Young massive clusters (YMCs) are dense aggregates of young stars, which are essential to galaxy evolution, owing to their ultraviolet radiation, stellar winds, and supernovae. The typical mass and radius of YMCs are M~10^4 M_sun and R~1 pc, respectively, indicating that many stars are located in a small region. The formation of YMC precursor clouds may be difficult because a very compact massive cloud should be formed before stellar feedback blows off the cloud. Recent observational studies suggest that YMCs can be formed as a consequence of the fast HI gas collision with a velocity of ~100 km s^-1, which is the typical velocity of the galaxy-galaxy interaction. In this study, we examine whether the fast HI gas collision triggers YMC formation using three-dimensional magnetohydrodynamics simulations, which include the effects of self-gravity, radiative cooling/heating, and chemistry. We demonstrate that massive gravitationally bound gas clumps with M >10^4 M_sun and L~4 pc are formed in the shock compressed region induced by the fast HI gas collision, which massive gas clumps can evolve into YMCs. Our results show that the YMC precursors are formed by the global gravitational collapse of molecular clouds, and YMCs can be formed even in low-metal environments, such as the Magellanic Clouds. Additionally, the very massive YMC precursor cloud, with M >10^5 M_sun, can be created when we consider the fast collision of HI clouds, which may explain the origin of the very massive stellar cluster R136 system in the Large Magellanic Cloud.
Young massive clusters (YMCs) are recently formed astronomical objects with unusually high star formation rates. We propose the collision of giant molecular clouds (GMCs) as a likely formation mechanism of YMCs, consistent with the YMC conveyor-belt formation mode concluded by other authors. We conducted smoothed particle hydrodynamical simulations of cloud-cloud collisions and explored the effect of the clouds' collision speed, initial cloud density, and the level of cloud turbulence on the global star formation rate and the properties of the clusters formed from the collision. We show that g…
Recent galaxy observations show that star formation activity changes depending on galactic environments. In order to understand the diversity of galactic-scale star formation, it is crucial to understand the formation and evolution of giant molecular clouds in an extreme environment. We focus on observational evidence that bars in strongly barred galaxies lack massive stars even though quantities of molecular gas are sufficient to form stars. In this paper, we present a hydrodynamical simulation of a strongly barred galaxy, using a stellar potential which is taken from observational results of…
Deep imaging programs, such as MATLAS which has just been completed at the CFHT, allows us to study with their diffuse light the outer stellar populations around large number of galaxies. We have carried out a systematic census of their fine structures, i.e. the collisional debris from past mergers. We have identified among them stellar streams from minor mergers, tidal tails from gas-rich major mergers, plumes from gas-poor major mergers, and shells from intermediate mass mergers. Having estimated the visibility and life time of each of these structures with numerical simulations, we can reco…
We performed sub-parsec ($\sim$ 0.014 pc) scale simulations of cloud-cloud collisions of two idealized turbulent molecular clouds (MCs) with different masses in the range of $0.76 - 2.67 \times 10^4$M$_{\odot}$ and with collision speeds of 5 $-$ 30 km/s. Those parameters are larger than Takahira, Tasker and Habe (2014) (paper I) in which the colliding system showed a partial gaseous arc morphology that supports the NANTEN observations of objects indicated to be colliding MCs by numerical simulations. Gas clumps with density greater than $10^{-20}$ g cm$^{-3}$ were identified as pre-stellar cor…
We present a multi-wavelength study of the massive ($M_{200\textrm{c}} \approx 1$-$2 \times 10^{15} M_\odot$) galaxy clusters RXC J2248.7$-$4431, MACS J0416.1$-$2403, and MACS J1206.2$-$0847 at $z \approx 0.4$. Using the X-ray surface brightness of the clusters from deep Chandra data to model their hot gas, we are able to disentangle this mass term from the diffuse dark matter in our new strong-lensing analysis, with approximately $50$-$100$ secure multiple images per cluster, effectively separating the collisional and collisionless mass components of the clusters. At a radial distance of $10\…
Massive Black Hole (MBH) binaries are considered to be one of the most important sources of Gravitational Waves (GW) that can be detected by GW detectors like LISA. However, there are a lot of uncertainties in the dynamics of MBH binaries in the stages leading up to the GW-emission phase. It has been recently suggested that Nuclear Star Clusters (NSCs) could provide a viable route to overcome the final parsec problem for MBH binaries at the center of galaxies. NSCs are collisional systems where the dynamics would be altered by the presence of a mass spectrum. In this study, we use a suite of h…
We investigate the thermal condensation caused by a massive object that passes through the interstellar medium with high velocity, and propose a mechanism for creating a filamentary gaseous object, or interstellar contrail. Our main result shows that a long interstellar contrail can form with a certain parameter; a compact object more massive than $10^4\ {\rm M_\odot}$ can make a filament whose length is larger than $100\ {\rm pc}$. Observation of interstellar contrails may provide information on the number, masses, and velocities of fast-moving massive objects, and can be a new method for pro…
We study effect of magnetic field on massive dense core formation in colliding unequal molecular clouds by performing magnetohydrodynamic simulations with sub-parsec resolution (0.015 pc) that can resolve the molecular cores. Initial clouds with the typical gas density of the molecular clouds are immersed in various uniform magnetic fields. The turbulent magnetic fields in the clouds consistent with the observation by Crutcher et al. (2010) are generated by the internal turbulent gas motion before the collision, if the uniform magnetic field strength is 4.0 $μ$G. The collision speed of 10 km s…
We explore here an scenario for massive black hole formation driven by stellar collisions in galactic nuclei, proposing a new formation regime of global instability in nuclear stellar clusters triggered by runaway stellar collisions. Using order of magnitude estimations, we show that observed nuclear stellar clusters avoid the regime where stellar collisions are dynamically relevant over the whole system, while resolved detections of massive black holes are well into such collision-dominated regime. We interpret this result in terms of massive black holes and nuclear stellar clusters being dif…
We present the results of analyses of the 12CO (J=1-0), 13CO (J=1-0), and 12CO (J=2-1) emission data toward Gum 31. Three molecular clouds separated in velocity were detected at -25, -20, and -10 km/s . The velocity structure of the molecular clouds in Gum 31 cannot be interpreted as expanding motion. Two of them, the -25 km/s cloud and the -20 km/s cloud, are likely associated with Gum 31, because their 12CO (J=2-1)/12CO (J=1-0) intensity ratios are high. We found that these two clouds show the observational signatures of cloud-cloud collisions (CCCs): a complementary spatial distribution and…
We report on a possible cloud-cloud collision in the DR 21 region, which we found through molecular observations with the Nobeyama 45-m telescope. We mapped an area of 8'x12' around the region with twenty molecular lines including the 12CO(J=1-0) and 13CO(J=1-0) emission lines, and sixteen of them were significantly detected. Based on the 12CO and 13CO data, we found five distinct velocity components in the observed region, and we call molecular gas associated with these components -42, -22, -3, 9, and 17 km/s clouds taking after their typical radial velocities. The -3 km/s cloud is the main f…
Star formation is a fundamental process for galactic evolution. One issue over the last several decades has been determining whether star formation is induced by external triggers or is self-regulated in a closed system. The role of an external trigger, which can effectively collect mass in a small volume, has attracted particular attention in connection with the formation of massive stellar clusters, which in the extreme may lead to starbursts. Recent observations have revealed massive cluster formation triggered by cloud-cloud collisions in nearby interacting galaxies, including the Magellan…
Models aiming to explain the formation of massive black hole seeds, and in particular the direct collapse scenario, face substantial difficulties. These are rooted in rather ad hoc and fine-tuned initial conditions, such as the simultaneous requirements of extremely low metallicities and strong radiation backgrounds. Here we explore a modification of such scenarios where a massive primordial star cluster is initially produced. Subsequent stellar collisions give rise to the formation of massive (10^4 - 10^5 solar mass) objects. Our calculations demonstrate that the interplay between stellar dyn…
Teen TITANS simulations -- I. Inefficient intermediate-mass black hole seeding via stellar collisions in young massive clusters
Gran Sasso Science Institute (GSSI)（格兰萨索科学研究所）；INFN, Laboratori Nazionali del Gran Sasso（意大利国家核物理研究所，格兰萨索国家实验室）；Institut für Theoretische Astrophysik, Zentrum für Astronomie, Universität Heidelberg（海德堡大学理论天体物理学研究所，天文中心）；Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen（海德堡大学跨学科科学计算中心）；Dipartimento di Fisica e Astronomia Galileo Galilei, Università di Padova（帕多瓦大学伽利略·伽利莱物理与天文学系）；INFN-Padova（意大利国家核物理研究所帕多瓦分部）；Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB)（巴塞罗那大学量子物理与天体物理学系）；Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB)（巴塞罗那大学宇宙科学研究所）；INAF Osservatorio Astronomico d’Abruzzo（意大利国家天体物理研究所阿布鲁佐天文台）
Collisions were suggested to potentially play a role in the formation of massive stars in present day clusters, and have likely been relevant during the formation of massive stars and intermediate mass black holes within the first star clusters. In the early Universe, the first stellar clusters were particularly dense, as fragmentation typically only occurred at densities above $10^9$cm$^{-3}$, and the radii of the protostars were enhanced due to the larger accretion rates, suggesting a potentially more relevant role of stellar collisions. We present here a detailed parameter study to assess h…
The runaway collision scenario is one of the most promising mechanisms to explain the formation of intermediate-mass black holes (IMBHs) in young dense star clusters. On the other hand, the massive stars that participate in the runaway collisions lose mass by stellar winds. In this paper, we discuss new N-body simulations of massive (6.5x10^4 Msun) star clusters, in which we added upgraded recipes for stellar winds and supernova explosion at different metallicity. We follow the evolution of the principal collision product (PCP), through dynamics and stellar evolution, till it forms a stellar r…
To explain the observed population of supermassive black holes at z~7, very massive seed black holes or, alternatively, super-Eddington scenarios are needed to reach final masses of the order of 10^9 solar masses. A popular explanation for massive seeds has been the direct collapse model, which predicts the formation of a single massive object due to the direct collapse of a massive gas cloud. Simulations over the last years have however shown that such a scenario is very difficult to achieve. A realistic model of black hole formation should therefore take fragmentation into account, and consi…
Herein, we present the 12CO (J=1-0) and 13CO (J=1-0) emission line observations via the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) toward a Spitzer bubble N4. We observed clouds of three discrete velocities: 16, 19, and 25 km/s. Their masses were 0.1x10^4 Msun, 0.3x10^4 Msun, and 1.4x10^4 Msun, respectively. The distribution of the 25-km/s cloud likely traces the ring-like structure observed at mid-infrared wavelength. We could not find clear expanding motion of the molecular gas in N4. On the contrary, we found a bridge feature and a complementary d…
Fragmentation often occurs in disk-like structures, both in the early Universe and in the context of present-day star formation. Supermassive black holes (SMBHs) are astrophysical objects whose origin is not well understood; they weigh millions of solar masses and reside in the centers of galaxies. An important formation scenario for SMBHs is based on collisions and mergers of stars in a massive cluster, in which the most massive star moves to the center of the cluster due to dynamical friction. This increases the rate of collisions and mergers since massive stars have larger collisional cross…
We present the results of a multiwavelength investigation of the very X-ray luminous galaxy cluster MACSJ0553.4-3342 ($z = 0.4270$; hereafter MACSJ0553). Combining high-resolution data obtained with the Hubble Space Telescope and the Chandra X-ray Observatory with ground-based galaxy spectroscopy, our analysis establishes the system unambiguously as a binary, post-collision merger of massive clusters. Key characteristics include perfect alignment of luminous and dark matter for one component, a separation of almost 650 kpc (in projection) between the dark-matter peak of the other subcluster an…
Rapid formation of a very massive star >50000 $M_\odot$ and subsequently an IMBH from runaway collisions. Direct N-body and Monte Carlo simulations of dense star clusters
Astronomisches Rechen-Institut, Zentrum für Astronomie（天文计算研究所，天文学中心）；Max-Planck-Institut für Astronomie（马克斯·普朗克天文学研究所）；Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences（尼古拉·哥白尼天文学中心，波兰科学院）；National Astronomical Observatories（国家天文台）；Kavli Institute for Astronomy and Astrophysics, Peking University（北京大学科维理天文与天体物理研究所）；Department of Physics, New York University Abu Dhabi（阿布扎比纽约大学物理系）；Center for Astrophysics and Space Science (CASS), New York University Abu Dhabi（阿布扎比纽约大学天体物理学与空间科学中心）；Dipartimento di Fisica, Sapienza Università di Roma（罗马智慧大学物理系）
The detection of the binary black hole merger GW190521, with primary black hole mass $85^{+21}_{-14}$ ${\rm M}_{\odot}$, proved the existence of black holes in the theoretically predicted pair-instability gap ($\sim60-120 \, {\rm M}_{\odot}$) of their mass spectrum. Some recent studies suggest that such massive black holes could be produced by the collision of an evolved star with a carbon-oxygen core and a main sequence star. Such a post-coalescence star could end its life avoiding the pair-instability regime and with a direct collapse of its very massive envelope. It is still not clear, howe…
A massive black hole exists in almost every galaxy. They occasionally radiate a vast amount of light by releasing gravitational energy of accreting gas, with a cumulative active period of only a few $10^8$ years, so-called the duty cycle of the Active Galactic Nuclei. Namely, many galaxies today host a starving massive black hole. Although galaxy collisions have been thought to enhance nucleus activity, the origin of the duty cycle, especially the shutdown process, is a still critical issue. Here we show that galaxy collisions are also capable of suppressing black hole fueling by using an anal…
