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Register a new user36 GHz methanol lines from nearby galaxies: maser or quasi-thermal emission?
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Methanol (CH3OH) is one of the most abundant interstellar molecules, offering a vast number of transitions to be studied, including many maser lines. While the strongest Galactic CH3OH lines, the so-called class II masers, show no indications for the presence of superluminous counterparts in external galaxies, the less luminous Galactic class I sources appear to be different. Here we report class I 36GHz CH3OH 4(-1) - 3(0) E line emission from the nearby galaxies Maffei2 and IC342, measured with the 100-m telescope at Effelsberg at three different epochs within a time span of about five weeks. The 36GHz methanol line of Maffei2 is the second most luminous among the sources detected with certainty outside the Local Group of galaxies. This is not matched by the moderate infrared luminosity of Maffei2. Higher resolution data are required to check whether this is related to its prominent bar and associated shocks. Upper limits for M82, NGC4388, NGC5728 and Arp220 are also presented. The previously reported detection of 36GHz maser emission in Arp220 is not confirmed. Non-detections are reported from the related class I 44GHz methanol transition towards Maffei2 and IC342, indicating that this line is not stronger than its 36GHz counterpart. In contrast to the previously detected 36GHz CH3OH emission in NGC253 and NGC4945, our 36GHz profiles towards Maffei2 and IC342 are similar to those of previously detected non-masing lines from other molecular species. However, by analogy to our Galactic center region, it may well be possible that the 36GHz methanol lines in Maffei~2 and IC~342 are composed of a large number of faint and narrow maser features that remain spatially unresolved. In view of this, a search for a weak broad 36GHz line component would also be desirable in NGC253 and NGC4945.
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We have used the Australia Telescope Compact Array (ATCA) to search for emission from the $4_{-1} rightarrow 3_{0}E$ transition of methanol (36.2 GHz) towards the center of the nearby starburst galaxy NGC253. Two regions of emission were detected, offset from the nucleus along the same position angle as the inner spiral arms. The emission is largely unresolved on a scale of 5 arcsec, has a full-width half maximum (FWHM) line width of < 30 km s$^{-1}$, and an isotropic luminosity orders of magnitude larger than that observed in any Galactic star formation regions. These characteristics suggest that the 36.2 GHz methanol emission is most likely a maser, although observations with higher angular and spectral resolution are required to confirm this. If it is a maser this represents the first detection of a class I methanol maser outside the Milky Way. The 36.2 GHz methanol emission in NGC253 has more than an order of magnitude higher isotropic luminosity than the widespread emission recently detected towards the center of the Milky Way. If emission from this transition scales with nuclear star formation rate then it may be detectable in the central regions of many starburst galaxies. Detection of methanol emission in ultra-luminous infra-red galaxies (ULIRGs) would open up a new tool for testing for variations in fundamental constants (in particular the proton-to-electron mass ratio) on cosmological scales.
We report the results of a search for class II methanol masers at 37.7, 38.3 and 38.5 GHz towards a sample of 70 high-mass star formation regions. We primarily searched towards regions known to show emission either from the 107 GHz class II methanol maser transition, or from the 6.035 GHz excited OH transition. We detected maser emission from 13 sources in the 37.7 GHz transition, eight of these being new detections. We detected maser emission from three sources in the 38 GHz transitions, one of which is a new detection. We find that 37.7 GHz methanol masers are only associated with the most luminous 6.7 and 12.2 GHz methanol maser sources, which in turn are hypothesised to be the oldest class II methanol sources. We suggest that the 37.7 GHz methanol masers are associated with a brief evolutionary phase (of 1000-4000 years) prior to the cessation of class II methanol maser activity in the associated high-mass star formation region.
22 GHz water and 6.7 GHz methanol masers are usually thought as signposts of early stages of high-mass star formation but little is known about their associations and the physical environments they occur in. The aim was to obtain accurate positions and morphologies of the water maser emission and relate them to the methanol maser emission recently mapped with Very Long Baseline Interferometry. A sample of 31 methanol maser sources was searched for 22 GHz water masers using the VLA and observed in the 6.7 GHz methanol maser line with the 32 m Torun dish simultaneously. Water maser clusters were detected towards 27 sites finding 15 new sources. The detection rate of water maser emission associated with methanol sources was as high as 71%. In a large number of objects (18/21) the structure of water maser is well aligned with that of the extended emission at 4.5 $mu$m confirming the origin of water emission from outflows. The sources with methanol emission with ring-like morphologies, which likely trace a circumstellar disk/torus, either do not show associated water masers or the distribution of water maser spots is orthogonal to the major axis of the ring. The two maser species are generally powered by the same high-mass young stellar object but probe different parts of its environment. The morphology of water and methanol maser emission in a minority of sources is consistent with a scenario that 6.7 GHz methanol masers trace a disc/torus around a protostar while the associated 22 GHz water masers arise in outflows. The majority of sources in which methanol maser emission is associated with the water maser appears to trace outflows. The two types of associations might be related to different evolutionary phases.
The Central Molecular Zone (CMZ) spans the inner ~450 pc (3 degrees) of our Galaxy. This region is defined by its enhanced molecular emission and contains 5% of the entire Galaxys molecular gas mass. However, the number of detected star forming sites towards the CMZ may be low for the amount of molecular gas that is present, and improved surveys of star formation indicators can help clarify this. With the Karl G Jansky Very Large Array (VLA), we conducted a blind survey of 6.7 GHz methanol masers spanning the inner 3deg x 40arcmin (450 pc x 100 pc) of the Galaxy. We detected 43 methanol masers towards 28 locations, 16 of which are new detections. The velocities of most of these masers are consistent with being located within the CMZ. A majority of the detected methanol masers are distributed towards positive Galactic longitudes, similar to 2/3 of the molecular gas mass distributed at positive Galactic longitudes. The 6.7 GHz methanol maser is an excellent indicator of high mass (>8 solar mass) star formation, with new detections indicating sites of massive star formation in the CMZ.
Class I methanol masers are believed to be produced in the shock-excited environment around star-forming regions. Many authors have argued that the appearance of various subsets of class I masers may be indicative of specific evolutionary stages of star formation or excitation conditions. Until recently, however, no major interferometer was capable of imaging the important 36 GHz transition. We report on Expanded Very Large Array observations of the 36 GHz methanol masers and Submillimeter Array observations of the 229 GHz methanol masers in DR21(OH), DR21N, and DR21W. The distribution of 36 GHz masers in the outflow of DR21(OH) is similar to that of the other class I methanol transitions, with numerous multitransition spatial overlaps. At the site of the main continuum source in DR21(OH), class I masers at 36 and 229 GHz are found in virtual overlap with class II 6.7 GHz masers. To the south of the outflow, the 36 GHz masers are scattered over a large region but usually do not appear coincident with 44 GHz masers. In DR21W we detect an S-curve signature in Stokes V that implies a large value of the magnetic field strength if interpreted as due to Zeeman splitting, suggesting either that class I masers may exist at higher densities than previously believed or that the direct Zeeman interpretation of S-curve Stokes V profiles in class I masers may be incorrect. We find a diverse variety of different maser phenomena in these sources, suggestive of differing physical conditions among them.
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