The rise of the first stars: Supersonic streaming, radiative feedback, and 21-cm cosmology

Abstract

Understanding the formation and evolution of the first stars and galaxies represents one of the most exciting frontiers in astronomy. Since the universe was filled with hydrogen atoms at early times, the most promising method for observing the epoch of the first stars is to use the prominent 21-cm spectral line of hydrogen. Current observational efforts are focused on the cosmic reionization era, but observations of the pre-reionization cosmic dawn are also beginning and promise exciting discoveries. While observationally unexplored, theoretical studies predict a rich variety of observational signatures from the astrophysics of the early galaxies that formed during cosmic dawn. As the first stars formed, their radiation (plus that from stellar remnants) produced feedback that radically affected both the intergalactic medium and the character of newly-forming stars. Lyman-α radiation from stars generated a strong 21-cm absorption signal, observation of which is currently the only feasible method of detecting the dominant population of galaxies at redshifts as early as z ∼ 25. Another major player is cosmic heating; if due to soft X-rays, then it occurred fairly early (z ∼ 15) and produced the strongest pre-reionization signal, while if it is due to hard X-rays, as now seems more likely, then it occurred later and may have dramatically affected the 21-cm sky even during reionization. In terms of analysis, much focus has gone to studying the angle-averaged power spectrum of 21-cm fluctuations, a rich dataset that can be used to reconstruct the astrophysical information of greatest interest. This does not, however, diminish the importance of finding additional probes that are complementary or amenable to a more model-independent analysis. Examples include the global (sky-averaged) 21-cm spectrum, and the line-of-sight anisotropy of the 21-cm power spectrum. Another striking feature may result from a recently recognized effect of a supersonic relative velocity between the dark matter and gas. This effect enhanced large-scale clustering and, if early 21-cm fluctuations were dominated by small galactic halos, it produced a prominent pattern on 100 Mpc scales. Work in this field, focused on understanding the whole era of reionization and cosmic dawn with analytical models and numerical simulations, is likely to grow in intensity and importance, as the theoretical predictions are finally expected to confront 21-cm observations in the coming years.

Constraining the intergalactic medium at z ≈ 9.1 using LOFAR Epoch of Reionization observations

“…Although we know that reionization took place, very few facts about it are known with certainty (see e.g. Morales & Wyithe 2010;Pritchard & Loeb 2012;Zaroubi 2013;Barkana 2016, for reviews).…”

First Results on the Epoch of Reionization from First Light with SARAS 2

“…INTRODUCTION The Epoch of Reionization (EoR), beginning with first light from the first stars and ultra-faint galaxies, and ending with almost complete reionization of the primordial gas, marks an important period in the cosmic evolution of baryons (Barkana & Loeb 2001;Ciardi & Ferrara 2005;Zaroubi 2013;McQuinn 2016;Haiman 2016;Barkana 2016). There is considerable uncertainty and limited observational constraints on the astrophysical evolution in this period, including the nature of the first sources of light, and the thermal and ionization state of the intergalactic medium (IGM).…”

Extracting the Global Signal from 21-cm Fluctuations: the Multi-Tracer Approach

“…where x is the comoving coordinate, TS is the spin temperature of the 21-cm transition (e.g., Barkana 2016, and references therein), TCMB = 2.725(1 + z) K is the CMB temperature at redshift z, and τ21 is the 21-cm optical depth given by…”

“…The 21-cm signal produced by neutral hydrogen atoms in the Intergalactic Medium (IGM) is predicted to be a powerful probe of the early Universe at z 6 and is complementary to the direct observations of bright sources (see Barkana 2016, for a recent review of the 21-cm line). The brightness temperature of the 21-cm line is driven by the thermal and ionization histories of the gas as well as by the Ly-α radia-tion of the first stars (Wouthuysen-Field effect, Wouthuysen 1952;Field 1958).…”