We present the first very-long-baseline interferometry (VLBI) detections of Zeeman splitting in another galaxy. We used Arecibo Observatory, the Green Bank Telescope, and the Very Long Baseline Array to perform dual-polarization observations of OH ma
ser lines in the merging galaxy Arp 220. We measured magnetic fields of $sim$1-5 mG associated with three roughly parsec-sized clouds in the nuclear regions of Arp 220. Our measured magnetic fields have comparable strengths and the same direction as features at the same velocity identified in previous Zeeman observations with Arecibo alone. The agreement between single dish and VLBI results provides critical validation of previous Zeeman splitting observations of OH megamasers that used a single large dish. The measured magnetic field strengths indicate that magnetic energy densities are comparable to gravitational energy in OH maser clouds. We also compare our total intensity results to previously published VLBI observations of OH megamasers in Arp 220. We find evidence for changes in both structure and amplitude of the OH maser lines that are most easily explained by variability intrinsic to the masing region, rather than variability produced by interstellar scintillation. Our results demonstrate the potential for using high-sensitivity VLBI to study magnetic fields on small spatial scales in extragalactic systems.
We present estimates of magnetic field strengths in the interstellar media of starburst galaxies derived from measurements of Zeeman splitting associated with OH megamasers. The results for eight galaxies with Zeeman detections suggest that the magne
tic energy density in the interstellar medium of starburst galaxies is comparable to their hydrostatic gas pressure, as in the Milky Way. We discuss the significant uncertainties in this conclusion, and possible measurements that could reduce these uncertainties. We also compare the Zeeman splitting derived magnetic field estimates to magnetic field strengths estimated using synchrotron fluxes and assuming that the magnetic field and cosmic rays have comparable energy densities, known as the minimum energy argument. We find that the minimum energy argument systematically underestimates magnetic fields in starburst galaxies, and that the conditions that would be required to produce agreement between the minimum energy estimate and the Zeeman derived estimate of interstellar medium magnetic fields are implausible. The conclusion that magnetic fields in starburst galaxies exceed the minimum energy magnetic fields is consistent with starburst galaxies adhering to the linearity of the FIR-radio correlation.
We report the results of a full-Stokes survey of all four 18 cm OH lines in 77 OH megamasers (OHMs) using the Arecibo Observatory. This is the first survey of OHMs that included observations of the OH satellite lines; only 4 of the 77 OHMs have exist
ing satellite line observations in the literature. In 5 sources, satellite line emission is detected, with 3 of the 5 sources re-detections of previously published sources. The 2 sources with new detections of satellite line emission are IRAS F10173+0829, which was detected at 1720 MHz, and IRAS F15107+0724, for which both the 1612 MHz and 1720 MHz lines were detected. In IRAS F15107+0724, the satellite lines are partially conjugate, as 1720 MHz absorption and 1612 MHz emission have the same structure at some velocities within the source, along with additional broader 1612 MHz emission. This is the first observed example of conjugate satellite lines in an OHM. In the remaining sources, no satellite line emission is observed. The detections and upper limits are generally consistent with models of OHM emission in which all of the 18 cm OH lines have the same excitation temperature. There is no evidence for a significant population of strong satellite line emitters among OHMs.
We present the results of a comprehensive survey using the Arecibo Observatory for Zeeman splitting of OH lines in OH megamasers (OHMs). A total of seventy-seven sources were observed with the Arecibo telescope. Of these, maser emission could not be
detected for eight sources, and two sources were only ambiguously detected. Another twenty-seven sources were detected at low signal-to-noise ratios or with interference that prevented placing any useful limits on the presence of magnetic fields. In twenty-six sources, it was possible to place upper limits on the magnitude of magnetic fields, typically between 10-30 mG. For fourteen sources, the Stokes V spectra exhibit features consistent with Zeeman splitting. Eleven of these fourteen are new detections, and the remaining three are re-detections of Stokes V detections in Robishaw et al. (2008). Among confident new detections, we derive magnetic fields associated with maser regions with magnitudes ranging from 6.1-27.6 mG. The distribution of magnetic field strengths suggests the magnetic fields in OH masing clouds in OHMs are larger than those in Galactic OH masers. The results are consistent with magnetic fields playing a dynamically important role in OH masing clouds in OHMs.
Models that reproduce the observed high-velocity clouds (HVCs) also predict clouds at lower radial velocities that may easily be confused with Galactic disk (|z| < 1 kpc) gas. We describe the first search for these low-velocity halo clouds (LVHCs) us
ing IRAS data and the initial data from the Galactic Arecibo L-band Feed Array survey in HI (GALFA-HI). The technique is based upon the expectation that such clouds should, like HVCs, have very limited infrared thermal dust emission as compared to their HI column density. We describe our displacement-map technique for robustly determining the dust-to-gas ratio of clouds and the associated errors that takes into account the significant scatter in the infrared flux from the Galactic disk gas. We find that there exist lower-velocity clouds that have extremely low dust-to-gas ratios, consistent with being in the Galactic halo - candidate LVHCs. We also confirm the lack of dust in many HVCs with the notable exception of complex M, which we consider to be the first detection of warm dust in HVCs. We do not confirm the previously reported detection of dust in complex C. In addition, we find that most Intermediate- and Low-Velocity clouds that are part of the Galactic disk have a higher 60 micron/100 micron flux ratio than is typically seen in Galactic HI, which is consistent with a previously proposed picture in which fast-moving Galactic clouds have smaller, hotter dust grains.
We measure the Galactic rotation curve and its first two vertical derivatives in the first and fourth quadrants of the Milky Way using the 21 cm VGPS and SGPS. We find tangent velocities of the atomic gas as a function of galactic longitude and latit
ude by fitting an analytic line profile to the edges of the velocity profiles. The shape of the analytic profile depends only on the tangent velocity and the velocity dispersion of the gas. We use two complementary methods to analyze the tangent velocities: a global model to fit typical parameter values and a local fitting routine to examine spatial variations. We confirm the validity of our fitting routines by testing simple models. Both the global and local fits are consistent with a vertical falloff in the rotation curve of -22 +/- 6 km/s/kpc within 100 pc of the Galactic midplane. The magnitude of the falloff is several times larger than what would be expected from the change in the potential alone, indicating some other physical process is important. The falloff we measure is consistent in magnitude with that measured in the halo gas of other galaxies.
We present results from neutral hydrogen (HI) observations of the tip of the Magellanic Stream (MS), obtained with the Arecibo telescope as a part of the on-going survey by the Consortium for Galactic studies with the Arecibo L-band Feed Array. We fi
nd four large-scale, coherent HI streams, extending continously over a length of 20 degrees, each stream possessing different morphology and velocity gradients. The newly discovered streams provide strong support for the tidal model of the MS formation by Connors et al. (2006), which suggested a spatial and kinematic bifurcation of the MS. The observed morphology and kinematics suggest that three of these streams could be interpreted as a 3-way splitting of the main MS filament, while the fourth stream appears much younger and may have originated from the Magellanic Bridge. We find an extensive population of HI clouds at the tip of the MS. Two thirds of clouds have an angular size in the range 3.5--10. We interpret this as being due to thermal instability, which would affect a warm tail of gas trailing through the Galactic halo over a characteristic timescale of a few Myrs to a few hundred Myrs. We show that thermal fragments can survive in the hot halo for a long time, especially if surrounded by a <10^6 K halo gas. If the observed clumpy structure is mainly due to thermal instability, then the tip of the MS is at a distance of ~70 kpc. A significant fraction of HI clouds at the tip of the MS show multi-phase velocity profiles, indicating the co-existence of cooler and warmer gas.
