arXiv subjects

R. Vio

Publications and source records attributed to R. Vio.

At least 19 recordsLinked to original sources

arXiv2020

The Molecular Mass function of the Local Universe

We construct the molecular mass function using the bivariate Kband-Mass Function of the Herschel Reference Survey, a volume-limited sample already widely studied at the entire electromagnetic spectrum. The molecular mass function is derived from the K-band and the gas mass cumulative distribution using a copula method described in detail in our previous papers. The H2 mass is relatively strong correlated with the K-band luminosity because of the tight relation between the stellar mass and the molecular gas mass within the sample with a scatter likely due to those galaxies which have lost their molecular content because of environmental effects or because of a larger gas consumption due to past star formation processes. The derived H2 Mass Function samples the molecular mass range from 4 10^6 to 10^10 solar masses, and when compared with theoretical models, it agrees well with the theoretical predictions at the lower end of the mass values, while at masses larger than 10^10 solar masses the HRS sample may miss galaxies with large content of molecular hydrogen and the outcomes are not conclusive. The value of the local density of the molecular gas mass inferred from our analysis is ~1.5x10^7 Mo Mpc^-3, and it is compared with the results at larger redshifts, confirming the lack of strong evolution of the molecular mass density between z=0 and z=4. This is the first Molecular Mass Function derived on a complete sample in the local Universe, which can be used as a reliable calibration at redshift $z$=0 for models aiming at predicting the evolution of the molecular mass density

astro-ph.GA↗

arXiv2019

Correct estimate of the probability of false detection of the matched filter in weak-signal detection problems. III (Peak distribution method versus the Gumbel distribution method)

The matched filter (MF) represents one of the main tools to detect signals from known sources embedded in the noise. In the Gaussian case the noise is assumed to be the realization of a Gaussian random field (GRF). The most important property of the MF, the maximization of the probability of detection subject to a constant probability of false detection or false alarm (PFA), makes it one of the most popular techniques. However, the MF technique relies upon the a priori knowledge of the number and the position of the searched signals in the GRF which usually are not available. A typical way out is to assume that the position of a signal coincides with one of the peaks in the matched filtered data. A detection is claimed when the probability that a given peak is due only to the noise (i.e. the PFA) is smaller than a prefixed threshold. In this case the probability density function (PDF) of the amplitudes has to be used for the computation of the PFA, which is different from the Gaussian. Moreover, the probability that a detection is false depends on the number of peaks present in the filtered GRF, the greater the number of peaks in a GRF, the higher the probability of peaks due to the noise that exceed the detection threshold. If not taken into account, the PFA can be severely underestimated. Many solutions proposed to this problem are non-parametric hence not able to exploit all the available information. This limitation has been overcome by means of two efficient parametric approaches, one based on the PDF of the peak amplitudes of a smooth and isotropic GRF whereas the other uses the Gumbel distribution (the asymptotic PDF of the corresponding extreme). Simulations and ALMA maps show that, although the two methods produce almost identical results, the first is more flexible and allows us to check the reliability of the detection procedure.

astro-ph.IM↗

arXiv2018

Comments on arXiv:1811.00154 [astro-ph.IM] "AGN Variability Analysis Handbook"

Why do we write this note? It is erroneous to pretend to extract physical information from the experimental light curves (time series) of astrophysical systems by means of linear stochastic differential equations (LSDE). In general, the time evolution of these systems is governed by a set of nonlinear differential equations. Hence, the LSDEs are not suitable to model their dynamics. In spite of this, recently the LSDEs have been proposed as tools for the analysis of AGN light curves. Their use in this context seems to be dictated by their simplicity rather than by a real physical argument. We stress in this note that the correct approach to the analysis of signals coming from systems with nonlinear dynamics is to tackle the problem using methodologies in well defined physical contexts.

astro-ph.IM↗

arXiv2018

A critical comparison of the Lomb-Scargle and the classical periodograms

The detection of signals hidden in noise is one of the oldest and common problems in astronomy. Various solutions have been proposed in the past such as the parametric approaches based on the least-squares fit of theoretical templates or the non-parametric techniques as the phase-folding method. Most of them, however, are suited only for signals with specific time evolution. For generic signals the spectral approach based on the periodogram is potentially the most effective. In astronomy the main problem in working with the periodogram is that often the sampling of the signals is irregular. This complicates its efficient computation (the fast Fourier transform cannot be directly used) but overall the determination of its statistical characteristics. The Lomb-Scargle periodogram (LSP) provides a solution to this last important issue, but its main drawback is the assumption of a very specific model of the datawhich is not correct for most of the practical applications. These issues are not always considered in literature with theoretical and practical consequences of no easy solution. Moreover, apart from pathological samplings, it is common believe that the LSP and the classical periodogram (CP) usually provide almost identical results. In general, this is true but here it is shown that there are situations where the LSP is less effective than the CP in the detection of signals in noise. There are no compelling reasons, therefore, to use the LSP instead of the CP which is directly connected to the correlation function of the observed signal with the sinusoidal functions at the various frequencies of interest.

astro-ph.IM↗

arXiv2018

The bivariate luminosity and mass functions of the local HRS galaxy sample. The stellar, dust, gas mass functions

We discuss the results of the relationships between the K-band and stellar mass, far-infrared luminosities, star formation rate, dust and gas masses of nearby galaxies computing the bivariate K-band Luminosity Function (BLF) and bivariate K-band Mass Function (BMF) of the Herschel Reference Survey (HRS), a volume-limited sample with full wavelength coverage. We derive the BLFs and BMFs from the K-band and stellar mass, far-infrared luminosities, star formation rate, dust and gas masses cumulative distributions using a copula method which is outlined in detail. The use of the bivariate computed taking into account the upper limits allows us to derive on a more solid statistical ground the relationship between the observed physical quantities. The analysis shows that the behaviour of the morphological (optically selected) subsamples is quite different. A statistically meaningful result can be obtained over the whole HRS sample only from the relationship between the K-band and the stellar mass, while for the remaining physical quantities (dust and gas masses, far-IR luminosity and star formation rate), the analysis is distinct for late-type (LT) and early-type galaxies (ETG). However, the number of ETGs is small to perform a robust statistical analysis, and in most of the case results are discussed only for the LTG subsample. The Luminosity and Mass Functions (LFs, MFs) of LTGs are generally dependent on the K-band and the various dependencies are discussed in detail. We are able to derive the corresponding LFs and MFs and compare them with those computed with other samples. Our statistical analysis allows us to characterise the HRS, that, although non homogeneously selected and partially biased towards low IR luminosities, may be considered as representative of the local LT galaxy population.

astro-ph.GA↗

arXiv2014

The bivariate K-band-submillimetre luminosity functions of the local HRS galaxy sample

We study the relationship between the K-band and the sub-millimetre (submm) emissions of nearby galaxies by computing the bivariate K-band-submm luminosity function (BLF) of the Herschel Reference Survey (HRS), a volume-limited sample observed in submm with Hersche/SPIRE. We derive the BLF from the K-band and submm cumulative distributions using a copula method. Using the BLF allows us to derive the relationship between the luminosities on more solid statistical ground. The analysis shows that over the whole HRS sample, no statistically meaningful conclusion can be derived for any relationship between the K-band and the submm luminosity. However, a very tight relationship between these luminosities is highlighted, by restricting our analysis to late-type galaxies. The luminosity function of late-type galaxies computed in the K-band and in the submm are dependent and the dependence is caused by the link, between the stellar mass and the cold dust mass, which has been already observed

astro-ph.CO↗

arXiv2013

An approach for the detection of point-sources in very high resolution microwave maps

This paper deals with the detection problem of extragalactic point-sources in multi-frequency, microwave sky maps that will be obtainable in future cosmic microwave background radiation (CMB) experiments with instruments capable of very high spatial resolution. With spatial resolutions that can be of order of 0.1-1.0 arcsec or better, the extragalactic point-sources will appear isolated. The same holds also for the compact structures due to the Sunyaev-Zeldovich (SZ) effect (both thermal and kinetic). This situation is different from the maps obtainable with instruments as WMAP or PLANCK where, because of the smaller spatial resolution (approximately 5-30 arcmin), the point-sources and the compact structures due to the SZ effect form a uniform noisy background (the "confusion noise"). Hence, the point-source detection techniques developed in the past are based on the assumption that all the emissions that contribute to the microwave background can be modeled with homogeneous and isotropic (often Gaussian) random fields and make use of the corresponding spatial power-spectra. In the case of very high resolution observations such an assumption cannot be adopted since it still holds only for the CMB. Here, we propose an approach based on the assumption that the diffuse emissions that contribute to the microwave background can be locally approximated by two-dimensional low order polynomials. In particular, two sets of numerical techniques are presented containing two different algorithms each. The performance of the algorithms is tested with numerical experiments that mimic the physical scenario expected for high Galactic latitude observations with the Atacama Large Millimeter/Submillimeter Array (ALMA).

astro-ph.IM↗

arXiv2013

Irregular time series in astronomy and the use of the Lomb-Scargle periodogram

Detection of a signal hidden by noise within a time series is an important problem in many astronomical searches, i.e. for light curves containing the contributions of periodic/semi-periodic components due to rotating objects and all other astrophysical time-dependent phenomena. One of the most popular tools for use in such studies is the "periodogram", whose use in an astronomical context is often not trivial. The "optimal" statistical properties of the periodogram are lost in the case of irregular sampling of signals, which is a common situation in astronomical experiments. Parts of these properties are recovered by the "Lomb-Scargle" (LS) technique, but at the price of theoretical difficulties, that can make its use unclear, and of algorithms that require the development of dedicated software if a fast implementation is necessary. Such problems would be irrelevant if the LS periodogram could be used to significantly improve the results obtained by approximated but simpler techniques. In this work we show that in many astronomical applications simpler techniques provide results similar to those obtainable with the LS periodogram. The meaning of the "Nyquist frequency" is also discussed in the case of irregular sampling.

astro-ph.IM↗

arXiv2012

Two modified ILC methods to detect point sources in Cosmic Microwave Background maps

We propose two detection techniques that take advantage of a small sky area approximation and are based on modifications of the "internal linear combination" (ILC) method, an approach widely used in Cosmology for the separation of the various components that contribute to the microwave background. The main advantage of the proposed approach, especially in handling multi-frequency maps of the same region, is that it does not require the "a priori" knowledge of the spatial power-spectrum of either the CMB and/or the Galactic foreground. Hence, it is more robust, easier and more intuitive to use. The performance of the proposed algorithms is tested with numerical experiments that mimic the physical scenario expected for high Galactic latitude observations with the Atacama Large Millimeter/submillimeter Array (ALMA).

astro-ph.IM↗

arXiv2010

Unevenly-sampled signals: a general formalism of the Lomb-Scargle periodogram

The periodogram is a popular tool that tests whether a signal consists only of noise or if it also includes other components. The main issue of this method is to define a critical detection threshold that allows identification of a component other than noise, when a peak in the periodogram exceeds it. In the case of signals sampled on a regular time grid, determination of such a threshold is relatively simple. When the sampling is uneven, however, things are more complicated. The most popular solution in this case is to use the "Lomb-Scargle" periodogram, but this method can be used only when the noise is the realization of a zero-mean, white (i.e. flat-spectrum) random process. In this paper, we present a general formalism based on matrix algebra, which permits analysis of the statistical properties of a periodogram independently of the characteristics of noise (e.g. colored and/or non-stationary), as well as the characteristics of sampling.

astro-ph.CO↗

arXiv2009

Considerations on some neglected but impotant issues concerning the In ternal Linear Combination method in Astronomy

Although the ``internal linear'' combination method (ILC) is a technique widely used for the separation of the Cosmic Microwave Background signal from the Galactic foregrounds, its characteristics are not yet well defined. This can lead to misleading conclusions about the actual potentialities and limits of such approach in real applications. Here we discuss briefly some facts about ILC that to our knowledge are not fully worked out in literature and yet have deep effects in the interpretation of the results.

astro-ph.CO↗

arXiv2009

A theoretical framework for the detection of point-sources in Cosmic Microwave Background maps

The detection of point-sources in microwave maps is a critical step in the analysis of the Cosmic Microwave Background (CMB) data. If not properly removed, these sources have adverse effects on the estimation of the power-spectrum and/or the test of Gaussianity of the CMB component. The techniques to extract point sources from an observed image used up to now do not show a general consensus about their real performance and properties. Their characteristics have been studied essentially through numerical simulations based on semi-empirical models of the CMB and the Galactic foreground. Such models often have different levels of sophistication and/or are based on different physical assumptions (e.g. the number of Galactic components and level of the noise). Moreover, the application of a given technique to a set of data (either simulated or experimental) requires the tuning of one or more parameters that unavoidably is a subjective operation. Hence, a reliable comparison is difficult. What is missing is a statistical analysis of the properties of the proposed methodologies. This is the aim of the present paper. The statistical properties of the detection techniques in the context of two different criteria, i.e. the Neyman-Pearson criterion and the maximization of the signal-to-noise ratio, are analyzed through an analytical approach. Various examples, techniques and the conditions under which they work satisfactorily are shown together with their limits and implementation details. Exploiting some a priori information, it is possible to develop simple algorithms with performances similar to those of more sophisticated but complex techniques. In this respect, a detection algorithm, useful in future ground-based experiments as, for example the Atacama Large Millimeter/submillimeter Array, is presented.

astro-ph↗

arXiv2008

"Internal Linear Combination" method for the separation of CMB from Galactic foregrounds in the harmonic domain

Foreground contamination is the fundamental hindrance to the cosmic microwave background (CMB) signals and its separation from it represents a fundamental question in Cosmology. One of the most popular algorithm used to disentangle foregrounds from the CMB signals is the "internal linear combination" method (ILC). In its original version, this technique is applied directly to the observed maps. In recent literature, however, it is suggested that in the harmonic (Fourier) domain it is possible to obtain better results since a separation can be attempted where the various Fourier frequencies are given different weights. This is seen as a useful characteristic in the case of noisy data. Here, we argue that the benefits of using such an approach are overestimated. Better results can be obtained if a classic procedure is adopted where data are filtered before the separation is carried out.

astro-ph↗

arXiv2008

A Statistical Analysis of the "Internal Linear Combination" Method in Problems of Signal Separation as in CMB Observations

AIMS: The separation of foreground contamination from cosmic microwave background (CMB) observations is one of the most challenging and important problem of digital signal processing in Cosmology. In literature, various techniques have been presented, but no general consensus about their real performances and properties has been reached. This is due to the characteristics of these techniques that have been studied essentially through numerical simulations based on semi-empirical models of the CMB and the Galactic foregrounds. Such models often have different level of sophistication and/or are based on different physical assumptions (e.g., the number of the Galactic components and the level of the noise). Hence, a reliable comparison is difficult. What actually is missing is a statistical analysis of the properties of the proposed methodologies. Here, we consider the "Internal Linear Combination" method (ILC) which, among the separation techniques, requires the smallest number of "a priori" assumptions. This feature is of particular interest in the context of the CMB polarization measurements at small angular scales where the lack of knowledge of the polarized backgrounds represents a serious limit. METHODS: The statistical characteristics of ILC are examined through an analytical approach and the basic conditions are fixed in a way to work satisfactorily. RESULTS: ILC provides satisfactory results only under rather restrictive conditions. This is a critical fact to take into consideration in planning the future ground-based observations (e.g., with ALMA) where, contrary to the satellite experiments, there is the possibility to have a certain control of the experimental conditions.

astro-ph↗

arXiv2007

Ly-alpha forest: efficient unbiased estimation of second-order properties with missing data

Context. One important step in the statistical analysis of the Ly-alpha forest data is the study of their second order properties. Usually, this is accomplished by means of the two-point correlation function or, alternatively, the K-function. In the computation of these functions it is necessary to take into account the presence of strong metal line complexes and strong Ly-alpha lines that can hidden part of the Ly-alpha forest and represent a non negligible source of bias. Aims. In this work, we show quantitatively what are the effects of the gaps introduced in the spectrum by the strong lines if they are not properly accounted for in the computation of the correlation properties. We propose a geometric method which is able to solve this problem and is computationally more efficient than the Monte Carlo (MC) technique that is typically adopted in Cosmology studies. The method is implemented in two different algorithms. The first one permits to obtain exact results, whereas the second one provides approximated results but is computationally very efficient. The proposed approach can be easily extended to deal with the case of two or more lists of lines that have to be analyzed at the same time. Methods. Numerical experiments are presented that illustrate the consequences to neglect the effects due to the strong lines and the excellent performances of the proposed approach. Results. The proposed method is able to remarkably improve the estimates of both the two-point correlation function and the K-function.

astro-ph↗

arXiv2006

Stochastic modeling of kHz QPO light curves

The Kluzniak & Abramowicz model explains high frequency, double peak, "3:2" QPOs observed in neutron star and black hole sources in terms of a non-linear parametric resonance between radial and vertical epicyclic oscillations of an almost Keplerian accretion disk. The 3:2 ratio of epicyclic frequencies occurs only in strong gravity. Rebusco (2004) and Horak (2004) studied the model analytically: they proved that a small forcing may indeed excite the parametric 3:2 resonance, but they have not explained the physical nature of the forcing. Here we integrate their equations numerically, dropping the ad hoc forcing, and adding instead a stochastic term to mimic the action of the very complex processes that occur in disks as, for example, MRI turbulence. We demonstrate that the presence of the stochastic term triggers the resonance in epicyclic oscillations of nearly Keplerian disks, and influences their pattern.

astro-ph↗

arXiv2005

Some further comments on the paper "biparametric scale adaptive filter design" for the detection of compact sources as proposed by M. Lopez-Caniego et. al 2005, MNRAS 359, 993

In this note we re-propose the arguments presented in Vio (2005) examining the superiority of the 'bi-parametric scale adaptive filter' (BSAF) when dealing with source detection as claimed by (Lopez-Caniego 2005a) and confirmed in Lopez-Caniego (2005b). We suggest here that the dispute can be easily solved if these authors provide the community with a detailed derivation of a basic equation.

astro-ph↗

arXiv2005

Some comments on the paper "Filter design for the detection of compact sources based on the Neyman-Pearson detector" by M. Lopez-Caniego et. al (2005, MNRAS 359, 993)

In this note we stress the necessity of a careful check of both the theoretical arguments and the numerical experiments used by Lopez-Caniego et al. (2005) to support the superior performances of the biparametric scale adaptive filter (BSAF) with respect to the classic matched filter (MF) in the detection of sources on a random Gaussian background.

astro-ph↗