Publications (3)0 Total impact
ABSTRACT: We report on the detection of optically thick free-free radio sources in the galaxies M33, NGC 253, and NGC 6946 using data in the literature. We interpret these sources as being young, embedded star birth regions, which are likely to be clusters of ultracompact HII regions. All 35 of the sources presented in this article have positive radio spectral indices alpha>0 suggesting an optically thick thermal bremsstrahlung emission arising in the HII region surrounding hot stars. Energy requirements indicate a range of a several to >500 O7V star equivalents powering each HII region. Assuming a Salpeter IMF, this corresponds to integrated stellar masses of 0.1--60,000 Msun. For roughly half of the sources in our sample, there is no obvious optical counterpart, giving further support for their deeply embedded nature. Their luminosities and radio spectral energy distributions are consistent with HII regions having electron densities from 1500 cm^-3 to 15000 cm^-3 and radii of 1 - 7 pc. We suggest that the less luminous of these sources are extragalactic ultracompact HII region complexes, those of intermediate luminosity are similar to W49 in the Galaxy, while the brightest will be counterparts to 30 Doradus. These objects constitute the lower mass range of extragalactic ``ultradense HII regions'' which we argue are the youngest stages of massive star cluster formation yet observed. This sample is beginning to fill in the continuum of objects between small associations of ultracompact HII regions and the massive extragalactic clusters that may evolve into globular clusters. Comment: 37 pages, uses AASTeX; scheduled to appear in ApJ v. 559 October 2001. Full postscript version available from http://www.astro.wisc.edu/~chip/Papers/Johnson_Kobulnicky_etal_ApJ559.ps.gz
ABSTRACT: Embedded massive starclusters have recently been identified in several nearby galaxies by means of the radio-wave thermal bremsstrahlung emission from their surrounding HII regions. Energy requirements imply that these optically-obscured starclusters contain 500-1000 O-type stars, making them similar to the ``super starclusters'' observed in many dwarf starbursts and mergers. Based on their high free-free optical depth and visual extinctions of A_V>>10 mag., these massive ``ultra-dense'' HII regions (UDHIIs) are distinct signatures of the youngest, most compact super starclusters. UDHII regions may represent the earliest stages of globular cluster formation. We review the properties of presently-known UDHIIs, and we outline a pictoral evolutionary taxonomy for massive cluster formation which is analogous to the more familiar evolutionary sequence for individual stars.
ABSTRACT: We present aperture synthesis observations of the prototype Wolf-Rayet
galaxy He 2-10 in the line of (12) CO(1-0). These observations represent
one of the first aperture synthesis maps of molecular gas in a blue
compact dwarf galaxy. He 2-10 contains two starburst regions, A and B,
separated by 8 arcsec which corresponds to 350 pc at at distance of 9
Mpc. Optical spectroscopy of region A indicates the presence of some 300
Wolf-Rayet and 4000 O-type stars, consistent with a very young starburst
(Vacca & Conti, 1992, ApJ 401, 543). From a line integral of of
165+/-8 K km s(-1) we derive a total molecular gas mass of 1.8 times
10(8) M_sun based on (12) CO(1-0) spectra from the 12m NRAO telescope.
The ratio of molecular to atomic gas mass, M(H_2)/M(HI)=0.54, is among
the highest of any late type or blue compact dwarf galaxy. (12) CO(1-0)
maps made with the Owens valley interferometer show two dynamical
systems, suggesting an interaction-triggered starburst. While the CO
peak is not conincident with either optical maximum, the CO is more
nearly centered on the brighter and younger of the two starburst
regions, A. There is no visible concentration of molecular gas near
starburst region B which contains only a few hundred O-type stars. A
significant fraction of the CO lies well outside the bright optical
core, and is thus unaffiliated with the site of active star formation.
We find a lower limit to the dynamical mass in the central 70 pc of
3.0times 10(6) M_sun inferred from the CO rotation curve. Conti &
Vacca (1994, ref) estimate the combined mass of nine blue starburst
knots revealed by HST UV imaging to be 4.5times 10(6) M_sun. Even if the
inclination of He 2-10 is as low as 30(deg) , the young clusters, termed
proto-globular clusters by Conti & Vacca, comprise at least 75% of
the dynamical mass in the inner 70 pc!
University of Minnesota Duluth