WSRT 1.4 GHz Observations of the Hubble Deep Field

We present WSRT 1.38 GHz observations of the Hubble Deep Field (and flanking fields). 72 hours of data were combined to produce the WSRT's deepest image yet, achieving an r.m.s. noise level of 8 microJy per beam. We detect radio emission from galaxies both in the HDF and HFF which have not been previously detected by recent MERLIN or VLA studies of the field.Comment: 2 pages, 1 figure, to appear in "The Universe at Low Radio Frequencies", IAU Symposium 199. For colour figures, see http://www.nfra.nl/~mag/hdf_wsrt.htm

Air/Sea Transfer of Highly Soluble Gases Over Coastal Waters

The deposition of soluble trace gases to the sea surface is not well studied due to a lack of flux measurements over the ocean. Here we report simultaneous air/sea eddy covariance flux measurements of water vapor, sulfur dioxide (SO2), and momentum from a coastal North Atlantic pier. Gas transfer velocities were on average about 20% lower for SO2 than for H2O. This difference is attributed to the difference in molecular diffusivity between the two molecules (DSO2/DH2O = 0.5), in reasonable agreement with bulk parameterizations in air/sea gas models. This study demonstrates that it is possible to observe the effect of molecular diffusivity on air-side resistance to gas transfer. The slope of observed relationship between gas transfer velocity and friction velocity is slightly smaller than predicted by gas transfer models, possibly due to wind/wave interactions that are unaccounted for in current models

SubmilliJansky Transients in Archival Radio Observations

[ABRIDGED] We report the results of a 944-epoch survey for transient sources with archival data from the Very Large Array spanning 22 years with a typical epoch separation of 7 days. Observations were obtained at 5 or 8.4 GHz for a single field of view with a full-width at half-maximum of 8.6' and 5.1', respectively, and achieved a typical point-source detection threshold at the beam center of ~300 microJy per epoch. Ten transient sources were detected with a significance threshold such that only one false positive would be expected. Of these transients, eight were detected in only a single epoch. Two transients were too faint to be detected in individual epochs but were detected in two-month averages. None of the ten transients was detected in longer-term averages or associated with persistent emission in the deep image produced from the combination of all epochs. The cumulative rate for the short timescale radio transients above 370 microJy at 5 and 8.4 GHz is 0.07 < R < 40 deg^-2 yr^-1, where the uncertainty is due to the unknown duration of the transients, 20 min < t_char < 7 days. A two-epoch survey for transients will detect 1.5 +/- 0.4 transient per square degrees above a flux density of 370 microJy. Two transients are associated with galaxies at z=0.040 and z=0.249. These may be similar to the peculiar Type Ib/c radio supernova SN 1998bw associated with GRB 980428. Six transients have no counterparts in the optical or infrared (R=27, Ks=18). The hosts and progenitors of these transients are unknown.Comment: Accepted for ApJ; full quality figures available at http://astro.berkeley.edu/~gbower/ps/rt.pd

Discovery of Radio Outbursts in the Active Nucleus of M81

The low-luminosity active galactic nucleus of M81 has been monitored at centimeter wavelengths since early 1993 as a by-product of radio programs to study the radio emission from Supernova 1993J. The extensive data sets reveal that the nucleus experienced several radio outbursts during the monitoring period. At 2 and 3.6 cm, the main outburst occurred roughly in the beginning of 1993 September and lasted for approximately three months; at longer wavelengths, the maximum flux density decreases, and the onset of the burst is delayed. These characteristics qualitatively resemble the standard model for adiabatically expanding radio sources, although certain discrepancies between the observations and the theoretical predictions suggest that the model is too simplistic. In addition to the large-amplitude, prolonged variations, we also detected milder changes in the flux density at 3.6 cm and possibly at 6 cm on short (less than 1 day) timescales. We discuss a possible association between the radio activity and an optical flare observed during the period that the nucleus was monitored at radio wavelengths.Comment: To appear in The Astronomical Journal. Latex, 18 pages including embedded figures and table

VLA Observations of J1228+441, a Luminous Supernova Remnant in NGC 4449

The luminous, oxygen-rich supernova remnant, J1228+441, is located in the irregular galaxy NGC 4449 and has been observed at radio wavelengths for thirty years. An analysis of recent VLA observations of NGC 4449, combined with VLA archive data and previously published VLA and WSRT observations, yields light curves at 6 and 20 cm from 1972 to 2002. The light curves at all radio frequencies exhibit a marked decline in radio emission, confirming past findings. This paper presents and discusses the radio light curves and spectral index, alpha, variations from 1972 to 2002 where nu propto nu^(-alpha) and compares J1228+441 with other radio supernovae. The spectral index of J1228+441 appears to have steepened in the last five years at higher frequencies from alpha=0.64 +- 0.02 in 1996 to alpha=1.01 +- 0.02 in 2001-2002.Comment: aastex, 19 pages, 6 figures, to be published in A

Statistical Properties of Radio Emission from the Palomar Seyfert Galaxies

We have carried out an analysis of the radio and optical properties of a statistical sample of 45 Seyfert galaxies from the Palomar spectroscopic survey of nearby galaxies. We find that the space density of bright galaxies (-22 mag <= M_{B_T} <= -18 mag) showing Seyfert activity is (1.25 +/- 0.38) X 10^{-3} Mpc^{-3}, considerably higher than found in other Seyfert samples. Host galaxy types, radio spectra, and radio source sizes are uncorrelated with Seyfert type, as predicted by the unified schemes for active galaxies. Approximately half of the detected galaxies have flat or inverted radio spectra, more than expected based on previous samples. Surprisingly, Seyfert 1 galaxies are found to have somewhat stronger radio sources than Seyfert 2 galaxies at 6 and 20 cm, particularly among the galaxies with the weakest nuclear activity. We suggest that this difference can be accommodated in the unified schemes if a minimum level of Seyfert activity is required for a radio source to emerge from the vicinity of the active nucleus. Below this level, Seyfert radio sources might be suppressed by free-free absorption associated with the nuclear torus or a compact narrow-line region, thus accounting for both the weakness of the radio emission and the preponderance of flat spectra. Alternatively, the flat spectra and weak radio sources might indicate that the weak active nuclei are fed by advection-dominated accretion disks.Comment: 18 pages using emulateapj5, 13 embedded figures, accepted by Ap

Early stages of ramified growth in quasi-two-dimensional electrochemical deposition

I have measured the early stages of the growth of branched metal aggregates formed by electrochemical deposition in very thin layers. The growth rate of spatial Fourier modes is described qualitatively by the results of a linear stability analysis [D.P. Barkey, R.H. Muller, and C.W. Tobias, J. Electrochem. Soc. {\bf 136}, 2207 (1989)]. The maximum growth rate is proportional to $(I/c)^\delta$ where $I$ is the current through the electrochemical cell, $c$ the electrolyte concentration, and $\delta = 1.37 \pm 0.08$. Differences between my results and the theoretical predictions suggest that electroconvection in the electrolyte has a large influence on the instability leading to ramified growth.Comment: REVTeX, four ps figure

Do Jet-Driven Shocks ionize the Narrow Line Regions of Seyfert Galaxies?

We consider a model in which the narrow line regions (NLRs) of Seyfert galaxies are photoionized ``in situ'' by fast (300 -- 1,000 km/s), radiative shock waves driven into the interstellar medium of the galaxy by radio jets from the active nucleus. Such shocks are powerful sources of soft X-rays. We compute the expected ratio of the count rates in the ROSAT PSPC and Einstein IPC detectors to the [OIII] \lambda 5007 flux as a function of shock velocity, and compare these ratios with observations of type 2 Seyferts. If most of the observed soft X-ray emission from these galaxies originates in the NLR and the absorbing hydrogen column is similar to that inferred from the reddening of the NLR, a photoionizing shock model with shock velocity $\simeq$ 400 -- 500 km/s is compatible with the observed ratios. High angular resolution observations with AXAF are needed to isolate the X-ray emission of the NLR and measure its absorbing column, thus providing a more conclusive test. We also calculate the expected coronal iron line emission from the shocks. For most Seyfert 2s, the [Fe X] \lambda 6374/H \beta$ ratio is a factor of 2 -- 14 lower than the predictions of 300 -- 500 km/s shock models, suggesting that less hot gas is present than required by these models.Comment: Astrophys J. Letters 1999 March 10 issue, Vol. 51

Electroconvection in a Suspended Fluid Film: A Linear Stability Analysis

A suspended fluid film with two free surfaces convects when a sufficiently large voltage is applied across it. We present a linear stability analysis for this system. The forces driving convection are due to the interaction of the applied electric field with space charge which develops near the free surfaces. Our analysis is similar to that for the two-dimensional B\'enard problem, but with important differences due to coupling between the charge distribution and the field. We find the neutral stability boundary of a dimensionless control parameter ${\cal R}$ as a function of the dimensionless wave number ${\kappa}$. ${\cal R}$, which is proportional to the square of the applied voltage, is analogous to the Rayleigh number. The critical values ${{\cal R}_c}$ and ${\kappa_c}$ are found from the minimum of the stability boundary, and its curvature at the minimum gives the correlation length ${\xi_0}$. The characteristic time scale ${\tau_0}$, which depends on a second dimensionless parameter ${\cal P}$, analogous to the Prandtl number, is determined from the linear growth rate near onset. ${\xi_0}$ and ${\tau_0}$ are coefficients in the Ginzburg-Landau amplitude equation which describes the flow pattern near onset in this system. We compare our results to recent experiments.Comment: 36 pages, 7 included eps figures, submitted to Phys Rev E. For more info, see http://mobydick.physics.utoronto.ca

Dynamics and Selection of Giant Spirals in Rayleigh-Benard Convection

For Rayleigh-Benard convection of a fluid with Prandtl number \sigma \approx 1, we report experimental and theoretical results on a pattern selection mechanism for cell-filling, giant, rotating spirals. We show that the pattern selection in a certain limit can be explained quantitatively by a phase-diffusion mechanism. This mechanism for pattern selection is very different from that for spirals in excitable media