Galaxy Collisions, Gas Stripping and Star Formation in the Evolution of Galaxies

A review of gravitational and hydrodynamical processes during formation of clusters and evolution of galaxies is given. Early, at the advent of N-body computer simulations, the importance of tidal fields in galaxy encounters has been recognized. Orbits are crowded due to tides along spiral arms, where the star formation is enhanced. Low relative velocity encounters lead to galaxy mergers. The central dominating galaxies in future clusters form before the clusters in a merging process in galaxy groups. Galaxy clusters are composed in a hierarchical scenario due to relaxation processes between galaxies and galaxy groups. As soon as the overall cluster gravitational potential is built, high speed galaxy versus galaxy encounters start to play a role. These harassment events gradually thicken and shorten spiral galaxy disks leading to the formation of S0 galaxies and ellipticals. Another aspect of the high speed motion in the hot and diluted Intracluster Medium (ICM) is the ram pressure exerted on the Interstellar Matter (ISM) leading to stripping of the ISM from parent spirals. The combinations of tides and ram pressure stripping efficiently removes the gas from spirals, quenching the star formation in galactic disks, while triggering it in the tidal arms and at the leading edge of gaseous disk. Gas stripping from disks transports the metals to the ICM. In some cases, the gas extracted from the galactic disks becomes self-gravitating forming tidal dwarf galaxies.Comment: 24 pages, 8 figures; to appear in Reviews in Modern Astronomy, Vol. 1

GS242-03+37: a lucky survivor in the galactic gravitational field

HI shells and supershells, found in discs of many galaxies including our own, are formed by the activity of young and massive stars (supernova explosions and stellar winds), but the formation of these structures may be linked to other energetic events, such as interactions of high-velocity clouds with the galactic disc. The larger structures in particular significantly influence their surroundings; their walls are often places where molecular clouds reside and where star formation happens. We explore the HI supershell GS242-03+37, a large structure in the outer Milky Way. Its size and position make it a good case for studying the effects of large shells on their surrounding. We perform numerical simulations of the structure with the simplified hydrodynamical code RING, which uses the thin-shell approximation. The best fit is found by a comparison with the HI data and then we compare our model with the distribution of star clusters near this supershell. The best model of GS242-03+37 requires, contrary to previous estimates, a relatively low amount of energy, and it has an old age of $\sim$ 100 Myr. We also find that the distribution of young star clusters (with ages $<$ 120 Myr) is correlated with walls of the supershell, while the distribution of older clusters is not. Clusters that have the highest probability of being born in the wall of the supershell show an age sequence along the wall. GS242-03+37 is a relatively old structure, shaped by the differential rotation, and its wall is a birthplace of several star clusters. The star formation started at a time when the supershell was not already supersonically expanding; it was a result of the density increase due to the galactic shear and oscillations perpendicular to the disc of the Milky Way.Comment: 18 pages, accepted by A&

Spatial motion of the Magellanic Clouds. Tidal models ruled out?

Recently, Kallivayalil et al. derived new values of the proper motion for the Large and Small Magellanic Clouds (LMC and SMC, respectively). The spatial velocities of both Clouds are unexpectedly higher than their previous values resulting from agreement between the available theoretical models of the Magellanic System and the observations of neutral hydrogen (HI) associated with the LMC and the SMC. Such proper motion estimates are likely to be at odds with the scenarios for creation of the large-scale structures in the Magellanic System suggested so far. We investigated this hypothesis for the pure tidal models, as they were the first ones devised to explain the evolution of the Magellanic System, and the tidal stripping is intrinsically involved in every model assuming the gravitational interaction. The parameter space for the Milky Way (MW)-LMC-SMC interaction was analyzed by a robust search algorithm (genetic algorithm) combined with a fast restricted N-body model of the interaction. Our method extended the known variety of evolutionary scenarios satisfying the observed kinematics and morphology of the Magellanic large-scale structures. Nevertheless, assuming the tidal interaction, no satisfactory reproduction of the HI data available for the Magellanic Clouds was achieved with the new proper motions. We conclude that for the proper motion data by Kallivayalil et al., within their 1-sigma errors, the dynamical evolution of the Magellanic System with the currently accepted total mass of the MW cannot be explained in the framework of pure tidal models. The optimal value for the western component of the LMC proper motion was found to be pm_w(LMC) > -1.3 mas/yr in case of tidal models. It corresponds to the reduction of the Kallivayalil et al. value for pm_w(LMC) by approx. 40% in its magnitude.Comment: ApJ accepted, 17 pages, 4 figure

Exploring GLIMPSE Bubble N107: Multiwavelength Observations and Simulations

Context. Bubble N107 was discovered in the infrared emission of dust in the Galactic Plane observed by the Spitzer Space Telescope (GLIMPSE survey: l ~ 51.0 deg, b ~ 0.1 deg). The bubble represents an example of shell-like structures found all over the Milky Way Galaxy. Aims. We aim to analyse the atomic and molecular components of N107, as well as its radio continuum emission. With the help of numerical simulations, we aim to estimate the bubble age and other parameters which cannot be derived directly from observations. Methods. From the observations of the HI (I-GALFA) and 13CO (GRS) lines we derive the bubble's kinematical distance and masses of the atomic and molecular components. With the algorithm DENDROFIND, we decompose molecular material into individual clumps. From the continuum observations at 1420 MHz (VGPS) and 327 MHz (WSRT), we derive the radio flux density and the spectral index. With the numerical code ring, we simulate the evolution of stellar-blown bubbles similar to N107. Results. The total HI mass associated with N107 is 5.4E3 Msun. The total mass of the molecular component (a mixture of cold gasses of H2, CO, He and heavier elements) is 1.3E5 Msun, from which 4.0E4 Msun is found along the bubble border. We identified 49 molecular clumps distributed along the bubble border, with the slope of the clump mass function of -1.1. The spectral index of -0.30 of a strong radio source located apparently within the bubble indicates nonthermal emission, hence part of the flux likely originates in a supernova remnant, not yet catalogued. The numerical simulations suggest N107 is likely less than 2.25 Myr old. Since first supernovae explode only after 3 Myr or later, no supernova remnant should be present within the bubble. It may be explained if there is a supernova remnant in the direction towards the bubble, however not associated with it.Comment: 15 pages, 11 figure

The fragmentation of expanding shells III: Oligarchic accretion and the mass spectrum of fragments

We use SPH simulations to investigate the gravitational fragmentation of expanding shells through the linear and non--linear regimes. The results are analysed using spherical harmonic decomposition to capture the initiation of structure during the linear regime; the potential-based method of Smith et al. (2009) to follow the development of clumps in the mildly non-linear regime; and sink particles to capture the properties of the final bound objects during the highly non-linear regime. In the early, mildly non--linear phase of fragmentation, we find that the clump mass function still agrees quite well with the mass function predicted by the analytic model. However, the sink mass function is quite different, in the sense of being skewed towards high-mass objects. This is because, once the growth of a condensation becomes non-linear, it tends to be growing non-competitively from its own essentially separate reservoir; we call this Oligarchic Accretion.Comment: 14 pages, accepted for publication in MNRA

Environmental Dependencies for Star Formation Triggered by Expanding Shell Collapse

Criteria for gravitational collapse of expanding shells in rotating, shearing galaxy disks were determined using three-dimensional numerical simulations in the thin shell approximation. The simulations were run over a grid of 7 independent variables, and the resultant probabilities for triggering and unstable masses were determined as functions of 8 dimensionless parameters. When the ratio of the midplane gas density to the midplane total density is small, an expanding shell reaches the disk scale height and vents to the halo before it collapses. When the Toomre instability parameter Q, or a similar shear parameter, Q_A, are large, Coriolis forces and shear stall or reverse the collapse before the shell accumulates enough mass to be unstable. With large values of C=c_sh/(GL)^0.2, for rms velocity dispersion c_sh in the swept-up matter and shell-driving luminosity L, the pressure in the accumulated gas is too large to allow collapse during the expansion time. Considering ~5000 models covering a wide range of parameter space, the common properties of shell collapse as a mechanism for triggered star formation are: (1) the time scale is 4*sqrt(C/2 pi G rho) for ambient midplane density rho, (2) the total fragment mass is ~2x10^7 Msun, of which only a small fraction is likely to be molecular, (3) the triggering radius is ~2 times the scale height, and the triggering probability is ~0.5 for large OB associations. Star formation triggered by shell collapse should be most common in gas-rich galaxies, such as young galaxies or those with late Hubble types.Comment: 7 pages, 6 figures, MNRAS in pres