H. V. Capelato

The Nature of the Activity in Hickson Compact Groups of Galaxies

We present the results of the spectral classification of the 82 brightest galaxies in a sample of 17 compact groups. We verify that the active galactic nuclei (AGNs) are preferentially located in the most early-type and luminous galaxies of the groups, as is usually observed in the field. But these AGNs also appear to be systematically concentrated toward the central parts of the groups. Our observations suggest a correlation between activity types, morphologies, and densities of galaxies in the compact groups. This is consistent with a scenario in which galaxies of compact groups evolve by interacting with their environment and are currently in a quiet phase of their activity.

Redshift Survey of Galaxies around a Selected Sample of Compact Groups

We report the results of a spectroscopic survey of faint galaxies in the regions surrounding Hickson compact groups. Our sample is composed of 17 groups within 9000 km s-1. The spectra were taken at the prime focus of the Tololo 4 m telescope, using the ARGUS fiber-fed spectrograph. From these observations, redshifts were determined for the faint galaxies previously identified by de Carvalho, Ribeiro, & Zepf in the surroundings of the groups. Statistical methods were applied to the resultant catalog in order to determine the kinematical structure of each group. This analysis confirms the idea that the Hickson sample of compact groups contains a wide variety of projection and dynamical configurations. Our results demonstrate the necessity of new spectroscopic surveys around compact groups in order to assess their complete velocity distribution.

On the kinematical behavior of galaxies in clusters

The sample of 15 galaxy clusters published by Dressler and Shectman (1988) has been analyzed to search for a tendency for spiral and irregular galaxies to have greater velocity dispersion than the ellipticals and lenticulars. It is shown that this trend is highly significant, with a probability of less than 0.05 percent that it may result from statistical fluctuations in the sample of clusters. An analogous but much less significant effect appears also to be present between lenticular and elliptical galaxies. These results are interpreted as evidence that late-type galaxies are still in the process of infalling toward the virialized cluster cores. 34 refs.

HCG 16: A High Concentration of Active Galaxies in the Nearby Universe

In the course of an extensive campaign to measure radial velocities of galaxies in a selected sample of compact groups photometrically studied by de Carvalho et al., we report the discovery of a system very rich in starburst galaxies and active galactic nuclei. This is the system HCG 16 of Hicksons catalog of compact groups. The seven brightest galaxies form a kinematical group with biweighted estimate mean velocity of VBI = 3959 ± 66 km s-1, dispersion σBI = 86 ± 55 km s-1, a median radius R = 0.197 Mpc, a mean density of D = 217 galaxies Mpc-3, and a total absolute magnitude of MB = -22.1. From their spectral characteristics, we have identified one Seyfert 2 galaxy, two LINERs, and three starburst galaxies. Thus, HCG 16 appears to be a dense concentration of active galaxies. In our sample of 17 Hickson groups, HCG 16 is unique in this regard, which suggests that it is an uncommon structure in the nearby universe.

X-RAY GALAXY CLUSTERS IN NoSOCS: SUBSTRUCTURE AND THE CORRELATION OF OPTICAL AND X-RAY PROPERTIES

We present a comparison of optical and X-ray properties of galaxy clusters in the northern sky, using literature data from BAX and optically selected clusters in DPOSS. We determine the recovery rate of X-ray-detected clusters in the optical as a function of richness, redshift, and X-ray luminosity, showing that the missed clusters are typically low-contrast systems when observed optically (either poor or at high redshifts). We employ four different statistical tests to test for the presence of substructure using optical two-dimensional data. We find that approximately 35% of the clusters show strong signs of substructure in the optical. However, the results are test-dependent, with variations also due to the magnitude range and radius utilized. We have also performed a comparison of X-ray luminosity and temperature with optical galaxy counts (richness). We find that the slope and scatter of the relations between richness and the X-ray properties are heavily dependent on the density contrast of the clusters. The selection of substructure-free systems does not improve the correlation between X-ray luminosity and richness, but this comparison also shows much larger scatter than one obtained using the X-ray temperature. In the latter case, the sample is significantly reduced because temperature measurements are available only for the most massive (and thus high-contrast) systems. However, the comparison between temperature and richness is very sensitive to the exclusion of clusters showing signs of substructure. The correlation of X-ray luminosity and richness is based on the largest sample to date (~750 clusters), while tests involving temperature use a similar number of objects as previous works (≾100). The results presented here are in good agreement with existing literature.

‘Fundamental Plane’-like relations from collisionless stellar dynamics: a comparison of mergers and collapses

We present a new set of dissipationless N-body simulations aiming to better understand the pure dynamical aspects of the “Fundamental Plane” (FP) of elliptical galaxies. We have extended our previous hierarchical merger scheme by considering the Hernquist profile for the initial galaxy model. Two-component Hernquist galaxy models were also used to study the effect of massive dark halos on the end-products characteristics. We have also performed new collapse simulations including initial spin. We found that the one-component Hernquist mergers give results similar to those found for the one-component King models, namely both were able to build-up small scatter FP-like correlations with slopes consistent with what is found for the near-infrared FP of nearby galaxies. The two-component models also reproduce a FP-like correlation, but with a significantly steeper slope. This is in agreement with what has been found for elliptical galaxies at higher redshift (0.1 < z < 0.6). We discuss some structural properties of the simulated galaxies and their ability to build-up FP-like correlations. We confirm that collapses generally do not follow a FP-like correlation regardless of the initial spin. We suggest that the evolution of gradients in the gravitational field of the merging galaxies may be the main ingredient dictating the final non-homology property of the end products.

The Evolution of Galaxies in Compact Groups

We present an analysis of the spectra of 62 galaxies in 15 compact groups. The galaxies are classified into four activity classes: galaxies without emission, starburst galaxies, luminous AGNs (Seyfert and LINERs), and low-luminosity AGNs (LLAGNs). The star formation in the Hickson compact group (HCG) starbursts is more intense than in normal spirals, but comparable to that observed in starburst-nucleus galaxies (SBNGs) in the field. In general, the HCG starbursts have mean solar gas metallicity and do not follow the metallicity-luminosity relation traced by the early-type SBNGs in the field, suggesting that most of them are late-type SBNGs. This morphology preference, coupled with the observation that the HCG starbursts are predominantly located in the halos of the groups, is consistent with the idea that compact groups are embedded in sparser structures. The stellar metallicities of the nonstarburst galaxies are comparable to those observed in normal galaxies with similar morphologies, but are relatively high for their luminosities. In these galaxies, the metal absorption line equivalent widths are slightly narrower than normal, while the Balmer absorption lines are relatively strong. All these observations suggest the presence of a population of intermediate-age stars. These galaxies could be poststarburst, but at a very advanced stage of evolution, the last bursts having happened more than 2 Gyr in the past. Our observations support a scenario in which the cores of the groups are slowly collapsing evolved systems embedded in more extended structures. In the cores of the groups, the interactions were more frequent and the galaxies evolved at a more rapid rate than in their halos.

Decision Tree Classifiers for Star/Galaxy Separation

We study the star/galaxy classification efficiency of 13 different decision tree algorithms applied to photometric objects in the Sloan Digital Sky Survey Data Release Seven (SDSS-DR7). Each algorithm is defined by a set of parameters which, when varied, produce different final classification trees. We extensively explore the parameter space of each algorithm, using the set of 884,126 SDSS objects with spectroscopic data as the training set. The efficiency of star-galaxy separation is measured using the completeness function. We find that the Functional Tree algorithm (FT) yields the best results as measured by the mean completeness in two magnitude intervals: 14 ? r ? 21 (85.2%) and r ? 19 (82.1%). We compare the performance of the tree generated with the optimal FT configuration to the classifications provided by the SDSS parametric classifier, 2DPHOT, and Ball et al. We find that our FT classifier is comparable to or better in completeness over the full magnitude range 15 ? r ? 21, with much lower contamination than all but the Ball et al. classifier. At the faintest magnitudes (r > 19), our classifier is the only one that maintains high completeness (>80%) while simultaneously achieving low contamination (~2.5%). We also examine the SDSS parametric classifier (psfMag ? modelMag) to see if the dividing line between stars and galaxies can be adjusted to improve the classifier. We find that currently stars in close pairs are often misclassified as galaxies, and suggest a new cut to improve the classifier. Finally, we apply our FT classifier to separate stars from galaxies in the full set of 69,545,326 SDSS photometric objects in the magnitude range 14 ? r ? 21.

Energy, entropy and mass scaling relations for elliptical galaxies. Towards a physical understanding of their photometric properties

In the present paper, we show that elliptical galaxies (Es) obey a scaling relation between potential energy and mass. Since they are relaxed systems in a post violent-relaxation stage, they are quasi-equilibrium gravitational systems and therefore they also have a quasi-constant specific entropy. Assuming that light traces mass, these two laws imply that in the space defined by the three S\\ersic law parameters (intensity Sigma_0, scale a and shape nu), elliptical galaxies are distributed on two intersecting 2-manifolds: the Entropic Surface and the Energy-Mass Surface. Using a sample of 132 galaxies belonging to three nearby clusters, we have verified that ellipticals indeed follow these laws. This also implies that they are distributed along the intersection line (the Energy-Entropy line), thus they constitute a one-parameter family. These two physical laws (separately or combined), allow to find the theoretical origin of several observed photometrical relations, such as the correlation between absolute magnitude and effective surface brightness, and the fact that ellipticals are located on a surface in the [log R_eff, -2.5 log Sigma_0, log nu] space. The fact that elliptical galaxies are a one-parameter family has important implications for cosmology and galaxy formation and evolution models. Moreover, the Energy-Entropy line could be used as a distance indicator.

SPIDER – IX. Classifying galaxy groups according to their velocity distribution

We introduce a new method to study the velocity distribution of galaxy systems, the Hellinger Distance (HD) - designed for detecting departures from a Gaussian velocity distribution. We define a relaxed galactic system as the one with unimodal velocity distribution and a normality deviation below a critical value (HD = 20) systems are significantly larger than in low multiplicity ones (N ) and the gaussianity of the velocity distribution of the groups. Bright galaxies (M_r <=-20.7) residing in the inner and outer regions of groups, do not show significant differences in the listed quantities regardless if the group has a Gaussian (G) or a Non-Gaussian (NG) velocity distribution. However, the situation is significantly different when we examine the faint galaxies (-20.7<M_r<=-17.9). In G groups, there is a remarkable difference between the galaxy properties of the inner and outer galaxy populations, testifying how the environment is affecting the galaxies. Instead, in NG groups there is no segregation between the properties of galaxies in the inner and outer regions, showing that the properties of these galaxies still reflect the physical processes prevailing in the environment where they were found earlier.