Absolute absorption and fluorescence measurements over a dynamic range of 106^6 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 $\sim$16,000 independent regions, each with $\sim$1,000 net counts. The excellent spatial resolution of the thermodynamic maps reveals the dramatic and complex temperature, pressure, entropy and metallicity structure of the system. Excluding the 'X-ray arms', the diffuse cluster gas at a given radius is strikingly isothermal. This suggests either that the ambient cluster gas, beyond the arms, remains relatively undisturbed by AGN uplift, or that conduction in the intracluster medium (ICM) is efficient along azimuthal directions. We confirm the presence of a thick ($\sim$40 arcsec or $\sim$3 kpc) ring of high pressure gas at a radius of $\sim$180 arcsec ($\sim$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 $\sim$40 arcsec ($\sim$3 kpc) surrounding the central AGN, with an estimated Mach number M > 1.2. As shown previously, if repeated shocks occur every $\sim$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 $\sim1.0\times10^6$ 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$^{3}$. 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 $\sim10^{12}M_{\odot}$ and we estimate that the cluster mass will exceed that of the Coma supercluster at $z \sim 0$. 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