977 research outputs found
Constraining fundamental constant evolution with HI and OH lines
We report deep Green Bank Telescope spectroscopy in the redshifted HI 21cm
and OH 18cm lines from the absorption system towards PMN
J0134-0931. A comparison between the "satellite" OH 18cm line redshifts, or
between the redshifts of the HI 21cm and "main" OH 18cm lines, is sensitive to
changes in different combinations of three fundamental constants, the fine
structure constant , the proton-electron mass ratio and the proton g-factor . We find that the satellite OH 18cm
lines are not perfectly conjugate, with both different line shapes and stronger
1612 MHz absorption than 1720 MHz emission. This implies that the satellite
lines of this absorber are not suitable to probe fundamental constant
evolution. A comparison between the redshifts of the HI 21cm and OH 18cm lines,
via a multi-Gaussian fit, yields the strong constraint , where and the
error budget includes contributions from both statistical and systematic
errors. We thus find no evidence for a change in the constants between and the present epoch. Incorporating the constraint from another absorber at a similar redshift and assuming
that fractional changes in are much smaller than those in , we
obtain over a
lookback time of 6.7 Gyrs.Comment: 5 pages, 4 figures; accepted for publication in Astrophysical Journal
Letter
On the detectability of HI 21-cm in MgII absorption system
We investigate the effect of two important, but oft neglected, factors which
can affect the detectability of HI 21-cm absorption in MgII absorption systems:
The effect of line-of-sight geometry of the coverage of the background radio
flux and any possible correlation between the 21-cm line strength and the rest
frame equivalent width of the MgII line. Regarding the former, while the
observed detection rate at small angular diameter distance ratios is a near
certainty, for an unbiased sample, where either a detection or a non-detection
are equally likely, at ratios > 0.8 the observed detection rate has an 8 sigma
significance, suggesting that the mix of ratios values at z < 1 is correlated
with the mix of detections and non-detections at low redshift, while the
exclusively high values of the ratio at z > 1 contribute to the low detection
rates at high redshift. In DLAs, the correlation between the 21-cm line
strength and the MgII equivalent width is dominated by the velocity spread of
the 21-cm line. This has recently been shown not to hold for MgII systems in
general. However, we do find the significance of the correlation to increase
when the MgII absorbers with MgI equivalent widths of >0.5 A are added to the
DLA sample. Large values of the angular diameter distance ratio may explain why
the absorbers which have similar equivalent widths to the detections remain
undetected. We do, however, also find the neutral hydrogen column densities of
the non-detections to be significantly lower. Applying the 21-cm line
strength/equivalent width correlation to yield column densities for the MgII
absorbers in which this is unmeasured, we find no evidence of a cosmological
evolution in the neutral hydrogen column density.Comment: 9 pages, accepted by MNRA
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
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 pc in pure super-luminal motion
models, and 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, 10 - 100 AU.Comment: 5 pages, 2 figures. Accepted for publication in MNRA
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- systems
We present Giant Metrewave Radio Telescope HI 21cm absorption observations,
of candidate and confirmed damped Lyman- systems (DLAS). The derived
spin temperatures (T_s) are in all cases 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
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