2,021 research outputs found
The Scale Height of NGC 1058 Measured from its HI Power Spectrum
We have measured the HI power spectrum of the nearly face-on spiral galaxy
NGC 1058 from radio-interferometric observations using a visibility based
estimator. The power spectrum is well fitted by two different power laws
, one with at small length-scales
and another with
at large length-scales . We interpret this change in the slope of the power spectrum as a
transition from 3D turbulence at small length-scales to 2D turbulence in the
plane of the galaxy's disk at large length-scales. We use the observed break in
the power spectrum to estimate the galaxy's scale-height, which we find to be pc.Comment: Accepted for publication in Mon. Not. of Royal Astron. Soc. Letter
GMRT observation towards detecting the Post-reionization 21-cm signal
We have analyzed 610 MHz GMRT observations towards detecting the redshifted
21-cm signal from z=1.32. The multi-frequency angular power spectrum C_l(Delta
nu) is used to characterize the statistical properties of the background
radiation across angular scales ~20" to 10', and a frequency bandwidth of 7.5
MHz with resolution 125 kHz. The measured C_l(Delta nu) which ranges from 7
mK^2 to 18 mK^2 is dominated by foregrounds, the expected HI signal
C_l^HI(Delta nu) ~10^{-6}- 10^{-7} mK^2 is several orders of magnitude smaller.
The foregrounds, believed to originate from continuum sources, is expected to
vary smoothly with Delta nu whereas the HI signal decorrelates within ~0.5 MHz
and this holds the promise of separating the two. For each l, we use the
interval 0.5 < Delta nu < 7.5 MHz to fit a fourth order polynomial which is
subtracted from the measured C_l(Delta nu) to remove any smoothly varying
component across the entire bandwidth Delta nu < 7.5 MHz. The residual
C_l(Delta nu), we find, has an oscillatory pattern with amplitude and period
respectively ~0.1 mK^2 and Delta nu = 3 MHz at the smallest l value of 1476,
and the amplitude and period decreasing with increasing l. Applying a suitably
chosen high pass filter, we are able to remove the residual oscillatory pattern
for l=1476 where the residual C_l(Delta nu) is now consistent with zero at the
3-sigma noise level. We conclude that we have successfully removed the
foregrounds at l=1476 and the residuals are consistent with noise. We use this
to place an upper limit on the HI signal whose amplitude is determined by x_HI
b where x_HI and b are the HI neutral fraction and the HI bias respectively. A
value of x_HI b greater than 7.95 would have been detected in our observation,
and is therefore ruled out at the 3-sigma level. (abridged)Comment: 29 pages, 13 figures, Accepted to MNRA
Turbulence in the Harassed Galaxy NGC 4254
Galaxy harassment is an important mechanism for the morphological evolution
of galaxies in clusters. The spiral galaxy NGC 4254 in the Virgo cluster is
believed to be a harassed galaxy. We have analyzed the power spectrum of HI
emission fluctuations from NGC 4254 to investigate whether it carries any
imprint of galaxy harassment. The power spectrum, as determined using the 16
central channels which contain most of the HI emission, is found to be well
fitted by a power law with at
length-scales to . This is similar to
other normal spiral galaxies which have a slope of and is
interpreted as arising from two dimensional turbulence at length-scales larger
than the galaxy's scale-height. NGC 4254 is hence yet another example of a
spiral galaxy that exhibits scale-invariant density fluctuations out to
length-scales comparable to the diameter of the HI disk. While a large variety
of possible energy sources like proto-stellar winds, supernovae, shocks, etc.
have been proposed to produce turbulence, it is still to be seen whether these
are effective on length-scales comparable to that of the entire HI disk. On
separately analyzing the HI power spectrum in different parts of NGC 4254, we
find that the outer parts have a different slope ()
compared to the central part of the galaxy (). Such a
change in slope is not seen in other, undisturbed galaxies. We suggest that, in
addition to changing the overall morphology, galaxy harassment also effects the
fine scale structure of the ISM, causing the power spectrum to have a steeper
slope in the outer parts.Comment: 6 figures, 1 table, accepted for publication in MNRAS letter
Power spectrum of HI intensity fluctuations in DDO 210
We measure the power spectrum of HI intensity fluctuations in the extremely
faint (M_B ~ -10.9) dwarf galaxy DDO 210 using a visibility based estimator
that is well suited in very low signal to noise ratio regimes. DDO 210's HI
power spectrum is well fit by a power law with
over the length-scales 80 pc to 500 pc. We also find
that the power spectrum does not change with an increase in the velocity
channel width, indicating that the measured fluctuations correspond mainly to
density fluctuations. However, Kolmogorov turbulence (with a velocity structure
function spectral slope of 2/3) cannot be ruled out from the present
observations. The value of the slope is similar to that obtained in
the Milkyway. In contrast to the Milkyway, DDO 210 has three orders of
magnitude less HI, no spiral arms, and also no measurable ongoing star
formation. The fact that the power spectrum slope is nonetheless similar in
these two galaxies (and also similar to the values measured for the LMC and
SMC) suggests that there is some universal, star formation independent,
phenomenon responsible for producing fine scale structure in the gas.Comment: 5 Pages, 3 Figures. Accepted for publication in MNRAS Letter
Guided Missile with an Intelligent Agent
Guided missiles involve the use of a conventional deviated pursuit course like proportionalnavigation algorithm and its variants, which is optimal when the speed advantage of the guided missile is veryhighandthe target maneouvering is minimal. Against the present-day aircraft,whichemploys fly-by-wire technology for high maneouverability andhigh speed, missiles needto have amuchhigher speed advantage or to use a combination of artificial intelligence and modern controlalgorithms. Results of simulation of pursuit and evasion with an autonomous intelligent agentincorporated in the control loop are presented
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