LB002	   Observing the Cosmic Dawn with the LWA
Bowman, J. (AsU), 

The formation of the first stars, galaxies, and black holes during
Cosmic Dawn is the next frontier in observational cosmology. During
this epoch, the intergalactic medium (IGM) is dominated by neutral
hydrogen gas, accessible to observations through its redshifted 21 cm
line. The thermal and ionization history of the early IGM is highly
anticipated to encode unique information about the properties of the
first luminous objects. Once the first stars form at z ≈ 30, they
produce a background of Lyman-α photons that couples the neutral
hydrogen spin temperature to the physical gas temperature in the
IGM. This causes the 21 cm line to become visible in absorption
against the warmer cosmic microwave background (CMB). Later, after the
early stars die, many will leave behind black holes that generate
X-rays through accretion. The X-rays travel long distances, depositing
their energy as heat and raising the IGM temperature. This heating
will drive the average 21 cm signal to switch into emission above the
CMB, until the neutral gas is eventually ionized during the epoch of
reionization, leaving no detectable signal.  

The evolution of these absorption and emission features in the early
IGM will be imprinted in the all-sky radio spectrum since redshift
maps to frequency for the 21 cm line. But significant observa- tional
challenges must be addressed to separate the ∼ 100 mK signal from the
dominant Galactic foreground emission. Existing efforts to measure
this signal have just recently demonstrated the first successful
results, but only for later times during the epoch of reionization at
z < 13.  

We propose a focused, two-year program utilizing a novel observing
strategy that has the potential to extend the probe of the IGM to much
earlier times of 15 < z < 50. We will use the existing first Long
Wavelength Array station (LWA1) in New Mexico, consisting of 256
dual-polarization dipole antennas, operating between 10 and 88 MHz,
that are digitally combined to form multiple beams on the sky. We will
exploit the unique beamforming capability to remove calibration
uncertainties that plague other instruments by simultaneously
targeting both science and calibrator fields. We plan to manipulate
the weighting coefficients of the individual antennas to carefully
control the frequency- dependent sidelobes of the beams, drastically
reducing the primary mechanism responsible for coupling angular sky
structure into deep radio spectra. Neither of these techniques are
possible with traditional radio telescopes. We expect their
application to lead to the first detection of the 21 cm absorption
signal at z ≈ 25, opening a new window on early star formation and the