Super Star Clusters Research Articles

The Red Stellar Population in NGC 1569

We present HST NICMOS photometry of the resolved stellar population in the dwarf irregular galaxy NGC 1569. The color-magnitude diagram (CMD) in the F110W and F160W photometric bands contains ~2400 stars with a formal photometric error 0.1 mag down to mF110W ≈ 23.5 and mF160W ≈ 22.5. The fiducial photometry has a completeness factor higher than 50% down to mF110W ≈ 21.5 and mF160W ≈ 20.0. We describe the data processing required to calibrate the instrumental peculiarities of NICMOS. Two different packages (DAOPHOT and StarFinder) for PSF-fitting photometry are used to strengthen the photometric results in the crowded stellar field of NGC 1569. The resulting CMD is discussed in terms of the major evolutionary properties of the resolved stellar populations. For a distance modulus of (m - M)0 = 26.71 and a reddening of E(B-V) = 0.56, our CMD samples stars down to ~0.8 M⊙, corresponding to look-back times of more than 15 Gyr (i.e., an entire Hubble time). This is a clear indication of star formation activity in NGC 1569 spanning an entire Hubble time. The metallicity of the reddest red giant branch (RGB) stars is in better agreement with Z = 0.004 as measured in H II regions than with Z = 0.0004, as expected from the stellar ages. However, the presence of—yet undetected—very metal-poor stars embedded in the stellar distribution around mF110W = 22.75 and mF110W - mF160W = 1.15 is not ruled out. The youngest stars (50 Myr) are preferentially found around the two central super star clusters, whereas the oldest population has a more uniform spatial distribution. A star formation rate per unit area of 1 M⊙ yr-1 kpc-2 and a mass formed in stars of ~ 1.4 × 106 M⊙ in the last 50 Myr are derived from the CMD. The near-infrared (NIR) CMD places strong constraints on the lower limit of the onset of star formation in NGC 1569. The exceptionally high crowding in the NICMOS images of NGC 1569 is a challenge for photometric analysis. As a result, optical and NIR images of NGC 1569 sample different populations and cannot be cross-correlated. Nevertheless, we demonstrate the consistency of the star formation histories derived from the optical and NIR CMDs.

The Star Clusters in the Starburst Irregular Galaxy NGC 1569

We examine star clusters in the irregular, starburst galaxy NGC 1569 from HST images. In addition to the two known super star clusters, we identify 45 other clusters that are compact but resolved. Integrated UVI colors of the clusters span a large range, and suggest that ages range from 3 Myrs to 1 Gyr. However, most of the clusters were formed at the tail end of the recent starburst. Numerous clusters in addition to the know super star clusters are similar in luminosity to a small globular cluster. We examined the radial surface brightness of four of the clusters. Their half-light radii and core radii are in the range observed in present-day globular clusters. Therefore, conditions that produced the recent starburst have also been those necessary for producing compact, bright star clusters. We examine resolved stars in the outer parts of the two super star clusters. Cluster A is dominated by bright blue stars with a small population of red supergiants. Sub-components A1 and A2 have similar colors and a two-dimensional color map does not offer evidence that one component is dominated by red supergiants and the other not. The contradiction of the presence of red super- giants with Wolf-Rayet stars may instead not be a contradiction at all since there coexistence in a coeval population is not inconsistent with the evolution of massive stars. Cluster B is dominated by red supergiants, and this is confirmed by the presence of the stellar CO absorption feature in an integrated spectrum. The various age indicators are consistent with a picture in which cluster B is of order 10--20 Myrs old, and cluster A is >4-5 Myrs old.

Circumnuclear Star Formation in the Early-Type Resonance Ring Barred Spiral Galaxy NGC 1326

We present multiband WFPC2 images of the nuclear ring of NGC 1326, an early-type southern barred spiral in the nearby Fornax galaxy cluster. The ring is a typical example of a 1 kpc–sized star-forming ring located in the region of the inner Lindblad resonances with the weak primary bar. The images reveal nearly 1000 point sources in and around the ring. Those sources brighter than MV = -9 are probably massive young clusters, while the fainter sources may include a mix of extremely luminous stars and fainter clusters. From an analysis of reddening-free parameters and two-color plots, we find evidence for a spread in ages of ring clusters, from less than 5 Myr to at least 200 Myr. The older still lie within the nuclear ring, with no evidence of migration of the ring being found over this time period. The luminosity function reveals no having an (uncorrected) absolute magnitude MV brighter than -11, and even after correction for extinction the most luminous cluster identified has only M = -12.6. The ring seems to lack the super–star clusters (SSCs) seen in starburst systems and in other nuclear rings, and the analysis suggests that SSCs are not a universal property of these rings. Complex dust structure is found inside the ring, south of the nucleus, and extinction is especially severe on the west side of the ring. An Hα image reveals hundreds of H II regions and emission complexes in the nuclear ring, but there is little correlation between these H II regions and the observed continuum sources, most likely owing to reddening and age differences in the ring.

ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] Observations of H[CLC]e[/CLC] 2-10: Outflows and Young Super–Star Clusters

We use Hubble Space Telescope (HST) WFPC2 optical images and GHRS ultraviolet spectroscopy to examine the recent star formation history and properties of the Wolf-Rayet galaxy He 2-10. The high spatial resolution afforded by HST has allowed the identification of a number of starburst knots, or super–star clusters (SSCs). The spatial morphology newly revealed by these HST images exhibits a prominent dust lane between the starburst regions known as A and B and a spur of material to the northwest of the nuclear region. Broadband photometry of the SSCs confirms that these objects have ages as young as a few Myr and masses up to 105 M☉. Narrowband Hα photometry of the SSCs indicates large equivalent widths for nearly half of them, consistent with ages less than 10 Myr. Since an appreciable fraction of SSCs are this young, the star formation history necessarily must be strongly peaked and not continuous. GHRS UV spectra were obtained for sections of the two starburst regions and were used to determine the parameters of the starburst regions. For a stellar population produced in a single burst with stellar masses between 1 and 100 M☉, the UV luminosities indicate regions A and B have masses between 1.6–2.6 × 106 M☉ and 2.6–6.6 × 104 M☉, respectively. The total starburst mass was between 1 and 100 M☉ consistent with the UV luminosity ranges between 1.6 and 2.6 × 106 M☉ and 2.6 and 6.6 × 104 M☉ for regions A and B, respectively. The observed integrated Hα flux is similar to that predicted by the models, implying that the leakage and/or dust absorption of Lyman continuum photons from the starburst regions is small. The integrated Hα equivalent widths of the two regions are substantially smaller than predicted. We attribute this to contamination by the continuum in the starburst regions (the starburst knots do not uniformly fill the GHRS apertures). The UV spectroscopy of starburst region A indicates a large-scale outflow of the interstellar medium of He 2-10 with speeds of at least 360 km s-1 and a total mass of at least 106 M☉. We estimate that the mass in metals which He 2-10 will expel is enough to raise primordial abundances in ≈1 Mpc3 to 10-3 to 10-2 solar.

Circumnuclear Supernova Remnants and H [CSC]ii[/CSC] Regions in NGC 253

Archival VLA data have been used to produce arcsecond-resolution 6 and 20 cm images of the region surrounding the nuclear 200 pc (~15'') starburst in NGC 253. Twenty-two discrete sources stronger than 0.4 mJy have been detected within ~2 kpc (~3') of the galaxy nucleus; almost all these sources must be associated with the galaxy. None of the radio sources coincides with a detected X-ray binary, so they appear to be due to supernova remnants and H II regions. The region outside the central starburst has a derived radio supernova rate of 0.1 yr-1 and may account for at least 20% of the recent star formation in NGC 253. Most of the newly identified sources have steep, nonthermal radio spectra, but several relatively strong thermal sources also exist, containing the equivalent of tens of O5 stars. These stars are spread over tens of parsecs and are embedded in regions having average ionized gas densities of ~20–200 cm-3, much lower than in the most active nuclear star-forming regions in NGC 253 or in the super–star clusters seen in other galaxies. The strongest region of thermal emission coincides with a highly reddened area seen at near-infrared wavelengths, possibly containing optically obscured H II regions.

Recent Star Formation in Several Galaxies of the Tidally Disturbed System HCG 31

High-resolution (07 to 08) Hα images of the Hickson Compact Group 31 (HCG 31) obtained with the WIYN telescope are used in conjunction with broadband optical images from the Hubble Space Telescope (HST) to examine the star formation history and properties of the component galaxies. The high spatial resolution of the WIYN telescope has allowed us to identify a large number of starburst regions from their Hα emission, which traces the recent star birth activity. The HST images of galaxies E and F reveal more detail within the starburst regions in which we have identified numerous super star clusters (SSCs). Photometry of these starburst regions and SSCs in the Hα and optical images indicates that there has been a substantial amount of star formation throughout HGC 31 over the past 10 Myr. The Hα equivalent widths suggest activity within the group as recently as a few megayears ago. There is evidence that galaxy F, the youngest member of the group, is possibly undergoing its initial episode of star formation, as no underlying stellar population has yet been detected.

B Stars as a Diagnostic of Star Formation at Low and High Redshift

We have extended the evolutionary synthesis models by Leitherer et al. (1999b) by including a new library of B stars generated from the IUE high-dispersion spectra archive. We present the library and show how the stellar spectral properties vary according to luminosity classes and spectral types. We have generated synthetic UV spectra for prototypical young stellar populations varying the IMF and the star formation law. Clear signs of age effects are seen in all models. The contribution of B stars in the UV line spectrum is clearly detected, in particular for greater ages when O stars have evolved. With the addition of the new library we are able to investigate the fraction of stellar and interstellar contributions and the variation in the spectral shapes of intense lines. We have used our models to date the spectrum of the local super star cluster NGC1705-1. Photospheric lines of CIII1247, SiIII1417, and SV1502 were used as diagnostics to date the burst of NGC 1705-1 at 10 Myr. We have selected the star-forming galaxy 1512-cB58 as a first application of the new models to high-z galaxies. This galaxy is at z=2.723, it is gravitationally lensed, and its high signal-to-noise Keck spectrum show features typical of local starburst galaxies, such as NGC 1705-1. Models with continuous star formation were found to be more adequate for 1512-cB58 since there are spectral features typical of a composite stellar population of O and B stars. A model with Z =0.4Z_solar and an IMF with alpha=2.8 reproduces the stellar features of the 1512-cB58 spectrum.

Synthetic Spectra of H Balmer and He i Absorption Lines. II. Evolutionary Synthesis Models for Starburst and Poststarburst Galaxies

We present evolutionary stellar population synthesis models to predict the spectrum of a single-metallicity stellar population, with a spectral sampling of 0.3 A in five spectral regions between 3700 and 5000 A. The models, which are optimized for galaxies with active star formation, synthesize the profiles of the hydrogen Balmer series (Hβ, Hγ, Hδ, H8, H9, H10, H11, H12, and H13) and the neutral helium absorption lines (He I λ4922, He I λ4471, He I λ4388, He I λ4144, He I λ4121, He I λ4026, He I λ4009, and He I λ3819) for a burst with an age ranging from 106 to 109 yr, and different assumptions about the stellar initial mass function (IMF). Continuous star formation models lasting for 1 Gyr are also presented. The input stellar library includes non-LTE absorption profiles for stars hotter than 25,000 K and LTE profiles for lower temperatures. The temperature and gravity coverage is 4000 K ≤ Teff ≤ 50,000 K and 0.0 ≤ log g ≤ 5.0, respectively. The metallicity is solar. It is found that the Balmer and He I line profiles are sensitive to the age, except during the first 4 Myr of the evolution, when the equivalent widths of these lines are constant. At these early stages of the evolution, the profiles of the lines are also sensitive to the IMF. However, strong H Balmer and He I lines are predicted even when the low-mass cutoff of the IMF is as high as 10 M☉. The equivalent widths of the Balmer lines range from 2 to 16 A and those of the He I lines from 0.2 to 1.2 A. During the nebular phase (cluster younger than about 10 Myr), Hβ ranges from 2 to 5 A and He I λ4471 ranges between 0.5 and 1.2 A. The strength of the lines is maximum when the cluster is a few hundred (for the Balmer lines) and a few tens (for the He I lines) of Myr old. In the continuous star formation scenario, the strength of the Balmer and He I lines increases monotonically with time until 500 and 100 Myr, respectively. However, the lines are weaker than in the burst models owing to the dilution of the Balmer and He I lines by the contribution from very massive stars. The high spectral resolution of the profiles is useful to reproduce the absorption wings observed in regions of recent star formation and to estimate the effect of the underlying absorption on the nebular emission lines. The strength of the nebular emission Balmer and He I lines compared with the stellar absorption components indicates that Hδ and the higher order terms of the Balmer series and He I are dominated by the stellar absorption component if an instantaneous burst is older than 5 Myr. Some of the He I lines (e.g., He I λ3819 and He I λ4922) are more favorable than others (e.g., He I λ4471) for the detection of stellar features in the presence of nebular emission. We estimate that the correction to the He I λ4471 nebular emission line due to the stellar absorption is between 5% and 25%, if the nebular emission has equivalent width between 10 and 2.5 A (corresponding to a burst age between 1 and 3 Myr). The models can be used to date starburst and poststarburst galaxies until 1 Gyr. They have been tested on data for clusters in the LMC, the super-star cluster B in the starburst galaxy NGC 1569, the nucleus of the dwarf elliptical NGC 205 and a luminous E+A galaxy. The full data set is available for retrieval at our websites or on request from the authors.

The [ITAL]BHK[/ITAL] Color Diagram: A Tool to Study Young Stellar Populations

A new method to derive age differences between the various super–star-clusters observed in starburst galaxies using the two-color diagram B-H versus H-K is presented. This method offers a quick and easy way to differentiate very young and intermediate-age stellar populations even if data on extinction are unavailable. In this case, discrimination of regions younger and older than 4 Myr is feasible. With the availability of data on extinction, the time resolution can be improved significantly. The application of the method to the starbursting system Arp 299 is presented. The validity of the method is confirmed by comparing the equivalent width of the Hα line with the chronological map of the northern part of NGC 3690.

A Super–Star Cluster in NGC 253: Mid‐Infrared Properties

We observed the nearby starburst galaxy NGC 253 in the mid-infrared to obtain a three-dimensional data set with arcsecond angular resolution and 0.2 km spectral resolution. The observations show the major spectral features in the upper half of the mid-IR window: the 11.3 km polycyclic aromatic hydro- carbon (PAH) line and the 12.8 km (Ne II) line as well as the broad silicate absorption feature at 9.7 km. We use the (Ne II) line to determine the emission measure of the ionized gas, and in combination with radio observations to predict the thermal and nonthermal contributions to the radio continuum. The amount of ionized gas is related to the rate of star formation. Based on the mid-IR spectra, we identify three major components in the nucleus of NGC 243: an AGN in the center of the galaxy, a superstar cluster also seen in optical images, and a larger scale diUuse envelope composed of an older population of supernova remnants and lower mass stars. Subject headings: galaxies: individual (NGC 253) ¨ galaxies: starburstgalaxies: star clusters

Stellar populations and ages of M82 super star clusters

We present high signal-to-noise optical spectra of two luminous super star clusters in the starburst galaxy M82. The data for cluster F and the nearby, highly reddened cluster L were obtained with the William Herschel Telescope (WHT) at a resolution of 1.6A. The blue spectrum (3250-5540A) of cluster F shows features typical of mid-B stars. The red spectra (5730-8790A) of clusters F and L show the Ca II triplet and numerous F and G-type absorption features. Strong Ca II and Na I interstellar absorption lines arising in M82 are also detected, and the 6283A diffuse interstellar band appears to be present. The quality of the WHT spectra allows us to considerably improve previous age estimates for cluster F. By comparing the blue spectrum with theoretical model cluster spectra using the PEGASE spectral synthesis code (Fioc & Rocca-Volmerange 1997), we derive an age of 60+/-20 Myr. The strength of the Ca II triplet is also in accord with this age. Cluster L appears to have a similar age, although this is much less certain. The measured radial velocities for the two clusters differ substantially, indicating that they are located in different regions of the M82 disk. Cluster F appears to be deep in M82, slightly beyond the main starburst region while the highly obscured cluster L lies near the outer edges of the disk. We derive an absolute V magnitude of -16.5 for F indicating that it is an extremely massive cluster. The presence of such a luminous super star cluster suggests that the M82 starburst experienced an episode of intense star formation approximately 60 Myr ago.

The Very Young Starburst Merger System NGC 1741

We use Hubble Space Telescope (HST) Faint Object Camera (FOC) ultraviolet (UV) and WFPC2 optical images in conjunction with UV spectroscopic observations taken with the Goddard High Resolution Spectrograph to examine the star formation history and properties of the interacting galaxy system NGC 1741 in the Hickson Compact Group 31. The high spatial resolution afforded by HST has allowed us to identify a large number of starburst knots, or super?star clusters (SSCs), in the starburst regions of this system. Photometry of these SSCs in the UV and optical bands indicates that most of these objects have ages of a few Myr, with a few up to ?100 Myr, and masses between 104 and 106 M?. The estimated age is confirmed by a spectral synthesis analysis of one knot for which we have obtained a UV spectrum. The V-band luminosity function of the SSCs is well represented by a power law (L) ~ L-? with an index of -1.85, with no evidence of a turnover brighter than the completeness limit. These properties are in good agreement with those found for SSCs in other starburst galaxies. Our results support the suggestion that some of these SSCs may be extremely young globular clusters formed in a relatively recent starburst episode that has been triggered by a merger event.

Massive Stars in the Quintuplet Cluster

We present near-infrared photometry and K-band spectra of newly-identified massive stars in the Quintuplet Cluster, one of the three massive clusters projected within 50 pc of the Galactic Center. We find that the cluster contains a variety of massive stars, including more unambiguously identified Wolf-Rayet stars than any cluster in the Galaxy, and over a dozen stars in earlier stages of evolution, i.e., LBV, Ofpe/WN9, and OB supergiants. One newly identified star is the second ``Luminous Blue Variable'' in the cluster, after the ``Pistol Star.'' Given the evolutionary stages of the identified stars, the cluster appears to be about 4 \pm 1 Myr old, assuming coeval formation. The total mass in observed stars is $\sim 10^3 \Msun$, and the implied mass is $\sim 10^4 \Msun$, assuming a lower mass cutoff of 1 \Msun and a Salpeter initial mass function. The implied mass density in stars is at least a few thousand $\Msun pc^{-3}$. The newly-identified stars increase the estimated ionizing flux from this cluster by about an order of magnitude with respect to earlier estimates, to 10^{50.9} photons/s, or roughly what is required to ionize the nearby ``Sickle'' HII region (G0.18 - 0.04). The total luminosity from the massive cluster stars is $\approx 10^{7.5}$ \Lsun, enough to account for the heating of the nearby molecular cloud, M0.20 - 0.033. We propose a picture which integrates most of the major features in this part of the sky, excepting the non-thermal filaments. We compare the cluster to other young massive clusters and globular clusters, finding that it is unique in stellar content and age, except, perhaps, for the young cluster in the central parsec of the Galaxy. In addition, we find that the cluster is comparable to small ``super star clusters.''

The star formation histories of two Wolf-Rayet galaxies: NGC 1741 and He2-10. Two butterflies in a collection

In an effort to better understand how the properties of star formation in starburst galaxies depend on various environmental parameters, I present a comparison between two well-known WR galaxies: the interacting galaxy system NGC 1741 in the Hickson Compact Group 31, and the dwarf galaxy He2-10. The high spatial resolution of HST has allowed identification of a large number of starburst knots, or ‘super star clusters’ in these WR galaxies. Broad-band photometry and the latest stellar synthesis models are used to estimate the ages and masses of the super star clusters. The properties of the clusters are then used to compare and contrast the overall star-formation histories in the two WR galaxies.

Are LINERs Starbursts or Mini-quasars? A Comparative Study of their Ultraviolet Spectra

Low Ionization Nuclear Emission Line Regions (LINERs) are found in ˜30% of all bright galaxies. The nuclear luminosities in these objects are such that they can be produced by a number of mechanisms and there have been heated debates on the nature of ionizing sources in LINERs. The variety of ionizing mechanisms suggested are low luminosity AGNs, starbursts, shocks, or any combination of these. We have studied Hubble Space Telescope (HST) ultraviolet (UV) spectra of seven LINERs having compact nuclear UV sources.The picture emerging from this comparison is that the compact source observed in these LINER galaxies, at least in some cases, is a nuclear star cluster rather than a low-luminosity active galactic nucleus (AGN). In these cases, the UV luminosity is driven by tens of thousands of O-type stars, depending on the assumed extinction for these objects. The O-stars could be the high-mass end of a bound stellar population, similar to those seen in super star clusters. Our data do not exclude the possibility that a similar stellar continuum source could dominate in all the LINERs. Alternatively, there may be two types of UV-bright LINERs: those where the UV continuum is produced by a starburst, and those where it is nonstellar.The “clearly-stellar”, weak [O I] emitters, LINERs have relatively weak X-ray emission, and their stellar populations probably provide enough ionizing photons to explain the observed optical emission-line flux. The other LINERs, strong [O I] emitters, have severe ionizing photon deficits, for reasonable extrapolations of their UV spectra beyond the Lyman limit, but have an X-ray/UV power ratio that is higher by two orders of magnitudes than that of the “clearly-stellar” LINERs. A component which emits primarily in the extreme-UV may be the main photoionizing agent in these objects.Recent results show that nuclear-starburst and quasar-like activity are often intermingled. Our results extend this result to the lower luminosities of the LINERs.

Models for Wolf-Rayet and O star populations in starbursts: new developments and applications to large samples of Wolf-Rayet galaxies

We summarise recent developments on synthesis models for massive star populations with a particular emphasis on Wolf-Rayet stars. Quantitative analyses of the stellar content of WR galaxies are reviewed. Comparing observations of WR galaxies from various samples with synthesis models we derive constraints on their burst properties. The observations indicate very short burst periods and are generally compatible with a Salpeter IMF and a large upper mass cut-off. The use of the Hβ equivalent width as an age indicator works well for WR galaxies. We briefly summarise comparisons of stellar populations in super star clusters which also provide useful contraints on evolutionary models for massive stars e.g., at very low metallicities inaccessible in the Local Group. In particular the observed WN and WC populations favour the high mass loss models of Meynet et al. (1994). Finally we review recent work on the origin of nebular He II emission. From the new catalogue of WR galaxies and high excitation HII regions of Schaerer et al. (1999) we find a close relation between the presence of nebular He II and WR stars. The analysis of individual WR galaxies including IZw 18 supports the suggestion of Schaerer (1996) that hot WR stars are responsible for the hard ionizing flux.

Super star clusters in the Galactic Center as revealed by HST-NICMOS

The three massive clusters in the Galactic Center are not only the most massive young clusters in the Galaxy, but they harbor more Wolf-Rayet stars than any other starburst region in the Local Group. An understanding of their stellar content will be valuable for extending models to starburst regions in other galaxies. We present HST-NICMOS images, luminosity functions, and color-magnitude diagrams of two of these: the Quintuplet and Arches clusters. The images allow the detection of stars over 6 magnitudes fainter than ever before and reveal previously undetected multiple star systems. For the first time, we clearly identify the main sequence in the Quintuplet cluster, and we extend earlier detections of the main sequence in the Arches cluster to Minitial < 10 M⊙. We estimate that the Arches cluster has an initial mass function slope which is greater than the Salpeter value. Given their stellar content, the Galactic Center clusters provide both the best nearby examples of super star clusters and the best nearby locale in which to investigate WR phenomena in starburst galaxies and galactic nuclei. We discuss the content of the Galactic Center clusters, with a particular emphasis on how they compare to other massive clusters of the local group. We expect that many of the massive stars in the Galactic Center will soon evolve to become WR stars, and eventually become supernovae at a rate of ∼ 1 per 20 000 years for the next several Myr. We note that our preliminary N-body simulations suggest that such dense clusters are short-lived in the strong tidal field of the Galactic Center, consistent with the fact that no older dense clusters are seen in the central 50 pc. This implies a star formation rate of 5(10−3) M⊙ yr−1 in the Galactic Center.

Synthesis models for starburst populations with Wolf-Rayet stars

The prospects of utilizing Wolf-Rayet populations in starburst galaxies to infer the stellar content are reviewed. I discuss which WR star features can be detected in an integrated stellar population. Specific examples are given where the presence of WR stars can help understand galaxy properties independent of the O-type star population. I demonstrate how populations with small age spread, such as super star clusters, permit observational tests to distinguish between single-star and binary models to produce WR stars. Different synthesis models for WR populations are compared. Predictions for WR properties vary dramatically between individual models. The current state of the models is such that a comparison with starburst populations is more useful for improving WR atmosphere and evolution models than for deriving the star-formation history and the initial mass function.

The near-infrared colours of H II galaxies

We present preliminary results of our near-infrared JHK images of a sample of H II galaxies observed at UKIRT. The presence of knots of enhanced emission in the near-IR may indicate the existence of luminous super star clusters which will serve to unveil the basic units which constitute the starburst phenomenon in galaxies. Based on near-IR colours we will further discuss the history of star formation in these bursting dwarf galaxies.

The stellar content of the Wolf-Rayet galaxy NGC 5253 from ISO-SWS spectroscopy

Wolf-Rayet galaxies are a subset of blue emission-line galaxies, whose spectra show the signature of large numbers of WR stars, with ages in the range 1-10 x 106 yr. Mid infra-red observations are well suited to the study of their hot, massive stars since this spectral region contains many fine-structure nebular lines which depend very sensitively on stellar content. The observations of ISO have opened-up this window, which we are exploiting through a Guest Observer program with the Short Wavelength Spectrometer (sws), that includes NGC 5253, a nearby (4 Mpc) WR galaxy. NGC 5253 contains several young super-star clusters, no older than a few million years and has been the focus of many recent studies which have shown that the extinction is very high and patchy in the UV and optical. ISO observations of NGC 5253 are particularly important since the interstellar extinction at mid-IR wavelengths is low and because of its remarkably hot and young stellar population.