Astronomical Applications Department, U.S. Naval Observatory thesis Page 56
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4.1.3 Radial Continuity of Star Formation
Hunter 1997 points out that similarities between the time-averaged level of
past star formation activity and current activity at a given radius is evidence that
star formation is a local rather than global process in irregular galaxies. The
general lack of correlation between star formation properties and other global
properties such as abundance or global gas parameters Hunter
et
al.
1982 in
irregular galaxies suggests that local rather than global conditions are important
in regulating star formation. This suggestion is supported by evidence such as
the similarity of observations tracking the radial dependence of star formation
over a signi cant portion of a galaxy's lifetime. Several comparisons between the
radial dependence of optical and H emission have been made that support this
hypothesis for both irregular Hunter & Gallagher 1985 and spiral galaxies Ryder
& Dopita 1994. The FUV data allow this trend to be extended since it tracks the
youngest population for longer timescales than H emission. Figure 4.6 displays
the FUV, U, B, and R-band pro les on the same scale. Beginning with the R-
band pro le and moving counterclockwise around the gure, the pro les become
less smooth as the wavelength of the pro le decreases. This is expected due
to the contribution from recent star formation. Only light from the most recent
population contributes to the FUV pro le while at increasingly longer wavelengths
the light is more complicated measure of the entire history of star formation. The
general shape of each pro le is same and the radii at which increased star formation
activity occurs remains relatively constant over the timescale which these pro les
track star formation activity, roughly a few Gyrs. Figure 4.6 illustrates that HoII
is currently forming stars at the same radii as it has in the past, supporting the
supposition that star formation in irregular galaxies is a local process and that
local conditions such as gas distributions and feedback from massive stars may
play a signi cant role in the star formation process.
