilitate scientific analysis and exploration. Astrophysical data is inherently multi-parameter, with the spatial-kinematic dimensions at the core of observations and simulations. The arrival of mainstream virtual-reality (VR) headsets and increased GPU power, as well as the availability of versatile development tools for video games, has enabled scientists to deploy such technology to effectively interrogate and interact with complex data. In this paper we present development and results from custom-built interactive VR tools, called the iDaVIE suite, that are informed and driven by research on galaxy evolution, cosmic large-scale structure, galaxy–galaxy interactions, and gas/kinematics of nearby galaxies in survey and targeted observations. In the new era of Big Data ushered in by major facilities such as the SKA and LSST that render past analysis and refinement methods highly constrained, we believe that a paradigm shift to new software, technology and methods that exploit the power of visual perception, will play an increasingly important role in bridging the gap between statistical metrics and new discovery. We have released a beta version of the iDaVIE software system that is free and open to the community.

The SAMI Galaxy Survey: Detection of Environmental Dependence of Galaxy Spin in Observations and Simulations Using Marked Correlation Functions

Abstract The existence of a kinematic morphology–density relation remains uncertain, and instead stellar mass appears the more dominant driver of galaxy kinematics. We investigate the dependence of the stellar spin parameter proxy λ R e on environment using a marked cross-correlation method with data from the Sydney Australian Astronomical Observatory Multi-object Integral-field Spectrograph (SAMI) Galaxy Survey. Our sample contains 710 galaxies with spatially resolved stellar velocity and velocity dispersion measurements. By utilizing the highly complete spectroscopic data from the Galaxy and Mass Assembly Survey, we calculate marked cross-correlation functions for SAMI galaxies using a pair count estimator and marks based on stellar mass and λ R e . We detect an anticorrelation of stellar kinematics with environment at the 3.2σ level, such that galaxies with low λ R e values are preferably located in denser galaxy environments. However, a significant correlation between stellar mass and environment is also found (correlation at 2.4σ), as found in previous works. We compare these results to mock observations from the cosmological Evolution and Assembly of Galaxies and their Environments (EAGLE) simulations, where we find a similar significant λ R e anticorrelation with environment, and a mass and environment correlation. We demonstrate that the environmental correlation of λ R e is not caused by the mass–environment relation. The significant relationship between λ R e and environment remains when we exclude slow rotators. The signals in SAMI and EAGLE are strongest on small scales (10–100 kpc) as expected from galaxy interactions and mergers. Our work demonstrates that the technique of marked correlation functions is an effective tool for detecting the relationship between λ R e and environment.

A meeting at z ∼ 3: Young massive galaxies and an AGN within 30 kpc of the luminous QSO LBQS 0302–0019

Context. Contrary to expectations from scenarios of black hole growth driven by galaxy interactions and mergers, dual active galactic nuclei (AGN) with kiloparsec separations are rarely observed and are very difficult to identify, in particular at high redshifts (i.e. z > 2). Aims. Focussing on the recently discovered dual AGN system LBQS 0302−0019 at z = 3.29, we seek to identify further group members in its environment and to understand their formation history through deep high-angular-resolution imaging. Methods. We present deep Hubble Space Telescope (HST) Wide-field Camera 3 near-infrared imaging of LBQS 0302−0019. In combination with ground-based VLT/HAWK-I imaging, we infer accurate sizes, colours, ages, and stellar masses of companion galaxies. Results. We clearly detect four companion objects close to LBQS 0302−0019 that also have faint signatures in the ground-based images. We constrain light-weighted ages and masses for the two most prominent companions, Jil1 and Jil2, to t⋆ = 252−109+222 Myr with log(M⋆/[M⊙]) = 11.2−0.1+0.3 and t⋆ = 19−14+74 Myr with log(M⋆/[M⊙]) = 9.4−0.4+0.9, respectively. The HST data also show that the obscured AGN, previously identified by strong nebular He II emission, is associated with the young massive companion Jil2. Because very massive stars of the starburst cannot be solely responsible for the He II emission, we strengthen our initial conclusion that Jil2 has been hosting an AGN. Conclusions. If the young starburst of Jil2 had been accompanied by sustained black hole growth, Jil2 may have contributed He II-ionising flux to create the large He II Lyα proximity zone around LBQS 0302−0019. Hence, the duration of the current luminous AGN episode of LBQS 0302−0019 may have been overestimated.

Gamma-ray bursts as probes of high-redshift Lyman-αemitters and radiative transfer models

Aims.We present the updated census and statistics of Lyman-αemitting long gamma-ray burst host galaxies (LAE-LGRBs). We investigate the properties of a subsample of LAE-LGRBs and test the shell model that is commonly used to fit Lyman-α(Lyα) emission line spectra.Methods.We considered all LGRB host galaxies with relevant publicly available information. We defined a golden sample of four LAE-LGRBs (GRBs 011211, 021004, 060926, and 070110) with afterglow and host galaxy observations allowing us to constrain the shell modeling of the Lyαline.Results.The census results in 29 detected LAE-LGRBs. We present 5 new Lyαemission detections in host-galaxy spectra and the corresponding unpublished VLT/X-shooter data (GRBs 060926, 070110, 081121, 081222, and 100424A). From the comparison of the statistics and properties of LAE-LGRBs to those of LAE samples in the literature, we find evidence of Lyαsuppression in dusty systems. The fraction of LAE-LGRBs in the overall LGRB hosts is lower than that found for Lyman-break galaxy (LBG) samples at similar redshift range. This result can arise because the selection criteria of the parent samples are different and the spectral observations of LGRB samples are shallower than those of LBG. However, we find that LAE-LGRBs are representative of Lyαemission from the bulk of UV-selected galaxies atz ∼ 2. We find that the golden sample of LAE-LGRBs we studied consists of complex systems characterized by multiple emission blobs and by signs of possible galaxy interactions. The fitting procedure recovers the HIcolumn densities (NHI) measured from the afterglow spectra and the other properties described by the shell-model parameters in the two low-NHIcases, but it fails to do this in the other two cases with highNHI. The afterglows of most LGRBs and LAE-LGRBs show log(NHI/cm−2) > 20.3, implying that statistically, the bulk of Lyαphotons that is expected to be produced by massive stars in the star-forming region hosting the GRB will be surrounded by these opaque lines of sight. We therefore interpret our results in the context of more sophisticated models and of different dominant Lyα-emitting regions. We also compare LAE-LGRBs to LAE Lyman continuum (LyC) leakers in the literature in terms of the properties that are identified as possible indirect indicators of LyC leakage. We find that only one LGRB (GRB 021004) would likely be a strong LyC leaker and discuss the validity of these indicators at high redshift. While our work shows that LGRBs are useful tools for probing LAEs and radiative transfer models, larger statistics are required to strengthen our findings.

Two interacting galaxies hiding as one, revealed by MaNGA

Given their prominent role in galaxy evolution, it is of paramount importance to unveil galaxy interactions and merger events and to investigate the underlying mechanisms. The use of high-resolution data makes it easier to identify merging systems, but it can still be challenging when the morphology does not show any clear galaxy pair or gas bridge. Characterising the origin of puzzling kinematic features can help reveal complicated systems. Here, we present a merging galaxy, MaNGA 1-114955, in which we highlighted the superimposition of two distinct rotating discs along the line of sight. These counter-rotating objects both lie on the star-forming main sequence but display perturbed stellar velocity dispersions. The main galaxy presents off-centred star formation as well as off-centred high-metallicity regions, supporting the scenario of recent starbursts, while the secondary galaxy hosts a central starburst that coincides with an extended radio emission, in excess with respect to star formation expectations. Stellar mass as well as dynamical mass estimates agree towards a mass ratio within the visible radius of 9:1 for these interacting galaxies. We suggest that we are observing a pre-coalescence stage of a merger. The primary galaxy accreted gas through a past first pericentre passage about 1 Gyr ago and more recently from the secondary gas-rich galaxy, which exhibits an underlying active galactic nucleus. Our results demonstrate how a galaxy can hide another one and the relevance of a multi-component approach for studying ambiguous systems. We anticipate that our method will be efficient at unveiling the mechanisms taking place in a sub-sample of galaxies observed by the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, all of which exhibit kinematic features of a puzzling origin in their gas emission lines.

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.

Dark Matter Deficient Galaxies and Their Member Star Clusters Form Simultaneously during High-velocity Galaxy Collisions in 1.25 pc Resolution Simulations

Abstract How diffuse dwarf galaxies that are deficient in dark matter—such as NGC1052-DF2 and NGC1052-DF4—formed remains a mystery. Along with their luminous member globular clusters (GCs), the so-called dark matter deficient galaxies (DMDGs) have challenged observers and theorists alike. Here we report a suite of galaxy collision simulations using the adaptive mesh refinement code Enzo with 1.25 pc resolution, which demonstrates that high-velocity galaxy collisions induce the formation of DMDGs and their star clusters (SCs) simultaneously. With a numerical resolution that is significantly better than our previous study, we resolve the dynamical structure of the produced DMDGs and the detailed formation history of their SCs, which are possible progenitors of the DMDG’s member GCs. In particular, we show that a galaxy collision with a high relative velocity of ∼300 km s−1, invoking severe shock compression, spawns multiple massive SCs M ⋆ ≳ 10 6 M ⊙ in <150 Myr after the collision. At the end of the ∼800 Myr evolution in our fiducial run, the resulting DMDG of M ⋆ ≃ 3.5 × 108 M ⊙ hosts 10 luminous M V ≲ −8.5 mag, gravitationally bound SCs with a line-of-sight velocity dispersion 11.2 km s−1. Our study suggests that DMDGs and their luminous member SCs could form simultaneously in high-velocity galaxy collisions while being in line with the key observed properties of NGC1052-DF2 and NGC1052-DF4.

On the H i Content of MaNGA Major Merger Pairs

AbstractTo study the role of H i content in galaxy interactions, we select galaxy pairs and control galaxies from the SDSS-IV MaNGA IFU survey, adopting kinematic asymmetry as a new effective indicator to describe the merger stage. With archival data from the HI-MaNGA survey and new observations from the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we investigate the differences in H i gas fraction (fH i), star formation rate (SFR), and H i star formation efficiency (SFEH i) between pairs and controls. Our results suggest that on average the H i gas fraction of major-merger pairs is marginally decreased by ∼ 15% relative to isolated galaxies, and paired galaxies during pericentric passage show weakly decreased fH i (−0.10 ± 0.05 dex), significantly enhanced SFR (0.42 ± 0.11 dex), and SFEH i (0.48 ± 0.12 dex). We propose the marginally detected H i depletion may originate from the gas consumption in fueling the enhanced H2 reservoir of galaxy pairs.

Break type and interactions from ultra-deep optical imaging of isolated galaxies

AbstractIn the standard cosmological model of galaxy evolution, mergers and interactions play a fundamental role in shaping galaxies. Galaxies that are currently isolated are thus interesting, allowing us to identify how internal or external processes impact galactic structure. However, current observational limits may be obscuring crucial information in the low-mass or low-brightness regime. We use the AMIGA catalog of isolated galaxies to explore the impact of different factors on the structure of these galaxies. In particular, we study the type of disk break based on the degree of isolation and the presence of interactions which are only detectable in the ultra-low surface brightness regime. We present the first results of an extensive observational campaign of ultra-deep optical imaging targeting a sample of 25 low-redshift (z < 0.035) isolated galaxies. The nominal surface brightness limits achieved are comparable to those to be obtained in the 10-year LSST coadds ( mag arcsec−2; 3σ ; 10” × 10”). We find that isolated galaxies have a considerably higher fraction of purely exponential disk profiles and a lower presence of up-bending breaks than field or cluster galaxies. Our extreme imaging depth allows us to detect the presence of previously unreported interactions with minor companions in some of the galaxies in our sample (∼40% of the galaxies show signs of interaction). The results of our work fit with the general framework of galactic structure in which up-bending breaks (Type III) would be produced by mergers and down-bending breaks (Type II) due to a threshold in star formation that would tend to become single exponential disk (Type I) in case of cessation or decrease of star formation.

A triple active galactic nucleus in the NGC 7733–7734 merging group

Context. Galaxy interactions and mergers can lead to supermassive black hole (SMBH) binaries, which become active galactic nucleus (AGN) pairs when the SMBHs start accreting mass. If there is a third galaxy involved in the interaction, then a triple-AGN system can form. Aims. Our goal is to investigate the nature of the nuclear emission from the galaxies in the interacting pair NGC 7733–NGC 7734 using archival VLT/MUSE integral field spectrograph data and study its relation to the stellar mass distribution traced by near-infrared (NIR) observations from the South African Astronomical Observatory (SAAO). Methods. We conducted NIR observations using the SAAO and identified the morphological properties of bulges in each galaxy. We used MUSE data to obtain a set of ionized emission lines from each galaxy and studied the ionization mechanism. We also examined the relation of the galaxy pair with any nearby companions with far-ultraviolet observations using the UVIT. Results. The emission line analysis from the central regions of NGC 7733 and NGC 7734 shows Seyfert and low ionization nuclear emission-line regions type AGN activity. The galaxy pair NGC 7733−34 also shows evidence of a third component, which has Seyfert-like emission. Hence, the galaxy pair NGC 7733−34 forms a triple-AGN system. We also detected an extended narrow-line region associated with the nucleus of NGC 7733.

Galaxy flybys: evolution of the bulge, disc, and spiral arms

ABSTRACT Galaxy flybys are as common as mergers in low-redshift Universe and are important for galaxy evolution as they involve the exchange of significant amounts of mass and energy. In this study, we investigate the effect of minor flybys on the bulges, discs, and spiral arms of Milky Way mass galaxies for two types of bulges – classical bulges and boxy/peanut pseudo-bulges. Our N-body simulations comprise of two disc galaxies of mass ratios 10:1 and 5:1, where the discs of the galaxies lie in their orbital plane and the pericentre distance is varied. We performed photometric and kinematic bulge–disc decomposition at regular time-steps and traced the evolution of the disc size, spiral structure, bulge sersic index, bulge mass, and bulge angular momentum. Our results show that the main effect on the discs is disc thickening, which is seen as the increase in the ratio of disc scale height to scale radius. The strength of the spiral structure A2/A0 shows small oscillations about the mean time-varying amplitude in the pseudo-bulge host galaxies. The flyby has no significant effect on non-rotating classical bulge, which shows that these bulges are extremely stable in galaxy interactions. However, the pseudo-bulges become dynamically hotter in flybys indicating that flybys may play an important role in accelerating the rate of secular evolution in disc galaxies. This effect on pseudo-bulges is a result of their rotating nature as part of the bar. Also, flybys do not affect the time and strength of bar buckling.

Simulations of the star-forming molecular gas in an interacting M51-like galaxy: cloud population statistics

ABSTRACT To investigate how molecular clouds react to different environmental conditions at a galactic scale, we present a catalogue of giant molecular clouds (GMCs) resolved down to masses of ∼10 M⊙ from a simulation of the entire disc of an interacting M51-like galaxy and a comparable isolated galaxy. Our model includes time-dependent gas chemistry, sink particles for star formation, and supernova feedback, meaning we are not reliant on star formation recipes based on threshold densities and can follow the physics of the cold molecular phase. We extract GMCs from the simulations and analyse their properties. In the disc of our simulated galaxies, spiral arms seem to act merely as snowplows, gathering gas, and clouds without dramatically affecting their properties. In the centre of the galaxy, on the other hand, environmental conditions lead to larger, more massive clouds. While the galaxy interaction has little effect on cloud masses and sizes, it does promote the formation of counter-rotating clouds. We find that the identified clouds seem to be largely gravitationally unbound at first glance, but a closer analysis of the hierarchical structure of the molecular interstellar medium shows that there is a large range of virial parameters with a smooth transition from unbound to mostly bound for the densest structures. The common observation that clouds appear to be virialized entities may therefore be due to CO bright emission highlighting a specific level in this hierarchical binding sequence. The small fraction of gravitationally bound structures found suggests that low galactic star formation efficiencies may be set by the process of cloud formation and initial collapse.

MeerKAT-64 discovers wide-spread tidal debris in the nearby NGC 7232 galaxy group

ABSTRACT We report the discovery of large amounts of previously undetected cold neutral atomic hydrogen (H i) around the core triplet galaxies in the nearby NGC 7232 galaxy group with MeerKAT. With a physical resolution of ∼1 kpc, we detect a complex web of low-surface-brightness H i emission down to a 4σ column density level of ∼1 × 1019 cm−2 (over 44 km s−1). The newly discovered H i streams extend over ∼20 arcmin corresponding to 140 kpc in projection. This is approximately three times the H i extent of the galaxy triplet (52 kpc). The H i debris has an H i mass of ∼6.6 × 109 M⊙, more than 50 per cent of the total H i mass of the triplet. Within the galaxy triplet, NGC 7233 and NGC 7232 have lost a significant amount of H i while NGC 7232B appears to have an excess of H i. The H i deficiency in NGC 7232 and NGC 7233 indicates that galaxy–galaxy interaction in the group concentrates on this galaxy pair while the other disc galaxies have visited them over time. In comparison to the AMIGA sample of isolated galaxies, we find that with regards to its total H i mass the NGC 7232/3 galaxy triplet is not H i-deficient. Despite the many interactions associated to the triplet galaxies, no H i seems to have been lost from the group (yet).