605 research outputs found
GRBs as Probes of the IGM
Gamma-ray Bursts (GRBs) are the most powerful explosions known, capable of
outshining the rest of gamma-ray sky during their short-lived prompt emission.
Their cosmological nature makes them the best tool to explore the final stages
in the lives of very massive stars up to the highest redshifts. Furthermore,
studying the emission from their low-energy counterparts (optical and infrared)
via rapid spectroscopy, we have been able to pin down the exact location of the
most distant galaxies as well as placing stringent constraints on their host
galaxies and intervening systems at low and high-redshift (e.g. metallicity and
neutral hydrogen fraction). In fact, each GRB spectrum contains absorption
features imprinted by metals in the host interstellar medium (ISM) as well as
the intervening intergalactic medium (IGM) along the line of sight. In this
chapter we summarize the progress made using a large dataset of GRB spectra in
understanding the nature of both these absorbers and how GRBs can be used to
study the early Universe, in particular to measure the neutral hydrogen
fraction and the escape fraction of UV photons before and during the epoch of
re-ionization.Comment: 18 pages; 5 Figures. Accepted for publication in Space Science
Review
Very Strong TeV Emission as Gamma-Ray Burst Afterglows
Gamma-ray bursts (GRBs) and following afterglows are considered to be
produced by dissipation of kinetic energy of a relativistic fireball and
radiation process is widely believed as synchrotron radiation or inverse
Compton scattering of electrons. We argue that the transfer of kinetic energy
of ejecta into electrons may be inefficient process and hence the total energy
released by a GRB event is much larger than that emitted in soft gamma-rays, by
a factor of \sim (m_p/m_e). We show that, in this case, very strong emission of
TeV gamma-rays is possible due to synchrotron radiation of protons accelerated
up to \sim 10^{21} eV, which are trapped in the magnetic field of afterglow
shock and radiate their energy on an observational time scale of \sim day. This
suggests a possibility that GRBs are most energetic in TeV range and such TeV
gamma-rays may be detectable from GRBs even at cosmological distances, i.e., z
\sim 1, by currently working ground-based telescopes. Furthermore, this model
gives a quantitative explanation for the famous long-duration GeV photons
detected from GRB940217. If TeV gamma-ray emission which is much more energetic
than GRB photons is detected, it provides a strong evidence for acceleration of
protons up to \sim 10^{21} eV.Comment: 10 pages, no figure. To appear in ApJ Letter
Infrared Spectral Energy Distribution of Galaxies in the AKARI All Sky Survey: Correlations with Galaxy Properties, and Their Physical Origin
We have studied the properties of more than 1600 low-redshift galaxies by
utilizing high-quality infrared flux measurements of the AKARI All-Sky Survey
and physical quantities based on optical and 21-cm observations. Our goal is to
understand the physics determining the infrared spectral energy distribution
(SED). The ratio of the total infrared luminosity L_TIR, to the star-formation
rate (SFR) is tightly correlated by a power-law to specific SFR (SSFR), and
L_TIR is a good SFR indicator only for galaxies with the largest SSFR. We
discovered a tight linear correlation for normal galaxies between the radiation
field strength of dust heating, estimated by infrared SED fits (U_h), and that
of galactic-scale infrared emission (U_TIR ~ L_TIR/R^2), where R is the optical
size of a galaxy. The dispersion of U_h along this relation is 0.3 dex,
corresponding to 13% dispersion in the dust temperature. This scaling and the
U_h/U_TIR ratio can be explained physically by a thin layer of heating sources
embedded in a thicker, optically-thick dust screen. The data also indicate that
the heated fraction of the total dust mass is anti-correlated to the dust
column density, supporting this interpretation. In the large U_TIR limit, the
data of circumnuclear starbursts indicate the existence of an upper limit on
U_h, corresponding to the maximum SFR per gas mass of ~ 10 Gyr^{-1}. We find
that the number of galaxies sharply drops when they become optically thin
against dust-heating radiation, suggesting that a feedback process to galaxy
formation (likely by the photoelectric heating) is working when dust-heating
radiation is not self-shielded on a galactic scale. Implications are discussed
for the M_HI-size relation, the Kennicutt-Schmidt relation, and galaxy
formation in the cosmological context.Comment: 29 pages including 28 figures. matches the published version (PASJ
2011 Dec. 25 issue). The E-open option was chosen for this article, i.e., the
official version available from PASJ site
(http://pasj.asj.or.jp/v63/n6/630613/630613-frame.html) without restrictio
Lyman-alpha Damping Wing Constraints on Inhomogeneous Reionization
One well-known way to constrain the hydrogen neutral fraction, x_H, of the
high-redshift intergalactic medium (IGM) is through the shape of the red
damping wing of the Lya absorption line. We examine this method's effectiveness
in light of recent models showing that the IGM neutral fraction is highly
inhomogeneous on large scales during reionization. Using both analytic models
and "semi-numeric" simulations, we show that the "picket-fence" absorption
typical in reionization models introduces both scatter and a systematic bias to
the measurement of x_H. In particular, we show that simple fits to the damping
wing tend to overestimate the true neutral fraction in a partially ionized
universe, with a fractional error of ~ 30% near the middle of reionization.
This bias is generic to any inhomogeneous model. However, the bias is reduced
and can even underestimate x_H if the observational sample only probes a subset
of the entire halo population, such as quasars with large HII regions. We also
find that the damping wing absorption profile is generally steeper than one
would naively expect in a homogeneously ionized universe. The profile steepens
and the sightline-to-sightline scatter increases as reionization progresses. Of
course, the bias and scatter also depend on x_H and so can, at least in
principle, be used to constrain it. Damping wing constraints must therefore be
interpreted by comparison to theoretical models of inhomogeneous reionization.Comment: 11 pages, 10 figures; submitted to MNRA
Cosmological Gamma-Ray Bursts and Evolution of Galaxies
Evolution of the rate density of cosmological gamma-ray bursts (GRBs) is
calculated and compared to the BATSE brightness distribution in the context of
binary neutron-star mergers as the source of GRBs, taking account of the
realistic star formation history in the universe and evolution of compact
binary systems. We tried two models of the evolution of cosmic star formation
rate (SFR): one is based on recent observations of SFRs at high redshifts,
while the other is based on a galaxy evolution model of stellar population
synthesis that reproduces the present-day colors of galaxies. It is shown that
the binary merger scenario of GRBs naturally results in the comoving
rate-density evolution of \propto (1+z)^{2-2.5} up to z ~ 1, that has been
suggested independently from the compatibility between the number-brightness
distribution and duration-brightness correlation. If the cosmic SFR has its
peak at z ~ 1--2 as suggested by recent observations, the effective power-index
of GRB photon spectrum, \alpha >~ 1.5$ is favored, that is softer than the
recent observational determination of \alpha = 1.1 \pm 0.3. However, high
redshift starbursts (z >~ 5) in elliptical galaxies, that have not yet been
detected, can alleviate this discrepancy. The redshift of GRB970508 is likely
about 2, just below the upper limit that is recently determined, and the
absorption system at z = 0.835 seems not to be the site of the GRB.Comment: ApJ Lett. in press, very minor change just making clear that the
predicted rate-density evolution is in a comoving sense. (Received 1997 May
15; Accepted 1997 July 2
Lyman Alpha Emitters in the Hierarchically Clustering Galaxy Formation
We present a new theoretical model for the luminosity functions (LFs) of
Lyman alpha (Lya) emitting galaxies in the framework of hierarchical galaxy
formation. We extend a semi-analytic model of galaxy formation that reproduces
a number of observations for local and high-z galaxies, without changing the
original model parameters but introducing a physically-motivated modelling to
describe the escape fraction of Lya photons from host galaxies (f_esc). Though
a previous study using a hierarchical clustering model simply assumed a
constant and universal value of f_esc, we incorporate two new effects on f_esc:
extinction by interstellar dust and galaxy-scale outflow induced as a star
formation feedback. It is found that the new model nicely reproduces all the
observed Lya LFs of the Lya emitters (LAEs) at different redshifts in z ~ 3-6.
Especially, the rather surprisingly small evolution of the observed LAE Lya LFs
compared with the dark halo mass function is naturally reproduced. Our model
predicts that galaxies with strong outflows and f_esc ~ 1 are dominant in the
observed LFs. This is also consistent with available observations, while the
simple universal f_esc model requires f_esc << 1 not to overproduce the
brightest LAEs. On the other hand, we found that our model significantly
overpredicts LAEs at z > 6, and absorption of Lya photons by neutral hydrogen
in intergalactic medium (IGM) is a reasonable interpretation for the
discrepancy. This indicates that the IGM neutral fraction x_HI rapidly evolves
from x_HI << 1 at z < 6 to a value of order unity at z ~ 6-7, which is broadly
consistent with other observational constraints on the reionization history.Comment: 14 pages, 7 figures, 1 table; accepted to ApJ; the html abstract is
replaced to match the accepted version, the .ps and .pdf files are strictly
identical between the 2nd and the 3rd version
Solving the Cooling Flow Problem of Galaxy Clusters by Dark Matter Neutralino Annihilation
Recent X-ray observations revealed that strong cooling flow of intracluster
gas is not present in galaxy clusters, even though predicted theoretically if
there is no additional heating source. I show that relativistic particles
produced by dark matter neutralino annihilation in cluster cores provide a
sufficient heating source to suppress the cooling flow, under reasonable
astrophysical circumstances including adiabatic growth of central density
profile, with appropriate particle physics parameters for dark matter
neutralinos. In contrast to other astrophysical heat sources such as AGNs, this
process is a steady and stable feedback over cosmological time scales after
turned on.Comment: 4 pages, no figure. Accepted to Phys. Rev. Lett. A few minor
revisions and references adde
The faint-galaxy hosts of gamma-ray bursts
The observed redshifts and magnitudes of the host galaxies of gamma-ray
bursts (GRBs) are compared with the predictions of three basic GRB models, in
which the comoving rate density of GRBs is (1) proportional to the cosmic star
formation rate density, (2) proportional to the total integrated stellar
density and (3) constant. All three models make the assumption that at every
epoch the probability of a GRB occuring in a galaxy is proportional to that
galaxy's broad-band luminosity. No assumption is made that GRBs are standard
candles or even that their luminosity function is narrow. All three rate
density models are consistent with the observed GRB host galaxies to date,
although model (2) is slightly disfavored relative to the others. Models (1)
and (3) make very similar predictions for host galaxy magnitude and redshift
distributions; these models will be probably not be distinguished without
measurements of host-galaxy star-formation rates. The fraction of host galaxies
fainter than 28 mag may constrain the faint end of the galaxy luminosity
function at high redshift, or, if the fraction is observed to be low, may
suggest that the bursters are expelled from low-luminosity hosts. In all
models, the probability of finding a z<0.008 GRB among a sample of 11 GRBs is
less than 10^(-4), strongly suggesting that GRB 980425, if associated with
supernova 1998bw, represents a distinct class of GRBs.Comment: 7 pages, ApJ in press, revised to incorporate yet more new and
revised observational result
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