199 research outputs found
When are extremely metal-deficient galaxies extremely metal-deficient?
Extremely metal-deficient (XMD) galaxies, by definition, have oxygen
abundances \le 1/10 solar, and form a very small fraction of the local
gas-rich, star-forming dwarf galaxy population. We examine their positions in
the luminousity-metallicity (L-Z) and mass-metallicity (M-Z) planes, with
respect to the L-Z and M-Z relations of other gas-rich, star-forming dwarf
galaxies, viz., blue compact galaxies (BCGs) and dwarf irregular (dI) galaxies.
We find that while the metallicities of some low-luminousity XMD galaxies are
consistent with those expected from the L-Z relation, other XMD galaxies are
deviant. We determine the 95 per cent confidence interval around the L-Z
relation for BCGs, and find that its lower boundary is given by 12 + log(O/H) =
-0.177 M_{B} + 4.87. We suggest that a galaxy should be regarded as XMD, in a
statistically significant manner, only if it lies below this boundary in the
L-Z plane. Of our sample of XMD galaxies, we find that more than half are XMD
by this criterion. We also determine the gas mass fractions and chemical yields
of galaxies in all three samples. We find that the effective chemical yield
increases with increasing baryonic mass, consistent with what is expected if
outflows of metal-enriched gas are important in determining the effective
yield. XMD galaxies have lower effective yield than BCG/dI galaxies of similar
baryonic mass. Motivated by the fact that interactions are common in XMD
galaxies, we suggest that improved (tidally-driven) mixing of the interstellar
media (ISM) in XMD galaxies leads to a lowering of both, the measured
metallicity and the calculated effective yield. We suggest that XMD galaxies
are deviant from the L-Z relation because of a combination of being gas-rich
(i.e., having processed less gas into stars) and having more uniform mixing of
metals in their ISM.Comment: Accepted for publication in MNRA
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
Non-linear Redundancy Calibration
For radio interferometric arrays with a sufficient number of redundant
spacings the multiplicity of measurements of the same sky visibility can be
used to determine both the antenna gains as well as the true visibilities. Many
of the earlier approaches to this problem focused on linearized versions of the
relation between the measured and the true visibilities. Here we propose to use
a standard non-linear minimization algorithm to solve for both the antenna
gains as well as the true visibilities. We show through simulations done in the
context of the ongoing upgrade to the Ooty Radio Telescope that the non-linear
minimization algorithm is fast compared to the earlier approaches. Further,
unlike the most straightforward linearized approach, which works with the
logarithms of the visibilities and the gains, the non-linear minimization
algorithm leads to unbiased solutions. Finally we present error estimates for
the estimated gains and visibilities. Monte-Carlo simulations establish that
the estimator is indeed statistically efficient, achieving the Cramer-Rao
bound.Comment: 9 pages, 5 figures. Accepted for publication in MNRAS. The definitive
version will be available at http://mnras.oxfordjournals.or
Widespread acetaldehyde near the Galactic Centre
We present Giant Meterwave Radio Telescope images of the 1065 MHz emission
from the 1_11 -> 1_10 rotational transition of acetaldehyde (CH3CHO) in the
molecular cloud complex Sgr B2. Our observations are unique in that they have a
high spatial resolution (~4"), while still being sensitive to large-scale
emission. Most complex organic molecules in this cloud (e.g. acetone, methyl
formate, acetic acid) are concentrated in a very small core, ~0.1pc across. In
contrast, acetaldehyde is found to be spread over a region at least 100 times
larger in extent. The line emission is confined to regions with radio continuum
emission and correlates well (in both position and velocity) with formaldehyde
absorption towards this continuum; this is consistent with earlier single dish
results suggesting that it is likely to be weakly mased. Our observations also
suggest that grain mantle destruction by shocks plays an important role in the
observed gas phase abundance of CH3CHO in Sgr B2.Comment: accepted for publication in A&A (letters
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