Astrophysical constraints on the proton-to-electron mass ratio with FAST

That the laws of physics are the same at all times and places throughout the Universe is one of the basic assumptions of physics. Astronomical observations provide the only means to test this basic assumption on cosmological time and distance scales. The possibility of variations in the dimensionless physical constant {\mu} - the proton-to-electron mass ratio, can be tested by comparing astronomical measurements of the rest frequency of certain spectral lines at radio wavelengths with laboratory determinations. Different types of molecular transitions have different dependencies on {\mu} and so observations of two or more spectral lines towards the same astronomical source can be used to test whether there is any evidence for either temporal or spatial changes in the physical fundamental constants. {\mu} will change if the relative strength of the strong nuclear force compared to the electromagnetic force varies. Theoretical studies have shown that the rotational transitions of some molecules which have transitions in the frequency range that will be covered by FAST (e.g., CH3OH, OH and CH) are sensitive to changes in {\mu}. A number of studies looking for possible variations in {\mu} have been undertaken with existing telescopes, however, the greater sensitivity of FAST means it will open new opportunities to significantly improve upon measurements made to date. In this paper, we discuss which molecular transitions and sources (both in the Galaxy and external galaxies) are likely targets for providing improved constraints on {\mu} with FAST

Detection of HC3_3N maser emission in NGC253

We report the detection of maser emission from the $J=4-3$ transition of HC$_3$N at 36.4~GHz towards the nearby starburst galaxy NGC253. This is the first detection of maser emission from this transition in either a Galactic or extragalactic source. The HC$_3$N maser emission has a brightness temperature in excess of 2500 K and is offset from the center of the galaxy by approximately 18 arcsec (300 pc), but close to a previously reported class~I methanol maser. Both the HC$_3$N and methanol masers appear to arise near the interface between the galactic bar and the central molecular zone, where it is thought that molecular gas is being transported inwards, producing a region of extensive low-velocity shocks.Comment: Accepted for publication in ApJ Letters, 7 pages, 3 figure

Detection of 84-GHz class I methanol maser emission towards NGC 253

We have investigated the central region of NGC 253 for the presence of 84.5-GHz ($5_{-1}\rightarrow4_0$E) methanol emission using the Australia Telescope Compact Array. We present the second detection of 84.5-GHz class~I methanol maser emission outside the Milky Way. This maser emission is offset from dynamical centre of NGC 253, in a region with previously detected emission from class~I maser transitions (36.2-GHz $4_{-1}\rightarrow3_0$E and 44.1-GHz $7_{0}\rightarrow6_1$A$^{+}$ methanol lines) . The emission features a narrow linewidth ($\sim$12 km s$^{-1}$) with a luminosity approximately 5 orders of magnitude higher than typical Galactic sources. We determine an integrated line intensity ratio of $1.2\pm0.4$ between the 36.2 GHz and 84.5-GHz class I methanol maser emission, which is similar to the ratio observed towards Galactic sources. The three methanol maser transitions observed toward NGC 253 each show a different distribution, suggesting differing physical conditions between the maser sites and that observations of additional class~I methanol transitions will facilitate investigations of the maser pumping regime.Comment: Accepted into ApJL 12 October 2018. 10 pages, 3 Figures and 2 Table

A New 95 GHz Methanol Maser Catalog: I. Data

The Purple Mountain Observatory 13.7 m radio telescope has been used to search for 95 GHz (8$_0$--7$_1$A$^+$) class I methanol masers towards 1020 Bolocam Galactic Plane Survey (BGPS) sources, leading to 213 detections. We have compared the line width of the methanol and HCO$^+$ thermal emission in all of the methanol detections and on that basis we find 205 of the 213 detections are very likely to be masers. This corresponds to an overall detection rate of 95 GHz methanol masers towards our BGPS sample of 20%. Of the 205 detected masers 144 (70%) are new discoveries. Combining our results with those of previous 95 GHz methanol masers searches, a total of four hundred and eighty-one 95 GHz methanol masers are now known, we have compiled a catalog listing the locations and properties of all known 95 GHz methanol masers.Comment: 18 pages, 7 figures, 8 tables, accepted for publication in ApJ

Detection of a methanol megamaser in a major-merger galaxy

We have detected emission from both the 4_{-1}-3_{0} E (36.2~GHz) class I and 7_{-2}-8_{-1} E (37.7~GHz) class II methanol transitions towards the centre of the closest ultra-luminous infrared galaxy Arp 220. The emission in both the methanol transitions show narrow spectral features and have luminosities approximately 8 orders of magnitude stronger than that observed from typical class I methanol masers observed in Galactic star formation regions. The emission is also orders of magnitude stronger than the expected intensity of thermal emission from these transitions and based on these findings we suggest that the emission from the two transitions are masers. These observations provides the first detection of a methanol megamaser in the 36.2 and 37.7 GHz transitions and represents only the second detection of a methanol megamaser, following the recent report of an 84 GHz methanol megamaser in NGC1068. We find the methanol megamasers are significantly offset from the nuclear region and arise towards regions where there is Ha emission, suggesting that it is associated with starburst activity. The high degree of correlation between the spatial distribution of the 36.2 GHz methanol and X-ray plume emission suggests that the production of strong extragalactic class I methanol masers is related to galactic outflow driven shocks and perhaps cosmic rays. In contrast to OH and H2O megamasers which originate close to the nucleus, methanol megamasers provide a new probe of feedback (e.g. outflows) processes on larger-scales and of star formation beyond the circumnuclear starburst regions of active galaxies.Comment: Accepted for publication in ApJ