Gravitational clustering in a D-dimensional Universe

We consider the problem of gravitational clustering in a D-dimensional expanding Universe and derive scaling relations connecting the exact mean two-point correlation function with the linear mean correlation function, in the quasi-linear and non-linear regimes, using the standard paradigms of scale-invariant radial collapse and stable clustering. We show that the existence of scaling laws is a generic feature of gravitational clustering in an expanding background, in all dimensions except D=2 and comment on the special nature of the 2-dimensional case. The D-dimensional scaling laws derived here reduce, in the 3-dimensional case, to scaling relations obtained earlier from N-body simulations. Finally, we consider the case of clustering of 2-dimensional particles in a 2-D expanding background, governed by a force -GM/R, and show that the correlation function does not grow (to first order) until much after the recollapse of any shell.Comment: 4 pages, no figures. Accepted by Physical Review

Tiny HI Clouds in the Local ISM

Very sensitive HI absorption spectra (tau RMS about 10^-4 over 1 km/s) toward high latitude QSOs have revealed a population of tiny discrete features in the diffuse ISM with peak tau of 0.1 - 2% and core line-widths corresponding to temperatures as low as 20 K. Imaging detections confirm linear dimensions of a few 1000 AU. We suggest these structures may be formed by the stellar winds of intermediate mass stars. A more speculative origin might involve molecular "dark matter".Comment: 6 pages, 8 figures, to appear in "The IMF at 50", eds. E. Corbelli, F. Palla, and H. Zinnecker, ASSL (Kluwer

ORT observations of the damped Lyman alpha system towards PKS 0201+113

We report a deep radio search with the Ooty Radio Telescope (ORT) for the redshifted 21 cm absorption line from the damped Lyman alpha system seen at redshift 3.388 against the quasar PKS 0201+113. This is currently the most distant system for which a detection of 21 cm absorption has been claimed. The present observations have a sensitivity comparable to the earlier ones and detect no statistically significant absorption. We use the non-detection to place an upper limit of ~ 0.011 on the optical depth of the damped Lyman alpha absorber. This corresponds to a lower limit of ~ 5600 K to the spin temperature of the system. This is considerably higher than the previous upper limit of ~ 1380 K.Comment: 5 pages, 1 figure. Accepted by MNRA

Implications of 21cm observations for damped Ly-α\alpha systems

We present Giant Metrewave Radio Telescope HI 21cm absorption observations, of candidate and confirmed damped Lyman-$\alpha$ systems (DLAS). The derived spin temperatures (T_s) are in all cases $\sim 1000$ K or higher. We have also collated from the literature a list of DLAS for which 21cm observations exist, and discuss their implications for the nature of the absorbers. A cross-comparison of the 21cm profiles with low ionization metal profiles shows that the 21cm absorption coincides in velocity with the deepest metal line feature. This is consistent with models in which the deep metal line features arise from discrete clouds but not with models where the deepest features are the result of velocity crowding. We also find that the typical derived spin temperatures of DLAS are considerably higher than those in the Galaxy or nearby spirals. The only exceptions are DLAS which are known to be associated with the disks of spirals; these do, in fact, show low spin temperatures. In a multi-phase medium,the derived T_s is a weighted average of the temperatures of the individual phases. High derived T_s values are hence to be expected from small, low metallicity objects, since these objects should have a lower fraction of the cold phase in their ISM as compared to large galaxies. The high T_s in DLAS is hence consistent with their observed low metallicities as well as with recent observations that DLAS are also associated with dwarf/LSB galaxies. Finally, we suggest that the following trend may be identified: at low redshift, damped absorption arises from a range of systems, including spiral galaxy disks, while, at high redshift, absorption occurs predominantly in smaller systems. (Abridged)Comment: 7 pages, 2 figure

Variable 21cm absorption at z=0.3127

We report multi-epoch GMRT HI observations of the z = 0.3127 damped absorber towards the quasar PKS 1127-145, which reveal variability in both the absorption profile and the flux of the background source, over a time-scale of a few days. The observed variations cannot be explained by simple inter-stellar scintillation (ISS) models where there are only one or two scintillating components and all of the ISS occurs in the Galaxy. More complicated models where there are either more scintillating components or some of the ISS occurs in the ISM of the z=0.3127 absorber may be acceptable. However, the variability can probably best be explained in models incorporating motion (on sub-VLBI scales) of a component of the background continuum source, with or without some ISS. All models for producing the variable 21cm absorption profile require small scale variations in the 21cm optical depth of the absorber. The length scale for the opacity variations is $\sim 0.1$ pc in pure super-luminal motion models, and $\sim$ 10 pc in pure ISS models. Models involving sub-luminal motion, combined with scintillation of the moving component, require opacity variations on far smaller scales, $\sim$ 10 - 100 AU.Comment: 5 pages, 2 figures. Accepted for publication in MNRA