Resolution of the Compact Radio Continuum Sources in Arp220

We present 2 cm and 3.6 cm wavelength very long baseline interferometry images of the compact radio continuum sources in the nearby ultra-luminous infrared galaxy Arp220. Based on their radio spectra and variability properties, we confirm these sources to be a mixture of supernovae (SNe) and supernova remnants (SNRs). Of the 17 detected sources we resolve 7 at both wavelengths. The SNe generally only have upper size limits. In contrast all the SNRs are resolved with diameters {\geq} 0.27 pc. This size limit is consistent with them having just entered their Sedov phase while embedded in an interstellar medium (ISM) of density 10^4 cm^{-3} . These objects lie on the diameter-luminosity correlation for SNRs (and so also on the diameter-surface brightness relation) and extend these correlations to very small sources. The data are consistent with the relation L {\propto} D^{-9/4}. Revised equipartition arguments adjusted to a magnetic field to relativistic particle energy density ratio of 1% combined with a reasonable synchrotron-emitting volume filling factor of 10% give estimated magnetic field strengths in the SNR shells of ~ 15-50 mG. The SNR shell magnetic fields are unlikely to come from compression of ambient ISM fields and must instead be internally generated. We set an upper limit of 7 mG for the ISM magnetic field. The estimated energy in relativistic particles, 2%-20% of the explosion kinetic energy, is consistent with estimates from models that fit the IR-radio correlation in compact starburst galaxies.Comment: 16 pages, 5 figure

Continuum and spectral line observations of the OH Megamaser galaxy Arp 220

We present MERLIN observations of the continuum (both 1.6 and 5 GHz) and OH maser emission towards Arp220. the correct spatial configuration of the various componnents of the galaxy is revealed. In the eastern component the masers are shown to be generally coincident with the larger scale continuum emission; in the west, the masers and continuum do not generally arise from the same location. A velocity gradient (0.32+/-0.03km/s/pc) is found in the eastern nuclear region in MERLIN scales; this gradient is three times smaller than seen in OH and implies that the OH gas lies inside the HI. A re-analysis of previously presented global VLBI data (Lonsdale et al. 1998) reveals a very high velocity gradient (18.67+/-0.12km/s/pc) in one component, possibly the site of a heavily obscured AGN.Comment: 10 pages, 11 figures, accepted by MNRA

Multiwavelength radio observations of the compact starburst in Arp 220

We report the first detection at multiple radio wavelengths (13, 6, and 3.6 cm) of 18 compact sources within both nuclei of the Ultra Luminous Infra-Red Galaxy (ULIRG) Arp 220. In just over half of the sources we find that the observed spectra are consistent with the standard model of powerful Type IIn supernovae interacting with their pre-explosion stellar wind. The rate of appearance of new radio sources ascribed to these supernova events suggests that a large fraction of core-collapse supernovae in Arp 220 are highly luminous, possibly implying a radically different stellar initial mass function (IMF) or stellar evolution compared to galactic disks. A second group of sources, consisting of the brightest and longest monitored sources at 18 cm, do not easily fit the radio supernova model. We propose that these are young supernova remnants that have just begun interacting with their surrounding dense ISM

VLBI Images of 49 Radio Supernovae in Arp 220

We have used a Very Long Baseline Interferometry (VLBI) array at 18cm wavelength to image the nucleus of the luminous IR galaxy Arp 220 at ~1 pc linear resolution, and with very high sensitivity. The resulting map has an rms of 5.5 microJy/beam, and careful image analysis results in 49 confirmed point sources ranging in flux density from 1.2 mJy down to ~60 microJy. Comparison with high sensitivity data from 12 months earlier reveals at least four new sources. The favored interpretation of these sources is that they are radio supernovae, and if all new supernovae are detectable at this sensitivity, a resulting estimate of the supernova rate in the Arp 220 system is 4 +/- 2 per year. The implied star formation rate is sufficient to power the entire observed far-infrared luminosity of the galaxy. The two nuclei of Arp 220 exhibit striking similarities in their radio properties, though the western nucleus is more compact, and appears to be ~3 times more luminous than the eastern nucleus. There are also some puzzling differences, and differential free-free absorption, synchrotron aging and expansion losses may all be playing a role. Comparison with the nearby starburst galaxy M82 supports the hypothesis that the activity in Arp 220 is essentially a scaled-up version of that in M82.Comment: 24 pages, 3 figures, 1 table. Accepted for publication in Ap.

The Grizzly, May 3, 1994

Spring Fling \u2794 • SERV Finishes Busy Semester • Alpha Sigma Nu Sponsors MS Walk-a-thon • A Bitter Farewell • Spade: A Waste • A Belated Farewell • Senior Spotlights: Cynthia Babcock; Erika Compton • Summer \u2794 Entertainment Preview • Concerts • The 1994 Phillies: What a Differencehttps://digitalcommons.ursinus.edu/grizzlynews/1338/thumbnail.jp

The Grizzly, April 26, 1994

Richter to Leave Presidency • Richter: Then and Now • S.T.A.R. Raising Awareness • Economics Conference Held at Ursinus • Vandalism in Olin • A Necessary Tension: Greeks and Administration • Final Exam Schedule • It\u27s Alive, It\u27s Alive • Moshing, Body Surfing and The Gin Blossoms • NBA Playoff Preview • Men\u27s Tennis Looks for Strong Finishhttps://digitalcommons.ursinus.edu/grizzlynews/1337/thumbnail.jp

The Grizzly, April 19, 1994

Erdrich Speaks of Cultural Struggles • Friendly Fire Over Iraq • Greeks Participate in Greek Week • U.S.G.A. Responds to Requests for a Wismer Meal Plan • Tropical Conservationist to Speak at Ursinus • President John Bartholomew Speaks Again • Tuning in to Talk Radio • Bands to Perform Saturday • It\u27s Not a Matter of Dryness • Women\u27s Lacrosse Crushes Swarthmore; Falls to Johns Hopkins • Eagles Draft Preview • George White Named New Men\u27s Basketball Coach at Ursinushttps://digitalcommons.ursinus.edu/grizzlynews/1336/thumbnail.jp

Multiwavelength radio observations of the compact starburst in Arp 220

We report the first detection at multiple radio wavelengths (13, 6, and 3.6 cm) of 18 compact sources within both nuclei of the Ultra Luminous Infra-Red Galaxy (ULIRG) Arp 220. In just over half of the sources we find that the observed spectra are consistent with the standard model of powerful Type IIn supernovae interacting with their pre-explosion stellar wind. The rate of appearance of new radio sources ascribed to these supernova events suggests that a large fraction of core-collapse supernovae in Arp 220 are highly luminous, possibly implying a radically different stellar initial mass function (IMF) or stellar evolution compared to galactic disks. A second group of sources, consisting of the brightest and longest monitored sources at 18 cm, do not easily fit the radio supernova model. We propose that these are young supernova remnants that have just begun interacting with their surrounding dense ISM