Building on the legacy of the Sloan Digital Sky Survey (SDSS-I and II), SDSS-III is a program of four spectroscopic surveys on three scientific themes: dark energy and cosmological parameters, the ...history and structure of the Milky Way, and the population of giant planets around other stars. In keeping with SDSS tradition, SDSS-III will provide regular public releases of all its data, beginning with SDSS DR8 (which occurred in Jan 2011). This paper presents an overview of the four SDSS-III surveys. BOSS will measure redshifts of 1.5 million massive galaxies and Lya forest spectra of 150,000 quasars, using the BAO feature of large scale structure to obtain percent-level determinations of the distance scale and Hubble expansion rate at z<0.7 and at z~2.5. SEGUE-2, which is now completed, measured medium-resolution (R=1800) optical spectra of 118,000 stars in a variety of target categories, probing chemical evolution, stellar kinematics and substructure, and the mass profile of the dark matter halo from the solar neighborhood to distances of 100 kpc. APOGEE will obtain high-resolution (R~30,000), high signal-to-noise (S/N>100 per resolution element), H-band (1.51-1.70 micron) spectra of 10^5 evolved, late-type stars, measuring separate abundances for ~15 elements per star and creating the first high-precision spectroscopic survey of all Galactic stellar populations (bulge, bar, disks, halo) with a uniform set of stellar tracers and spectral diagnostics. MARVELS will monitor radial velocities of more than 8000 FGK stars with the sensitivity and cadence (10-40 m/s, ~24 visits per star) needed to detect giant planets with periods up to two years, providing an unprecedented data set for understanding the formation and dynamical evolution of giant planet systems. (Abridged)
Recent evidence on the metal content of the high-redshift Lyman alpha forest
seen in quasar spectra suggests that an early generation of galaxies enriched
the intergalactic medium (IGM) at z ~ 5. We ...calculate the number of supernovae
that need to have taken place to produce the observed metallicity. The
progenitor stars of the supernovae should have emitted $\sim 20$ ionizing
photons for each baryon in the universe, i.e., more than enough to ionize the
IGM. We calculate that the rate of these supernovae is such that about one of
them should be observable at any time per square arc minute. Their fluxes are,
of course, extremely faint: at z=5, the peak magnitude should be K=27 with a
duration of ~ 1 year. However, these supernovae should still be the brightest
objects in the universe beyond some redshift, because the earliest galaxies
should form before quasars and they should have very low mass, so their
luminosities should be much lower than that of a supernova.
We also show that, under the assumption of a standard initial mass function,
a significant fraction of the stars in the Galactic halo should have formed in
the early galaxies that reionized and enriched the IGM, and which later must
have merged with our Galaxy. These stars should have a more extended radial
distribution than the observed halo stars.
Astrophys.J. 530 (2000) 1-16 A model of the density distribution in the intergalactic medium, motivated by
that found in numerical simulations, is used to demonstrate the effect of a
clumpy IGM and ...discrete sources on the reionization of the universe. In an
inhomogeneous universe reionization occurs outside-in, starting in voids and
gradually penetrating into overdense regions. Reionization should not be sudden
but gradual, with a continuous rise of the photon mean free path over a fair
fraction of the Hubble time as the emissivity increases. We show that a
hydrogen Gunn-Peterson trough should be present at $z\simeq 6$ unless the
emissivity increases with redshift at $z>4$. However, the epoch of overlap of
cosmological \hii regions could have occurred at a higher redshift if sources
of low luminosity reionized the IGM; the Gunn-Peterson trough at $z\sim 6$
would then appear because even the most underdense voids have a large enough
neutral fraction in ionization equilibrium to be optically thick to \lya
photons. Cosmological \hii regions near the epoch of overlap can produce gaps
of transmitted flux only if luminous quasars contributed to the reionization.
Despite the clumpiness of the matter distribution, recombinations are not very
important during the reionization of hydrogen because the high density gas is
not ionized until a late time. We show that the \heii reionization was most
likely delayed relative to the hydrogen reionization, but should be completed
by $z\sim 3$, the redshift where observations are available. The reported large
optical depth fluctuations of \heii are probably not due to an incomplete \heii
reionization, but arise from a combination of density fluctuations and the
variations in the intensity of the ionizing background due to luminous QSO's.
If the universe was reionized by O and B stars in an early population of galaxies, the associated supernovae should have enriched the universe to a mean metallicity \(\bar Z = 10^{-5} (1+ n_{rec})\), ...where \(n_{rec}\) is the mean number of times that each baryon recombined during the reionization era. This is consistent with recent observations of the metallicity in the Lya forest at \(z\simeq 3\). The mean surface brightness observable at present from the galaxies that produced these heavy elements, in the rest-frame wavelengths \(1216\AA < \lambda \lesssim 2500\AA\), should be \(\sim 10^{-6} (\bar Z/10^{-4}) (\Omega_b h^2/0.02) ~ photons \cm^{-2} s^{-1} arcsec^{-2}\). Most of this radiation was emitted at \(z>5\), before reionization was complete. These high-redshift galaxies may be detectable in near-infrared photometric surveys, identifying them via the Gunn-Peterson trough (analogous to the use of the Lyman limit cutoff to search for galaxies at \(z\sim 3\), where the Lya forest blanketing is smaller). Their spectrum may also be characterized by a strong Lya emission line. However, the spectra of galaxies seen behind intervening gas that is still neutral should show the red damping wing of the Gunn-Peterson trough, with a predictable profile that obstructs part of the Lya emission. The low-mass galaxies formed before reionization might constitute a distinctive population; we discuss the signature that this population could have in the faint number counts. Although most of these galaxies should have merged into larger ones, those that survived to the present could be dwarf spheroidals.
Recent evidence on the metal content of the high-redshift Lyman alpha forest seen in quasar spectra suggests that an early generation of galaxies enriched the intergalactic medium (IGM) at z ~ 5. We ...calculate the number of supernovae that need to have taken place to produce the observed metallicity. The progenitor stars of the supernovae should have emitted \(\sim 20\) ionizing photons for each baryon in the universe, i.e., more than enough to ionize the IGM. We calculate that the rate of these supernovae is such that about one of them should be observable at any time per square arc minute. Their fluxes are, of course, extremely faint: at z=5, the peak magnitude should be K=27 with a duration of ~ 1 year. However, these supernovae should still be the brightest objects in the universe beyond some redshift, because the earliest galaxies should form before quasars and they should have very low mass, so their luminosities should be much lower than that of a supernova. We also show that, under the assumption of a standard initial mass function, a significant fraction of the stars in the Galactic halo should have formed in the early galaxies that reionized and enriched the IGM, and which later must have merged with our Galaxy. These stars should have a more extended radial distribution than the observed halo stars.
A model of the density distribution in the intergalactic medium, motivated by that found in numerical simulations, is used to demonstrate the effect of a clumpy IGM and discrete sources on the ...reionization of the universe. In an inhomogeneous universe reionization occurs outside-in, starting in voids and gradually penetrating into overdense regions. Reionization should not be sudden but gradual, with a continuous rise of the photon mean free path over a fair fraction of the Hubble time as the emissivity increases. We show that a hydrogen Gunn-Peterson trough should be present at \(z\simeq 6\) unless the emissivity increases with redshift at \(z>4\). However, the epoch of overlap of cosmological \hii regions could have occurred at a higher redshift if sources of low luminosity reionized the IGM; the Gunn-Peterson trough at \(z\sim 6\) would then appear because even the most underdense voids have a large enough neutral fraction in ionization equilibrium to be optically thick to \lya photons. Cosmological \hii regions near the epoch of overlap can produce gaps of transmitted flux only if luminous quasars contributed to the reionization. Despite the clumpiness of the matter distribution, recombinations are not very important during the reionization of hydrogen because the high density gas is not ionized until a late time. We show that the \heii reionization was most likely delayed relative to the hydrogen reionization, but should be completed by \(z\sim 3\), the redshift where observations are available. The reported large optical depth fluctuations of \heii are probably not due to an incomplete \heii reionization, but arise from a combination of density fluctuations and the variations in the intensity of the ionizing background due to luminous QSO's.
