Characterization of the intergalactic medium (IGM) and early generations of luminous objects is an exciting frontier in observational cosmology. Sometime between the redshifts of 40 and 15 (when the Universe was between 100 and 300 million years old), it is thought that the Universe transitioned from a pristine landscape, where just gravity dominated the evolution of all the matter, to a vastly more complex system in which stars abound and in which their radiation heated and ionized the cosmic gas.
The first stars were thought to be very massive, perhaps as massive as 100s of solar masses, and upon their death they likely enriched the Universe with the metals that they synthesized. This enrichment is believed to dramatically change the character of stars that formed subsequently to be more like those in our galactic neighborhood. In addition, the death of the first stars created the first black holes. These black holes may have been the seeds that grew into the supermassive black holes seen at the centers of most galaxies.
There exists no data with which to directly constrain the evolution of the IGM prior to (the “dark ages”) and during (the “first light”) the formation of the first luminous objects. Currently, the highest redshift spectroscopically confirmed galaxy, composed of millions of second-generation stars, is z = 8.6. In addition, cosmologists can study the production of ionizing photons produced by the first generations of stars by studying the cosmic microwave background (CMB). CMB observations with the WMAP satellite reveal that a lot of star formation was happening in the Universe even as early as z=10. Lastly, astronomers directly observe accreting supermassive (billion solar mass) black holes, called “quasars”, at redshifts as high as z=7. The presence of such massive black holes suggests that the first black holes must have formed long before this redshift. The radiation from the first stars and black holes impacted the intensity of the 21cm transition of the atomic hydrogen, allowing low-frequency radio observatories to study these early times. Such radio observations have the potential to dramatically enhance humanity’s knowledge about the early universe and to constrain sparser stellar populations than is possible with other techniques.
At 15 < z < 200, the gas in the IGM cooled adiabatically with the expansion of the Universe. Initially during this period, the Universe was dense enough that collisions between hydrogen atoms could excite the 21cm transition to emit at the temperature of the intergalactic gas. Because the gas temperature was colder than that of the CMB, the 21 cm line would appear in absorption against the CMB. This first absorption period ended at z=40, before the first stars formed, when the Universe was no longer dense enough for collisions to excite the 21cm line. However, with the onset of early star formation, the UV photons these stars produced leaked into the IGM and recoupled the 21cm line to the kinetic temperatures of the gas via scattering in the Rydberg states of hydrogen. This recoupling resulted in a second absorption trough, which is forecast to occur at z~20 but admittedly there is significant uncertainty in current models (e.g., Pritchard & Loeb 2010). The depth and redshifts of this absorption depend on the UV and X-ray radiation backgrounds from the first stars and black holes.
This absorption at z~20 is precisely what LEDA is being built to detect for the first time, looking through the 21cm window to glimpse a very important moment in the cosmic history. Eventually, IGM heating was extensive and prolonged enough to cause the 21cm line to transition into emission, closing this window. The sharper an absorption feature in frequency, the easier it will be for LEDA to isolate from the foreground emission, which is 10,000 times larger. However, most models predict that this absorption feature is much sharper in frequency than the foregrounds. Thus, LEDA’s measurement should provide invaluable insights into the first stars and black holes.
Adapted from contributions by M. McQuinn, G. Bernardi, L. Greenhill