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A γ-ray burst at a redshift of z ≈ 8.2

29 Oct 2009-Nature (Nature Publishing Group)-Vol. 461, Iss: 7268, pp 1254-1257
TL;DR: In this paper, the authors reported that GRB 090423 lies at a redshift of z approximate to 8.2, implying that massive stars were being produced and dying as GRBs similar to 630 Myr after the Big Bang.
Abstract: Long-duration gamma-ray bursts (GRBs) are thought to result from the explosions of certain massive stars(1), and some are bright enough that they should be observable out to redshifts of z > 20 using current technology(2-4). Hitherto, the highest redshift measured for any object was z = 6.96, for a Lyman-alpha emitting galaxy(5). Here we report that GRB 090423 lies at a redshift of z approximate to 8.2, implying that massive stars were being produced and dying as GRBs similar to 630 Myr after the Big Bang. The burst also pinpoints the location of its host galaxy.

Summary (1 min read)

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Summary

  • Hitherto, the highest redshift measured for any object was z 5 6.96, for a Lyman-a emitting galaxy5.
  • The full grizYJHK spectral energy distribution (SED) obtained ,17 h after burst gives a photometric redshift of z 5 8:06z0:21{0:28, assuming a simple intergalactic medium (IGM) absorption model.
  • 21CRESST and NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.
  • Finding such events is not an unreasonable hope: the most extreme GRBs have had afterglows that were intrinsically significantly brighter than that of GRB 090423 at the same rest-frame time3,4, and their first spectra were recorded more than 15 h after the burst.
  • The infrared light curve was obtained using UKIRT, Gemini North, the MPI/ESO 2.2-m telescope and the VLT.
  • Error bars are 1s (68% confidence level) and the absolute magnitude scale corresponds to absolute AB magnitudes at 0.136mm.
  • Probing the neutral fraction of the IGM with GRBs during the epoch of reionization.

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LETTERS
A c-ray burst at a redshift of z < 8.2
N. R. Tanvir
1
, D. B. Fox
2
, A. J. Levan
3
, E. Berger
4
, K. Wiersema
1
, J. P. U. Fynbo
5
, A. Cucchiara
2
, T. Kru
¨
hler
6,7
,
N. Gehrels
8
, J. S. Bloom
9
, J. Greiner
6
, P. A. Evans
1
, E. Rol
10
, F. Olivares
6
, J. Hjorth
5
, P. Jakobsson
11
, J. Farihi
1
,
R. Willingale
1
, R. L. C. Starling
1
, S. B. Cenko
9
, D. Perley
9
, J. R. Maund
5
, J. Duke
1
, R. A. M. J. Wijers
10
, A. J. Adamson
12
,
A. Allan
13
, M. N. Bremer
14
, D. N. Burrows
2
, A. J. Castro-Tirado
15
, B. Cavanagh
12
, A. de Ugarte Postigo
16
,
M. A. Dopita
17
, T. A. Fatkhullin
18
, A. S. Fruchter
19
, R. J. Foley
4
, J. Gorosabel
15
, J. Kennea
2
, T. Kerr
12
, S. Klose
20
,
H. A. Krimm
21,22
, V. N. Komarova
18
, S. R. Kulkarni
23
, A. S. Moskvitin
18
, C. G. Mundell
24
, T. Naylor
13
, K. Page
1
,
B. E. Penprase
25
, M. Perri
26
, P. Podsiadlowski
27
, K. Roth
28
, R. E. Rutledge
29
, T. Sakamoto
21
, P. Schady
30
, B. P. Schmidt
17
,
A. M. Soderberg
4
, J. Sollerman
5,31
, A. W. Stephens
28
, G. Stratta
26
, T. N. Ukwatta
8,32
, D. Watson
5
, E. Westra
4
,
T. Wold
12
& C. Wolf
27
Long-duration c-ray bursts (GRBs) are thought to result from the
explosions of certain massive stars
1
, and some are bright enough
that they should be observable out to redshifts of z . 20 using
current technology
2–4
. Hitherto, the highest redshift measured
for any object was z 5 6.96, for a Lyman-a emitting galaxy
5
.
Here we report that GRB 090423 lies at a redshift of z < 8.2, imply-
ing that massive stars were being produced and dying as GRBs
630 Myr after the Big Bang. The burst also pinpoints the location
of its host galaxy.
GRB 090423 was detected by the Burst Alert Telescope (BAT) on
NASA’s Swift satellite
6
at 07:55:19 UT on 23 April 2009. Observations
with Swift’s X-ray Telescope (XRT), which began 73 s after the trig-
ger, revealed a variable X-ray counterpart and localized its position to
a precision of 2.3 arcsec (at the 90% confidence level). Ground-based
optical observations in the r, i and z filters starting within a few min-
utes of the burst revealed no counterpart at these wavelengths
(Supplementary Information).
The United Kingdom Infrared Telescope (UKIRT), Hawaii, began
imaging about 20 min after the burst, in response to an automated
request, and provided the first infrared (2.15-m m) detection of the
GRB afterglow. In parallel, observations in other near-infrared (NIR)
filters using the Gemini North 8-m telescope, Hawaii, showed that
the counterpart was only visible at wavelengths greater than about
1.2 mm (Fig. 1). In this range, the afterglow was relatively bright and
exhibited a shallow spectral slope, F
n
/ n
20.26
, in contrast to the deep
limit on any flux at 1.02 mm. Later observations from Chile using the
MPI/ESO 2.2-m telescope, Gemini South and the Very Large
Telescope (VLT) confirmed this finding. Such a sharp spectral break
cannot be produced by dust absorption at any redshift, and is a
textbook case of a short-wavelength ‘drop-out’ source. The full
grizYJHK spectral energy distribution (SED) obtained ,17 h after
burst gives a photometric redshift of z 5 8:06
z0:21
{0:28
, assuming a simple
intergalactic medium (IGM) absorption model. Complete details of
our imaging campaign are given in Supplementary Table 1.
Our first NIR spectroscopy was performed with the European
Southern Observatory (ESO) 8.2-m VLT, starting about 17.5 h after
the burst. These observations revealed a flat continuum that abruptly
disappeared at wavelengths less than about 1.13 mm, confirming the
origin of the break as being due to Lyman-a absorption by neutral
1
Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK.
2
Department of Astronomy & Astrophysics, Pennsylvania State University,
University Park, Pennsylvania 16802, USA.
3
Department of Physics, University of Warwick, Coventry CV4 7AL, UK.
4
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,
Cambridge, Massachusetts 02138, USA.
5
Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark.
6
Max-Planck-
Institut fu¨r Extraterrestrische Physik, Giessenbachstraße 1, 85740 Garching, Germany.
7
Universe Cluster, Technische Universita
¨
tMu¨nchen, Boltzmannstrasse 2, 85748 Garching,
Germany.
8
NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.
9
Department of Astronomy, University of California, Berkeley, California 94720-3411, USA.
10
Astronomical Institute ‘‘Anton Pannekoek’’, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands.
11
Centre for Astrophysics and Cosmology, Science
Institute, University of Iceland, Dunhagi 5, 107 Reykjavı
´
k, Iceland.
12
Joint Astronomy Centre, 660 North A’ohoku Place, University Park, Hilo, Hawaii 96720, USA.
13
School of Physics,
University of Exeter, Stocker Road, Exeter EX4 4QL, UK.
14
H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK.
15
Instituto de Astrofı
´
sica de
Andalucı
´
a del Consejo Superior de Investigaciones Cientı
´
ficas, PO Box 03004, 18080 Granada, Spain.
16
European Southern Observatory, Casilla 19001, Santiago 19, Chile.
17
Research
School of Astronomy & Astrophysics, The Australian National University, Cotter Road, Weston Creek, Australian Capital Territory 2611, Australia.
18
Special Astrophysical
Observatory, Nizhnij Arkhyz, Karachai-Cirkassian Republic, 369167, Russia.
19
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA.
20
Thu¨ringer
Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany.
21
CRESST and NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.
22
Universities Space
Research Association, 10211 Wincopin Circle, Suite 500, Columbia, Maryland 21044, USA.
23
Department of Astronomy, California Institute of Technology, MC 249-17, Pasadena,
California 91125, USA.
24
Astrophysics Research Institute, Liverpool John Moores University, Birkenhead CH41 1LD, UK.
25
Department of Physics and Astronomy, Pomona College,
Claremont, California 91711, USA.
26
ASI Science Data Center, Via Galileo Galilei, 00044 Frascati, Italy.
27
Department of Physics, Oxford University, Keble Road, Oxford OX1 3RH, UK.
28
Gemini Observatory, Hilo, Hawaii 96720, USA.
29
Physics Department, McGill University, 3600 Rue University, Montreal, Quebec H3A 2T8, Canada.
30
The UCL Mullard Space
Science Laboratory, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK.
31
The Oskar Klein Centre, Department of Astronomy, Stockholm University, 106 91 Stockholm, Sweden.
32
The
George Washington University, Washington DC 20052, USA.
Figure 1
|
Multiband images of the afterglow of GRB 090423. The right-
most panel shows the discovery image made using the UKIRT Wide Field
Infrared Camera with the K filter (centred at 2.15 mm) at a mid-time of about
30 min after the burst. The other three images (Y, 1.02 mm; J, 1.26 mm;
H, 1.65 mm) were obtained approximately 1.5 h after the burst using Gemini
North’s Near Infrared Imager and Spectrometer (NIRI). The main panels are
40 arcsec to a side, oriented with north to the top and east to the left. Insets,
regions around the GRB, smoothed and at higher contrast. The absence of
any flux in Y implies a power-law spectral slope between Y and J steeper than
F
n
/ n
218
and, coupled with the blue colour at longer wavelengths
(J2H(AB) < 0.15 mag), immediately implies a redshift greater than about
7.8 for GRB 090423.
Vol 461
|
29 October 2009
|
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Macmillan Publishers Limited. All rights reserved
©2009

hydrogen, with a redshift of z < 8.2. The spectrum and broadband
photometric observations, plotted over model data, are shown in
Fig. 2. To obtain a more quantitative estimate of the redshift, we fit
the spectra in redshift versus log[N
H
I
(cm
22
)] space, assuming a flat
prior likelihood value for log[N
H
I
(cm
22
)] of between 19 and 23,
which is broadly consistent with the distribution observed for
lower-redshift GRB hosts
7–9
. We take the neutral fraction of the
IGM to be 10%, although our conclusions depend only weakly on
this assumption. We find the redshift from ISAAC spectroscopy to be
z 5 8:19
z0:03
{0:06
. An additional spectrum, recorded ,40 h after the
burst using the VLT’s Spectrograph for INtegral Field Observations
in the Near Infrared confirms this analysis, yielding z 5 8:33
z0:06
{0:11
(Supplementary Information). Fitting simultaneously to both spec-
tra and the photometric data points gives our best estimate of the
redshift, z 5 8:23
z0:06
{0:07
. The low signal-to-noise ratio means we that
are unable to detect metal absorption features in either spectrum—
which would provide a more precise value of the redshift—and pre-
vents a meaningful attempt to measure the IGM H
I column density
in this instance. Our three independent redshift measures are con-
sistent with that reported from a low-resolution spectrum obtained
with the Telescopio Nazionale Galileo, La Palma
10
.
The X-ray and NIR light curves of GRB 090423 (Fig. 3) show a
broken power-law decay, with evidence of flares in both the X-ray
and the infrared bands. The spectral energy distribution is consistent
with the presence of the cooling break between the X-ray and optical
bands. Apart from the unusually shallow spectral slope of the con-
tinuum at wavelengths greater than 1.2 mm, its afterglow properties in
general appear to be consistent with the bulk GRB population (see
Supplementary Information for further discussion).
With the standard cosmological parameters (Hubble parameter,
H
0
5 71 km s
21
Mpc
21
; total matter density, V
M
5 0.27; dark-
energy density, V
L
5 0.73) a redshift of z 5 8.2 corresponds to a time
of only 630 Myr after the Big Bang, when the Universe was just 4.6%
of its current age. GRB 090423’s inferred isotropic equivalent energy,
E
iso
5 1 3 10
53
erg (8–1,000 keV)
11
, indicates that it was a bright, but
not extreme, GRB. Thus, we find no evidence of exceptional beha-
viour that might indicate an origin in a population III progenitor.
First-generation stars are thought more likely to collapse into par-
ticularly massive black holes, which in turn may produce unusually
long-lived GRBs
12
; this seems not to be the case for GRB 090423.
Indeed, we note that the c-ray duration of GRB 090423,
t
90
5 10.3 s, corresponds in the rest frame to only 1.1 s, and the peak
energy measured by BAT, 49 keV, is moderately hard in the rest
frame. Two other GRBs with z . 5 (GRB 060927 and GRB 080913)
had similarly short rest-frame durations, leading to some debate
13
as
to whether their progenitors were similar to those of the ‘short-hard’
class of GRBs, which are not thought to be directly related to core
collapse. However, in the case of GRB 090423, a more careful extra-
polation of the observed c-ray and X-ray light curves to lower red-
shifts shows that its duration would have appeared significantly
longer than suggested by naive time-dilation considerations
14
.In
any event, short GRBs probably have their origins in compact objects
that are themselves the end products of massive stars, so the above
conclusions will hold irrespective of the population from which
GRB 090423 derives.
It has long been recognized that GRBs have the potential to be power-
ful probes of the early Universe
15
. Their association with individual stars
means that they serve as a signpost of star formation, even if their host
0.1
12
Flux density at 16 h (μJy)
20
10
0
–10
–20
0.2
Rest-frame wavelength (μm)
Observed wavelength (μm)
SZ J
23
22
21
20
19
8 8.5
Redshift
H
K
J
z
Y
log[N
H I
(cm
–2
)]
Figure 2
|
The composite infrared spectrum of the GRB 090423 afterglow.
SZ-band (0.98–1.1 mm) and J-band (1.1–1.4 mm) one- and two-dimensional
spectra obtained with the VLT using the Infrared Spectrometer And Array
Camera (ISAAC). Also plotted are the sky-subtracted photometric data
points obtained using Gemini North’s NIRI (red) and the VLT’s High Acuity
Wide field K-band Imager and Gemini South’s Gemini Multi-Object
Spectrograph (blue) (scaled to 16 h after the burst and expressed in
microjanskys; 1 Jy 5 10
226
Wm
22
Hz
21
). The vertical error bars show the
2s (95%) confidence level, and the horizontal lines indicate the widths of the
filters. The shorter-wavelength measurements are non-detections, and
emphasize the tight constraints on any transmitted flux below the break. The
break itself, at an observed wavelength of about 1.13 mm, is seen to occur
close to the short-wavelength limit of the J-band spectrum, below which,
although noisy, the spectrum shows no evidence of any detected continuum.
Details of the data-reduction steps and adaptive binning used to construct
these spectra are given in Supplementary Information. A model spectrum
showing the H
I damping wing for a host galaxy with a hydrogen column
density of N
H
I
5 10
21
cm
22
at a redshift of z 5 8.23 is also plotted (solid
black line), and provides a good fit to the data. Inset, allowing for a wider
range in possible host N
H
I
values gives the 1s (68%) and 2s confidence
contours shown. The fact that no deviation is seen from a power-law
spectrum at wavelengths greater than 1.2 mm, together with its shallow
spectral slope, suggests that there is little or no dust along the line of sight
through the GRB host galaxy (unless it is ‘grey’), consistent with the galaxy
being relatively unevolved, and having a low abundance of metals.
NATURE
|
Vol 461
|
29 October 2009 LETTERS
1255
Macmillan Publishers Limited. All rights reserved
©2009

galaxies are too faint to detect directly. Equally important, precise deter-
mination of the hydrogen Lyman-a absorption profile can provide a
measure of the neutral fraction of the IGM at the location of the
burst
16–20
. With multiple GRBs at redshifts of z . 7, and the associated
information about the IGM, we could therefore trace the process of
reionization from its early stages
21
.
The high redshift of GRB 090423 has several crucial implications.
Predictions based on extrapolating the global star-formation-rate
density suggest that the observed rate of GRBs at z < 8 should be about
40% of that at z < 6 (ref. 12). Given the extra difficulty of identifying
afterglows at higher redshifts, our finding is broadly consistent with
these predictions. This is extremely encouraging for the prospects of
future initiatives aimed at finding high-redshift GRBs and using them
to locate and study primordial galaxies and measure the history of star
formation at early times
22–24
. Furthermore, it is close to the redshift
range in which the bulk of the cosmic reionization is thought to have
taken place
25–27
. Very high-redshift GRBs for which infrared spectro-
scopy was possible earlier, or which had brighter afterglows, would
provide a direct probe of the progress of reionization. Finding such
events is not an unreasonable hope: the most extreme GRBs have had
afterglows that were intrinsically significantly brighter than that of
GRB 090423 at the same rest-frame time
3,4
, and our first spectra were
recorded more than 15 h after the burst. Spectroscopy with a high
signal-to-noise ratio would also provide a measure of the metallicity
of the host galaxy, which potentially offers important clues to the
nature of any earlier generations of stars. Because the massive stars
that yield GRBs are also likely to belong to the same population that is
responsible for reionization, this suggests that GRBs will ultimately be
used to constrain both sides—supply and demand—of the cosmic
ionization budget in the early Universe.
Received 3 June; accepted 19 August 2009.
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0.1
10
5
10
4
1,000
100
10
1
0.1
0.01
19
20
21
22
23
24
25
26
1 10 100 1,000
J band
H band
K band
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Rest-frame time since GRB 090423 (s)
Observer time since GRB 090423 (s)
X-ray
Luminosity (erg s
–1
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Flux (10
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AB magnitude
M
AB
Infrared
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Figure 3
|
The X-ray and infrared light curves of GRB 090423. The axes
show both observed (left-hand and bottom axes) and rest-frame (right-hand
and top axes) quantities. The X-ray light curve was obtained using Swift’s
BAT (cyan) and XRT (magenta), where the BAT observations have been
extrapolated into the X-ray band. The fitted function represents a
phenomenological model
28
of the prompt and afterglow components. The
infrared light curve was obtained using UKIRT, Gemini North, the MPI/ESO
2.2-m telescope and the VLT. For consistency, although individual bands are
plotted, they have been transformed into absolute magnitudes in the J band
by means of the best-fitting SED (F
n
/ n
20.26
). We show two illustrative fits
to the infrared light curve. The solid line shows a plateau, breaking at
24,000 s to a steeper slope proportional to ,t
21.4
. This underestimates the
late time points, which must then be interpreted as a flare. The dashed line
shows an alternative model, in which mid-time points at ,60,000 s are
instead interpreted as a flare; this is more consistent with the later time
points and the X-ray break time at the end of the plateau. However, in this
case the post-break slope, proportional to ,t
20.7
, is much slower than the
X-ray decay at comparable times, and it further requires a additional break in
the light curve to accommodate the late-time upper limit. Error bars are 1s
(68% confidence level) and the absolute magnitude scale corresponds to
absolute AB magnitudes at 0.136 mm. See Supplementary Information for
further details.
LETTERS NATURE
|
Vol 461
|
29 October 2009
1256
Macmillan Publishers Limited. All rights reserved
©2009

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Supplementary Information is linked to the online version of the paper at
www.nature.com/nature.
Acknowledgements We thank Ph. Yock, B. Allen, P. Kubanek, M. Jelinek and
S. Guziy for their assistance with the BOOTES-3 YA telescope observations
(Supplementary Information). This work was partly based on observations
obtained at the Gemini Observatory, which is operated by the Association of
Universities for Research in Astronomy, Inc., under a cooperative agreement with
the US National Science Foundation on behalf of the Gemini partnership: the
National Science Foundation (United States), the Science and Technology Facilities
Council (United Kingdom), the National Research Council (Canada), CONICYT
(Chile), the Australian Research Council (Australia), the Ministe
´
rio da Cie
ˆ
ncia e
Tecnologia (Brazil) and SECYT (Argentina). This work was also partly based on
observations made using ESO telescopes at the La Silla or Paranal observatories by
G. Carraro, L. Schmidtobreick, G. Marconi, J. Smoker, V. Ivanov, E. Mason and
M. Huertas-Company. The UKIRT is operated by the Joint Astronomy Centre on
behalf of the UK Science and Technology Facilities Council. R.J.F. acknowledges a
Clay Fellowship.
Author Contributions Triggering observations: N.R.T., D.B.F., A.J.L., E.B., J.S.B.,
D.P., J. Greiner, A.J.C.-T., A.d.U.P.; analysis of ground-based data: N.R.T., D.B.F.,
A.J.L., E.B., K.W., J.P.U.F., A.C., J.S.B., J.F., J.D., J. Gorosabel, B.C., D.P., J.R.M.,
T. Kru¨hler, A.J.C.-T., A.d.U.P., C.G.M.; Swift analysis: P.A.E., R.L.C.S., K.P., R.W.,
A.J.L., N.R.T., N.G., D.W., P.S., T.S.; observations at various observatories and their
automation to accept GRB overrides: A.J.A., A.A., T. Kerr, T.N., A.W.S., K.R., T.W.
All authors made contributions through their involvement in the programmes from
which the data derive, and contributed to the interpretation, content and
discussion presented here. Writing was led by N.R.T., A.J.L., D.B.F. and E.B.
Author Information Reprints and permissions information is available at
www.nature.com/reprints. Correspondence and requests for materials should be
addressed to N.R.T. (nrt3@star.le.ac.uk).
NATURE
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Vol 461
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29 October 2009 LETTERS
1257
Macmillan Publishers Limited. All rights reserved
©2009
Citations
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Journal ArticleDOI
30 Jun 2011-Nature
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Abstract: Quasars have historically been identified in optical surveys, which are insensitive to sources at z > 6.5. Infrared deep-sky survey data now make it possible to explore higher redshifts, with the result that a luminous quasar (ULAS J1120+0641) with a redshift z = 7.085, beyond the previous high of z = 6.44, has now been identified. Further observations of this and other distant quasars should reveal the ionization state of the Universe as it was only about 0.75 billion years after the Big Bang. The intergalactic medium was not completely reionized until approximately a billion years after the Big Bang, as revealed1 by observations of quasars with redshifts of less than 6.5. It has been difficult to probe to higher redshifts, however, because quasars have historically been identified2,3,4 in optical surveys, which are insensitive to sources at redshifts exceeding 6.5. Here we report observations of a quasar (ULAS J112001.48+064124.3) at a redshift of 7.085, which is 0.77 billion years after the Big Bang. ULAS J1120+0641 has a luminosity of 6.3 × 1013L⊙ and hosts a black hole with a mass of 2 × 109M⊙ (where L⊙ and M⊙ are the luminosity and mass of the Sun). The measured radius of the ionized near zone around ULAS J1120+0641 is 1.9 megaparsecs, a factor of three smaller than is typical for quasars at redshifts between 6.0 and 6.4. The near-zone transmission profile is consistent with a Lyα damping wing5, suggesting that the neutral fraction of the intergalactic medium in front of ULAS J1120+0641 exceeded 0.1.

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TL;DR: In this article, the authors show that a complete Gunn-Peterson trough is most likely to continue to be observed through the epoch where the intergalactic medium is partially ionized.
Abstract: Observations of high-redshift quasars show that the intergalactic medium (IGM) must have been reionized at some redshift z > 5. If a source of radiation could be observed at the rest-frame Lyα wavelength, at a sufficiently high redshift where some of the IGM in the line of sight was not yet reionized, the Gunn-Peterson trough should be present. Longward of the Lyα wavelength, a damping wing should be observed, caused by the neutral IGM whose absorption profile can be predicted. Measuring the shape of this damping wing would provide irrefutable evidence of the observation of the IGM before reionization and a determination of the density of the neutral IGM. This measurement might be hindered by the possible presence of a dense absorption system associated with the source. Shortward of the Lyα wavelength, absorption should be seen from the patchy structure of the IGM in the process of reionization, intersected in the line of sight. We show that a complete Gunn-Peterson trough is most likely to continue to be observed through the epoch where the IGM is partially ionized. The damping wings of the neutral patches around an ionized region should overlap in the spectrum if the proper path length through the ionized region is less than 1 h-1 Mpc; even in larger ionized regions, the characteristic background intensity should be low enough to yield a very high optical depth due to the residual neutral fraction, although occasionally some flux may be transmitted through large, underdense voids within an ionized region. In the case of the He II reionization, the ionization fronts are much thicker than in the case of hydrogen, and the profile of this front determines the shape of the absorption at the edge of a He III region. Analogous to the case of hydrogen, windows of transmitted flux are not likely to be observed until after the low-density IGM has been completely reionized. Therefore, the observation of these transmission windows by Reimers et al. at z 2.85 suggests that the helium reionization was complete by this redshift. The recently discovered afterglows of gamma-ray bursts might soon be observed at the very high redshifts required for these observations. Their featureless continuum spectrum and high luminosities make them ideal sources for studying absorption by the IGM.

355 citations

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TL;DR: In this paper, the authors combine published surveys of widely varying depths and areas to construct luminosity functions at z = 6.5 and 5.7, where the characteristic luminosity L_star and density phi_star are well constrained while the faint-end slope of the luminosity function is essentially unconstrained.
Abstract: Lyman-alpha emission from galaxies should be suppressed completely or partially at redshifts beyond reionization. Without knowing the instrinsic properties of galaxies at z = 6.5, this attenuation is hard to infer in any one source, but can be infered from a comparison of luminosity functions of lyman-alpha emitters at redshifts just before and after reionization. We combine published surveys of widely varying depths and areas to construct luminosity functions at z=6.5 and 5.7, where the characteristic luminosity L_star and density phi_star are well constrained while the faint-end slope of the luminosity function is essentially unconstrained. Excellent consistency is seen in all but one published result. We then calculate the likelihood of obtaining the z=6.5 observations given the z=5.7 luminosity function with (A) no evolution and (B) an attenuation of a factor of three. Hypothesis (A) gives an acceptable likelihood while (B) does not. This indicates that the z=6.5 lyman-alpha lines are not strongly suppressed by a neutral intergalactic medium and that reionization was largely complete at z = 6.5.

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TL;DR: In this paper, a spectral modeling analysis of the optical afterglow spectrum taken by the Subaru Telescope is presented, aiming to constrain the reionization history of the gamma-ray burst (GRB) 050904 at z =6.3 provides the first opportunity to probe the intergalactic medium (IGM) by GRBs at the epoch of reionisation.
Abstract: The gamma-ray burst (GRB) 050904 at z =6 .3 provides the first opportunity to probe the intergalactic medium (IGM) by GRBs at the epoch of reionization. Here, we present a spectral modeling analysis of the optical afterglow spectrum taken by the Subaru Telescope, aiming to constrain the reionization history. The spectrum shows a clear damping wing at wavelengths redward of the Lyman break, and the wing shape can be fitted either by a damped Lyα system with a column density of log[NHI(cm −2 )] ∼ 21.6 at a redshift close to the detected metal absorption lines (zmetal =6 .295), or by almost neutral IGM extending to a slightly higher redshift of zIGM,u ∼ 6.36. In the latter case, the difference between the two redshifts may be explained by the acceleration of metal absorbing shells in the activities of the GRB or its progenitor. However, we exclude this possibility by using the light transmission feature around the Lyβ resonance, leading to a firm upper limit of zIGM,u ≤ 6.314. We then show evidence that the IGM was already largely ionized at z =6 .3 with the best-fit neutral fraction of IGM, xHI (≡ nHI/nH )=0 .00, and upper limits of xHI 6. Various systematic uncertainties are examined, but none of them appears large enough to change our conclusion. To get further information on the reionization, it is important to increase the sample size of z 6 GRBs, in order to find GRBs with low column densities (log NHI 20) within their host galaxies and to make statistical studies of Lyα line emission from host galaxies.

337 citations

Journal ArticleDOI
TL;DR: In this paper, the authors investigate the implications of the intergalactic opacity for the evolution of the cosmic UV luminosity density and its sources and show that star-forming galaxies likely dominate the photoionization rate at ≈ 3.
Abstract: We investigate the implications of the intergalactic opacity for the evolution of the cosmic UV luminosity density and its sources. Our main constraint is our measurement of the Ly? forest opacity at redshifts -->2 ? z ? 4.2 from 86 high-resolution quasar spectra. In addition, we impose the requirements that H I must be reionized by -->z = 6 and He II by -->z ~ 3 and consider estimates of the hardness of the ionizing background from H I-to-He II column density ratios. The derived hydrogen photoionization rate is remarkably flat over the Ly? forest redshift range covered. Because the quasar luminosity function is strongly peaked near -->z ~ 2, the lack of redshift evolution indicates that star-forming galaxies likely dominate the photoionization rate at -->z 3. Combined with direct measurements of the galaxy UV luminosity function, this requires only a small fraction -->fesc ~ 0.5% of galactic hydrogen-ionizing photons to escape their source for galaxies to solely account for the entire ionizing background. Under the assumption that the galactic UV emissivity traces the star formation rate, current state-of-the-art observational estimates of the star formation rate density appear to underestimate the total photoionization rate at -->z ~ 4 by a factor of ~4, are in tension with recent determinations of the UV luminosity function, and fail to reionize the universe by -->z ~ 6 if extrapolated to arbitrarily high redshift. A theoretical star formation history peaking earlier fits the Ly? forest photoionization rate well, reionizes the universe in time, and is in better agreement with the rate of -->z ~ 4 gamma-ray bursts observed by Swift. Quasars suffice to doubly ionize helium by -->z ~ 3 and likely contribute a nonnegligible and perhaps dominant fraction of the hydrogen-ionizing background at their -->z ~ 2 peak.

316 citations

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20 Jun 2007
TL;DR: In this article, the decay curves of γ-ray bursts measured by Swift can be fitted using one or two components, both of which have exactly the same functional form comprised of an early falling exponential phase followed by a power-law decay.
Abstract: We show that all X-ray decay curves of γ-ray bursts (GRBs) measured by Swift can be fitted using one or two components, both of which have exactly the same functional form comprised of an early falling exponential phase followed by a power-law decay. The first component contains the prompt γ-ray emission and the initial X-ray decay. The second component appears later, has a much longer duration, and is present for ≈80% of GRBs. It most likely arises from the external shock that eventually develops into the X-ray afterglow. In the remaining ≈20% of GRBs the initial X-ray decay of the first component fades more slowly than the second and dominates at late times to form an afterglow. The temporal decay parameters and γ/X-ray spectral indices derived for 107 GRBs are compared to the expectations of the standard fireball model including a search for possible jet breaks. For ~50% of GRBs the observed afterglow is in accord with the model, but for the rest the temporal and spectral indices do not conform to the expected closure relations and are suggestive of continued, late, energy injection. We identify a few possible jet breaks, but there are many examples where such breaks are predicted but are absent. The time Ta at which the exponential phase of the second component changes to a final power-law decay afterglow is correlated with the peak of the γ-ray spectrum, Epeak. This is analogous to the Ghirlanda relation, indicating that this time is in some way related to optically observed break times measured for pre-Swift bursts.

312 citations

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