We present 5. ′′ 2 × 2. ′′ 6 resolution interferometry of CO J = 1 → 0 emission from the starburst galaxy NGC 253. The high spatial resolution of these new data, in combination with recent high resolution maps of 13 CO, HCN and near-infrared emission, allow us for the first time to link unambiguously the gas properties in the central starburst of NGC 253 with its bar dynamics. We confirm that the star formation results from bar-driven gas flows as seen in "twin peaks" galaxies. Two distinct kinematic features are evident from the CO map and position-velocity diagram: a group of clouds rotating as a solid body about the kinematic center of the galaxy, and a more extended gas component associated with the nearinfrared bar. We model the line intensities of CO, HCN and 13 CO to infer the physical conditions of the gas in the nucleus of NGC 253. The results indicate increased volume densities around the radio nucleus in a twin-peaks morphology. Compared with the CO kinematics, the gas densities appear highest near the radius of a likely inner Linblad resonance, and slightly lead the bar minor axis. This result is similar to observations of the face-on, twin-peaks galaxy NGC 6951, and is consistent with models of starburst generation due to gas inflow along a bar.

It is shown that the low-mass groups obey the L x ∼ σ 4 v law deduced for galaxy clusters. The impression of the more shallow slope of the L x − σ v correlation for groups is created not by enhanced X-ray emission, but by underestimation of the radial velocity dispersion of some groups.

Sierra Negra, one of the highest peaks in central Mexico, is the site of the Large Millimeter Telescope. We describe the first results of a comprehensive analysis of the weather data measured in situ from 2000 October to 2008 February to be used as a reference for future activity in the site. We compare the data from two different stations at the summit considering the accuracy of both instruments. We analysed the diurnal, seasonal and annual cycles for all the parameters. The thermal stability is remarkably good, crucial for a good performance of the telescopes. From the solar radiation data, we developed a new method to estimate the fraction of time when the sky is clear of clouds. We show that our measurements are consistent with a warm standard atmosphere model. The conditions at the site are benign and stable given its altitude, showing that Sierra Negra is an extremely good site for millimeter and high‐energy observations.

We show that compact groups (CGs) are real physical entities, that almost all HCGs are dynamically associated with generally elongated loose groups of galaxies, that HCGs are compact cores of loose groups, and that they are longer living formations.

ABSTRACT. Optical seeing measurements carried out at Sierra Negra, the site of the Large Millimeter Telescope, are reported. The site, one of the highest peaks in central Mexico, offers good coverage of the northern and southern hemispheres, and we have undertaken several campaigns to investigate the astronomical potential of the site in the optical. Here we report on our campaign to establish the seeing quality of the site. We present data from the first three campaigns of optical seeing monitoring covering 2000 February to 2002 May, carried out with a Differential Image Motion Monitor. The results clearly indicate subarcsecond seeing, better statistics during the dry season, and no dependence on the time of night. We find no dependence of our results on the integration time used.