6 The host galaxy

In the images obtained on October 6 the emission at the position of the OT appeared elongated although at low signal-to-noise ratio. We therefore obtained further deep imaging on October 27 and November 3 and 4. A total of 3 hours of R-band imaging was obtained under dark/grey sky conditions and a seeing varying between 1.0 and 1.3 arcsec. We subsequently combined these images using the code described in Møller & Warren (1993). The FWHM of point sources in the combined image was 1.18 arcsec. In the combined image an extended object directly underneath the position of the OT was clearly detected. A contour image showing this extended emission is shown as Fig. 6. Although the seeing was not optimal, the object is resolved into several compact knots covering a region of total extension of about 5 arcsec. This is more clearly visible in images of the host galaxy + OT obtained with the Hubble Space Telescope presented by Price et al. (2001). In order to determine the position of the OT relative to these compact knots we measured the position of the OT relative to point-sources in the vicinity in the first image obtained on September 27 and in the combined image. The position of the OT in the combined image could in this way be determined with a conservatively estimated uncertainty of 0.02 arcsec (10% of a pixel). The position of the OT is within the errors spatially coincident with one of the knots of the extended emission. As the OT, based on an extrapolation of the light-curve, is expected to be much fainter than this knot (by several magnitudes) we conclude that at least this particular knot is related to the host galaxy of GRB 000926. An underlying supernova (SN) even if similar to SN1998bw would also be much fainter than the observed knot, especially since the R-band corresponds to the rest-frame UV around 2100 Å where SNe are intrinsically faint. Whereas it is possible that all the compact knots are either part of the host galaxy or several galaxy sub-clumps in the process of merging, we cannot exclude that some of the emission is from objects lying at other distances along the line of sight (although there are no intervening absorption systems in the spectrum of the OT, Møller et al., in prep). If all the emission is from the host galaxy of GRB 000926 then the extension of the galaxy would be similar to the Ly-$\alpha$ emitting region of the galaxy S4 at z=1.93 towards Q0151+048A (Fynbo et al. 1999).

The total magnitude of the extended emission in a circular aperture with diameter 4.7 arcsec is $R=23.87\pm0.15$. Assuming that all this emission comes from the host galaxy we can get an estimate of the star-formation-rate (SFR) of the galaxy. The restframe UV continuum in the range 1500 Å-2800 Å can be used as a SFR estimator if one assume that the star-formation is continuous over a time scale of more than 10⁸ years. Kennicutt (1998) provides the relation

$${\rm SFR}(M_{\odot}\,{\rm yr}^{-1}) = 1.4\times10^{-28} \times L_{\nu},$$ (2)

where $L_{\nu}$ is the luminosity in the 1500 Å-2800 Å range measured in erg s^-1 Hz^-1. The observed R-band corresponds to the rest-frame UV continuum around 2100 Å, which falls well within this range. To derive $L_{\nu}$ we first converted our R-band magnitude to R(AB) using R(AB)=R+0.17 (Fukugita et al. 1995). Then we used the definition of the AB magnitude to derive the observed flux ( $F_{\nu} = 10^{-0.4\times(R(AB)+48.6)}$) and finally the luminosity distance in our assumed cosmology ( $d_{\rm lum}=5.28\times10^{28}$ cm) to derive $L_{\nu}$ :

$$L_{\nu} F_{\nu} \times 4 \pi d_{\rm lum}^2/(1+z) {}$$ =

$${} (1.02\pm0.15)\times 10^{29}\: {\rm erg~s}^{-1} {\rm Hz}^{-1} {},$$ =

where the factor (1+z)^-1 corrects for the fact that $L_{\nu}$ is a specific luminosity (not a bolometric luminosity). Using the relation of Kennicutt (1998) we find a SFR of 14 $M_{\odot}$ yr^-1. If the extinction derived in Sect. 5 is valid for the galaxy as a whole then we estimate an extinction in the observed R-band of $0.56\pm0.20$ mag, which means that the SFR should be increased by a factor of $\sim$1.7. This SFR is a high compared to that of the Milky Way, but it falls in the low end of the range of SFRs of Lyman-Break galaxies (Pettini et al. 1998).

Figure 4: A contour plot of a 9.5$\times$9.5 arcsec2 region centred on the probable host galaxy of GRB 000926. The image has been smoothed with a Gaussian filter with a width of 2 pixels (0.39 arcsec). The contour levels are -2, 2, 3.5, 5, 6.5, ... $\times$ 1$\sigma$ of the sky-noise, with the dotted contours being negative. East is left and north is up. The cross marks the position of the OT. The error on the position of the OT is 0.10 pixels corresponding to 0.02 arcsec.