arXiv subjects

Simon Ellingsen

Publications and source records attributed to Simon Ellingsen.

At least 19 recordsLinked to original sources

arXiv2026

Cosmic Rulers: Masers as Tools for Probing Structure in the Galaxy and Beyond, from AU to kpc

Maser emission provides an unique window into astronomical sources across vast spatial scales - from tens to hundreds of astronomical units around protostars and evolved stars up to kiloparsecs in distant galaxies. These natural microwave amplifiers penetrate dust shells in star-forming regions, revealing the dynamics of accretion disks and outflows, trace envelopes and winds of evolved stars, map Galactic structure, while also allowing us to follow the evolution of all these systems. Owing to their compactness and brightness masers provide precise astrometry as cosmic rulers: measuring positions, structures and kinematics in dense regions, not easily accessible at other wavelengths. SKA-Mid will observe hydroxyl, methanol, and formaldehyde masers using bands 2, 5a and 5b, and later methylidyne radical masers in band 4. SKA-Mid's sensitivity and broad frequency coverage will support discoveries of new maser types and allow for simultaneous multi-transition and multi-species maser observations. In addition, bright masers can serve in the science verification of the SKA-Mid array during the deployment phase.

astro-ph.GA↗

arXiv2023

The Bright Supernova 1996cr in the Circinus Galaxy Imaged with VLBI: Shell Structure with Complex Evolution

We present broadband radio flux-density measurements supernova (SN) 1996cr, made with MeerKAT, ATCA and ALMA, and images made from very long baseline interferometry (VLBI) observations with the Australian Long Baseline Array. The spectral energy distribution of SN 1996cr in 2020, at age, $t \sim$8700 d, is a power-law, with flux density, $S \propto ν^{-0.588 \pm 0.011}$ between 1 and 34 GHz, but may steepen at $>35$ GHz. The spectrum has flattened since $t = 5370$ d (2010). Also since $t = 5370$ d, the flux density has declined rapidly, with $S_{\rm 9 \, GHz} \propto t^{-2.9}$. The VLBI image at $t = 8859$ d shows an approximately circular structure, with a central minimum reminiscent of an optically-thin spherical shell of emission. For a distance of 3.7 Mpc, the average outer radius of the radio emission at $t = 8859$ d was $(5.1 \pm 0.3) \times 10^{17}$ cm, and SN 1996cr has been expanding with a velocity of $4650 \pm 1060$ km s$^{-1}$ between $t=4307$ and 8859 d. It must have undergone considerable deceleration before $t = 4307$ d. Deviations from a circular shell structure in the image suggest a range of velocities up to $\sim$7000 km s$^{-1}$, and hint at the presence of a ring- or equatorial-belt-like structure rather than a complete spherical shell.

astro-ph.HE↗

arXiv2021

Molecular line search toward the flaring 6.7-GHz methanol masers of G24.33+0.13 and G359.6-0.243: rare maser transitions detected

We have performed a molecular line search toward the flaring 6.7-GHz masers G24.33+0.13 and G359.62-0.24 using the Australia Telescope Compact Array. We present spectra of the 6.7-GHz class~II methanol and 22.2-GHz water masers toward these sources and provide comparison with other recent flaring events these sources have experienced. We also detect the fourth example of a 23.4-GHz class~I methanol maser, and the eleventh example of a 4.8-GHz formaldehyde maser toward G24.33+0.13. Alongside these results, we observe the previously detected ammonia (3,3) emission and report upper limits on the presence of various other cm-wavelength methanol, ammonia and OH transitions. Our results are consistent with the flaring of G24.33+0.13 being driven by a variable accretion rate in the host high-mass young stellar object.

astro-ph.GA↗

arXiv2021

Variability in extragalactic class I methanol masers: New maser components toward NGC4945 and NGC253

We have used the Australia Telescope Compact Array (ATCA) to make new observations of the 36.2-GHz ($4_{-1}\rightarrow3_0$E) methanol transition toward NGC 4945 and NGC 253. These observations have revealed the presence of new maser components toward these galaxies, and have provided the first clear evidence for variability in extragalactic class~I methanol masers. Alongside the new observations of NGC 4945 and NGC 253, we present the results of recent 36.2-GHz methanol maser searches toward 12 galaxies, placing upper limits on the emission from the 36.2-GHz class~I transition and the 37.7-GHz ($7_{2}\rightarrow8_1$E) class~II maser line toward these sources. Flux density values for the 7-mm continuum emission toward these sources are also reported where applicable. A re-analysis of the published 36.2-GHz methanol observations of Arp 220 undertaken as part of the search revealed some issues with previous imaging procedures. The re-analysis, combined with non-detections in independent follow-up observations suggest that there is no 36.2-GHz methanol emission toward Arp 220 stronger than 3.5 mJy in a 10 km s$^{-1}$ channel (5$σ$ upper limit).

astro-ph.GA↗

arXiv2018

84-GHz methanol masers, their relationship to 36-GHz methanol masers and their molecular environments

We present observations of the 36- (4_-1 -> 3_0E) and 84-GHz (5_-1 -> 4_0 E) class I methanol maser transitions towards a sample of 94 known class I sites. These observations resulted in 93 and 92 detections in the 84- and 36-GHz transitions. While the majority of the 36-GHz sources have been previously reported, many of the sites are observed in the 84-GHz transition for the first time. The near-simultaneous observations of the two transitions revealed strikingly similar spectral profiles and a mean and median 36- to 84-GHz integrated flux density ratio of 2.6 and 1.4. Alongside the 36- and 84-GHz observations, we included rare class II methanol masers at 37.7-, 38.3-, 38.5-, 86.6- and 86.9-GHz, a number of recombination lines, and thermal molecular transitions. We detect one new site of 86.6- and 86.9-GHz methanol masers, as well as six maser candidates in one or more of 37.7-, 38.3-, 38.5-, 86.6- and 86.9-GHz methanol maser transitions. We detect a relatively higher rate of HC3N compared to that reported by MALT90 (once the respective detection limits were taken into account) who targeted dense dust clumps, suggesting that the class I methanol maser targets incorporate a relatively higher number of warm protostellar sources. We further find that there are similar relationships between the integrated flux density of both class I transitions with the integrated intensity of HC3N, HNC, HCO+, HNC, SiO and H13CO+. We suggest that this indicates that the integrated flux density of the 36- and 84-GHz transitions are closely linked to the available gas volume.

astro-ph.GA↗

arXiv2018

Detection of a 6.7 GHz methanol kilomaser toward NGC4945

We report the detection of emission from the 6.7 GHz 5(1)-6(0)A+ transition of methanol towards the center of the nearby galaxy NGC4945. This is the first detection of emission in this transition beyond the local group. The isotropic luminosity of the integrated 6.7 GHz methanol emission is approximately a factor of 10000 greater than that for 6.7 GHz methanol masers associated with Galactic high-mass star formation regions. The methanol emission is resolved on scales smaller than 40 pc and it appears unlikely that it could be due to a large concentration of Galactic-style star formation masers within a small region. Comparison with observations of other methanol transitions suggests that the 6.7 GHz methanol emission is due to a diffuse, low-gain maser, amplifying the background continuum radiation from the nuclear region. The methanol emission is blueshifted with respect to the the systemic velocity of the galaxy by several hundred kilometers per second and lies outside the velocity range associated with the dense gas and neutral hydrogen in the central region of NGC4945. We speculate that it may be associated with gas entrained in a superwind outflow from the nuclear region.

astro-ph.GA↗

arXiv2018

The first high-resolution observations of 37.7-, 38.3- and 38.5-GHz methanol masers

We have used the Australia Telescope Compact Array (ATCA) to undertake the first high angular resolution observations of 37.7-GHz ($7_{-2} - 8_{-1}E$) methanol masers towards a sample of eleven high-mass star formation regions which host strong 6.7-GHz methanol masers. The 37.7-GHz methanol sites are coincident to within the astrometric uncertainty (0.4 arcseconds) with the 6.7-GHz methanol masers associated with the same star formation region. However, spatial and spectral comparison of the 6.7- and 37.7-GHz maser emission within individual sources shows that the 37.7-GHz masers are less often, or to a lesser degree co-spatial than are the 12.2-GHz and 6.7-GHz masers. We also made sensitive, high angular resolution observations of the 38.3- and 38.5-GHz class II methanol transitions ($6_{2} - 5_{3}A^{-}$ and $6_{2} - 5_{3}A^{+}$, respectively) and the 36.2-GHz ($4_{-1} - 3_{0}E$) class I methanol transition towards the same sample of eleven sources. The 37.7-, 38.3- and 38.5-GHz methanol masers are unresolved in the current observations, which implies a lower limit on the brightness temperature of the strongest masers of more than $10^6$K. We detected the 38.3-GHz methanol transition towards 7 sources, 5 of which are new detections and detected the 38.5-GHz transition towards 6 sources, 4 of which are new detections. We detected 36.2-GHz class I methanol masers towards all eleven sources, 6 of these are new detections for this transition, of which 4 sources do not have previously reported class I methanol masers from any transition.

astro-ph.GA↗

arXiv2018

H2O MegaMasers: a RadioAstron success story

The RadioAstron space-VLBI mission has successfully detected extragalactic H2O MegaMaser emission regions at very long Earth to space baselines ranging between 1.4 and 26.7 Earth Diameters (ED). The preliminary results for two galaxies, NGC3079 and NGC4258, at baselines longer than one ED indicate masering environments and excitation conditions in these galaxies that are distinctly different. Further observations of NGC4258 at longer baselines will reveal more of the physics of individual emission regions.

astro-ph.GA↗

arXiv2017

The Maser-Starburst connection in NGC253

NGC253 is one of the closest starburst galaxies to the Milky Way and as such it has been studied in detail across the electromagnetic spectrum. Recent observations have detected the first extragalactic class I methanol masers at 36 and 44 GHz and the first extragalactic HC$_3$N (cyanoacetylene) masers in this source. Here we discuss the location of the masers with respect to key morphological features within NGC253 and the association between the masers and the ongoing starburst.

astro-ph.GA↗

arXiv2017

Class I methanol masers in NGC253: Alcohol at the end of the bar

We have used the Australia Telescope Compact Array (ATCA) to observe the 36.2-GHz class I methanol maser emission towards NGC253 and find that it is located at the interface between the nuclear ring and both ends of the galactic bar. This is thought to be the location of the inner Linblad resonance and we suggest that the maser emission in this region is likely due to large-scale cloud-cloud collisions. We have detected the first extragalactic 44.1-GHz class I methanol maser and find that it is associated with the 36.2-GHz maser emission. In contrast to the class I methanol masers found in Galactic star formation regions, the 44.1-GHz emission in NGC253 is two orders of magnitude weaker than the 36.2-GHz masers. Both the 36.2- and 44.1- GHz emission is orders of magnitude stronger than expected from typical high-mass star formation regions. This demonstrates that the luminous class I methanol masers observed in NGC253 are significantly different from those associated with Galactic star formation.

astro-ph.GA↗

arXiv2017

Detection of HC$_3$N 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.

astro-ph.GA↗

arXiv2016

A sensitive search for predicted methanol maser transitions with the Australia Telescope Compact Array

We have used the Australia Telescope Compact Array (ATCA) to search for a number of centimetre wavelength methanol transitions which are predicted to show weak maser emission towards star formation regions. Sensitive, high spatial and spectral resolution observations towards four high-mass star formation regions which show emission in a large number of class II methanol maser transitions did not result in any detections. From these observations we are able to place an upper limit of <~1300K on the brightness temperature of any emission from the $3_1$A$^+$-$3_1$A$^-$, $17_{-2}$-$18_{-3}$E ($v_t=1$), $12_4$-$13_3$A$^-$, $12_4$-$13_3$A$^+$ and $4_1$A$^+$-$4_1$A$^-$ transitions of methanol in these sources on angular scales of 2 arcseconds. This upper limit is consistent with current models for class II methanol masers in high-mass star formation regions and better constraints than those provided here will likely require observations with next-generation radio telescopes.

astro-ph.GA↗

arXiv2015

First scientific VLBI observations using New Zealand 30 metre radio telescope WARK30M

We report the results of a successful 24 hour 6.7 GHz VLBI experiment using the 30 meter radio telescope WARK30M near Warkworth, New Zealand, recently converted from a radio telecommunications antenna, and two radio telescopes located in Australia: Hobart 26-m and Ceduna 30-m. The geocentric position of WARK30M is determined with a 100 mm uncertainty for the vertical component and 10 mm for the horizontal components. We report correlated flux densities at 6.7 GHz of 175 radio sources associated with Fermi gamma-ray sources. A parsec scale emission from the radio source 1031-837 is detected, and its association with the gamma-ray object 2FGL J1032.9-8401 is established with a high likelihood ratio. We conclude that the new Pacific area radio telescope WARK30M is ready to operate for scientific projects.

astro-ph.IM↗

arXiv2015

Maser Astrometry with VLBI and the SKA

We discuss the unique opportunities for maser astrometry with the inclusion of the Square Kilometre Array (SKA) in Very Long Baseline Interferometry (VLBI) networks. The first phase of the SKA will enable observations of hydroxyl and methanol masers, positioning the latter to an accuracy of 5 microarcseconds, and the second phase may allow water maser observations. These observations will provide trigonometric distances with errors as small as 1%. The unrivalled sensitivity of the SKA will enable large-scale surveys and, through joint operations, will turn any VLBI network into a fast astrometry device. Both evolved stars and high mass star formation regions will be accessible throughout the (Southern) Milky Way, completing our understanding of the content, dynamics and history of our Galaxy. Maser velocities and proper motions will be measurable in the Local Group of galaxies and beyond, providing new insights into their kinematics and evolution.

astro-ph.GA↗

arXiv2015

OH masers in the Milky Way and Local Group galaxies in the SKA era

The intense line emission of OH masers is a perfect tracer of regions where new stars are born aswell as of evolved stars, shedding large amounts of processed matter into the interstellar medium. From SKA deep surveys at 18 cm, where the maser lines from the ground-state of the OH molecule arise, we predict the discovery of more than 20000 sources of stellar and interstellar origin throughout the Galaxy. The study of this maser emission has many applications, including the determination of magnetic field strengths from polarisation measurements, studies of stellar kinematics using the precisely determined radial velocities, and distance determinations from VLBI astrometry. A new opportunity to study shocked gas in different galactic environments is expected to arise with the detection of lower luminosity masers. For the first time, larger numbers of OH masers will be detected in Local Group galaxies. New insights are expected in structure formation in galaxies by comparing maser populations in galaxies of different metallicity, as both their properties as well as their numbers depend on it. With the full capabilities of SKA, further maser transitions such as from excited OH and from methanol will be accessible, providing new tools to study the evolution of star-forming regions in particular.

astro-ph.SR↗

arXiv2014

Tomography of Galactic star-forming regions and spiral arms with the Square Kilometer Array

Very Long Baseline Interferometry (VLBI) at radio wavelengths can provide astrometry accurate to 10 micro-arcseconds or better (i.e. better than the target GAIA accuracy) without being limited by dust obscuration. This means that unlike GAIA, VLBI can be applied to star-forming regions independently of their internal and line-of-sight extinction. Low-mass young stellar objects (particularly T Tauri stars) are often non-thermal compact radio emitters, ideal for astrometric VLBI radio continuum experiments. Existing observations for nearby regions (e.g. Taurus, Ophiuchus, or Orion) demonstrate that VLBI astrometry of such active T Tauri stars enables the reconstruction of both the regions' 3D structure (through parallax measurements) and their internal kinematics (through proper motions, combined with radial velocities). The extraordinary sensitivity of the SKA telescope will enable similar "tomographic mappings" to be extended to regions located several kpc from Earth, in particular to nearby spiral arm segments. This will have important implications for Galactic science, galactic dynamics and spiral structure theories.

astro-ph.SR↗

arXiv2014

Detection of 36 GHz class I methanol maser emission towards NGCS253

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.

astro-ph.GA↗

arXiv2013

12.2-GHz methanol maser MMB follow-up catalogue - III. Longitude range 10 to 20 degrees

We present the third instalment of a series of catalogues presenting 12.2-GHz methanol maser observations made towards each of the 6.7-GHz methanol masers detected in the Methanol Multibeam (MMB) Survey. The current portion of the catalogue includes the Galactic longitude region 10 to 20 degrees, where we detect 47 12.2-GHz methanol masers towards 99 6.7-GHz targets. We compare the occurrence of 12.2-GHz methanol masers with water maser emission, for which all 6.7-GHz methanol masers in the 6 to 20 degrees longitude range have now been searched. We suggest that the water masers follow a more complicated evolutionary scenario than has been found for the methanol and OH masers, likely due to their different pumping mechanisms. Comparisons of the 6.7-GHz methanol to OH maser peak flux density ratio and the luminosity of the associated 12.2-GHz sources suggests that the 12.2-GHz maser luminosity begins to decline around the time that an OH maser becomes detectable.

astro-ph.GA↗