arXiv:astro-ph/9701062v1 13 Jan 1997 Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 13 July 2011 (MN L A T E X style file v1.4) THE MORPHOLOGY OF HII GALAXIES Eduardo Telles 1,2⋆ Jorge Melnick 3 Roberto Terlevich 2 1. Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, U.K. 2. Royal Greenwich Observatory, Madingley Road, Cambridge CB3 0EZ, U.K. 3. European Southern Observatory, La Silla, Chile etelles@cosmos.iagusp.usp.br, jmelnick@eso.org & rjt@ast.cam.ac.uk accepted January 7, 1997 ABSTRACT We present CCD images of a sample of 39 HII galaxies taken at the Danish 1.54m telescope on La Silla. The images are used to analyse the morphology of these emission line dwarfs, and the structural properties of the knots of star formation and of the underlying galaxy. The sizes of the starbursts are measured. We propose a morpho- logical classification based on the presence or absence of signs of tails, extensions, or distorted outer isophotes. This criterion segregates the objects into two broad mor- phological types with different physical properties: the more disturbed and extended (type I) HII galaxies having larger luminosities and velocity dispersions than the more compact and regular (type II) objects. The relative position of HII galaxies and of a sample of dwarf elliptical galaxies in the [R – σ] diagram support the hypothesis of a possible evolutionary link between the two types of galaxy. Key words: HII region – galaxies: dwarf – galaxies: starburst – galaxies: structure. 1 INTRODUCTION HII galaxies are narrow emission line dwarf galaxies under- going violent star formation (Melnick, Terlevich & Eggle- ton 1985) whose spectroscopic properties are indistinguish- able from extragalactic giant HII regions in normal late type galaxies (e.g. 30 Dor in LMC, NGC 604 in M33) (Sar- gent & Searle 1970). Their high rates of star formation and low heavy element abundances imply that the star forma- tion history must be simple and episodic (i.e. few burst of short duration followed by long quiescent periods). A recent review on the global properties of HII galaxies is given by Telles (1995, and references therein). The possi- ble links of HII galaxies with other types of known dwarf galaxies have been discussed by Thuan (1983); Loose & Thuan (1985); Bothun et al. (1986); Kunth, Maurogordato & Vigroux (1988); Davies & Phillipps (1988), Drinkwater & Hardy (1991). However, no conclusive answer has been given to the questions of what these systems will resemble when the present period of violent star formation ends, or what triggered the burst. It has been suggested that in their qui- ⋆ present address: Instituto Astronˆ omico e Geof´ ısico - USP, Caixa Postal 9638, 01065-970 - S˜ ao Paulo - BRASIL escent phase HII galaxies may be related to dwarf irregulars (dI) or dwarf elliptical galaxies (dE). Bothun et al. (1986) made a comparative study of dIs and dEs in the Virgo clus- ter based on the colour distributions and structural proper- ties derived from exponential fits to the surface brightness profiles (e.g. scale length and central brightness). They con- clude that dIs are not progenitors of dEs, but they seem to form a parallel sequence of dwarf galaxies. The fading of dIs would make them very diffuse and place them below the detection threshold of photographic plates. They propose that Blue Compact Galaxies (BCG’s, of which HII galax- ies are a subset) could probably be gas-rich analog of dEs. Meurer, Mackie & Carignan (1994) have studied the struc- tural properties of the dwarf amorphous galaxy NGC 2915 and compared with the properties of NGC 1705 (Meurer, Freeman & Dopita 1992) and NGC 5253 from the work of and S´ ersic & Donzelli (1992). They find that their luminosity profiles show two components indicating the presence of two distinct stellar populations. The inner component represents the fraction of the galaxy dominated by hydrogen gas pho- toionized by the embedded massive star clusters. Its (B-R) colour profile is increasingly bluer inwards. The outer com- ponent has an exponential luminosity (also found for dE’s and dIrr’s) and a constant redder colour likely to represent c  0000 RAS