arXiv subjects

F. Fraternali

Publications and source records attributed to F. Fraternali.

At least 73 records · Page 4Linked to original sources

arXiv2011

Continuum limits of bistable spring models of carbon nanotube arrays accounting for material damage

Using chains of bistable springs, a model is derived to investigate the plastic behavior of carbon nanotube arrays with damage. We study the preconditioning effect due to the loading history by computing analytically the stress-strain pattern corresponding to a fatigue-type damage of the structure. We identify the convergence of the discrete response to the limiting case of infinitely many springs, both analytically in the framework of Gamma-convergence, as well as numerically.

math-ph↗

arXiv2011

Fountain-driven gas accretion by the Milky Way

Accretion of fresh gas at a rate of ~ 1 M_{sun} yr^{-1} is necessary in star-forming disc galaxies, such as the Milky Way, in order to sustain their star-formation rates. In this work we present the results of a new hydrodynamic simulation supporting the scenario in which the gas required for star formation is drawn from the hot corona that surrounds the star-forming disc. In particular, the cooling of this hot gas and its accretion on to the disc are caused by the passage of cold galactic fountain clouds through the corona.

astro-ph.GA↗

arXiv2011

Supernova-driven gas accretion in the Milky Way

We use a model of the Galactic fountain to simulate the neutral-hydrogen emission of the Milky Way Galaxy. The model was developed to account for data on external galaxies with sensitive HI data. For appropriate parameter values, the model reproduces well the HI emission observed at Intermediate Velocities. The optimal parameters imply that cool gas is ionised as it is blasted out of the disc, but becomes neutral when its vertical velocity has been reduced by ~30 per cent. The parameters also imply that cooling of coronal gas in the wakes of fountain clouds transfers gas from the virial-temperature corona to the disc at ~2 Mo/yr. This rate agrees, to within the uncertainties with the accretion rate required to sustain the Galaxy's star formation without depleting the supply of interstellar gas. We predict the radial profile of accretion, which is an important input for models of Galactic chemical evolution. The parameter values required for the model to fit the Galaxy's HI data are in excellent agreement with values estimated from external galaxies and hydrodynamical studies of cloud-corona interaction. Our model does not reproduce the observed HI emission at High Velocities, consistent with High Velocity Clouds being extragalactic in origin. If our model is correct, the structure of the Galaxy's outer HI disc differs materially from that used previously to infer the distribution of dark matter on the Galaxy's outskirts.

astro-ph.GA↗

arXiv2011

Galactic fountains and the rotation of disc-galaxy coronae

In galaxies like the Milky Way, cold (~ 10^4 K) gas ejected from the disc by stellar activity (the so-called galactic-fountain gas) is expected to interact with the virial-temperature (~ 10^6 K) gas of the corona. The associated transfer of momentum between cold and hot gas has important consequences for the dynamics of both gas phases. We quantify the effects of such an interaction using hydrodynamical simulations of cold clouds travelling through a hot medium at different relative velocities. Our main finding is that there is a velocity threshold between clouds and corona, of about 75 km/s, below which the hot gas ceases to absorb momentum from the cold clouds. It follows that in a disc galaxy like the Milky Way a static corona would be rapidly accelerated: the corona is expected to rotate and to lag, in the inner regions, by ~ 80-120 km/s with respect to the cold disc. We also show how the existence of this velocity threshold can explain the observed kinematics of the cold extra-planar gas.

astro-ph.GA↗

arXiv2010

An optical and HI study of the dwarf Local Group galaxy VV124=UGC4879. A gas-poor dwarf with a stellar disk?

We present a detailed study of the dwarf galaxy VV124, recently recognized as a isolated member of the Local Group. We have obtained deep (r=26.5) wide-field g,r photometry of individual stars with the LBT under sub-arcsec seeing conditions. The Color-Magnitude Diagram suggests that the stellar content of the galaxy is dominated by an old, metal-poor population, with a significant metallicity spread. A very clean detection of the RGB tip allows us to derive an accurate distance of D=1.3 +/- 0.1 Mpc. Combining surface photometry with star counts, we are able to trace the surface brightness profile of VV124 out to ~ 5' = 1.9 kpc radius (where mu_r=30 mag/arcsec^2), showing that it is much more extended than previously believed. Moreover, the surface density map reveals the presence of two symmetric flattened wings emanating from the central elongated spheroid and aligned with its major axis, resembling a stellar disk seen nearly edge-on. We also present HI observations obtained with the WSRT, the first ever of this object. A total amount of 10^6 M_sun of HI gas is detected in VV124. Compared to the total luminosity, this gives a value of M_HI/L_V=0.11, which is particularly low for isolated Local Group dwarfs. The spatial distribution of the gas does not correlate with the observed stellar wings. The systemic velocity of the HI in the region superposed to the stellar main body of the galaxy is V_h=-25 km/s. The velocity field shows substructures typical of galaxies of this size but no sign of rotation. The HI spectra indicates the presence of a two-phase interstellar medium, again typical of many dwarf galaxies.

astro-ph.CO↗

arXiv2010

The mode of gas accretion onto star-forming galaxies

It is argued that galaxies like ours sustain their star formation by transferring gas from an extensive corona to the star-forming disc. The transfer is effected by the galactic fountain -- cool clouds that are shot up from the plane to kiloparsec heights above the plane. The Kelvin-Helmholtz instability strips gas from these clouds. If the pressure and the the metallicity of the corona are high enough, the stripped gas causes a similar mass of coronal gas to condense in the cloud's wake. Hydrodynamical simulations of cloud-corona interaction are presented. These confirm the existence of a critical ablation rate above which the corona is condensed, and imply that for the likely parameters of the Galactic corona this rate lies near the actual ablation rate of clouds. In external galaxies trails of HI behind individual clouds will not be detectable, although the integrated emission from all such trails should be significant. Parts of the trails of the clouds that make up the Galaxy's fountain should be observable and may account for features in targeted 21-cm observations of individual high-velocity clouds and surveys of Galactic HI emission. Taken in conjunction with the known decline in the availability of cold infall with increasing cosmic time and halo mass, the proposed mechanism offers a promising explanation of the division of galaxies between the blue cloud to the red sequence in the colour-luminosity plane.

astro-ph.GA↗

arXiv2010

Galactic fountains and gas accretion

Star-forming disc galaxies such as the Milky Way need to accrete $\gsim$ 1 $M_{\odot}$ of gas each year to sustain their star formation. This gas accretion is likely to come from the cooling of the hot corona, however it is still not clear how this process can take place. We present simulations supporting the idea that this cooling and the subsequent accretion are caused by the passage of cold galactic-fountain clouds through the hot corona. The Kelvin-Helmholtz instability strips gas from these clouds and the stripped gas causes coronal gas to condense in the cloud's wake. For likely parameters of the Galactic corona and of typical fountain clouds we obtain a global accretion rate of the order of that required to feed the star formation.

astro-ph.GA↗

arXiv2009

Stationary models for the extra-planar gas in disc galaxies

The kinematics of the extra-planar neutral and ionised gas in disc galaxies shows a systematic decline of the rotational velocity with height from the plane (vertical gradient). This feature is not expected for a barotropic gas, whilst it is well reproduced by baroclinic fluid homogeneous models. The problem with the latter is that they require gas temperatures (above $10^5$ K) much higher than the temperatures of the cold and warm components of the extra-planar gas layer. In this paper, we attempt to overcome this problem by describing the extra-planar gas as a system of gas clouds obeying the Jeans equations. In particular, we consider models having the observed extra-planar gas distribution and gravitational potential of the disc galaxy NGC 891: for each model we construct pseudo-data cubes and we compare them with the HI data cube of NGC 891. In all cases the rotational velocity gradients are in qualitative agreement with the observations, but the synthetic and the observed data cubes of NGC 891 show systematic differences that cannot be accommodated by any of the explored models. We conclude that the extra-planar gas in disc galaxies cannot be satisfactorily described by a stationary Jeans-like system of gas clouds.

astro-ph.GA↗

arXiv2009

Tailored graph ensembles as proxies or null models for real networks I: tools for quantifying structure

We study the tailoring of structured random graph ensembles to real networks, with the objective of generating precise and practical mathematical tools for quantifying and comparing network topologies macroscopically, beyond the level of degree statistics. Our family of ensembles can produce graphs with any prescribed degree distribution and any degree-degree correlation function, its control parameters can be calculated fully analytically, and as a result we can calculate (asymptotically) formulae for entropies and complexities, and for information-theoretic distances between networks, expressed directly and explicitly in terms of their measured degree distribution and degree correlations.

cond-mat.dis-nn↗

arXiv2009

Do high-velocity clouds form by thermal instability?

We examine the proposal that the HI "high-velocity" clouds (HVCs) surrounding the Milky Way and other disc galaxies form by condensation of the hot galactic corona via thermal instability. Under the assumption that the galactic corona is well represented by a non-rotating, stratified atmosphere, we find that for this formation mechanism to work the corona must have an almost perfectly flat entropy profile. In all other cases the growth of thermal perturbations is suppressed by a combination of buoyancy and thermal conduction. Even if the entropy profile were nearly flat, cold clouds with sizes smaller than 10 kpc could form in the corona of the Milky Way only at radii larger than 100 kpc, in contradiction with the determined distances of the largest HVC complexes. Clouds with sizes of a few kpc can form in the inner halo only in low-mass systems. We conclude that unless even slow rotation qualitatively changes the dynamics of a corona, thermal instability is unlikely to be a viable mechanism for formation of cold clouds around disc galaxies.

astro-ph.GA↗

arXiv2009

Life at the Periphery of the Local Group: the kinematics of the Tucana Dwarf Galaxy

Dwarf spheroidal galaxies in the Local Group are usually located close to the Milky Way or M31. Currently, there are two clear exceptions to this rule, and the Tucana dwarf galaxy is the most distant at almost 1 Mpc from the Milky Way. Using the VLT/FORS2 spectrograph in multi-object mode we were able to measure the velocities of 23 individual Red Giant Branch stars in and around Tucana using the Ca triplet absorption lines. From this sample 17 reliable members have been identified. We measured the systemic velocity and dispersion of Tucana to be v_hel = +194.0+-4.3 km/s and sigma_l.o.s. = 15.8+4.1-3.1 km/s respectively. These measures are derived after removing the signature of rotation using a linear gradient of 6.5 x R/R_core+-2.9 km/s, which corresponds to a rotation of 16 km/s at the reliable limit of our data. Our systemic velocity corresponds to a receding velocity from the barycentre of the Local Group of v_LG = +73.3 km/s. We also determined the mean metallicity of Tucana to be [Fe/H] = -1.95+-0.15 with a dispersion of 0.32+-0.06 dex. Our study firmly excludes any obvious association of Tucana with the HI emission in the vicinity and shows that Tucana is a genuine dwarf spheroidal, with low metallicity stars, no gaseous ISM and no recent star formation. The present location and relatively high recession velocity are consistent with Tucana having been an isolated Local Group galaxy for the majority of its existence.

astro-ph.CO↗

arXiv2008

Ultra-High Energy Cosmic Rays, Spiral galaxies and Magnetars

We measure the correlation between the arrival directions of the highest energy cosmic rays detected by the Pierre Auger Observatory with the position of the galaxies in the HI Parkes All Sky Survey (HIPASS) catalogue, weighted for their HI flux and Auger exposure. The use of this absorption-free catalogue, complete also along the galactic plane, allows us to use all the Auger events. The correlation is significant, being 86.2% for the entire sample of HI galaxies, and becoming 99% when considering the richest galaxies in HI content, or 98% with those lying between 40-55. We interpret this result as the evidence that spiral galaxies are the hosts of the producers of UHECR and we briefly discuss classical (i.e energetic and distant) long Gamma Ray Burst (GRBs), short GRBs, as well as newly born or late flaring magnetars as possible sources of the Auger events. With the caveat that these events are still very few, and that the theoretical uncertainties are conspicuous, we found that newly born magnetars are the best candidates. If so, they could also be associated with sub-energetic, spectrally soft, nearby, long GRBs. We finally discuss why there is a clustering of Auger events in the direction on the radio-galaxy Cen A and an absence of events in the direction of the radio-galaxy M87.

astro-ph↗

arXiv2008

HI holes and high-velocity clouds in the spiral galaxy NGC 6946

We present a study of the distribution and kinematics of the neutral gas in the low-inclination Scd galaxy NGC 6946. The galaxy has been observed for 192 hours at 21-cm with the Westerbork Synthesis Radio Telescope. These are among the deepest observations ever obtained for a nearby galaxy. We detect widespread high-velocity HI (up to about 100 km/s) and find 121 HI holes, most of which are located in the inner regions where the gas density and the star formation rate are higher. Much of the high-velocity gas appears to be related to star formation and to be, in some cases, associated with HI holes. The overall kinematics of the high-velocity gas is characterized by a slower rotation as compared with the regular disk rotation. We conclude that the high-velocity gas in NGC 6946 is extra-planar and has the same properties as the gaseous halos observed in other spiral galaxies including the Milky Way. Stellar feedback (galactic fountain) is probably at the origin of most of the high-velocity gas and of the HI holes. There are also indications, especially in the outer regions, -an extended HI plume, velocity anomalies, sharp edges, and large-scale asymmmetries- pointing to tidal encounters and recent gas accretion.

astro-ph↗

arXiv2008

Cold gas accretion in galaxies

Evidence for the accretion of cold gas in galaxies has been rapidly accumulating in the past years. HI observations of galaxies and their environment have brought to light new facts and phenomena which are evidence of ongoing or recent accretion: 1) A large number of galaxies are accompanied by gas-rich dwarfs or are surrounded by HI cloud complexes, tails and filaments. It may be regarded as direct evidence of cold gas accretion in the local universe. It is probably the same kind of phenomenon of material infall as the stellar streams observed in the halos of our galaxy and M31. 2) Considerable amounts of extra-planar HI have been found in nearby spiral galaxies. While a large fraction of this gas is produced by galactic fountains, it is likely that a part of it is of extragalactic origin. 3) Spirals are known to have extended and warped outer layers of HI. It is not clear how these have formed, and how and for how long the warps can be sustained. Gas infall has been proposed as the origin. 4) The majority of galactic disks are lopsided in their morphology as well as in their kinematics. Also here recent accretion has been advocated as a possible cause. In our view, accretion takes place both through the arrival and merging of gas-rich satellites and through gas infall from the intergalactic medium (IGM). The infall may have observable effects on the disk such as bursts of star formation and lopsidedness. We infer a mean ``visible'' accretion rate of cold gas in galaxies of at least 0.2 Msol/yr. In order to reach the accretion rates needed to sustain the observed star formation (~1 Msol/yr), additional infall of large amounts of gas from the IGM seems to be required.

astro-ph↗

arXiv2008

A solvable model of the genesis of amino-acid sequences via coupled dynamics of folding and slow genetic variation

We study the coupled dynamics of primary and secondary structure formation (i.e. slow genetic sequence selection and fast folding) in the context of a solvable microscopic model that includes both short-range steric forces and and long-range polarity-driven forces. Our solution is based on the diagonalization of replicated transfer matrices, and leads in the thermodynamic limit to explicit predictions regarding phase transitions and phase diagrams at genetic equilibrium. The predicted phenomenology allows for natural physical interpretations, and finds satisfactory support in numerical simulations.

q-bio.BM↗

arXiv2005

The gaseous haloes of disc galaxies

The study of gas outside the plane of disc galaxies is crucial to understanding the circulation of material within a galaxy and between galaxies and the intergalactic environment. We present new HI observations of the edge-on galaxy NGC891, which show an extended halo component lagging behind the disc in rotation. We compare these results for NGC891 with other detections of gaseous haloes. Finally, we present a dynamical model for the formation of extra-planar gas.

astro-ph↗

arXiv2005

HI study of the warped spiral galaxy NGC5055: a disk/dark matter halo offset?

We present a study of the HI distribution and dynamics of the nearby spiral galaxy NGC5055 based on observations with the Westerbork Synthesis Radio Telescope. The gaseous disk of NGC5055 extends out to about 40 kpc, equal to 3.5 R_25, and shows a pronounced warp that starts at the end of the bright optical disk (R_25= 11.6 kpc). This very extended warp has large-scale symmetry, which along with the rotation period of its outer parts (~1.5 Gyr at 40 kpc), suggests a long-lived phenomenon. The rotation curve rises steeply in the central parts up to the maximum velocity (v_max ~ 206 km/s). Beyond the bright stellar disk (R_25), it shows a decline of about 25 km/s and then remains flat out to the last measured point. The standard analysis with luminous and dark matter components shows the dynamical importance of the disk. The best fit to the rotation curve is obtained with a ``maximum disk''. Less satisfactory fits with lighter disks help to set a firm lower limit of 1.4 to the mass-to-light ratio in F band of the disk. Such a ``minimum disk'' contributes about 60% of the observed maximum rotational velocity. NGC5055 shows remarkable overall regularity and symmetry. A mild lopsidedness is noticeable, however, both in the distribution and kinematics of the gas. The tilted ring analysis of the velocity field led us to adopt different values for the kinematical centre and for the systemic velocity for the inner and the outer parts of the system. This has produced a remarkable result: the kinematical and geometrical asymmetries disappear, both at the same time. These results point at two different dynamical regimes: an inner region dominated by the stellar disk and an outer one, dominated by a dark matter halo offset with respect to the disk.

astro-ph↗

arXiv2005

Hydrostatic models for the rotation of extra-planar gas in disk galaxies

We show that fluid stationary models are able to reproduce the observed, negative vertical gradient of the rotation velocity of the extra-planar gas in spiral galaxies. We have constructed models based on the simple condition that the pressure of the medium does not depend on density alone (baroclinic instead of barotropic solutions: isodensity and isothermal surfaces do not coincide). As an illustration, we have successfully applied our method to reproduce the observed velocity gradient of the lagging gaseous halo of NGC 891. The fluid stationary models discussed here can describe a hot homogeneous medium as well as a "gas" made of discrete, cold HI clouds with an isotropic velocity dispersion distribution. Although the method presented here generates a density and velocity field consistent with observational constraints, the stability of these configurations remains an open question.

astro-ph↗