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Register a new userStellar Masses of Lyman Break Galaxies, Lyman Alpha Emitters and Radio Galaxies in Overdense Regions at z=4-6
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We present new information on galaxies in the vicinity of luminous radio galaxies and quasars at z=4,5,6. These fields were previously found to contain overdensities of Lyman Break Galaxies (LBGs) or spectroscopic Lyman alpha emitters. We use HST and Spitzer data to infer stellar masses, and contrast our results with large samples of LBGs in more average environments as probed by the Great Observatories Origins Deep Survey (GOODS). The following results were obtained. First, LBGs in both overdense regions and in the field at z=4-5 lie on a very similar sequence in a z-[3.6] versus [3.6] color-magnitude diagram. This is interpreted as a sequence in stellar mass (log[M*/Msun] = 9-11) in which galaxies become increasingly red due to dust and age as their star formation rate (SFR) increases. Second, the two radio galaxies are among the most massive objects (log[M*/Msun]~11) known to exist at z~4-5, and are extremely rare based on the low number density of such objects as estimated from the ~25x larger area GOODS survey. We suggest that the presence of these massive galaxies and supermassive black holes has been boosted through rapid accretion of gas or merging inside overdense regions. Third, the total stellar mass found in the z=4 ``proto-cluster TN1338 accounts for <30% of the stellar mass on the cluster red sequence expected to have formed at z>4, based on a comparison with the massive X-ray cluster Cl1252 at z=1.2. Although future near-infrared observations should determine whether any massive galaxies are currently being missed, one possible explanation for this mass difference is that TN1338 evolves into a smaller cluster than Cl1252. This raises the interesting question of whether the most massive protocluster regions at z>4 remain yet to be discovered.
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We study the properties of Lyman-alpha emitters (LAEs) and Lyman-break galaxies (LBGs) at z=3-6 using cosmological SPH simulations. We investigate two simple scenarios for explaining the observed Ly-a and rest-frame UV luminosity functions (LFs) of LAEs: (i) the escape fraction scenario, in which the effective escape fraction (including the IGM attenuation) of Ly-a photons is f_Lya ~0.1 (0.15) at z=3 (6), and (ii) the stochastic scenario, in which the fraction of LAEs that are turned on at z=3 (6) is Cstoc ~0.07 (0.2) after correcting for the IGM attenuation. Our comparisons with a number of different observations suggest that the stochastic scenario is preferred over the escape fraction scenario. We find that the mean values of stellar mass, metallicity and black hole mass hosted by LAEs are all smaller in the stochastic scenario than in the escape fraction scenario. In our simulations, the galaxy stellar mass function evolves rapidly, as expected in hierarchical structure formation. However, its evolution is largely compensated by a beginning decline in the specific star formation rate, resulting in little evolution of the rest-frame UV LF from z=6 to 3. The rest-frame UV LF of both LAEs and LBGs at z=3 & 6 can be described well by the stochastic scenario provided the extinction is moderate, E(B-V) ~0.15, for both populations, although our simulation might be overpredicting the number of bright LBGs at z=6. We also discuss the correlation function and bias of LAEs. The Ly-a LFs at z=6 in a field-of-view of 0.2 deg^2 show a significantly larger scatter owing to cosmic variance relative to that in a 1 deg^2 field, and the scatter seen in the current observational estimates of the Ly-a LF can be accounted for by cosmic variance.
We perform Monte-Carlo radiative transfer calculations to model the Lyman alpha properties of galaxies in high-resolution, zoom-in cosmological simulations at z ~ 6.6. The simulations include both constrained and unconstrained runs, representing respectively a highly overdense region and an average field. Different galactic wind models are used in the simulations in order to investigate the effects of these winds on the apparent Ly alpha properties of galaxies. We find that, for models including galactic winds, the Ly alpha properties of massive galaxies residing in the overdense region match well recent observations of luminous Ly alpha emitters (LAEs) at z ~ 6-7, in terms of apparent Ly alpha luminosity, Ly alpha line width and Ly alpha equivalent width distributions. Without winds, the same galaxies appear less Ly alpha bright as a result of both differences in the line profile emerging from galaxies themselves, and, in the distributions of neutral gas in the circumgalactic (CGM) and intergalactic medium (IGM). We also study the relations between apparent Ly alpha luminosity and various galaxy properties: stellar mass, star formation rate (SFR) and host halo mass. At fixed halo mass, the apparent Ly alpha luminosity of galaxies appears to depend on the large-scale environment while this is no longer true for galaxies at a given stellar mass or SFR. We provide simple linear fits to these relations that can be used for quickly constructing mock LAE samples from N-body simulations. Our results suggest that the observed luminous LAEs at z ~ 6.6 are hosted by ~10^{12} h^{-1} Mo, dark matter haloes, residing in large, overdense ionized regions.
We perform a spectrophotometric analysis of galaxies at redshifts z = 4 - 6 in cosmological SPH simulations of a Lambda CDM universe. Our models include radiative cooling and heating by a uniform UV background, star formation, supernova feedback, and a phenomenological model for galactic winds. Analysing a series of simulations of varying boxsize and particle number allows us to isolate the impact of numerical resolution on our results. Specifically, we determine the luminosity functions in B, V, R, i, and z filters, and compare the results with observed galaxy surveys done with the Subaru telescope and the Hubble Space Telescope. We find that the simulated galaxies have UV colours consistent with observations and fall in the expected region of the colour-colour diagrams used by the Subaru group. Assuming a uniform extinction of E(B-V) = 0.15, we also find reasonable agreement between simulations and observations in the space density of UV bright galaxies at z = 3 - 6, down to the magnitude limit of each survey. For the same moderate extinction level of E(B-V) ~ 0.15, the simulated luminosity functions match observational data, but have a steep faint-end slope with alpha ~ -2.0. We discuss the implications of the steep faint-end slope found in the simulations.
We present the results of a photometric and spectroscopic survey of 321 Lyman break galaxies (LBGs) at z ~ 3 to investigate systematically the relationship between Lya emission and stellar populations. Lya equivalent widths (EW) were calculated from rest-frame UV spectroscopy and optical/near-infrared/Spitzer photometry was used in population synthesis modeling to derive the key properties of age, dust extinction, star formation rate (SFR), and stellar mass. We directly compare the stellar populations of LBGs with and without strong Lya emission, where we designate the former group (EW > 20 AA) as Lya emitters (LAEs) and the latter group (EW < 20 AA) as non-LAEs. This controlled method of comparing objects from the same UV luminosity distribution represents an improvement over previous studies in which the stellar populations of LBGs and narrowband-selected LAEs were contrasted, where the latter were often intrinsically fainter in broadband filters by an order of magnitude simply due to different selection criteria. Using a variety of statistical tests, we find that Lya equivalent width and age, SFR, and dust extinction, respectively, are significantly correlated in the sense that objects with strong Lya emission also tend to be older, lower in star formation rate, and less dusty than objects with weak Lya emission, or the line in absorption. We accordingly conclude that, within the LBG sample, objects with strong Lya emission represent a later stage of galaxy evolution in which supernovae-induced outflows have reduced the dust covering fraction. We also examined the hypothesis that the attenuation of Lya photons is lower than that of the continuum, as proposed by some, but found no evidence to support this picture.
We present the results of a study of a large sample of luminous (z{AB}<26) Lyman break galaxies (LBGs) in the redshift interval 4.7<z<6.3, selected from a contiguous 0.63 square degree area covered by the UKIDSS Ultra Deep Survey (UDS) and the Subaru XMM-Newton Survey (SXDS). Utilising the large area coverage and the excellent available optical+nearIR data, we use a photometric redshift analysis to derive a new, robust, measurement of the bright end (L>L*) of the UV-selected luminosity function at high redshift. When combined with literature studies of the fainter LBG population, our new sample provides improved constraints on the luminosity function of redshift 5<z<6 LBGs over the luminosity range 0.1L*<L<10L*. A maximum likelihood analysis returns best-fitting Schechter function parameters of M*_1500=-20.73, phi*=0.0009 Mpc^-3 and alpha=-1.66 for the luminosity function at z=5, and M*_1500 = -20.04, phi*=0.0018 Mpc^-3 and alpha=-1.71 at z=6. In addition, an analysis of the angular clustering properties of our LBG sample demonstrates that luminous 5<z<6 LBGs are strongly clustered (r_0 = 8.1 Mpc), and are consistent with the occupation of dark matter halos with masses of ~10^{11.5-12.0} Msun. Moreover, by stacking the available multi-wavelength imaging data for the high-redshift LBGs it is possible to place useful constraints on their typical stellar mass. The results of this analysis suggest that luminous LBGs at 5<z<6 have an average stellar mass of ~10^10 Msun, consistent with the results of the clustering analysis assuming plausible values for the ratio of stellar to dark matter. Finally, by combining our luminosity function results with those of the stacking analysis we derive estimates of ~1x10^7 Msun Mpc^-3 and 4x10^6 Msun Mpc^-3 for the stellar mass density at z~5 and z~6 respectively.
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