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G. Gentile

Publications and source records attributed to G. Gentile.

At least 73 records · Page 4Linked to original sources
arXiv2006

On the radio properties of the highest redshift quasars

We present deep radio observations of the most distant complete quasar sample drawn from the Sloan Digital Sky Survey. Combining our new data with those from literature we obtain a sample which is ~100 per cent complete down to S_1.4GHz = 60 μJy over the redshift range 3.8 < z < 5. The fraction of radio detections is relatively high (~43 per cent), similar to what observed locally in bright optical surveys. Even though the combined radio and optical properties of quasars remain overall unchanged from z ~ 5 to the local Universe, there is some evidence for a slight over-abundance of radio-loud sources at the highest redshifts when compared with the lower-z regime. Exploiting the deep radio VLA observations we present the first attempt to directly derive the radio luminosity function of bright quasars at z ~ 4. The unique depth -- both in radio and optical -- allows us to thoroughly explore the population of optically bright FR~II quasars up to z ~ 5 and opens a window on the behaviour of the brightest FR~I sources. A close investigation of the space density of radio loud quasars also suggests a differential evolution, with the more luminous sources showing a less pronounced cut-off at high z when compared with the less luminous ones.

astro-ph↗
arXiv2006

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.

astro-ph↗
arXiv2006

Borel summability and Lindstedt series

Resonant motions of integrable systems subject to perturbations may continue to exist and to cover surfaces with parametric equations admitting a formal power expansion in the strength of the perturbation. Such series may be, sometimes, summed via suitable sum rules defining $C^\infty$ functions of the perturbation strength: here we find sufficient conditions for the Borel summability of their sums in the case of two-dimensional rotation vectors with Diophantine exponent $τ=1$ (e. g. with ratio of the two independent frequencies equal to the golden mean).

math-ph↗
arXiv2005

Comment on "Scalar-tensor gravity coupled to a global monopole and flat rotation curves" by Lee and Lee

The recent paper by Lee and Lee (2004) may strongly leave the impression that astronomers have established that the rotation curves of spiral galaxies are flat. We show that the old paradigm of Flat Rotation Curves lacks, today, any observational support and following it at face value leads to intrinsically flawed alternatives to the Standard Dark Matter Scenario. On the other side, we claim that the rich systematics of spiral galaxy rotation curves, that reveals, in the standard Newtonian Gravity framework, the phenomenon of dark matter, in alternative scenarios, works as a unique benchmark.

astro-ph↗
arXiv2005

The dwarf galaxy DDO 47: testing cusps hiding in triaxial halos

We present HI data of the dwarf galaxy DDO 47, aimed at testing the hypothesis that dark halo triaxiality might induce non-circular motions resulting in rotation curves best fitted by cored halos, even if the dark matter halo is intrinsically cuspy. We performed a harmonic decomposition of the velocity field in order to search for alleged non-circular motions needed to ``hide'' a cusp: in DDO 47 non-circular motions are globally at a level of 2-3 km s$^{-1}$, far from being sufficient to reconcile the observed rotation curve with the $Λ$CDM predictions. We conclude that the dark matter halo around DDO 47 is truly cored and that a cusp cannot be hidden by non-circular motions. More details are shown in Gentile et al. (2006, ApJL in press, astro-ph/0506538).

astro-ph↗
arXiv2005

The dwarf galaxy DDO 47 as a dark matter laboratory: testing cusps hiding in triaxial halos

We present an analysis of high resolution HI data of the dwarf galaxy DDO 47, aimed at testing the hypothesis that dark halo triaxiality might induce non-circular motions resulting in rotation curves best fitted by cored halos, even if the dark matter halo is intrinsically cuspy. This hypothesis could be invoked in order to reconcile the predictions of the standard LambdaCDM theory with the rotation curves of disk galaxies. DDO 47 is an ideal case to test this hypothesis because it has a very regular velocity field, its rotation curve is best fitted by a cored halo and a NFW halo is inconsistent with the data. We analysed the velocity field through the higher-order harmonic terms in order to search for kinematical signatures of alleged non-circular motions needed to "hide" a cusp: the result is that globally non-circular motions are at a level of 2-3 km/s, and they are more likely to be associated to the presence of some spiral structure than to a global elongated potential (e.g. a triaxial halo). These non-circular motions are far from being sufficient to account for the discrepancy with the LambdaCDM predictions. We therefore conclude that the dark matter halo around the dwarf galaxy DDO 47 is truly cored and that a cusp cannot be hidden by non-circular motions.

astro-ph↗
arXiv2004

Cores of Dark Matter Halos Correlate with Disk Scale Lengths

We investigate in detail the mass distribution obtained by means of high resolution rotation curves of 25 galaxies of different morphological types. The dark matter contribution to the circular rotation velocity is well-described by resorting to a dark component whose density shows an inner core, i.e. a central constant density region. We find a very strong correlation between the core radius size $R_C$ and the stellar exponential scale length $R_D$: $R_C \simeq 13 (\frac {R_D} {5 {\rm kpc}})^{1.05} {\rm kpc} $, and between $R_C$ and the galaxy dynamical mass at this distance, $M_{dyn}(R_C)$. These relationships would not be expected if the core radii were the product of an incorrect decomposition procedure, or the biased result of wrong or misunderstood observational data. The very strong correlation between the dark and luminous scale lengths found here seems to hold also for different Hubble types and opens new scenarios for the nature of the dark matter in galaxies.

astro-ph↗
arXiv2004

The cored distribution of dark matter in spiral galaxies

We present the HI data for 5 spiral galaxies that, along with their Halpha rotation curves, are used to derive the distribution of dark matter within these objects. A new method for extracting rotation curves from HI data cubes is presented; this takes into account the existence of a warp and minimises projection effects. The rotation curves obtained are tested by taking them as input to construct model data cubes that are compared to the observed ones: the agreement is excellent. On the contrary, the model data cubes built using rotation curves obtained with standard methods, such as the first-moment analysis, fail the test. The HI rotation curves agree well with the Halpha data, where they coexist. Moreover, the combined Halpha + HI rotation curves are smooth, symmetric and extended to large radii. The rotation curves are decomposed into stellar, gaseous and dark matter contributions and the inferred density distribution is compared to various mass distributions: dark haloes with a central density core, $Λ$ Cold Dark Matter ($Λ$CDM) haloes (NFW, Moore profiles), HI scaling and MOND. The observations point to haloes with constant density cores of size $r_{core} \sim r_{opt}$ and central densities scaling approximately as $ρ_0 \propto r_{core}^{-2/3}$. $Λ$CDM models (which predict a central cusp in the density profile) are in clear conflict with the data. HI scaling and MOND cannot account for the observed kinematics: we find some counter-examples.

astro-ph↗
arXiv2003

The line-of-sight warp of the spiral galaxy ESO 123-G23

We present 3-D modelling of the distribution and kinematics of the neutral hydrogen in the spiral galaxy ESO 123-G23. The optical appearance of this galaxy is an almost perfectly edge-on disk, while the neutral hydrogen is found to extend vertically out to about 15 kpc on either side of the galactic plane. The HI layer and the major features of the HI data cube can be successfully explained by a model dominated by a strong (about 30 degrees) line-of-sight warp. Other models were tried, including a flare model and a two-component model, but they clearly do not reproduce the data. This is the first unambiguous detection of a galactic warp that has the maximum deviation from the central plane almost along the line-of-sight. No evidence for the presence of any companion galaxy is found in the HI data cube. Line-of-sight warps in edge-on galaxies are probably frequent, but escape detection as they are too weak. Moreover they may easily be mistaken as flares or 'thick disks'. A 3-D modelling of the HI layer as the one presented here is needed in order to distinguish between these possibilities.

astro-ph↗
arXiv2002

Unveiling Dark Matter with HI and Halpha Data - Observational Problems

We present combined Halpha+HI rotation curves for a sample of spiral galaxies. Most of the velocity profiles (spectra at single points) in these galaxies are asymmetric, preventing the use of standard methods like the first moment analysis and the single Gaussian fitting. We thus propose a method similar to the Envelope-Tracing method (Sofue & Rubin 2001) to analyse those profiles from which we obtain HI rotation curves in good agreement with the Halpha rotation curves. These final rotation curves provide the required high resolution in the inner parts of the galaxies, but also extend out to typically 2-3 optical radii. They will hence allow us to investigate the distribution of dark matter.

astro-ph↗
arXiv2002

Warps and Rotation Curves in Edge-On Galaxies

We present our investigation on the effect of warps on the extraction of rotation curves in edge-on galaxies. The method to derive the rotation curve from the position-velocity diagram in warped edge-on systems yields underestimated velocities, and the tilted-ring model is not reliable in highly inclined, poorly resolved galaxies. In a warped system the kinematical major axis is different from the optical major axis. While this is generally a limit in optical slit spectroscopy, in the HI emission which extends far from the optical body where self-gravity is weaker and the effect of warping is more pronounced, this represents a severe effect to be considered in the procedure to extract the rotation curve. We propose a new approach to extract the rotation curve in highly inclined, warped galaxies. Based on this method we are able to trace accurately the frequency of peculiarities in our sample of Thick Boxy Bulge (TBB) galaxies. We report an increasing trend of kinematical lopsidedness from spheroidal bulge galaxies towards TBB galaxies. Concerning the question whether interactions contribute significantly to the bar formation and to the subsequent evolution in a box/peanut (b/p) structure, we confirm these theoretical predictions. Based on our sample, galaxy interaction is the likely formation mechanism to trigger bars in TBB galaxies.

astro-ph↗
arXiv2001

Hyperbolic low-dimensional invariant tori and summations of divergent series

We consider a class of a priori stable quasi-integrable analytic Hamiltonian systems and study the regularity of low-dimensional hyperbolic invariant tori as functions of the perturbation parameter. We show that, under natural nonresonance conditions, such tori exist and can be identified through the maxima or minima of a suitable potential. They are analytic inside a disc centered at the origin and deprived of a region around the positive or negative real axis with a quadratic cusp at the origin. The invariant tori admit an asymptotic series at the origin with Taylor coefficients that grow at most as a power of a factorial and a remainder that to any order N is bounded by the (N+1)-st power of the argument times a power of $N!$. We show the existence of a summation criterion of the (generically divergent) series, in powers of the perturbation size, that represent the parametric equations of the tori by following the renormalization group methods for the resummations of perturbative series in quantum field theory

math-ph↗
arXiv1999

On a conjecture for the critical behaviour of KAM tori

At the light of recent results in literature we review a conjecture formulated in Math. Phys. Electron. J. 1 (1995), paper 5, 1--13, about the mechanism of breakdown of invariant sets in KAM problems and the identification of the dominant terms in the perturbative expansion of the conjugating function. We show that some arguments developed therein can be carried out further only in some particular directions, so limiting a possible future research program, and that the mechanism of break down of invariant tori has to be more complicated than as conjectured in the quoted paper.

chao-dyn↗
arXiv1998

Melnikov's approximation dominance. Some examples

We continue a previous paper to show that Mel'nikov's first order formula for part of the separatrix splitting of a pendulum under fast quasi periodic forcing holds, in special examples, as an asymptotic formula in the forcing rapidity.

chao-dyn↗
arXiv1998

Peierls instability for the Holstein model

We consider the static Holstein model, describing a chain of Fermions interacting with a classical phonon field, when the interaction is weak and the density is a rational number. We show that the energy of the system, as a function of the phonon field, has two stationary points, defined up to a lattice translation, which are local minima in the space of fields periodic with period equal to the inverse of the density.

cond-mat.stat-mech↗
arXiv1997

Pendulum: separatrix splitting

An exact expression for the determinant of the splitting matrix is derived: it allows us to analyze the asympotic behaviour needed to amend the large angles theorem proposed in Ann. Inst. H. Poincaré, B-60, 1, 1994. The asymptotic validity of Melnokov's formulae is proved for the class of models considered, which include polynomial perturbations.

chao-dyn↗