41 research outputs found
A Size of ~10 Mpc for the Ionized Bubbles at the End of Cosmic Reionization
The first galaxies to appear in the universe at redshifts z>20 created
ionized bubbles in the intergalactic medium of neutral hydrogen left over from
the Big-Bang. It is thought that the ionized bubbles grew with time, surrounded
clusters of dwarf galaxies and eventually overlapped quickly throughout the
universe over a narrow redshift interval near z~6. This event signaled the end
of the reionization epoch when the universe was a billion years old. Measuring
the hitherto unknown size distribution of the bubbles at their final overlap
phase is a focus of forthcoming observational programs aimed at highly
redshifted 21cm emission from atomic hydrogen. Here we show that the combined
constraints of cosmic variance and causality imply an observed bubble size at
the end of the overlap epoch of ~10 physical Mpc, and a scatter in the observed
redshift of overlap along different lines-of-sight of ~0.15. This scatter is
consistent with observational constraints from recent spectroscopic data on the
farthest known quasars. Our novel result implies that future radio experiments
should be tuned to a characteristic angular scale of ~0.5 degrees and have a
minimum frequency band-width of ~8 MHz for an optimal detection of 21cm flux
fluctuations near the end of reionization.Comment: Accepted for publication in Nature. Press embargo until publishe
Cosmic Hydrogen Was Significantly Neutral a Billion Years After the Big Bang
The ionization fraction of cosmic hydrogen, left over from the big bang,
provides crucial fossil evidence for when the first stars and quasar black
holes formed in the infant universe. Spectra of the two most distant quasars
known show nearly complete absorption of photons with wavelengths shorter than
the Ly-alpha transition of neutral hydrogen, indicating that hydrogen in the
intergalactic medium (IGM) had not been completely ionized at a redshift z~6.3,
about a billion years after the big bang. Here we show that the radii of
influence of ionizing radiation from these quasars imply that the surrounding
IGM had a neutral hydrogen fraction of tens of percent prior to the quasar
activity, much higher than previous lower limits of ~0.1%. When combined with
the recent inference of a large cumulative optical depth to electron scattering
after cosmological recombination from the WMAP data, our result suggests the
existence of a second peak in the mean ionization history, potentially due to
an early formation episode of the first stars.Comment: 14 Pages, 2 Figures. Accepted for publication in Nature. Press
embargo until publishe
Dwarf Galaxy Formation Was Suppressed By Cosmic Reionization
A large number of faint galaxies, born less than a billion years after the
big bang, have recently been discovered. The fluctuations in the distribution
of these galaxies contributed to a scatter in the ionization fraction of cosmic
hydrogen on scales of tens of Mpc, as observed along the lines of sight to the
earliest known quasars. Theoretical simulations predict that the formation of
dwarf galaxies should have been suppressed after cosmic hydrogen was reionized,
leading to a drop in the cosmic star formation rate. Here we present evidence
for this suppression. We show that the post-reionization galaxies which
produced most of the ionizing radiation at a redshift z~5.5, must have had a
mass in excess of ~10^{10.6+/-0.4} solar masses or else the aforementioned
scatter would have been smaller than observed. This limiting mass is two orders
of magnitude larger than the galaxy mass that is thought to have dominated the
reionization of cosmic hydrogen (~10^8 solar masses). We predict that future
surveys with space-based infrared telescopes will detect a population of
smaller galaxies that reionized the Universe at an earlier time, prior to the
epoch of dwarf galaxy suppression.Comment: 19 pages, 3 figures. Accepted for publication in Nature; press
embargo until publishe
The Formation of the First Low-Mass Stars From Gas With Low Carbon and Oxygen Abundances
The first stars in the Universe are predicted to have been much more massive
than the Sun. Gravitational condensation accompanied by cooling of the
primordial gas due to molecular hydrogen, yields a minimum fragmentation scale
of a few hundred solar masses. Numerical simulations indicate that once a gas
clump acquires this mass, it undergoes a slow, quasi-hydrostatic contraction
without further fragmentation. Here we show that as soon as the primordial gas
- left over from the Big Bang - is enriched by supernovae to a carbon or oxygen
abundance as small as ~0.01-0.1% of that found in the Sun, cooling by
singly-ionized carbon or neutral oxygen can lead to the formation of low-mass
stars. This mechanism naturally accommodates the discovery of solar mass stars
with unusually low (10^{-5.3} of the solar value) iron abundance but with a
high (10^{-1.3} solar) carbon abundance. The minimum stellar mass at early
epochs is partially regulated by the temperature of the cosmic microwave
background. The derived critical abundances can be used to identify those
metal-poor stars in our Milky Way galaxy with elemental patterns imprinted by
the first supernovae.Comment: 14 pages, 2 figures (appeared today in Nature
Gravitational waves from binary supermassive black holes missing in pulsar observations.
This is the author accepted manuscript. The final version is available from AAAS via http://dx.doi.org/10.1126/science.aab1910Gravitational waves are expected to be radiated by supermassive black hole binaries formed during galaxy mergers. A stochastic superposition of gravitational waves from all such binary systems would modulate the arrival times of pulses from radio pulsars. Using observations of millisecond pulsars obtained with the Parkes radio telescope, we constrained the characteristic amplitude of this background, A(c,yr), to be <1.0 × 10(-15) with 95% confidence. This limit excludes predicted ranges for A(c,yr) from current models with 91 to 99.7% probability. We conclude that binary evolution is either stalled or dramatically accelerated by galactic-center environments and that higher-cadence and shorter-wavelength observations would be more sensitive to gravitational waves.The PPTA project was initiated with support from R.N.M.’s Australian Research Council (ARC) Federation Fellowship (grant FF0348478) and from CSIRO under that fellowship program. The PPTA project has also received support from ARC through Discovery Project grants DP0985272 and DP140102578. N.D.R.B. acknowledges support from a Curtin University research fellowship. G.H. and Y.L. are recipients of ARC Future Fellowships (respectively, grants FT120100595 and FT110100384). S.O. is supported by the Alexander von Humboldt Foundation. R.M.S. acknowledges travel support from CSIRO through a John Philip Award for excellence in early-career research. The authors declare no conflicts of interest. Data used in this analysis can be accessed via the Australian National Data Service (www.ands.org.au)
The Formation of the First Massive Black Holes
Supermassive black holes (SMBHs) are common in local galactic nuclei, and
SMBHs as massive as several billion solar masses already exist at redshift z=6.
These earliest SMBHs may grow by the combination of radiation-pressure-limited
accretion and mergers of stellar-mass seed BHs, left behind by the first
generation of metal-free stars, or may be formed by more rapid direct collapse
of gas in rare special environments where dense gas can accumulate without
first fragmenting into stars. This chapter offers a review of these two
competing scenarios, as well as some more exotic alternative ideas. It also
briefly discusses how the different models may be distinguished in the future
by observations with JWST, (e)LISA and other instruments.Comment: 47 pages with 306 references; this review is a chapter in "The First
Galaxies - Theoretical Predictions and Observational Clues", Springer
Astrophysics and Space Science Library, Eds. T. Wiklind, V. Bromm & B.
Mobasher, in pres
Black hole growth in the early Universe is self-regulated and largely hidden from view
The formation of the first massive objects in the infant Universe remains
impossible to observe directly and yet it sets the stage for the subsequent
evolution of galaxies. While some black holes with masses > billion solar
masses? have been detected in luminous quasars less than one billion years
after the Big Bang, these individual extreme objects have limited utility in
constraining the channels of formation of the earliest black holes. The initial
conditions of black hole seed properties are quickly erased during the growth
process. From deep, optimally stacked, archival X-ray observations, we measure
the amount of black hole growth in z=6-8 galaxies (0.7-1 billion years after
the Big Bang). Our results imply that black holes grow in tandem with their
hosts throughout cosmic history, starting from the earliest times. We find that
most copiously accreting black holes at these epochs are buried in significant
amounts of gas and dust that absorb most radiation except for the highest
energy X-rays. This suggests that black holes grow significantly more than
previously thought during these early bursts, and due to obscuration they do
not contribute to the re-ionization of the Universe with their ultraviolet
emission.Comment: Nature, in pres
Early star-forming galaxies and the reionization of the Universe
Star forming galaxies represent a valuable tracer of cosmic history. Recent
observational progress with Hubble Space Telescope has led to the discovery and
study of the earliest-known galaxies corresponding to a period when the
Universe was only ~800 million years old. Intense ultraviolet radiation from
these early galaxies probably induced a major event in cosmic history: the
reionization of intergalactic hydrogen. New techniques are being developed to
understand the properties of these most distant galaxies and determine their
influence on the evolution of the universe.Comment: Review article appearing in Nature. This posting reflects a submitted
version of the review formatted by the authors, in accordance with Nature
publication policies. For the official, published version of the review,
please see http://www.nature.com/nature/archive/index.htm
The JWST FRESCO Survey: Legacy NIRCam/Grism Spectroscopy and Imaging in the two GOODS Fields
We present the JWST Cycle 1 53.8hr medium program FRESCO, short for “First Reionization Epoch Spectroscopically Complete Observations”. FRESCO covers 62 arcmin2 in each of the two GOODS/CANDELS fields for a total area of 124 arcmin2 exploiting JWST’s powerful new grism spectroscopic capabilities at near-infrared wavelengths. By obtaining ∼2hr deep NIRCam/grism observations with the F444W filter, FRESCO yields unprecedented spectra at R ∼ 1600 covering 3.8 to 5.0 μm for most galaxies in the NIRCam field-of-view. This setup enables emission line measurements over most of cosmic history, from strong PAH lines at z ∼ 0.2 − 0.5, to Paα and Paβ at z ∼ 1 − 3, HeI and [SIII] at z ∼ 2.5 − 4.5, Hα and [NII] at z ∼ 5 − 6.5, up to [OIII] and Hβ for z∼7-9 galaxies. FRESCO’s grism observations provide total line fluxes for accurately estimating galaxy stellar masses and calibrating slit-loss corrections of NIRSpec/MSA spectra in the same field. Additionally, FRESCO results in a mosaic of F182M, F210M, and F444W imaging in the same fields to a depth of ∼28.2 mag (5 σ in 032 diameter apertures). Here, we describe the overall survey design and the key science goals that can be addressed with FRESCO. We also highlight several, early science results, including: spectroscopic redshifts of Lyman break galaxies that were identified almost 20 years ago, the discovery of broad-line active galactic nuclei at z &gt; 4, and resolved Paα maps of galaxies at z ∼ 1.4. These results demonstrate the enormous power for serendipitous discovery of NIRCam/grism observations
The JWST FRESCO survey: legacy NIRCam/grism spectroscopy and imaging in the two GOODS fields
We present the JWST cycle 1 53.8 h medium program FRESCO, short for 'First Reionization Epoch Spectroscopically Complete Observations'. FRESCO covers 62 arcmin2 in each of the two GOODS/CANDELS fields for a total area of 124 arcmin2 exploiting JWST's powerful new grism spectroscopic capabilities at near-infrared wavelengths. By obtaining ∼2 h deep NIRCam/grism observations with the F444W filter, FRESCO yields unprecedented spectra at R ∼1600 covering 3.8-5.0 μm for most galaxies in the NIRCam field of view. This setup enables emission line measurements over most of cosmic history, from strong PAH lines at z ∼0.2-0.5, to Pa α and Pa β at z ∼1-3, He i and [S iii] at z ∼2.5-4.5, H α and [N ii] at z ∼5-6.5, up to [O iii] and H β for z ∼7-9 galaxies. FRESCO's grism observations provide total line fluxes for accurately estimating galaxy stellar masses and calibrating slit-loss corrections of NIRSpec/MSA spectra in the same field. Additionally, FRESCO results in a mosaic of F182M, F210M, and F444W imaging in the same fields to a depth of ∼28.2 mag (5σ in 032 diameter apertures). Here, we describe the overall survey design and the key science goals that can be addressed with FRESCO. We also highlight several, early science results, including: spectroscopic redshifts of Lyman break galaxies that were identified almost 20 yr ago, the discovery of broad-line active galactic nuclei at z > 4, and resolved Pa α maps of galaxies at z ∼1.4. These results demonstrate the enormous power for serendipitous discovery of NIRCam/grism observations