6 Conclusions

We have presented an intensive UBVRIZJsHKs broad band photometry of the GRB 000210 host galaxy which has allowed us to determine its photometric redshift. The derived photometric redshift is z=0.842^+0.054_-0.042, in excellent agreement with the spectroscopic redshift ( $z=0.8463\pm 0.0002$) proposed by Piro et al. (2002). The inferred redshift is basically independent of the extinction law and IMF assumed, although (at least in the particular case of GRB 000210) the Scalo (1986) IMF provides slightly worse results than Miller & Scalo (1979) and Salpeter (1955) IMFs. The SED of the host galaxy is consistent with a starburst template with an age of $\sim$0.181 Gyr and a very low extinction ( $A_{\rm V} \sim 0$). Based on the restframe UV flux a star formation rate of $2.1 \pm 0.2 \ M_{\odot}$ yr^-1 is estimated.

The absolute restframe B-band magnitude of the host ( M_B = -20.16) is consistent with the distribution of the M_B host galaxy values measured to date (see Djorgovski et al. 2001, Fig. 2). We determine a value of $L = 0.5 \pm 0.2\ L^{\star}$ for the luminosity of the host, in agreement with the value estimated by Piro et al. (2002). We have tried to the explore the role played by galactic interactions triggering the GRB phenomena. Many host galaxies observed to date appear as part of complex and interacting systems (GRB 980613, Hjorth et al. 2002; GRB 001007, Castro Cerón et al. 2002). According to our study the GRB 000210 host galaxy is a subluminous galaxy with no interacting companions above $0.18 \pm 0.06 \ L^{\star}$.

The low value of the extinction obtained in the SED fit ( $A_{\rm V} \sim 0$) makes difficult to explain the optical darkness of GRB 000210 in terms of the global host galaxy dust extinction. If dust extinction is the reason of the lack of optical afterglow emission, then the circumburst region has to be very compact and localised around the progenitor. This hypothesis would agree with observations carried out for the optically-faint GRB 990705. Andersen et al. (2002) have localised the optically dim GRB 990705 (but NIR bright, see Masetti et al. 2000) in a face-on spiral galaxy. Thus given the thin disk of a spiral galaxy ($\sim$0.3 kpc), the optical extinction of GRB 990705 can not be attributed to the global ISM in its host. This clumpy and fragmented ISM would also explain the apparent discrepancy between our SFR estimate (derived from the galaxy UV flux) and the one recently reported based on the sub-millimeter range (Berger et al. 2003; Barnard et al. 2003).

Several progenitor models have been discussed in order to explain the inferred stellar population age and the low host galaxy extinction. Both the collapsar and the binary merging models show severe limitations to explain the visible stellar age and the line of sight H I column density (derived from the afterglow X-ray spectrum) respectively. A solution to this problem would be the existence of a younger population of stars (several Myr of age) hidden by the clumpy ISM. Such population (which would include the progenitor massive star) would not have any impact in the host galaxy SED. Morphological information derived by HST could verify the proposed ISM clumpy scenario present in the host galaxy of the dark GRB 000210.

Acknowledgements

J. Gorosabel acknowledges the receipt of a Marie Curie Research Grant from the European Commission. This work was supported by the Danish Natural Science Research Council (SNF). J.M. Castro Cerón acknowledges the receipt of a FPI doctoral fellowship from Spain's Ministerio de Ciencia y Tecnología. J.U. Fynbo acknowledges support from the Carlsberg Foundation. We thank our referee L. Piro for fruitful comments. The observations presented in this paper were obtained under the ESO Large Programmes 165.H-0464(E), 165.H-0464(I) and 265.D-5742(C).