Formation History of Galaxies and Large-Scale Structures in the Subaru Deep Fields

We investigate galaxy formation and structure formation from 2,600 high redshift galaxies at z=3.5-5.2 detected in two large contiguous blank fields, the Subaru Deep Field (SDF) and the Subaru/XMM Deep Field (SXDF). We developed a wide-field optical imager Subaru Prime Focus Camera (Suprime-Cam) mounted on 8m Subaru, so as to detect a large number of these faint high-z galaxies distributed in a large volume. From November 2000 to May 2002, we carried out very deep (i'~26.5-27) and wide-field (1200 arcmin² in total) imaging covering the whole optical window with broad-band filers (B, V, R, i' and z') and a narrow-band filter (NB711;lambda_c=7126A, Delta_lambda=73A). We detect about 10⁵ objects, in total, in the SDF and SXDF with i'<26.5 and i'<26.0, respectively.

We make three photometric samples of Lyman break galaxies (LBGs) and one photometric sample of Lyman alpha emitters (LAEs) from the deep and wide-field images. The three LBG samples are BRi-LBGs at z=4.0+/-0.5, Viz-LBGs at z=4.7+/-0.5 and Riz-LBGs at z=4.9+/-0.3 selected with two-color diagrams of B-R vs. R-i', V-i' vs. i'-z', and R-i' vs. i'-z', respectively. The one LAE sample is NB711-LAE at z=4.86+/-0.03. We find 2,600 LBGs and LAEs. The selection criteria for these samples are examined with 85 spectroscopically identified objects in the SDF, and the completeness and contamination of the samples are estimated from Monte-Carlo simulations based on the photometric-redshift catalog of the Hubble Deep Field-North (HDF-N).

We calculate luminosity functions (LFs) of LBGs with the estimated completeness and contamination, and find that (i) the LFs of LBGs indicate a significant decrease in the number density of bright galaxies (M₁₇₀₀<-22) from z=4 to 5 and (ii) the slope of the luminosity functions of LBGs becomes steeper towards higher redshifts. We derive LFs of the LAEs at Ly alpha luminosity and at UV-continuum (rest-frame 1700A) luminosity. The LFs at Ly alpha luminosity do not show clear evolution between z=3.4 and z=4.86. The amplitude of the LAE LF at UV-continuum luminosity tends to decline at the bright magnitudes more rapidly than that of the LBG LF.

We estimate dust extinction of LBGs from UV-continuum slopes to obtain E(B-V)=0.15+/-0.03 as the mean value. E(B-V) may depend on luminosity, but the difference is as small as Delta E(B-V)<0.05 on average over -23<M₁₇₀₀< -21, if any.

We investigate the evolution of UV-luminosity density by integrating the luminosity functions of LBGs. We find that the UV-luminosity density at 2000A does not change from z~2 to z~5, and that the cosmic star-formation rate density (with and without correction for dust extinction) remains constant within the error bars from z=3 to z=5. We set an constraint on the escape fraction of UV-ionizing photons (i.e. UV continuum in 900A) produced by LBGs. We find that the escape fraction should be larger than ~0.13, if we assume the galaxy spectrum of Bruzual & Charlot (1993) with Salpeter IMF and Solar metallicity.

We study the clustering properties of these high-z galaxies. We calculate the angular correlation functions of these galaxies, and find clear clustering signals in BRi-LBGs, Viz-LBGs, and LAEs at z=4.9. The correlation lengths are estimated to be r₀= 4.1 +0.2/-0.2, 5.9 +1.3/-1.7, and 6.2 +0.5/-0.5 h(100)^-1 Mpc (in comoving units) for BRi-LBGs, Viz-LBGs, and LAEs at z=4.9, respectively. These correlation lengths significantly exceed that of the underlying-dark matter. We calculate the galaxy- dark matter bias of these galaxies to be b_g= 2.9 +0.1/-0.1, 4.6 +0.9/-1.2}, and 4.9 +0.4/-0.4}, respectively. We find that the bias of LBGs with L>~ L* monotonically increases with redshift at z>~3, which supports the biased galaxy formation picture. We find the luminosity segregation with UV-continuum luminosity for LBGs at z=4 (BRi-LBGs), and with Lyman alpha luminosity for LAEs at z=4.9, which are similarly found in present-day galaxies.

It is also found that the distribution of LAEs at z=4.86 shows a large density gradient with a scale of >15 arcmin. We carry out additional imaging for LAEs at z=4.86 in the northern field flanking the SDF. We discover a proto-large scale structure traced by LAEs which distribute in a filamentary region with 20 h(70)^-1 Mpc width and >50 h(70)^-1 Mpc length which extends from east-southeast to west-northwest. This is the first discovery of a large-scale structure at high redshifts.

We estimate the mass of dark halos hosting these high-z galaxies with the analytic model given by Sheth & Tormen. We find that the hosting dark halos have masses of 10¹¹-10¹³ h(70)^-1 Msol for LBGs at z=4 and ~10¹² h(70)^-1 Msol for LBGs and LAEs at z=5. We compare the number densities of the high-z galaxies with that of all dark halos existing at z=4-5. It is implied that a single dark halo hosts less than one LBG but more than one LAE on average. We estimate the mass of hosting dark halos for various types of galaxies at z=3; LBGs, K-band selected galaxies, and SCUBA sources whose correlation lengths are given in the literature. Then we find that both LBGs and K-band selected galaxies reside in dark halos with ~10¹² h(70)^-1 Msol. However, the number density of K-band selected galaxies exceeds that of all dark halos existing at z=3, while the number density of LBGs is below that of all dark halos. It is indicated that galaxies in such dark halos have an episodic star formation, and that about 1/10 of these galaxies have an active-star formation whose star-formation rate (SFR) exceeds their bright UV continuum. Dark halos hosting SCUBA sources are massive, that SCUBA sources could reside only in proto-cluster regions.

The descendants of these hosting dark halos are predicted with the analytic model given by Sheth et al. We find that most of the descendants of the high-z galaxies, observed so far (i.e., LBGs, LAEs, K-selected galaxies, and SCUBA sources) are probably taken a part in dark halos of present-day clusters and groups with >~10¹³ h(70)^-1 Msol. Thus, the most of the observed galaxies at high-z are the progenitors of member galaxies in present-day clusters and groups. We find that either K-band selected galaxies or faint LBGs should experience merging in a few times from high-z to the present day, if they evolve into normal galaxies with M_bg <~ -19.

Finally, we propose a unified picture for the observed high-z galaxies, and discuss a scenario for galaxy formation through the structure formation in the Universe.