18,953 research outputs found
Absolute absorption and fluorescence measurements over a dynamic range of 10 with cavity-enhanced laser-induced fluorescence
We describe a novel experimental setup that combines the advantages of both
laser-induced fluorescence and cavity ring-down techniques. The simultaneous
and correlated measurement of the ring-down and fluorescence signals yields
absolute absorption coefficients for the fluorescence measurement. The combined
measurement is conducted with the same sample in a single, pulsed laser beam.
The fluorescence measurement extends the dynamic range of a stand-alone cavity
ring-down setup from typically three to at least six orders of magnitude. The
presence of the cavity improves the quality of the signal, in particular the
signal-to-noise ratio. The methodology, dubbed cavity-enhanced laser-induced
fluorescence (CELIF), is developed and rigorously tested against the
spectroscopy of 1,4-bis(phenylethynyl)benzene in a molecular beam and density
measurements in a cell. We outline how the method can be utilised to determine
absolute quantities: absorption cross sections, sample densities and
fluorescence quantum yields.Comment: 12 pages, 6 figures, submitted to J. Chem. Phy
Memetic Multilevel Hypergraph Partitioning
Hypergraph partitioning has a wide range of important applications such as
VLSI design or scientific computing. With focus on solution quality, we develop
the first multilevel memetic algorithm to tackle the problem. Key components of
our contribution are new effective multilevel recombination and mutation
operations that provide a large amount of diversity. We perform a wide range of
experiments on a benchmark set containing instances from application areas such
VLSI, SAT solving, social networks, and scientific computing. Compared to the
state-of-the-art hypergraph partitioning tools hMetis, PaToH, and KaHyPar, our
new algorithm computes the best result on almost all instances
The X-ray coronae of the two brightest galaxies in the Coma cluster
We use deep Chandra X-ray Observatory observations to examine the coronae of
the two brightest cluster galaxies in the Coma cluster of galaxies, NGC 4874
and NGC 4889. We find that NGC 4889 hosts a central depression in X-ray surface
brightness consistent with a cavity or pair of cavities of radius 0.6 kpc. If
the central cavity is associated with an AGN outburst and contains relativistic
material, its enthalpy should be around 5x10^55 erg. The implied heating power
of this cavity would be around an order of magnitude larger than the energy
lost by X-ray emission. It would be the smallest and youngest known cavity in a
brightest cluster galaxy and the lack of over pressuring implies heating is
still gentle. In contrast, NGC 4874 does not show any evidence for cavities,
although it hosts a well-known wide-angle-tail radio source which is visible
outside the region occupied by the X-ray corona. These two galaxies show that
AGN feedback can behave in varied ways in the same cluster environment.Comment: 11 pages, 11 figures, accepted by MNRA
Magnetic Fields in the Center of the Perseus Cluster
We present Very Long Baseline Array (VLBA) observations of the nucleus of NGC
1275, the central, dominant galaxy in the Perseus cluster of galaxies. These
are the first observations to resolve the linearly polarized emission from
3C84, and from them we determine a Faraday rotation measure (RM) ranging from
6500 to 7500 rad/m^2 across the tip of the bright southern jet component. At 22
GHz some polarization is also detected from the central parsec of 3C84,
indicating the presence of even more extreme RMs that depolarize the core at
lower frequencies. The nature of the Faraday screen is most consistent with
being produced by magnetic fields associated with the optical filaments of
ionized gas in the Perseus Cluster.Comment: Accepted for publication in MNRA
Kinematics of Gas Near the Galactic Center
The number of similarities between the nuclei of some ordinary galaxies and the quasars, radio galaxies, and Seyfert nuclei has become impressively large. Ambartsumian's recent account of the resemblances makes it hard not to believe that these energetic objects are galaxies passing through a difficult age, or perhaps suffering a not-so-rare galactic malady. And very few would deny that to understand this exceptional behavior it would be well to know how normal galaxies function in their innermost regions
Feedback under the microscope: thermodynamic structure and AGN driven shocks in M87
(abridged) Using a deep Chandra exposure (574 ks), we present high-resolution
thermodynamic maps created from the spectra of 16,000 independent
regions, each with 1,000 net counts. The excellent spatial resolution of
the thermodynamic maps reveals the dramatic and complex temperature, pressure,
entropy and metallicity structure of the system. Excluding the 'X-ray arms',
the diffuse cluster gas at a given radius is strikingly isothermal. This
suggests either that the ambient cluster gas, beyond the arms, remains
relatively undisturbed by AGN uplift, or that conduction in the intracluster
medium (ICM) is efficient along azimuthal directions. We confirm the presence
of a thick (40 arcsec or 3 kpc) ring of high pressure gas at a
radius of 180 arcsec (14 kpc) from the central AGN. We verify that
this feature is associated with a classical shock front, with an average Mach
number M = 1.25. Another, younger shock-like feature is observed at a radius of
40 arcsec (3 kpc) surrounding the central AGN, with an estimated
Mach number M > 1.2. As shown previously, if repeated shocks occur every
10 Myrs, as suggested by these observations, then AGN driven weak shocks
could produce enough energy to offset radiative cooling of the ICM. A high
significance enhancement of Fe abundance is observed at radii 350 - 400 arcsec
(27 - 31 kpc). This ridge is likely formed in the wake of the rising bubbles
filled with radio-emitting plasma that drag cool, metal-rich gas out of the
central galaxy. We estimate that at least solar masses of
Fe has been lifted and deposited at a radius of 350-400 arcsec; approximately
the same mass of Fe is measured in the X-ray bright arms, suggesting that a
single generation of buoyant radio bubbles may be responsible for the observed
Fe excess at 350 - 400 arcsec.Comment: 18 pages, 16 figures. Accepted to MNRA
A massive, distant proto-cluster at z=2.47 caught in a phase of rapid formation?
Numerical simulations of cosmological structure formation show that the
Universe's most massive clusters, and the galaxies living in those clusters,
assemble rapidly at early times (2.5 < z < 4). While more than twenty
proto-clusters have been observed at z > 2 based on associations of 5-40
galaxies around rare sources, the observational evidence for rapid cluster
formation is weak. Here we report observations of an asymmetric, filamentary
structure at z = 2.47 containing seven starbursting, submillimeter-luminous
galaxies and five additional AGN within a comoving volume of 15000 Mpc.
As the expected lifetime of both the luminous AGN and starburst phase of a
galaxy is ~100 Myr, we conclude that these sources were likely triggered in
rapid succession by environmental factors, or, alternatively, the duration of
these cosmologically rare phenomena is much longer than prior direct
measurements suggest. The stellar mass already built up in the structure is
and we estimate that the cluster mass will exceed that
of the Coma supercluster at . The filamentary structure is in line
with hierarchical growth simulations which predict that the peak of cluster
activity occurs rapidly at z > 2.Comment: 7 pages, 3 figures, 2 tables, accepted in ApJL (small revisions from
previous version
Eddington-Born-Infeld action for dark energy and dark matter
We argue that Einstein gravity coupled to a Born-Infeld theory provides an
attractive candidate to represent dark matter and dark energy. For cosmological
models, the Born-Infeld field has an equation of state which interpolates
between matter, w=0 (small times), and a cosmological constant w=-1 (large
times). On galactic scales, the Born-Infeld field predicts asymptotically flat
rotation curves.Comment: A sign mistake in section on galactic scales is pointed out. This
sign invalidates the content of that section. See comment on manuscrip
The Distribution of Mass in the Orion Dwarf Galaxy
Dwarf galaxies are good candidates to investigate the nature of Dark Matter,
because their kinematics are dominated by this component down to small
galactocentric radii. We present here the results of detailed kinematic
analysis and mass modelling of the Orion dwarf galaxy, for which we derive a
high quality and high resolution rotation curve that contains negligible
non-circular motions and we correct it for the asymmetric drift. Moreover, we
leverage the proximity (D = 5.4 kpc) and convenient inclination (47{\deg}) to
produce reliable mass models of this system. We find that the Universal
Rotation Curve mass model (Freeman disk + Burkert halo + gas disk) fits the
observational data accurately. In contrast, the NFW halo + Freeman disk + gas
disk mass model is unable to reproduce the observed Rotation Curve, a common
outcome in dwarf galaxies. Finally, we attempt to fit the data with a MOdified
Newtonian Dynamics (MOND) prescription. With the present data and with the
present assumptions on distance, stellar mass, constant inclination and
reliability of the gaseous mass, the MOND "amplification" of the baryonic
component appears to be too small to mimic the required "dark component". The
Orion dwarf reveals a cored DM density distribution and a possible tension
between observations and the canonical MOND formalism.Comment: 8 pages, 9 figures, accepted for publication in MNRA
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