358 research outputs found
Critical Metallicities for Second-Generation Stars
The first massive stars may influence the formation of second-generation
stars, in part by their metal enrichment of the surrounding gas. We investigate
the "critical metallicity", defined as the the value, Z_crit, at which
primordial gas cools more efficiently by fine-structure lines of O I (63.18
microns, Si II 34.8 microns, Fe II (25.99 and 35.35 microns), and C II (157.74
microns) than by either H I or H2 line emission. We explore the time-dependent
thermodynamics and fragmentation of cooling gas at redshifts z = 10-30, seeded
by trace heavy elements expelled from early supernovae. Because different modes
of nucleosynthesis (alpha-process, Fe-group) produce abundance ratios far from
solar values, these early stellar populations are likely to be influenced by O,
Si, and Fe cooling. Our models also include radiative coupling of the fine
structure lines and H2 to the cosmic microwave background (CMB), which sets a
temperature floor (70-80K at z = 25-30) that may increase the Jeans mass. The
H2 forms from catalytic effects of electrons left over from the recombination
epoch or produced during virialization. These electrons form the H^- ion (H + e
-> H- + gamma), which in turn forms H2 through associative detachment (H- + H
-> H2 + e). In virialized halos at z = 10-30, the gas densities (n = 1-100
cm^{-3}) are well below the critical densities, n_cr = 10^{5-6} cm^{-3}, at
which (O, Si, Fe) fine-structure lines reach LTE populations and produce their
most efficient cooling. Thus, Z_crit may initially exceed 0.01 Z_sun at n =
1-100 cm^{-3}, and then drop to 10^{-3.5} Z_sun at n = 10^6 cm^{-3}, where the
Jeans mass may be imprinted on the stellar mass function. Primordial clouds of
10^8 M_sun at 0.01 Z_sun and 200K will produce redshifted fine structure lines,
with fluxes between 10^{-22} and 10^{-21} Watts/m^2 at z = 4.Comment: From "First Stars III" Conference (6 pages incl 4 figures
HST/COS Observations of the Quasar Q0302-003: Probing the He II Reionization Epoch and QSO Proximity Effects
Q0302-003 () was the first quasar discovered that showed
a He II Gunn-Peterson trough, a sign of incomplete helium reionization at . We present its HST/Cosmic Origins Spectrograph far-UV medium-resolution
spectrum, which resolves many spectral features for the first time, allowing
study of the quasar itself, the intergalactic medium, and quasar proximity
effects. Q0302-003 has a harder intrinsic extreme-UV spectral index than
previously claimed, as determined from both a direct fit to the spectrum
(yielding ) and the helium-to-hydrogen ion ratio in the
quasar's line-of-sight proximity zone. Intergalactic absorption along this
sightline shows that the helium Gunn-Peterson trough is largely black in the
range , apart from ionization due to local sources, indicating
that helium reionization has not completed at these redshifts. However, we
tentatively report a detection of nonzero flux in the high-redshift trough when
looking at low-density regions, but zero flux in higher-density regions. This
constrains the He II fraction to be a few percent, suggesting helium
reionization has progressed substantially by . The Gunn-Peterson
trough recovers to a He II Ly forest at . We confirm a
transmission feature due to the ionization zone around a quasar just
off the sightline, and resolve the feature for the first time. We discover a
similar such feature possibly caused by a luminous quasar further
from the sightline, which suggests that this quasar has been luminous for >34
Myr.Comment: ApJ accepted version; 20 pages, 16 figure
Cosmic Metal Production and the Contribution of QSO Absorption Systems to the Ionizing Background
The recent discovery by Cowie \etal and Tytler \etal of metals in the \Lya
clouds shows that the intergalactic medium (IGM) at high redshift is
contaminated by the products of stars, and suggests that ionizing photons from
massive star formation may be a significant contributor to the UV background
radiation at early epochs. We assess the validity of the stellar
photoionization hypothesis. Based on recent computations of metal yields and
O-star Lyman continuum (LyC) fluxes, we find that 0.2\% of the rest-mass energy
of the metals produced is radiated as LyC. By modeling the transfer of ionizing
radiation through the IGM and the rate of chemical enrichment, we demonstrate
that the background intensity of photons at 1 Ryd that accompanies the
production of metals in the \Lya forest clouds may be significant, approaching
0.5\times 10^{-21}\uvunits at if the LyC escape fraction is
\gta 0.25. Together with quasars, massive stars could then, in principle,
provide the hydrogen and helium LyC photons required to ionize the universe at
high redshifts. We propose that observations of the \HeII Gunn-Peterson effect
and of the metal ionization states of the \Lya forest and Lyman-limit absorbers
should show the signature of a stellar spectrum. We also note that the stellar
photoionization model fails if a large fraction of the UV radiation emitted
from stars cannot escape into the IGM, as suggested by the recent {\it Hopkins
Ultraviolet Telescope} observations by Leitherer \etal of low-redshift
starburst galaxies, or if most of the metals observed at were
produced at much earlier epochs.Comment: ApJ, in press; uuencoded, compressed, PS fil
The Low-z Intergalactic Medium. III. HI and Metal Absorbers at z<0.4
We conduct an ultraviolet (HST and FUSE) spectroscopic survey of HI (Lyman
lines) and seven metal ions (OVI, NV, CIV, CIII, SiIV, SiIII, FeIII) in the
low-redshift intergalactic medium (IGM) at z<0.4. We analyzed 650 Lya absorbers
over redshift pathlength Delta z=5.27, detecting numerous absorbers: 83 OVI
systems, 39 CIII, 53 SiIII, 24 CIV, 24 NV, and so on. Our survey yields
distributions in column density and estimates of the IGM baryon content and
metallicities of C, N, O in the IGM. In the low-z IGM, we have accounted for
~40% of the baryons: 30% in the photoionized Lya forest and 10% in the
(T=10^5-6 K) warm-hot intergalactic medium (WHIM) traced by OVI. Statistical
metallicities of C, N, O ions are consistent with the canonical (z=0) value of
10% solar, with considerable scatter. Improved statistics for weak OVI
absorbers allows us to estimate Omega_WHIM/Omega_b=0.073+-0.008 down to
logN_OVI=13.4. NV absorption is well-correlated with OVI and both ions show
similarly steep power-law indices dN/dz N^-beta with beta_OVI beta_NV 2 while
beta_HI=1.7. We conclude that OVI and NV are reliable tracers of the portion of
the WHIM at T=10^5-6 K. CIV may be present in both collisional and photoionized
phases; N_CIV correlates poorly with both N_HI and N_OVI and
beta_HI<beta_CIV<beta_OVI. The ions CIII, SiIII, and SiIV are well correlated
with HI and show patterns typical of photoionization. Adjacent ion stages of
the same element (CIII/IV and SiIII/IV) provide useful constraints on the
photoionization parameter, logU=-1.5+-0.5. Comparison of SiIV and CIV with
high-z surveys shows a modest increase in line density, consistent with
increasing IGM metallicity at recent epochs.Comment: Submitted to ApJ, 27 pages in ApJ format (figure and discussion
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