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S. Andreon

Publications and source records attributed to S. Andreon.

At least 343 records · Page 19Linked to original sources
arXiv2001

Dissecting the luminosity function of the Coma cluster of galaxies using CFHT wide field images

[Abridged] We determined the luminosity function (LF) of the Coma cluster galaxies selected by luminosity, and the LF bi-variate in central brightness. The Coma cluster and control fields were imaged using the CFH12K (42x28 arcmin) and UH8K (28x28 arcmin) wide-field cameras at the CFHT. Selected HST images were used for testing. Quantities were derived from measurements in at least two colors, which have the following features: (1) Galaxies as faint as three times the luminosity of the brightest globular clusters are in the completeness region of our data. (2) We have a complete census (in the explored region) of low surface brightness galaxies with central surface brightness galaxies almost as low as the faintest so far cataloged ones. (3) The explored area is among the largest ever sampled with CCDs at comparable depth for any cluster of galaxies. (4) The error budget includes all sources of errors known to date. Using HST images we also discovered that blends of globular clusters, not resolved in individual components due to seeing, look like dwarf galaxies when observed from the ground and are numerous and bright. The derived Coma LF is relatively steep (alpha=-1.4) over the 11 magnitudes sampled, but the slope and shape depend on color. A large population of faint low surface brightness galaxies was discovered, representing the largest contributor (in number) to the LF at faint magnitudes. We found a clear progression for a faintening of the LF from high surface brightness galaxies (mu~20 mag/arcsec2) to galaxies of very faint central brightnesses (mu~24.5 mag/arcsec2), and some evidence for a steepening. Compact galaxies, usually classified as stars and therefore not included in the LF, are found to be a minor population in Coma.

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arXiv2001

Dim galaxies and outer halos of galaxies missed by 2MASS ? The near-infrared luminosity function and density

By using high-resolution and deep Ks band observations of early-type galaxies of the nearby Universe and of a cluster at z=0.3 we show that the two luminosity functions (LFs) of the local universe derived from 2MASS data miss a fair fraction of the flux of the galaxies (more than 20 to 30%) and a whole population of galaxies of central brightness fainter than the isophote used for detection, but bright enough to be included in the published LFs. In particular, the fraction of lost flux increases as the galaxy surface brightness become fainter. Therefore, the so far derived LF slopes and characteristic luminosity as well as luminosity density are underestimated. Other published near-infrared LFs miss flux in general, including the LF of the distant field computed in a 3 arcsec aperture.

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arXiv2001

Photometric validation of a model independent procedure to extract galaxy clusters

By means of CCD photometry in three bands (Gunn g, r, i) we investigate the existence of 12 candidate clusters extracted via a model independent peak finding algorithm (\cite{memsait}) from DPOSS data. The derived color-magnitude diagrams allow us to confirm the physical nature of 9 of the cluster candidates, and to estimate their photometric redshifts. Of the other candidates, one is a fortuitous detection of a true cluster at z~0.4, one is a false detection and the last is undecidable on the basis of the available data. The accuracy of the photometric redshifts is tested on an additional sample of 8 clusters with known spectroscopic redshifts. Photometric redshifts turn out to be accurate within z~0.01 (interquartile range).

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arXiv2000

Luminosity function of clusters of galaxies

The composite galaxy luminosity function (hereafter LF) of 39 Abell clusters of galaxies is derived by computing the statistical excess of galaxy counts in the cluster direction with respect to control fields. Due to the wide field coverage of the digitised POSS-II plates, we can measure field counts around each cluster in a fully homogeneous way. Furthermore, the availability of virtually unlimited sky coverage allows us to directly compute the LF errors without having to rely on the estimated variance of the background. The wide field coverage also allows us to derive the LF of the whole cluster, including galaxies located in the cluster outskirts. The global composite LF has a slope alpha ~ -1.1+/-0.2 with minor variations from blue to red filters, and M* ~ -21.7,-22.2,-22.4 mag (H_0=50 km/s/Mpc) in g, r and i filters, respectively. These results are in quite good agreement with several previous determinations and in particular with the LF determined for the inner region of a largely overlapping set of clusters, but derived making use of a completely different method for background subtraction. The similarity of the two LFs suggests the existence of minor differences between the LF in the cluster outskirts and in the central region, or a negligible contribution of galaxies in the cluster outskirts to the global LF.

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arXiv2000

Dissecting the luminosity function of the Coma cluster of galaxies by means of CFHT wide field images

By means of wide-field (42x28 arcmin) CCD images of the Coma cluster of galaxies and of control fields, taken at the Canada-France-Hawaii telescope with the CFH12K and UH8K cameras, we determine the luminosity function bi-variate in central brightness. We found a clear progression for a steeping and weakening of the FL going from high surface brightness galaxies (mu~20$ mag/arcsec2) to galaxies of very-faint central brightnesses (mu 24.5 mag/arcsec2). Compact galaxies, usually rejected in the star/galaxy classification, are found to be a minor population in Coma. A huge population of low surface brightness galaxies red and faint are discovered, representing the largest contributor to the LF at faint magnitudes. Among them, there could be the remnant of high redshift starbursting galaxies responsible for the Butcher-Oemler effect.

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arXiv2000

The Luminosity Function of 81 Abell Clusters from the CRoNaRio catalogues

We present the composite luminosity function (hereafter LF) of galaxies for 81 Abell clusters studied in our survey of the Northern Hemisphere, using DPOSS data processed by the CRoNaRio collaboration. The derived LF is very accurate due to the use of homogeneous data both for the clusters and the control fields and to the local estimate of the background, which takes into account the presence of large-scale structures and of foreground clusters and groups. The global composite LF is quite flat down to $M^*+5$ has a slope $α\sim-1.0\pm0.2$ with minor variations from blue to red filters, and $M^*\sim-21.8,-22.0,-22.3$ mag ($H_0=50$ km s$^{-1}$ Mpc$^{-1}$) in the $g, r$ and $i$ filters, respectively (errors are detailed in the text). We find a significant difference between rich and poor clusters thus arguing in favour of a dependence of the LF on the properties of the environment.

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arXiv2000

The Coma cluster luminosity function from ultraviolet to near-infrared

[Abridged] The Coma cluster luminosity function (LF) from ultraviolet (2000 AA) to the near-infrared (H band) is summarized. In the UV the LF is very steep, much steeper than in the optical. The steep Coma UV LF implies that faint and bright galaxies give similar contributions to the total UV flux and to the total metal production rate. The Coma UV LF is dominated by star forming galaxies, not by massive and large galaxies. The optical Coma LF is relatively steep (alpha=-1.4) over the 11 magnitudes sampled. We found a clear steeping of the FL going from B to R bands, indicative of the presence of a large number of red dwarfs, as faint as three bright globular clusters. Furthermore, using Hubble Space Telescope images, we discover that blends of globular clusters, not resolved in individual components due to seeing, look like dwarf galaxies when observed from the ground and are numerous and bright. The existence of these fake extended sources increases the steepness of the LF at faint magnitudes, if not deal on. The near-infrared LF was computed on a near-infrared selected sample of galaxies which photometry is complete down to the typical dwarf (M^*+5) luminosity. The Coma LF can be described by a Schechter function with intermediate slope (alpha~-1.3), plus a dip at M_H~-22 mag. The shape of the Coma LF in H band is quite similar to the one found in the B band. The similarity of the LF in the optical and H bands implies that in the central region of Coma there is no new population of galaxies which is too faint to be observed in the optical band (because dust enshrouded, for instance), down to the magnitudes of dwarfs.

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arXiv2000

Deep near-infrared luminosity function of a cluster of galaxies at z=0.3

The deep near-infrared luminosity function of AC118, a cluster of galaxies at z=0.3, is presented. AC118 is a bimodal cluster, as evidenced both by our near-infrared images of lensed galaxies, by public X-ray Rosat images and by the spatial distribution of bright galaxies. Taking advantage of the extension and depth of our data, which sample an almost unexplored region in the depth vs. observed area diagram, we derive the luminosity function (LF), down to the dwarf regime (M*+5), computed in several cluster portions. The overall LF, computed on a 2.66 Mpc2 areas (H_0=50 km/s/Mpc), has an intermediate slope (alpha=-1.2). However, the LF parameters depend on the surveyed cluster region: the central concentration has 2.6^{+5.1}_{-1.7} times more bright galaxies and 5.3^{+7.2}_{-2.3} times less dwarfs per typical galaxy than the outer region, which includes galaxies at an average projected distance of ~580 kpc (errors are quoted at the 99.9 % confidence level). The LF in the secondary AC118 clump is intermediate between the central and outer one. In other words, the near-infrared AC118 LF steepens going from high to low density regions. At an average clustercentric distance of ~580 kpc, the AC118 LF is statistically indistinguishable from the LF of field galaxies at similar redshift, thus suggesting that the hostile cluster environment plays a minor role in shaping the LF at large clustercentric distances, while it strongly affects the LF at higher galaxy density.

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arXiv2000

The Luminosity Function of 80 Abell Clusters from the CRoNaRio catalogues

We present the composite luminosity function (hereafter LF) of galaxies for 80 Abell clusters studied in our survey of the Northern Hemisphere, using DPOSS data in the framework of the CRoNaRio collaboration. Our determination of the LF has been computed with very high accuracy thanks to the use of homogeneous data both for the clusters and the control fields and to a local estimate of the background, which takes into account the presence of large-scale structures and of foreground clusters and groups. The global composite LF is quite flat down to M^+5 and it has a slope alpha=-1.0 +/- 0.2 with minor variations from blue to red filters, and M^* = -21.9,-22.0,-22.3 mag (H_0=50 km s^-1 Mpc^-1) in g, r and i filters, respectively (errors are detailed in the text). We find a significant difference between rich and poor clusters supporting the existence of an LF's dependence on the environment.

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arXiv2000

Wide Field Imaging. I. Applications of Neural Networks to object detection and star/galaxy classification

[Abriged] Astronomical Wide Field Imaging performed with new large format CCD detectors poses data reduction problems of unprecedented scale which are difficult to deal with traditional interactive tools. We present here NExt (Neural Extractor): a new Neural Network (NN) based package capable to detect objects and to perform both deblending and star/galaxy classification in an automatic way. Traditionally, in astronomical images, objects are first discriminated from the noisy background by searching for sets of connected pixels having brightnesses above a given threshold and then they are classified as stars or as galaxies through diagnostic diagrams having variables choosen accordingly to the astronomer's taste and experience. In the extraction step, assuming that images are well sampled, NExt requires only the simplest a priori definition of "what an object is" (id est, it keeps all structures composed by more than one pixels) and performs the detection via an unsupervised NN approaching detection as a clustering problem which has been thoroughly studied in the artificial intelligence literature. In order to obtain an objective and reliable classification, instead of using an arbitrarily defined set of features, we use a NN to select the most significant features among the large number of measured ones, and then we use their selected features to perform the classification task. In order to optimise the performances of the system we implemented and tested several different models of NN. The comparison of the NExt performances with those of the best detection and classification package known to the authors (SExtractor) shows that NExt is at least as effective as the best traditional packages.

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arXiv2000

Use of DPOSS data to study globular cluster halos: an application to M 92

We exploited the large areal coverage offered by the DPOSS to analyze the outermost regions of the galactic globular cluster M 92. Two independent photometric reduction programs (SKICAT and DAOPHOT) were used to construct a color-magnitude diagram and a surface density profile for this cluster, based on J- and F-band DPOSS plates. A strong similarity has been found in the performance of the two programs in the low--crowded outermost cluster regions. After removing the background contribution, we obtained the cluster outer surface density profile down to a surface brightness magnitude of mu_V ~ 31 mag/sq.arcsec and matched it with the inner profile of Trager et al. 1995. The profile shapes match very well: since our data are uncalibrated, the shift in magnitude between the profiles has been also used to calibrate our profile. The analysis shows that the cluster has an extra tidal halo extending out to ~ 30 arcmin from the cluster center at a 3 sigma level over the background noise. This halo is revealed to be almost circular.

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arXiv1999

Near-infrared Luminosity Function in the Coma cluster

We present the near-infrared H band luminosity function (hereafter LF) of the Coma cluster of galaxies. It is the deepest ever computed in the near-infrared, for any type of environment, extending over 7 magnitudes, down to ~M_H*+6. The LF was computed on a near-infrared selected sample of galaxies which photometry, complete down to the typical dwarf luminosity, is presented in a companion paper. The Coma LF can be described by a Schechter function with intermediate slope (alpha~-1.3), plus a dip at M_H~-22 mag. The shape of the Coma LF in H band is quite similar to the one found in the B band and, with less confidence, to the R band LF as well. The similarity of the LF in the optical and H bands implies that in the central region of Coma there is no new population of galaxies which is too faint to be observed in the optical band (because dust enshrouded, for instance), down to the magnitudes of dwarfs. The exponential cut of the LF at the bright end is in good agreement with the one derived from shallower near-infrared samples of galaxies, both in clusters and in the field. This fact is suggestive of a similarity of the tip of the mass function of galaxies, irrespective of the environment where they are found. The dip at M_H~-22 mag is instead unique among all the so far measured near-infrared LF, although several published observations are not deep enough or spanning a suitable wide field to distinctly detect this feature. The faint end of the LF, reaching M_H~-19 mag (roughly M_B~-15), is steep, but less than previously suggested from shallower near-infrared observations of an adjacent region in the Coma cluster. The differences between our measured LF and that measured previously in other regions suggests a dependency on environment of the faint end of the mass function (below M*+2.5).

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arXiv1999

Extensive near-infrared (H-band) photometry in Coma

We present extensive and accurate photometry in the near-infrared H band of a complete sample of objects in an area of about 400 arcmin2 toward the Coma cluster of galaxies. The sample, including about 300 objects, is complete down to H~17 mag, the exact value depending on the type of magnitude (isophotal, aperture, Kron) and the particular region studied. This is six magnitudes below the characteristic magnitude of galaxies, well into the dwarfs' regime at the distance of the Coma cluster. For each object (star or galaxy) we provide aperture magnitudes computed within five different apertures, the magnitude within the 22 mag arcsec2 isophote, the Kron magnitude and radius, magnitude errors, as well as the coordinates, the isophotal area, and a stellarity index. Photometric errors are 0.2 mag at the completness limit. This sample is meant to be the zero-redshift reference for evolutionary studies of galaxies.

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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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