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S. Andreon

Publications and source records attributed to S. Andreon.

355 records · Page 20Linked to original sources
arXiv1999

UV (2000 A) luminosity function of Coma cluster galaxies

The UV (2000 A) luminosity function (hereafter UV LF) of Coma cluster galaxies, based on more than 120 members, is computed as the statistical difference between counts in the Coma direction and in the field. Our UV LF is an up-date of a preliminary constrain on the UV LF previously computed without the essential background counts. The UV LF is well described by a power law with slope alpha~0.46, or equivalently, by a Schechter function with M* much brighter than the brightest cluster galaxy and with a slope alpha_S~-2.0 or larger. In spite of what happens in the optical band, low luminosity galaxies give a large contribution to the integral luminosity, and by inference, to the total metal production rate. Galaxies blue in UV-b and/or b-r dominate the Coma cluster UV LF, both in number and luminosity. The major source of error in the estimate of the UV LF cames from the background determination in the Coma direction, which is still uncertain, even though constrained at high and low amplitudes by redshift surveys covering the studied field.

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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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arXiv1999

Is the Butcher-Oemler effect a function of the cluster redshift ?

Using PSPC {\it Rosat} data, we measure x-ray surface brightness profiles, size and luminosity of the Butcher-Oemler (BO) sample of clusters of galaxies. The cluster x-ray size, as measured by the Petrosian r_{η=2} radius, does not change with redshift and is independent from x-ray luminosity. On the other hand, the x-ray luminosity increases with redshift. Considering that fair samples show no-evolution, or negative luminosity evolution, we conclude that the BO sample is not formed from the same class of objects observed at different look-back times. This is in conflict with the usual interpretation of the Butcher-Oemler as an evolutionary (or redshift-dependent) effect, based on the assumption that we are comparing the same class of objects at different redshifts. Other trends present in the BO sample reflect selection criteria rather than differences in look-back time, as independently confirmed by the fact that trends loose strength when we enlarge the sample with x-ray selected sample of clusters. The variety of optical sizes and shapes of the clusters in the Butcher-Oemler sample, and the Malmquist-like bias, are the reasons for these selection effects that mimic the trends usually interpreted as changes due to evolution.

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arXiv1998

The age of early-type galaxies in clusters: an observational perspective

Contradictory results on the evolution of lenticular galaxies (S0s) at intermediate redshift have been recently published. A careful analysis of the used methodology shows that in order to gain insights on the evolution of the morphological types it is necessary: (1) to use quantitative morphological types, (2) to propagate morphological errors in the quantity used to measure the evolution of the S0s and (3) to adopt the same morphological classes at high as at low redshift, testing the correspondence with standard types. Some published analysis fail to satisfy at least one of the above three conditions, thus undermining the claim for a young age of S0s of intermediate redshift clusters.

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arXiv1998

On the universality of the type-dependent luminosity functions

We find that the bright part (M_B<-18.5 mag) of luminosity function of each Hubble type has the same shape in three poor clusters (Virgo, Fornax and Centaurus) and in two rich clusters (Coma and Cl0939+4713). The fact that these type-dependent luminosity functions are invariant in shape from poor to rich environments gives support to the hypothesis that they may be universal. We also present in tabular form improved type-dependent luminosity functions, based on a much larger sample than previous works.

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arXiv1997

What is the primary source of the morphological segregation?

In Coma and two other clusters of galaxies the primary morphological segregation is the one with respect to a privileged direction. Only when this segregation is not considered it appears that the morphological types are segregated in density or in clustercentric distance.

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arXiv1997

An optical and near-IR survey of nearby clusters of galaxies

We present an optical and near-infrared survey of galaxies in nearby clusters aimed at determining fundamental quantities of galaxies, such as multivariate luminosity function and color distribution for each Hubble type. The main characteristics of our survey are completeness in absolute magnitude, wide wavelength coverage and faint limiting magnitudes.

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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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arXiv1997

Morphological classification and structural parameter of galaxies in the Coma and Perseus clusters

We present the results of an isophotal shape analysis of galaxies in the Coma and Perseus clusters. These data, together with those of two previous papers, provide two complete samples of galaxies with reliable Hubble types in rich clusters: 1) all galaxies brighter than m_b = 16.5 falling within one degree (=2.3 Mpc) from the center of the Coma cluster (187 galaxies), 2) all galaxies brighter than m_{Zwicky}=15.7 in a region of 5 deg 3' times 5 deg 27' around the center of the Perseus cluster (139 galaxies). These two complete samples cover 5 orders of magnitude in galaxy density and span areas of 91 and 17 Mpc^2, clustercentric radii up to 2.3 and 6.4 Mpc, for Perseus and Coma respectively. They will be used in subsequent papers to study the dependence of galaxy types on cluster environment and as reference samples in comparisons with distant clusters.

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arXiv1996

The morphological segregation of galaxies in clusters. III. The distant cluster Cl0939+4713 (Abell 851)

We have performed an isophotal analysis of galaxies in the distant (z=0.4) cluster Cl0939+4713 (Abell 851), using post-refurbished Hubble Space Telescope images. Morphological type estimates for the galaxies are given. A rigorous comparison of the properties of the types shows that early-type galaxies in Cl0939+4713 are, within the present statistical or systematic errors, indistinguishable from their counterparts in Coma in all their studied properties, namely ellipticity profile, slope of the color-magnitude relation from the near-ultraviolet to the near-infrared colors, mean surface brightness and luminosity function in the restframe photographic J band, homogeneity in color (around the color-magnitude relation). Furthermore ellipticals and lenticulars are separately homogeneous in their mean surface brightness, and have similar relative mean surface brightnesses in both clusters. Spirals are overabundant in Cl0939+4713 with respect to Coma, but twice less than previously estimated, and are more similar to field spirals than to cluster spirals. We suggest that the differences in the photometric properties of the spirals in the two clusters arise from differences in cluster gas density distribution which ultimately bring star formation in the spirals to a stop in Coma, but not in Cl0939+4713. The morphological types are segregated along a privileged direction that coincides with the position angle of the major axis of the outer X-ray isophotes in the cluster, just like in Coma and Perseus.

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arXiv1995

Morphological classification and structural parameters for early-type galaxies in the Coma cluster

We present the results of an isophotal shape analysis of three samples of galaxies in the Coma cluster. Quantitative morphology, together with structural and photometric parameters, is given for each galaxy. Special emphasis has been placed on the detailed classification of early-type galaxies. The three samples are: i) a sample of 97 early-type galaxies brighter than m$_B$ = 17.00 falling within one degree from the center of the Coma cluster; these galaxies were observed with CCD cameras, mostly in good to excellent resolution conditions; ii) a magnitude complete sample of 107 galaxies of all morphological types down to m$_B$ = 17.00 falling in a circular region of 50 arcmin diameter, slightly offcentered to the North-West of the cluster center; the images for this and the next sample come from digitized photographic plates; iii) a complete comparison sample of 26 galaxies of all morphological types down to m$_R$ = 16.05 (or m$_B$ $\simeq$ 17.5), also in a region of 50 arcmin diameter, but centered 2.6 degrees West of the cluster center. The reliability of our morphological classifications and structural parameters of galaxies, down to the adopted magnitude limits, is assessed by comparing the results on those galaxies for which we had images taken with different instrumentation and/or seeing conditions, and by comparing our results with similar data from other observers.

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arXiv1995

Multicolor surface photometry of brightest cluster galaxies: II. Enlarging the sample and improving the analysis

Intrinsic color profiles of a sample of nine Brightest Cluster Galaxies (BCGs) are recovered from the observed color profiles by evaluating spurious gradients introduced by errors in the determination of the sky levels and by different seeing conditions between the observations. Isophote shapes and surface brightness profiles are presented for the four newly observed BCGs. Three out of nine BCGs show color gradients of the order of 0.10 mag per decade in radius. Five BCGs do not possess any color gradient larger than 0.01 mag per decade in radius. We do not see any correlation between the presence (or the sign) of a color gradient and the BCG morphology (slope and shape of its surface brightness profile), or the cluster richness. This argues against a strong and recent influence of the environment on the BCG morphology. The slope of the BCG surface brightness profile is correlated to the cluster richness, posing a constraint on the initial conditions, or on the physical mechanism that is responsible for the present BCG morphology. Finally, only one BCG can be a cD despite visual classification as such of all studied BCGs.

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