Astron. Astrophys. 344, L67-L70 (1999)

4. Star-forming regions near GRB 990123

In order to examine the substructure near the OT we used DAOPHOT II (Stetson 1987) to subtract the light from the OT. We constructed a point-spread function (PSF) using STIS CCD observations of a star field in the Galactic globular star cluster Omega Centaurus. These images were taken with the STIS in the 50CCD (clear) imaging mode as part of the Cycle 7 proposal 7079 and were combined and drizzled in exactly the same way that the images of GRB 990123 were (output scale /pixel, "pixfrac" ). Fig. 2 shows the STIS image of the region around the OT with the light from the OT subtracted.

Fig. 2. This is the same image as Fig. 1 except the OT has been subtracted. The location of the OT is indicated by the circle. Three knots are visible to the north and northwest of the OT (see Table 1). Knots #2 and #3 were identified as A2 and A1 respectively by Bloom et al. (1999b). The scale is /pixel, north is to the top of the image, and east is towards the left.

The host galaxy appears to be a large irregular galaxy with several non-axisymmetric components. The total AB magnitude of the galaxy (excluding the OT) is . Castro-Tirado et al. (1999) find a flat spectrum () consistent with a starburst galaxy. Adopting this spectrum, and assuming no internal reddening in the galaxy, we find and for the host galaxy, corresponding to fluxes of µJy and µJy respectively. The total luminosity is . There is a large amount of unresolved light around the galaxy, and an arc-like feature can be seen ( kpc) west of the nucleus. This structure is ( kpc) long and ( kpc) wide with a bright knot (#3 A1 [Bloom et al. 1999b]) at its southern end. Two more small knots can be seen to the north (#2 A2 [Bloom et al. 1999b]) and northwest of the OT (#1). The locations, and distances from the OT, for these knots are listed in Table 1. The final column gives the Bloom et al. (1999b) identifications. The magnitudes, fluxes, and rest-frame V-band luminosities for each knot are listed in Table 2. The uncertainties in the magnitudes and fluxes are mag and nJy respectively. The uncertainties in the total luminosities are .

Table 1. The locations of possible star-forming regions near the OT.

Table 2. Properties of the possible star-forming regions near the OT.

The central regions of all three knots are well fit by the PSF, which suggests that the knots are centrally concentrated. The drizzled image has a resolution of /pixel ( kpc/pixel). Therefore, we conclude that most of the light is concentrated in the inner 0.2 kpc of each knot. This is comparable with the sizes of star-forming regions and H II regions in the local Universe. The PSF does not, however, provide a good fit to the outer regions of the knots. There is excess light left in the images of each knot after the scaled PSFs have been subtracted. This suggests that the knots consist of dense, centrally concentrated cores embedded in extended structures. The apparent full-width at half-maximum (FWHM) of each knot is while the PSF has FWHM . Correcting the apparent knot diameters for the width of the PSF gives intrinsic FWHMs of ( kpc) for the knots. This is consistent with the knots containing embedded star-forming regions.

Knot #1 is partially covered by the OT and not clearly visible until the OT is subtracted. We believe that this knot is a real feature and not an artifact of the PSF subtraction since similar features are not seen when the PSF is used to subtract stars from the Omega Centaurus images. Fig. 2 shows no systematic change in intensity between the northeast half of the knot, which was not obscured by the OT, and the southwest half, which was obscured by the OT. This also suggests that the knot is not an artifact of the PSF subtraction. To test the ability of PSF subtraction to reveal structure under the OT we generated a series of artificial stars with the same magnitude as the OT. These stars were put on the three knots, and in empty parts of the image near the OT, then PSFs were fit and subtracted for each artificial star. We found that the knots were clearly visible after the artificial stars were removed, although the knots located under artificial stars appeared less centrally concentrated after the artificial stars were subtracted. Comparing the recovered magnitudes of the isolated artificial stars with those of the artificial stars situated on the knots suggests that we are able to detect knots with that are located under the OT.

The OT for GRB 990123 is located on the southeast edge of knot #1. This knot is the most likely source of the metallic absorption lines seen in the spectra of the OT (Andersen et al. 1999; Kulkarni et al. 1999), although we can not rule out the possibility that the absorption lines are due to a very small, undetected, faint knot located under the OT. Such absorption systems are often associated with high column densities of hydrogen, which in turn are associated with star formation. We used Eq. 2 of Madau et al. (1998) to estimate the star-formation rate (SFR) in each of the knots listed in Table 1, assuming a flat () spectrum. For a rest-frame wavelength of Å corresponds to an observed wavelength of Å, so we computed the observed flux at Å by extrapolating between the V- and R-band fluxes. These were computed for each knot in the same manner as was done for the host galaxy (see Sect. 3). The estimated SFRs for each knot, assuming a Salpeter initial mass function, are listed in Table 2. For a Scalo initial mass function multiply the SFR by 1.55. The uncertainty in each SFR is yr^-1. These SFRs assume that there is no dust, or obscured star formation, in the knots. This is probably a poor assumption if the knots are star-forming regions. Therefore, our derived SFRs should be considered a lower limit on the true SFR in each knot.

The OT is located at a projected distance of ( kpc) from the nucleus of the host galaxy. Bloom et al. (1998) calculated that % of the GRBs from NS-NS and BS-NS progenitors in galaxies with a shallow gravitational potential will occur within 5 kpc of the nucleus of the host galaxy and % will occur within 30 kpc. This is consistent with the location of the OT relative to the nucleus of the host galaxy. If the progenitor is a failed supernova, or a hypernova, then the OT should be located within a few hundred parsecs of the star-forming region (Paczynski 1998). The GRB 990123 OT is located at a projected distance of ( kpc) southeast of the centre of the nearest knot (#1), which is larger than the expected separation if the GRB was due to the explosion of a massive star, yet consistent with the NS-NS and BS-NS hypotheses. We wish to stress that this conclusion depends on there being no faint star-forming region directly under the OT. Further observations will be needed, after the OT has faded, to determine if there are other small star formation regions that are currently hidden by the OT.

© European Southern Observatory (ESO) 1999

Online publication: March 29, 1999
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