A few hundred thousand years after the Big Bang, objects of a million solar masses and larger began to condense out of the universal expansion and collapse under their own gravity. Larger objects took longer to collapse, so there was probably a hierarchy where galaxies built up from the coalescence of smaller structures. If galaxies were formed in this rather violent manner, then protogalaxies at redshifts between 1 and 10 should resemble the disturbed starburst irregular galaxies seen at the present day at low redshift. It is reasonable to imagine that protogalaxies underwent a series of starburst events as successive clouds of material fell into the deepening protogalactic potential.A piece of the Hubble Deep Field
Recent evidence supports this scenario. High-resolution images made with the Hubble Space Telescope show that the faint blue galaxies that are detectable only in long observations on large optical telescopes are indeed irregular in form. Since these galaxies are near the limit of photometry with large telescopes, they are beyond the limit for spectroscopy, but it is known that some are gravitationally lensed by foreground clusters of galaxies and are therefore believed to fall at redshifts between 1 and 3. At low redshifts, there are few objects which appear to be galaxies in formation. Star formation in normal galaxies, extrapolated back in time at the current rate, is insufficient to produce the quantity of stars or metals that they now contain. It is clear that sometime well after z ~ 1000, but before z ~ 1 (between a million and seven billion years after the Big Bang), galaxies formed and had relatively high star-formation rates. Exactly when they formed, how they were distributed in space, and the mechanisms of their formation remain unknown, but are at present on the verge of observational possibility.
Guhathakurta, Tyson, and Majewski (1990), Steidel et al. (1996), and Yee et al. (1996) have detected some galaxies at z ~ 3 which show relatively high rates of star formation. These objects may be the first direct observational evidence of a protogalactic age. But so far, only a handful of these high-redshift protogalaxies have been observed. Their redshift distribution and properties remain largely unknown, and there are few observational constraints on theory. Cold dark matter galaxy formation scenarios predict violent starbursts at relatively late times (z ~ 2.2; Katz, Hernquist, and Weinberg 1992). These have not been observed, nor do current observations rule them out. Potentially there are millions of observable protogalaxies, about one per square arcminute. Their number, distribution, and properties are a new and exciting field of study which will open up vast volumes of the Universe and let us look back in time more than halfway to the beginning. Any information about the formation of galaxies is fundamental, because it ties together and completes what is known about cosmology at high and low redshift and connects to every aspect of astronomy.
Statistical studies of galaxy formation activity at visual and near-infrared wavelengths are seriously complicated by the presence or possible presence of dust. Charlot and Fall (1993) have argued that there may be only a brief period at the beginning of star formation between the generation of significant Lyman-a emission and the shrouding of that light by dust. Ostriker and Cowie (1981) suggest that all objects at z > 3 may be significantly attenuated by dust. Most of the luminosity resulting from the collapse energy of galaxies and the first generations of stars may not appear at visual or near-infrared wavelengths (even in the rest frame of the distant galaxy), but may instead be reradiated by dust.
In the broadband rest-frame spectrum of normal galaxies are two roughly equal flux peaks: visible light produced by stars, and far-infrared radiation near 100 microns produced by dust and the fine-structure cooling lines of the interstellar medium. This figure shows the broadband spectrum of normal spiral galaxies at low and high redshifts. At high redshift, the rest-frame visible light peak is shifted into the near-infrared, and the rest-frame far-infrared peak is shifted into the submillimeter. In both cases, the peaks are shifted into cosmological windows coinciding with minima in zodiacal and galactic foreground light. In the case of the millimeter/submillimeter, the window is bracketed by galactic synchrotron emission which rises toward longer wavelengths and Galactic dust emission which increases steeply toward shorter wavelengths. In the case of the near-infrared, the window occurs at the minimum between the thermal emission and scattered sunlight from dust in the inner solar system.
To understand the nature of protogalaxies, it will be necessary to observe them at submillimeter, near-infrared, and visible wavelengths. The submillimeter luminosity is derived from the collapse energy of protogalactic gas and ultraviolet light from massive blue stars through kinetic and radiative heating of dust and gas in the interstellar medium. For objects at redshifts less than about 3 or 4, the K-band infrared light is a robust measure of the light from the later type stars which account for most of the galaxian mass. The ratios of the intensities in various infrared and visible bands can give additional information about stellar populations and dust absorption. Galaxies and protogalaxies at redshifts between 1 and 5 all subtend about an arcsecond as seen from the Earth, because of the cosmological distance effect. At visual and near-infrared wavelengths, the arcsecond-sized diffuse patches corresponding to the highest redshift galaxies are very much fainter than the arcsecond-sized diffuse patches corresponding to foreground galaxies. At visible and near-infrared wavelengths, the deepest images are crowded with galaxies, and no more than a few percent of them have redshifts greater than 3 (Guhathakurta, Tyson, and Majewski 1990). Finding young and distant objects among this welter of images is difficult, time-consuming, and prone to selection bias.
At submillimeter wavelengths, however, high-redshift galaxies have surface brightness comparable to galaxies at lower redshift. This occurs because across the submillimeter band, the emissivity of dust falls dramatically with increasing wavelength. Moreover, starbursts associated with the rapid production of metals in the first generations of stars and the energy of the initial gravitational collapse make young galaxies orders of magnitude more luminous in their rest frames than galaxies are today, so in the submillimeter they may actually outshine their more nearby and more evolved cousins. Young galaxies at high redshift (z ~ 1 to 5) can be found with broadband photometric observations in the 350 and 450 micron atmospheric windows with large submillimeter-wave telescopes. In the submillimeter photometric bands, all low-redshift objects have steep color temperatures, whereas high-redshift objects have flat or inverted colors.
Across the electromagnetic spectrum, from X-rays to radio waves, the wavelength of highest flux density in a galaxian spectrum is the peak of the 158 micron fine-structure line of ionized carbon (C II); as much as 0.5% of the total luminosity of a galaxy can be emitted in this single spectral line (Stacey et al. 1991; Wright et al. 1991). At redshifts of 1 to 10, C II line emission is shifted into the submillimeter to wavelengths where the atmosphere at the South Pole is fairly transparent. The C II line is so bright that it will provide a way to measure the redshifts of protogalaxies that are too faint or too dust shrouded for near-infrared redshift measurements. In addition, this line may provide dynamical information on protogalaxies and the star-formation processes within them. The submillimeter may offer an efficient means through which to identify high-redshift protogalaxies from the foreground blanket of nearby galaxies.
Extragalactic submillimeter-wave observations are being pursued from the JCMT and CSO on Mauna Kea. A few nearby galaxies have so far been detected at 350 microns (e.g. Thronson; Stark et al. 1989), and detection of higher-redshift objects have been enabled by new instrumentation. A 10 meter diameter telescope instrumented with modern submillimeter-wave detector technology and operating at the South Pole in winter is capable of detecting both line and continuum radiation from protogalaxies at z ~ 2.5 in less than an hour. An L* protogalaxy produces a continuum antenna temperature of about 1 mK and a [CII] line power of about 1 K km/s at the focus of a 10 meter telescope (Stark 1997). AST/RO has already detected submillimeter spectral lines having 0.5 K km/s power (Stark et al. 1997b). Even though this power level corresponds to a significantly larger flux in the beam of a 1.7 meter telescope, detection of these lines with AST/RO demonstrates that an offset 10 meter antenna operating under the winter South Pole sky can overcome systematic noise problems at this power level and is capable of making observations of the requisite sensitivity for observing protogalaxies. This may not be true of other telescopes at other sites: the AST/RO spectra are an order-of-magnitude deeper than any published submillimeter wave lines. Both the excellent quality of the South Pole sky and the offset optics of AST/RO contribute to its sensitivity.
A 10 meter submillimeter telescope with a 20 element array receiver and 2000 channel filter bank consisting of twenty 50 MHz filters per array element would detect several hundred "normal" galaxies at z > 2 during each winter of operation, and could measure both dust continuum and CII line radiation (Stark 1997). This sample would be independent of the near-infrared brightness of the protogalaxies, and be sensitive to protogalactic activity at redshifts beyond the reach of even the largest optical telescopes. The 10 meter telescope at Pole may be the most sensitive instrument for detecting early galaxies, and will provide a valuable complement to protogalactic studies in the near-infrared.
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