Proceedings of the Workshop
"The Magellanic Clouds and Other Dwarf Galaxies"
of the Bonn/Bochum-Graduiertenkolleg

Multiband Analysis of the Brightness Distribution

of a Sample of Blue Compact Dwarf Galaxies

L.M. Cairós and J.M. Vílchez

Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain

Received 14th March 1998

1. Introduction and Goals

Blue Compact Dwarf galaxies (BCD) are low luminosity (MB≥-18) systems where intense star formation is presently occurring, as evidenced by their blue colours and their optical spectra, which exhibit strong narrow emission lines superimposed on a nearly featureless stellar continuum. In these objects, formation rates of massive stars range between 0.1 and 1 Msun yr-1 (Fanelli et al. 1988). Such rates mean that the current burst can not last more than ≅108 yr before depleting the gas supply (typically 108 Msun, Thuan & Martin 1981). This fact, together with the low metal content found in BCDs - values ranging between 1/5 to 1/40 Zsun - supports the generally accepted idea that star formation in these objects is episodic.

We are involved in an extensive study of a sample of BCDs. Our work includes aspects related with the morphology, structure, stellar content and star-formation histories of the galaxies. The first step will be to determine the morphology and structure of the BCD by deriving surface brightness profiles, SBP, and color profiles. We report here on the preliminary results of this analysis.

The observations were obtained at the German 2.2 m and 3.5 m telescopes at Calar Alto, Almería, the 2.5 m Isaac Newton Telescope and 4.2 m William Herschel telescopes at the Roque de los Muchachos Observatory, La Palma and at the Danish 1.5 m telescope at ESO. We have obtained data for a total of ∼40 galaxies.

2. Surface Photometry: Luminosity Profiles

Since the BCDs present irregular morphologies, we prefer not to perform the study of the SBP using the standard elliptical isophote fitting. Instead, we employed an alternative method that has allowed us to obtain the surface brightness distribution for a given galaxy without any assumption about its morphology. The equivalenth radius, re, at a given intensity level, Ie, has been calculated as re=(A/π)1/2, being A defined as the area of the ensemble of pixels with I≥Ie, where I is the intensity level of each pixel.

In Fig. 1 the SBP of 3 objects of our sample are shown. They are displayed over three radial coordinates r, r1/4 and log (r) to allow direct comparison with the three standard analytical profiles representative of galactic light distribution:

[Click here to see Fig. 1!]

3. Results

The main conclusions obtained from the preliminary analysis of the SBP are:
I) All the BCDs of our sample are well fitted by an exponential function at low surface brightness levels (below a turn off which appears aproximatly in the range 23 to 25 mag arcsec-2).

II) Attempting to the appearance of the SBP at high surface levels (where high means over the low surface brightness level component) we can distinguish three major groups of objects:
a) Objects whose profiles are well fitted by an exponential function.
b) Galaxies showing profiles that can be well described by a R1/4 or a power-law function.
c) Objects which present an extra structure at intermediate intensity levels, and cannot be described by a single fitting law.

III) Those galaxies with a dominant star formation knot show SBP belonging to types a) or b), whereas galaxies with profiles belonging to type c) tend to present a number of intense star forming knots spread over a larger region (this result is consistent with the findings of Papaderos et al. 1996).

References

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First version: 08thAugust,1998
Last update: 29thSeptember,1998
Jochen M. Braun   &   Tom Richtler
 (E-Mail: jbraun|richtler@astro.uni-bonn.de)