The flaring HI disk of the nearby spiral galaxy NGC 2683

New deep VLA D array HI observations of the highly inclined nearby spiral galaxy NGC 2683 are presented. Archival C array data were processed and added to the new observations. To investigate the 3D structure of the atomic gas disk, we made different 3D models for which we produced model HI data cubes. The main ingredients of our best-fit model are (i) a thin disk inclined by 80 degrees; (ii) a crude approximation of a spiral and/or bar structure by an elliptical surface density distribution of the gas disk; (iii) a slight warp in inclination; (iv) an exponential flare; and (v) a low surface-density gas ring. The slope of NGC 2683's flare is comparable, but somewhat steeper than those of other spiral galaxies. NGC 2683's maximum height of the flare is also comparable to those of other galaxies. On the other hand, a saturation of the flare is only observed in NGC 2683. Based on the comparison between the high resolution model and observations, we exclude the existence of an extended atomic gas halo around the optical and thin gas disk. Under the assumption of vertical hydrostatic equilibrium we derive the vertical velocity dispersion of the gas. The high turbulent velocity dispersion in the flare can be explained by energy injection by (i) supernovae, (ii) magneto-rotational instabilities, (iii) ISM stirring by dark matter substructure, or (iv) external gas accretion. The existence of the complex large-scale warping and asymmetries favors external gas accretion as one of the major energy sources that drives turbulence in the outer gas disk. We propose a scenario where this external accretion leads to turbulent adiabatic compression that enhances the turbulent velocity dispersion and might quench star formation in the outer gas disk of NGC 2683.Comment: accepted for publication in A&

Fermi acceleration and suppression of Fermi acceleration in a time-dependent Lorentz Gas

We study some dynamical properties of a Lorentz gas. We have considered both the static and time dependent boundary. For the static case we have shown that the system has a chaotic component characterized with a positive Lyapunov Exponent. For the time-dependent perturbation we describe the model using a four-dimensional nonlinear map. The behaviour of the average velocity is considered in two situations (i) non-dissipative and (ii) dissipative. Our results show that the unlimited energy growth is observed for the non-dissipative case. However, when dissipation, via damping coefficients, is introduced the senary changes and the unlimited engergy growth is suppressed. The behaviour of the average velocity is described using scaling approach

Optical Tracking of Anomalous Diffusion Kinetics in Polymer Microspheres

In this Letter we propose the use of whispering gallery mode resonance tracking as a label-free optical means to monitor diffusion kinetics in glassy polymer microspheres. Approximate solutions to the governing diffusion equations are derived for the case of slow relaxation and small Stefan number. Transduction of physical changes in the polymer, including formation of a rubbery layer, swelling, and dissolution, into detectable resonance shifts are described using a perturbative approach. Concrete examples of poly(methyl methacrylate) and polystyrene spheres in water are considered

Whispering-gallery mode (WGM) sensors: review of established and WGM-based techniques to study protein conformational dynamics

This is the final version. Available on open access from Elsevier via the DOI in this recordMonitoring the conformational dynamics of proteins is crucial for a better understanding of their biological functions. To observe the structural dynamics of proteins, it is often necessary to study each molecule individually. To this end, single-molecule techniques have been developed such as Förster resonance energy transfer and optical tweezers. However, although powerful, these techniques do have their limitations, for example, limited temporal resolution, or necessity for fluorescent labelling, and they can often only access a limited set of all protein motions. Here, within the context of established structural biology techniques, we review a new class of highly sensitive optical devices based on WGM, which characterise protein dynamics on previously inaccessible timescales, visualise motions throughout a protein, and track movements of single atoms.Engineering and Physical Sciences Research Council (EPSRC

Optical observation of single atomic ions interacting with plasmonic nanorods in aqueous solution

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this recordPlasmonic nanoparticles provide the basis for a multitude of applications in chemistry, health care and optics because of their unique properties. Nanoparticle-based techniques have evolved into powerful tools for studying molecular interactions with single-molecule resolution. Here we show that this sensing capability can be used to detect single atomic ions in aqueous medium. We monitored interactions of single zinc and mercury ions with plasmonic gold nanorods (NRs) resonantly coupled to our whispering gallery mode sensor. Our system's ability to discern permanent binding and transient interaction allows us to study the different interaction kinetics of both ion species. The detection of transient interactions enables us to confirm statistically that the sensor signals originate from single ions. Furthermore, we reveal how the ion–NR interactions evolve with respect to the medium's ionic strength as mercury ions amalgamate with gold and zinc ions eventually turn into probes of highly localized surface potentials.Max Planck Societ

Shear-flow transition: the basin boundary

The structure of the basin of attraction of a stable equilibrium point is investigated for a dynamical system (W97) often used to model transition to turbulence in shear flows. The basin boundary contains not only an equilibrium point Xlb but also a periodic orbit P, and it is the latter that mediates the transition. Orbits starting near Xlb relaminarize. We offer evidence that this is due to the extreme narrowness of the region complementary to basin of attraction in that part of phase space near Xlb. This leads to a proposal for interpreting the 'edge of chaos' in terms of more familiar invariant sets.Comment: 11 pages; submitted for publication in Nonlinearit

In situ observation of single-molecule surface reactions from low to high affinities

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this recordUnderstanding reactions occurring between ligand molecules and nanomaterial surfaces is essential in the field of nanoscience. The conventional methods for characterizing such surface-based reactions allow only for the analysis of the end product of a reaction, although the reaction path proceeds through the transient interaction of reactants and with kinetics dependent on environmental parameters. Here we study single molecule reaction kinetics associated with gold nanoparticle surfaces in an aqueous medium by utilizing whispering-gallery-mode microcavity sensors. Our approach resolves transient as well as permanent interaction kinetics of ligand molecules at the nanoparticle interface in situ, over a broad range of affinities and even under conditions where no net product is formed. This enables us to monitor and characterize reactions during the entire procedure of a bottom-up surface modification, ranging from the deposition of ligands to the confirmation of their functionality. We demonstrate this prospect by studying surface reaction kinetics with respect to the species of ligand head groups, tethered molecules, and inhibitors in addition to subsequent bio-specific reactions between tethered molecules and analytes.Max Planck Societ

Simultaneous multi-band detection of Low Surface Brightness galaxies with Markovian modelling

We present an algorithm for the detection of Low Surface Brightness (LSB) galaxies in images, called MARSIAA (MARkovian Software for Image Analysis in Astronomy), which is based on multi-scale Markovian modeling. MARSIAA can be applied simultaneously to different bands. It segments an image into a user-defined number of classes, according to their surface brightness and surroundings - typically, one or two classes contain the LSB structures. We have developed an algorithm, called DetectLSB, which allows the efficient identification of LSB galaxies from among the candidate sources selected by MARSIAA. To assess the robustness of our method, the method was applied to a set of 18 B and I band images (covering 1.3 square degrees in total) of the Virgo cluster. To further assess the completeness of the results of our method, both MARSIAA, SExtractor, and DetectLSB were applied to search for (i) mock Virgo LSB galaxies inserted into a set of deep Next Generation Virgo Survey (NGVS) gri-band subimages and (ii) Virgo LSB galaxies identified by eye in a full set of NGVS square degree gri images. MARSIAA/DetectLSB recovered ~20% more mock LSB galaxies and ~40% more LSB galaxies identified by eye than SExtractor/DetectLSB. With a 90% fraction of false positives from an entirely unsupervised pipeline, a completeness of 90% is reached for sources with r_e > 3" at a mean surface brightness level of mu_g=27.7 mag/arcsec^2 and a central surface brightness of mu^0 g=26.7 mag/arcsec^2. About 10% of the false positives are artifacts, the rest being background galaxies. We have found our method to be complementary to the application of matched filters and an optimized use of SExtractor, and to have the following advantages: it is scale-free, can be applied simultaneously to several bands, and is well adapted for crowded regions on the sky.Comment: 39 pages, 18 figures, accepted for publication in A

Ram pressure stripping and galaxy orbits: The case of the Virgo cluster

We investigate the role of ram pressure stripping in the Virgo cluster using N-body simulations. Radial orbits within the Virgo cluster's gravitational potential are modeled and analyzed with respect to ram pressure stripping. The N-body model consists of 10000 gas cloud complexes which can have inelastic collisions. Ram pressure is modeled as an additional acceleration on the clouds located at the surface of the gas distribution in the direction of the galaxy's motion within the cluster. We made several simulations changing the orbital parameters in order to recover different stripping scenarios using realistic temporal ram pressure profiles. We investigate systematically the influence of the inclination angle between the disk and the orbital plane of the galaxy on the gas dynamics. We show that ram pressure can lead to a temporary increase of the central gas surface density. In some cases a considerable part of the total atomic gas mass (several 10^8 M_solar) can fall back onto the galactic disk after the stripping event. A quantitative relation between the orbit parameters and the resulting HI deficiency is derived containing explicitly the inclination angle between the disk and the orbital plane. The comparison between existing HI observations and the results of our simulations shows that the HI deficiency depends strongly on galaxy orbits. It is concluded that the scenario where ram pressure stripping is responsible for the observed HI deficiency is consistent with all HI 21cm observations in the Virgo cluster.Comment: 29 pages with 21 figures. Accepted for publication in Ap