Over the past decade, it has become clear that interaction induced formation of compact young star clusters is a ubiquitous pheonomenon, and the understanding of this process is thought to also shed light on galaxy evolution in general, because these young clusters are widely believed to be the progenitors of a part of the globular cluster systems seen in local elliptical galaxies. We have observed the prototypical merger NGC 4038/4039 using near-infrared broad- and narrow band imaging, integral field spectroscopy and medium and high resolution spectroscopy. We find that all of the bright star clusters are young (<20 Myrs), but the “overlap region” hosts the youngest clusters (∼5 Myrs), while the nuclear starbursts started ∼100 Myrs ago. Photometric and dynamical masses range from 105 to a few x106M⊙. However, mass-to-light ratios vary from cluster to cluster and suggest differences in the contribution of low-mass stars. While clusters with a deficiency in low-mass stars are likely to evaporate before they are a Hubble time old, those with a high mass-to-light-ratio could represent young globulars.

M82 is the nearest starburst galaxy. It contains two large systems of super star clusters, one being spawned today in the active starforming core, and one produced by an earlier starburst event which coincided with the last orbital passage of its neighbor, M81. The proximity of M82 makes it uniquely valuable for a wide range of studies of massive young clusters and their environments.

Star formation is triggered in essentially three ways: (1) the pressures from existing stars collect and squeeze nearby dense gas into gravitationally unstable configurations, (2) random compression from supersonic turbulence makes new clouds and clumps, some of which are gravitationally unstable, and (3) gravitational instabilities in large parts of a galaxy disk make giant new clouds and spiral arms that fragment by the other two processes into a hierarchy of smaller star-forming pieces. Examples of each process are given. Most dense clusters in the solar neighborhood were triggered by external stellar pressures. Most clusters and young stars on larger scales are organized into hierarchical patterns with an age-size correlation, suggestive of turbulence. Beads-on-a-string of star formation in spiral arms and resonance rings indicate gravitational instabilities. The turbulence model explains the mass spectrum of clusters, the correlation between the fraction of star formation in the form of clusters and the star formation rate, found by Larsen & Richtler, and the correlation between the size of the largest cluster and the number of clusters in a galaxy.

Existence of complexes of clusters and complexes of isolated stars is the key to understand the cluster and isolated modes of star formation. Examples of such complexes in the LMC are considered. New kind of complexes with the circular shape or including arc-like structures is described, first of all in the LMC and NGC 6946. They may be of other origin than most HI supershells. The system of the giant stellar arcs (partiall spheres seen in projection) in the NE region of the LMC might be formed by GRB events, progenitors of which might be ejected from the massive nearby cluster NGC 1978. Otherwise the arcs might be results of cloud impacts, being concentrate at the NE side of the LMC because this side is leading in the orbital motion of the LMC. The strong arguments are given for the NE (and not Eastward) direction of the LMC movement. Also, the shock wave from a superexplosion might give the shape of the partial sphere to the facing surface of a nearby dense cloud.

We review what is known about the large-scale trends in the sizes, luminosities, and mass functions of young stellar associations and clusters in nearby galaxies. We also describe an ongoing survey of massive young cluster populations in nearby galaxies.

Populations of young star clusters show significant differences even among “normal” disk galaxies. In this contribution I discuss how properties of young cluster systems are related to those of their host galaxies, based on a recent study of clusters in a sample of 22 nearby spiral galaxies. Luminous young clusters similar to the “super” star clusters observed in starbursts and mergers exist in several of these galaxies, and it is found that the luminosity of the brightest star cluster as well as the specific luminosity of the cluster systems both correlate well with the host galaxy star formation rate. When considering star clusters in different environments the traditional distinction between “open”, “massive” and “globular” clusters breaks down, underscoring the need for a universal physical description of cluster formation.

This review deals with properties of very young (≤ 10–20 Myr) stellar clusters which are in the nebular phase and are embedded in photoionized regions (classical extragalactic HII regions (RHIIs) or starburst galaxies). Based on the analysis of the integrated light of these clusters at the UV and optical wavelengths, different techniques are discussed that allow to estimate in consistent the stellar content and the evolutionary state of the ionizing clusters. Figures illustrating this contribution can be found in: http://www.iaa.csic.es/∼rosa/

In this paper we study the relationship between the star formation efficiency and luminosity of Hα emission, Lyman continuum radiation, and Hβ emission on 35 giant extragalactic HII regions in seven galaxies. Using the observational results we obtain the relationship, and find that the star formation efficiency is correlation with Halpha luminocity, and Lyman continuum luminosity, and Hβ lumonosity, respectively. Key words: external galaxy—giant HII region—star cluster

Resonance rings are intriguing sites of organized star formation in some galaxies. The Hubble Space Telescope Wide Field and Planetary Camera 2 has been used to image several resonance rings at high resolution in order to study the star clusters in the rings. Here I summarize results on inner Lindblad resonance rings in ESO 565–11 and NGC 1326, and on an inner 4:1 resonance ring in the Seyfert 2 galaxy NGC 3081. The latter ring provides one of the strongest cases illustrating the connection between star formation and dynamics in disk galaxies.

An analysis of parameters of the 566 young star clusters in the 35 late type spiral and irregular galaxies is presented. The disposition, the sizes and the colors of the star clusters are studied, based on the multicolor surface photometry data. The ages of the young star clusters are estimated using evolutionary synthesis method. A problem of taking into account the dust contribution is discussed. Most of the clusters are located in the Inner Lindblad Resonance ring and in other resonance rings. In some ring galaxies the distances between neighbouring star clusters are similar. For some of investigated galaxies, the star clusters belonging to a given galaxy have similar ages. A correlation between age and size of star forming regions exists. The younger objects have smaller average sizes. There are no single star forming regions with sizes > 550 pc. In general, the star forming regions in a given galaxy have similar sizes. A correlation between maximal diameter of star clusters and absolute blue magnitude of parental galaxy exists.

The simultaneous multiple constraints of the IMF and SFR history based on the observed UBV R colours, Lyman continuum fluxes and chemical abundances Z resolve the IMF-SFR ambiguity in 100 extragalctic star formation complexes in 20 galaxies (SFCs). The separate study of SFCs with different SFR history shows links between IMF parameters and observed properties of SFCs as well as local and global properties of a host galaxy. The SFR history depends on the star density of SFCs. Low density regions demonstrate instantaneous starburst, while in high density SFCs extended star formation bursts are detected. Constrained IMF slopes are in a good agreement with direct observations of IMFs in OB associations of the Milky Way and LMC.

Moderately deep ground based UBV images are used to investigate the OB associations and star clusters in IC 1613. Fifty eight OB associations with sizes between 30 and 130 pc are outlined. The iteration between associations and superbubbles in IC 1613 was analyzed. The lack of star clusters was confirmed.

We show that X-ray emission and dynamical properties of superbubbles around OB associations are affected by metal ejection from the enclosed Type II supernovae (SNe). The SN and massive star yields may significantly change the superbubble interior metallicity and enhance its X-ray luminosity.

We present new infrared observations of the low-metallicity blue compact galaxy SBS 0335-052. We find that the NIR emission is consistent with star formation occurring in clusters younger than 5 Myr. We detect optically hidden star formation at 4 μm obscured by more than 15 mag. of visual extinction. We quantify the possible contribution from an evolved stellar population and from warm dust. We finally discuss the possible origin of the dust observed.

We present an analysis of super star cluster populations using WFPC 2 on the Hubble Space Telescope for two starburst galaxies: NGC 7673 and Markarian 8. Both galaxies are luminious and have signatures of massive star formation, such as emission-line dominated optical spectra. Their optical appearances are highly disturbed, featuring prominent knots of intense star formation. Using HST F255W, F555W, and F814W images, we perform aperture photometry for a select sample of star clusters in different regions of each galaxy, and compare the results with evolutionary population synthesis models.

As part of an ongoing program to better understand the early stages of massive star cluster evolution and the physical conditions required for their formation, we have obtained 10 μm (N-band) images with Gemini North of the nuclear region of the starburst galaxy He 2-10. Five massive star clusters still enshrouded in their natal cocoons with no optical counterparts were previously discovered by Kobulnicky & Johnson (1999) as optically thick thermal radio sources. Three of these five radio sources have strong 10 μm detections in only 10 minutes of integration time with Gemini. The blackbody temperatures of the dust cocoons are estimated to range from ∼ 40 — 150 K at their outer and inner edges, and the mass of these dust shells is ∼ 107 M⊙. The ages of the embedded stellar clusters must be < 106 years. The bolometric luminosities of the exciting clusters can be constrained to ∼ 108–9 L⊙, and the implied masses are > 106 M⊙. These three embedded clusters are responsible for at least 60% of the IRAS flux of the entire galaxy He 2-10.

We summarize recent results from our study of the massive young super star clusters (SSCs) in the dwarf galaxy NGC 1569, and present new high-resolution NIRSPEC spectra that permit the measurement of a cluster's intrinsic stellar velocity dispersion. Thus we derive kinematic masses for the two brightest SSCs: 2.3 × 105 Msun for SSC B, and 3.9 × 105 and 4.4 × 105 Msun for the two components of SSC A.

We present early results from the analysis of HST imaging observations for several pairs of interacting galaxies. We include two cases that were specifically chosen to represent a strong early (young) encounter and a weak late (old) encounter. The goals of the project include a determination of the timing, frequency, strength, and characteristics of the young star clusters formed in these two limiting cases of tidal encounters.

Using new HST imaging, we identify a large, evolved system of super star clusters in a disk region just outside the starburst core in the prototypical starburst galaxy M82, “M82 B.” This region has been suspected to be a fossil starburst site in which an intense episode of star formation occurred over 100 Myr ago, which is now confirmed by our derived age distribution. It suggests steady, continuing cluster formation at a modest rate at early times (> 2 Gyr ago), followed by a concentrated formation episode ∼ 600 Myr ago and more recent suppression of cluster formation. The peak episode coincides with independent dynamical estimates for the last tidal encounter with M81.

We derive a dynamical mass of 1.2 ± 0.1 × 106 M⊙ for the young SSC M82-F and determine that the current (LV/M)⊙ is a factor of ∼ 5 higher than that predicted by a standard Kroupa (2001) IMF. This suggests that M82-F has a deficit of low mass stars and will not survive to become an old globular cluster.