The Growth of Structure

One of the most important problems in astrophysics is understanding how structures in our Universe have evolved from the tiny spatial fluctuations observed in the cosmic microwave background to the clumpy and highly non-Gaussian matter distributions that we observe today. Thus far, our understanding of structure formation, lacking comprehensive observational data, has been largely guided by theoretical studies. Numerical simulations of structure formation have now attained a high degree of sophistication, and demonstrate that clear differences between competing cosmological structure formation models may be observable at large lookback times (z>1; figure 2). Confronting these models with observations of the matter distribution at a range of lookback times remains one of the most robust methods of discriminating between present competing models and constraining future theories of structure formation. This requires obtaining redshifts for galaxies over very large areas of the sky (to probe the characteristic scales on which structures exist) to very faint limiting magnitudes (to probe the distant universe). Redshift surveys would not only probe the overall topology of the matter distribution, but also the detailed dynamical history of galaxies in a variety of environments from the sparse voids to the densest clusters.

  

Figure 1: Numerical simulations of the dark matter distribution predict structures on scales of clumps and filaments (arcminutes) to large voids and walls (several degrees). Characterizing this structure from observations of galaxy clustering requires a wide-field, highly multiplexed spectroscopic capability.

  

Figure 2: In numerical simulations of the growth of large scale structure, differences between competing models are especially obvious at large lookback times. In these simulations from the VIRGO Consortium (e.g., White 1997), the boxes (240h^-1Mpc on a side, corresponding to ~8^o at z=1) show the dark matter distributions at different epochs and predict discernible differences at z>1.

Need for Large Area / Depth / Large Samples: Current surveys suggest that there is significant structure on Mpc scales (corresponding to at z=1), comparable to the largest scales thus far investigated (e.g., Landy et al. 1996, Doroshkevich et al. 1996). The mass distribution may be comprised of large filamentary or sheet-like structures which are inferred from pencil-beam surveys -- little is known about the true large scale angular distribution at each epoch due to the paucity of redshift information over large angular scales for all but the very nearest galaxies (z<0.2; e.g., Davis et al. 1982, Shechtman et al. 1996, Ellis et al. 1996, Geller et al. 1997, da Costa et al. 1998). This implies the need for spectroscopic surveys over several times this characteristic scale (i.e., 10^ox10^o) in order to sample the large volumes that contain these structures (e.g., Figures 1 and 2).

  

Figure 3: The need for dense sampling: when all the galaxies in the Las Campanas Redshift Survey (Top) are sampled at 10% (Bottom), the strongest features disappear.

A deep survey (AB) is needed to densely sample the high redshift populations and characterize the three-dimensional matter distribution to redshifts beyond 4. Dense sampling is needed to detect and highlight compact structures: most existing surveys of the sky are non-contiguous and sparsely sample the galaxy distribution. Sparse sampling is insensitive to strong clustering (Figure 3) and results in significant uncertainties in the derived structure statistics due to both sampling and `cosmic' errors (e.g., Szapudi & Colomb 1996, Szapudi & Szalay 1996). Since the simulations show structures on scales ranging from tens of kpc to Mpc, the observations must be sensitive enough to sample the velocity fields on small and large scales, as well as comprehensive enough to have sufficient numbers of objects on all scales in each redshift interval to provide robust statistical measures of the spatial structures.

 

Figure 4: Simulation by M. Pierre (1998) of an XMM observation of an X-ray emitting filament located between two z=0.5 clusters (not shown). By measuring the velocities of galaxies in the different regions, SWIFT will locate the filaments and clumps in velocity space and measure the total mass in the virialized regions. The region shown is 1.25^ox1.82^o, and is well matched to the FOV of SWIFT.

A Representative SWIFT Project

To study the growth of structure from to , we would map out galaxy redshifts over 10^ox10^o. Dividing the redshift range crudely into 10 bins of dz=0.3, and requiring ~ 1000 galaxies/sq.deg./dz bin for an accurate estimate of a clustering statistic, we would need a total sample size of about 10⁶ galaxies. To AB mag, the surface density of galaxies is 10⁵/sq.deg., the majority of which lie at z>1. This program will reveal the evolution of the matter distribution as traced by the galaxies as a function of redshift, and provide a relatively unbiased discovery tool for high redshift clusters and groups. An investigation of the velocity fields of galaxies in regions imaged at X-ray wavelengths (e.g., figure 4) will isolate the hot X-ray emitting gas in redshift space, determine the fractional baryonic mass in galaxies versus that in hot gas, and provide a direct measure of the ratio of baryonic to total mass in filaments and clusters. Measuring redshifts of ~10⁶ galaxies at a resolution ~500 requires 200 clear nights with SWIFT, whose field of view is well matched to the angular scale of structures predicted from simulations (figure 1). The efficiency gain compared with other large telescopes with MOS capability is shown in table 1.

In addition to mapping out the evolution of the large-scale topology of the matter distribution, a project of this scale would simultaneously yield detailed dynamical information on clusters of galaxies over a large range in redshift. Since the densest clusters identify sites of the strongest initial perturbations and the earliest ones to collapse, interactions play an important role in shaping the evolution of galaxies in clusters and perhaps in the chemical and stellar enrichment of the inter-cluster medium as a whole. The dynamical analysis of member galaxies in clusters at high redshift, when combined with detailed kinematic and chemical studies of galaxies and intracluster stars and gas in clusters at the present epoch, can yield a comprehensive understanding of the chemical enrichment of the Universe in these over-dense regions. In particular, the intracluster stellar component in nearby clusters contains a fossil record of the past dynamical history imprinted in the kinematics of the intra-cluster stellar population. A chemical and kinematic study of the intra-cluster planetary nebulae (a good velocity tracer population that is relatively easy to identify from the ground) in these clusters, would reveal the entire tidal history of many cluster galaxies over the evolutionary history of the clusters! For example, the areal density of intracluster planetaries in Virgo (with line fluxes F([OIII]5007)>10^-17erg s^-1 cm^-2) is ~1000/sq.deg., and the cluster spans an area of ~100 sq.deg. Obtaining moderate resolution, moderate signal-to-noise ratio spectra of ~10⁵ planetaries requires ~100 nights with SWIFT.

 

Table 1: Comparison of Completion Times for Some Science Projects

Program	m	Nobj	Ntot	Keck+DEIMOS	IMACS	SWIFT
	AB mag	/sq.deg.		(nights)	(nights)	(nights)
Large-scale Structure	25	105	106	12,500	4,000	200
Galaxy Evolution	25	104		6,250	1,975	100
Virgo Intracluster	``29''	>103	105	4,500	1,000	100
Galactic Plane KIII	23			18,000	4,700	225