Propagating Star Formation Research Articles

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.

Reexamining the Helium Abundance of I Zw 18

We present high signal-to-noise 4 m KPNO Mayall telescope optical spectrophotometry of the two brightest northwest (NW) and southeast (SE) star-forming components in I Zw 18, the most metal-deficient blue compact dwarf (BCD) galaxy known. In addition, the flux-calibrated spectrum of the faint C component, a blue irregular star-forming region ~22'' northwest of the NW component, is presented. The Hβ equivalent width increases from the C component, through the NW component, to the SE component, indicating an age sequence and propagating star formation from the oldest C component (age ~2 × 108 yr) to the youngest SE component (age ~5 × 106 yr). The spatial distribution of the nebular He II λ4686 emission is investigated. It is confined to the older NW component, suggesting that hot post-main-sequence stars may be responsible for this emission.

The Nearby Young Dwarf Galaxy SBS 0335-052: Primordial Gas and Lyman-Alpha Emission

Ever since their discovery, blue compact dwarf (BCD) galaxies have been thought of as excellent candidates for being young galaxies, i.e. systems presently undergoing one of their very first bursts of star formation. This is mainly because BCDs are very metal-deficient, the metallicities of their ionized gas ranging between Zʘ/50 and Zʘ/3, which makes them the least chemically evolved galaxies in the Universe. Other evidence, such as the very high fractional neutral hydrogen gas content and the lack of an evident underlying old stellar population on optical images, also point to the relative youth of some BCDs.Thuan, Izotov h Lipovetsky (1997) have argued SBS 0335-052 to be a young galaxy on the basis of the following evidence: 1) HST imaging of the BCD shows its underlying extended low surface brightness component to have an irregular and filamentary structure, suggesting that a significant part of the emission (~1/3) comes not from an underlying stellar population, but from ionized gas. Any underlying stellar population must be younger than ~108yr. Propagating star formation occurs in a chain of 6 super-star clusters with ages ranging between 4 and 30 Myr. 2) The underlying component shows unusually blue colors consistent with gaseous emission, in contrast to most BCDs which possess an underlying red component. 3) VLA 21 cm observations show the BCD to be embedded in an extraordinarily large HI cloud with dimensions some 64 × 24 kpc (the typical size of HI envelopes around BCDs is more like a few kpc in each dimension).

The Intrinsic Properties of the Stellar Clusters in the M82 Starburst Complex: Propagating Star Formation?

Near-Infrared spectroscopy combined with high spatial resolution imaging have been used in this work to probe the central 500 pc of M82. Imaging observations in the 2.36 μm CO band head are added to our previously published near-infrared hydrogen recombination line imaging, near-infrared broadband imaging, and 3.29 μm dust feature imaging observations, in order to study the nature of the starburst stellar population. A starburst model is constructed and compared with the observations of the stellar clusters in the starburst complex. Our analysis implies that the typical age for the starburst clusters is 107 yr. In addition, our high spatial resolution observations indicate that there is an age dispersion within the starburst complex that is correlated with projected distance from the center of the galaxy. The inferred age dispersion is 6 × 106 yr. If the starburst in M82 is propagating outward from the center, this age dispersion corresponds to a velocity of propagation, originating in the center, of ~50 km s-1. Our quantitative analysis also reveals that a Salpeter initial mass function, extending from 0.1 to 100 M☉, can fit the observed properties of M82 without using up more than 30% of the total dynamical mass in the starburst.

Hubble Space TelescopeObservations of the Blue Compact Dwarf SBS 0335−052: A Probable Young Galaxy

We present HST WFPC2 V and I images and GHRS UV spectrophotometry of the spectral regions around Lyα and O I λ1302 of the extremely metal-deficient (Z ~ Z☉/41) blue compact dwarf galaxy (BCD) SBS 0335-052. All the star formation in the BCD occurs in six super-star clusters (SSCs) with ages ≤25 Myr, within a region of ~2'' or 520 pc in size. Dust is clearly present and mixed spatially with the SSCs. The SSCs are roughly aligned in the southeast-northwest direction, and there is a systematic increase in reddening of the clusters away from the brightest one. The observed color dependence on position may be the combined effects of differential extinction by dust and color evolution with time due to sequential propagating star formation. There is a supershell of radius ~380 pc, delineating a large supernova cavity. The instantaneous star formation rate is ~0.4 M☉ yr-1. Strong narrow Lyα emission is not observed. Rather there is low-intensity broad (FWZI = 20 A) Lyα emission superposed on even broader Lyα absorption by the H I envelope. This broad low-intensity emission is caused by resonant scattering of Lyα photons. The absence of strong Lyα emission may be due partly to dust absorption, but is due mainly to multiple scattering that removes Lyα photons from the small HST aperture. As the H I cloud is seen nearly edge-on, geometrical effects may also play a role as photons escape more easily in a direction perpendicular to the plane than along it. The BCD appears to be a young galaxy, undergoing one of its very first bursts of star formation. This conclusion is based on the following evidence: (1) The underlying extended low surface brightness component is irregular and filamentary, suggesting that a significant part of the emission comes from ionized gas. Any underlying stellar population must be younger than ~108 yr. (2) The underlying component has very blue colors [-0.34 ≤ (V-I)0 ≤ 0.16], consistent with gaseous emission colors. (3) The O I λ1302 line is not detected in absorption in the Goddard High Resolution Spectrograph spectrum, setting an upper limit for N(O)/N(H) in the H I envelope of the BCD of more than 3000 times smaller than the value in Orion.

A new model of the structure of spiral galaxies based on propagating star formation -- II. The effect of a spiral density wave

A new model of the structure of spiral galaxies based on propagating star formation -- II. The effect of a spiral density wave

Cellular Automaton experiments on local galactic structure. I. Model assumptions

The purpose of the present paper, combined with the companion paper (Lejeune & Perdang 1995, hereinafter Paper II), is to demonstrate that a Cellular Automaton (CA) framework incorporating detailed physical evolutionary mechanisms of the galactic components provides a straightforward approach for simulating local structural features in galaxies (such as those of flocculent spiral galaxies). Conversely, and more important, the observed local irregularities may give information on the relevant timescales of the evolutionary processes operating in these galaxies. In this paper we start out with a critical review of the more standard methods in use in galactic modelling. We insist on the fact that these models do not lend themselves to a straightforward inclusion of both the galactic dynamics and the physical evolution of the galactic components. We show that the Cellular Automaton approach can combine both effects, on condition that the dynamics is approximated by a stationary, in general space–dependent velocity field of the galactic matter. The main part of the paper addresses an extension of the Stochastic Propagating Star Formation scheme originally devised by Mueller & Arnett (1976). The model consists in a multi–state CA specifically designed to deal with the evolutionary behaviour of an off-centre region of a galaxy, of an area of a few . The model incorporates a detailed sequence of in part parametrised stellar evolutionary processes. In the version discussed here it includes as dynamical effects the motions of galactic matter due to a stationary circulation and, to some extent, due to the proper motions of the stars. The model we present is a first nontrivial instance of a CA defined over a lattice lacking geometric symmetries (crystal symmetries of standard CA, or rotational symmetry of the Mueller–Arnett CA). The precise geometry of the CA network of cells is imposed in our model by the space–dependent stationary galactic velocity field. Numerical results are discussed in the companion paper.

THE STELLAR CONTENT AND DYNAMICS OF SUPERBUBBLES IN THE LARGE MAGELLANIC CLOUD

THE STELLAR CONTENT AND DYNAMICS OF SUPERBUBBLES IN THE LARGE MAGELLANIC CLOUD

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.

A new model of the structure of spiral galaxies based on propagating star formation – I. The Galactic star formation rate and Schmidt Law

We have constructed a new model for galactic structure based on the principles of propagating star formation. Unlike earlier simulations using this mechanism, it represents the galactic molecular clouds as distinct entities which move, interacting with one another through collisions, in a constant potential field. This allows us to superimpose on the propagation a spiral density wave and include more realistic dynamics for the interstellar medium. Using as input parameters values appropriate for our own Galaxy, we are able to reproduce the Galactic star formation rate to within the uncertainties of our model. Furthermore, we predict a Schmidt-type law for the integrated star formation rate which is in excellent agreement with observations.

Star formation in differentially rotating galactic discs: the physics of self-propagation

Large-scale propagating star formation in galaxies is studied as a self-regulatin process. The model connects the energy injection by star formation with the resulting interstellar structures in a differentially rotating disc. The star formation cycl includes the formation of new stars in groups, multisupernova remnants agglomerafin the gas into supershells, formation of clouds and repeated birth of stars. We investigat the evolution of a galaxy dominated by this cycle and conclude that the predicted radial distributions of H I and H2, the numbers of multisupernova remnants and massive cloud complexes, the surface filling factors of shells, the star formation rate (SFR), and the location of the molecular rings are in agreement with the properties of the observed galaxies.

The distribution and nature of star formation regions in NGC 1792

A strong asymmetry with respect to the major axis has been found in the spatial distribution of H II regions in NGC 1792 detected on our H(alpha) image. In the radio continuum, this asymmetry is even stronger than in H(alpha), indicating that supernova activity set in earlier in the northeastern half of the disk than southwest of the nucleus. This suggests that we are viewing spatially propagating star formation (SF) in the disk, which set in about 10(exp 7) yr ago. The most luminous H II regions are distinctly different from the rest of the star-forming regions in NGC 1792, showing up as a peak in the H(alpha) luminosity near L(H(alpha)) approximately equals 3 x 10(exp 39) ergs/s. Around these giant H II regions large amounts of diffuse ionized gas are found, suggesting that this gas is heated by UV radiation escaping these regions. Attributing the radiation coming from the luminous H II regions to a temporarily increased level of SF activity, we estimate that on the order of 1200 OB stars were formed in each of them, with integrated Lyman-continuum luminosites in the range of 6 x 10(exp 51)/s. Although increased significantly near the galactic turnover of rotation, the global SF rate in NGC 1792 is only marginally affected (approximately 20%) by this phenomenon. The most plausible explanation of these results and those obtained for NGC 1808, the interaction partner of NGC 1792, is that the currently on-going SF was triggered by an interaction some 10(exp 8) yr ago.

A Q condition for long-range propagating star formation

view Abstract Citations (86) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Q Condition for Long-Range Propagating Star Formation Elmegreen, B. G. Abstract Collapse conditions for large expanding shells and rings in a disk galaxy are derived and shown to lead to a condiditon for star formation that is similar to the condition Q less than 1 for spontaneous instabilities. This result implies that both spontaneous and stimulated star formation are sensitive to Q, and that the observation of a critical surface density for star formation that is based on Q does not necessarily imply that star formation results from large-scale quiescent instabilities. The results also suggest that in regions with high gas densities and high rotation rates, such as starburst galaxy nuclei, the normal balance between stimulated and spontaneous star formation mechanisms could shift to give a higher proportion of stars forming in shells and other swept-up debris, and less in giant cloud complexes containing the local Jeans mass. Publication: The Astrophysical Journal Pub Date: May 1994 DOI: 10.1086/174147 Bibcode: 1994ApJ...427..384E Keywords: Applications Of Mathematics; Magellanic Clouds; Molecular Clouds; Star Formation; Angular Velocity; Space Density; Thermal Radiation; Velocity Distribution; Astrophysics; STARS: FORMATION full text sources ADS | data products SIMBAD (1)

Coherent waves of star formation in a galactic disc

A model for generation of non-linear spatial structures in a rigidly rotating star-forming disc is proposed. The model takes into consideration non-local propagating star formation, infall of gas and the formation of molecular clouds from the gas component. The linear stability of the disc is determined by the rate of accretion so that the disc is unstable in its central parts. Numerical simulations demonstrate the development of coherent spiral waves of enhanced stellar density in the inner parts of the disc

Supershells

AbstractRepeated supernovae from an OB association will, in a few ×107 yr, create a cavity of coronal gas in the interstellar medium, with radius > 100 pc, surrounded by a dense expanding shell of cool interstellar gas. Such a cavity will likely burst through the gas layer of a disk galaxy. Such holes and “supershells” have been observed in optical and H I radio emission maps of the Milky Way and other nearby galaxies. The gas swept up in the supershell is likely to become gravitationally unstable, providing a mechanism for propagating star formation that may be particularly effective in irregular galaxies.

Vacuum ultraviolet images of the Large Magellanic Cloud

Images with 50arcsec resolution of the Large Magellanic Cloud (LMC), obtained with sounding-rocket instrumentation in two vacuum ultraviolet (VUV) bandpasses, are presented. The bandpasses are each ≡200 Å wide and are centered, for hot stars, near 1500 Å and 1900 Å. Photometry was done on the digitized images for all associations in the list of Lucke and Hodge. The authors discuss the results and their relationship to the overall characteristics of star formation in the LMC. They present a simple model for propagating star formation in the LMC whose results closely resemble the distribution of associations as revealed by VUV images.

Supershells and propagating star formation

Stellar winds and repeated supernovae from an OB association will create a cavity of coronal gas in the interstellar medium, with radius greater than 100 pc, surrounded by a dense, expanding shell of cool interstellar gas. If the association has a typical initial mass function, its supernovae explosions will inject energy into the supershell at a nearly constant rate for about 50 Myr. The supershell loses its interior pressure and enters the snowplow phase when radiative cooling becomes important or when the shell bursts through the gas disk of a galaxy, typically after a few times 10 Myr and with a radius of 100-300 pc. At approximately the same time, the supershell becomes gravitationally unstable, forming giant molecular clouds which are sites for new star formation. There is widespread evidence for supershells in the Galaxy and other spiral and irregular galaxies from 21-cm emission-line surveys, optical emission-line surveys, and studies of supernova remnants. The gravitational instability of the supershells provides a physical mechanism for induced star formation and may account for bursts of star formation, especially in irregular galaxies.

Supershells and Propagating Star Formation

Correlated supernovae from an OB association can carve large cavities (greater than 100 pc) in the interstellar medium (ISM), and can punch holes completely through the disk of a spiral galaxy. Supernova remnant energy within such a cavity is thermalized before the shock reaches the supershell. Thus stellar wind theory may be used to model these superbubbles. We describe how the evolution of the superbubble depends on the density distribution of the galactic disk gas and the rate of supernovae in the OB association. At a radius of 100 to 300 pc, the supershell becomes gravitationally unstable, forming giant molecular clouds which are the sites for new star formation. This gravitational instability of the supershells provides a physical mechanism for propagating star formation and may account for the observation of bursts of star formation in galaxies.

Percolation and Galaxies

A theory is presented in which much of the structure of spiral galaxies arises from a percolation phase transition that underlies the phenomenon of propagating star formation. According to this view, the appearance of spiral arms is a consequence of the differential rotation of the galaxy and the characteristic divergence of correlation lengths for continuous phase transitions. Other structural properties of spiral galaxies, such as the distribution of the gaseous components and the luminosity, arise directly from a feedback mechanism that pins the star formation rate close to the critical point of the phase transition. The approach taken in this article differs from traditional dynamical views. The argument is presented that, at least for some galaxies, morphological and other features are already fixed by general properties of phase transitions, irrespective of detailed dynamic or other considerations.

Propagating Star Formation Induced by Superbubbles

A supernova explosion in the galactic disk creates a hot, low density cavity which persists much longer than typical lifetimes of supernova remnants. McCray and Snow (1979), Bruhweiler et al. (1980), Tomisaka, Habe and Ikeuchi (1980) and Kafatos et al. (1980) pointed out that repeated supernovae from a stellar association will produce an expanding shell of gas (R ≳ 100 pc) and offered extensive evidence for such shells -primarily in the form of H I shells.