Pericenter Passage Research Articles

Unveiling the internal structure of the Hercules supercluster

ABSTRACT We have investigated the structure of the Hercules supercluster (SCL160) based on data originally extracted from the Sloan Digital Sky Survey (SDSS-DR7). We have traced the mass distribution in the field through the numerical density, weighted by the r′ luminosity of the galaxies, and classified objects based on their spatial position and redshift. This has allowed us to address not only the kinematics of the supercluster as a whole, but also the internal kinematics of each cluster, which had not been explored further before. We have confirmed that the Hercules supercluster is composed of the galaxy clusters A2147, A2151, and A2152. A2151 consists of five subclusters, A2147 two, and A2152 at least two. They form the heart of the Hercules supercluster. We also have found two other gravitationally bound clusters, therefore increasing the known members of the supercluster. We have estimated a total mass of 2.1 ± 0.2 × 1015 M⊙ for the Hercules supercluster. To determine the dynamical masses in this work, we have resorted to the M200−σ scaling relation and the caustic technique. Comparing both methods with simulated data of bimodal merging clusters, we found that the caustic, as well as the σ-based masses, is biased through the merger age, showing a boost just after pericentric passage. This is not in line with the principle of the caustic method, which affirms it does not depend on the cluster dynamical state.

The Magellanic Edges Survey – II. Formation of the LMC’s northern arm

ABSTRACT The highly-substructured outskirts of the Magellanic Clouds provide ideal locations for studying the complex interaction history between both Clouds and the Milky Way (MW). In this paper, we investigate the origin of a >20° long arm-like feature in the northern outskirts of the Large Magellanic Cloud (LMC) using data from the Magellanic Edges Survey (MagES) and Gaia EDR3. We find that the arm has a similar geometry and metallicity to the nearby outer LMC disc, indicating that it is comprised of perturbed disc material. Whilst the azimuthal velocity and velocity dispersions along the arm are consistent with those in the outer LMC, the in-plane radial velocity and out-of-plane vertical velocity are significantly perturbed from equilibrium disc kinematics. We compare these observations to a new suite of dynamical models of the Magellanic/MW system, which describe the LMC as a collection of tracer particles within a rigid potential, and the SMC as a rigid Hernquist potential. Our models indicate the tidal force of the MW during the LMC’s infall is likely responsible for the observed increasing out-of-plane velocity along the arm. Our models also suggest close LMC/SMC interactions within the past Gyr, particularly the SMC’s pericentric passage ∼150 Myr ago and a possible SMC crossing of the LMC disc plane ∼400 Myr ago, likely do not perturb stars that today comprise the arm. Historical interactions with the SMC prior to ∼1 Gyr ago may be required to explain some of the observed kinematic properties of the arm, in particular its strongly negative in-plane radial velocity.

Estimating the Parameters of Extended Gravity Theories with the Schwarzschild Precession of S2 Star

After giving a short overview of previous results on constraining of Extended Gravity by stellar orbits, we discuss the Schwarzschild orbital precession of S2 star assuming the congruence with predictions of General Relativity (GR). At the moment, the S2 star trajectory is remarkably fitted with the first post-Newtonian approximation of GR. In particular, both Keck and VLT (GRAVITY) teams declared that the gravitational redshift near its pericenter passage for the S2 star orbit corresponds to theoretical estimates found with the first post-Newtonian (pN) approximation. In 2020, the GRAVITY Collaboration detected the orbital precession of the S2 star around the supermassive black hole (SMBH) at the Galactic Center and showed that it is close to the GR prediction. Based on this observational fact, we evaluated parameters of the Extended Gravity theories with the Schwarzschild precession of the S2 star. Using the mentioned method, we estimate the orbital precession angles for some Extended Gravity models including power-law f(R), general Yukawa-like corrections, scalar–tensor gravity, and non-local gravity theories formulated in both metric and Palatini formalism. In this consideration, we assume that a gravitational field is spherically symmetric, therefore, alternative theories of gravity could be described only with a few parameters. Specifically, considering the orbital precession, we estimate the range of parameters of these Extended Gravity models for which the orbital precession is like in GR. Then we compare these results with our previous results, which were obtained by fitting the simulated orbits of S2 star to its observed astrometric positions. In case of power-law f(R), generic Yukawa-like correction, scalar–tensor gravity and non-local gravity theories, we were able to obtain a prograde orbital precession, like in GR. According to these results, the method is a useful tool to evaluate parameters of the gravitational potential at the Galactic Center.

Accretion of tidally disrupted asteroids on to white dwarfs: direct accretion versus disc processing

ABSTRACT Atmospheric heavy elements have been observed in more than a quarter of white dwarfs (WDs) at different cooling ages, indicating ongoing accretion of asteroidal material, whilst only a few per cent of the WDs possess a dust disc, and all these WDs are accreting metals. Here, assuming that a rubble-pile asteroid is scattered inside a WD’s Roche lobe by a planet, we study its tidal disruption and the long-term evolution of the resulting fragments. We find that after a few pericentric passages, the asteroid is shredded into its constituent particles, forming a flat, thin ring. On a time-scale of Myr, tens of per cent of the particles are scattered on to the WD, and are therefore directly accreted without first passing through a circularized close-in disc. Fragment mutual collisions are most effective for coplanar fragments, and are thus only important in 103−104 yr before the orbital coplanarity is broken by the planet. We show that for a rubble pile asteroid with a size frequency distribution of the component particles following that of the near earth objects, it has to be roughly at least 10 km in radius such that enough fragments are generated and $\ge 10{{\ \rm per\ cent}}$ of its mass is lost to mutual collisions. At relative velocities of tens of km s−1, such collisions grind down the tidal fragments into smaller and smaller dust grains. The WD radiation forces may shrink those grains’ orbits, forming a dust disc. Tidal disruption of a monolithic asteroid creates large km-size fragments, and only parent bodies ≥100 km are able to generate enough fragments for mutual collisions to be significant. Hence, those large asteroids experience a disc phase before being accreted.

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.

The Three Hundred Project: The stellar angular momentum evolution of cluster galaxies

Using 324 numerically modelled galaxy clusters as provided by THETHREEHUNDREDproject, we study the evolution of the kinematic properties of the stellar component of haloes on first infall. We selected objects withMstar > 5 × 1010 h−1 M⊙within 3R200of the main cluster halo atz = 0 and followed their progenitors. We find that although haloes are stripped of their dark matter and gas after entering the main cluster halo, there is practically no change in their stellar kinematics. For the vast majority of our ‘galaxies’ – defined as the central stellar component found within the haloes that form our sample – their kinematic properties, as described by the fraction of ordered rotation, and their position in the specific stellar angular momentum−stellar mass planejstar − Mstarare mostly unchanged by the influence of the central host cluster. However, for a small number of infalling galaxies, stellar mergers and encounters with remnant stellar cores close to the centre of the main cluster, particularly during pericentre passage, are able to spin up their stellar component byz = 0.

Tides in the High-eccentricity Migration of Hot Jupiters: Triggering Diffusive Growth by Nonlinear Mode Interactions

Abstract High-eccentricity migration is a possible formation channel for hot Jupiters. However, in order for it to be consistent with the observed population of planets, tides must circularize the orbits in an efficient manner. A potential mechanism for such rapid circularization is the diffusive growth of the tidally driven planetary f-mode. Such growth occurs if the f-mode phase at pericenter varies chaotically from one pericenter passage to the next. Previous studies focused on the variation of the orbital period due to tidal back-reaction on the orbit as the source of chaos. Here we show that nonlinear mode interactions can also be an important source. Specifically, we show that nonlinear interactions between a parent f-mode and daughter f-/p-modes induce an energy-dependent shift in the oscillation frequency of the parent. This frequency shift varies randomly from orbit to orbit because the parents’ energy varies. As a result, the parents’ phase at pericenter varies randomly, which we find can trigger it to grow diffusively. We show that the phase shift induced by nonlinear mode interactions in fact dominates the shift induced by tidal back-reaction and lowers the one-kick energy threshold for diffusive growth by about a factor of 5 compared to the prediction from the linear theory. For a given pericenter distance, this reduces the semimajor axis needed to trigger diffusive growth from ≳2 au to ≃1 au. Nonlinear interactions could thus enhance the formation rate of hot Jupiters through the high-eccentricity migration channel and potentially mitigate the discrepancy between the observed and predicted occurrence rates for close-in gas giants as compared to those farther from the star.

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.

Neutron stars in the effective fly-by framework: f-mode re-summation

Eccentric compact binaries pose not only a challenge for gravitational wave (GW) detectors, but also provide a probe into the nuclear equation of state if one of the objects is a neutron star. At the short pericenter passage, tidal interactions excite f-modes on the star, which in turn emit their own GWs. We derive an analytic waveform for these stellar oscillations within the effective fly-by framework, modeling the emission to leading post-Newtonian order. At this order, the f-mode response can be written in a Fourier decomposition in terms of orbital harmonics, with the amplitudes of each harmonic depending on Hansen coefficients. Re-summing the harmonics of the f-mode results in a simple decaying harmonic oscillator, with the amplitude now determined by a Hansen coefficient of complex harmonic number. We compute the match between the re-summed f-mode and numerical integrations of the tidal response, and find for systems with high orbital eccentricity (e > 0.8) and low semi-latus rectum (p < 15M) for three equations of state. We further compare our model to modes generated from subsequent pericenter passages under the effect of radiation reaction, and develop an accurate model to time pericenter passages. We show how the timing model can be used to specify initial conditions to accurate track the f-mode excitation across multiple pericenter passages.

Pericentric passage-driven star formation in satellite galaxies and their hosts: CLUES from local group simulations

ABSTRACT Local Group satellite galaxies show a wide diversity of star formation histories (SFHs) whose origin is yet to be fully understood. Using hydrodynamical simulations from the Constrained Local UniversE project, we study the SFHs of satellites of Milky Way-like galaxies in a cosmological context: while in the majority of the cases the accretion on to their host galaxy causes the satellites to lose their gas, with a subsequent suppression in SF, in about 25 per cent of our sample we observe a clear enhancement of SF after infall. Peaks in SF clearly correlate with the satellite pericentric passage around its host and, in one case, with a satellite–satellite interaction. We identify two key ingredients that result in enhanced SF after infall: galaxies must enter the host’s virial radius with a reservoir of cold gas Mgas, inf/Mvir, inf ≳ 10−2 and with a minimum pericentric distance ≳10 kpc (mean distance ∼50 kpc for the full sample), in order to form new stars due to compression of cold gas at pericentric passage. On the other hand, satellites that infall with little gas or whose pericentric distance is too small, have their gas ram-pressure stripped and subsequent SF quenched. The pericentric passage of satellites likewise correlates with SF peaks in their hosts, suggesting that this mechanism induces bursts of SF in satellites and central galaxies alike, in agreement with recent studies of our Galaxy’s SFH. Our findings can explain the recently reported multiple stellar populations observed in dwarf galaxies such as Carina and Fornax, and should be taken into account in semi-analytic models of galaxy formation and satellite quenching.

Star formation histories of Coma cluster galaxies matched to simulated orbits hint at quenching around first pericenter

Context. The star formation in galaxies in present-day clusters has almost entirely been shut down, but the exact mechanism that quenched these galaxies is still uncertain. Aims. By tracing the orbital and star formation histories of galaxies within the Coma cluster, we seek to understand the role of the high-density cluster environment in quenching these galaxies. Methods. We combine star formation histories extracted from high-signal-to-noise spectra of 11 early-type galaxies around the center of the Coma cluster with probability distributions for their orbital parameters obtained using an N-body simulation to connect their orbital and star formation histories. Results. We find that all 11 galaxies likely quenched near their first pericentric approach. Higher stellar mass galaxies (log(M⋆/M⊙) &gt; 10) had formed a higher fraction of their stellar mass (more than ∼90%) than their lower mass counterparts (∼80−90%) by the time they fell into the cluster (when they cross 2.5rvir). We find that the expected infall occurred around z ∼ 0.6, followed by the first pericentric passage ∼4 Gyr later. Galaxies in our sample formed a significant fraction of their stellar mass, up to 15%, between infall and first pericenter, and had assembled more than ∼98% of their cumulative stellar mass by first pericenter. Conclusions. Unlike previous low-redshift studies that suggest that star formation continues until about first apocenter or later, the high percentage of stellar mass already formed by first pericenter in our sample galaxies points to star formation ceasing within a gigayear after the first pericentric passage. We consider the possible physical mechanisms driving quenching and find that our results resemble the situation in clusters at z ∼ 1, where active stripping of gas (ram-pressure or tidally driven) seems to be required to quench satellites by their first pericentric passage. However, a larger sample will be required to conclusively account for the unknown fraction of preprocessed satellites in the Coma cluster.

Brought to Light. II. Revealing the Origins of Cloaked Spiral Features in Cluster Passive Dwarf Galaxies

Abstract In our companion paper (Brought to Light I: Michea et al.), we reveal spectacular spiral-galaxy-like features in deep optical imaging of nine Virgo early-type dwarf galaxies, hidden beneath a dominating smooth stellar disk. Using a new combination of approaches, we find that bar- and spiral-like features contribute 2.2%–6.4% of the total flux within 2 R eff. In this study, we conduct high-resolution simulations of cluster harassment of passive dwarf galaxies. Following close pericenter passages of the cluster core, tidal triggering generates features in our model disks that bear a striking resemblance to the observed features. However, we find the disks must be highly rotationally supported (V peak/σ 0 ∼ 3), much higher than typically observed. We propose that some early-type dwarfs may contain a few percent of their mass in a cold, thin disk that is buried in the light of a hot, diffuse disk and only revealed when they undergo tidal triggering. The red optical colors of our sample do not indicate any recent significant star formation, and our simulations show that very plunging pericenter passages (r peri &lt; 0.25r vir) are required for tidal triggering. Thus, many cluster early-type dwarfs with less-plunging orbits may host a yet-undetected cold stellar disk component. We discuss possible origin scenarios and consider why similar-mass star-forming galaxies in the field are significantly more thin-disk dominated than in our cluster sample.

The Merger Dynamics of the Galaxy Cluster A1775: New Insights from Chandra and XMM-Newton for a Cluster Simultaneously Hosting a Wide-angle Tail and a Narrow-angle Tail Radio Source

Abstract We present a new study of the merger dynamics of A1775 by analyzing the high-quality Chandra and XMM-Newton archival data. We confirm/identify an arc-shaped edge (i.e., the head) at ∼48 kpc west of the X-ray peak, a split cold gas tail that extends eastward to ∼163 kpc, and a plume of spiral-like X-ray excess (within about 81–324 kpc northeast of the cluster core) that connects to the end of the tail. The head, across which the projected gas temperature rises outward from to keV, is found to be a cold front with a Mach number of . Along the surfaces of the cold front and tail, typical Kelvin–Helmholtz instability features (noses and wings, etc.) are found and are used to constrain the upper limit of the magnetic field (∼11.2 μG) and the viscosity suppression factor (∼0.01). Combining optical and radio evidence, we propose a two-body merger (instead of systematic motion in a large-scale gas environment) scenario and have carried out idealized hydrodynamic simulations to verify it. We find that the observed X-ray emission and temperature distributions can be best reproduced with a merger mass ratio of 5 after the first pericentric passage. The NAT radio galaxy is thus more likely to be a single galaxy falling into the cluster center at a relative velocity of 2800 km s−1, a speed constrained by its radio morphology. The infalling subcluster is expected to have a relatively low gas content, because only a gas-poor subcluster can cause central-only disturbances as observed in such an off-axis merger.

AzTEC survey of the central molecular zone: data reduction, analysis, and preliminary results

ABSTRACT We present a large-scale survey of the central molecular zone (CMZ) of our Galaxy, as well as a monitoring program of Sgr A*, with the AzTEC/Large Millimeter Telescope in the 1.1 mm continuum. Our 1.1 mm map covers the main body of the CMZ over a field of 1.6 × 1.1 deg2 with an angular resolution of 10.5 arcsec and a depth of 15 mJy beam−1. To account for the intensity loss due to the background removal process, we combine this map with lower resolution CSO/Bolocam and Planck/HFI data to produce an effective full intensity 1.1 mm continuum map. With this map and existing Herschel surveys, we have carried out a comprehensive analysis of the spectral energy distribution of dust in the CMZ. A key component of this analysis is the implementation of a model-based deconvolution approach, incorporating the Point Spread Functions (PSFs) of the different instruments, and hence recovering a significant amount of spatial information on angular scales larger than 10.5 arcsec. The monitoring of Sgr A* was carried out as part of a worldwide, multiwavelength campaign when the so-called G2 object was undergoing the pericentre passage around the massive black hole. Our preliminary results include (1) high-resolution maps of column density, temperature and dust spectral index across the CMZ; (2) a 1.1 mm light curve of Sgr A* showing an outburst of $140{{\ \rm per\ cent}}$ maximum amplitude on 2014 May 9th May, 2014 but otherwise only stochastic variations of $10{{\ \rm per\ cent}}$ and no systematic long-term change, consistent with other observations.

A study of cool core resiliency and entropy mixing in simulations of galaxy cluster mergers

ABSTRACT We present results from a suite of binary merging cluster simulations. The hydrodynamical cluster simulations are performed employing a smoothed particle hydrodynamics formulation in which gradient errors are strongly reduced by means of an integral approach. We consider adiabatic as well as radiative simulations, in which we include gas cooling, star formation, and energy feedback from supernovae. We explore the effects of merging on the thermodynamic structure of the intracluster gas of the final merger remnant. In particular, we study how core entropy is generated during the merging and the stability properties of the initial cool-core profile against disruption. To this end, we consider a range of initial mass ratio and impact parameters. Final entropy profiles of our adiabatic merging simulations are in good accord with previous findings, with cool-cores being disrupted for all of the initial merging setups. For equal-mass off-axis mergers, we find that a significant contribution to the final primary core entropy is due to hydrodynamic instabilities generated by rotational motions, which are induced by tidal torques during the first pericentre passage. In radiative simulations, cool-cores are more resilient against heating processes; none the less, they are able to maintain their integrity only in the case of off-axis mergers with very unequal masses. We suggest that these results are robust against changes in the gas physical modelling, in particular to the inclusion of AGN thermal feedback. Our findings support the view that the observed core cluster morphology emerges naturally in a merging cluster context, and conclude that the merging angular momentum is a key parameter in shaping the thermodynamical properties of the final merger remnant.

A tale of two tails: insights from simulations into the formation of the peculiar dwarf galaxy NGC 1427A

ABSTRACT We present a scenario for the formation and the morphology of the arrow-shaped dwarf irregular galaxy NGC 1427A in the Fornax Cluster. This galaxy shows intriguing stellar and gaseous tails pointing in different directions for which alternative but not conclusive formation scenarios have been proposed in the literature. We performed N-body/SPH simulations of dwarf galaxies falling into a model of the Fornax cluster, exhibiting a jellyfish-like appearance while undergoing ram-pressure stripping. We noted that some of our models show interesting tail morphologies similar to that of NGC 1427A. In this way, the peculiar NGC 1427A structure can be studied using models whose stellar and neutral gas photometry and kinematics are in good agreement with the observed ones, without the need of invoking an interaction with a nearby galaxy. Thanks to the tails, we can identify the requirements for a galaxy to expose such a structure and assess the possible position and velocity of the galaxy in the cluster. This puts constraints on the orbit of the galaxy, its position in the cluster and the time since its pericentre passage. From the statistics of identified snapshots following our modelling, we found that the most likely position of the galaxy is around 200 kpc in front of the cluster centre, travelling towards the cluster with a velocity angle with respect to the line-of-sight direction of around 50 deg. This analysis can be useful in future observations of similar galaxies in clusters to characterize their position and velocity in the cluster and their formation.

Galaxies with kinematically distinct cores in Illustris

Context. The growing amount of integral-field spectroscopic data creates an increased demand for understanding kinematic peculiarities that carry valuable information about the evolution of host galaxies. Aims. For kinematically distinct cores (KDCs), a number of formation mechanisms have been proposed, but it is still unclear as to which of them commonly occur in the Universe. We aim to address the KDC formation in the cosmological context. Methods. We used the publicly available data of the large-scale hydrodynamic cosmological simulation Illustris. We identified 134 KDCs, studied their properties, and followed their evolution back in time. Examples of four galaxies hosting KDCs are presented and described in detail. Results. The masses of the KDC hosts follow the general distribution of the Illustris galaxies, with a possible slight preference for massive galaxies. KDCs can be long-lived features, with their formation epochs distributed, roughly uniformly, between look-back times 0–11.4 Gyr, and they can survive even major or multiple subsequent mergers. There is no single channel of KDC formation, but mergers seem to be the formation mechanism for about 60% of KDCs with a significant preference for major mergers and with the percentage being higher among massive hosts. Other KDCs formed during a pericentric passage or flyby of another galaxy by the precession of a previously formed rapidly rotating core, or without an obvious external cause. The mean mass-weighted stellar age inside the KDC radius is either about the same as the look-back time of the KDC formation or older. Although the radii of our KDCs are, on average, larger than observed, we find that younger stellar ages are typically associated with smaller KDCs. A significant fraction of KDC hosts possess stellar shells that formed during mergers, which led to KDCs within the last 5 Gyr, or double peaks in their velocity dispersion maps.

Discovery of a Candidate Hypervelocity Star Originating from the Sagittarius Dwarf Spheroidal Galaxy

Abstract In this Letter, we report the discovery of an intriguing hypervelocity star (HVS; J1443+1453) candidate that is probably from the Sagittarius Dwarf Spheroidal galaxy (Sgr dSph). The star is an old and very metal-poor low-mass main-sequence turn-off star (age ∼14.0 Gyr and [Fe/H] = −2.23 dex) and has a total velocity of km s−1 in the Galactic rest frame and a heliocentric distance of kpc. The velocity of J1443+1453 is larger than the escape speed at its position, suggesting that it is a promising HVS candidate. By reconstructing its trajectory in the Galactic potential, we find that the orbit of J1443+1453 intersects closely with that of Sgr dSph Myr ago, when the latter has its latest pericentric passage through the Milky Way. The encounter occurs at a distance kpc from the center of Sgr dSph, a distance that is smaller than the size of the Sgr dSph. The chemical properties of this star are also consistent with those of an Sgr dSph-associated globular cluster, or of the Sgr stream member stars. Our finding suggests that J1443+1453 is an HVS that is either tidally stripped from the Sgr dSph or ejected from the Sgr dSph by the gravitational slingshot effect, requiring a (central) massive/intermediate-mass black hole or a (central) massive primordial black hole in the Sgr dSph.

Fate of stellar bars in minor merger of galaxies

ABSTRACT Minor merger of galaxies is common during the evolutionary phase of galaxies. Here, we investigate the dynamical impact of a minor merger (mass ratio 1:10) event on the final fate of a stellar bar in the merger remnant. To achieve that, we choose a set of minor merger models from the publicly available GalMer library of galaxy-merger simulations. The models differ in terms of their orbital energy, orientation of the orbital spin vector, and morphology of the satellite galaxy (discy/spheroidal). We demonstrate that the central stellar bar, initially present in the host galaxy, undergoes a transient bar amplification phase after each pericentre passage of the satellite; in concordance with past studies of bar excitation due to tidal encounter. However, once the merger happens, the central stellar bar weakens substantially in the post-merger remnants. The accumulation of satellite’s stars in the central region of merger remnant plays a key role in the bar-weakening process; causing a net increase in the central mass concentration as well as in the specific angular momentum content. We find that the efficiency of mass accumulation from the satellite in the central parts of merger remnants depends on the orbital parameters as well as on the satellite’s morphology. Consequently, different minor merger models display different degrees of bar-weakening event. This demonstrates that minor merger of galaxies is a plausible avenue for bar weakening in disc galaxies.

The Dawes Review 9: The role of cold gas stripping on the star formation quenching of satellite galaxies

AbstractOne of the key open questions in extragalactic astronomy is what stops star formation in galaxies. While it is clear that the cold gas reservoir, which fuels the formation of new stars, must be affected first, how this happens and what are the dominant physical mechanisms involved is still a matter of debate. At least for satellite galaxies, it is generally accepted that internal processes alone cannot be responsible for fully quenching their star formation, but that environment should play an important, if not dominant, role. In nearby clusters, we see examples of cold gas being removed from the star-forming discs of galaxies moving through the intracluster medium, but whether active stripping is widespread and/or necessary to halt star formation in satellites, or quenching is just a consequence of the inability of these galaxies to replenish their cold gas reservoirs, remains unclear. In this work, we review the current status of environmental studies of cold gas in star-forming satellites in the local Universe from an observational perspective, focusing on the evidence for a physical link between cold gas stripping and quenching of the star formation. We find that stripping of cold gas is ubiquitous in satellite galaxies in both group and cluster environments. While hydrodynamical mechanisms such as ram pressure are important, the emerging picture across the full range of dark matter halos and stellar masses is a complex one, where different physical mechanisms may act simultaneously and cannot always be easily separated. Most importantly, we show that stripping does not always lead to full quenching, as only a fraction of the cold gas reservoir might be affected at the first pericentre passage. We argue that this is a key point to reconcile apparent tensions between statistical and detailed analyses of satellite galaxies, as well as disagreements between various estimates of quenching timescales. We conclude by highlighting several outstanding questions where we expect to see substantial progress in the coming decades, thanks to the advent of the Square Kilometre Array and its precursors, as well as the next-generation optical and millimeter facilities.