Variability of sub-mJy radio sources

We present 1.4 GHz VLA observations of the variability of radio sources in the Lockman Hole region at the level of > 100 uJy on timescales of 17 months and 19 days. These data indicate that the areal density of highly variable sources at this level is < 0.005 arcmin^{-2}. We set an upper limit of 2% to the fraction of 50 to 100uJy sources that are highly variable (> 50%). These results imply a lower limit to the beaming angle for GRBs of 1deg, and give a lower limit of 200 arcmin^2 to the area that can be safely searched for GRB radio afterglows before confusion might become an issue.Comment: aastex 2 postscript figures. to appear in the Astrophysical Journa

Searching for high-redshift centimeter-wave continuum, line and maser emission using the Square Kilometer Array

We discuss the detection of redshifted line and continuum emission at radio wavelengths using a Square Kilometer Array (SKA), specifically from low-excitation rotational molecular line transitions of CO and HCN (molecular lines), the recombination radiation from atomic transitions in almost-ionized hydrogen (radio recombination lines; RRLs), OH and water maser lines, as well as from synchrotron and free-free continuum radiation and HI 21-cm line radiation. The detection of radio lines with the SKA offers the prospect to determine the redshifts and thus exact luminosities for some of the most distant and optically faint star-forming galaxies and active galactic nuclei (AGN), even those galaxies that are either deeply enshrouded in interstellar dust or shining prior to the end of reionization. Moreover, it provides an opportunity to study the astrophysical conditions and resolved morphologies of the most active regions in galaxies during the most active phase of star formation at redshift z~2. A sufficiently powerful and adaptable SKA correlator will enable wide-field three-dimensional redshift surveys at chosen specific high redshifts, and will allow new probes of the evolution of large-scale structure (LSS) in the distribution of galaxies. The detection of molecular line radiation favours pushing the operating frequencies of SKA up to at least 26 GHz, and ideally to 40 GHz, while very high redshift maser emissions requires access to about 100 MHz. To search for LSS the widest possible instantaneous field of view would be advantageous.Comment: 12 pages, 2 figures. To appear in "Science with the Square Kilometer Array," eds. C. Carilli and S. Rawlings, New Astronomy Reviews (Elsevier: Amsterdam

Intensity Mapping of Molecular Gas During Cosmic Reionization

I present a simple calculation of the expected mean CO brightness temperature from the large-scale distribution of galaxies during cosmic reionization. The calculation is based on the cosmic star formation rate density required to reionize, and keep ionized, the intergalactic medium, and uses standard relationships between star formation rate, IR luminosity, and CO luminosity derived for star-forming galaxies over a wide range in redshift. I find that the mean CO brightness temperature resulting from the galaxies that could reionize the universe at z = 8 is T_B ∼ 1.1(C/5)(fesc/0.1)−1μK, where fesc is the escape fraction of ionizing photons from the first galaxies and C is the IGM clumping factor. Intensity mapping of the CO emission from the large-scale structure of the star-forming galaxies during cosmic reionization on scales of order 102 to 103 deg^2, in combination with H i 21 cm imaging of the neutral IGM, will provide a comprehensive study of the earliest epoch of galaxy formation