Cosmological Effects of the First Stars: Evolving Spectra of Population III

Abstract

The first stars hold intrinsic interest for their uniqueness and for their potentially important contributions to galaxy formation, chemical enrichment, and feedback on the intergalactic medium (IGM). Although the sources of cosmological reionization are unknown at present, the declining population of large bright quasars at redshifts z > 3 implies that stars are the leading candidates for the sources that reionized the hydrogen in the IGM by z $ 6. The metal-free composition of the first stars restricts the stellar energy source to protonproton burning rather than the more efficient CNO cycle. Consequently, they are hotter, smaller, and have harder spectra than their present-day counterparts of finite metallicity. We present new results from a continuing study of metal-free stars from a cosmological point of view. We have calculated evolving spectra of Population III clusters, derived from a grid of zero-metallicity stellar evolutionary tracks. We find that H-ionizing photon production from metal-free stellar clusters takes twice as long as that of Population II to decline to 1/10 its peak value. In addition, metal-free stars produce substantially more photons than Population II in the He ii (E > 4 ryd) continuum. We suggest that large Ly equivalent widths (W Ly > 400 Å ) may provide a means of detecting metal-free stellar populations at high redshift and that He ii recombination lines (1640, 4686) may confirm identifications of Population III. While Population III clusters are intrinsically bluer than their Population II counterparts, nebular continuum emission makes up this difference and may confuse attempts to discern Population III stars with broadband colors. In a companion paper, we explore the consequences of evolving spectra of Population III for the reionization of the IGM in both H and He.

High-redshift Ly$\mathsf{\alpha}$ emitters with a large equivalent width

“…In particular, the equivalent width (EW) of the Lyα emission provides clues on the stellar population of high-z galaxies, for which it is generally difficult to investigate stellar spectral features due to the limited observational sensitivity of currently available instruments. Malhotra & Rhoads (2002) present their theoretical estimates that large Lyα equivalent widths (EW 0 > ∼ 150 Å for a Salpeter initial-mass function (IMF) and EW 0 > ∼ 240 Å for a flatter IMF) suggest a very young (<10 7 years) stellar population (e.g., Tumlinson et al 2003). More interestingly, it has been theoretically predicted that galaxies hosting zero-metallicity stars (or Population III stars, hereafter PopIII) should show huge Lyα equivalent widths that could reach up to EW 0 ∼ 1000 Å (e.g., Schaerer 2002Schaerer , 2003Tumlinson et al 2003;Scannapieco et al 2003).…”

First stars and the extragalactic background light: how recentγ-ray observations constrain the early universe

“…Emissivities for primordial/zero metallicity (ZM) stars are based on the stellar models from Tumlinson et al (2003) and Tumlinson (2006). Tumlinson et al (2003) for a synthetic stellar cluster have been constructed from evolutionary tracks.…”

“…Tumlinson et al (2003) for a synthetic stellar cluster have been constructed from evolutionary tracks. Tumlinson (2006) connected present day observational data, like the galactic metal distribution function, with properties of the PopIII stars via galaxy chemical evolution models.…”

“…2 top panel. Nuclear burning in ZM stars occurs at higher temperatures than in PopII stars (Tumlinson et al 2003), which results in a large number of H and He ionizing photons in the UV. High mass stars have a short lifetime, so the overall emission of a high mass stellar population is significantly reduced at stellar population ages 0.1 Gyr.…”

“…They burn at higher temperatures than their successors (Tumlinson et al 2003), producing copious amounts of H and He ionizing photons. The nebula absorption and emission can therefore severely alter the SPS spectra (Santos et al 2002;Schaerer 2002;Fernandez & Komatsu 2006).…”