Analyzing global starburst properties in various kinds of starburst and post-starburst galaxies and relating them to the properties of the star cluster populations they form, I explore the conditions for the formation of massive, compact, long-lived star clusters. The aim is to find out whether the relative amount of star formation that goes into star cluster formation as opposed to field star formation, and into the formation of massive long-lived clusters in particular, is universal or scales with star formation rate, burst strength, star formation efficiency, galaxy or gas mass, and whether or not there are special conditions or some threshold for the formation of star clusters that merit to be called globular clusters a few gigayears later.
Using a quantitative model for bipolar outflows driven by hydromagnetic protostellar winds, we calculate the efficiency of star formation assuming that available gas is either converted into stars or ejected in outflows. We estimate the efficiency of a single star formation event in a protostellar core, finding 25%-70% for cores with various possible degrees of flattening. The core mass function and the stellar initial mass function have similar slopes, because the efficiency is not sensitive to its parameters. We then consider the disruption of gas from a dense molecular clump in which a clus…
$UBV$ photometry is used to re-identify the OB associations which power the two most luminous HII regions in M33, NGC 604 and NGC 595. There is a significant difference (2-3 Myr) in the ages of the most recent star formation episode in these two regions, while NGC 595 also has undergone a prior episode of star formation (10-15 Myr ago). These data, combined with the presence of molecular clouds in the heart of NGC 604, suggest that molecular clouds may survive at least one intense episode of massive star formation. The star formation efficiencies (mass of stars per mass of gas) of these two HI…
Starbursts produce large numbers of Young Star Clusters (YSCs). Multi-color photometry in combination with a dedicated SED analysis tool allows to derive ages, metallicities, ${\rm E_{B-V}}$, and masses including $1 σ$ uncertainties for individual clusters and, hence, mass functions for YSC systems. The mass function, known to be Gaussian for old Globular Cluster (GC) systems, is still controversial for YSC systems. GC formation is expected in massive gas-rich spiral -- spiral mergers because of their high global star formation efficiencies and observed in $\geq 1$ Gyr old merger remnants. Yet…
A new method of evaluating relative (arm with respect to the interarm disk) star formation rates and relative star formation efficiencies, together with spiral arm-amplitudes, as a function of the galactocentric radius, using broad-band photometry is derived. The classical method for obtaining star formation rates from H_alpha photometry is discussed, and a new method is derived for diagnosing the possible presence of biased star formation due to different initial mass functions in the arms and in the interarm disk. As an example, these methods are applied to the spiral galaxies NGC 4321 and N…
After the stars of a new, embedded star cluster have formed they blow the remaining gas out of the cluster. Especially winds of high mass stars and definitely the on-set of the first super novae can remove the residual gas from a cluster. This leads to a very violent mass-loss and leaves the cluster out of virial equilibrium. Standard models predict that the star formation efficiency (SFE) has to be about 33 per cent for sudden (within one crossing-time of the cluster) gas expulsion to retain some of the stars in a bound cluster. If the efficiency is lower the stars of the cluster disperse com…
Globular clusters are often assumed to be good tracers of major star formation episodes in their host galaxies. While observations over the past 2 decades have confirmed the presence of young objects with globular cluster-like properties in many galaxies, it is still not well understood exactly how the formation efficiency of bound star clusters relative to field stars and their mass spectrum depend on external factors. The cluster initial mass function typically appears to be consistent with a power-law with a slope of about -2, but most attempts to constrain any upper limit on the CIMF have …
(Abridged) The c2d Spitzer Legacy project obtained images and photometry with both IRAC and MIPS instruments for five large, nearby molecular clouds. This paper combines information drawn from studies of individual clouds into a combined and updated statistical analysis of star formation rates and efficiencies, numbers and lifetimes for SED classes, and clustering properties. Current star formation efficiencies range from 3% to 6%. Taken together, the five clouds are producing about 260 solar masses of stars per Myr. The star formation surface density is more than an order of magnitude larger …
The fraction of stellar mass contained in globular clusters (GCs), also measured by number as the specific frequency, is a fundamental quantity that reflects both a galaxy's early star formation and its entire merging history. We present specific frequencies, luminosities, and mass fractions for the globular cluster systems of 100 early-type galaxies in the ACS Virgo Cluster Survey, the largest homogeneous catalog of its kind. We find that 1) GC mass fractions can be high in both giants and dwarfs, but are universally low in galaxies with intermediate luminosities. 2) The behavior of specific …
We present new J=1-0 12-CO observations along the northern spiral arm of the grand-design spiral galaxy M100 (NGC 4321), and study the distribution of molecular hydrogen as derived from these observations, comparing the new data with a set of data points on the southern arm published previously. We compare these measurements on both spiral arms and on the interarm regions with observations of the atomic and ionized hydrogen components. We determine massive star formation efficiency parameters, defined as the ratio of H alpha luminosity to total gas mass, along the arms and compare the values t…
We model how repeated supernova explosions in high-redshift dwarf starburst galaxies drive superbubbles and winds out of the galaxies. We compute the efficiencies of metal and mass ejection and energy transport from the galactic potentials, including the effect of cosmological infall of external gas. The starburst bubbles quickly blow out of small, high-redshift, galactic disks, but must compete with the ram pressure of the infalling gas to escape into intergalactic space. We show that the assumed efficiency of the star formation rate dominates the bubble evolution and the metal, mass, and ene…
We present CO(J=3-2) emission observations with the Atacama Submillimeter Telescope Experiment (ASTE) toward the 5' x 5' (or 6.6 x 6.6 kpc at the distance D = 4.5 Mpc) region of the nearby barred spiral galaxy M 83. We successfully resolved the major structures, i.e., the nuclear starburst region, bar, and inner spiral arms in CO(J=3-2) emission at a resolution of 22'' (or 480 pc), showing a good spatial coincidence between CO(J=3-2) and 6 cm continuum emissions. We found a global CO(J=3-2) luminosity L'_CO(3-2) of 5.1 x 10^8 K km s^-1 pc^2 within the observed region. We also found L'_CO(3-2) …
The main objective of the present work is to ckeck if the star formation efficiency plays a relevant role in the evolution of the relative abundance N/O. We analyze the evolution of the nitrogen-to-oxygen ratio as predicted by a set of computed theoretical models of simulated galaxies with different total masses which are evolved assuming different collapse time scales and different star formation efficiencies, thus producing different star formation histories. The stellar yield set we use for all of them (Gavilán et al. 2005) have an important contribution of primary nitrogen proceeding from …
I briefly review models for the formation of globular cluster systems, and summarize the observational properties (i.e., formation efficiencies, metallicity distributions, kinematics and ages) of the globular cluster systems of M87 and M49: two thoroughly studied elliptical galaxies. Many of the properties of the metal-poor clusters in these and other galaxies appear to be consistent with their formation in low-mass, proto-galactic fragments, as proposed by several different formation models. A number of outstanding questions concerning the formation of the metal-rich clusters in these galaxie…
Star clusters are observed to form in a highly compact state and with low star-formation efficiencies. If the residual gas is expelled on a dynamical time the clusters disrupt thereby (i) feeding a hot kinematical stellar component into their host-galaxy's field population, and (ii) if the gas-evacuation time-scale depends on cluster mass, then a power-law embedded-cluster mass function transforms within ten to a few dozen Myr to a mass function with a turnover near 10^5 M_sun, thereby possibly explaining this universal empirical feature.
We investigate how the dynamical state of a turbulently supported, 1000 solar mass, molecular cloud affects the properties of the cluster it forms, focusing our discussion on the star formation efficiency (SFE) and the initial mass function (IMF). A variety of initial energy states are examined in this paper, ranging from clouds with PE = 0.1 KE to clouds with PE = 10 KE, and for both isothermal and piece-wise polytropic equations of state (similar to that suggested by Larson). It is found that arbitrary star formation efficiencies are possible, with strongly unbound clouds yielding very low s…
There are several physical processes to remove gas from galaxies in clusters, with subsequent starvation and star formation quenching: tidal interactions between galaxies, or tidal stripping from the cluster potential itself, interactions with the hot intra-cluster medium (ICM) through ram pressure, turbulent or viscous stripping, or also outflows from star formation of nuclear activity, We review the observational evidence for all processes, and numerical simulations of galaxies in clusters which support the respective mechanisms. This allows to compare their relative efficiencies, all along …
We analyze the evolution of nitrogen resulting from a set of spiral and irregular galaxy models computed for a large number of input mass radial distributions and with various star formation efficiencies. We show that our models produce a nitrogen abundance evolution in good agreement with the observational data. Differences in the star formation histories of the regions and galaxies modeled are essential to reproduce the observational data in the N/O-O/H plane and the corresponding dispersion.
We use a sample of 43,690 galaxies selected from the Sloan Digital Sky Survey Data Release 4 to study the systematic effects of specific star formation rate (SSFR) and galaxy size (as measured by the half light radius, r_h) on the mass-metallicity relation. We find that galaxies with high SSFR or large r_h for their stellar mass have systematically lower gas phase-metallicities (by up to 0.2 dex) than galaxies with low SSFR or small r_h. We discuss possible origins for these dependencies, including galactic winds/outflows, abundance gradients, environment and star formation rate efficiencies.
We argue that rich star clusters take at least several local dynamical times to form, and so are quasi-equilibrium structures during their assembly. Observations supporting this conclusion include morphologies of star-forming clumps, momentum flux of protostellar outflows from forming clusters, age spreads of stars in the Orion Nebula Cluster (ONC) and other clusters, and the age of a dynamical ejection event from the ONC. We show that these long formation timescales are consistent with the expected star formation rate in turbulent gas, as recently evaluated by Krumholz & McKee. Finally, we di…
We present a census of the population of deeply embedded young stellar objects (YSOs) in the Ophiuchus molecular cloud complex based on a combination of Spitzer Space Telescope mid-infrared data from the "Cores to Disks" (c2d) legacy team and JCMT/SCUBA submillimeter maps from the COMPLETE team. We have applied a method developed for identifying embedded protostars in Perseus to these datasets and in this way construct a relatively unbiased sample of 27 candidate embedded protostars with envelopes more massive than our sensitivity limit (about 0.1 M_sun). Embedded YSOs are found in 35% of the …
It is well known that cosmic rays (CRs) contribute significantly to the pressure of the interstellar medium in our own Galaxy, suggesting that they may play an important role in regulating star formation during the formation and evolution of galaxies. We will present a novel numerical treatment of the physics of CRs and its implementation in the parallel smoothed particle hydrodynamics (SPH) code GADGET-2. In our methodology, the non-thermal CR population is treated self-consistently in order to assess its dynamical impact on the thermal gas as well as other implications on cosmological observ…
Aims: The production of molecular hydrogen and its deuterated forms onto carbonaceous dust grains is investigated in detail. The goal of this study is to estimate the importance of the chemistry occuring on grain surfaces for the deuteration of H2. Furthermore, we aim to find a robust and general surface chemical model which can be used in different astrophysical environments. Methods: Surface processes are described for the cases of graphitic and amorphous--carbon grains, where laboratory work is available. Langmuir--Hinshelwood as well as Eley--Rideal surface chemistries are included in the …
N-body simulations are used to model the early evolution of globular clusters. These simulations include residual gas which was not turned into stars which is expelled from the globular cluster by the actions of massive stars. The results of these simulations are compared to observations of 8 LMC globular clusters less than 100 Myr old. These observations are used to constrain the initial conditions that may have produced these clusters. It is found that the entire variety of young LMC globular clusters may be explained in a model where they form from a fairly uniform population of roughly sph…
I briefly review some measures of star formation rates in galaxies and discuss their respective uncertainties before outlining the range of star formation rates encountered in starbursts from isolated dwarf through massive gas-rich interacting systems. I present our current understanding of molecular cloud masses and structures and on star formation processes and efficiencies in starburst and interacting galaxies. Star cluster formation is an important mode of star formation, in particular in strong star formation regimes. I discuss the role of star clusters and their properties in helping us …
(Abridged) The building blocks of galaxies are star clusters. These form with low-star formation efficiencies and, consequently, loose a large part of their stars that expand outwards once the residual gas is expelled by the action of the massive stars. Massive star clusters may thus add kinematically hot components to galactic field populations. This kinematical imprint on the stellar distribution function is estimated here by calculating the velocity distribution function for ensembles of star-clusters distributed as power-law or log-normal initial cluster mass functions (ICMFs). The resulti…
The discovery of young globular clusters in merging galaxies and other environments provides an opportunity to study directly the process of globular cluster formation. Empirically it appears that globular cluster formation occurs preferentially in regions in which star formation occurs at a high rate and efficiency. Further, the interstellar medium in such regions is likely to be at a higher pressure than less active star-forming environments. An additional observational clue to the globular cluster formation process is that young globular clusters have little or no mass-radius relationship. …
We present high resolution interferometric and single dish observations of molecular gas in the Serpens cluster-forming core. Star formation does not appear to be homogeneous throughout the core, but is localised in spatially- and kinematically-separated sub-clusters. The stellar (or proto-stellar) density in each of the sub-clusters is much higher than the mean for the entire Serpens cluster. This is the first observational evidence for the hierarchical fragmentation of proto-cluster cores suggested by cluster formation models.
A great burst of cluster formation increased the rate at which open clusters were formed in the Large Magellanic Cloud 3-5 Gyr ago by at least an order of magnitude. On the other hand the rate of star formation ~ 4 Gyr appears to have increased by a factor of only 2-4. This shows that the rate of cluster formation is not a good tracer of the rate at which stars are formed. Normalized to the same rate of star formation, the Large Cloud is presently forming / 600 times more star clusters than the Local Group dwarf irregular IC 1613. The high rate of cluster formation in merging gas-rich galaxies…
Despite the rapid observational progress in the study of young massive star clusters, the formation of globular clusters still remains poorly understood. Yet, it is emerging that globular cluster formation is intimately linked to the formation of the Galaxy. I discuss a generic scenario of the cluster formation within progenitor galaxies, based on the available observational constraints. The oldest clusters formed around redshift z = 7, but the process continued at least until z = 3. Because of their high density, globular clusters survived when their progenitor galaxies were disrupted by the …
This is the written account of the first of two panel discussions, on "Star formation in galaxies: how do we continue?" The chair of the panel was Phil James, and panel members were John Beckman, Torsten Boeker, Daniela Calzetti, Angeles Diaz, and Rob Kennicutt. The panel and audience discussed the four following questions: 1) What are the most critically needed techniques to give accurate measurements of total rates and efficiencies of star formation? 2) Do we understand the form of the initial mass function and its variation as a function of redshift and environment? 3) Are there multiple mo…
Photoevaporation driven by the ultraviolet radiation from massive stars severely limits the lifetime of protoplanetary discs around stars formed within stellar clusters. I investigate the resulting influence of clustered environments on the probability of giant planet formation, and show that for clusters as rich, or richer than, Orion, the time available for planet formation is likely to be limited to the length of any delay between low mass and high mass star formation. Under popular models for the formation of massive planets, the fraction of stars with giant planets in rich clusters is exp…
We explore how the expulsion of gas from star-cluster forming cloud-cores due to supernova explosions affects the shape of the initial cluster mass function, that is, the mass function of star clusters when effects of gas expulsion are over. We demonstrate that if the radii of cluster-forming gas cores are roughly constant over the core mass range, as supported by observations, then more massive cores undergo slower gas expulsion. Therefore, for a given star formation efficiency, more massive cores retain a larger fraction of stars after gas expulsion. The initial cluster mass function may thu…
In this work we concentrate on the evolution of the cluster population of the interacting galaxy M51 (NGC 5194), namely the timescale of cluster disruption and possible variations in the cluster formation rate. We present a method to compare observed age vs. mass number density diagrams with predicted populations including various physical input parameters like the cluster initial mass function, cluster disruption, cluster formation rate and star bursts. If we assume that the cluster formation rate increases at the moments of the encounters with NGC 5195, we find an increase in the cluster for…
A phenomenological model for chemical enrichment in early-type galaxies is presented, in which the process of star formation is reduced to a set of four parameters: star formation efficiency (Ceff), fraction of ejected gas in outflows (Bout), formation redshift (zF) and infall timescale (Tf). Out of these four parameters, only variations of Bout or Ceff can account for the color-magnitude relation. A range of outflows results in a metallicity sequence, whereas a range of star formation efficiencies will yield a mixed age + metallicity sequence. The age-metallicity degeneracy complicates the is…
We studied the stellar population and star clusters around six regions in the Large Magellanic Cloud (LMC) in order to understand the correlation between star formation and cluster formation episodes. We used the stellar data base of OGLE II LMC survey and the star cluster catalogues. The analysis of the colour-magnitude diagrams (CMDs) were used to estimate the ages of the stellar population. It is found that most of the regions have undergone three major star formation episodes. The star formation which began about 4 Gyr ago, continued upto 1 Gyr, or continued further. The other two events h…
We propose and investigate a new formation mechanism for globular clusters in which they form within molecular clouds that are formed in the shocked regions created by galactic winds driven by successive supernova explosions shortly after the initial burst of massive star formation in the galactic centers. The globular clusters have a radial distribution that is more extended than that of the stars because the clusters form as pressure-confined condensations in a shell that is moving outward radially at high velocity. In addition the model is consistent with existing observations of other glob…
The stellar population and star clusters around six regions in the Large Magellanic Cloud (LMC) are studied to understand the correlation between star formation and cluster formation rates. We used the stellar database of the OGLE II LMC survey and the star cluster catalogues. The observed distribution of stellar density on the colour-magnitude diagrams (CMDs) were compared with the synthetic ones generated from the stellar evolutionary models. By minimising the reduced chi-square values, the star formation history of the regions were obtained in terms of the star formation rates (SFRs). All t…
I 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.
We review recent work that investigates the formation of stellar clusters, ranging in scale from globular clusters through open clusters to the small scale aggregates of stars observed in T associations. In all cases, recent advances in understanding have been achieved through the use of state of the art stellar dynamical and gas dynamical calculations, combined with the possibility of intercomparison with an increasingly large dataset on young clusters. Among the subjects that are highlighted are the frequency of cluster-mode star formation, the possible relationship between cluster density a…
We present the results of a numerical simulation in which star formation proceeds from an initially unbound molecular cloud core. The turbulent motions, which dominate the dynamics, dissipate in shocks leaving a quiescent region which becomes gravitationally bound and collapses to form a small multiple system. Meanwhile, the bulk of the cloud escapes due to its initial supersonic velocities. In this simulation, the process naturally results in a star formation efficiency of 50%. The mass involved in star formation depends on the gas fraction that dissipates sufficient kinetic energy in shocks.…
A brief review on globular cluster sub-populations in galaxies, and their constraints on galaxy formation and evolution is given. The metal-poor and metal-rich sub-populations are put in a historical context, and their properties, as known to date, are summarized. We review why the study of these sub-populations is extremely useful for the study of galaxy formation and evolution, but highlight a few caveats with the current interpretations. We re-visit the current globular cluster system formation scenarios and show how they boil down to a single scenario for the metal-poor clusters (namely th…
We demonstrate that single and binary star clusters can be formed during cloud-cloud collisions triggered by the tidal interaction between the Large and Small Magellanic clouds. We run two different sets of self-consistent numerical simulations which show that compact, bound star clusters can be formed within the centers of two colliding clouds due to strong gaseous shocks, compression, and dissipation, providing the clouds have moderately large relative velocities (10-60 km s$^{-1}$). The impact parameter determines whether the two colliding clouds become a single or a binary cluster. The sta…
There has been plenty of observational evidence of cluster galaxy evolution such as the Butcher-Oemler effect and the decrease in S0 fraction with increasing redshift. As a modern version, the redshift evolution of total star formation rate (SFR) in clusters has been actively debated recently. However, these studies of redshift trend have been always hampered by the possible mass dependence; due to the observational selection effects, clusters found at higher redshift inevitably tend to be more massive and luminous than local counterparts. Consequently, one has to correct for the mass trend be…
Star clusters are observed to form in a highly compact state and with low star-formation efficiencies, and only 10 per cent of all clusters appear to survive to middle- and old-dynamical age. If the residual gas is expelled on a dynamical time the clusters disrupt. Massive clusters may then feed a hot kinematical stellar component into their host-galaxy's field population thereby thickening galactic disks, a process that theories of galaxy formation and evolution need to accommodate. If the gas-evacuation time-scale depends on cluster mass, then a power-law embedded-cluster mass function may t…
I present recent observations and analyses of star cluster formation in a wide variety of environments -- from young star clusters and super star clusters in normal actively star-forming spirals and irregulars to starbursting dwarfs and spiral-spiral mergers. Star cluster formation in interacting galaxies can be restricted to central starburst region, extend over the entire body of the merger, or even all along extended tidal structures. I address methods and results for the determination of star cluster ages, metallicities, masses, and sizes and discuss the nature, possible lifetimes and futu…
We study the evolution of nitrogen resulting from a set of spiral and irregular galaxy models computed for a large number of input mass radial distributions and with various star formation efficiencies. We show that our models produce a nitrogen abundance evolution in good agreement with the observational data. In particular, low N/O values for high-redshift objects, such as those obtained for Damped Lyman Alpha galaxies can be obtained with our models simultaneously to higher and constant values of N/O as those observed for irregular and dwarf galaxies, at the same low oxygen abundances $\rm …
Recent developments relevant to the formation of globular cluster systems in galaxies are discussed, including (1) The age distribution of the oldest Milky Way clusters, (2) Subgroups and possible accretion and merger remnants in the Galactic halo, (3) Key features of globular cluster systems in giant elliptical galaxies (metallicity distribution functions, luminosity distribution functions, specific frequencies). Processes contributing to the formation of giant ellipticals are summarized.
Recent observations demonstrate that dwarf elliptical (dE) galaxies in clusters, despite their faintness, are likely a critical galaxy type for understanding the processes behind galaxy formation. Dwarf ellipticals are the most common galaxy type, and are particularly abundant in rich galaxy clusters. The dwarf to giant ratio is in fact highest in rich clusters of galaxies, suggesting that cluster dEs do not form in groups that later merge to form clusters. Dwarf ellipticals are potentially the only galaxy type whose formation is sensitive to global, rather than local, environment. The dominan…
The properties of old globular cluster systems (GCSs) in galaxy halos offer unique insight into the physical processes that conspire to form any generic star cluster, at any epoch. Presented here is a summary of the information obtained from (1) the specific frequencies (total populations) and spatial structures (density vs. galactocentric radius) of GCSs in early-type galaxies, as they relate to the efficiency (or probability) of bound cluster formation, and (2) the fundamental role of a scaling between cluster mass and energy among Galactic globulars in setting their other structural correla…
The causal interpretation of quantum mechanics is applied to the universe as a whole and the problem of cluster formation is studied in this framework. It is shown that the quantum effects be the source of the cluster formation.
