Search for quantum criticality in a ferromagnetic system UNi1-xCoxSi2

Polycrystalline samples of the isostructural alloys UNi1-xCoxSi2 (0 <= x <= 1) were studied by means of x-ray powder diffraction, magnetization, electrical resistivity and specific heat measurements, at temperatures down to 2 K and in magnetic fields up to 5 T. The experimental data revealed an evolution from strongly anisotropic ferromagnetism with pronounced Kondo effect, observed for the alloys with x < 0.98 and being gradually suppressed with rising Co-content, to spin-glass-like states with dominant spin fluctuations, seen for the sample with x = 0.98. Extrapolation of the value of TC(x) yields a critical concentration xc = 1, at which the magnetic ordering entirely disappears. This finding is in line with preliminary data collected for stoichiometric UCoSi2.Comment: to appear in Phys. Rev.

The Origin of X-shaped Radio Galaxies: Clues from the Z-symmetric Secondary Lobes

Existing radio images of a few X-shaped radio galaxies reveal Z-symmetric morphologies in their weaker secondary lobes which cannot be naturally explained by either the galactic merger or radio-lobe backflow scenarios, the two dominant models for these X-shaped radio sources. We show that the merger picture can explain these morphologies provided one takes into account that, prior to the coalescence of their supermassive black holes, the smaller galaxy releases significant amounts of gas into the ISM of the dominant active galaxy. This rotating gas, whose angular momentum axis will typically not be aligned with the original jets, is likely to provide sufficient ram pressure at a distance ~10 kpc from the nucleus to bend the extant jets emerging from the central engine, thus producing a Z-symmetry in the pair of radio lobes. Once the two black holes have coalesced some 10^7 yr later, a rapid reorientation of the jets along a direction close to that of the orbital angular momentum of the swallowed galaxy relative to the primary galaxy would create the younger primary lobes of the X-shaped radio galaxy. This picture naturally explains why such sources typically have powers close to the FR I/II break. We suggest that purely Z-symmetric radio sources are often en route to coalescence and the concomitant emission of substantial gravitational radiation, while X-shaped ones have already merged and radiated.Comment: 12 pages, 1 compressed figure; accepted for publication in ApJ Letter

Weakly commensurable arithmetic groups, lengths of closed geodesics and isospectral locally symmetric spaces

We introduce the notion of weak commensurabilty of arithmetic subgroups and relate it to the length equivalence and isospectrality of locally symmetric spaces. We prove many strong consequences of weak commensurabilty and derive from these many interesting results about isolength and isospectral locally symmetric spaces.Comment: 62 page

High Latitude HI in NGC 2613: Buoyant Disk-Halo Outflow

We combine new VLA D array HI data of NGC 2613 with previous high resolution data to show new disk-halo features in this galaxy. The global HI distribution is modeled in detail using a technique which can disentangle the effects of inclination from scale height and can also solve for the average volume density distribution in and perpendicular to the disk. The model shows that the galaxy's inclination is on the low end of the range given by Chaves & Irwin (2001) and that the HI disk is thin (z_e = 188 pc), showing no evidence for halo. Numerous discrete disk-halo features are observed, however, achieving z heights up to 28 kpc from mid-plane. One prominent feature in particular, of mass, 8X10^7 Msun and height, 22 kpc, is seen on the advancing side of the galaxy at a projected galactocentric radius of 15.5 kpc. If this feature achieves such high latitudes because of events in the disk alone, then input energies of order ~ 10^{56} ergs are required. We have instead investigated the feasibility of such a large feature being produced via buoyancy (with drag) within a hot, pre-existing X-ray corona. Reasonable plume densities, temperatures, stall height (~ 11 kpc), outflow velocities and ages can indeed be achieved in this way. The advantage of this scenario is that the input energy need only be sufficient to produce blow-out, a condition which requires a reduction of three orders of magnitude in energy. If this is correct, there should be an observable X-ray halo around NGC 2613.Comment: 32 pages 7 gif figures, accepted by Ap

Discovery Of A Molecular Outflow in the Haro 6-10 Star-Forming Region

We present high sensitivity 12CO and 13CO (1-0) molecular line maps covering the full extent of the parsec scale Haro~6-10 Herbig-Haro (HH) flow. We report the discovery of a molecular CO outflow along the axis of parsec-scale HH flow. Previous molecular studies missed the identification of the outflow probably due to their smaller mapping area and the confusing spectral features present towards the object. Our detailed molecular line study of the full 1.6 pc extent of the optical flow shows evidence for both blueshifted and redshifted gas set in motion by Haro~6-10 activity. The molecular outflow is centered at Haro~6-10, with redshifted gas being clumpy and directed towards the northeast, while blueshifted gas is in the southwest direction. The molecular gas terminates well within the cloud, short of the most distant HH objects of the optical flow. Contamination from an unrelated cloud along the same line of sight prevents a thorough study of the blueshifted outflow lobe and the mass distribution at the lowest velocities in both lobes. The cloud core in which Haro~6-10 is embedded is filamentary and flattened in the east-west direction. The total cloud mass is calculated from 13CO(1-0) to be ~200Msun. The lower limit of the mass associated with the outflow is ~0.25Msun.Comment: ApJ Accepted; 9 pages, 8 figures. For high resolution ps file use: http://www.astro.umass.edu/~irena/haro.p