We study the luminosity gap, Δm12, between the first- and second-ranked galaxies in a sample of 59 massive (∼1015 M⊙) galaxy clusters, using data fro

We study the distribution and evolution of the stellar mass and the star formation rate (SFR) of the brightest group galaxies (BGGs) over 0.04 < z < 1.3 using a large sample of 407 X-ray galaxy groups selected from the COSMOS, AEGIS, and XMM–LSS fields. We compare our results with predictions from the semi-analytic models based on the Millennium simulation. In contrast to model predictions, we find that, as the Universe evolves, the stellar mass distribution evolves towards a normal distribution. This distribution tends to skew to low-mass BGGs at all redshifts implying the presence of a star-forming population of the BGGs with M_S ∼ 10^10.5 M_⊙ which results in the shape of the stellar mass distribution deviating from a normal distribution. In agreement with the models and previous studies, we find that the mean stellar mass of BGGs grows with time by a factor of ∼2 between z = 1.3 and z = 0.1, however, the significant growth occurs above z = 0.4. The BGGs are not entirely a dormant population of galaxies, as low-mass BGGs in low-mass haloes are more active in forming stars than the BGGs in more massive haloes, over the same redshift range. We find that the average SFR of the BGGs evolves steeply with redshift and fraction of the passive BGGs increases as a function of increasing stellar mass and halo mass. Finally, we show that the specific SFR of the BGGs within haloes with M₂₀₀ ≤ 10^13.4 M_⊙ decreases with increasing halo mass at z < 0.4.

We study the radio emission of the most massive galaxies in a sample of dynamically relaxed and unrelaxed galaxy groups from the Galaxy and Mass Assembly survey. The dynamical state of the group is defined by the stellar dominance of the brightest group galaxy (BGG), e.g., the luminosity gap between the two most luminous members, and the offset between the position of the BGG and the luminosity centroid of the group. We find that the radio luminosity of the largest galaxy in the group strongly depends on its environment, such that the BGGs in dynamically young (evolving) groups are an order of magnitude more luminous in the radio than those with a similar stellar mass but residing in dynamically old (relaxed) groups. This observation has been successfully reproduced by a newly developed semi-analytic model that allows us to explore the various causes of these findings. We find that the fraction of radio-loud BGGs in the observed dynamically young groups is ∼2 times that of the dynamically old groups. We discuss the implications of this observational constraint on the central galaxy properties in the context of galaxy mergers and the super massive black hole accretion rate.

We report on the X-ray and optical observations of galaxy groups selected from the 2dfGRS group catalogue to explore the possibility that galaxy groups hosting a giant elliptical galaxy and a large optical luminosity gap present between the two brightest group galaxies can be associated with an extended X-ray emission, similar to that observed in fossil galaxy groups. The X-ray observations of four galaxy groups were carried out with Chandra telescope with 10–20 ksec exposure time. Combining the X-ray and the optical observations we find evidence for the presence of a diffuse extended X-ray emission beyond the optical size of the brightest group galaxy. Taking both the X-ray and the optical criteria, one of the groups is identified as a fossil group and one is ruled out because of the contamination in the earlier optical selection. For the two remaining systems, the X-ray luminosity threshold is close to the convention known for fossil groups. In all cases the X-ray luminosity is below the expected value from the X-ray selected fossils for a given optical luminosity of the group. A rough estimation for the comoving number density of fossil groups is obtained to be 4 to 8 × 10⁻⁶ Mpc⁻³, in broad agreement with the estimations from observations of X-ray selected fossils and predictions of cosmological simulations.

The results of the site selection campaign conducted for the proposed 3.4 m optical telescope of the Iranian National Observatory are reported. During the first three years, among 33 nominated regions throughout the country, the potential regions were confined to four provinces, namely, Southern Khorasan located in the east, Kerman in the south-east and Qom and Esfahan in the central part, by examining long-term meteorological and geographical parameters. Over the following three years, after astro-climate considerations and short-term atmospheric seeing measurements using a differential image motion monitor (DIMM) technique were carried out, four candidate sites were selected among these regions. Simultaneous seeing measurements were carried out on the four candidate sites and, finally, Dinava located at the common border of the Esfahan and Qom provinces and Gargash in the Kashan region were selected as the most promising sites. Continuous seeing measurements over 23 months yielded a median value of about 1.0″ for both sites. The latitude and longitude of the Dinava site are 50,54 E and 34,09 N, respectively and it is situated at an altitude of about 3000 m and about 57 km due south from Qom. The latitude and longitude of the Gargash site are 51,19 E and 33,40 N, respectively and it is situated at an altitude of about 3600 m and about 37 km due south from Kashan. With two sites in hand, further comparative measurements of seeing and weather data continued at the two sites, which resulted in the selection of the Gargash site for the construction of the Iranian National Observatory.