177 research outputs found
Fluctuations of the intergalactic ionization field at redshift z ~ 2
(Abridged) Aims. To probe the spectral energy distribution (SED) of the
ionizing background radiation at z ~ 2 and to specify the sources contributing
to the intergalactic radiation field. Methods. The spectrum of a bright quasar
HS1103+6416 (zem = 2.19) contains five successive metal-line absorption systems
at zabs = 1.1923, 1.7193, 1.8873, 1.8916, and 1.9410. The systems are optically
thin and reveal multiple lines of different metal ions with the ionization
potentials lying in the extreme ultraviolet (EUV) range (1 Ryd to 0.2 keV). For
each system, the EUV SED of the underlying ionization field is reconstructed by
means of a special technique developed for solving the inverse problem in
spectroscopy. For the zabs = 1.8916 system, the analysis also involves the HeI
resonance lines of the Lyman series and the HeI 504 A continuum, which are seen
for the first time in any cosmic object except the Sun. Results. From one
system to another, the SED of the ionizing continuum changes significantly,
indicating that the intergalactic ionization field at z ~ 2 fluctuates at the
scale of at least Delta_z ~ 0.004. This is consistent with Delta_z ~ 0.01
estimated from HeII and HI Lyman-alpha forest measurements between the
redshifts 2 and 3.Comment: 29 pages, 18 figures, 3 tables, accepted for publication in A\&
QSOs and Absorption Line Systems Surrounding the Hubble Deep Field
We have imaged a 45x45 sq. arcmin. area centered on the Hubble Deep Field
(HDF) in UBVRI passbands, down to respective limiting magnitudes of
approximately 21.5, 22.5, 22.2, 22.2, and 21.2. The principal goals of the
survey are to identify QSOs and to map structure traced by luminous galaxies
and QSO absorption line systems in a wide volume containing the HDF. We have
selected QSO candidates from color space, and identified 4 QSOs and 2 narrow
emission-line galaxies (NELGs) which have not previously been discovered,
bringing the total number of known QSOs in the area to 19. The bright z=1.305
QSO only 12 arcmin. away from the HDF raises the northern HDF to nearly the
same status as the HDF-S, which was selected to be proximate to a bright QSO.
About half of the QSO candidates remain for spectroscopic verification.
Absorption line spectroscopy has been obtained for 3 bright QSOs in the field,
using the Keck 10m, ARC 3.5m, and MDM 2.4m telescopes. Five heavy-element
absorption line systems have been identified, 4 of which overlap the
well-explored redshift range covered by deep galaxy redshift surveys towards
the HDF. The two absorbers at z=0.5565 and z=0.5621 occur at the same redshift
as the second most populated redshift peak in the galaxy distribution, but each
is more than 7Mpc/h (comoving, Omega_M=1, Omega_L=0) away from the HDF line of
sight in the transverse dimension. This supports more indirect evidence that
the galaxy redshift peaks are contained within large sheet-like structures
which traverse the HDF, and may be precursors to large-scale ``pancake''
structures seen in the present-day galaxy distribution.Comment: 36 pages, including 9 figures and 8 tables. Accepted for publication
in the Astronomical Journa
Deuteronomy and Numbers
Four light isotopes - D, ^3He, ^4He and ^7Li - were produced by nuclear
reactions a few seconds after the big bang. New measurements of ^3He in the ISM
by Gloeckler and Geiss and of deuterium in high redshift hydrogen clouds by
Tytler and his collaborators provide further confirmation of big-bang
nucleosynthesis and new insight about the density of ordinary matter (baryons).Comment: 6 pages LaTeX with 1 eps Figur
Non-Gaussian Features of Transmitted Flux of QSO's Ly Absorption: Intermittent Exponent
We calculate the structure function and intermittent exponent of the 1.) Keck
data, which consists of 29 high resolution, high signal to noise ratio (S/N)
QSO Ly absorption spectra, and 2.)the Ly forest simulation
samples produced via the pseudo hydro scheme for the low density cold dark
matter (LCDM) model and warm dark matter (WDM) model with particle mass
and 1000 eV. These two measures detect not only
non-gaussianities, but also the type of non-gaussianty in the the field. We
find that, 1.) the structure functions of the simulation samples are
significantly larger than that of Keck data on scales less than about 100
h kpc, 2.) the intermittent exponent of the simulation samples is more
negative than that of Keck data on all redshifts considered, 3.) the
order-dependence of the structure functions of simulation samples are closer to
the intermittency of hierarchical clustering on all scales, while the Keck data
are closer to a lognormal field on small scales. These differences are
independent of noise and show that the intermittent evolution modeled by the
pseudo-hydro simulation is substantially different from observations, even
though they are in good agreement in terms of second and lower order
statistics. (Abridged)Comment: 17 pages, 13 figures. Accepted by Ap
Morphological Evolution and the Ages of Early-Type Galaxies in Clusters
Morphological and spectroscopic studies of high redshift clusters indicate
that a significant fraction of present-day early-type galaxies was transformed
from star forming galaxies at z<1. On the other hand, the slow luminosity
evolution of early-type galaxies and the low scatter in their color-magnitude
relation indicate a high formation redshift of their stars. In this paper we
construct models which reconcile these apparently contradictory lines of
evidence, and we quantify the effects of morphological evolution on the
observed photometric properties of early-type galaxies in distant clusters. We
show that in the case of strong morphological evolution the apparent luminosity
and color evolution of early-type galaxies are similar to that of a single age
stellar population formed at z=infinity, irrespective of the true star
formation history of the galaxies. Furthermore, the scatter in age, and hence
the scatter in color and luminosity, is approximately constant with redshift.
These results are consequences of the ``progenitor bias'': the progenitors of
the youngest low redshift early-type galaxies drop out of the sample at high
redshift. We construct models which reproduce the observed evolution of the
number fraction of early-type galaxies in rich clusters and their color and
luminosity evolution simultaneously. Our modelling indicates that approx. 50%
of early-type galaxies were transformed from other galaxy types at z<1, and
their progenitor galaxies may have had roughly constant star formation rates
prior to morphological transformation. After correcting the observed evolution
of the mean M/L_B ratio for the maximum progenitor bias we find that the mean
luminosity weighted formation redshift of stars in early-type galaxies
z_*=2.0^{+0.3}_{-0.2} for Omega_m=0.3 and Omega_Lambda=0.7. [ABRIDGED]Comment: Accepted for publication in The Astrophysical Journal. 13 pages, 6
figure
Spectral shape of the UV ionizing background and HeII absorption at redshifts 1.8 < z < 2.9
The shape of the UV ionizing background is reconstructed from optically thin
metal absorption-line systems identified in spectra of HE2347-4342, Q1157+3143,
and HS1700+6416 in the redshift interval 1.8 < z < 2.9. The systems are
analyzed by means of the Monte Carlo Inversion method completed with the
spectral shape recovering procedure. The UVB spectral shape fluctuates at 2.4 <
z < 2.9 mostly due to radiative transfer processes in the clumpy IGM. At z <
1.8, the IGM becomes almost transparent both in the HI and HeII Lyman continua
and the variability of the spectral shape comes from diversity of spectral
indices describing the QSO/AGN intrinsic radiation. At z > 2.4, the recovered
spectral shapes show intensity depression between 3 and 4 Ryd due to HeII
Ly-alpha absorption in the IGM clouds (line blanketing) and continuous medium
(true Gunn-Petersen effect). The mean HeII Ly-alpha opacity estimated from the
depth of this depression corresponds within 1-2sigma to the values directly
measured from the HI/HeII Ly-alpha forest towards the quasars studied. The
observed scatter in eta = N(HeII)/N(HI) and anti-correlation between N(HI) and
eta can be explained by the combined action of variable spectral softness and
differences in the mean gas density between the absorbing clouds. Neither of
the recovered spectral shapes show features which can be attributed to the
putative input of radiation from soft sources like starburst galaxies.Comment: 20 pages, 20 figures. Accepted for publication in A&
The Type Ic Supernova 1994I in M51: Detection of Helium and Spectral Evolution
We present a series of spectra of SN 1994I in M51, starting 1 week prior to maximum brightness. The nebular phase began about 2 months after the explosion; together with the rapid decline of the optical light, this suggests that the ejected mass was small. Although lines of He I in the optical region are weak or absent, consistent with the Type Ic classification, we detect strong He I λ10830 absorption during the first month past maximum. Thus, if SN 1994I is a typical Type Ic supernova, the atmospheres of these objects cannot be completely devoid of helium. The emission-line widths are smaller than predicted by the model of Nomoto and coworkers, in which the iron core of a low-mass carbon-oxygen star collapses. They are, however, larger than in Type Ib supernovae
The CORALS Survey I: New Estimates of the Number Density and Gas Content of Damped Lyman Alpha Systems Free from Dust Bias
(Abridged) We present the first results from the Complete Optical and Radio
Absorption Line System (CORALS) survey. We have compiled a homogeneous sample
of radio-selected QSOs from the Parkes Catalogue and searched for damped Lyman
alpha systems (DLAs) towards every target, irrespective of its optical
magnitude. This approach circumvents selection effects -- particularly from
intervening dust -- which have long been suspected to affect DLA surveys in
optically-selected, magnitude-limited QSO samples. The CORALS data set consists
of 66 z_em > 2.2 QSOs in which 22 DLAs with absorption redshifts 1.8 < z_abs <
z_em have been identified over a total redshift interval Delta z = 55.46. In
this first paper of the CORALS series we describe the sample, present
intermediate resolution spectroscopy and determine the population statistics of
DLAs. We deduce a value of the neutral gas mass density traced by DLAs
(expressed as a fraction of the closure density) log Omega_DLA h =
-2.59^{+0.17}_{-0.24}, and a number density of DLAs per unit redshift n(z) =
0.31^{+0.09}_{-0.08}, both at a mean redshift = 2.37. Taking into account
the errors, we conclude that dust-induced bias in previous surveys may have led
to an underestimate of these quantities by at most a factor of two and we have
not uncovered a previously unrecognised population of high column density DLAs
in front of faint QSOs.Comment: 25 pages, accepted for publication in A&
Spectral shape of the UV ionizing background and OVI absorbers at z ~ 1.5 towards HS0747+4259
We report on high resolution spectra of the bright QSO HS0747+4259 (zem =
1.90, V = 15.8) observed to search for intermediate redshift OVI absorption
systems. The spectra were obtained by means of the Space Telescope Imaging
Spectrograph (STIS) at the Hubble Space Telescope (HST) and the High Resolution
Echelle Spectrometer (HIRES) at the W. M. Keck telescope. We identify 16 OVI
systems in the range 1.07 <= z <= 1.87. Among them, six systems with zabs =
1.46-1.8 exhibit a sufficient number of lines of different ionic transitions to
estimate the shape of the ionizing radiation field in the range 1 Ryd < E < 10
Ryd. All recovered UV ionizing spectra are characterized by the enhanced
intensity at E > 3 Ryd compared to the model spectrum of Haardt and Madau
(1996). This is in line with the observational evidence of a deficiency of
strong Ly-alpha absorbers with N(HI) > 10^{15} cm^{-2}, at z < 2. The UV
background shows significant local variations: the spectral shape estimated at
z = 1.59 differs from that obtained at z = 1.81 and 1.73. A possible cause of
these variations is the presence of a QSO/AGN at z ~= 1.54-1.59 close to the
line of sight. No features favoring the input of stellar radiation to the
ionizing background are detected, limiting the escape fraction of the galactic
UV photons to f_esc < 0.05.Comment: 14 pages, 7 figures, 5 tables; accepted for publication in A&
Geometrical Effects of Baryon Density Inhomogeneities on Primordial Nucleosynthesis
We discuss effects of fluctuation geometry on primordial nucleosynthesis. For
the first time we consider condensed cylinder and cylindrical-shell fluctuation
geometries in addition to condensed spheres and spherical shells. We find that
a cylindrical shell geometry allows for an appreciably higher baryonic
contribution to be the closure density (\Omega_b h_{50}^2 \la 0.2) than that
allowed in spherical inhomogeneous or standard homogeneous big bang models.
This result, which is contrary to some other recent studies, is due to both
geometry and recently revised estimates of the uncertainties in the
observationally inferred primordial light-element abundances. We also find that
inhomogeneous primordial nucleosynthesis in the cylindrical shell geometry can
lead to significant Be and B production. In particular, a primordial beryllium
abundance as high as [Be] = 12 + log(Be/H) is possible while still
satisfying all of the light-element abundance constraints.Comment: Latex, 20 pages + 11 figures(not included). Entire ps file with
embedded figures available via anonymous ftp at
ftp://genova.mtk.nao.ac.jp/pub/prepri/bbgeomet.ps.g
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