Observations of neutral hydrogen in spiral galaxies reveal a sharp cutoff in the radial density profile at some distance from the center .
Using 22 galaxies with known HI distributions as an example , we discuss the question of whether this effect can be associated exclusively with external ionizing radiation , as is commonly assumed .
We show that before the surface density reaches \sigma _ { \textrm { HI } } \leq 0.5 \leavevmode \nobreak { \cal M } _ { \odot } / { \textrm { pc } % } ^ { 2 } ( the same for galaxies of different types ) , it is hard to expect the gas to be fully ionized by background radiation .
For two of 13 galaxies with a sharp drop in the HI profile , the steepening can actually be caused by ionization .
At the same time , for the remaining galaxies , the observed cutoff in the radial HI profile is closer to the center than if it was a consequence of ionization by background radiation and , therefore , it should be caused by other factors . Article is published in Astronomy Letters , 2012 , Volume 38 , Number 4 , Pages 222-230 .