Licentiate Seminar: Hunting for the first stars and galaxies

by Mr Claes-Erik Rydberg (Astronomy)

Friday 24 Aug 2012, 13:15 → 15:00 Europe/Stockholm

FA31

This thesis investigates the potential for detection and identification of high- redshift primordial stars and galaxies. It also serves as an introduction to pop- ulation III stars, their formation and properties, the first galaxies, and to grav- itational lensing. Simulations indicate that the very first population III stars should appear in minihalos at z ≈ 10–30. To assess the detectability of these objects, we use the- oretical models for stellar atmosphere and the surrounding H ii region. The case for detection through the gravitational lens constituting of the galaxy cluster MACS J0717.5+3745 using the upcoming James Webb Space Telescope (JWST) is also considered. We find that the detection of these objects is improbable but not impossible. The detection prospects also have a strong dependence on the typical mass of the stars. To investigate the prospects of detecting and identifying the first galaxies, we introduce the spectral synthesis code Yggdrasil. According to this code, JWST may be able to detect population III galaxies with stellar masses as low as 105 M at z ≈ 10. We also find that, over limited redshift intervals, it could be possible to use JWST broadband color criteria to single out population III galaxy candidates. When considering high Lyα-emission from population III galaxies, we find them to be identifiable even with the Hubble Space Telescope (HST). We then use gravitational lensing to assess the prospects of detecting lensed population III galaxies with JWST and HST. Observations in the unlensed ultradeep fields with HST are first used to set constraints on the star formation efficiency. By comparing simulated and observed luminosity functions, we find an upper limit to the baryon fraction converted to population III stars in a host halo. The limit is of the order of 10−1 , even though a strong dependence on model parameters, such as IMF and H ii nebula coverage, is found. We then calculate the lower limit to the star formation efficiency for detection of ∼ 1 population III galaxy, including the whole HST/CLASH survey. We find it to be of the order of 10−2 . A JWST survey of MACS J0717.5+3745 will in principle be able to detect ∼ 1 population III galaxy, even if as little as ∼ 10−3 of the baryons in the host halos turn into population III stars.