35 research outputs found
The complex multiscale structure in simulated and observed emission maps of the proto-cluster cloud G0.253+0.016 (\u27the Brick\u27)
The Central Molecular Zone (the central ∼500 pc of the Milky Way) hosts molecular clouds in an extreme environment of strong shear, high gas pressure and density, and complex chemistry. G0.253+0.016, also known as \u27the Brick\u27, is the densest, most compact, and quiescent of these clouds. High-resolution observations with the Atacama Large Millimetre/submillimetre Array (ALMA) have revealed its complex, hierarchical structure. In this paper we compare the properties of recent hydrodynamical simulations of the Brick to those of the ALMA observations. To facilitate the comparison, we post-process the simulations and create synthetic ALMA maps of molecular line emission from eight molecules. We correlate the line emission maps to each other and to the mass column density and find that HNCO is the best mass tracer of the eight emission lines within the simulations. Additionally, we characterize the spatial structure of the observed and simulated cloud using the density probability distribution function (PDF), spatial power spectrum, fractal dimension, and moments of inertia. While we find good agreement between the observed and simulated data in terms of power spectra and fractal dimensions, there are key differences in the density PDFs and moments of inertia, which we attribute to the omission of magnetic fields in the simulations. This demonstrates that the presence of the Galactic potential can reproduce many cloud properties, but additional physical processes are needed to fully explain the gas structure
HD/H2 Molecular Clouds in the Early Universe: The Problem of Primordial Deuterium
We have detected new HD absorption systems at high redshifts, z_abs=2.626 and
z_abs=1.777, identified in the spectra of the quasars J0812+3208 and Q1331+170,
respectively. Each of these systems consists of two subsystems. The HD column
densities have been determined: log(N(HD),A)=15.70+/-0.07 for z_A=2.626443(2)
and log(N(HD),B)=12.98+/-0.22 for z_B=2.626276(2) in the spectrum of J0812+3208
and log(N(HD),C)=14.83+/-0.15 for z_C=1.77637(2) and log(N(HD),D)=14.61+/-0.20
for z_D=1.77670(3) in the spectrum of Q1331+170. The measured HD/H2 ratio for
three of these subsystems has been found to be considerably higher than its
values typical of clouds in our Galaxy. We discuss the problem of determining
the primordial deuterium abundance, which is most sensitive to the baryon
density of the Universe \Omega_{b}. Using a well-known model for the chemistry
of a molecular cloud, we have estimated the isotopic ratio
D/H=HD/2H_2=(2.97+/-0.55)x10^{-5} and the corresponding baryon density
\Omega_{b}h^2=0.0205^{+0.0025}_{-0.0020}. This value is in good agreement with
\Omega_{b}h^2=0.0226^{+0.0006}_{-0.0006} obtained by analyzing the cosmic
microwave background radiation anisotropy. However, in high-redshift clouds,
under conditions of low metallicity and low dust content, hydrogen may be
incompletely molecularized even in the case of self-shielding. In this
situation, the HD/2H_2 ratio may not correspond to the actual D/H isotopic
ratio. We have estimated the cloud molecularization dynamics and the influence
of cosmological evolutionary effects on it
Erratum: "A Gravitational-wave Measurement of the Hubble Constant Following the Second Observing Run of Advanced LIGO and Virgo" (2021, ApJ, 909, 218)
[no abstract available
Search for Tensor, Vector, and Scalar Polarizations in the Stochastic Gravitational-Wave Background
The detection of gravitational waves with Advanced LIGO and Advanced Virgo has enabled novel tests of general relativity, including direct study of the polarization of gravitational waves. While general relativity allows for only two tensor gravitational-wave polarizations, general metric theories can additionally predict two vector and two scalar polarizations. The polarization of gravitational waves is encoded in the spectral shape of the stochastic gravitational-wave background, formed by the superposition of cosmological and individually unresolved astrophysical sources. Using data recorded by Advanced LIGO during its first observing run, we search for a stochastic background of generically polarized gravitational waves. We find no evidence for a background of any polarization, and place the first direct bounds on the contributions of vector and scalar polarizations to the stochastic background. Under log-uniform priors for the energy in each polarization, we limit the energy densities of tensor, vector, and scalar modes at 95% credibility to Ω0T<5.58×10-8, Ω0V<6.35×10-8, and Ω0S<1.08×10-7 at a reference frequency f0=25 Hz. © 2018 American Physical Society
Search for gravitational waves from Scorpius X-1 in the second Advanced LIGO observing run with an improved hidden Markov model
We present results from a semicoherent search for continuous gravitational waves from the low-mass x-ray binary Scorpius X-1, using a hidden Markov model (HMM) to track spin wandering. This search improves on previous HMM-based searches of LIGO data by using an improved frequency domain matched filter, the J-statistic, and by analyzing data from Advanced LIGO's second observing run. In the frequency range searched, from 60 to 650 Hz, we find no evidence of gravitational radiation. At 194.6 Hz, the most sensitive search frequency, we report an upper limit on gravitational wave strain (at 95% confidence) of h095%=3.47×10-25 when marginalizing over source inclination angle. This is the most sensitive search for Scorpius X-1, to date, that is specifically designed to be robust in the presence of spin wandering. © 2019 American Physical Society
On the progenitor of binary neutron star merger GW170817
On 2017 August 17 the merger of two compact objects with masses consistent with two neutron stars was discovered through gravitational-wave (GW170817), gamma-ray (GRB 170817A), and optical (SSS17a/AT 2017gfo) observations. The optical source was associated with the early-type galaxy NGC 4993 at a distance of just ∼40 Mpc, consistent with the gravitational-wave measurement, and the merger was localized to be at a projected distance of ∼2 kpc away from the galaxy's center. We use this minimal set of facts and the mass posteriors of the two neutron stars to derive the first constraints on the progenitor of GW170817 at the time of the second supernova (SN). We generate simulated progenitor populations and follow the three-dimensional kinematic evolution from binary neutron star (BNS) birth to the merger time, accounting for pre-SN galactic motion, for considerably different input distributions of the progenitor mass, pre-SN semimajor axis, and SN-kick velocity. Though not considerably tight, we find these constraints to be comparable to those for Galactic BNS progenitors. The derived constraints are very strongly influenced by the requirement of keeping the binary bound after the second SN and having the merger occur relatively close to the center of the galaxy. These constraints are insensitive to the galaxy's star formation history, provided the stellar populations are older than 1 Gyr
3115 Topical ophthalmic pharmacological agents in post operative management after ablative and incisional refractive corneal procedures
The geometry of the gas surrounding the Central Molecular Zone: on the origin of localised molecular clouds with extreme velocity dispersions
Observations of molecular gas near the Galactic centre (,
) reveal the presence of a distinct population of enigmatic
compact clouds which are characterised by extreme velocity dispersions (). These Extended Velocity Features (EVFs) are very
prominent in the datacubes and dominate the kinematics of molecular gas just
outside the Central Molecular Zone (CMZ). The prototypical example of such a
cloud is Bania Clump 2. We show that similar features are naturally produced in
simulations of gas flow in a realistic barred potential. We analyse the
structure of the features obtained in the simulations and use this to interpret
the observations. We find that the features arise from collisions between
material that has been infalling rapidly along the dust lanes of the Milky Way
bar and material that belongs to one of the following two categories: (i)
material that has `overshot' after falling down the dust lanes on the opposite
side; (ii) material which is part of the CMZ. Both types of collisions involve
gas with large differences in the line-of-sight velocities, which is what
produces the observed extreme velocity dispersions. Examples of both categories
can be identified in the observations. If our interpretation is correct, we are
directly witnessing (a) collisions of clouds with relative speeds of and (b) the process of accretion of fresh gas onto the CMZ.Comment: 12 pages, 10 figures. Movies of the simulation are available at the
link provided in the Supplementary Information sectio