Early Universe Studies

The discovery of CO in the z = 2.3 galaxy IRAS F10214+4724 dramatically opened up the distant universe to mm and submm astronomy. Since then, CO has been detected in the gravitationally lensed Cloverleaf quasar at z = 2.56(Fig. 2.1a), and in several other high redshift objects. The most remarkable discovery is that large amounts of dust and CO molecules are present already at z=4.7 (Fig. 2.1b). This redshift corresponds to a look-back time of 92% of the age of the universe and shows that enrichment of the interstellar medium occured at very early epochs. These lines make it possible to estimate the mass, density, velocity spread, and kinetic temperature of the cold molecular gas. As well as the emission lines, numerous molecular lines have been detected at z = 0.3 to 0.9 in absorption against distant background radio sources. The absorption lines allow us to measure the temperature of the cosmic background radiation at intermediate redshifts (z=0.9), and may be valuable for deriving differential time delays in gravitational lenses. In the millimeter and submm bands, we can detect not only molecular lines but potentially also the atomic fine-structure lines of carbon, oxygen, and nitrogen. These lines have rest frequencies in the far infrared, but at high-z, they are redshifted into the submm bands. An advantage of the mm and submm bands over other radio bands is that for spectral lines with the same brightness temperatures and velocity linewidths, the line power varies as $\nu^2\, T_b\, \Delta \nu$, and hence as $\nu^3$. A CO(3-2) line redshifted to 100 GHz emits 3 10⁷ times more power than an H I line shifted to 400  MHz. Even if the H I line could be detected at z>2, it is redshifted to the meter band where there is high radio noise from our Galaxy, strong ionospheric effects on interferometer phases, and much man-made radio interference. Another advantage of the mm/sub-mm bands is that most molecules have a ladder of spectral lines. If a redshift is so high that a spectral line is shifted out of a given mm window, there is a good chance the next line up the ladder will be shifted into it. It is thus imperative that the receivers cover all of the mm/submm bands.

A crucial advantage is that we can study the mm and sub-mm emission from dust, which is too weak to detect at cm or meter wavelengths. At present, the mm/sub-mm continuum from dust has been detected in quasars with redshifts as large as 4.7 (Fig. 2.1b). From the dust flux, one may estimate the mass of dust and gas in the central regions of these objects. The mm/sub-mm thermal dust continuum may be one of the best tracers for finding primeval galaxies at $z \geq 5$; indeed the LSA/MMA may be the only instrument that will be capable of finding such galaxies. If starbursts injected large amounts of dust into the disks of young galaxies, the resulting far IR continuum will be detectable at high z -- the increasing distance is compensated by increased flux as the far IR bump is redshifted into the mm and sub-mm bands. Studies in these bands can potentially determine the redshift range in which most of the early-universe star formation and dust injection occurred. Tantalizing examples of this possibility are now being obtained with the SCUBA bolometer array on the James Clerk Maxwell Telescope. Figure 2.2 shows a map made with this instrument at 850 $\mu$m toward the galaxy cluster Abell 370 (Smail, Ivison, & Blain 1997, ApJ, 490, L5), and an optical image of the same patch of sky. The foreground cluster, seen in the optical image, has a redshift of 0.37, and acts as a gravitational lens that distorts and magnifies the images of distant galaxies well beyond the cluster. The prominent arc in the optical image is a distant galaxy at z=0.73.However, the objects on the submillimeter map, from optical spectra taken so far, seem to be even farther away, and are possibly dusty young protogalaxies in the early universe. With existing facilities, we are thus able to detect distant dusty protogalaxies when their emission is amplified by gravitational lenses; with the LSA/MMA, we will be able to detect them everywhere in the sky. Because of the much greater sensitivity and much higher resolution of the LSA/MMA, such results will greatly improve our knowledge of the timescales of galaxy and structure formation in the universe, and the chemical evolution as a function of redshift. The study of the early epochs of galaxy evolution is one of the main goals of a new mm/submm array, and it is one of the main reasons to have a huge collecting area.