30 research outputs found
Compact Radio Sources and Nuclear Activity in Seyfert Galaxies
Using recent high-resolution radio observations of a large sample of Seyfert
galaxies (Roy et al. 1994), we analyze the relations between the compact radio
core emission and several nuclear and host galaxy properties. Seyfert nuclei
hosted in early-type galaxies or in object with nearby companions show stronger
radio cores than the norm. Radio core emission show some correlation with total
radio emission and with tracers of nuclear activity (mid-IR, hard X-ray and
narrow-line emissions). This favours the view that Seyfert radio cores are
powered by AGN rather than by radio supernovae.Comment: 20 pages, latex file with macros included, 7 figures and 6 tables
(automatically included
Nearby Optical Galaxies: Selection of the Sample and Identification of Groups
In this paper we describe the Nearby Optical Galaxy (NOG) sample, which is a complete, distance-limited (6000 km/s) and magnitude-limited (B14) sample of 7000 optical galaxies. The sample covers 2/3 (8.27 sr) of the sky () and appears to have a good completeness in redshift (98%). We select the sample on the basis of homogenized corrected total blue magnitudes in order to minimize systematic effects in galaxy sampling. We identify the groups in this sample by means of both the hierarchical and the percolation {\it friends of friends} methods. The resulting catalogs of loose groups appear to be similar and are among the largest catalogs of groups presently available. Most of the NOG galaxies (60%) are found to be members of galaxy pairs (580 pairs for a total of 15% of objects) or groups with at least three members (500 groups for a total of 45% of objects). About 40% of galaxies are left ungrouped (field galaxies). We illustrate the main features of the NOG galaxy distribution. Compared to previous optical and IRAS galaxy samples, the NOG provides a denser sampling of the galaxy distribution in the nearby universe. Given its large sky coverage, the identification of groups, and its high-density sampling, the NOG is suited for the analysis of the galaxy density field of the nearby universe, especially on small scales
Environmental Effects on Local Active Galactic Nuclei
Using an extensive sample of nearby galaxies (the Nearby Galaxies Catalog, by
Tully), we investigate the environment of the galaxies hosting low-luminosity
AGNs (Seyferts and LINERs). We define the local galaxy density, adopting a new
correction for the incompleteness of the galaxy sample at large distances. We
consider both a complete sample of bright and nearby AGNs, identified from the
nuclear spectra obtained in available wide optical spectroscopic surveys, and a
complete sample of nearby Seyferts. Basically, we compare the local galaxy
density distributions of the AGNs with those of non-AGN samples, chosen in
order to match the magnitude and morphological type distributions of the AGN
samples. We find, only for the early-type spirals more luminous than , that both LINERs and Seyferts tend to reside in denser environments on
all the scales tested, from tenths of Mpc to a few Mpc; moreover Seyferts show
an enhanced small-scale density segregation with respect to LINERs. This gives
support to the idea that AGNs can be stimulated by interactions. On larger
scales, tens of Mpc, we find that the AGNs hosted in luminous early-type
spirals show a tendency to stay near the center of the Local Supercluster.
Finally we discuss the interpretations of our findings and their consequences
for some possible scenarios of AGN formation and evolution and for the problem
of how AGNs trace the large-scale structures.Comment: 16 pages+3 figures, uuencoded postscript file, preprint SISSA 76/94/A
, ApJ November 20, 199
THE LOCAL GALAXY DENSITY AND THE BULGE-TO-DISK RATIO OF DISK GALAXIES
Relying on samples of disk galaxies for which a detailed photometric
bulge/disk decomposition has been provided in the literature, we examine the
dependence of the bulge-to-disk luminosity ratio (B/D) on the blue absolute
luminosity and on the environmental density. In our statistical analysis of
various B/D data sets we pay particular attention to disentangling the role
played by the galaxy morphology--galaxy density relation. Besides, we focus our
attention on nearby () galaxies, for which we can provide a
three-dimensional characterization of the local galaxy density. We find that
the observed tendency of galaxies to have greater B/D with increasing galaxy
density simply reflects the average decline of B/D towards later morphological
types together with the morphology--density relation. This relation tends to
give rise also to a greater proportion of bright bulges in denser regions,
because the decrease of B/D towards later types is mostly due to a dimming of
the bulge rather than to a brightening of the disk. But when we remove the
effect induced by the morphology--density relation, we detect no clear evidence
of a dependence of B/D on galaxy density. Furthermore, B/D turns out to be
substantially unrelated to the blue absolute magnitude of the galaxy. We
briefly discuss to what extent our results (partially) disagree with previous
claims.Comment: LATEX file, 19 pages, figures available on reques
Observational Mass-to-Light Ratio of Galaxy Systems: from Poor Groups to Rich Clusters
We study the mass-to-light ratio of galaxy systems from poor groups to rich
clusters, and present for the first time a large database for useful
comparisons with theoretical predictions. We extend a previous work, where B_j
band luminosities and optical virial masses were analyzed for a sample of 89
clusters. Here we also consider a sample of 52 more clusters, 36 poor clusters,
7 rich groups, and two catalogs, of about 500 groups each, recently identified
in the Nearby Optical Galaxy sample by using two different algorithms. We
obtain the blue luminosity and virial mass for all systems considered. We
devote a large effort to establishing the homogeneity of the resulting values,
as well as to considering comparable physical regions, i.e. those included
within the virial radius. By analyzing a fiducial, combined sample of 294
systems we find that the mass increases faster than the luminosity: the linear
fit gives M\propto L_B^{1.34 \pm 0.03}, with a tendency for a steeper increase
in the low--mass range. In agreement with the previous work, our present
results are superior owing to the much higher statistical significance and the
wider dynamical range covered (about 10^{12}-10^{15} M_solar). We present a
comparison between our results and the theoretical predictions on the relation
between M/L_B and halo mass, obtained by combining cosmological numerical
simulations and semianalytic modeling of galaxy formation.Comment: 25 pages, 12 eps figures, accepted for publication in Ap
Nearby Optical Galaxies: Selection of the Sample and Identification of Groups
In this paper we describe the Nearby Optical Galaxy (NOG) sample, which is a
complete, distance-limited (6000 km/s) and magnitude-limited
(B14) sample of 7000 optical galaxies. The sample covers 2/3 (8.27
sr) of the sky () and appears to have a good completeness in
redshift (98%). We select the sample on the basis of homogenized corrected
total blue magnitudes in order to minimize systematic effects in galaxy
sampling.
We identify the groups in this sample by means of both the hierarchical and
the percolation {\it friends of friends} methods. The resulting catalogs of
loose groups appear to be similar and are among the largest catalogs of groups
presently available. Most of the NOG galaxies (60%) are found to be
members of galaxy pairs (580 pairs for a total of 15% of objects)
or groups with at least three members (500 groups for a total of
45% of objects). About 40% of galaxies are left ungrouped (field
galaxies). We illustrate the main features of the NOG galaxy distribution.
Compared to previous optical and IRAS galaxy samples, the NOG provides a denser
sampling of the galaxy distribution in the nearby universe. Given its large sky
coverage, the identification of groups, and its high-density sampling, the NOG
is suited for the analysis of the galaxy density field of the nearby universe,
especially on small scales.Comment: 47 pages including 6 figures. Accepted for publication in Ap
The Nearby Optical Galaxy Sample: The Local Galaxy Luminosity Function
In this paper we derive the galaxy luminosity function from the Nearby
Optical Galaxy (NOG) sample, which is a nearly complete, magnitude-limited
(B<14 mag), all-sky sample of nearby optical galaxies (~6400 galaxies with
cz<5500 km/s). For this local sample, we use galaxy distance estimates based on
different peculiar velocity models. Therefore, the derivation of the luminosity
function is carried out using the locations of field and grouped galaxies in
real distance space.
The local field galaxy luminosity function in the B system is well described
by a Schechter function. The exact values of the Schechter parameters slightly
depend on the adopted peculiar velocity field models.
The shape of the luminosity function of spiral galaxies does not differ
significantly from that of E-S0 galaxies. On the other hand, the late-type
spirals and irregulars have a very steeply rising luminosity function towards
the faint end, whereas the ellipticals appreciably decrease in number towards
low luminosities.
The presence of galaxy systems in the NOG sample does not affect
significantly the field galaxy luminosity function, since environmental effects
on the total luminosity function appear to be marginal.Comment: 35 pages including 7 figures and 4 tables. Accepted for publication
in Ap
The Near- and Mid-Infrared Continuum Emission of Seyfert Nuclei: Constraints on the Models of Obscuring Tori
For an extended sample of Seyfert galaxies we compile from the literature the
infrared fluxes in the four IRAS bands, the ground-based small-beam (~ 5-10")
fluxes in the standard Q, N, M, L (or L') bands, and the nuclear (non-stellar)
estimated fluxes in the JHK bands. We estimate nuclear fluxes in the L band by
applying a correction for stellar light. From the statistical study of the
infrared colors and luminosities, we derive the typical SEDs of Seyfert 1 and 2
nuclei and the typical differences in luminosities between the two types of
objects in the mid- and near-infrared spectral ranges.
The observational data appear to severely challenge many models of dusty
tori, which hardly account for the shapes of the SEDs and the degree of
anisotropy observed in Seyfert galaxies. In particular, at variance with some
earlier claims, very thick and compact tori are basically inconsistent with
these observational constraints. The most successful models, though having
problems in accounting for several details, can fit the major infrared
observational data of both Seyfert 1 and Seyfert 2 nuclei with tori which
extend up to several hundreds pc and have fairly low optical thickness.Comment: 18 pages, LaTeX2e, accepted by The Astrophysical Journa