1,833 research outputs found
HST imaging and Keck Spectroscopy of z~6 I-band Drop-Out Galaxies in the ACS GOODS Fields
We measure the surface density of i'-band dropout galaxies at z~6 through
wide field HST/ACS imaging and ultra-deep Keck/DEIMOS spectroscopy. Using deep
HST/ACS SDSS-i' (F775W) and SDSS-z' (F850LP) imaging from GOODS-N (200
arcmin^2), we identify 9 i'-drops satisfying an (i'-z')_AB>1.5 selection
criterion to a depth of z'_AB=25.6 (corresponding to L*_UV at z~3-4). We use
HK' imaging data to improve the fidelity of our sample, discriminating against
lower redshift red galaxies and cool Galactic stars. Three i'-drops are
consistent with M/L/T dwarf stars. We present ultra-deep Keck/DEIMOS
spectroscopy of 10 objects from our combined GOODS-N and GOODS-S i'-drop
sample. We detect Lyman-alpha emission at z=5.83 from one object in the GOODS-S
field, which lies only 8arcmin away (i.e. 3Mpc/h_70) from the z=5.78 object
already confirmed by Bunker et al. (2003). One possible Lyman-alpha emitter at
z=6.24 is found in the GOODS-N field (although identification of this
spatially-offset emission line is ambiguous). Using the rest-frame UV continuum
from our 6 candidate z~6 galaxies from the GOODS-N field, we determine a lower
limit to the unobscured volume-averaged global star formation rate at z~6 of
(5.4+/-2.2)x10^-4 h_70 M_sun/yr/Mpc^3. We find that the cosmic star formation
density in galaxies with unobscured star formation rates 15M_sun/yr/h_70^2
falls by a factor of 8 between z~3 and z~6. Hence the luminosity function of
LBGs must evolve in this redshift interval: a constant integrated star
formation density at requires a much steeper faint-end slope, or a
brighter characteristic luminosity. This result is in agreement with our
previous measurement from the Chandra Deep Field South (Stanway et al. 2003),
indicating that cosmic variance is not a dominant source of uncertainty.Comment: to appear in ApJ; replaced with accepted versio
Heavily reddened type 1 quasars at z > 2 I: Evidence for significant obscured black-hole growth at the highest quasar luminosities
We present a new population of z>2 dust-reddened, Type 1 quasars with
0.5<E(B-V)<1.5, selected using near infra-red (NIR) imaging data from the
UKIDSS-LAS, ESO-VHS and WISE surveys. NIR spectra obtained using the Very Large
Telescope (VLT) for 24 new objects bring our total sample of spectroscopically
confirmed hyperluminous (>10^{13}L_0), high-redshift dusty quasars to 38. There
is no evidence for reddened quasars having significantly different H
equivalent widths relative to unobscured quasars. The average black-hole masses
(~10^9-10^10 M_0) and bolometric luminosities (~10^{47} erg/s) are comparable
to the most luminous unobscured quasars at the same redshift, but with a tail
extending to very high luminosities of ~10^{48} erg/s. Sixty-six per cent of
the reddened quasars are detected at at 22um by WISE. The average
6um rest-frame luminosity is log10(L6um/erg/s)=47.1+/-0.4, making the objects
among the mid-infrared brightest AGN currently known. The extinction-corrected
space-density estimate now extends over three magnitudes (-30 < M_i < -27) and
demonstrates that the reddened quasar luminosity function is significantly
flatter than that of the unobscured quasar population at z=2-3. At the
brightest magnitudes, M_i < -29, the space density of our dust-reddened
population exceeds that of unobscured quasars. A model where the probability
that a quasar becomes dust-reddened increases at high luminosity is consistent
with the observations and such a dependence could be explained by an increase
in luminosity and extinction during AGN-fuelling phases. The properties of our
obscured Type 1 quasars are distinct from the heavily obscured, Compton-thick
AGN that have been identified at much fainter luminosities and we conclude that
they likely correspond to a brief evolutionary phase in massive galaxy
formation.Comment: 16 pages, 9 figures (+ 2 appendices), Accepted for publication in
MNRA
Cosmological Evolution of the Universe Neutral Gas Mass Measured by Quasar Absorption Systems
The cosmological evolution of neutral hydrogen is an efficient way of tracing
structure formation with redshift. It indicates the rate of evolution of gas
into stars and hence the gas consumption and rate star formation history of the
Universe. In measuring HI, quasar absorbers have proven to be an ideal tool and
we use observations from a recent survey for high-redshift quasar absorption
systems together with data gathered from the literature to measure the
cosmological comoving mass density of neutral gas. This paper assumes
Omega_M=0.3, Omega_lambda=0.7 and h=0.65.Comment: 3 pages, 2 figures. To appear in the proceedings of the "Cosmic
Evolution" conference, held at l'Institut d'Astrophysique de Paris, November
13-17, 200
An Extremely Luminous Galaxy at z=5.74
We report the discovery of an extremely luminous galaxy lying at a redshift
of z=5.74, SSA22-HCM1. The object was found in narrowband imaging of the SSA22
field using a 105 Angstrom bandpass filter centered at 8185 Angstroms during
the course of the Hawaii narrowband survey using LRIS on the 10 m Keck II
Telescope, and was identified by the equivalent width of the emission
W_lambda(observed)=175 Angstroms, flux = 1.7 x 10^{-17} erg cm^{-2} s^{-1}).
Comparison with broadband colors shows the presence of an extremely strong
break (> 4.2 at the 2 sigma level) between the Z band above the line, where the
AB magnitude is 25.5, and the R band below, where the object is no longer
visible at a 2 sigma upper limit of 27.1 (AB mags). These properties are only
consistent with this object's being a high-z Ly alpha emitter. A 10,800 s
spectrum obtained with LRIS yields a redshift of 5.74. The object is similar in
its continuum shape, line properties, and observed equivalent width to the
z=5.60 galaxy, HDF 4-473.0, as recently described by Weymann et al. (1998), but
is 2-3 times more luminous in the line and in the red continuum. For H_0 = 65
km s^{-1} Mpc^{-1} and q_0 = (0.02, 0.5) we would require star formation rates
of around (40, 7) solar masses per year to produce the UV continuum in the
absence of extinction.Comment: 5 pages, 4 figures, Latex with emulateapj style file; to appear in
the Astrophysical Journal (Letters
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