Galactic OH Research Articles

Detection of an OH 1665 MHz Maser in M33

We report one OH 1665-MHz detection in new observations with the NSF's Karl G. Jansky Very Large Array. To the authors' knowledge, this is the first OH maser detection in M33. The source occurs at a position of $\alpha=23.50069(9)^{\circ}$ and $\delta = 30.67984(9)^{\circ}$ (J2000) and is unresolved at the beam size ($9.7$). Two H$\alpha$ sources from the Moody et al. (2017) catalogue --- C1-1 and C1-2 --- are consistent within the beam size. We find a peak flux density of 24.1 mJy in a 1.5 km s$^{-1}$ channel. At the distance of M33, the source's specific luminosity is consistent with the brightest Galactic OH 1665-MHz masers.

Spatially resolving the OH masers in M82

AbstractWith luminosities between those of typical Galactic OH masers and more distant OH megamasers, the masers in the nearby galaxy M82 are an interesting population which can be used to probe the physical conditions in the central starburst region of this irregular galaxy. Following on from previous low spatial resolution studies, here we present the initial results of two high-resolution observations separated by eight years. We find that some of the maser spots are resolved into multiple spatial components when observed with the EVN, as predicted by our previous studies, but that significantly less flux is recovered that that seen with the previous VLA observations. We conclude that some of this flux difference is likely due to variability but that, in common with the results seen in Arp220, there may also be a significant diffuse component.

AN ARECIBO SURVEY FOR ZEEMAN SPLITTING IN OH MEGAMASER GALAXIES

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.

Very Large Array Observations of Galactic Center OH 1720 MHz Masers in Sagittarius A East and in the Circumnuclear Disk

We present Very Large Array (VLA) radio interferometry observations of the 1720 MHz OH masers in the Galactic Center (GC). Most 1720 MHz OH masers arise in regions where the supernova remnant Sgr A East is interacting with the interstellar medium. The majority of the newly found 1720 MHz OH masers are located to the northeast, independently indicating and confirming an area of shock interaction with the +50 km/s molecular cloud (M-0.02-0.07) on the far side of Sgr A East. The previously known bright masers in the southeast are suggested to be the result of the interaction between two supernova remnants, instead of between Sgr A East and the surrounding molecular clouds as generally found elsewhere in the Galaxy. Together with masers north of the circumnuclear disk (CND) they outline an interaction on the near side of Sgr A East. In contrast to the interaction between the +50 km/s cloud and Sgr A East, OH absorption data do not support a direct interaction between the CND material and Sgr A East. We also present three new high-negative velocity masers, supporting a previous single detection. The location and velocities of the high-negative and high-positive velocity masers are consistent with being near the tangent points of, and physically located in the CND. We argue that the high velocity masers in the CND are pumped by dissipation between density clumps in the CND instead of a shock generated by the supernova remnant. That is, the CND masers are not coupled to the supernova remnant and are sustained independently.

The Effect of 53 μm IR Radiation on 18 cm OH Megamaser Emission

OH megamasers (OHMs) emit primarily in the main lines at 1667 and 1665 MHz and differ from their Galactic counterparts due to their immense luminosities, large line widths, and 1667/1665 MHz flux ratios, which are always greater than 1. We find that these maser properties result from strong 53 μm radiative pumping combined with line overlap effects caused by turbulent line widths ~20 km s−1; pumping calculations that do not include line overlap are unreliable. A minimum dust temperature of ~45 K is needed for inversion, and maximum maser efficiency occurs for dust temperatures ~80-140 K. We find that warmer dust can support inversion at lower IR luminosities, in agreement with observations. Our results are in good agreement with a clumpy model of OHMs, with clouds sizes 1 pc and OH column densities ~5 × 1016 cm−2, that is able to explain both the diffuse and compact emission observed for OHMs. We suggest that all OH main-line masers may be pumped by far-IR radiation, with the major differences between OHMs and Galactic OH masers caused by differences in line width produced by line overlap. Small Galactic maser line widths tend to produce stronger 1665 MHz emission. The large OHM line widths lead to inverted ground-state transitions having approximately the same excitation temperature, producing 1667/1665 MHz flux ratios greater than 1 and weak satellite line emission. Finally, the small observed ratio of pumping radiation to dense molecular gas, as traced by HCN and HCO+, is a possible reason for the lack of OH megamaser emission in NGC 6240.

Extragalactic Zeeman Detections in OH Megamasers

We have measured the Zeeman splitting of OH megamaser emission at 1667 MHz from five (ultra)luminous infrared galaxies ([U]LIRGs) using the 305 m Arecibo telescope and the 100 m Green Bank Telescope. Five of eight targeted galaxies show significant Zeeman-splitting detections, with 14 individual masing components detected and line-of-sight magnetic field strengths ranging from ~0.5-18 mG. The detected field strengths are similar to those measured in Galactic OH masers, suggesting that the local process of massive star formation occurs under similar conditions in (U)LIRGs and the Galaxy, in spite of the vastly different large-scale environments. Our measured field strengths are also similar to magnetic field strengths in (U)LIRGs inferred from synchrotron observations, implying that milligauss magnetic fields likely pervade most phases of the interstellar medium in (U)LIRGs. These results provide a promising new tool for probing the astrophysics of distant galaxies.

Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo

(Abridged) We investigate the hypothesis that some fraction of the globular clusters presently observed in the Galactic halo formed in external dwarf galaxies, by means of a detailed comparison between Zinn's halo, young halo, and bulge/disk sub-systems and the globular clusters in local dwarf galaxies. The Galactic sub-systems possess distinct spatial and age distributions, age-metallicity relationships, and orbital parameters, although we observe some OH clusters with ages and orbits more similar to those of YH objects. Almost all Galactic globulars with large core radii (r_c) fall into the YH sub-system, while the OH and BD ensembles are characterized by compact clusters. The majority of the external clusters are indistinguishable from the Galactic YH objects in terms of HB morphology, but 20-30% of external clusters have HB morphologies most similar to the Galactic OH clusters. In addition, the external clusters have an r_c distribution which very closely matches that for the YH objects. The old halo r_c distribution can be very well represented by a composite distribution formed from 83-85% of objects with structures typical of BD clusters, and 15-17% of objects with structures typical of external clusters. Taken together our results fully support the accretion hypothesis. We conclude that all 30 YH clusters and 15-17% of the OH clusters (10-12 objects) are of external origin. The Galaxy may therefore have experienced ~7 merger events with cluster-bearing dSph-type galaxies during its lifetime, building up 45-50% of the mass of the stellar halo. Finally, we identify a number of OH objects which have properties characteristic of accreted clusters. Several of the clusters associated with the recently proposed CMa dwarf fall into this category.

A survey of SiO emission towards interstellar masers

We have conducted a survey of SiO emission towards galactic H2O and OH masers and ultracompact HII regions using the 15-m SEST and the 20-m Onsala telescope. With the SEST the transitions (, ) and (,) of SiO at 3 and 2 mm were measured simultaneously. With Onsala only the (,) line was accessible. Altogether 369 objects were observed and SiO was detected towards 137 of them. The detection rate is highest towards the most intense H2O masers, which probably require powerful shocks to be excited. The SiO detection rate correlates also with the integrated far-infrared flux density and the FIR luminosity of the associated IRAS point source, indicating that the occurrence of shocks is related to the amount of radiation from the central stellar source(s). For flux and luminosity limited samples the SiO detection rate is higher in the inner 7 kpc from the galactic centre than elsewhere. This suggests that dense cores belonging to the so called “molecular ring" provide particularly favourable conditions for the production of gaseous SiO. The full widths above of the SiO() lines, which are likely to be related to the associated shock velocities, range from 2 to 60 kms-1 except for the line in Ori KL which has a full width of about 100 kms-1. The median of our sample is 19 kms-1. The SiO lines are single-peaked and the peak velocities are always close to the ambient cloud velocity as determined from published CS observations. These line characteristics are compared with the predictions of kinematical bow-shock models. The SiO line shapes correspond with the model of [Raga & Cabrit (1993)] where the emission arises from turbulent wakes behind bow-shocks. However, the number of symmetric, relatively narrow profiles indicates that at least in some of the observed sources SiO emission arises also from the quiescent gas component. We suggest that this is due to evaporation of silicon compounds from grain mantles and their reprocessing to SiO in dense quiescent gas according to the model of [McKay (1995, 1996).] These reactions may be initiated and sustained by ionizing radiation from shocks, in the same way as the enhancement of HCO+ near Herbig-Haro objects has been explained in the model of [Wolfire & Königl (1993).] The excitation temperatures of SiO() and () transitions were determined towards three strong sources using measurements in isotopically substituted SiO. In all three sources the transitions are clearly subthermally excited, implying moderate densities ( cm-3) in the SiO emission regions.

Detection of Formula-methanol masers towards southern Galactic OH masers

A search for 6.668-GHz A+-methanol masers toward all Galactic star-forming regions lying in the longitude range 260○ to 359○ with OH masers and no known methanol masers has been completed. Forty new A-methanol masers have been detected above a 3σ detection limit of 5 Jy. This brings the total number now detected to 143.

On the nature of OH/IR stars

OH/IR stars are the infrared counterparts of galactic OH maser sources which show a characteristic double-peaked emission-line profile. Their strong radio emission can be detected at large distances, making them excellent tracers of distribution and kinematics of evolved stars in the Milky Way. The OH maser profile is typical for line emission from an expanding circumstellar shell. The circumstellar shells of OH/IR stars absorb the optical emission of the central star nearly completely and reemit the energy in the infrared. Having luminosities ~ 105 L⊙ and energy distributions peaking around 10μm, they may make a major contribution to the interstellar radiation field beyond 5μm. With mass loss rates of 10-5 to 10-4 M⊙/yr they lose several solar masses in a few hundred thousand years. OH/IR stars are therefore important objects for recycling stellar matter into the interstellar medium.Progress has been made in understanding the nature of OH/IR stars. They are Mira-like large-amplitude variables with periods up to 5 years long. It is proposed that they are stars of intermediate mass (2–10 M⊙) on the asymptotic giant branch (AGB). They have not only larger masses than Mira variables proper, but also longer periods of pulsation and larger mass loss rates. As a result optically thick circumstellar dust shells are formed, which prevent the detection of these more massive Mira-like variables at optical wavelengths. Radial pulsation (Mira variability) is thus thought to occur for all intermediate-mass stars in the course of their evolution on the AGB. In view of their high mass-loss rates, these stars may be key objects in the study of the formation of planetary nebulae.

Widespread galactic OH emission at 1720 MHz - A new tracer of spiral arms

Widely extended, weak OH emission has been observed at 1720 MHz in 53 galactic clouds whose sizes are those of giant molecular clouds and whose distribution appears confined to two inner spiral arms. Collision pumping of the OH at low (15--45 K) temperatures seems the most likely excitation mechanism.

Maser radiometer observations of water vapor and OH in weak galactic OH sources

view Abstract Citations (34) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Maser radiometer observations of water vapor and OH in weak galactic OH sources. Yngvesson, K. S. ; Cardiasmenos, A. G. ; Shanley, J. F. ; Rydbeck, O. E. H. ; Ellder, J. Abstract Observations of water vapor and ground-state hydroxyl lines in weak galactic OH sources were performed with maser-equipped radio telescopes at the NEROC Haystack Observatory and Onsala Space Observatory, Sweden, respectively. In general, the weak sources exhibit simpler spectra than the strong sources, and detailed correlation studies are facilitated. New water vapor sources have been detected in W37, W43 N, NGC 2438, and ON-3. W12 and W37 are examples of sources in which H2O masses while OH is seen only in absorption. In other sources, e.g., W31 and W43 N, the velocities of H2O and OH correlate well. Interferometer measurements of the positions of these fairly weak H2O sources are required for further correlation between OH and H2O. Publication: The Astrophysical Journal Pub Date: January 1975 DOI: 10.1086/153307 Bibcode: 1975ApJ...195...91Y Keywords: Galactic Radiation; Hydroxyl Emission; Line Spectra; Spectrum Analysis; Water Vapor; Absorption Spectra; Masers; Microwave Radiometers; Nebulae; Radiometers; Ruby Lasers; Astrophysics full text sources ADS | data products SIMBAD (1)

Very Long Baseline Interferometry of Galactic OH Sources

Several hydroxyl radical (OH) microwave emission sources were studied in July 1969 with very long baseline interferometry (VLBI). The elements of the interferometer were the 84‐foot telescope of the Onsala Space Observatory at Chalmers University of Technology, Sweden, and the 120‐foot telescope of the Haystack Microwave Research Facility at Lincoln Laboratory, MIT; a baseline of 31.1×106 wavelengths, or a fringe spacing of 0.0066 sec of arc at a frequency of 1665 MHz, resulted. Earlier spectral line interferometric observations have given detailed information on the angular sizes and spatial separations of the 1665‐MHz OH sources in the W3 source. The measurements reported here confirm these results and give additional information on the complex structure of the various features. Four other galactic OH emission sources with unknown angular sizes were also investigated. Only one (the 5‐km/sec component in the 1667‐MHz spectrum of W49) gave reliable fringes.

18 cm Observations of Galactic OH from Longitudes 128° to 300°

Observations have been made of 18 cm OR emission and/or absorption for 12 sources between galactic longitudes 128� and 300�. The OR is seen in emission in five of the sources, and the four Stokes parameters of the emission profiles have been measured. A new type of strong wideband OR emission has been found near III = 284�. There are two sources where weak emission at 1612 MHz accompanies absorption on the other three lines.

The hydroxyl molecule in interstellar space

A historical introduction and an outline of the radio frequency spectroscopy of the OH molecule is followed by a description of radio frequency absorption by galactic OH and of radio emission from OH in dust clouds. The outstanding features of anomalous radio emission from sources associated with H II regions are described and an account of the interpretation in terms of stimulated emission is given, emphasis being laid on a phenomenological description of the observations. Finally the implications of the observations for the dynamics and constitutions of interstellar gas and for the process going on within it, are considered and the extent of our ignorance of the properties of the OH molecule and the further need for laboratory investigation are indicated.

Pumping Mechanisms for Maser Action in Galactic OH

VARIOUS pumping mechanisms depending on the absorption of radiation have been proposed as competent to invert the Λ-doublet of the 2II3/2 ground state of the OH molecule and so to explain the origin of anomalous 18 cm emission from galactic OH as stimulated emission. I argue here that the available power of radiation is likely to be too small for the purpose.

Observations of Galactic OH Emission

view Abstract Citations (24) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of Galactic OH Emission Ball, J. A. ; Meeks, M. L. Abstract The 120-foot Haystack antenna of Massachussetts Institute of Technology Lincoln Laboratory was used to observe polarized OH emission in the galactic sources W49, NGC 6334, W51, and W75 Both the 1665 and 1667 MHz transitions were observed in W49 and NGC 6334, and oniy the 1665 MHz transition was observed in W51 and W75 The Stokes parameters were determined as functions of radial velocity, and the resulting spectra were decomposed into a set of independent, uniformly polarized features with Gaussian profiles. Half of the fifty-one observed features have a degree of polarization »= 90 per cent. The polarization was circular in 70 per cent of the features Two bright, unpolarized features appear in the spectra of W51, and one feature in NGC 6334 appears to have nearly 100 per cent linear polarization. Observations of NGC 6334 at irregular intervals from March 1966 through June 1967 disclosed intrinsic time variations in the more southern of a pair of OH source points in this region. Not all features showed variations, but two features with radial velocities -6 6 and 7.9 km sec1 (1665 MHz) and one feature at -8 7 km sec' (1667 MHz) varied significantly in intervals of 9 days or longer. The time variations in the 1665 MHz features appear to be correlated. There is little apparent correlation in the radial velocities of 1665 and 1667 Mhz features originating at the same position in W49 and NGC 6334. Of the four cases where such near-coincidence is found, three cases show the same sense of circular polarization in both transitions Publication: The Astrophysical Journal Pub Date: August 1968 DOI: 10.1086/149688 Bibcode: 1968ApJ...153..577B full text sources ADS |

Observations of Galactic OH

Observations of Galactic OH

Brightness and Temporal Variation of Radio Emission from Galactic OH

THE recent interferometric observations of the angular sizes of OH radio sources1–3, which imply minimum brightness temperatures of between 106 and 1011 °K, remove any theoretical difficulty in accepting the temporal variations of intensity that the radio astronomy group at Berkeley claim to have detected4. The time constant for a variation of intensity may, in the absence of a rigorous solution of the equations of transfer and population densities for even a simple model, be estimated in two ways that lead to the same result. A simplified equation of transfer for any one of the four radio frequency lines is where i denotes the upper level of the Λ-doublet as split by hyperfine interaction, j denotes the lower level, Iij is the intensity of the radiation, vij is the difference of the number of densities in the two levels, c is the velocity of light and (Aij is the Einstein coefficient for spontaneous emission and δv the width of the line.) This equation of transfer incorporates a number of approximations which seem entirely justified in view of the very high intensity of the radiation, namely, that the rates of spontaneous emission and of non-radiative creation and destruction of excited states can be ignored in comparison with the rate of stimulated emission, and that the radiation is propagating in one direction only. The four equations for the four radio frequency lines are coupled through four simultaneous equations for the rates of change of the populations. It has not so far been possible to solve the eight equations except for very artificial cases. If it can be assumed that there are circumstances in which ∂Iij/∂x is dominated by the other two terms in the equation of transfer, the time constant τ would be

Determination of Direction, Frequency and Polarization of Radio Emission from Galactic OH

A MAJOR problem in the study of galactic radio emission from OH is to understand how it is that the emissions appear to be localized in very small bright regions each characterized by a particular frequency1; in addition, it has been found that emissions can show both linear and circular polarization2,3. The localization of emission at a particular frequency can readily be explained on the hypothesis of stimulated emission4 if there are regions in which the velocities of the OH molecules are correlated over large distances. Should the velocities vary considerably along a line of sight, so will the Doppler shifts relative to some local standard and stimulated emission will not occur because the optical depth at any particular frequency would be inadequate. For a sufficient optical depth, the velocities along the line of sight must be correlated over a distance of about 1019 cm. In the absence of shifts of wave-length from any other cause, the velocity must be constant along the line of sight over such a distance. Only in those directions along which there is such a systematic motion will intense emission be observed. The frequencies of peaks in observed spectra indicate that velocities in different parts of a source may differ by 100 km/s (ref. 1).