4 Concluding remarks

We have presented the B, V, R, I photometry of 337 resolved stars in the blue compact dwarf galaxy NGC 6789. The CM diagram shows two stellar populations distinct in structure and colour: an underlying older low-surface-brightness stellar population formed previously to the present burst, and an inner high-surface-brightness young population within 150 pc from the center of the galaxy.

The tip of the first ascent RGB is used to estimate a metallicity ([Fe/H] $\sim -1$ dex) and distance of the NGC 6789 (about 2.1 Mpc).

NGC 6789 has a light distribution which is well described by combination of the Gaussian and exponential laws. Its central surface brightness is $\mu_0(V)=21.1~{\rm mag/\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\no... ... } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi\hbox{$^{\prime\prime}$ }}$, and its isophotal equivalent diameter is $\theta_{\rm 26,equiv}=2\hbox{$.\mkern-4mu^\prime$ }1$ or $D_{\rm 26,equiv}=1.2$ kpc, making it an intrinsically small high surface brightness galaxy. NGC 6789's integrated absolute magnitude within $\mu_V=25~{\rm mag/\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak... ... } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi\hbox{$^{\prime\prime}$ }}$ is $M_{V,0}=-12\hbox{$.\!\!^{\rm m}$ }36\pm0\hbox{$.\!\!^{\rm m}$ }35$. It is also quite blue: ( $(V-I)\!=\!0\hbox{$.\!\!^{\rm m}$ }67$) in its central region, but its halo much redder, so the total colour index of NGC 6789 inside the $\mu_V=25~{\rm mag/\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak... ... } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi\hbox{$^{\prime\prime}$ }}$ isophote is $(V-I)\!=\!0\hbox{$.\!\!^{\rm m}$ }90$.

The total H$\alpha$ flux of NGC 6789 is $F_{{\rm H}{\alpha}} \simeq 1.36\allowbreak 10^{-12}$ erg/(cm² s). A corresponding current star formation rate, $SFR \simeq 7.6~10^{-3} {\cal M}_{\hbox{$\odot$ }}/{\rm year}$. To account for the observed radiation of the ionized gas it is necessary about 50 O7 V stars. Such a high star formation rate could not be constant during the long time due to absence gas supply supporting this process (let us remind that H I flux was undetected which indicates an absence of large gas stock). After $\sim 10^6$ years this galaxy should turn to the another evolution phase.

Recent observations have shown that the division of dwarf galaxies into irregulars (dIrs) and ellipticals (dEs + dSphs) is often quite ambiguous. Dwarf galaxies vary widely in shape and properties and call to a revision of our current understanding of their nature and evolution.

In accordance to this results we can conclude that NGC 6789 belongs to the group of blue compact dwarf (BCD) galaxies, which have an irregular patchy blue central part with large H II regions and bright blue stars. They show no prominent nucleus while their outer regions have a smooth spheroidal shape and an old red stellar population, as dwarf ellipticals. According to the classification scheme of Loose & Thuan ([1985]), NGC 6789 may be classified of the "iE'' type, comprising $\sim\!70\%$ of BCD, which combines irregular inner isophotes with accurately elliptical outer isophotes. The intensive star formation observed in the central region of NGC 6789 contrasts with the absence of the significant young population in the outer regions and suggests a two-component (core-halo or disk-halo) structure of the galaxy. This seems to be common not only in large spirals but also in dwarfs (see for example the case of WLM; Minniti & Zijstra [1996], and Antlia; Aparicio et al. [1998]). The nearby galaxies GR 8 (Tikhonov & Drozdovsky [1998b]), NGC 4163 (Tikhonov & Karachentsev [1998a]), NGC 1705 (Meurer et al. [1992]), NGC 2915 (Meurer et al. [1996]), and UGC 1104 (Sharina et al. [1996]) are probably the most similar to NGC 6789.

The detection of an extended faint stellar underlying component (Loose & Thuan [1985], [1986]; Kunth et al. [1988]), in the majority of BCDs supports the idea that they are not truly primordial galaxies, but older LSB dwarf galaxies undergoing transient periods of star formation. In the case of NGC 6789 a continuous gas infall from the halo and its accumulation in the central part could lead to the present burst of the star formation. All accumulated gas proved to be involved in this burst. This can explain the present picture: a presence of rather large amounts of the ionized hydrogen in the central region of the galaxy despite of the lack of detected H I gas.

Acknowledgements

We are grateful to Dr. A. Aparicio for his participation in the observations with NOT and basic CCD frames reduction from this run. This work is partially supported by RFBR grant No. 97-02-17163 and INTAS-RFBR grant No. 95-IN-RU-1390. ID thanks the Special Astrophysical Observatory for its hospitality. Dr. Lequeux comments and corrections to the previous version of this paper were of invaluable help. This research has made use of the NASA/IPAC extragalactic database (NED) which is operated by the Jet Propulsion Laboratory, Caltech, under contract with the National Aeronautics and Space Administration.