1,186 research outputs found
An IR-Selected Galaxy Cluster at z=1.27
We report the discovery of a galaxy cluster at z=1.27. ClG J0848+4453 was
found in a near-IR field survey as a high density region of objects with very
red J-K colors. Optical spectroscopy of a limited number of 24 < R < 25 objects
in the area shows that 6 galaxies within a 90 arcsec (0.49/h Mpc, q_O = 0.1)
diameter region lie at z=1.273 +/- 0.002. Most of these 6 member galaxies have
broad-band colors consistent with the expected spectral energy distribution of
a passively-evolving elliptical galaxy formed at high redshift. An additional 2
galaxies located ~2 arcmin from the cluster center are also at z=1.27. Using
all 8 of these spectroscopic members, we estimate the velocity dispersion is
700 +/- 180 km/s, similar to that of Abell R=1 clusters in the present epoch. A
deep Rosat PSPC observation detects X-ray emission at the 5 sigma level
coincident with the nominal cluster center. Assuming that the X-ray flux is
emitted by hot gas trapped in the potential well of a collapsed system (no AGN
is known to exist in the area), the resulting X-ray luminosity in the rest
frame 0.1-2.4 keV band of L_x = 1.5 x 10^44 ergs/s suggests the presence of a
moderately large mass. ClG J0848+4453 is the highest redshift cluster found
without targetting a central active galaxy.Comment: Accepted for publication in The Astronomical Journal; 22 pages, 6
figures; corrected titl
Discovery of a Color-Selected Quasar at z=5.50
We present observations of RD J030117+002025, a quasar at z=5.50 discovered
from deep, multi-color, ground-based observations covering 74 square arcmin.
This is the most distant quasar or AGN currently known. The object was targeted
as an R-band dropout, with R(AB)>26.3 (3-sigma limit in a 3 arcsec diameter
region), I(AB)=23.8, and z(AB)=23.4. The Keck/LRIS spectrum shows broad
Lyman-alpha/NV emission and sharp absorption decrements from the
highly-redshifted hydrogen forests. The fractional continuum depression due to
the Lyman-alpha forest is D(A)=0.90. RD J030117+002025 is the least luminous,
high-redshift quasar known (M(B)~-22.7).Comment: 9 pages, 2 figures; to appear in the The Astrophysical Journal
Letter
Gemini-South + FLAMINGOS Demonstration Science: Near-Infrared Spectroscopy of the z=5.77 Quasar SDSS J083643.85+005453.3
We report an infrared 1-1.8 micron (J+H-bands), low-resolution (R=450)
spectrogram of the highest-redshift radio-loud quasar currently known, SDSS
J083643.85+005453.3, obtained during the spectroscopic commissioning run of the
FLAMINGOS multi-object, near-infrared spectrograph at the 8m Gemini-South
Observatory. These data show broad emission from both CIV 1549 and CIII] 1909,
with strengths comparable to lower-redshift quasar composite spectra. The
implication is that there is substantial enrichment of the quasar environment,
even at times less than a billion years after the Big Bang. The redshift
derived from these features is z = 5.774 +/- 0.003, more accurate and slightly
lower than the z = 5.82 reported in the discovery paper based on the
partially-absorbed Lyman-alpha emission line. The infrared continuum is
significantly redder than lower-redshift quasar composites. Fitting the
spectrum from 1.0 to 1.7 microns with a power law f(nu) ~ nu^(-alpha), the
derived power law index is alpha = 1.55 compared to the average continuum
spectral index = 0.44 derived from the first SDSS composite quasar.
Assuming an SMC-like extinction curve, we infer a color excess of E(B-V) = 0.09
+/- 0.01 at the quasar redshift. Only approximately 6% of quasars in the
optically-selected Sloan Digital Sky Survey show comparable levels of dust
reddening.Comment: 10 pages, 1 figure; to appear in the Astrophysical Journal Letter
SPICES II. Optical and Near-Infrared Identifications of Faint X-Ray Sources from Deep Chandra Observations of Lynx
We present our first results on field X-ray sources detected in a deep, 184.7
ks observation with the ACIS-I camera on Chandra. The observations target the
Lynx field of SPICES, and contains three known X-ray-emitting clusters out to
z=1.27. Not including the known clusters, in the 17'x17' ACIS-I field we detect
132 sources in the 0.5-2 keV (soft) X-ray band down to a limiting flux of
\~1.7e-16 erg/cm2/s and 111 sources in the 2-10 keV (hard) X-ray band down to a
limiting flux of ~1.3e-15 erg/cm2/s. The combined catalog contains a total of
153 sources, of which 42 are detected only in the soft band and 21 are detected
only in the hard band. Confirming previous Chandra results, we find that the
fainter sources have harder X-ray spectra, providing a consistent solution to
the long-standing `spectral paradox'. From deep optical and near-infrared
follow-up data, 77% of the X-ray sources have optical counterparts to I=24 and
71% of the X-ray sources have near-infrared counterparts to K=20. Four of the
24 sources in the near-IR field are associated with extremely red objects
(EROs; I-K>4). We have obtained spectroscopic redshifts with the Keck
telescopes of 18 of the Lynx Chandra sources. These sources comprise a mix of
broad-lined active galaxies, apparently normal galaxies, and two late-type
Galactic dwarfs. Intriguingly, one Galactic source is identified with an M7
dwarf exhibiting non-transient, hard X-ray emission. We review non-AGN
mechanisms to produce X-ray emission and discuss properties of the Lynx Chandra
sample in relation to other samples of X-ray and non-X-ray sources.Comment: 42 pages, 16 figures. Accepted for publication in the May 2002
Astronomical Journa
An X-ray Selected Galaxy Cluster at z=1.26
We report the discovery of an X-ray luminous galaxy cluster at z=1.26.
RXJ0848.9+4452 was selected as an X-ray cluster candidate in the ROSAT Deep
Cluster Survey, on the basis of its spatial extent. Deep optical and near-IR
imaging have revealed a galaxy overdensity around the peak of the X-ray
emission, with a significant excess of red objects with J-K colors typical of
elliptical galaxies at z>1. Spectroscopic observations at the Keck II telescope
have secured 6 galaxy redshifts in the range 1.257=1.261), within
a 35 arcsec radius around the peak X-ray emission. This system lies only 4.2
arcmin away (5.0 h^{-1}_{50} comoving Mpc, q_0=0.5) from the galaxy cluster ClG
J0848+4453, which was identified by Stanford et al. (1997) at z=1.273 in a
near-IR field galaxy survey, and is also known to be X-ray luminous. Assuming
that the X-ray emission is entirely due to hot intra-cluster gas, both these
systems have similar rest frame luminosities L_x ~=1x10^{44} ergs/s (0.5-2.0
keV band). In combination with our spectrophotometric data for the entire 30
arcmin^2 field, this suggests the presence of a superstructure, consisting of
two collapsed, possibly virialized clusters, the first detected to date at z>1.Comment: To appear in The Astronomical Journal, 24 pages, 8 figures, 1 color
jpg plate (fig.7), see http://www.eso.org/~prosati/lynx/plate_fig7.jp
Recommended from our members
Plasma Aβ40 and Aβ42 and Alzheimer's Disease: Relation to Age, Mortality, and Risk
BACKGROUND: Plasma amyloid [beta]-peptide (A[beta]) 40 and A[beta]42 levels are increased in persons with mutations causing early-onset familial Alzheimer's disease (AD). Plasma A[beta]42 levels were also used to link microsatellite genetic markers to a putative AD genetic locus on chromosome 10 and were observed in patients with incipient sporadic AD. METHODS: The authors measured plasma A[beta]40 and A[beta]42 levels using a sandwich ELISA after the initial examination of 530 individuals participating in an epidemiologic study of aging and dementia. Participants were examined at 18-month intervals, and plasma A[beta]40 and A[beta]42 levels were repeated in 307 subjects 3 years after baseline. RESULTS: Compared with individuals who never developed AD, patients with AD at baseline and those who developed AD during the follow-up had significantly higher A[beta]42, but not A[beta]40, plasma levels. The risk of AD in the highest quartile of plasma A[beta]42 was increased by more than twofold over that in the lowest quartile. The highest plasma A[beta]42 levels were observed in patients with AD who died during the follow-up. Plasma A[beta]42, but not A[beta]40, levels decreased over time in patients with newly acquired AD. CONCLUSIONS: Plasma A[beta]40 and A[beta]42 increase with age and are strongly correlated with each other. Plasma A[beta]40 and A[beta]42 levels are elevated in some patients before and during the early stages of AD but decline thereafter. High plasma A[beta]42 levels may also be associated with mortality in patients with AD
Thermal Energy for Lunar In Situ Resource Utilization: Technical Challenges and Technology Opportunities
Oxygen production from lunar raw materials is critical for sustaining a manned lunar base but is very power intensive. Solar concentrators are a well-developed technology for harnessing the Sun s energy to heat regolith to high temperatures (over 1375 K). The high temperature and potential material incompatibilities present numerous technical challenges. This study compares and contrasts different solar concentrator designs that have been developed, such as Cassegrains, offset parabolas, compound parabolic concentrators, and secondary concentrators. Differences between concentrators made from lenses and mirrors, and between rigid and flexible concentrators are also discussed. Possible substrate elements for a rigid mirror concentrator are selected and then compared, using the following (target) criteria: (low) coefficient of thermal expansion, (high) modulus of elasticity, and (low) density. Several potential lunar locations for solar concentrators are compared; environmental and processing-related challenges related to dust and optical surfaces are addressed. This brief technology survey examines various sources of thermal energy that can be utilized for materials processing on the lunar surface. These include heat from nuclear or electric sources and solar concentrators. Options for collecting and transporting thermal energy to processing reactors for each source are examined. Overall system requirements for each thermal source are compared and system limitations, such as maximum achievable temperature are discussed
- …