An old galaxy group: Chandra X-ray observations of the nearby fossil group NGC 6482

We present the first detailed X-ray observations, using Chandra, of NGC 6482 - the nearest known `fossil group'. The group is dominated by an optically luminous giant elliptical galaxy and all other known group members are at least two magnitudes fainter. The global X-ray properties (luminosity, temperature, extent) of NGC 6482 fall within the range of other groups, but the detailed properties show interesting differences. We derive the gas temperature and total mass profiles for the central 30 kpc using ACIS spatially resolved spectroscopy. The temperature profile shows a continuous decrease outward, dropping to 0.63 of its central value at 0.1 r_200. The derived total mass profile is strongly centrally peaked, suggesting an early formation epoch. These results support a picture in which fossil groups are old, giving time for the most massive galaxies to have merged to produce a central giant elliptical galaxy. Although the cooling time within 0.1 r_200 is less than a Hubble time, no decrease in central temperature is detected. The entropy of the system lies toward the low side of the distribution seen in poor groups, and it drops all the way into the centre of the system, reaching very low values. No isentropic core, such as those predicted in simple preheating models, is present. Given the lack of any central temperature drop in the system, it seems unlikely that radiative cooling can be invoked to explain this low central entropy. We find that the centrally peaked temperature profile is consistent with a steady-state cooling flow solution with an accretion rate of 2 solar mass per year, given the large PdV work arising from the cuspy mass profile. However, solutions involving distributed or non-steady heating cannot be ruled out.Comment: 11 pages, 12 postscript figures. Accepted for publication in MNRA

The central elliptical galaxy in fossil groups and formation of BCGs

We study the dominant central giant elliptical galaxies in ``Fossil groups'' using deep optical (R-band) and near infrared (Ks-band) photometry. These galaxies are as luminous as the brightest cluster galaxies (BCGs), raising immediate interest in their link to the formation of BCGs and galaxy clusters. However, despite apparent similarities, the dominant fossil galaxies show non-boxy isophotes, in contrast to the most luminous BCGs. This study suggests that the structure of the brightest group galaxies produced in fossil groups are systematically different to the majority of BCGs. If the fossils do indeed form from the merger of major galaxies including late-types within a group, then their disky nature is consistent with the results of recent numerical simulations of semi-analytical models which suggest that gas rich mergers result in disky isophote ellipticals. We show that fossils form a homogeneous population in which the velocity dispersion of the fossil group is tightly correlated with the luminosity of the dominant elliptical galaxy. This supports the scenario in which the giant elliptical galaxies in fossils can grow to the size and luminosity of BCGs in a group environment. However, the boxy structure of luminous BCGs indicate that they are either not formed as fossils, or have undergone later gas-free mergers within the cluster environment.Comment: 5 pages, 4 figures, Accepted for publication in MNRAS letter

Sweet Memories of Dixie / words by Laurence C. Jones

Cover: drawing of an African American couple in a rowboat, the male is playing banjo and singing to the female; photo inset of Laurence C. Jones; [Laurence Jones was the founder and principal of The Piney Woods Country Life School in Mississippi]; Publisher: xhttps://egrove.olemiss.edu/sharris_d/1057/thumbnail.jp