Extragalactic H II regions, direct tracers of luminous young
stars, are traditionally studied in H
, H
, and other
lines characteristic of gas at 
K. Radio observations,
though difficult, offer two key advantages.
A single O5 star creates an H II region with a flux density of
about 1 mJy at 1 Mpc, or 10 microJy at 10 Mpc; a B0 star is 100 times
less bright. The enhanced VLA at 40 GHz will reach a sensitivity
of 2 microJy in 12 hrs with 1 GHz bandwidth; the
super-wide 10 GHz bandwidth would make this much easier (1.3 hrs),
but in any case we could count individual stars down to the B0
class in nearby galaxies. Even with the narrow 1 GHz band we
could see a single O5 star at 10 Mpc in 12 hrs, and 10 GHz would
allow either very fast imaging of distant sources, or a more
involved but still plausible project to look at even smaller H II
regions out to Virgo. One should pick up many more of the
optically-detected H II regions, allowing the determination of
extinctions, the direct measurement of temperatures and densities
(useful for example in abundance work), and high-resolution
imaging (see below). This might also be a good approach to
finding possible star-forming regions in dwarf, elliptical, and
low (optical) surface brightness galaxies, although direct
H
imaging probably wins when looking simply for
detections.
Long-wavelength observations would be useful in distinguishing H II regions from faint background sources and SNRs, requiring a sensitive long-wavelength system (somewhere between 0.6 and 2.4 GHz). One would of course also get SNRs for free (see above), making this a wonderful star formation project.
The high angular resolution of the A+ configuration would be especially
useful for this work. A typical H II region is a few to tens of
parsecs across, and can be resolved in nearby galaxies even with the
current VLA. Compact (
pc) and ultra-compact (
pc)
H II regions, corresponding to much younger sources, are much more
difficult to study outside our own galaxy. At 
9mm the
A configuration gives 
resolution, or 0.3 pc at 1 Mpc, and 3 pc at
10 Mpc; the extended A+ configuration is a factor 7 better, or 8 mas
(0.04 pc at 1 Mpc, 0.4 pc at 10 Mpc, 0.55 pc at Virgo). Given the
sensitivities discussed above, one could easily detect and image
hundreds of individual ultra-compact H II regions in nearby galaxies,
and even resolve compact H II regions to the distance of Virgo. This
means knowing the ages and (high end) initial mass functions for star
forming regions across entire galactic disks, in unprecedented detail,
with no worries about extinction. This simply cannot be done now.
(The MMA would do about as well, although in both cases, to eliminate
confusing sources, the expanded VLA with high-sensitivity
low-frequency systems would be needed.)