Regional Meeting of the International Astronomical Union
    Puebla, Mexico 9-13 November 1998

The Near-Infrared Colours of HII Galaxies

Eduardo Telles¹, Leda Sampson¹, Mauricio Tapia², Roberto Terlevich³ & Daniel Kunth⁴

1. Observatório Nacional, Rio de Janeiro, Brasil

2. Instituto de Astronomia, UNAM, Ensenada, Mexico

3. Institute of Astronomy, Cambridge, UK

4. Institute d'Astrophysique de Paris, France

Abstract:

We present preliminary results of our near-infrared J H K images of a sample of HII galaxies observed at UKIRT. The presence of knots of enhanced emission in the near-IR may indicate the existence of luminous super stellar 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.
 keywords: Dwarf Galaxy, Starburst, Stellar Population

Introduction

HII galaxies comprise a class of small, gas-rich, metal-poor, starburst galaxies whose optical appearance is dominated by one or more high surface brightness star-forming regions. These galaxies are excellent examples of the ``starburst'' phenomenon, in which a vigorous but short-lived burst of massive star formation has recently occurred.

Despite the indications of youth, optical CCD imaging of a large sample of HII galaxies has revealed that in the majority of objects a faint extended elliptical envelope of late type stars surrounds the compact central starburst regions (e.g., Thuan 1983; Kunth et al. 1988; Papaderos et al. 1997; Doublier et al. 1997; Telles, Melnick & Terlevich 1997). Optical observations, combined with evolutionary synthesis models have shown that these faint components are probably composed of stars older than a few Gyrs (Telles & Terlevich 1997). With the use of near-infrared imaging the contamination due to the emission from the hot young stars and line emission from ionized gas of the burst will be minimized, allowing stronger constraints on their star formation histories.
 

 

Images

Figure 1 J band images of two famous examples of HII galaxies.

Our near-IR images of H II galaxies have revealed supercluster-sized objects (SSC) within the star forming regions (Figure 1) similar to those detected in HST ultraviolet images of starburst galaxies (Meurer et al. 1995; Vacca 1997, Ostlin et al.998). In many cases these knots form continuous structures, which suggests that star formation may have propagated across the starburst region.

Integrated colours of HII galaxies seem to be overall not compatible with stellar evolutionary model predictions. The infrared magnitudes and colours of individual starburst knots will greatly enhance our knowledge of their ages and stellar populations. The analysis of these data is underway and the results will be presented in a forthcoming paper.

Understanding the formation and evolution of these Super Star Clusters may play an fundamental role in the study of the starburst phenomena in galaxies.
 

references

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Telles & Terlevich 1997, Mon. Not. R. astr. Soc. 286, 183
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Vacca, W. D. 1997, in The Ultraviolet Universe at Low and High Redshift: Probing the Progress of Galaxy Evolution, ed. W. H. Waller, M. N. Fanelli, J. E. Hollis, & A. C. Danks, (New York: AIP Press), in press

1999-01-13