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 $P(U)=AU^{\alpha}$ with $\alpha\ =-\ 1.7\pm 0.2$ at
length-scales $1.7 \, {\rm k pc}$ to $ 8.4 \, {\rm kpc}$. This is similar to
other normal spiral galaxies which have a slope of $\sim -1.5$ 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 ($ \alpha = -2.0\pm0.3$)
compared to the central part of the galaxy ($\alpha = -1.5\pm0.2$). 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.
