GRB afterglows and host galaxies at very high redshifts

GRB afterglows and host galaxies at very high redshifts Cosmology is beginning to constrain the nature of the earliest stars and galaxies to form in the universe, but direct observation of galaxies at z>6 remains highly challenging due to their scarcity, intrinsically small size, and high luminosity distance. GRB afterglows, thanks to their extreme luminosities, offer the possibility of circumventing these normal constraints by providing redshifts and spectral information which couldn't be obtained by direct observation of the hosts themselves. In addition, the association of GRBs with massive stars means that they are a tracer of star formation, and that their hosts are likely responsible for a large proportion of the ionizing radiation during that era. Our collaboration is mounting a campaign to rapidly identify and study candidate very high redshift bursts, bringing to bear a network of 2, 4 and 8m telescopes with nIR instrumentation. The capabilities of Swift to detect faint, distant GRBs, and to report accurate positions for many bursts in near real-time makes our program now feasible. HST is crucial to this endeavour, allowing us (a) to monitor the late time afterglows and hence compare them to lower-z bursts and test the use of GRBs as standard candles; and (b) characterise the basic properties, luminosities, and in some cases morphologies, of the hosts, which is essential to understanding these primordial galaxies and their relationship to other populations.

We will concentrate efforts on the NICMOS/f160w band which, while still rest-frame UV, is as red as one can go with HST with reasonable sensitivity, and therefore mitigates as much as possible against dust extinction. A range of plausible light curves (afterglow and host) is illustrated in Fig. 3. Where possible (pointing constraints permitting) we will rst make two visits 1 orbit at roughly 2 weeks and 3 weeks post burst, to monitor the post-break light curve for suf ciently bright afterglows (on suf ciently faint hosts) and to provide precise localisation of the GRB within its host. As also shown in Fig. 3, the host, if detectable at all, should dominate the likely afterglow contribution after about 3 months. At this time we will revisit to obtain a deep host observation. With 4 orbits we will reach a 5B detection limit of about H(AB)=26.5. This is below the LC -level for z=3 galaxies redshifted to z=10 and hence offers a fair chance of a successful detection. Note that late time host observations are also necessary to interpret the earlier afterglow observations, since contaminating host light must be removed. For up to 2 targets if found to be ?D E ?6.5 we will also obtain a 2 orbit ACS/WFC z-band (F850LP) exposure (reaching FHGJI $: for a z(AB)=26.8 point source) to provide more detailed colour and morphological information for the host and any other structures in its environment. Given the big uncertainties in expected numbers, and afterglow and galaxy properties, as discussed above, we request 4 triggers in cycle 14, with IR and in some cases optical followup, typically 7 orbits per trigger. This will be adequate to acquire basic information on the population of very high redshift GRB selected galaxies. GALAXIES 12 false 0.0 false false false false false SUPPORTED STAR GAMMA RAY BURSTER J mag -- 21-24 J2000 00 00 0.0000 +00 00 0.00 J2000 EXT-STAR GAMMA RAY BURSTER J2000 09 09 52.1300 +45 39 44.90 J2000 GALAXY AMORPHOUS IRREGULAR RA = 0H 0M 0S, DEC = 0d 0m 0s GALAXY AMORPHOUS IRREGULAR RA = 0H 0M 0S, DEC = 0d 0m 0s GALAXY AMORPHOUS IRREGULAR RA = 0H 0M 0S, DEC = 0d 0m 0s EXT-STAR GAMMA RAY BURSTER RA = 0H 0M 0S, DEC = 0d 0m 0s EXT-STAR GAMMA RAY BURSTER RA = 0H 0M 0S, DEC = 0d 0m 0s EXT-STAR GAMMA RAY BURSTER RA = 0H 0M 0S, DEC = 0d 0m 0s NIC3 Imaging No No F160W NIC3 Imaging No No F160W NIC3 Imaging No No F160W ACS/WFC Imaging No No F850LP NIC3 Imaging No No F160W NIC3 Imaging No No F160W NIC3 Imaging No No F160W ACS/WFC Imaging No No F850LP NIC3 Imaging No No F160W NIC3 Imaging No No F160W NIC3 Imaging No No F160W NIC3 Imaging No No F160W NIC3 Imaging No No F160W NIC3 Imaging No No F160W POS-TARG DITHER 0.265 LINE 0.187 false false 69.050003 4 20.67 ACS-WFC-DITHER-BOX 1

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