NGC 4214#1

This bright cluster (diameter 5), contains OB stars which lie inside a 10 ring of H emission. HST imaging and spectra appear to show 4-8 more UV flux than can be accounted for by the H emission. H equivalent width (normally well correlated with UV flux) is lower than expected for the observed UV flux (Leitherer etal 1996). Is the H emission density-bounded?

HI data are consistent with this, but any swept-out region must be small or incompletely cleared. Local HI surface density is  cm. For gaussian disk profile, corresponding mean density is 2.6 cm. The H emission implies a local ionized gas density of 2-3 cm.

For a uniform density of 2.5 cm, the UV flux of NGC 4214#1 would produce a Strömgren sphere 260 pc (13) in diameter. Since this is comparable to the scale height, the ionized volume should be even larger. But no signature of an H I hole is visible in column density or position-velocity maps. The two most likely interpretations of this are; a) there is neutral gas in self-shielding filaments or clumps, b) some coincidence of viewing geometry.

Self-shielding is consistent with the presence of CO emission nearby (Becker etal 1995), and with the observations of dense filaments (Kim etal 1996) and cold absorbing clouds (Dickey etal 1994) near 30 Dor.

Luks & Rohlfs (1992) made observations of the LMC with spatial resolution comparable to the data shown here, and found no evidence of a hole around the central cluster of 30 Dor. They speculated the cluster lies in front of or behind the main disk of the galaxy. A similar model for NGC 4214#1 cannot easily be ruled out, but seems improbable given the cluster's central location.

© Copyright Astronomical Society of Australia 1997