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Register a new userFirst Light Sources at the End of the Dark Ages: Direct Observations of Population III Stars, Proto-Galaxies, and Supernovae During the Reionization Epoch
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The cosmic dark ages are the mysterious epoch during which the pristine gas began to condense and ultimately form the first stars. Although these beginnings have long been a topic of theoretical interest, technology has only recently allowed the beginnings of observational insight into this epoch. Many questions surround the formation of stars in metal-free gas and the history of the build-up of metals in the intergalactic medium: (1) What were the properties of the first stellar and galactic sources to form in pristine (metal-free) gas? (2) When did the epoch of Population III (metal-free) star formation take place and how long did it last? (3) Was the stellar initial mass function dramatically different for the first stars and galaxies? These questions are all active areas of theoretical research. However, new observational constraints via the direct detection of Population III star formation are vital to making progress in answering the broader questions surrounding how galaxies formed and how the cosmological properties of the universe have affected the objects it contains.
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Population III star formation during the dark ages shifted from minihalos (~10^6 Msun) cooled via molecular hydrogen to more massive halos (~10^8 Msun) cooled via Ly-alpha as Lyman-Werner backgrounds progressively quenched molecular hydrogen cooling. Eventually, both modes of primordial star formation were suppressed by the chemical enrichment of the IGM. We present a comprehensive model for following the modes of Population III star formation that is based on a combination of analytical calculations and cosmological simulations. We characterize the properties of the transition from metal-free star formation to the first Population II clusters for an average region of the Universe and for the progenitors of the Milky Way. Finally, we highlight the possibility of observing the explosion of Population III stars within Ly-alpha cooled halos at redshift z~6 in future deep all sky surveys such as LSST.
The highly neutral inter-galactic medium (IGM) during the Epoch of Reionization (EoR) is expected to suppress Ly$alpha$ emission with damping-wing absorption, causing nearly no Ly$alpha$ detection from star-forming galaxies at $z{sim}8$. However, spectroscopic observations of the 4 brightest galaxies (${rm H}_{160}{sim}25$ mag) at these redshifts do reveal prominent Ly$alpha$ line, suggesting locally ionised IGM. In this paper, we explore the Ly$alpha$ IGM transmission and environment of bright galaxies during the EoR using the Meraxes semi-analytic model. We find brighter galaxies to be less affected by damping-wing absorption as they are effective at ionizing surrounding neutral hydrogen. Specifically, the brightest sources (${rm H}_{160}{lesssim}25.5$ mag) lie in the largest ionized regions in our simulation, and have low attenuation of their Ly$alpha$ from the IGM (optical depth ${<}1$). Fainter galaxies (25.5 mag${<}{rm H}_{160}{<}27.5$ mag) have transmission that depends on UV luminosity, leading to a lower incidence of Ly$alpha$ detection at fainter magnitudes. This luminosity-dependent attenuation explains why Ly$alpha$ has only been observed in the brightest galaxies at $z{sim}8$. Follow-up observations have revealed counterparts in the vicinity of these confirmed $z{sim}8$ Ly$alpha$ emitters. The environments of our modelled analogues agree with these observations in the number of nearby galaxies, which is a good indicator of whether Ly$alpha$ can be detected among fainter galaxies. At the current observational limit, galaxies with ${ge}2$--5 neighbours within $2{times}2$ are ${sim}2$--3 times more likely to show Ly$alpha$ emission. JWST will discover an order of magnitude more neighbours, revealing ${gtrsim}50$ galaxies in the largest ionizing bubbles and facilitating direct study of reionization morphology.
We investigate how the hydrostatic suppression of baryonic accretion affects the growth rate of dark matter halos during the Epoch of Reionization. By comparing halo properties in a simplistic hydrodynamic simulation in which gas only cools adiabatically, with its collisionless equivalent, we find that halo growth is slowed as hydrostatic forces prevent gas from collapsing. In our simulations, at the high redshifts relevant for reionization (between ${sim}6$ and ${sim}11$), halos that host dwarf galaxies ($lesssim 10^{9} mathrm{M_odot}$) can be reduced by up to a factor of 2 in mass due to the hydrostatic pressure of baryons. Consequently, the inclusion of baryonic effects reduces the amplitude of the low mass tail of the halo mass function by factors of 2 to 4. In addition, we find that the fraction of baryons in dark matter halos hosting dwarf galaxies at high redshift never exceeds ${sim}90%$ of the cosmic baryon fraction. When implementing baryonic processes, including cooling, star formation, supernova feedback and reionization, the suppression effects become more significant with further reductions of ${sim}30%$ to 60%. Although convergence tests suggest that the suppression may become weaker in higher resolution simulations, this suppressed growth will be important for semi-analytic models of galaxy formation, in which the halo mass inherited from an underlying N-body simulation directly determines galaxy properties. Based on the adiabatic simulation, we provide tables to account for these effects in N-body simulations, and present a modification of the halo mass function along with explanatory analytic calculations.
Long wavelength spectral distortions in the Cosmic Microwave Background arising from the 21-cm transition in neutral Hydrogen are a key probe of Cosmic Dawn and the Epoch of Reionization. These features may reveal the nature of the first stars and ultra-faint galaxies that transformed the spin temperature and ionization state of the primordial gas. SARAS~2 is a spectral radiometer purposely designed for precision measurement of these monopole or all-sky global 21-cm spectral distortions. We use 63~hr night time observing of the radio background in the frequency band 110-200~MHz with the radiometer deployed at the Timbaktu Collective in Southern India to derive likelihoods for plausible redshifted 21-cm signals predicted by theoretical models. First light with SARAS 2 disfavors the class of models that feature weak X-ray heating (with $f_X leq 0.1$) and rapid reionization (with peak $frac{dT_b}{dz} geq 120~textrm{mK per unit redshift interval}$ ).
While most simulations of the epoch of reionization have focused on single-stellar populations in star-forming dwarf galaxies, products of binary evolution are expected to significantly contribute to emissions of hydrogen-ionizing photons. Among these products are stripped stars (or helium stars), which have their envelopes stripped from interactions with binary companions, leaving an exposed helium core. Previous work has suggested these stripped stars can dominate the LyC photon output of high-redshift low luminosity galaxies. Other sources of hard radiation in the early universe include zero-metallicity Population III stars, which may have similar SED properties to galaxies with radiation dominated by stripped star emissions. Here, we use two metrics (the power-law exponent over wavelength intervals 240-500 r{A}, 600-900 r{A}, and 1200-2000 r{A}, and the ratio of total luminosity in FUV wavelengths to LyC wavelengths) to compare the SEDs of simulated galaxies with only single-stellar evolution, galaxies containing stripped stars, and galaxies containing Population III stars, with four different IMFs. We find that stripped stars significantly alter the SEDs in the LyC range of galaxies at the epoch of reionization. SEDs in galaxies with stripped stars present have lower power-law indices in the LyC range and lower FUV to LyC luminosity ratios. These differences in SEDs are present at all considered luminosities ($M_{UV} > -15$, AB system), and are most pronounced for lower luminosity galaxies. We also find that SEDs of galaxies with stripped stars and Pop III stars are distinct from each other for all tested IMFs.
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