27 research outputs found
Particle decay in the early universe: predictions for 21 cm
The influence of ultra-high energy cosmic rays (UHECRs) and decaying dark
matter particles on the emission and absorption characteristics of neutral
hydrogen in 21 cm at redshifts is considered. In presence of UHECRs
21 cm can be seen in absorption with the brightness temperature
mK in the range . Decayng particles can stimulate a 21 cm signal in
emission with mK at , and mK at . Characteristics of the fluctuations of the brightness temperature, in
particular, its power spectrum are also calculated. The maps of the power
spectrum of the brightness temperature on the plane {\it wavenumber-redshift}
are shown to be sensitive to the parameters of UHECRs and decaying dark matter.
Observational possibilities to detect manifestations of UHECRs and/or decaying
particles in 21 cm with the future radio telescopes (LOFAR, 21CMA and SKA), and
to distinguish contributions from them are briefly discussed.Comment: 10 pages, 9 figures, accepted in MNRA
Observing H2 Emission in Forming Galaxies
We study the H2 cooling emission of forming galaxies, and discuss their
observability using the future infrared facility SAFIR. Forming galaxies with
mass >10^11 Msun emit most of their gravitational energy liberated by
contraction in molecular hydrogen line radiation, although a large part of
thermal energy at virialization is radiated away by the H Ly alpha emission.
For more massive objects, the degree of heating due to dissipation of kinetic
energy is so great that the temperature does not drop below 10^4 K and the
gravitational energy is emitted mainly by the Ly alpha emission. Therefore, the
total H2 luminosity attains the peak value of about 10^42 ergs/s for forming
galaxies whose total mass 10^11 Msun. If these sources are situated at redshift
z=8, they can be detected by rotational lines of 0-0S(3) at 9.7 micron and
0-0S(1) at 17 micron by SAFIR. An efficient way to find such H2 emitters is to
look at the Ly alpha emitters, since the brightest H2 emitters are also
luminous in the Ly alpha emission.Comment: 20 pages, 7 figures, ApJ accepte
Early reionization by decaying particles in the light of three year WMAP data
We study the reionization histories where ionizing UV photons are emitted
from decaying particles, in addition to usual contributions from stars and
quasars, taking account of the fact that the universe is not fully ionized
until z = 6 as observed by Sloan Digital Sky Survey. Likelihood analysis of the
three-year data from the WMAP (Wilkinson Microwave Anisotropy Probe) severely
constrains the decaying particle scenario.In particular, the decaying particle
with relatively short lifetime is not favored by the polarization data.Comment: 9 pages, 11 figure
Observational Manifestations of the First Protogalaxies in the 21 cm Line
The absorption properties of the first low-mass protogalaxies (mini-halos)
forming at high redshifts in the 21-cm line of atomic hydrogen are considered.
The absorption properties of these protogalaxies are shown to depend strongly
on both their mass and evolutionary status. The optical depths in the line
reach 0.1-0.2 for small impact parameters of the line of sight. When a
protogalaxy being compressed, the influence of gas accretion can be seen
manifested in a non-monotonic frequency dependence of the optical depth. The
absorption characteristics in the 21-cm line are determined by the thermal and
dynamical evolution of the gas in protogalaxies. Since the theoretical line
width in the observer's reference frame is 1-6 kHz and the expected separation
between lines 8.4 kHz, the lines from low mass protogalaxies can be resolved
using ongoing and future low frequency interferometers.Comment: 12 pages, 5 figure
Simulations of M87 and Sgr A* imaging with the Millimetron Space Observatory on near-Earth orbits
High resolution imaging of supermassive black holes shadows is a direct way
to verify the theory of general relativity at extreme gravity conditions. Very
Long Baseline Interferometry (VLBI) observations at millimeter/sub-millimeter
wavelengths can provide such angular resolution for supermassive black holes,
located in Sgr A* and M87. Recent VLBI observations of M87 with the Event
Horizon Telescope (EHT) has shown such capabilities. The maximum obtainable
spatial resolution of EHT is limited by Earth diameter and atmospheric phase
variations. In order to improve the image resolution longer baselines are
required. Radioastron space mission has successfully demonstrated the
capabilities of Space-Earth VLBI with baselines much larger than Earth
diameter. Millimetron is a next space mission of the Russian Space Agency that
will operate at millimeter wavelengths. Nominal orbit of the observatory will
be located around Lagrangian L2 point of the Sun-Earth system. In order to
optimize the VLBI mode, we consider a possible second stage of the mission that
could use near-Earth high elliptical orbit (HEO). In this contribution a set of
near-Earth orbits is used for the synthetic space-ground VLBI observations of
Sgr A* and M87 in joint Millimetron and EHT configuration. General-relativistic
magnetohydrodynamic models (GRMHD) for black hole environment of Sgr A* and M87
are used for static and dynamic imaging simulations at 230 GHz. A comparison
preformed between ground and space-ground baselines demonstrates that joint
observations with Millimetron and EHT significantly improve the image
resolution and allow the EHT+Millimetron to obtain snapshot images of Sgr A*
probing dynamics at fast timescales.Comment: 12 pages, 6 figure
Cosmic 21-cm Fluctuations as a Probe of Fundamental Physics
Fluctuations in high-redshift cosmic 21-cm radiation provide a new window for
observing unconventional effects of high-energy physics in the primordial
spectrum of density perturbations. In scenarios for which the initial state
prior to inflation is modified at short distances, or for which deviations from
scale invariance arise during the course of inflation, the cosmic 21-cm power
spectrum can in principle provide more precise measurements of exotic effects
on fundamentally different scales than corresponding observations of cosmic
microwave background anisotropies.Comment: 8 pages, 2 figure
The Universe Was Reionized Twice
We show the universe was reionized twice, first at z~15-16 and second at z~6.
Such an outcome appears inevitable, when normalizing to two well determined
observational measurements, namely, the epoch of the final cosmological
reionization at z~6 and the density fluctuations at z~6, which in turn are
tight ly constrained by Lyman alpha forest observations at z~3. These two
observations most importantly fix the product of star formation efficiency and
ionizing photon escape fraction from galaxies at high redshift. To the extent
that the relative star formation efficiencies in gaseous minihalos with H2
cooling and large halos with atomic cooling at high redshift are still unknown,
the primary source for the first reionization could be Pop III stars either in
minihalos or in large halos. We show that gas in minihalos can be cooled
efficiently by H2 molecules and star formation can continue to take place
largely unimpeded throughout the first reionization period, thanks to two new
mechanisms for generating a high X-ray background during the Pop III era, put
forth here. Moreover, an important process for producing a large number of H2
molecules in relic HII regions of Pop III galaxies, first pointed out by
Ricotti, Gnedin, & Shull, is quantified here. It is shown that the Lyman-Werner
background may never build up during the Pop III era. The long cosmological
reionization and reheating history is complex. We discuss a wide range of
implications and possible tests for this new reionization picture. In
particular, Thomson scattering optical depth is increased to 0.10 +- 0.03,
compared to 0.027 for the case of only one rapid reionization at z=6. Upcoming
Microwave Anisotropy Probe observation of the polarization of the cosmic
microwave background should be able to distinguish between these two scenarios.Comment: submitted to ApJ, 69 pages, substantial revision made and conclusions
strengthene
Synchrotron emission from circumstellar disks around massive stars
We argue that the interaction of stellar wind with the surface of a
circumstellar accretion (or protoplanetary) disk can result in the acceleration
of relativistic electrons in an external layer of the disk, and produce
synchrotron radiation. Conservative estimates give a total synchrotron
luminosity L_s\sim 10^{-5}\lsun for a central star with \dot M=10^{-6}\msun
yr, comparable with the value observed around the TW object in the
W3(OH) region.Comment: 7 pages, no figs, accepted in A&
A Hubble Space Telescope WFPC2 Investigation of the Disk-Halo Interface in NGC891
We present deep narrowband observations with high spatial resolution of
extraplanar diffuse ionized gas in the halo of NGC891, obtained with the WFPC2
on-board the HST. Our H-alpha observations, centered on the northern part of
NGC891, reveal an extended gaseous halo, which fills almost the entire field of
view of our WFPC2 observation. The majority of the H-alpha emission is diffuse.
Several discrete features (e.g., filaments) are visible as well. Some of the
filaments reach distances of up to 2.2 kpc above the galactic plane, and are
extremely collimated, even at high galactic latitudes. We compare the
morphology of these filaments with theoretical models, which describe possible
transport mechanisms in a general way. We also investigate extraplanar dust
features, which are best visible in unsharp-masked images of our broadband
F675W image, and we compare them to the spatial distribution of DIG filaments.
Individual dust features, however, are not spatially correlated with diffuse
ionized gas counterparts, such as individual filaments. Quite interestingly,
the orientation of the dust features changes from being mostly aligned
perpendicular to the disk at low galactic latitudes, to a parallel alignment at
high |z|. We compare the diffuse ionized gas distribution to the hot ionized
medium, traced by X-ray observations performed by Chandra. There exists a good
correlation of the presence of the warm and hot ionized gas, in particular, an
X-ray bright region at |z| ~ 1-1.5 kpc fills the entire northern halo region,
whereas the intensity in the midplane is considerably depressed. We also
compare the sizes of individual H-alpha emission line features in the midplane
of NGC891 with similar structures that are known in our Milky Way and in the
LMC.Comment: LaTeX, 15 pages, 11 figures, accepted for publication in The
Astronomical Journal, paper with high resolution figures can be accessed at
http://www.astro.rub.de/jrossa/ngc891