14,690 research outputs found
Searches For Primeval Galaxies
A primeval galaxy represents the earliest stages of a galaxy's life and as
such provides clues to the early history of the Universe and the evolution of
stars and galaxies. Over the last 20 years astronomers have been engaged in the
quest to detect the faint signals from these objects, believed to lie at a
distance comparable with the size of the Universe. A wide variety of
observational techniques have been employed in this search, with astronomers
eagerly awaiting each new generation of astronomical telescope or detector in
the hope of finally solving the mystery to the origin of galaxies -- or at
least placing new and interesting constraints. Until recently, primeval
galaxies have eluded detection in these searches, however experiments over the
last couple of years which use either 10m-class optical telescopes or
state-of-the-art submillimetre array detectors, may hold the clue to the origin
of structure as they have finally uncovered what appears to be a widespread
population of young galaxies.Comment: 17 pages, 5 figures. Paper to be published in Contemporary Physic
Improved Searches for HI in Three Dwarf Spheroidal Galaxies
Previous searches for HI in our Galaxy's dwarf spheroidal companions have not
been complete enough to settle the question of whether or not these galaxies
have HI, especially in their outer parts. We present VLA observations of three
dwarf spheroidals: Fornax, Leo II, and Draco, all of which have known stellar
velocities. The new data show no HI emission or absorption. Column density
limits in emission are 4--7 x 10^18 atoms/cm^2 in the centers of the galaxies.
The importance of the new observations is that they cover larger areas than
previous searches and they are less plagued by confusion with foreground
(Galactic) HI. The apparent absence of neutral gas in the Fornax dwarf
spheroidal is especially puzzling because recent photometry shows evidence of
stars only 10^8 years old. We discuss whether the VLA observations could have
missed significant amounts of HI.Comment: 20 pages, 7 figures; to appear in the Astronomical Journa
A candidate redshift z ~ 10 galaxy and rapid changes in that population at an age of 500 Myr
Searches for very-high-redshift galaxies over the past decade have yielded a
large sample of more than 6,000 galaxies existing just 900-2,000 million years
(Myr) after the Big Bang (redshifts 6 > z > 3; ref. 1). The Hubble Ultra Deep
Field (HUDF09) data have yielded the first reliable detections of z ~ 8
galaxies that, together with reports of a gamma-ray burst at z ~ 8.2 (refs 10,
11), constitute the earliest objects reliably reported to date. Observations of
z ~ 7-8 galaxies suggest substantial star formation at z > 9-10. Here we use
the full two-year HUDF09 data to conduct an ultra-deep search for z ~ 10
galaxies in the heart of the reionization epoch, only 500 Myr after the Big
Bang. Not only do we find one possible z ~ 10 galaxy candidate, but we show
that, regardless of source detections, the star formation rate density is much
smaller (~10%) at this time than it is just ~200 Myr later at z ~ 8. This
demonstrates how rapid galaxy build-up was at z ~ 10, as galaxies increased in
both luminosity density and volume density from z ~ 8 to z ~ 10. The 100-200
Myr before z ~ 10 is clearly a crucial phase in the assembly of the earliest
galaxies.Comment: 41 pages, 14 figures, 2 tables, Nature, in pres
The Cosmic Gamma-Ray Bursts
Cosmic gamma-ray bursts are one of the great frontiers of astrophysics today.
They are a playground of relativists and observers alike. They may teach us
about the death of stars and the birth of black holes, the physics in extreme
conditions, and help us probe star formation in the distant and obscured
universe. In this review we summarise some of the remarkable progress in this
field over the past few years. While the nature of the GRB progenitors is still
unsettled, it now appears likely that at least some bursts originate in
explosions of very massive stars, or at least occur in or near the regions of
massive star formation. The physics of the burst afterglows is reasonably well
understood, and has been tested and confirmed very well by the observations.
Bursts are found to be beamed, but with a broad range of jet opening angles;
the mean gamma-ray energies after the beaming corrections are ~ 10^51 erg.
Bursts are associated with faint ~ 25 mag) galaxies at cosmological
redshifts, with ~ 1. The host galaxies span a range of luminosities and
morphologies, but appear to be broadly typical for the normal, actively
star-forming galaxy populations at comparable redshifts and magnitudes. Some of
the challenges for the future include: the nature of the short bursts and
possibly other types of bursts and transients; use of GRBs to probe the
obscured star formation in the universe, and possibly as probes of the very
early universe; and their detection as sources of high-energy particles and
gravitational waves.Comment: An invited review, to appear in: Proc. IX Marcel Grossmann Meeting,
eds. V. Gurzadyan, R. Jantzen, and R. Ruffini, Singapore: World Scientific,
in press (2001); Latex file, 33 pages, 22 eps figures, style files include
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
The Evolution of X-ray Clusters of Galaxies
Considerable progress has been made over the last decade in the study of the
evolutionary trends of the population of galaxy clusters in the Universe. In
this review we focus on observations in the X-ray band. X-ray surveys with the
ROSAT satellite, supplemented by follow-up studies with ASCA and Beppo-SAX,
have allowed an assessment of the evolution of the space density of clusters
out to z~1, and the evolution of the physical properties of the intra-cluster
medium out to z~0.5. With the advent of Chandra and Newton-XMM, and their
unprecedented sensitivity and angular resolution, these studies have been
extended beyond redshift unity and have revealed the complexity of the
thermodynamical structure of clusters. The properties of the intra-cluster gas
are significantly affected by non-gravitational processes including star
formation and Active Galactic Nucleus (AGN) activity. Convincing evidence has
emerged for modest evolution of both the bulk of the X-ray cluster population
and their thermodynamical properties since redshift unity. Such an
observational scenario is consistent with hierarchical models of structure
formation in a flat low density universe with Omega_m=0.3 and sigma_8=0.7-0.8
for the normalization of the power spectrum. Basic methodologies for
construction of X-ray-selected cluster samples are reviewed and implications of
cluster evolution for cosmological models are discussed.Comment: 40 pages, 15 figures. Full resolution figures can be downloaded from
http://www.eso.org/~prosati/ARAA
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