Formation Of Massive Star Clusters Research Articles

THE “NESSIE” NEBULA: CLUSTER FORMATION IN A FILAMENTARY INFRARED DARK CLOUD

The "Nessie" Nebula is a filamentary infrared dark cloud (IRDC) with a large aspect ratio of over 150:1 (1.5 degrees x 0.01 degrees, or 80 pc x 0.5 pc at a kinematic distance of 3.1 kpc). Maps of HNC (1-0) emission, a tracer of dense molecular gas, made with the Australia Telescope National Facility Mopra telescope, show an excellent morphological match to the mid-IR extinction. Moreover, because the molecular line emission from the entire nebula has the same radial velocity to within +/- 3.4 km/s, the nebula is a single, coherent cloud and not the chance alignment of multiple unrelated clouds along the line of sight. The Nessie Nebula contains a number of compact, dense molecular cores which have a characteristic projected spacing of ~ 4.5 pc along the filament. The theory of gravitationally bound gaseous cylinders predicts the existence of such cores, which, due to the "sausage" or "varicose" fluid instability, fragment from the cylinder at a characteristic length scale. If turbulent pressure dominates over thermal pressure in Nessie, then the observed core spacing matches theoretical predictions. We speculate that the formation of high-mass stars and massive star clusters arises from the fragmentation of filamentary IRDCs caused by the "sausage" fluid instability that leads to the formation of massive, dense molecular cores. The filamentary molecular gas clouds often found near high-mass star-forming regions (e.g., Orion, NGC 6334, etc.) may represent a later stage of IRDC evolution.

Simulations of Massive Star Cluster Formation and Feedback in Turbulent Giant Molecular Clouds

AbstractUsing the AMR code ENZO we are simulating the formation of massive star clusters within turbulent Giant Molecular Clouds (GMCs). Here we discuss the simulations from the first stages of building realistic turbulent GMCs, to accurate star formation, and ultimately comprehensive feedback. These simulations aim to build a better understanding of how stars affect GMCs, helping to answer the questions of how long GMCs live and why only a small fraction of the GMC gas becomes stars.

THE DYNAMICS OF RADIATION-PRESSURE-DOMINATED H II REGIONS

We evaluate the role of radiation pressure in the dynamics of H II regions. We first determine under what conditions radiation pressure is significant in comparison to gas pressure and show that, while radiation pressure is generally unimportant for H II regions driven by a handful of massive stars, it is dominant for the larger H II regions produced by the massive star clusters found near the Galactic center and in starburst environments. We then provide a solution for the problem of how H II regions expand when radiation pressure influences their behavior. Finally, we compare radiation-dominated H II regions to other sources of stellar feedback, and argue that H II regions are probably the primary mechanism for regulating the formation of massive star clusters.

Super star clusters in Hii galaxies

AbstractWe summarize our results based on observations with the NIRI camera on the Gemini North telescope of three Hii galaxies (Mrk 36, UM 408 and UM 461), obtained to identify and determine the ages and masses of the elementary components (the star cluster population) of the starburst regions in compact Hii galaxies. Our preliminary results indicate that the masses of the stellar clusters in these galaxies range from ~104 to ~106 M⊙, with associated ages of a few Myr. The most massive star clusters fall in the so-called super star cluster category. The identification of these clusters suggests that the formation and evolution of massive star clusters is the dominant mode of star formation in these galaxies. Their spatial distribution and ages seem to indicate that star formation is simultaneous over these timescales in some of our objects. We also review our recent description of the spatial distribution of physical conditions in the Hii galaxy UM 408 using the GMOS integral-field unit on Gemini South. The spatial distribution of the oxygen abundance does not show any significant variation or gradient across the galaxy on scales of hundreds of parsecs, within our observational uncertainties, confirming that this compact Hii galaxy, like other previously studied dwarf irregular galaxies, is chemically homogeneous.

Colliding galaxies and the rapid assembly of clouds into proto-super star clusters

AbstractWe present a short summary of several 2D hydrodynamic calculations that suggest that upon the collision of galaxies two physical mechanisms lead to the formation of proto-super star clusters. These are condensation, induced by radiative cooling, and implosion caused by the shocked intercloud medium. Even in the absence of gravity, these lead to storage and compression of the dense cloud component into massive and compact gravitationally unstable condensations. The resulting entities exhibit enhanced surface densities that are several hundred times higher than their initial values. These are here postulated as the cradles of very efficient and rapid star-formation episodes, able to withstand the negative feedback effects associated with star formation, while leading to the formation of massive and compact super star clusters.

High-resolution simulations of galaxy mergers: resolving globular cluster formation

Abstract Massive star clusters observed in galaxy mergers are often suggested to be progenitors of globular clusters. To study this hypothesis, we performed the highest resolution simulation of a gas-rich galaxy merger so far. The formation of massive star clusters of 105 to 107 M⊙, triggered by the galaxy interaction, is directly resolved in this model. We show that these clusters are tightly bound structures with little net rotation, due to evolve into compact long-lived stellar systems. Massive clusters formed in galaxy mergers are thus robust candidates for progenitors of long-lived globular clusters. The simulated cluster mass spectrum is consistent with theory and observations. Tidal dwarf galaxies of 108–9 M⊙ can form at the same time, and appear to be part of a different class of objects, being more extended and rotating.

NORMAL GLOBULAR CLUSTER SYSTEMS IN MASSIVE LOW SURFACE BRIGHTNESS GALAXIES

We present the results of a study of the globular cluster systems of six massive spiral galaxies, originally cataloged as low surface brightness (LSB) but here shown to span a wide range of surface brightness values, including two intermediate to LSB galaxies. We used the Advanced Camera for Surveys on-board Hubble Space Telescope to obtain photometry in the F475W and F775W bands and select sources with photometric and morphological properties consistent with those of globular clusters. A total of 206 candidates were identified in our target galaxies. From a direct comparison with the Galactic globular cluster system we derive specific frequency values for each galaxy that are in the expected range for late-type galaxies. We show that the globular cluster candidates in all galaxies have properties consistent with globular cluster systems of previously studied galaxies in terms of luminosity, sizes and color. We establish the presence of globular clusters in the two intermediate to LSB galaxies in our sample and show that their properties do not have any significant deviation from the behavior observed in the other sample galaxies, implying that these properties do not evolve with the surface brightness of the galaxies. Our results are broadly consistent with a scenario in which low surface brightness galaxies follow roughly the same evolutionary history as normal (i.e. high surface) brightness galaxies except at a much lower rate, but require the presence of an initial period of star formation intense enough to allow the formation of massive star clusters.

Triggered cluster formation in the RMC

AbstractA comprehensive study of clustered star formation in the Rosette Molecular Complex was carried out based on archived data from the 2 Micron All Sky Survey. We presented strong evidence that triggered formation of embedded clusters and stellar aggregates took place in the working shells of the Rosette Nebula, a spectacular HII region excavated by the dozens of OB stars of the emerging massive cluster NGC 2244. Surprisingly, we have identified, within the confines of NGC 2244, a distinct congregation of young stellar objects showing prominent NIR excess that forms an arc like structure in appearance. Its location right to the south-east of the center of the main cluster and its strange morphology indicate most likely an origin from a former working shell of the HII region. This relic arc and the large, fragmented working surface layer of Rosette with the ambient cloud roughly show a concentric origin in morphology. This implies also a common origin of the clusters or stellar aggregates in association. The formation of massive star clusters was evidenced further into the heart of the molecular complex, and structured clustering star formation seemed to have taken place toward the south-east edge of the complex.

Massive star and star cluster formation

AbstractI review the status of massive star formation theories: accretion from collapsing, massive, turbulent cores; competitive accretion; and stellar collisions. I conclude the observational and theoretical evidence favors the first of these models. I then discuss: the initial conditions of star cluster formation as traced by infrared dark clouds; the cluster formation timescale; and comparison of the initial cluster mass function in different galactic environments.

M82, Starbursts, Star Clusters, and the Formation of Globular Clusters

We observed the nearby starburst galaxy M82 in CO in the higher frequency (2-1) transition to achieve an angular resolution below 1'' or 17 pc at the target. We resolved the molecular gas into a large number of compact clouds, with masses ranging from ~2 × 103 to 2 × 106 M☉. The mass spectrum scales as N(M) ∝ M-1.5±0.1, similar to the mass spectra of young massive star clusters, suggesting that individual molecular clouds are transformed in the starburst into individual star clusters. The larger clouds are surrounded by supernovae and H II regions, suggesting that star formation proceeds from the outside of the clouds and progresses inward, consistent with triggering by a sudden increase in external pressure. The clouds with internal star formation have velocity gradients and inverse P Cygni spectral line profiles indicating inward motions of 35 km s-1, consistent with shock-driven compression. Diffuse free-free radio emission and X-ray emission around the clouds provide evidence for superheated ionized gas sufficient to drive the compression. Clouds with spectral lines indicating expansion show little internal star formation, suggesting that the dynamics precedes and is responsible for the star formation rather than the inverse. M82 is known to be in interaction with the neighboring galaxy M81. The overall picture is consistent with the formation of massive star clusters from individual giant molecular clouds crushed by a sudden galactic-scale increase in external pressure generated by the changing dynamics that result from a near-collision with a neighboring galaxy. Present-day globular clusters may have formed in a similar fashion in primordial galaxies.

Massive star formation in the W49 giant molecular cloud: Implications for the formation of massive star clusters

We present results from JHKs imaging of the densest region of the W49 molecular cloud. In a recent paper (Alves & Homeier 2003), we reported the detection of (previously unknown) massive stellar clusters in the well-known giant radio HII region W49A, and here we continue our analysis. We use the extensive line-of-sight extinction to isolate a population of objects associated with W49A. We constrain the slope of the stellar luminosity function by constructing an extinction-limited luminosity function, and use this to obtain a mass function. We find no evidence for a top-heavy MF, and the slope of the derived mass function is -1.6 \pm 0.3. We identify candidate massive stars from our color-magnitude diagram, and we use these to estimate the current total stellar mass of 5-7x10^4 M_sun in the region of the W49 molecular cloud covered by our survey. Candidate ionizing stars for several ultra-compact HII regions are detected, with many having multipe candidate sources. On the global molecular cloud scale in W49, massive star formation apparently did not proceed in a single concentrated burst, but in small groups, or subclusters. This may be an essential physical description for star formation in what will later be termed a 'massive star cluster'.

The temporal and spatial evolution of the starburst in ESO 338-IG04 as probed by its star clusters

In this paper we use ultra-violet (UV) and optical HST photometry in five bands, and an extensive set of spectral evolutionary synthesis scenarios to investigate the age and masses of 124 star clusters in the luminous blue compact galaxy ESO338-IG04 (Tololo 1924-416). The very small internal reddening makes ESO 338-IG04 an excellent laboratory for studying the formation of massive star clusters. We have used the star clusters to trace the temporal and spatial evolution of the starburst, and to put constraints on the star formation activity over a cosmological time-scale. The present starburst has been active for about 40 Myr. A standard Salpeter initial mass function (IMF) extending up to 120 solar masses provides the best fit to the data, although a flatter IMF cannot be excluded. The compact star clusters provide 30-40 percent of the UV luminosity and star formation activity. We find no evidence for dust obscuration even among the youngest (< 1 Myr) clusters. The fraction of stellar mass contained in compact star clusters is found to be several percent, which is an unusually high value. The intermediate age clusters show a flattened space distribution which agrees with the isophotal shape of the galaxy, whereas the oldest clusters seem to have a spherical distribution.(abridged)

The Mass Function of Supergiant Molecular Complexes and Implications for Forming Young Massive Star Clusters in the Antennae (NGC 4038/4039)

We have used previously published observations of the CO emission from the Antennae (NGC 4038/4039) to study the detailed properties of the supergiant molecular complexes with the goal of understanding the formation of young massive star clusters. Over a mass range from 5 × 106 to 9 × 108 M☉ the molecular complexes follow a power-law mass function with a slope of -1.4 ± 0.1, which is very similar to the slope seen at lower masses in molecular clouds and cloud cores in the Galaxy. Compared with the spiral galaxy M51, which has a similar surface density and total mass of molecular gas, the Antennae contain clouds that are an order of magnitude more massive. Many of the youngest star clusters lie in the gas-rich overlap region, where extinctions as high as AV ~ 100 mag imply that the clusters must lie in front of the gas. Young clusters found in other regions of the galaxies can be as far as 2 kpc from the nearest massive cloud, which suggests that either young clusters can form occasionally in clouds less massive than 5 × 106 M☉ or that these young clusters have already destroyed their parent molecular clouds. Combining data on the young clusters, thermal and nonthermal radio sources, and the molecular gas suggests that young massive clusters could have formed at a constant rate in the Antennae over the last 160 Myr and that sufficient gas exists to sustain this cluster formation rate well into the future. However, this conclusion requires that a very high fraction of the massive clusters that form initially in the Antennae do not survive as long as 100 Myr. Furthermore, if most young massive clusters do survive for long periods, the Antennae must be experiencing a relatively short burst of cluster formation to prevent the final merger remnant from exceeding the observed specific frequency of star clusters in elliptical galaxies by a wide margin. Finally, we compare our data with two models for massive star cluster formation and conclude that the model in which young massive star clusters form from dense cores within the observed supergiant molecular complexes is most consistent with our current understanding of this merging system.

A Sample of Clusters of Extragalactic Ultracompact HiiRegions

We report on the detection of optically thick free-free radio sources in the galaxies M33, NGC 253, and NGC 6946 using data in the literature. We interpret these sources as being young embedded star birth regions that are likely to be clusters of ultracompact H II regions. All 35 of the sources presented in this article have positive radio spectral indices (α > 0 for Sν ∝ να), suggesting an optically thick thermal bremsstrahlung origin from the H II region surrounding the hot stars. The estimated emission measures for these sources are EM6 cm 108 cm-6 pc, and energy requirements indicate that the sources in our sample have a range of a few to ~560 O7 V star equivalents powering their H II regions. Assuming a Salpeter initial mass function with lower and upper mass cutoffs of 1 and 100 M☉, respectively, this range in NLyc corresponds to integrated stellar masses of 0.1-60 × 103 M☉. For roughly half of the sources in our sample there is no obvious optical counterpart, which gives further support for their deeply embedded nature; for most of the remaining sources, the correspondence to an optical source is insecure owing to relative astrometric uncertainty. Their luminosities and radio spectral energy distributions are consistent with H II regions modeled as spheres of plasma with electron densities from ne ~ 1.5 × 103 to ~1.5 × 104 cm-3 and radii of ~1-7 pc. Because of the high densities required to fit the data, we suggest that the less luminous of these sources are extragalactic ultracompact H II region complexes, those of intermediate luminosity are similar to W49 in the Galaxy, and the brightest will be counterparts to 30 Doradus when they emerge from their birth material. These objects constitute the lower mass range of extragalactic ultradense H II regions, which we argue are the youngest stages of massive star cluster formation yet observed. The sample presented in this paper is beginning to fill in the continuum of objects between small associations of ultracompact H II regions and the analogous massive extragalactic clusters that may evolve into globular clusters.

Millimeter-Wavelength Molecular Lines and Far-Infrared Sources

view Abstract Citations (127) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Millimeter-Wavelength Molecular Lines and Far-Infrared Sources Morris, M. ; Palmer, P. ; Turner, B. E. ; Zuckerman, B. Abstract Millimeter-wavelength emission lines of HCN and CS have been detected in many galactic sources of farinfrared radiation. These sources can be divided into two classes: (I) those which do not have associated with them radio continuum emission from a compact H ir region, and (Il) those which do. An extensive map of the J = 1 0 HCN and CS emission from the W75 region suggests that molecules have comparable concentrations in the two classes of sources which may represent different stages in the formation of massive stars and star clusters. Consequences of the association of molecular sources and far-infrared sources are discussed in terms of early stellar evolution. The W75 map, and similar ones of W3, NGC 2024, and NGC 2264, show that the molecular emission is extended over much larger regions than previously realized, and consequently very large masses (> l0 -10 M0) are implied if the molecules are collisionally excited throughout the cloud. Subject headings: infrared sources - molecules, interstellar - radio lines - star formation Publication: The Astrophysical Journal Pub Date: July 1974 DOI: 10.1086/152973 Bibcode: 1974ApJ...191..349M full text sources ADS |