Nearby Optical Galaxy Research Articles

Galaxies in group and field environments: A comparison of optical–NIR luminosities and colors

We compare properties of galaxies in loose groups with those in field environment by analyzing the Nearby Optical Galaxy (NOG) catalog of galaxy systems. We consider as group galaxies objects belonging to systems with at least five members identified by means of the of friends method, and, as field galaxies, all galaxies with no companions. We analyze both a magnitude-limited sample of 959 and 2035 galaxies (groups vs. field galaxies, respectively, B < 14 mag, and 2000 < cz < 6000 km s - 1 ) and a volume-limited sample (M B < -19.01 + 5 log 1 0 h h mag, 2000 < cz < 4000 km s - 1 369 group and 548 field galaxies). For all these galaxies, blue corrected magnitudes and morphological types are available. The cross-correlation of NOG with the 2MASS second release allow us to assign K magnitudes and obtain B - K colors for about half of the galaxies in our samples. We analyze luminosity and color segregation-effects in relation with the morphological segregation. For both B and K bands, we find that group galaxies are, on average, more luminous than field galaxies and this effect is not entirely a consequence of the morphological segregation. After taking into account the morphological segregation, the luminosity difference between group and field galaxies is about 10%. When considering only very early-type galaxies (T < -2) the difference is larger than 30%. We also find that group galaxies are redder than field galaxies, A(B - K) ∼ 0.4 mag. However, after taking into account the morphological segregation, we find a smaller B - K difference, poorly significant (only at the c.l. of ∼80%). We discuss our results considering that the analyzed groups define a very low density environment (projected mean density ∼5-6 h 2 Mpc - 2 galaxies).

Morphology and luminosity segregation of galaxies in nearby loose groups

We study morphology and luminosity segregation of galaxies in loose groups. We analyze the two catalogs of groups identified in the Nearby Optical Galaxy (NOG) sample, by means of hierarchical and percolation “friends-of-friends” methods (HG and PG catalogs, respectively). In the first part of our analysis we consider 387 and 436 groups of HG and PG and compare morphology- (luminosity-) weighted to unweighted group properties: velocity dispersion, mean pairwise distance, and mean groupcentric distance of member galaxies. The second part of our analysis is based on two ensemble systems, one for each catalog, built by suitably combining together galaxies of all groups (1584 and 1882 galaxies for HG and PG groups). We find that earlier-type (brighter) galaxies are more clustered and lie closer to the group centers, both in position and in velocity, than later-type (fainter) galaxies. Spatial segregations are stronger than kinematical segregations. These effects are generally detected at the $\mathrel{\hbox{\rlap{\hbox{\lower4pt\hbox{~}}}\hbox{$>$}}}$3-sigma level. Luminosity segregation is shown to be independent of morphology segregation. Our main conclusions are strengthened by the detection of segregation in both hierarchical and percolation catalogs. Our results agree with a continuum of segregation properties of galaxies in systems, from low-mass groups to massive clusters.

Morphology and Luminosity Segregations in Galaxy Groups

We study morphology and luminosity segregation of galaxies in groups. We analyze the two catalogs of (∼2×400) groups which have been identified in the Nearby Optical Galaxy sample, by means of hierarchical and percolation `friends-of-friends' methods. We find that earlier-type (brighter) galaxies are more clustered and lie closer to the group centers, both in position and in velocity, than later-type (fainter) galaxies. Spatial segregations are stronger than kinematical segregations. These effects are generally detected at the ≳3-sigma level, with the exception of morphological segregation in velocity, which is the weakest effect. Our main results are confirmed by the analysis of statistically more reliable groups (with at least five members), and are strengthened by the detection of segregation in both hierarchical and percolation catalogs. Luminosity segregation is shown to be independent of morphology segregation. Our conclusions agree with a continuum of segregation properties of galaxies in systems, from low-mass groups to massive clusters.

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.

The Optical Luminosity Function of Virialized Systems

We determine the optical luminosity function of virialized systems over the full range of density enhancements, from single galaxies to clusters of galaxies. The analysis is based on galaxy systems identified from the Nearby Optical Galaxy (NOG) sample, which is the largest all-sky catalog of objectively identified bound objects presently available. We find that the B-band luminosity function of systems is insensitive to the choice of group-finding algorithms and is well described, over the absolute magnitude range -24.5 ≤ M - 5 log h75 ≤ -18.5, by a Schechter function with αs = -1.4 ± 0.03, M - 5 log h75 = -23.1 ± 0.06, and = 4.8 × 10-4 h Mpc-3, or by a double power law: pl(Ls) ∝ L for Ls Lpl, with Lpl = 8.5 × 1010 h L☉, corresponding to Ms - 5 log h75 = -21.85. The characteristic luminosity of virialized systems, Lpl, is ~3 times that (L) of the NOG galaxies. Our results show that half of the luminosity of the universe is generated in systems with Ls 30L. We find a significant environmental dependence in the luminosity function of systems, in the sense that overdense regions, as measured on scales of 5 h-1 Mpc, preferentially host brighter, and presumably more massive, virialized systems.

The Redshift‐Space Two‐Point Correlation Functions of Galaxies and Groups in the Nearby Optical Galaxy Sample

We use the two-point correlation function in redshift space, ξ(s), to study the clustering of the galaxies and groups of the Nearby Optical Galaxy (NOG) sample, which is a nearly all-sky (|b| > 20°), complete, magnitude-limited sample of ~7000 bright and nearby optical galaxies (cz ≤ 6000 km s-1). The correlation function of galaxies is well described by a power law, ξ(s) = (s/s0)-γ, with slope γ ~ 1.5 and s0 ~ 6.4 h-1 Mpc (on scales 2.7-12 h-1 Mpc), in substantial agreement with previous results of several redshift surveys of optical galaxies. Splitting NOG into different morphological subsamples, we confirm the existence of morphological segregation between early- and late-type galaxies (out to 20 h-1 Mpc), and, in particular, we find a gradual decreasing of the strength of clustering from the S0 galaxies to the late-type spirals on intermediate scales (around 5 h-1 Mpc). The relative bias factor between early- and late-type galaxies appears to be substantially constant with scale. Furthermore, luminous galaxies turn out to be more clustered than dim galaxies. The luminosity segregation, which is significant for both early- and late-type objects, starts to become appreciable only for galaxies brighter than MB ~ -19.5 + 5 log h (~0.6L*) and is independent of scale. The NOG groups identified with the hierarchical and percolation algorithms show similar clustering properties, with a degree of clustering that is intermediate between galaxies and clusters. The group correlation functions are characterized by s0-values ranging from ~8 h-1 Mpc (for groups with at least three members) to ~10 h-1 Mpc (for groups with at least five members). The degree of group clustering depends on the physical properties of groups. Specifically, groups with greater velocity dispersions, sizes, and masses tend to be more clustered than those with lower values of these quantities.

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 ($cz\leq$6000 km/s) and magnitude-limited (B$\leq$14) sample of $\sim$7000 optical galaxies. The sample covers 2/3 (8.27 sr) of the sky ($|b|>20^{\circ}$) and appears to have a good completeness in redshift (98%). We select the sample on the basis of homogenized corrected total blue magnitudes in order to minimize systematic effects in galaxy sampling. We identify the groups in this sample by means of both the hierarchical and the percolation {\it friends of friends} methods. The resulting catalogs of loose groups appear to be similar and are among the largest catalogs of groups presently available. Most of the NOG galaxies ($\sim$60%) are found to be members of galaxy pairs ($\sim$580 pairs for a total of $\sim$15% of objects) or groups with at least three members ($\sim$500 groups for a total of $\sim$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.

Ellipticity of Nearby Elliptical Galaxies Identified with Extended Extragalactic Radio Sources

The dependence of the ellipticity of nearby optical galaxies associated with extragalactic radio sources on the type of radio galaxy is considered. It is shown that elliptical galaxies identified with radio galaxies of the FRI type and with radio sources having a radio image with a small elongation have a smaller ellipticity, on the average, than those associated with radio galaxies of the FRII type and with radio sources with greater elongation. As one possible explanation for this phenomenon, one can assume that the optical galaxies associated with radio galaxies of type FRI and with radio sources of small elongation are oriented in space so that they are observed edge-on relatively rarely.

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.