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G. Longo

Publications and source records attributed to G. Longo.

121 records · Page 7Linked to original sources
arXiv1999

Automated search for LSB galaxies

Low Surface Brightness Galaxies (LSBGs) today constitute the less known fraction of galaxies: their number density and their physical properties (e.g. luminosity, colour, dynamics) are still largely uncertain. This is mainly due to the difficoulty in finding them (since their presence goes often unnoticed when astronomical images are analyzed with standard methods) and in measuring their properties. We present here the first results of a novel application to DPOSS plates, a project within the CRoNaRio collaboration. The aim is to build a large catalog of LSBGs, with much better understanding of completness and biases. First results are quite encouraging: in the same regions covered by existing catalogues, done on the same plates, we find several new LSBGs in addition to the known ones.

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arXiv1999

The Luminosity Function of 24 Abell Clusters from the CRoNaRio catalogues

We present the composite luminosity function of galaxies for 24 Abell clusters studied in our survey of the Northern Hemisphere, using DPOSS data in the framework of the CRoNaRio collaboration. Our determination of the luminosity function has been computed with very high accuracy thanks to 1) the use of homogeneous data both in the cluster and in the control field, 2) a local estimate of the background, which takes into account the presence of large-scale structures (which tipically make the background contribution larger) and of foreground clusters and groups; 3) the inclusion in the error calculation of the variance of background counts, which is tipically larger than Poissonian fluctuation. We plan at least a tenfold increase of the number of clusters.

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arXiv1999

An automatic procedure to extract galaxy clusters from CRoNaRio catalogs

We present preliminary results of a simple peak finding algorithm applied to catalogues of galaxies, extracted from the Second Palomar Sky Survey in the framework of the CRoNaRio project. All previously known Abell and Zwicky clusters in a test region of 5x5 sq. deg. are recovered and new candidate clusters are also detected. This algorithm represents an alternative way of searching for galaxy clusters with respect to that implemented and tested at Caltech on the same type of data (Gal et al. 1998).

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arXiv1999

The Multiplicity Function of groups of galaxies from CRoNaRio catalogues

The projected multiplicity function of galaxies gives (per square degree) the density of galaxy aggregates formed by N members. We use the CRoNaRio matched catalogues to derive the low N (N<15) multiplicity function from D-POSS data. The van Albada (1982) algorithm was implemented and used to identify candidate groups in the CRoNaRio catalogues. In absence of redshift surveys complete down to the magnitude limit of the DPOSS material and covering a wide enough area, the performances of the algorithm were tested on realistically simulated catalogues. The application to a set of 13 CRoNaRio catalogues allowed us to derive a multiplicity function as accurate as those available in literature obtained from redshift surveys.

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arXiv1999

Spectral Analysis of Stellar Light Curves by Means of Neural Networks

Periodicity analysis of unevenly collected data is a relevant issue in several scientific fields. In astrophysics, for example, we have to find the fundamental period of light or radial velocity curves which are unevenly sampled observations of stars. Classical spectral analysis methods are unsatisfactory to solve the problem. In this paper we present a neural network based estimator system which performs well the frequency extraction in unevenly sampled signals. It uses an unsupervised Hebbian nonlinear neural algorithm to extract, from the interpolated signal, the principal components which, in turn, are used by the MUSIC frequency estimator algorithm to extract the frequencies. The neural network is tolerant to noise and works well also with few points in the sequence. We benchmark the system on synthetic and real signals with the Periodogram and with the Cramer-Rao lower bound.

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arXiv1999

Neural Nets and Star/Galaxy Separation in Wide Field Astronomical Images

One of the most relevant problems in the extraction of scientifically useful information from wide field astronomical images (both photographic plates and CCD frames) is the recognition of the objects against a noisy background and their classification in unresolved (star-like) and resolved (galaxies) sources. In this paper we present a neural network based method capable to perform both tasks and discuss in detail the performance of object detection in a representative celestial field. The performance of our method is compared to that of other methodologies often used within the astronomical community.

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arXiv1998

The Palomar Digital Sky Survey (DPOSS)

We describe DPOSS, a new digital survey of the northern sky, based on the POSS-II photographic sky atlas. The survey covers the entire sky north of delta = -3 deg in 3 bands, calibrated to the Gunn $gri$ system, reaching to equivalent limiting magnitude of B_lim ~ 22 mag. As a result of the state-of-the-art digitisation of the plates, detailed processing of the scans, and a very extensive CCD calibration program, the data quality exceedes that of the previous photographically-based efforts. The end product of the survey will be the Palomar-Norris Sky Catalog, anticipated to contain > 50 million galaxies and > 2 billion stars, down to the survey classification limit, ~ 1 mag above the flux detection limit. Numerous scientific projects utilising these data have been started, and we describe briefly some of them; they illustrate the scientific potential of the data, and serve as the scientific verification tests of the survey. Finally, we discuss some general issues posed by the advent of multi-terabyte data sets in astronomy.

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arXiv1998

Extended stellar kinematics of elliptical galaxies in the Fornax cluster

We present extended stellar kinematics for a sample of 13 elliptical galaxies in the Fornax cluster. Major-axis velocity dispersion profiles (VDPs) and rotation curves (RCs) are given for 12 of the galaxies. A major feature of this data is the spatial extension: for 8 galaxies the data extends beyond 1 R_e, and for 5 it extends beyond 2 R_e. Compared to the previously available data, this corresponds to an increase in spatial coverage by a factor from 1 to 5. Five of the ellipticals in the sample turn out to be rotationally-supported systems, having positive rotation parameter log (V/sigma)*. One of these five, and another 3 galaxies from the remaining sample, display evidence for bar-like kinematics. The data indicate that the true number of `dynamically hot' stellar systems, is much lower than previously thought: of the Es in the present sample only 1/4 are confirmed as `pressure-supported' systems. The data reveal a host of individual peculiarities, like: wiggles, strong gradients, and asymmetries in the rotation curve and/or in the velocity dispersion profile, thus showing that the presence of kinematically distinct components and/or triaxiality is a common characteristic of this class of object.

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arXiv1998

Early versus late type galaxies in compact groups

We find a strong correlation between the effective radius of the largest early-type galaxies in compact groups of galaxies and the velocity dispersion of the groups. The lack of a similar correlation for late type galaxies is supportive of the so called second generation merging scenario which predicts that ellipticals should dominate the internal dynamics of the groups, while late-type galaxies are mainly recent interlopers which are still in an early stage of interaction with the group potential.

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arXiv1997

The CRONA (Caltech-Roma-Napoli) project

The CRONA project is a joint enterprise between Caltech and the astronomical observatory of Rome and Naples aimed to the photometric calibration of the Second Palomar Sky Survey (POSS-II) plates and to the construction of a complete photometric and astrometric catalogue of all objects present on the POSS-II. The calibrated final catalogues of objects will be made available to the community. We present here a comparison between the performances of the SKICAT and DAOPHOT packages applied to POSS-II images of the globular cluster M92.

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arXiv1996

Cataloging of the Digitized POSS-II, and Some Initial Scientific Results From It

We are conducting an effort to catalog all sources detected in the digitized POSS-II (DPOSS). This is now becoming an international collaboration including Caltech and the Observatories of Rome and Naples (project CRONA). There are also ongoing extensive CCD calibration efforts at Palomar. The resulting Palomar-Norris Sky Catalog (PNSC) is expected to contain > 5 x 10^7 galaxies, and > 2 x 10^9 stars, in 3 colors (photographic JFN bands, calibrated to CCD gri system), down to the limiting magnitude equivalent of B approx 22 mag. The star-galaxy classification is accurate to 90 - 95% down to the equivalent of B approx 21 mag. The catalog will be made available to the community via computer networks or other suitable media, probably in installments, as soon as scientific validation and quality checks are completed and the funding allows it. Analysis and manipulation software will also be freely available. A great variety of scientific projects will be possible with this vast new data base, including studies of the large-scale structure in the universe, the Galactic structure, automatic optical identifications of sources from radio through x-ray bands, generation of objectively defined catalogs of clusters and groups of galaxies, generation of statistically complete catalogs of galaxies to be used in future redshift surveys, searches for high-redshift quasars and other active objects, searches for variable or extreme-color objects, etc. We have already started work on a number of such projects, and we summarize briefly some of the results obtained to date.

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arXiv1994

Major axis kinematics of 15 early-type galaxies in the Fornax cluster

Major axis rotation curves and velocity dispersions profiles, extending out to about one effective radius, are presented for 15 ellipticals and S0's out of the photometric sample of Fornax cluster galaxies studied by Caon \etal\ (1994). A brief description of the spectroscopic and photometric characteristics of each galaxy is provided, together with a comparison with previous studies. Six of the nine E's are possibly misclassified S0's or ellipticals harboring a disk-like component. Two galaxies (NGC~1399 and 1404) show a hint of counter-rotation. Evidence is given that the \bright\ and the \ordinary\ families of early-type galaxies, first introduced by Capaccioli \etal\ (1992), look distinct also in term of the anisotropy parameter $(\Vm/σ)^*$.

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