149 research outputs found
The nature of gas and stars in the circumnuclear regions of AGN: a chemical approach
Aim of this communication is to describe the first results of a
work-in-progress regarding the chemical properties of gas and stars in the
circumnuclear regions of nearby galaxies. Different techniques have been
employed to estimate the abundances of chemical elements in the gaseous and
stellar components of nuclear surroundings in different classes of galaxies
according to the level of activity of the nucleus (normal or passive, star
forming galaxies and AGNs).Comment: 19 pages, proceedings of the 1st International Workshop:
Astrophysical winds and disk 2009 (Platamonas
Photometric redshifts for Quasars in multi band Surveys
MLPQNA stands for Multi Layer Perceptron with Quasi Newton Algorithm and it
is a machine learning method which can be used to cope with regression and
classification problems on complex and massive data sets. In this paper we give
the formal description of the method and present the results of its application
to the evaluation of photometric redshifts for quasars. The data set used for
the experiment was obtained by merging four different surveys (SDSS, GALEX,
UKIDSS and WISE), thus covering a wide range of wavelengths from the UV to the
mid-infrared. The method is able i) to achieve a very high accuracy; ii) to
drastically reduce the number of outliers and catastrophic objects; iii) to
discriminate among parameters (or features) on the basis of their significance,
so that the number of features used for training and analysis can be optimized
in order to reduce both the computational demands and the effects of
degeneracy. The best experiment, which makes use of a selected combination of
parameters drawn from the four surveys, leads, in terms of DeltaZnorm (i.e.
(zspec-zphot)/(1+zspec)), to an average of DeltaZnorm = 0.004, a standard
deviation sigma = 0.069 and a Median Absolute Deviation MAD = 0.02 over the
whole redshift range (i.e. zspec <= 3.6), defined by the 4-survey cross-matched
spectroscopic sample. The fraction of catastrophic outliers, i.e. of objects
with photo-z deviating more than 2sigma from the spectroscopic value is < 3%,
leading to a sigma = 0.035 after their removal, over the same redshift range.
The method is made available to the community through the DAMEWARE web
application.Comment: 38 pages, Submitted to ApJ in February 2013; Accepted by ApJ in May
201
Steps toward a classifier for the Virtual Observatory. I. Classifying the SDSS photometric archive
Modern photometric multiband digital surveys produce large amounts of data
that, in order to be effectively exploited, need automatic tools capable to
extract from photometric data an objective classification. We present here a
new method for classifying objects in large multi-parametric photometric data
bases, consisting of a combination of a clustering algorithm and a cluster
agglomeration tool. The generalization capabilities and the potentialities of
this approach are tested against the complexity of the Sloan Digital Sky Survey
archive, for which an example of application is reported.Comment: To appear in the Proceedings of the "1st Workshop of Astronomy and
Astrophysics for Students" - Naples, 19-20 April 200
Unidentifed gamma-ray sources: hunting gamma-ray blazars
One of the main scientific objectives of the ongoing Fermi mission is
unveiling the nature of the unidentified gamma-ray sources (UGSs). Despite the
large improvements of Fermi in the localization of gamma-ray sources with
respect to the past gamma-ray missions, about one third of the Fermi-detected
objects are still not associated to low energy counterparts. Recently, using
the Wide-Field Infrared Survey Explorer (WISE) survey, we discovered that
blazars, the rarest class of Active Galactic Nuclei and the largest population
of gamma-ray sources, can be recognized and separated from other extragalactic
sources on the basis of their infrared (IR) colors. Based on this result, we
designed an association method for the gamma-ray sources to reognize if there
is a blazar candidate within the positional uncertainty region of a generic
gamma-ray source. With this new IR diagnostic tool, we searched for gamma-ray
blazar candidates associated to the UGS sample of the second Fermi gamma-ray
catalog (2FGL). We found that our method associates at least one gamma-ray
blazar candidate as a counterpart each of 156 out of 313 UGSs analyzed. These
new low-energy candidates have the same IR properties as the blazars associated
to gamma-ray sources in the 2FGL catalog.Comment: 24 pages, 4 figures, Accepted for publication on the Astrophysical
Journa
The WISE gamma-ray strip parametrization: the nature of the gamma-ray Active Galactic Nuclei of Uncertain type
Despite the large number of discoveries made recently by Fermi, the origins
of the so called unidentified gamma-ray sources remain unknown. The large
number of these sources suggests that among them there could be a population
that significantly contributes to the isotropic gamma-ray background and is
therefore crucial to understand their nature. The first step toward a complete
comprehension of the unidentified gamma-ray source population is to identify
those that can be associated with blazars, the most numerous class of
extragalactic sources in the gamma-ray sky. Recently, we discovered that
blazars can be recognized and separated from other extragalactic sources using
the infrared (IR) WISE satellite colors. The blazar population delineates a
remarkable and distinctive region of the IR color-color space, the WISE blazar
strip. In particular, the subregion delineated by the gamma-ray emitting
blazars is even narrower and we named it as the WISE Gamma-ray Strip (WGS). In
this paper we parametrize the WGS on the basis of a single parameter s that we
then use to determine if gamma-ray Active Galactic Nuclei of the uncertain type
(AGUs) detected by Fermi are consistent with the WGS and so can be considered
blazar candidates. We find that 54 AGUs out of a set 60 analyzed have IR colors
consistent with the WGS; only 6 AGUs are outliers. This result implies that a
very high percentage (i.e., in this sample about 90%) of the AGUs detected by
Fermi are indeed blazar candidates.Comment: 22 pages, 13 figures, Astrophysical Journal in pres
New blazars from the cross-match of recent multi-frequency catalogs
Blazars are radio-loud active galactic nuclei well known for their non
thermal emission spanning a wide range of frequencies. The Roma-BZCAT is, to
date, the most comprehensive list of these sources. We performed the
cross-match of several catalogs obtained from recent surveys at different
frequencies to search for new blazars. We cross-matched the 1 Swift-XRT
Point Source catalog with the spectroscopic sample of the 9 Data Release
of the Sloan Digital Sky Survey. Then, we performed further cross-matches with
the catalogs corresponding to the Faint Images of the Radio Sky at Twenty cm
survey and to the AllWISE Data release, focusing on sources with infrared
colors similar to those of confirmed -ray blazars included in the
Second Fermi-LAT catalog. As a result, we obtained a preliminary list of
objects with all the elements needed for a proper blazar classification
according to the prescriptions of the Roma-BZCAT. We carefully investigated
additional properties such as their morphology and the slope of their spectral
energy distribution in the radio domain, the features shown in their optical
spectrum, and the luminosity in the soft X rays to exclude generic active
galactic nuclei and focus on authentic blazar-like sources. At the end of our
screening we obtained a list of 15 objects with firmly established blazar
properties.Comment: 8 pages, 3 figures, 1 table. Accepted for publication in Astrophysics
and Space Science on 2015 April 25. Corrected typo in Section
Low-mass X-ray binaries and globular clusters streamers and ARCS in NGC 4278
We report significant inhomogeneities in the projected two-dimensional spatial distributions of low-mass X-ray binaries (LMXBs) and globular clusters (GCs) of the intermediate mass elliptical galaxy NGC 4278. In the inner region of NGC 4278, a significant arc-like excess of LMXBs extending south of the center at ∼50″ in the western side of the galaxy can be associated with a similar overdensity of the spatial distribution of red GCs from Brassington et al. Using a recent catalog of GCs produced by Usher et al. and covering the whole field of the NGC 4278 galaxy, we have discovered two other significant density structures outside the D 25 isophote to the W and E of the center of NGC 4278, associated with an overdensity and an underdensity, respectively. We discuss the nature of these structures in the context of the similar spatial inhomogeneities discovered in the LMXBs and GCs populations of NGC 4649 and NGC 4261, respectively. These features suggest streamers from disrupted and accreted dwarf companions.Peer reviewe
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