1155749
High Resolution Radio Maps of Four Nearby Spiral Galaxies
We report subarcsecond-resolution VLA imaging of the centers of four nearby spiral galaxies: IC 342, Maffei II, NGC 2903, and NGC 6946. In each galaxy, 7 - 12 compact radio continuum sources were identified within the central ∼ 15 ′′ × 15 ′′ . Slightly over half of the compact sources appear to be H ii regions with flat or positive spectral indices (�> ∼ −0.1). The H ii regions with rising spectra are optically thick at centimeter wavelengths, and thus dense (ni ∼ 10 4 cm −3 ) and young. The largest of these H ii regions require the excitation of 500 - 800 O stars, within regions of only few parsecs extent. These clusters approach the sizes expected for globular clusters.
8606630
Subarcsecond-Resolution Radio Maps of Nearby Spiral Galaxies
We report subarcsecond-resolution Very Large Array imaging of four nearby spiral galaxies: IC 342, Maffei II, NGC 2903, and NGC 6946. In each galaxy, compact radio continuum sources are identified in the central ~15'' × 15'' region. These compact sources are responsible for 20%-30% of the total emission from the central kiloparsec of the host galaxies at 2 cm but only ~5%-10% at 6 cm. More than half of the compact sources appear to be H II regions. The H II regions with rising spectra must be fairly dense (ni ~ 104 cm-3) and are presumably very young. The largest of these H II regions require the excitation of 500-800 O stars within regions only a few parsecs in extent. These clusters approach the sizes expected for globular clusters. Thermal free-free emission from compact sources contributes more significantly at 2 cm, while diffuse synchrotron emission dominates at 6 cm. The radio H II regions are found near the centers of giant molecular clouds in projection and do not have obvious visual counterparts.
15435454
RADIO-INFRARED SUPERNEBULAE IN II Zw 40
We report subarcsecond-resolution Very Large Array and Keck mid-infrared imaging of the dwarf starburst galaxy II Zw 40. II Zw 40 contains a bright compact thermal radio and infrared source with all the characteristics of a collection of dense H ii regions ionized by at least 14,000 O stars. The supernebula is revealed to consist of multiple sources within an envelope of weaker emission. The radio emission is dominated by free-free emission at 2 cm, and the spectrum of this emission appears to be rising. This suggests that the free-free emission is optically thick at 2 cm and that the individual H ii regions are � 1 pc in size. This complex of ‘‘ supernebulae ’’ dominates the total infrared luminosity of II Zw 40, although the radio source is less than � 150 pc in diameter. Multiple super–star clusters appear to be forming here, the much larger analogs of large Galactic H ii region complexes.
117289722
Watching the Birth of Super Star Clusters
Subarcsecond infrared and radio observations yield important information about the formation of super star clusters from their surrounding gas. We discuss the general properties of ionized and molecular gas near young, forming SSCs, as illustrated by the prototypical young forming super star cluster nebula in the dwarf galaxy, NGC 5253. This super star cluster appears to have a gravitationally bound nebula. The lack of molecular gas suggests a very high star formation efficiency, consistent with the formation of a large, bound star cluster.
119408453
MERLIN and VLA Observations of VII Zw 19: Distant Cousin of M82
We have observed the starburst dwarf galaxy VII Zw 19 at subarcsecond resolution with MERLIN and MERLIN+European VLBI Network (EVN) at 18 cm, with MERLIN at 6 cm, and with the Very Large Array (VLA) in the A configuration at 6 and 2 cm. The galaxy is detected at all VLA wavelengths and is resolved. It is also resolved at 18 cm by MERLIN but is not detected by the EVN or by MERLIN at 6 cm. VII Zw 19 has a complex structure of nonthermal radio emission at 18 cm extended over ~1200–1800 pc (4''–6''). That the EVN did not detect this emission indicates that there are no obvious point sources that could be radio supernovae. The 2 cm emission, predominantly thermal free-free emission, has a markedly different spatial distribution from the nonthermal emission. The radio colors show that the galaxy contains numerous supernova remnants, as well as many young H II regions, many of which are optically thick at 6 cm. Two extended regions of 2 cm emission have little 18 cm flux and are probably emission from young star clusters. VII Zw 19 resembles M82 in its radio and infrared spectrum, but the starburst region in VII Zw 19 is twice the size of the one in M82 and twice as luminous.
53662967
Supersymmetry of AdS and flat IIB backgrounds
A bstractWe present a systematic description of all warped AdSn ×wM10−n and ℝn−1,1×wM10−n$$ {\mathbb{R}}^{n-1,1}{\times}_w{M}^{10-n} $$ IIB backgrounds and identify the a priori number of supersymmetries N preserved by these solutions. In particular, we find that the AdSn backgrounds preserve N=2n2k$$ N={2}^{\left[\frac{n}{2}\right]}k $$ for n ≤ 4 and N=2n2+1k$$ N={2}^{\left[\frac{n}{2}\right]+1}k $$ for 4 < n ≤ 6 supersymmetries and for k∈ℕ+$$ k\in {\mathbb{N}}_{+} $$ suitably restricted. In addition under some assumptions required for the applicability of the maximum principle, we demonstrate that the Killing spinors of AdSn backgrounds can be identified with the zero modes of Dirac-like operators on M10−n establishing a new class of Lichnerowicz type theorems. Furthermore, we adapt some of these results to ℝn−1,1×wM10−n$$ {\mathbb{R}}^{n-1,1}{\times}_w{M}^{10-n} $$ backgrounds with fluxes by taking the AdS radius to infinity. We find that these backgrounds preserve N=2n2k$$ N={2}^{\left[\frac{n}{2}\right]}k $$ for 2 < n ≤ 4 and N=2n+12k$$ N={2}^{\left[\frac{n+1}{2}\right]}k $$ for 4 < n ≤ 7 supersymmetries. We also demonstrate that the Killing spinors of AdSn ×wM10−n do not factorize into Killing spinors on AdSn and Killing spinors on M10−n.