Stochastic Self-propagating Star Formation Research Articles

Stochastic 2-D Models of Galaxy Disk Evolution. The Galaxy M33

Abstract We have developed a fast numerical 2-D model of galaxy disk evolution (resolved along the galaxy radius and azimuth) by adopting a scheme of parameterized stochastic self-propagating star formation. We explore the parameter space of the model and demonstrate its capability to reproduce 1-D radial profiles of the galaxy M33: gas surface density, surface brightness in the i and GALEX FUV passbands, and metallicity.

RADIO CONTINUUM AND H I STUDY OF BLUE COMPACT DWARF GALAXIES

The multifrequency radio continuum and 21cm HI observations of five blue compact dwarf (BCD) galaxies, Mrk 104, Mrk 108, Mrk 1039, Mrk 1069 and I Zw 97 using the Giant Meterwave Radio Telescope (GMRT) are presented here. Radio continuum emission at 610 MHz and 325 MHz is detected from all the observed galaxies whereas only a few are detected at 240 MHz. In our sample, three galaxies are members of groups and two galaxies (Mrk 1069 and I Zw 97) are isolated galaxies. The radio emission from Mrk 104 and Mrk 108 is seen to encompass the entire optical galaxy whereas the radio emission from Mrk 1039, Mrk 1069, I Zw 97 is confined to massive HII regions. This, we suggest, indicates that the star formation in the latter group of galaxies has recently been triggered and that the environment in which the galaxy is evolving plays a role. Star formation rates (SFR) calculated from 610 MHz emission is in the range 0.01-0.1 M_sun/yr; this is similar to the SFR obtained for individual star forming regions in BCDs. The integrated radio spectra of four galaxies are modelled over the frequency range where data is available. We find that two of the galaxies Mrk 1069 and Mrk 1039, show a turnover at low frequencies which is well fitted by free-free absorption whereas the other two galaxies, Mrk 104 and Mrk 108, show a power law at the lowest GMRT frequencies. The flatter spectrum, localized star formation and radio continuum in isolated galaxies lend support to stochastic self-propagating star formation (SSPSF). The HI observations of four galaxies Mrk 104, Mrk 108, Mrk 1039 and Mrk 1069 show extended disks as large as ~1.1-6 times the optical size. All the observed BCDs (except Mrk 104) show rotating disk with a half power width of ~50-124 km/s. Solid body rotation is common in our sample. We note that the tidal dwarf (TD) origin is possible for two of the BCDs in our sample.

Disk Galaxy Models Driven by Stochastic Self-Propagating Star Formation

AbstractWe present a model of chemical and spectrophotometric evolution of disk galaxies based on a stochastic self-propagating star formation scenario. The model incorporates galaxy formation through the process of accretion, chemical and photometric evolution treatment, based on simple stellar populations (SSP), and parameterized gas dynamics inside the model. The model reproduces observational data of the late-type spiral galaxy M33 reasonably well. Promising test results prove the applicability of the model and the adequate accuracy for the interpretation of disk galaxy properties.

Distribution of star-forming complexes in dwarf irregular galaxies

We study the distribution of bright star-forming complexes in a homogeneous sample of 72 late-type (“irregular”) dwarf galaxies located within the 10 Mpc volume. Star-forming complexes are identified as bright lumps in B -band galaxy images and isolated by means of the unsharp-masking method. For the sample as a whole the radial number distribution of bright lumps largely traces the underlying exponential-disk light profiles, but peaks at a 10 percent smaller scale length. Moreover, the presence of a tail of star forming regions out to at least six optical scale lengths provides evidence against a systematic star formation truncation within that galaxy extension. Considering these findings, we apply a scale length-independent concentration index, taking into account the implied non-uniform random spread of star formation regions throughout the disk. The number profiles frequently manifest a second, minor peak at about two scale lengths. Relying on a two-dimensional stochastic self-propagating star formation model, we show these secondary peaks to be consistent with triggered star formation; for a few of the brighter galaxies a peculiar peak distribution is observed that is conceivably due to the onset of shear provided by differential rotation. On scales between 100 and 1000 pc, and by taking into account exponential-disk structure, bright lumps reveal cluster dimensions between 1.3 and 2, with a weak trend to higher dimensions for brighter galaxies. Cluster dimension weakly anticorrelates with the lumpiness index (the fraction of the total galaxy light due to the light contributed by the lumps), the latter index showing no dependence on luminosity. Lump spreading within the disk, as measured by the concentration index, and lump clustering, as given by the cluster dimension, are not linked to each other. Interpreting cluster dimension in terms of porosity of a self-similar intragalactic medium, we derive a relation between current star formation rate, scale length, and porosity.

Morphology of star formation regions in irregular galaxies

The location of H ii regions, which indicates the locus of present star formation in galaxies, is analysed for a large collection of 110 irregular galaxies (Irr) imaged in Hα and nearby continuum. The analysis is primarily by visual inspection, although a two-dimensional quantitative measure is also employed. The two different analyses yield essentially identical results. H ii regions appear preferentially at the edges of the light distribution, predominantly on one side of the galaxy, contrary to what is expected from stochastic self-propagating star formation scenarios. This peculiar distribution of star-forming regions cannot be explained by a scenario of star formation triggered by an interaction with extragalactic gas, or by a strong one-armed spiral pattern.

Star Formation History in Shapley Constellation III

The observational stellar age is multiple in Shapley constellation III (SCIII) of the Large Magellanic Cloud (LMC). Previously, the stochastic self-propagating star formation (SSPSF) model was favored to explain the age spread in SCIII since it was reported that there was a correlation between age of stars and distance from the center of the supershell. Unfortunately, such a star formation history due to SSPSF is not confirmed by any subsequent observations. We find only the multiple age of stars in SCIII. In this paper, we present a first attempt to describe multiple star formation history in the context of SSPSF. As a result, we find this criterion: only when the cooling time of coronal gas is longer than the propagation timescale of star formation is SSPSF possible. The strictest conclusion is that if the cooling time is shorter than the propagation timescale, we must abandon the SSPSF model to explain the star formation history in SCIII. Checking these two timescales, we find ~106 yr for the propagation timescale and ~108 yr for the cooling timescale. Thus, there are plenty of chances to describe the star formation history in SCIII in the context of a modified SSPSF.

A Hybrid N-Body/Cellular Automaton Scheme for Modelling Propagating Star Formation in Galaxies

We present a numerical scheme for modelling star formation in galaxies based on the dual operation of gravitational instabilities and propagating star formation. Star formation is superposed on a mechanical N-body simulation of a collisional particle system by using a cellular automaton framework. The simulations involve 5000 particles representing gas clouds which can collide inelastically, and have been performed on a 2D square surface with quasiperiodic boundary conditions. The input of kinetic energy from star-forming regions leads to self-regulation of the global dynamics of star formation. The model naturally generates cloud complexes assembled from groups of cloud particles and simulates the ignition and propagation of star formation in such complexes leading to the formation of low density bubbles and expanding shells. The percentage of the model surface undergoing star formation and the ratio of the rates of stimulated to spontaneous star formation are shown to be consistent with models based on the theory of Stochastic Self-Propagating Star Formation (SSPSF). The code is shown to lead to star formation that propagates very nearly isotropically and can thus be adapted to the case of rotating disc galaxies without geometrical modification.

Morphology of star formation regions in irregular galaxies

The location of HII regions, which indicates the locus of present star formation in galaxies, is analyzed for a large collection of 110 irregular galaxies (Irr) imaged in H$\alpha$ and nearby continuum. The analysis is primarily by visual inspection, although a two-dimensional quantitative measure is also employed. The two different analyses yield essentially identical results. HII regions appear preferentially at the edges of the light distribution, predominantly on one side of the galaxy, contrary to what is expected from stochastic self-propagating star formation scenarios. This peculiar distribution of star forming regions cannot be explained by a scenario of star formation triggered by an interaction with extragalactic gas, or by a strong one-armed spiral pattern.

On a modification to the stochastic self propacating star formation model

The stochastic self propagating star formation (SSPSF) model is modified by assuming that a star is formed if a neighbouring star explodes. This is a more reasonable assumption than the original model. The basic features of SSPSF are shown to be preserved hence one still can explain the galactic spiral arms and luminosity.

Bifurcation analysis of a simple analytic model of self-propagating star formation

We investigate the structure and stability of rotationally symmetric nonhomogeneous time-independent solutions derived from a simple analytic model of self-propagating star formation. For this purpose we employ two methodologies: We use bifurcation theoretical methods to prove the existence of nonhomogeneous axisymmetric stationary solutions of an appropriate nonlinear evolution equation for the stellar density. We show that the nonhomogeneous solution branch bifurcates from the homogeneous one at a critical parameter value of the star formation rate. Further, the analytical theory allows us to show that the new solution set is stable in the weakly nonlinear regime near the bifurcation point. To follow the solution branch further, we use numerical methods. The numerical calculation shows the structure and stability of these solutions. We conclude that no periodic time-dependent solutions of this special model exist, and no further bifurcations can be found. The same results have been found in simulations of stochastic self-propagating star formation based on similar models. Therefore, our findings provide a natural explanation, why long-lived large-scale structure have not been found in those simulations.

Turbulence and stochastic star formation in the Large Magellanic Cloud

A recent turbulence analysis of the Large Magellanic Cloud (LMC) is summarized. New model calculations on stochastic self-propagating star formation taking into account the multi-phase structure of the interstellar medium are presented. The relationship between star formation and turbulence is discussed.

An overview of the structure and kinematics of the Magellanic Clouds

A vast amount of observational data concerning the structure and kinematics of the Magellanic Clouds is now available. Many basic quantities (e.g. distances and geometry) are, however, not yet sufficiently well determined. Interactions between the Small Magellanic Cloud (SMC), the Large Magellanic Cloud (LMC) and our Galaxy have dominated the evolution of the Clouds, causing bursts of star formation which, together with stochastic self-propagating star formation, produced the observed structures. In the youngest generation in the LMC it is seen as an intricate pattern imitating a fragmented spiral structure. In the SMC much of the fragmentation is along the line of sight complicating the reconstruction of its history. The violent events in the past are also recognizable in complex velocity patterns which make the analysis of the kinematics of the Clouds difficult.

Percolation model of galactic structure

Abstract Galactic structure is generally studied from the point of view of Newtonian dynamics. The goal of this paper is to show that modern statistical physics also has an important contribution to make in understanding the structure of galaxies. In particular, we show how the process of star formation can be modelled as a percolation process, and how the phase transition associated with this percolation process plays a critical role in the stabilization and control of star formation. For galaxies well described by our model, the dominant morphological feature, the presence of spiral arms, is a consequence of the proximity to the second-order phase transition associated with the percolation threshold. We present an introduction to the astrophysical problem we are investigating and an analysis of the directed percolation problem, called stochastic self-propagating star formation, that controls star formation. We also describe a simulation of the galactic evolution problem in terms of a cellular automaton that provides a representation of the percolation process of interest in an astrophysical environment. Lastly, we provide a number of examples of how the model gives a good account of much of the observed nature of spiral galaxies.

Numerical simulations of galactic structure and helium abundance evolution in disk galaxies

view Abstract Citations (4) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Numerical Simulations of Galactic Structure and Helium Abundance Evolution in Disk Galaxies Balser, Dana S. ; Comins, Neil F. Abstract We have developed a three-dimensional, two gas-phase, stochastic self-propagating star formation (SSPSF) model of galaxies which allows gas to move from cell to cell, permanently removes some gas in the form of long-lived, low-mass stars, and follows the change in galactic chemistry, at least as far as to track changes from primordial abundances of hydrogen and helium to abundances of hydrogen and helium (plus metals) after many generations of star formation. The results of this model compare reasonably well with observation of the Milky Way: relative quantities of neutral and molecular gas, ionized gas, and stars are in excellent agreement with observations as is global helium plus metal formation history. Spiral structure is not sensitive to the parameters of the model and may be recovered in most cases. However, profiles of hot gas density increase from the center of the disk to a peak at 1 kpc and then decline with increasing radii whereas observations of our Galaxy indicate that the peak occurs at 5 kpc. Radial profiles of gas that is cold enough to undergo immediate star formation are consistently flat and therefore do not resemble gradients of the galaxy, although methods for determining densities of molecular hydrogen are still controversial. Parameters which determine the rate at which hydrogen is processed into helium may be set to match present radial helium density gradients. A strongly decreasing star formation rate must exist for the model to reproduce correct gas fractions of the galaxy. Global, galactic infall is plausible, assuming higher depletion rates. Publication: The Astrophysical Journal Pub Date: May 1988 DOI: 10.1086/166308 Bibcode: 1988ApJ...328..499B Keywords: Computerized Simulation; Disk Galaxies; Galactic Evolution; Galactic Structure; Helium; Star Formation; Abundance; Interstellar Gas; Ionized Gases; Stellar Mass; Vapor Phases; Astrophysics; STARS: FORMATION full text sources ADS |

Large-scale filaments of star formation in the Large Magellanic Cloud

A new high sensitivity radio survey of the Large Magellanic Cloud (LMC) has been undertaken at 1.4 GHz. Evidence for large-scale star-forming filaments has come from this new data. The radio results are complemented by an IRAS image of the LMC at 100 μm which shows the same large-scale filaments. Additional calculations in the frame of stochastic selfpropagating star formation models are presented. The implications for the morphology of the LMC are discussed. 1. 1.|The immediate past history of star formation and the distribution of dust in the LMC show coherent regular strings. As many as six or seven very extensive star-forming features can be recognized. 2. 2.|The optical bar of the LMC is not directly related to the star-forming pattern; it appears to reflect an earlier period in the evolution of the galaxy. 3. 3.|The stochastic selfpropagating star formation models simulate very realistically the observed distribution of young stars and molecular cloud or dust features but they do not explain the function of the 30 Doradus nebula which is also a site of a burst of star formation.

Star Formation in the Large Magellanic Cloud

Methods used in pattern recognition and cluster analysis are applied to investigate the spatial distribution of the star forming regions. The fractal dimension of these structures is deduced. The new 21 cm, radio continuum (1.4 GHz) and IRAS surveys reveal scale structures of 700 pc to 1500 pc being identical with the optically identified star forming sites. The morphological structures delineated by young stars reflect physical parameters which determine the star formation in this galaxy. The formation of spiral arm filaments is understandable by stochastic selfpropagating star formation processes.

C. Spiral Structure

The spiral structure of galaxies is probably related to density waves, primarily governed by gravitational forces. Density waves may result from an inherent instability of galaxies against spiral perturbations as conceived in the conventional density-wave theory or may be forced by other internal or external perturbations of the gravitational field, such as neighbouring galaxies or oval distortions and bars in the inner regions of the galaxies. Reviews of recent developments on the various aspects of gravitational theories of spiral structure have been given by Ambastha and Varma (30.151.029), Athanassoula (33.151.051, 1984), Contopoulos (32.151.021, 34.151.103), Donner (30.151.085), Hunter (34.151.053), James and Wilkinson (29.151.023), Kalnajs (33.151.024), Kormendy (32.151.049), Lin (32.151.040, 33.151.025, 33.151.071), Lin and Bertin (30.151.068, 1984), Lin and Roberts (30.151.045), Martinet (30.151.043), McElroy (34.157.160), Norman (33. 157.088), Pasha and Tsitsin (34.151.042), Sorensen (29.151.024), Thonnard (31.158. 357), and Toomre (30.151.021). Seiden and Gerola (31.151.084) reviewed the theory of formation of spiral structures by stochastic self-propagating star formation in galaxies.

Stochastic star formation and spiral structure

Most approaches to explaining the long-range order of the spiral arms in galaxies assume that it is induced by the long-range gravitational interaction. However, it is well-known in many fields of physics that long-range order may be induced by short-range interactions. A typical example is magnetism, where the exchange interaction between magnetic spins has a range of only 10 ångströms, yet a bar magnet can be made as large as one likes. Stochastic self-propagating star formation (SSPSF) starts from the point of view of a short-range interaction and examines the spiral structure arising from it (Seiden and Gerola 1982). We assume that the energetic processes of massive stars, stellar winds, ionization-front shocks and supernova shocks, in an OB association or open cluster can induce the creation of a new molecular cloud from cold interstellar atomic hydrogen. In turn this new molecular cloud will begin to form stars that will allow the process to repeat, creating a chain reaction. The differential rotation existing in a spiral galaxy will stretch the aggregation of recently created stars into spiral features.

Bursts of star formation in computer simulations of dwarf galaxies

view Abstract Citations (17) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Bursts of star formation in computer simulations of dwarf galaxies. Comins, N. F. Abstract A three-dimensional Stochastic Self-Propagating Star Formation (SSPSF) model of compact galaxies is presented. Two phases of gas, active and inactive, are present, and permanent depletion of gas in the form of long lived, low mass stars and remnants occurs. Similarly, global infall of gas from a galactic halo or through galactic cannibalism is permitted. The parameters are based on the observed properties of the compact blue galaxy I Zw 36. The results indicate that bursts of star formation occur much more frequently in these runs than continuous nonbursting star formation, suggesting that the blue compact galaxies are probably undergoing bursts rather than continuous, nonbursting low-level star formation activity. Publication: The Astrophysical Journal Pub Date: September 1984 DOI: 10.1086/162387 Bibcode: 1984ApJ...284...90C Keywords: Astronomical Models; Compact Galaxies; Computational Astrophysics; Computerized Simulation; Dwarf Galaxies; Star Distribution; Star Formation; Cosmic Gases; Stellar Mass Accretion; Stellar Mass Ejection; Stochastic Processes; Astrophysics full text sources ADS | data products SIMBAD (1) NED (1)

A galactic disk is not a true exponential

view Abstract Citations (44) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A galactic disk is not a true exponential Seiden, P. E. ; Schulman, L. S. ; Elmegreen, B. G. Abstract Stochastic self-propagating star formation predicts that the distribution of atomic hydrogen in a galactic disk should be relatively independent of radius. The molecular hydrogen density should therefore vary as the total gas density minus a constant. The most likely distribution for the total gas density is approximately 1/r, the same as the distribution of column density in a halo that produces a flat rotation curve. The authors propose that the approximately exponential surface brightness distribution observed in disk galaxies results from the intrinsic r-1-constant distribution produced by propagating star formation. The exponential fit has no intrinsic physical meaning. Publication: The Astrophysical Journal Pub Date: July 1984 DOI: 10.1086/162179 Bibcode: 1984ApJ...282...95S Keywords: Disk Galaxies; Interstellar Gas; Stellar Evolution; Brightness; Density Distribution; Stochastic Processes; Astrophysics full text sources ADS |