Turbulence in the harassed galaxy NGC 4254

Abstract

Galaxy harassment is an important mechanism for the morphological evolution of galaxies in clusters. The spiral galaxy NGC4254 in the Virgo cluster is believed to be a harassed galaxy. We have analysed the power spectrum of HI emission fluctuations from NGC4254 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) =AUα with α=−1.7 ± 0.2 at length-scales 1.7 to 8.4kpc. This is similar to other normal spiral galaxies which have a slope of ∼−1.5 and is interpreted as arising from two-dimensional turbulence at length-scales larger than the galaxy's scaleheight. NGC4254 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 disc. While a large variety of possible energy sources like protostellar 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 disc. On separately analysing the HI power spectrum in different parts of NGC4254, we find that the outer parts have a different slope (α=−2.0 ± 0.3) compared to the central part of the galaxy (α=−1.5 ± 0.2). 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 affects the fine scale structure of the interstellar medium, causing the power spectrum to have a steeper slope in the outer parts.