3 GRB host galaxies

  

Figure: Cumulative distribution of extinction-corrected R-band magnitudes of the eight GRB host galaxies identified so far (solid histogram), compared to that expected for the fiducial star-formation-rate model (solid curve), total-stellar-density model (dotted curve) and constant-comoving-volume model (dashed curve). From Hogg & Fruchter (1999)

The detection of burst X-ray and optical afterglows has led in eight cases to identification of the likely host galaxy by positional coincidence with the optical afterglow. At R = 25.5 -26, the typical R-band magnitudes of these galaxies, galaxies cover $10-15\%$ of the sky for ground-based observations, because of smearing of the galaxy images due to seeing. Therefore one expects 1/10 to 1/7 of ground-based "identifications'' to be incorrect. If we are lucky, all of the identifications made to date are correct, but if we are unlucky, one or two are incorrect. On the other hand, it is highly probable that all of the host galaxies identified from HST observations are correct (e.g., the host galaxies for GRBs 970228, 970508, 971214, and 980329), since HST images are free of the effects of seeing that bedevil observations from the ground. It is also reassuring that in two cases (GRBs 970508 and 971214), the host galaxy identified from ground-based observations has been confirmed by later HST observations.

Let me mention a related concern. Until very recently, all GRB host galaxies had $R = 25.7 \pm 0.3$, no matter what their redshift and no matter how the afterglow on which the identification is based was discovered (i.e., whether detected in the optical, NIR, or radio); that is, the R-band magnitude of the GRB host galaxy appeared to be a kind of "cosmological constant''. In contrast, if the GRB rate is proportional to the star formation rate (SFR) (see below), one expects a relatively broad distribution of R-band magnitudes for GRB host galaxies (Hogg & Fruchter 1999; Fruchter 1999; Madau 1999) (see Fig. 2). The recent discovery of the host galaxy of GRB 980703 at R=22.6 broadens the observed distribution of host galaxy R-band magnitudes, provided the identification is correct. However, it also increases the asymmetry of the R-band magnitude distribution, which exhibits a tail toward the bright end and a cutoff toward the dim. This is the opposite of what one expects if the GRB rate is proportional to the SFR.

This raises the possibility that in some cases we are merely finding the first galaxy along the line-of-sight to the burst. If so, in some cases the galaxy found may be a foreground galaxy, and the actual host galaxy may lie behind it. Or it might be that the GRB rate is not proportional to the SFR (see the discussion below). Or most likely of all, the asymmetry may merely reflect the fact that we are still very much in the regime of small number statistics. Additional confirmations and/or identifications of host galaxies using HST will resolve this question.

  

Figure: The weighted average spectrum of the host galaxy of GRB 970508. Prominent emission lines of [O II] and [Ne III] are labeled. From Bloom et al. (1998)

Castander & Lamb (1998) showed that the light from the host galaxy of GRB 970228, the first burst for which X-ray and optical afterglows were detected, is very blue, implying that the host galaxy is undergoing copious star formation and suggesting an association between GRB sources and star forming galaxies. Subsequent analyses of the color of this galaxy (Castander & Lamb 1999; Fruchter et al. 1999; Lamb et al. 1999) and other host galaxies (see, e.g., Kulkarni et al. 1998; Fruchter 1999) have strengthened this conclusion, as does the morphology and the detection of [OII] and Ly${\alpha}$ emission lines from several host galaxies (see, e.g., Metzger et al. 1997b; Kulkarni et al. 1998; Bloom et al. 1998) (see Fig. 3). The positional coincidences between several burst afterglows and the bright blue regions of the host galaxies (see Fig. 4), and the evidence for extinction by dust of some burst afterglows (see, e.g., Reichart 1998; Kulkarni et al. 1998; Lamb et al. 1999), suggests that these GRB sources lie near or in the star-forming regions themselves.

  

Figure: HST image of the afterglow and host galaxy of GRB 970508 taken 1998 August. From Fruchter (1999)

The inferred size ( kpc) and the morphology of GRB host galaxies strongly suggest that they are primarily low-mass () but not necessarily sub-luminous galaxies, because of the ongoing star formation in them (most have , but some have $L \sim L_{\rm Galaxy}$; here $M_{\rm Galaxy}$ and $L_{\rm Galaxy}$ are the mass and luminosity of a galaxy like the Milky Way). A point sometimes not fully appreciated is that, while the total star formation rate in GRB host galaxies is often modest (resulting in modest [OII] and Ly${\alpha}$ emission line strengths), the star formation per unit mass in them is very large.