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 $\sim M^*$, 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 ($z<0.01$) 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

Cluster mass estimation through Fair Galaxies

We analyse a catalogue of simulated clusters within the theoretical framework of the Spherical Collapse Model (SCM), and demonstrate that the relation between the infall velocity of member galaxies and the cluster matter overdensity can be used to estimate the mass profile of clusters, even though we do not know the full dynamics of all the member galaxies. In fact, we are able to identify a limited subset of member galaxies, the 'fair galaxies', which are suitable for this purpose. The fair galaxies are identified within a particular region of the galaxy distribution in the redshift (line-of-sight velocity versus sky-plane distance from the cluster centre). This 'fair region' is unambiguously defined through statistical and geometrical assumptions based on the SCM. These results are used to develop a new technique for estimating the mass profiles of observed clusters and subsequently their masses. We tested our technique on a sample of simulated clusters; the mass profiles estimates are proved to be efficient from 1 up to 7 virialization radii, within a typical uncertainty factor of 1.5, for more than 90 per cent of the clusters considered. Moreover, as an example, we used our technique to estimate the mass profiles and the masses of some observed clusters of the Cluster Infall Regions in the Sloan Digital Sky Survey catalogue. The technique is shown to be reliable also when it is applied to sparse populated clusters. These characteristics make our technique suitable to be used in clusters of large observational catalogues.Comment: 11 pages, 11 figures, 5 tables - Slightly revised to match the version published on MNRAS; abstract update

Photometric Elements of the Eclipsing Binary FZ-Canis

2nonenoneGIURICIN G.; MARDIROSSIAN F.Giuricin, G.; Mardirossian, Fabi

Redshift Asymmetry and Color-VelocityCorrelation in Nearby Galaxy Groups: Evidence of Dust?

An excess of higher redshift galaxies, relative to the brightest member, in loose groups, was discovered by Arp in 1970, and was later confirmed by several authors. This effect is found to be present also in nearby small groups, identified by Tully in the Nearby Galaxy Catalogue. The asymmetry found is not likely to be ascribed to contamination by interlopers. A significant anticorrelation has also been found between the color excess E(B-V)(0)T and the velocity difference, with an associated E(B-V)(0)T variation on the order of 0.1-0.2. Such anticorrelation is not induced by obvious selection effects, such as the color-magnitude relation. A possible explanation is that groups are still collapsing and contain diffuse dust in the intragroup medium, with a mean optical depth, in the blue bland, on the order of 0.1-0.2. The mean dust density inside groups would be about 4 x 10 exp -31 g/cu cm; if the presence of dust is not only a local effect, the deduced cosmological density Omega(dust) would be in the range 10 exp -4 to 10 exp -5

THE OBSERVATIONAL MASS FUNCTION OF NEARBY GALAXY CLUSTERS

We present a new determination of the mass function of galaxy clusters, based on optical virial mass estimates for a large sample of 152 nearby (z ≤ 0.15) Abell–ACO clusters, as provided by Girardi et al. (1998). This sample includes both data from the literature and the new ENACS data. The resulting mass function is reliably estimated for masses larger than Mlim ≃ 4 × 10 14 h −1 M ⊙ , while it is affected by sample incompleteness at smaller masses. We find N(&gt; Mlim) = (6.3 ± 1.2)10 −6 (h −1 Mpc) −3 for cluster masses estimated within a 1.5 h −1 Mpc radius. Our mass function is intermediate between the two previous estimates by Bahcall &amp; Cen (1993) and by Biviano et al. (1993). Based on the Press–Schechter approach, we use this mass function to constrain the amplitude of the fluctuation power spectrum at the cluster scale. After suitably convolving the PS predictions with observational errors on cluster masses and COBE–normalizing the fluctuation power spectrum, we find σ8 = (0.60 ±0.04)Ω −0.46+0.09Ω0 0 for flat low–density models and σ8 = (0.60 ±0.04)Ω −0.48+0.17Ω0 0 for open models (at the 90 % c.l.)

Optical Luminosities and Mass-to-Light Ratios of Nearby Galaxy Clusters

5We analyze a sample of 105 clusters having virial mass homogeneously estimated and for which galaxy magnitudes are available with a well-defined high degree of completeness. In particular, we consider a subsample of 89 clusters with Bj-band galaxy magnitudes taken from the COSMOS/UKST Southern Sky Object Catalog. After suitable magnitude corrections and uniform conversions to Bj band, we compute cluster luminosities LBj within several clustercentric distances, 0.5, 1.0, 1.5 h-1 Mpc and within the virialization radius Rvir. In particular, we use the luminosity function and background counts estimated by Lumsden et al. on the Edinburgh/Durham Southern Galaxy Catalog, which is the well-calibrated part of the COSMOS catalog. We analyze the effect of several uncertainties connected to photometric data, fore/background removal, and extrapolation below the completeness limit of the photometry, in order to assess the robustness of our cluster luminosity estimates. We draw our results on the relations between luminosity and dynamical quantities from the COSMOS sample by considering mass and luminosities determined within the virialization radius. We find a very good correlation between cluster luminosity, LBj, and galaxy velocity dispersion, σv, with LBj~σ2.1-2.3v. Our estimate of the typical value for the mass-to-light ratio is M/LBj~250 h Msolar/Lsolar. We do not find any correlation of M/LBj with cluster morphologies, i.e., Rood-Sastry and Bautz-Morgan types, and only a weak significant correlation with cluster richness. We find that mass has a slight, but significant, tendency to increase faster than the luminosity does, M~L1.2-1.3Bj. We verify the robustness of this relation against a number of possible systematics. We verify that this increasing trend of M/L with cluster mass cannot be entirely due to a higher spiral fraction in poorer clusters, thus suggesting that a similar result would also be found by using R-band galaxy magnitudes.We analyze a sample of 105 clusters having virial mass homogeneously estimated and for which galaxy magnitudes are available with a well-defined high degree of completeness. In particular, we consider a subsample of 89 clusters with Bj-band galaxy magnitudes taken from the COSMOS/UKST Southern Sky Object Catalog. After suitable magnitude corrections and uniform conversions to Bj band, we compute cluster luminosities LBj within several clustercentric distances, 0.5, 1.0, 1.5 h-1 Mpc and within the virialization radius Rvir. In particular, we use the luminosity function and background counts estimated by Lumsden et al. on the Edinburgh/Durham Southern Galaxy Catalog, which is the well-calibrated part of the COSMOS catalog. We analyze the effect of several uncertainties connected to photometric data, fore/background removal, and extrapolation below the completeness limit of the photometry, in order to assess the robustness of our cluster luminosity estimates. We draw our results on the relations between luminosity and dynamical quantities from the COSMOS sample by considering mass and luminosities determined within the virialization radius. We find a very good correlation between cluster luminosity, LBj, and galaxy velocity dispersion, σv, with LBj~σ2.1-2.3v. Our estimate of the typical value for the mass-to-light ratio is M/LBj~250 h Msolar/Lsolar. We do not find any correlation of M/LBj with cluster morphologies, i.e., Rood-Sastry and Bautz-Morgan types, and only a weak significant correlation with cluster richness. We find that mass has a slight, but significant, tendency to increase faster than the luminosity does, M~L1.2-1.3Bj. We verify the robustness of this relation against a number of possible systematics. We verify that this increasing trend of M/L with cluster mass cannot be entirely due to a higher spiral fraction in poorer clusters, thus suggesting that a similar result would also be found by using R-band galaxy magnitudes.nonemixedGIRARDI M.; BORGANI S.; GIURICIN G.; MARDIROSSIAN F.; MEZZETTI M.Girardi, Marisa; Borgani, Stefano; Giuricin, Giuliano; Mardirossian, Fabio; Mezzetti, Marin

Environmental effects on local active galactic nuclei

The Astrophysical Journal, in pressConsiglio Nazionale delle Ricerche (CNR). Biblioteca Centrale / CNR - Consiglio Nazionale delle RichercheSIGLEITItal