2 GRB 970228 and the nature of its host galaxy

We have determined the local galactic extinction toward the GRB 970228 field by comparing galaxy counts in two bands in this field to those in the HDF, and by comparing the observed broad band colors of stars in the GRB 970228 field to the colors of library spectra of the same spectral type. We also estimate the extinction using the neutral hydrogen column density and the amount of infrared dust emission toward this field. Combining the results of these methods, we find a best-fit galactic extinction in the optical of A_V=1.09^+0.10_-0.20, which implies a substantial dimming and change of the spectral slope of the intrinsic GRB 970228 afterglow. Further details can be found in Castander & Lamb (1998, 1999a); see also González et al. (1999).

Re-analyzing the HST images, we measure a color $(V_{606}-I_{814})_{\rm ST} = -0.18^{+0.51}_{-0.61}$ for the extended source. We constrain the nature of the likely host galaxy of GRB 970228 by comparing this color to those obtained from synthetic galaxy spectra computed with PEGASE (Fioc & Rocca-Volmerange 1997), taking into account the measured extinction. The top panel of Fig. 1 shows the expected colors of an Sa, an Sc and an Irregular galaxy with and without evolution included. Galaxies are only consistent with the observed color if they are at high redshift (), or have active star formation (like the evolving Irr shown).

  

Figure 1: WFPC2 colors for synthesized Spectral Energy Distributions (SEDs) versus redshift. The top panel shows the expected colors for spirals and irregular galaxies including a local extinction of AV=1.09. The thick lines include a K-correction, the thin lines include evolutionary plus K-corrections. The thick horizontal line is the best value observed color and the horizontal dotted lines its $1\sigma$ error. The bottom panel shows models of 1 Gyr (solid line), 100 Myr (dotted line) and 10 Myr (dashed line) starbursts observed at redshift z. The dot-dashed line represents a 1 Gyr starburst observed at redshift z, 200 Myr after the cessation of star formation. The cessation of star formation, with the resulting disappearance of the most massive stars, produces redder colors incompatible with the observations

The bottom panel of Fig. 1 better illustrates this point. It shows that on-going bursts of star formation of duration shorter than 1 Gyr produce acceptable V₆₀₆-I₈₁₄ colors; longer duration bursts are disfavored for redshifts . If we include the H and K magnitudes of Fruchter et al. (1998) in our analysis, our conclusions are strengthened (see Castander & Lamb 1999b for further details). We conclude that the host galaxy must be undergoing star formation, in agreement with our earlier result (Castander & Lamb 1998; see also Castander & Lamb 1999b). If there is any extinction present due to the host galaxy, this conclusion would be strengthened.

If the extended source is a galaxy with ongoing star formation, strong emission lines are expected. Tonry et al. (1997) and Kulkarni et al. (1997) have tried to obtain the spectrum of the GRB 970228 afterglow and its associated nebulosity. Neither observation revealed any obvious emission lines. The lack of observed [OII] and Ly${\alpha}$ emission lines suggests that the redshift of the galaxy may lie in the range , considering the spectral coverage of the observations.