Formation of Structure in the Universe

1.4.7 Early Structure Formation

In linear theory, adiabatic density fluctuations grow linearly with the scale factor in an Omega = 1 universe, but more slowly if Omega < 1 with or without a cosmological constant. As a result, if fluctuations of a certain size in an Omega = 1 and an Omega ₀ = 0.3 theory are equal in amplitude at the present epoch (z = 0), then at higher redshift the fluctuations in the low- Omega model had higher amplitude. Thus, structures typically form earlier in low- Omega models than in Omega = 1 models.

Since quasars are seen at the highest redshifts, they have been used to try to constrain Omega = 1 theories, especially CHDM which because of the hot component has additional suppression of small-scale fluctuations that are presumably required to make early structure (e.g., Haehnelt 1993). The difficulty is that dissipationless simulations predict the number density of halos of a given mass as a function of redshift, but not enough is known about the nature of quasars - for example, the mass of the host galaxy - to allow a simple prediction of the number of quasars as a function of redshift in any given cosmological model. A more recent study (Katz et al. 1994) concludes that very efficient cooling of the gas in early structures, and angular momentum transfer from it to the dark halo, allows for formation of at least the observed number of quasars even in models where most galaxy formation occurs late (cf. Eisenstein & Loeb 1995).

Observers are now beginning to see significant numbers of what may be the central regions of galaxies in an early stage of their formation at redshifts z = 3-3.5 (Steidel et al. 1996; Giavalisco, Steidel, & Macchetto 1996) - although, as with quasars, a danger in using systems observed by emission is that they may not be typical. As additional observations (e.g., Lowenthal et al. 1996) clarify the nature of these objects, they can perhaps be used to constrain cosmological parameters and models. (This data is discussed in more detail in Section 1.7.5.)

Another sort of high redshift object which may hold more promise for constraining theories is damped Lyman alpha systems (DLAS). DLAS are high column density clouds of neutral hydrogen, generally thought to be protogalactic disks, which are observed as wide absorption features in quasar spectra (Wolfe 1993). They are relatively common, seen in roughly a third of all quasar spectra, so statistical inferences about DLAS are possible. At the highest redshift for which data was published in 1995, z = 3-3.4, the density of neutral gas in such systems in units of critical density was reported to be Omega _gas approx 0.006, comparable to the total density of visible matter in the universe today (Lanzetta, Wolfe, & Turnshek 1995). Several papers (Mo & Miralda-Escude 1994, Kauffmann & Charlot 1994, Ma & Bertschinger 1994) pointed out that the CHDM model with Omega = 0.3 could not produce such a high Omega _gas. However, it has been shown that CHDM with Omega = 0.2 could do so (Klypin et al. 1995, cf. Ma 1995). The power spectrum on small scales is a very sensitive function of the total neutrino mass in CHDM models. This theory makes two crucial predictions: Omega _gas must fall off at higher redshifts, and the DLAS at z gtapprox 3 mostly correspond to systems of internal rotation velocity or velocity dispersion less than about 100 km s^-1. This velocity can perhaps be inferred from the Doppler widths of the metal line systems associated with the DLAS. Preliminary reports regarding the amount of neutral hydrogen in such systems deduced from the latest data at redshifts above 3.5 appear to be consistent with the first of these predictions (Storrie-Lombardi et al. 1996). But a possible problem for the second (Wolfe 1997) is the large velocity widths and other statistical properties (Prochaska & Wolfe 1997) of the metal line systems associated with the highest-redshift DLAS (e.g., Lu et al. 1996, at z = 4.4); if these actually indicate that a massive disk galaxy is already formed at such a high redshift, and if discovery of other such systems shows that they are not rare, that would certainly disfavor CHDM and other theories with relatively little power on small scales. However, other interpretations of such data which would not cause such problems for theories like CHDM are perhaps more plausible, since they are based on fairly realistic hydrodynamic simulations (Haehnelt, Steinmetz, & Rauch 1996). More data will help resolve this question, along with DLAS models including both dust absorption (Pei & Fall 1995) and lensing (Bartelmann & Loeb 1996, Maller et al. 1997).

One of the best ways of probing early structure formation would be to look at the main light output of the stars of the earliest galaxies, which is redshifted by the expansion of the universe to wavelengths beyond about 5 microns today. Unfortunately, it is not possible to make such observations with existing telescopes; since the atmosphere blocks almost all such infrared radiation, what is required is a large infrared telescope in space. The Space Infrared Telescope Facility (SIRTF) has long been a high priority, and it will be great to have access to the data such an instrument will produce when it is launched sometime in the next decade. In the meantime, the Near Infrared Camera/Multi-Object Spectrograph (NICMOS), installed on Hubble Space Telescope in spring 1997, will help. Infrared spectrographs on the largest ground-baed telescopes will also be of great value.

An alternative method is to look for the starlight from the earliest stars as extragalactic background infrared light (EBL). Although it is difficult to see this background light directly because our Galaxy is so bright in the near infrared, it may be possible to detect it indirectly through its absorption of TeV gamma rays (via the process gamma gamma -> e⁺ e^-). Of the more than twenty active galactic nuclei (AGNs) that have been seen at ~ 10 GeV by the EGRET detector on the Compton Gamma Ray Observatory, only two of the nearest, Mk421 and Mk501, have also been clearly detected in TeV gamma rays by the Whipple Atmospheric Cerenkov Telescope (Quinn et al. 1996, Schubnell et al. 1996). Absorption of ~ TeV gamma rays from AGNs at redshifts z ~ 0.2 has been shown to be a sensitive probe of the EBL and thus of the era of galaxy formation (MacMinn & Primack 1996).