The Local Galaxy Density and the Arm Class of Spiral Galaxies

We have examined the effect of the environmental density on the arm classification of an extensive sample of spiral galaxies included in the Nearby Galaxy Catalog (Tully, 1988a). We have also explored the dependence of the arm class of a galaxy on other factors, such as its blue absolute magnitude and its disk-to-total mass ratio, inferred in the literature either from the gradient of a good galaxy rotation curve or from a photometric mass decomposition method. We have found that the arm class is strongly related to the absolute magnitude in the mid-type spirals (in the sense that grand design galaxies are, on average, more luminous than flocculent objects), whilst this relation is considerably weaker in the early and late types. In general the influence of the local density on the arm structure appears to be much weaker than that of the absolute magnitude. The local density acts essentially in strengthening the arm class--absolute magnitude relation for the mid types, whereas no environmental density effects are observed in the early and late types. Using the most recent estimates of the disk-to-total mass ratio, we do not confirm this ratio to be a significant factor which affects the arm class; nevertheless, owing to poor statistics and large uncertanties, the issue remains open. Neither a local density effect nor an unambiguous bar effect on the disk-to-total mass ratio is detectable; the latter finding may challenge some theoretical viewpoints on the formation of bar structures.Comment: 15 pages, Latex, SISSA 102/93/A openbib.sty and 4 POSTSCRIPT figures appende

The Mass Function of Nearby Galaxy Clusters

We present the distribution of virial masses for nearby galaxy clusters, as obtained from a data-set of 75 clusters, each having at least 20 galaxy members with measured redshifts within 1 Abell radius. After having accounted for problems of incompleteness of the data-set, we fitted a power-law to the cluster mass distribution.Comment: 10 pages (2 figures not included, available upon request), LATEX, Ref.SISSA 54/93/

Structures in Galaxy Clusters

The analysis of the presence of substructures in 16 well-sampled clusters of galaxies suggests a stimulating hypothesis: Clusters could be classified as unimodal or bimodal, on the basis of to the sub-clump distribution in the {\em 3-D} space of positions and velocities. The dynamic study of these clusters shows that their fundamental characteristics, in particular the virial masses, are not severely biased by the presence of subclustering if the system considered is bound.Comment: (16 pages in LATEX, 4 tables in LATEX are at the end of the file, the figures not included are available upon request), REF SISSA 158/93/

The Local Galaxy Density and the Bars of Spiral Galaxies

Using a variety of parameters of local galaxy density, we have examined the effects of the environmental density on the presence of bar structures in spiral galaxies of various morphological types. For an extensive sample of nearby galaxies, listed in the ``Nearby Galaxies Catalogue'' (Tully, 1988a), we have found that the spirals characterized by a high local density tend to be barred if they are early-type and early-type if they are barred (at the significance level of $\sim$3 sigma), confirming some earlier suggestion of low statistical significance. This fact, which is observed substantially in low-luminosity spirals, indicates that galaxy interactions can stimulate the formation of bars, primarily in early-type, low-luminosity spirals. This is in partial, qualitative agreement with the most recent relevant N-body simulations. On the other hand, no significant density segregation is observed between pure S-shaped (S(s)) spirals and spirals with inner rings (S(r)), which are often associated with bars. {\it Subject headings:} galaxies: general --- galaxies: structure --- galaxies: interactions --- galaxies: clusteringComment: 23 pages, LaTeX, SISSA Ref. 178/92/

Optical Mass Estimates of Galaxy Clusters

We evaluate in a homogeneous way the optical masses of 170 nearby clusters (z< 0.15). The sample includes both data from the literature and the new ENACS data (Katgert et al. 1996, 1998). On the assumption that mass follows the galaxy distribution, we compute the masses of each cluster by applying the virial theorem to the member galaxies. We constrain the masses of very substructured clusters (about 10% of our clusters) between two limiting values. After appropriate rescaling to the X-ray radii, we compare our optical mass estimates to those derived from X-ray analyses, which we compiled from the literature (for 66 clusters). We find a good overall agreement. This agreement is expected in the framework of two common assumptions: that mass follows the galaxy distribution, and that clusters are not far from a situation of dynamical equilibrium with both gas and galaxies reflecting the same underlying mass distribution. We stress that our study strongly supports the reliability of present cluster mass estimates derived from X-ray analyses and/or (appropriate) optical analyses.Comment: 13 pages, 7 eps figures, tables are not included, USE LaTeX2e !!, accepted by Ap

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

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 B_j band galaxy magnitudes taken from the COSMOS/UKST Southern Sky Object Catalogue. We compute cluster luminosities L_{B_j} within several clustercentric distances and within the virialization radius R_{vir}. In particular, we use the luminosity function and background counts estimated by Lumsden et al. (1997) on the Edinburgh/Durham Southern Galaxy Catalogue. 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, L_{B_j}, and galaxy velocity dispersion, sigma_v, with L_{B_j} proportional to sigma_v^{2.1--2.3}. Our estimate of typical value for the mass-to-light ratio is M/L_{B_j} about 250 (in solar units). We do not find any correlation of M/L_{B_j} 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 proportional to L_{B_j}^{1.2--1.3}. We verify the robustness of this relation against a number of possible systematics

Study od a Slice at +9 to +15 degrees of Declination: I. The Neutral Hydrogen Content of Galaxies in Loose Groups

We examine the H1 content of spiral galaxies in groups by using a catalog of loose groups of galaxies identified in a magnitude limited sample m < 15.7 spanning the range 8 h to 18 h in right ascension and +9 to +15 in declination. The redshift completeness of the galaxy sample is ~95%. No significant effect of H1 depletion is found, although there may be a hint that the earliest type spirals are slightly deficient.Comment: 10 pages, Latex, 3 tables, 5 figures, to appear in the Astronomical Journa

The Nuclear Ten Micron Emission of Spiral Galaxies

We examine the 10\m\ emission of the central regions of 281 spiral galaxies, after having compiled all ground-based, small-aperture ($\sim$5") broad-band photometric observations at \l$\sim$10\m\ (N magnitudes) published in the literature. We evaluate the compactness of the $\sim$10\m\ emission of galaxy nuclei by comparing these small-beam measures with the large-beam IRAS 12\m\ fluxes. In the analysis of different subsets of objects, we apply survival analysis techniques in order to exploit the information contained in ``censored'' data (i.e., upper limits on the fluxes). Seyferts are found to contain the most powerful nuclear sources of mid-infrared emission, which in $\sim$1/3 of cases provide the bulk of the emission of the entire galaxy; thus, mid-infrared emission in the outer disc regions is not uncommon in Seyferts. The 10\m\ emission of Seyferts appears to be unrelated to their X-ray emission. HII region-like nuclei are stronger mid-infrared sources than normal nuclei and LINER nuclei (whose level of emission is not distinguishable from that of normal nuclei). Interacting objects have, on average, greater 10\m\ luminosities than non-interacting ones and exhibit more compact emission. Early-type spirals have stronger and more compact 10\m\ emission than late-type ones. Barred spirals are brighter at $\sim$10\m\ than unbarred systems, essentially because they more frequently contain HII region-like nuclei. The results of our detailed comparison between the behaviour of various categories of objects stress that the 10\m\ emission of spiral nuclei is closely linked to the (predominantly non-thermal synchrotron) radioComment: 20 pages, Latex, SISSA preprint, 8 figures and 4 tables available upon reques