![[Picture of J.T.]](VLA_JT_ondish2_sm.jpg)
Jean L. Turner
UCLAPhysics & Astronomy
turner@astro.ucla.edu
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UCLA |
Pat Crosthwaite and I have mapped the total neutral gas content of the nearby
bright spiral galaxy, NGC 6946. Our 1' resolution image of NHI +
NH2 was obtained using data from the National Radio Astronomy Observatory's
12 Meter Telescope and the Very Large Array. There are not too many maps of
total gas content; interferometer maps miss the extended emission in spiral
disks, and generally do not detect interarm gas. We do. And the interarm gas
appears to be optically thin. We also find that, point by point, the star
formation rate on arcminute scales (about a kpc) in these galaxies is
proportional to the first power of the total gas surface density, that is,
a Schmidt law with n=1.
This is our image of NGC 6946, with HI in red, CO in green (the HI/H2overlap regions look yellowish), and stars in blue. We've also done IC342 and M83; M83 shows a sharp edge, perhaps due to tidal interaction. (For astro-ph links, see Publications. Crosthwaite & Turner 2007)
Chao-Wei Tsai and I and others (including Crosthwaite, Dave Meier, Sara Beck, Paul Ho) have been using the Very Large Array to map the pc-scale structure around young star-forming regions in nearby galaxies. We have published our work on the nuclear star-forming regions of IC342, Maffei 2, NGC 2903, and NGC 6946, and are working on M82. These HII regions are excited by compact clusters of up to 1000 O stars, within regions of about 1-5 pc extent. In our work we aim to locate these super star cluster nebulae, obtain estimates of their luminosity, and try to obtain morphological clues as to how star formation is triggered in different kinds of galaxies. (Tsai et al. 2006)
David S. Meier
and I have been working on the chemical properties of the central
regions of nearby galaxies. We find that the chemical properties of molecular
clouds vary dramatically with environment, even on sizescales of 50-100 pc. This
has been inferred from spectra of nearby galaxies (M82 has been characterized
as a "giant PDR" cloud, for example), but is clear from these spatially
resolved images from the Owens Valley Millimeter Array (now part of CARMA):
The underlying contours are 13CO emission as a reference for the molecules presented in color, labeled at the upper right. The mapped region is about 400 pc in extent, and our beam was 4" (about 60 pc at the 3 Mpc distance of IC342). A principal component analysis of the emission reveals correlations between the molecules, which can be roughly classified as 1) good overall gas tracers (CO and isotopomers, N2H+, HNC, HCN, and for dense regions, HC3N), 2) PDR tracers (C2H and C34S) and 3) grain chemistry tracers (HNCO and CH3OH aka methanol). The grain chemistry tracers lie along the arms of the nuclear bar, and may trace shocks there. The PDR tracer molecules reflect gas-phase chemistry in the high-radiation environment of the central nuclear cluster. We also find that HCN is extremely well correlated with 3mm continuum emission, which is free-free emission from HII regions. This confirms that the global correlation of HCN and star formation (IR) of Gao & Solomon holds down to the scales of GMCs. We are pursuing this work in other nearby galaxies to see if these correlations hold there too. (Meier & Turner 2005 and Turner & Meier 2007).
With Sara Beck (Tel Aviv), I have imaged the "supernebula" in the dwarf galaxy NGC 5253 with the Very Large Array and the Pie Town link. The long baselines of the Pie Town-VLA link allowed us to achieve nearly diffraction-limited performance at Q band (7mm) and gave us a beamsize of 75 mas x 17 mas in our highest resolution images. A slightly lower resolution, but higher signal to noise, map is shown here:
Above is an overlay of the radio image, in green, on a two color NICMOS false color image. The blue channel is 1.1 microns, and the red is 1.9 microns. The image on the right hand side is a blow-up of the central region; this panel is only 2" across! Visible in the 1.9 micron image (and therefore very red) is an embedded IR cluster. We believe that it is this IR cluster that powers the supernebula in NGC 5253. The high density and volume of this nebula (Beck et al. 1996; Turner et al. 1998, 2000) require the input of the equivalent of ~1200 O7 stars within the core of the nebula, which is only 0.72 x 1.8 pc across (about 2 x 6 light years). This is about the size of a globular cluster core. The central 1" region (corresponding to the double IR source) contains a total of 7000 O7 stars - the total number of stars depends on the mass function but is probably half a million to 2 million for this region. See Publications for the reference to our 2004 paper with this image. (Turner & Beck 2004)
Another unusual aspect of this nebula, aside from the remarkable concentration of stellar luminosity, is that it appears to be gravitationally bound. This may be true for super star cluster nebulae in general but has not been seen before. With the NIRSPEC near-IR spectrometer of the Keck Observatory. I and my colleagues (Beck, Crosthwaite, Larkin, McLean, and Meier) detected supersonic wind motions in the supernebula. The wind motions are probably caused by the combined stellar winds from the estimated 4000 O stars that are located within the small (3-4 light years) cluster. Even though the gas motions are supersonic, and undoubtedly the gas is very turbulent within this cluster, because of the small size and large mass, the nebula is gravitationally bound. This could have implications for starburst evolution, since it would confine the effects of stellar winds from super star clusters for some time, allowing other clusters to form nearby. (Turner et al. 2003)