High-mass Groups Research Articles

Structural properties of central galaxies in groups and clusters

Using a statistically representative sample of 911 central galaxies (CENs) from the Sloan Digital Sky Survey (SDSS) Data Release 4 group catalogue, we study how the structure (shape and size) of the first rank (by stellar mass) group and cluster members depends on (1) galaxy stellar mass (M(star)), (2) the global environment defined by the dark matter halo mass (M(halo)) of the host group and (3) the local environment defined by their special halocentric position. We quantify the structure of SDSS galaxies with a GALFIT-based pipeline that fits two-dimensional Sersic models to the r-band image data. Through tests with simulated and real galaxy images, we demonstrate that our pipeline can recover Sersic parameters without significant bias. We find that the fitting results are most sensitive to the background sky level determination, and we strongly recommend using the SDSS global value. We also find that uncertainties in the background sky level translate into a strong covariance between the total magnitude, the half-light radius (r(50)) and the Sersic index (n), especially for bright/massive galaxies. Applying our pipeline to the CEN sample, we find that n of CENs depends strongly on M(star), but only weakly or not at all on M(halo). The n -M(star) relation holds for CENs over the full range of halo masses that we consider. Less massive CENs tend to be disc like and high-mass systems are typically spheroids, with a considerable scatter in n at all galaxy masses. Similarly, CEN sizes depend on galaxy stellar mass and luminosity, with early-and late-type galaxies exhibiting different slopes for the size-luminosity (r(50)-L) and the size-stellar mass (r50 -M(star)) scaling relations. Moreover, to test the impact of local environment on CENs, we compare the structure of CENs with that satellite galaxies (SATs) of comparable M(star). We find that low-mass (<10(10.75) h(-2) M(circle dot)) SATs have somewhat larger median Sersic indices than CENs of a similar M(star). Furthermore, low-mass, late-type SATs are moderately smaller in size than late-type CENs of the same stellar mass. However, we find no size differences between early-type CENs and SATs and no structural differences between CENs and SATs when they are matched in both optical colour and stellar mass. The similarity in the structure of massive SATs and CENs demonstrates that the halocentric distinction has no significant impact on the structure of spheroids. We conclude that M(star) is the most fundamental property determining the basic structural shape and size of a galaxy. In contrast, the lack of a significant n-M(halo) relation rules out a clear distinct group mass for producing spheroids. The existence of spheroid CENs in low- and high-mass groups suggests that the morphological transformation processes that produce spheroids must occur at the centres of groups spanning a wide range of masses.

Groups of galaxies: relationship between environment and galaxy properties

We analyse how the properties of galaxies in groups identified in the Sloan Digital Sky Survey depend on the environment. In particular, we study the relationship between galaxy properties and group mass and group-centric distance. Among the galaxy properties we have considered here, we find that the $g-r$ colour is the most predictive parameter for group mass, while the most predictive pair of properties are $g-r$ colour and $r-$band absolute magnitude. Regarding the position inside the systems, the $g-r$ colour is the best tracer of group-centric distance and the most predictive pair of properties are $g-r$ colour and spectral type taken together. These results remain unchanged when a subsample of high mass groups is analysed. The same happens if the brightest group galaxies are excluded.

A study of high velocity molecular outflows with an up-to-date sample

A statistical study of the properties of molecular outflows is performed based on an up-to-date sample. 391 outflows were identified in published articles or preprints before February 28, 2003. The parameters of position, morphology, mass, energy, outflow dynamics and central source luminosity are presented for each outflow source. Outflow lobe polarity is known for all the sources, and 84% are found to be bipolar. The sources are divided into low mass and high mass groups according to either the available bolometric luminosity of the central source or the outflow mass. The pace of discovery of outflows over the past seven years has increased much more rapidly than in previous periods. Surveys for outflows are still continuing. The number of high-mass outflows detected (139) has considerably increased, showing that they are commonly associated with massive as well as low mass stars. Energetic mass ejection may be a common aspect of the formation of high mass as well as low mass stars. Outflow masses are correlated strongly with bolometric luminosity of the center sources, which was obtained for the first time. There are also correlations between the central source luminosity and the parameters of mechanical luminosity and the thrust or force necessary to drive the outflow. The results show that flow mass, momentum and energy depend on the nature of the central source. Despite their similarity, there are differences between the high mass and low mass outflows. Low mass outflows are more collimated than high mass outflows. On average, the mass of high mass sources can be more than two orders of magnitude larger than those of low mass outflows. The relation between flow mass and dynamical time appears to differ for the two types of outflows. Low mass sources make up 90% of outflows associated with HH objects while high mass outflows make up 61% of the sources associated with H 2 O masers. Sources with characteristics of collapse or infall comprise 12% of the entire outflow sample. The spatial distribution of the outflow sources in the Galaxy is presented and the local occurrence rate is compared with the stellar birth rate.

Galaxy groups in the 2dF Galaxy Redshift Survey: galaxy spectral type segregation in groups

The behaviour of the relative fraction of galaxies with different spectral types in groups is analysed as a function of projected local galaxy density and the group-centric distance. The group sample was taken from the 2dF Group Galaxy Calatogue constructed by Merchan & Zandivarez. Our group sample was constrained to have a homogeneous virial mass distribution with redshift. Galaxies belonging to this group sample were selected in order to minimize possible biases, such as preferential selection of high-luminosity objects. We find a clear distinction between high virial mass groups (MV≳ 1013.5 M⊙) and the less massive ones. While the massive groups show a significant dependence of the relative fraction of low star formation galaxies on local galaxy density and group-centric radius, groups with lower masses show no significant trends. We also cross-correlate our group subsample with the previously identified clusters, finding that this sample shows a very similar behaviour to that observed in the high virial mass group subsample.

Mass-Metallicity Relation for the Local Group Dwarf Spheroidal Galaxies: A New Picture for the Chemical Enrichment of Galaxies in the Lowest Mass Range

The virial mass (Mvir)-metallicity relation among the Local Group dwarf spheroidal (dSph) galaxies is examined. Hirashita, Takeuchi, & Tamura showed that the dSph galaxies can be divided into two distinct classes with respect to the relation between their virial masses and luminosities: low-mass (Mvir ≲ 108 M☉) and high-mass (Mvir ≳ 108 M☉) groups. We see that both the mass-metallicity and mass-luminosity relations of the high-mass dSph galaxies are understood as a low-mass extension of giant elliptical galaxies. On the contrary, we find that the classical galactic wind model is problematic when applied to the low-mass dSph galaxies, whose low binding energy is comparable to that released by several supernova explosions. A strongly regulated star formation in their formation phase is required to reproduce their observed metallicity. Such regulation is naturally expected in a gas cloud with the primordial elemental abundance according to Nishi & Tashiro. A significant scatter in the mass-metallicity relation for the low-mass dSph galaxies is also successfully explained along with the scenario of Hirashita and coworkers. We not only propose a new picture for a chemical enrichment of the dSph galaxies but also suggest that the mass-metallicity and mass-luminosity relations be understood in a consistent context.

White dwarf masses and nova efficiency in magnetic cataclysmic variables

We show that current ideas for forming the period gap in cataclysmic variables (CVs) give a simple explanation for the spike in the observed distribution of magnetic CVs, provided that they have a bimodal distribution of white dwarf masses M1 at the start of their secular evolution. The majority should have M1 very close to a value of ∼ 0.6−0.7 |$M_\odot$| and form at periods of ∼ 4–6 hr, with a much smaller group having larger M1 (up to the Chandrasekhar limit), and forming at periods up to 10 hr. Nova explosions remove almost exactly the accreted mass for the low-mass group, so that M1 remains at the initial value of ∼ 0.6−0.7 |$M_\odot$| while the high-mass group may lose small amounts of mass. These conclusions are supported by nova theory and offer the possibility of an observational test of gap forming hypotheses. We tentatively identify the low- and high-mass groups as the products of case C and case BB evolution. Non-magnetic systems may have the same initial mass distribution, but nova explosions are more efficient and reduce the white dwarf mass, smearing out the distribution.