B. Kelm

The spectral-type/luminosity and the spectral type/satellite-density relations in the 2dFGRS

We examine the relative fractions of passive (type 1), quiet-SF (type 2) and active-SF (type 3+4) galaxies as a function of luminosity and number of neighbours in several volume-limited samples selected from the 2dFGRS. Neighbours are counted within 1

The diverse X-ray properties of four truly isolated elliptical galaxies: NGC 2954, NGC 6172, NGC 7052, and NGC 7785

h_{75}^{-1}

A new sample of bright galaxy pairs in UZC

 Mpc projected distance and ± 1000 km s -1 depth. We apply a maximum magnitude difference criterion and require neighbours to be fainter than the galaxy itself. We show that, whatever the environment, passive galaxies dominate in bright samples and active-SF galaxies in faint samples, whereas quiet-SF galaxies never dominate. We further show that in bright samples ( M B  – 5

Compact groups in the UZC galaxy sample - II. Connections between morphology, luminosity and large-scale density

\log

Seyfert galaxies in UZC-Compact Groups

h 75 ≤

Small-Scale Systems of Galaxies. II. Properties of the NGC 4756 Group of Galaxies

\,-\,19

Dominant galaxies in 2dF groups

) the fraction of passive galaxies grows steadily with fainter neighbour density, whereas in faint samples a threshold-like dependence is observed. This suggests that the spectral-type/density ( ≈ morphology/density) relation extends to the intermediate dense environment, but only in the surroundings of luminous galaxies and that it reflects an enhancement of the number of satellites rather than stronger clustering among galaxies themselves. Our analysis indicates that, in general, luminosity is a good tracer of galaxy halo mass and that it dominates over environment (satellite density) in setting the spectral type mix of a population. However, minority populations exist, such as luminous SF galaxies and faint passive galaxies, whose luminosity is an inaccurate tracer of halo mass.

Single Star‐Forming Galaxies and Star‐Forming Galaxies in SF + SF and Mixed Pairs

Aims. We investigate the X-ray properties of four isolated elliptical galaxies, selected from the Updated Zwicky Catalog according to strict isolation criteria. Isolated galaxies are not influenced by the group/cluster environment, and their X-ray emission can be studied independently of the often overwhelming contribution of the hot intergalactic medium. They are therefore suited to studying the X-ray characteristics relative to their intrinsic properties. Methods. We analyzed our own XMM- Newton and archival Chandra data in detail for three objects, and derived, when possible, the spatial and spectral characteristics of each source. An upper limit for the fourth one was obtained from archival ASCA data. We compared their characteristics with those of other 23 isolated objects for which X-ray and optical data are available in the literature. We explored possible theoretical explanations to interpret our results. Results. In spite of our attempt to select very homogeneous objects, both in terms of optical properties and environmental characteristics, we find a wide range in X-ray luminosities and L X / L B ratios for the four objects: two of them show a hot gaseous halo, whereas no gas is detected in the other two, to a factor >10 in luminosity. In fact, we find a large spread in the L X / L B for all galaxies considered, suggesting that the presence of hot gas is not easily related to the optical luminosity or to the mass, even in isolated systems. Younger objects tend to be less luminous in X-rays than older systems. However, it appears that older objects could span a wide range in luminosities.

ACTIVE STAR-FORMING GALAXIES IN PAIRS IN THE 2DF

Aims. We present a new sample of bright galaxy pairs extracted applying an objective selection code to the UZC catalog. The sample is volume-limited to Mzw = −18.9 + 5l ogh and contains 89 galaxy pairs. Methods. We analyze the kinematical, morphological, and photometrical properties of galaxies belonging to this sample. Results. We show that velocity separation, |∆vr|, between pair members is significantly lower in spiral type (S+S) pairs than in earlytype (E+E) and mixed (E+S) pairs. This indicates that truly isolated galaxy pairs are more likely to be found among S+ Sp airs. We show that ellipticals are rare and underluminous in B and that late spirals (T ≥ 4) are overluminous. Conclusions. We confirm that the formation of bright ellipticals is a phenomenon linked to group/cluster environment. while galaxy– galaxy interaction may enhance the blue luminosity of disk galaxies through SF phenomena. This last statement is supported by the presence of strong FIR emission from early spirals in this sample and by the high frequency of the AGN/SB phenomenon, revealed mainly in pairs of low relative radial velocity separation showing signs of interaction.

Environmental Effects on Galaxy Properties

The nature of Compact Groups (CGs) is investigated by comparing the luminosities and morphologies of CG galax- ies, CG Neighbours and Isolated galaxies. CGs turn out to include more early type galaxies than Isolated galaxies and fewer low-luminosity galaxies. The 33 CGs with a dominant E/S0 and the 30 CGs with a dominant spiral have similar LF parameters. Spiral dominated CGs have fewer galaxies at high and low luminosity in comparison with their Neighbours, while E-S0 dom- inated CGs seem to lack only faint galaxies when compared to their Neighbours. Ellipticals which are the dominant galaxy of a CG are also brighter than all their Neighbours, while this holds true for only half of the dominant spirals and S0s. Relations linking the number of Neighbours of dominant E-S0s to the luminosity of E-S0s and to the difference between the first and the second ranked CG members do suggest a link between the formation of bright early-type galaxies and the presence of a group- like potential. No similar relations are found for dominant spirals. These tentative results are compatible with the assumption that CG dominant Ellipticals are anomalous galaxies whose formation might have been a secondary outcome during the process of groups formation.The nature of Compact Groups (CGs) is investigated by comparing the luminosities and morphologies of CG galaxies, CG Neighbours and Isolated galaxies. CGs turn out to include more early type galaxies than Isolated galaxies and fewer low-luminosity galaxies. The 33 CGs with a dominant E/S0 and the 30 CGs with a dominant spiral have similar LF parameters. Spiral dominated CGs have fewer galaxies at high and low luminosity in comparison with their Neighbours, while E-S0 dominated CGs seem to lack only faint galaxies when compared to their Neighbours. Ellipticals which are the dominant galaxy of a CG are also brighter than all their Neighbours, while this holds true for only half of the dominant spirals and S0s. Relations linking the number of Neighbours of dominant E-S0s to the luminosity of E-S0s and to the difference between the first and the second ranked CG members do suggest a link between the formation of bright early-type galaxies and the presence of a group-like potential. No similar relations are found for dominant spirals. These tentative results are compatible with the assumption that CG dominant Ellipticals are anomalous galaxies whose formation might have been a secondary outcome during the process of groups formation.