The Intrinsic Size of Sagittarius A* from 0.35 cm to 6 cm

We present new high-resolution observations of Sagittarius A* at wavelengths of 17.4 to 23.8 cm with the Very Large Array in A configuration with the Pie Town Very Long Baseline Array antenna. We use the measured sizes to calibrate the interstellar scattering law and find that the major axis size of the scattering law is smaller by ~6% than previous estimates. Using the new scattering law, we are able to determine the intrinsic size of Sgr A* at wavelengths from 0.35 cm to 6 cm using existing results from the VLBA. The new law increases the intrinsic size at 0.7 cm by ~20% and <5% at 0.35 cm. The intrinsic size is 13^{+7}_{-3} Schwarzschild radii at 0.35 cm and is proportional to lambda^gamma, where gamma is in the range 1.3 to 1.7.Comment: ApJL, in pres

VLA H92 Alpha and H53 Alpha Radio Recombination Line Observations of M82

We present high angular resolution (0.6'') observations made with the VLA of the radio continuum at 8.3 and 43 GHz as well as H92 Alpha and H53 Alpha radio recombination lines from the nearby (3 Mpc) starburst galaxy M82. In the continuum we report 19 newly identified sources at 8.3 GHz and 5 at 43 GHz that were unknown previously. The spatial distribution of the H92 Alpha line is inhomogeneous; we identify 27 features. The line and continuum emission are modeled using a collection of HII regions at different distances from the nucleus assuming a single-density component and two-density components. The high-density component has a density of 4 X 10^{4} cm^{-3}. However, the bulk of the ionization is in regions with densities which are typically a factor 10 lower. The gas kinematics, using the H92 Alpha line, confirms the presence of steep velocity gradient (26 km s^{-1} arcsec^{-1}) in the nuclear region. As this steep gradient is observed not only on the major axis but also at large distances along a band of PA of 150 degrees, the interpretation in terms of x2 orbits elongated along the minor axis of the bar, which would be observed at an angle close to the inclination of the main disk, seems inadequate. Ad-hoc radial motions must be introduced to reproduce the pattern of the velocity field. Different families of orbits are indicated as we detect a signature in the kinematics at the transition between the two plateaus observed in the NIR light distribution. The H92 Alpha line also reveals the base of the outflow where the injection towards the halo on the Northern side occurs. The kinematical pattern suggests a connection between the gas flowing in the plane of M82 towards the center; this behavior most likely originates due to the presence of a bar and the outflow out of the plane.Comment: 45 pages, 11 figures and 6 tables. Accepted for publication in Ap

NGC 3576 and NGC 3603: Two Luminous Southern HII Regions Observed at High Resolution with the Australia Telescope Compact Array

NGC 3576 (G291.28-0.71; l=291.3o, b=-0.7o) and NGC 3603 (G291.58-0.43; l=291.6o, b=-0.5o) are optically visible, luminous HII regions located at distances of 3.0 kpc and 6.1 kpc, respectively. We present 3.4 cm Australian Telescope Compact Array (ATCA) observations of these two sources in the continuum and the H90a, He90a, C90a and H113b recombination lines with an angular resolution of 7" and a velocity resolution of 2.6 km/s. All four recombination lines are detected in the integrated profiles of the two sources. Broad radio recombination lines are detected in both NGC 3576 (DV_{FWHM}>= 50 km/s) and NGC 3603 (DV_{FWHM}>=70 km/s). In NGC 3576 a prominent N-S velocity gradient (~30 km/s/pc) is observed, and a clear temperature gradient (6000 K to 8000 K) is found from east to west, consistent with a known IR color gradient in the source. In NGC 3603, the H90a, He90a and the H113b lines are detected from 13 individual sources. The Y^+ (He/H) ratios in the two sources range from 0.08+/-0.04 to 0.26+/-0.10. We compare the morphology and kinematics of the ionized gas at 3.4 cm with the distribution of stars, 10 micron emission and H_2O, OH, and CH_3OH maser emission. These comparisons suggest that both NGC 3576 and NGC 3603 have undergone sequential star formation.Comment: 24 pages, 12 Postscript figure

OH Zeeman Magnetic Field Detections Toward Five Supernova Remnants Using the VLA

We have observed the OH (1720 MHz) line in five galactic SNRs with the VLA to measure their magnetic field strengths using the Zeeman effect. We detected all 12 of the bright ($S_{\nu} > 200$ mJy) OH (1720 MHz) masers previously detected by Frail et al. (1996) and Green et al. (1997) and measured significant magnetic fields (i.e. $> 3\sigma$) in ten of them. Assuming that the ``thermal'' Zeeman equation can be used to estimate $\mid\vec{B}\mid$ for OH masers, our estimated fields range from 0.2 to 2 mG. These magnetic field strengths are consistent with the hypothesis that ambient molecular cloud magnetic fields are compressed via the SNR shock to the observed values. Magnetic fields of this magnitude exert a considerable influence on the properties of the cloud with the magnetic pressures ($10^{-7} - 10^{-9}$ erg cm$^{-3}$) exceeding the pressure in the ISM or even the thermal pressure of the hot gas interior to the remnant. This study brings the number of galactic SNRs with OH (1720 MHz) Zeeman detections to ten.Comment: 23 pages, 14 figures, accepted to ApJ, for higher resolution images of Figs 4,11, and 12 see http://www.pa.uky.edu/~brogan/brog_publ.htm

Orion\u27s Veil: Magnetic Field Strengths and Other Properties of a PDR in Front of the Trapezium Cluster

We present an analysis of physical conditions in the Orion Veil, an atomic photon-dominated region (PDR) that lies just in front (≈2 pc) of the Trapezium stars of Orion. This region offers an unusual opportunity to study the properties of PDRs, including the magnetic field. We have obtained 21 cm H i and 18 cm (1665 and 1667 MHz) OH Zeeman effect data that yield images of the line-of-sight magnetic field strength B los in atomic and molecular regions of the Veil. We find B los ≈ −50 to −75 μG in the atomic gas across much of the Veil (25\u27\u27 resolution) and B los ≈ −350 μG at one position in the molecular gas (40\u27\u27 resolution). The Veil has two principal H i velocity components. Magnetic and kinematical data suggest a close connection between these components. They may represent gas on either side of a shock wave preceding a weak-D ionization front. Magnetic fields in the Veil H i components are 3–5 times stronger than they are elsewhere in the interstellar medium where N(H) and n(H) are comparable. The H i components are magnetically subcritical (magnetically dominated), like the cold neutral medium, although they are about 1 dex denser. Comparatively strong fields in the Veil H i components may have resulted from low-turbulence conditions in the diffuse gas that gave rise to OMC-1. Strong fields may also be related to magnetostatic equilibrium that has developed in the Veil since star formation. We also consider the location of the Orion-S molecular core, proposing a location behind the main Orion H+ region

Very large array H53α and H92α line observations of the central region of NGC 253

We present new VLA observations toward NGC 253 of the recombination line H53&#945; (43 GHz) at an angular resolution of 1".5 &#215; 1".0. The free-free emission at 43 GHz is estimated to be ~140 mJy, implying a star formation rate of 2 M&#X02299; yr-1 in the nuclear region of this starburst galaxy. A reanalysis is made for previously reported H92&#945; observations carried out with angular resolution of 1".5 &#215; 1".0 and 0".36 &#215; 0".21. Based on the line and continuum emission models used for the 1".5 &#215; 1".0 angular resolution observations, the RRLs H53&#945; and H92&#945; are tracers of the high-density (~105 cm-3) and low-density (~103 cm-3) thermally ionized gas components in NGC 253, respectively. The velocity fields observed in the H53&#945; and H92&#945; lines (1".5 &#215; 1".0) are consistent. The velocity gradient in the central ~18 pc of the NE component, as observed in both the H53&#945; and H92&#945; lines, is in the opposite direction to the velocity gradient determined from the CO observations. The enclosed virial mass, as deduced from the H53&#945; velocity gradient over the NE component, is ~5 &#215; 106 M&#x02299; in the central ~18 pc region. The H92&#945; line observations at high angular resolution (0".36 &#215; 0".21) reveal a larger velocity gradient, along a P.A. ~ -45&#176; on the NE component, of ~110 km s-1 arcsec-1. The dynamical mass estimated using the high angular resolution H92&#945; data (~7 &#215; 106 M&#x02299;) supports the existence of an accreted massive object in the nuclear region of NGC 253

Very large array H92α and H53α radio recombination line observations of M82

We present high angular resolution (0".6) observations made with the Very Large Array of the radio continuum at 8.3 and 43 GHz, as well as H92&#945; and H53&#945; radio recombination lines from the nearby (~3 Mpc) starburst galaxy M82. In the continuum we identify 58 sources at 8.3 GHz, of which 19 have no counterparts in catalogs published at other frequencies. At 43 GHz we identify 18 sources, unresolved at 0".6 resolution, of which five were unknown previously. The spatial distribution of the H92&#945; line is inhomogeneous; we identify 27 features; about half of them are associated with continuum emission sources. Their sizes are typically in the range 2-10 pc. Although observed with poorer signal-to-noise ratio, the H53&#945; line is detected. The line and continuum emission are modeled using a collection of H II regions at different distances from the nucleus. The observations can be interpreted assuming a single-density component, but equally well with two components if constraints originating from previous high-resolution continuum observations are used. The high-density component has a density of ~4&#215; 104 cm-3. However, the bulk of the ionization is in regions with densities that are typically a factor of 10 lower. The gas kinematics, using the H92&#945; line, confirms the presence of steep velocity gradient (26 km s-1 arcsec-1) in the nuclear region, as previously reported, in particular from observations of the [Ne II] line at 12 &#181;m. This gradient has about the same amplitude on both sides of the nucleus. Since this steep gradient is observed not only on the major axis but also at large distances along a band at P.A. ~ 150&#176;, the interpretation in terms of x2 orbits elongated along the minor axis of the bar, which would be observed at an angle close to the inclination of the main disk, seems inadequate. The observed kinematics cannot be modeled using a simple model that consists of a set of circular orbits observed at different tilt angles. Ad hoc radial motions must be introduced to reproduce the pattern of the velocity field. Different families of orbits are indicated since we detect a signature in the kinematics at the transition between the two plateaus observed in the NIR light distribution. These H92&#945; data also reveal the base of the outflow where the injection toward the halo on the northern side occurs. The outflow has a major effect on the observed kinematics, present even in the disk at distances close to the nucleus. The kinematic pattern suggests a connection between the gas flowing in the plane of M82 toward the center; this behavior most likely is due to the presence of a bar and the outflow out of the plane

Deep Radio Imaging of Globular Clusters and the Cluster Pulsar Population

We have obtained deep multifrequency radio observations of seven globular clusters using the Very Large Array and the Australia Telescope Compact Array. Five of these, NGC 6440, NGC 6539, NGC 6544, NGC 6624 and Terzan 5 had previously been detected in a shallower survey for steep spectrum radio sources in globular clusters (Fruchter and Goss 1990). The sixth, the rich globular cluster, Liller 1, had heretofore been undetected in the radio, and the seventh, 47 Tucanae, was not included in our original survey. High resolution 6 and 20 cm images of three of the clusters, NGC 6440, NGC 6539, NGC 6624 reveal only point sources coincident with pulsars which have been discovered subsequent to our first imaging survey. 21 and 18 cm images reveal several point sources within a few core-radii of the center of 47 Tuc. Two of these are identified pulsars, and a third, which is both variable and has a steep spectrum, is also most likely a pulsar previously identified by a pulsed survey. However, the 6, 20 and 90 cm images of NGC 6544, Liller 1 and Terzan 5 display strong steep-spectrum emission which cannot be associated with known pulsars. The image of the rich cluster Terzan 5 displays numerous point sources within $30''$, or 4 core radii of the cluster center. The density of these objects rises rapidly toward the core, where an elongated region of emission is found. The brightest individual sources, as well as the extended emission, possess the steep spectra expected of pulsars. Furthermore, the flux distribution of the sources agrees well with the standard pulsar luminosity function. The total luminosity and number of objects observed suggest that Terzan 5 contains more pulsars than any other Galactic globular cluster.Comment: 33 pages, 6 Postscript figures; Accepted for publication in the Astrophysical Journal; abstract abridged. PDF version also available at http://nemesis.stsci.edu/~fruchter/fg99/fg99.pd