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

Structure and stellar content of dwarf galaxies:

Surface photometry of dwarf galaxies in the M81 & M101 groups

Torbjørn Bremnes1, Bruno Binggeli1, and Philippe Prugniel2

1Astronomical Institute, University of Basel, Venusstrasse 7, CH-4102 Binningen, Switzerland
2Observatoire de Lyon, F-64561 St. Genis-Laval Cedex, France

Received 05th March 1998
Abstract. Dwarf galaxies are by now widely recognized as prime laboratories for the study of issues such as structure formation, galaxy evolution, star formation, and dark matter. Until now, studies of dwarf galaxies have concentrated either on clusters of galaxies, such as Virgo and Fornax, or the Local Group (LG). The clusters, although rich in dwarfs, are relatively distant. The LG dwarfs can be studied in great detail, but there are only a few of them and these are intrinsically much fainter than the known cluster dwarfs. Hence there is a gap of data and knowledge between LG and cluster dwarfs.

We have therefore started an observing program at the Observatoire de Haute Provence (OHP) to do systematic imaging of dwarf galaxies in nearby groups and the general field, based on the ``10 Mpc Catalogue'' of galaxies by Kraan-Korteweg & Tammann (1979), updated by Schmidt & Boller (1992). Our goal is to derive all relevant spectrophotometric parameters for these dwarfs of intermediate brightness and to compare them with existing data on the dwarf galaxy populations of the LG and the Virgo and Fornax clusters.

1. Photometric parameters & Parameter relations

We have imaged dwarf galaxy members and suspected members of the nearby M81 and M101 groups of galaxies in the Cousins B and R and partially in V and I bands (Bremnes et al. 1998a, 1998b; Lesaffre & Prugniel 1998). Total magnitudes, effective radii, effective surface brightnesses, as well as galaxy diameters at various isophotal levels have been derived. Best-fitting exponential parameters and B - R color gradients have also been determined for these galaxies. An example photometric parameter relation for the M81 dwarfs is shown in Fig. 1.

One clearly sees that the region between cluster dwarfs and the LG is spanned by the M81 group dwarfs. With data from the M101 group and other field and group dwarfs one will be able to study environmental dependencies in the parameter relations as well as general properties of dwarf irregulars more closely.

[Click here to see Fig. 1!]

2. Luminosity function

The luminosity function of the M81 group compared to the LF of the local group is sketched in Fig. 2. The absolute magnitude scale is based on a distance of ∼4 Mpc for the M81 group. The limit of completeness for this LF depends on the depth of the photographic survey of Börngen et al. (1982), which is not well defined. We guess a magnitude of completeness is MlimB≅-12. The data for the local LF are taken from Côté (1995). The estimated limit of completeness for the LG is MlimB≅-10.

Besides a factor of ∼2 difference in richness - the M81 group being richer than the LG - the two LF's are clearly compatible with each other.

[Click here to see Fig. 2!]

3. Future work

The collection of data from as many ``local'' dwarfs as possible will allow the exploration of parameter dependencies on the specific environments of these mainly irregular galaxies, as well as it gives a handle on the faint end of the luminosity function. Such studies have previously mostly only covered dE's in clusters or just a few irregulars.

References

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First version: 22ndJuly,1998
Last update: 08thOctober,1998
Jochen M. Braun   &   Tom Richtler
 (E-Mail: jbraun|richtler@astro.uni-bonn.de)