HST Images and Grism Spectrum of GRB 030329

A color image of the field of GRB 030329. The WFC/ACS images were taken without dithering and only a section of one chip was read-out. The field is approximately 100" across. North is approximately to the right. This image was taken on the 15 April, when the underlying supernova was near its peak.

An enlarged view of the region around the GRB. The region shown is about 20" across. This image was taken during our third epoch on the 12 May. The OT is seen at the center. The faint blue extension of the OT to the lower left is the host.

GRB 030329 has been observed at three epochs by HST: 15/16 April, 21/23 April and 12/13 May. During all epochs UV (ACS/HRC), optical (ACS/WFC) and NIR (NICMOS) images were obtained. Additionally, in the first epoch an ACS grism spectrum was taken, and in the second epoch a STIS optical spectrum, delayed from the first epoch by a gyroscope failure, was also obtained. Here we report on aspects of the imaging and grism spectroscopy which provide a unique contribution from HST: information on the nature of the host.

All the ACS images show the host extending to approximately ~0."5 from the OT to the west, with a PA of approximately 230 degrees. In the most recent ACS/WFC images the OT has faded sufficiently to allow a reasonable subtraction of the OT from the host, particularly in the F435W and F606W filters where the blue color of the host provides greater contrast against the redder OT than in the F814W image. We find the magnitude of the host to be approximately V=22.7 +/- 0.3, where the uncertainty is dominated by the subtraction of the OT and the unknown contribution to the host magnitude by faint outerlying areas. The mean apparent magnitude corresponds to an absolute magnitude of about -16.5, very similar to that of the SMC.

At left an enlarged view of the ACS/WFC F435W (blue) image taken on 12 May. At right a point source has been subtracted from the location of the OT. This is a maximal subtraction -- subtracting a brighter point source produces negative residuals. This image may therefore underestimate the true brightness of the host at the location of the optical transient. The "stretch" of the image at the right has been increased to better show the fainter host. The two images are vertically aligned.

The host can also be seen clearly in the UV ACS/HRC F250W images taken on May 12. The OT appears to lie at the end of a bar-like structure approximately 0."25 across, which at the ~600 Mpc angular-diameter distance of the GRB corresponds to a length of about 750 pc.

At left an enlarged view of the ACS/HRC F250W (UV) image taken on 12 May. The image was drizzled with a final pixel size of approximately 0."018. At right is the same image after convolution with a 3x3 boxcar, to bring out the faint extended structure. No attempt has been made to subtract the point source.

The separation of the host from the OT can also be discerned in the grism spectrum, where the emission lines of the host are seen to be offset from the spectral continuum of the OT. Although the spatial projection of the grism makes this somewhat uncertain, the Halpha emission of the host appears to be fairly well centered on host's blue light.

Top: The ACS two-dimensional grism spectrum of GRB 030329 taken on the 15 April, when the underlying supernova is believe to have been near peak. The spectrum extends from approximately 550 to 1100 nm. Bottom: The two-dimensional spectrum with the continuum subtracted. The Halpha emission of the host is clearly visible as a region of emission below the (now largely removed) OT spectrum. Emission from [O III] can also be discerned at the left.