794 research outputs found
Molecular Lines as Diagnostics of High Redshift Objects
Models are presented for CO rotational line emission by high redshift
starburst galaxies. The influence of the cosmic microwave background on the
thermal balance and the level populations of atomic and molecular species is
explicitly included. Predictions are made for the observability of starburst
galaxies through line and continuum emission between z=5 and z=30. It is found
that the Millimeter Array could detect a starburst galaxy with ~10^5 Orion
regions, corresponding to a star formation rate of about 30 Mo yr^{-1}, equally
well at z=5 or z=30 due to the increasing cosmic microwave background
temperature with redshift. Line emission is a potentially more powerful probe
than dust continuum emission of very high redshift objects.Comment: 15 pages LaTex, uses aasms4.sty, Accepted by ApJ
Far-Infrared and Sub-Millimeter Observations and Physical Models of the Reflection Nebula Ced 201
ISO [C II] 158 micron, [O I] 63 micron, and H_2 9 and 17 micron observations
are presented of the reflection nebula Ced 201, which is a photon-dominated
region illuminated by a B9.5 star with a color temperature of 10,000 K (a cool
PDR). In combination with ground based [C I] 609 micron, CO, 13CO, CS and HCO+
data, the carbon budget and physical structure of the reflection nebula are
constrained. The obtained data set is the first one to contain all important
cooling lines of a cool PDR, and allows a comparison to be made with classical
PDRs. To this effect one- and three-dimensional PDR models are presented which
incorporate the physical characteristics of the source, and are aimed at
understanding the dominant heating processes of the cloud. The contribution of
very small grains to the photo-electric heating rate is estimated from these
models and used to constrain the total abundance of PAHs and small grains.
Observations of the pure rotational H_2 lines with ISO, in particular the S(3)
line, indicate the presence of a small amount of very warm, approximately 330
K, molecular gas. This gas cannot be accommodated by the presented models.Comment: 32 pages, 7 figures, in LaTeX. To be published in Ap
Detection of a large fraction of atomic gas not associated with star-forming material in M17 SW
We probe the column densities and masses traced by the ionized and neutral
atomic carbon with spectrally resolved maps, and compare them to the diffuse
and dense molecular gas traced by [C I] and low- CO lines toward the
star-forming region M17SW. We mapped a 4.1pc x 4.7pc region in the [C I] 609
m line using the APEX telescope, as well as the CO isotopologues with the
IRAM 30m telescope. We analyze the data based on velocity channel maps that are
1 km/s wide. We correlate their spatial distribution with that of the [C II]
map obtained with SOFIA/GREAT. Optically thin approximations were used to
estimate the column densities of [C I] and [C II] in each velocity channel. The
spatial distribution of the [C I] and all CO isotopologues emission was found
to be associated with that of [C II] in about 20%-80% of the mapped region,
with the high correlation found in the central (15-23 km/s ) velocity channels.
The excitation temperature of [C I] ranges between 40 K and 100 K in the inner
molecular region of M17 SW. Column densities in 1 km/s channels between
~10 and ~10 cm were found for [C I]. Just ~20% of the
velocity range (~40 km/s) that the [C II] line spans is associated with the
star-forming material traced by [C I] and CO. The total gas mass estimated from
the [C II] emission gives a lower limit of ~4.4x10 . At least
64% of this mass is not associated with the star-forming material in M17SW. We
also found that about 36%, 17%, and 47% of the [C II] emission is associated
with the HII, HI, and H_2 regimes, respectively. Comparisons with the
H41 line shows an ionization region mixed with the neutral and part of
the molecular gas, in agreement with the clumped structure and dynamical
processes at play in M17SW. These results are also relevant to extra-galactic
studies in which [C II] is often used as a tracer of star-forming material.Comment: 21 pages + 6 pages of appendix, 32 figures in total, accepted for
publication on A&A (10/12/2014) Relevant calibrated data cubes are available
on CD
Search for Interstellar Water in the Translucent Molecular Cloud toward HD 154368
We report an upper limit of 9 x 10^{12} cm-2 on the column density of water
in the translucent cloud along the line of sight toward HD 154368. This result
is based upon a search for the C-X band of water near 1240 \AA carried out
using the Goddard High Resolution Spectrograph of the Hubble Space Telescope.
Our observational limit on the water abundance together with detailed chemical
models of translucent clouds and previous measurements of OH along the line of
sight constrain the branching ratio in the dissociative recombination of H_3O+
to form water. We find at the level that no more than 30% of
dissociative recombinations of H_3O+ can lead to H_2O. The observed spectrum
also yielded high-resolution observations of the Mg II doublet at 1239.9 \AA
and 1240.4 \AA, allowing the velocity structure of the dominant ionization
state of magnesium to be studied along the line of sight. The Mg II spectrum is
consistent with GHRS observations at lower spectral resolution that were
obtained previously but allow an additional velocity component to be
identified.Comment: Accepted by ApJ, uses aasp
Models for Dusty Lyman alpha Emitters at High Redshift
Models are presented for the Lyman alpha emission of dusty high-redshift
galaxies by combining the Press-Schechter formalism with a treatment of the
inhomogeneous dust distribution inside galaxies. It is found that the amount of
Lyman alpha radiation escaping from the galaxies strongly depends on the time
over which the dust is produced through stellar activity, and on the ambient
inhomogeneity of the HII regions that surround the ionizing OB stars. Good
agreement is found with recent observations, as well as previous
non-detections. Our models indicate that the dust content builds up in no more
than approximately 5x10^8 yr, the galactic HII regions are inhomogeneous with a
cloud covering factor of order unity, and the overall star formation efficiency
is at least about 5%. It is predicted that future observations can detect these
Lyman alpha galaxies upto redshifts of about 8.Comment: 16 pages, 4 figures, submitted to Ap
The nature of the ISM in galaxies during the star-formation activity peak of the Universe
We combine a semi-analytic model of galaxy formation, tracking atomic and
molecular phases of cold gas, with a three-dimensional radiative-transfer and
line tracing code to study the sub-mm emission from atomic and molecular
species (CO, HCN, [CI], [CII], [OI]) in galaxies. We compare the physics that
drives the formation of stars at the epoch of peak star formation (SF) in the
Universe (z = 2.0) with that in local galaxies. We find that normal
star-forming galaxies at high redshift have much higher CO-excitation peaks
than their local counterparts and that CO cooling takes place at higher
excitation levels. CO line ratios increase with redshift as a function of
galaxy star-formation rate, but are well correlated with H2 surface density
independent of redshift. We find an increase in the [OI]/[CII] line ratio in
typical star-forming galaxies at z = 1.2 and z = 2.0 with respect to
counterparts at z = 0. Our model results suggest that typical star-forming
galaxies at high redshift consist of much denser and warmer star-forming clouds
than their local counterparts. Galaxies belonging to the tail of the SF
activity peak at z = 1.2 are already less dense and cooler than counterparts
during the actual peak of SF activity (z = 2.0). We use our results to discuss
how future ALMA surveys can best confront our predictions and constrain models
of galaxy formation.Comment: 19 pages, 14 figures, accepted for publication in MNRA
L-band ATS 5/Orion/S. S. Manhattan marine navigation and communication experiment Final report
L-band signals relayed by synchronous satellite for navigation and data communicatio
Search for grain growth towards the center of L1544
In dense and cold molecular clouds dust grains are surrounded by thick icy
mantles. It is however not clear if dust growth and coagulation take place
before the switch-on of a protostar. This is an important issue, as the
presence of large grains may affect the chemical structure of dense cloud
cores, including the dynamically important ionization fraction, and the future
evolution of solids in protoplanetary disks. To study this further, we focus on
L1544, one of the most centrally concentrated pre-stellar cores on the verge of
star formation, and with a well-known physical structure. We observed L1544 at
1.2 and 2 mm using NIKA, a new receiver at the IRAM 30 m telescope, and we used
data from the Herschel Space Observatory archive. We find no evidence of grain
growth towards the center of L1544 at the available angular resolution.
Therefore, we conclude that single dish observations do not allow us to
investigate grain growth toward the pre-stellar core L1544 and high sensitivity
interferometer observations are needed. We predict that dust grains can grow to
200 m in size toward the central ~300 au of L1544. This will imply a dust
opacity change by a factor of ~2.5 at 1.2 mm, which can be detected using the
Atacama Large Millimeter and submillimeter Array (ALMA) at different
wavelengths and with an angular resolution of 2".Comment: 12 pages, 14 figures. Accepted for publication in A&
The first frost in the Pipe Nebula
Spectroscopic studies of ices in nearby star-forming regions indicate that
ice mantles form on dust grains in two distinct steps, starting with polar ice
formation (H2O rich) and switching to apolar ice (CO rich). We test how well
the picture applies to more diffuse and quiescent clouds where the formation of
the first layers of ice mantles can be witnessed. Medium-resolution
near-infrared spectra are obtained toward background field stars behind the
Pipe Nebula. The water ice absorption is positively detected at 3.0 micron in
seven lines of sight out of 21 sources for which observed spectra are
successfully reduced. The peak optical depth of the water ice is significantly
lower than those in Taurus with the same visual extinction. The source with the
highest water-ice optical depth shows CO ice absorption at 4.7 micron as well.
The fractional abundance of CO ice with respect to water ice is 16+7-6 %, and
about half as much as the values typically seen in low-mass star-forming
regions. A small fractional abundance of CO ice is consistent with some of the
existing simulations. Observations of CO2 ice in the early diffuse phase of a
cloud play a decisive role in understanding the switching mechanism between
polar and apolar ice formation.Comment: 17 pages, 8 figures, accepted by A&
The Response of Metal Rich Gas to X-Ray Irradiation from a Massive Black Hole at High Redshift: Proof of Concept
Observational studies show that there is a strong link between the formation
and evolution of galaxies and the growth of supermassive black holes (SMBH) at
their centers. However, the underlying physics of this observed relation is
poorly understood. In order to study the effects of X-ray radiation on the
surroundings of the black hole, we implement X-ray Dominated Region (XDR)
physics into Enzo and use the radiation transport module Moray to calculate the
radiative transfer for a polychromatic spectrum. In this work, we investigate
the effects of X-ray irradiation, produced by a central massive black hole
(MBH) with a mass of M = 5x10^4 M_(solar), on ambient gas with solar and zero
metallicity. We find that in the solar metallicity case, due to high opacity of
the metals, the energy deposition rate in the central region (< 20 pc) is high
and hence the temperatures in the center are on the order of 10^(5-7) K.
Moreover, due to the cooling ability and high intrinsic opacity of solar
metallicity gas, column densities of 10^(24) cm^(-2) are reached at a radius of
20 pc from the MBH. These column densities are about 3 orders of magnitudes
higher than in the zero metallicity case. Furthermore, in the zero metallicity
case an X-ray induced H II region is formed already after 5.8 Myr. This causes
a significant outflow of gas (~8x10^6 M_(solar) from the central region, with
the gas reaching outflow velocities up to ~100 km s^(-1). At later times, ~23
Myr after we insert the MBH, we find that the solar metallicity case also
develops an X-ray induced H II region, but delayed by ~17 Myr.Comment: 27 pages, 11 figures. Resubmitted to Ap
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