Mount Stromlo Observatory

Weston, Australia

Mount Stromlo Observatory

Weston, Australia

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Pisano D.J.,West Virginia University | Pisano D.J.,U.S. National Radio Astronomy Observatory | Barnes D.G.,Swinburne University of Technology | Staveley-Smith L.,University of Western Australia | And 4 more authors.
Astrophysical Journal, Supplement Series | Year: 2011

We have conducted an H I 21cm emission-line survey of six loose groups of galaxies chosen to be analogs to the Local Group. The survey was conducted using the Parkes multibeam instrument and the Australia Telescope Compact Array (ATCA) over a 1Mpc2 area and covering the full depth of each group, with an M H I sensitivity of 7 × 105 M. Our survey detected 110sources, 61 of which are associated with the six groups. All of these sources were confirmed with ATCA observations or were previously cataloged by HIPASS. The sources all have optical counterparts and properties consistent with dwarf irregular or late-type spiral galaxies. We present here the H I properties of the groups and their galaxies. We derive an H I mass function (HIMF) for the groups that is consistent with being flatter than the equivalent field HIMF. We also derive a circular velocity distribution function, tracing the luminous dark matter halos in the groups, that is consistent with those of the Local Group and HIPASS galaxies, both of which are shallower than that of clusters or predictions from cold dark matter models of galaxy formation. © 2011. The American Astronomical Society. All rights reserved.


Jensen H.,The Oskar Klein Center | Datta K.K.,The Oskar Klein Center | Mellema G.,The Oskar Klein Center | Chapman E.,University College London | And 27 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

One of the most promising ways to study the epoch of reionization (EoR) is through radio observations of the redshifted 21-cm line emission from neutral hydrogen. These observations are complicated by the fact that the mapping of redshifts to line-of-sight positions is distorted by the peculiar velocities of the gas. Such distortions can be a source of error if they are not properly understood, but they also encode information about cosmology and astrophysics. We study the effects of redshift space distortions on the power spectrum of 21-cm radiation from the EoR using large-scale N-body and radiative transfer simulations. We quantify the anisotropy introduced in the 21-cm power spectrum by redshift space distortions and show how it evolves as reionization progresses and how it relates to the underlying physics. We go on to study the effects of redshift space distortions on LOFAR observations, taking instrument noise and foreground subtraction into account.We find that LOFAR should be able to directly observe the power spectrum anisotropy due to redshift space distortions at spatial scales around k ~ 0.1Mpc-1 after ≳1000 h of integration time. At larger scales, sample errors become a limiting factor, while at smaller scales detector noise and foregrounds make the extraction of the signal problematic. Finally, we show how the astrophysical information contained in the evolution of the anisotropy of the 21-cm power spectrum can be extracted from LOFAR observations, and how it can be used to distinguish between different reionization scenarios. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


News Article | February 23, 2017
Site: www.eurekalert.org

Glowing nebula found at the heart of a huge "rotocluster' of early galaxies appears to be part of the cosmic web of filaments connecting galaxies, but what's lighting it up? Astronomers have found an enormous, glowing blob of gas in the distant universe, with no obvious source of power for the light it is emitting. Called an "enormous Lyman-alpha nebula" (ELAN), it is the brightest and among the largest of these rare objects, only a handful of which have been observed. ELANs are huge blobs of gas surrounding and extending between galaxies in the intergalactic medium. They are thought to be parts of the network of filaments connecting galaxies in a vast cosmic web. Previously discovered ELANs are likely illuminated by the intense radiation from quasars, but it's not clear what is causing the hydrogen gas in the newly discovered nebula to emit Lyman-alpha radiation (a characteristic wavelength of light absorbed and emitted by hydrogen atoms). The newly discovered nebula was found at a distance of 10 billion light years in the middle of a region with an extraordinary concentration of galaxies. Researchers found this massive overdensity of early galaxies, called a "protocluster," through a novel survey project led by Zheng Cai, a Hubble Postdoctoral Fellow at UC Santa Cruz. "Our survey was not trying to find nebulae. We're looking for the most overdense environments in the early universe, the big cities where there are lots of galaxies," said Cai. "We found this enormous nebula in the middle of the protocluster, near the peak density." Cai is first author of a paper on the discovery accepted for publication in the Astrophysical Journal and available online at arxiv.org/abs/1609.04021. His survey project is called Mapping the Most Massive Overdensities Through Hydrogen (MAMMOTH), and the newly discovered ELAN is known as MAMMOTH-1. Coauthor J. Xavier Prochaska, professor of astronomy and astrophysics at UC Santa Cruz, said previously discovered ELANs have been detected in quasar surveys. In those cases, the intense radiation from a quasar illuminated hydrogen gas in the nebula, causing it to emit Lyman-alpha radiation. Prochaska's team discovered the first ELAN, dubbed the "Slug Nebula," in 2014. MAMMOTH-1 is the first one not associated with a visible quasar, he said. "It's extremely bright, and it's probably larger than the Slug Nebula, but there's nothing else visible except the faint smudge of a galaxy. So it's a terrifically energetic phenomenon without an obvious power source," Prochaska said. Equally impressive is the enormous protocluster in which it resides, he said. Protoclusters are the precursors to galaxy clusters, which consist of hundreds to thousands of galaxies bound together by gravity. Because protoclusters are spread out over a much larger area of the sky, they are much harder to find than galaxy clusters. The protocluster hosting the MAMMOTH-1 nebula is massive, with an unusually high concentration of galaxies in an area about 50 million light years across. Because it is so far away (10 billion light years), astronomers are in effect looking back in time to see the protocluster as it was 10 billion years ago, or about 3 billion years after the big bang, during the peak epoch of galaxy formation. After evolving for 10 billion more years, this protocluster would today be a mature galaxy cluster perhaps only one million light years across, having collapsed down to a much smaller area, Prochaska said. The standard cosmological model of structure formation in the universe predicts that galaxies are embedded in a cosmic web of matter, most of which is invisible dark matter. The gas that collapses to form galaxies and stars traces the distribution of dark matter and extends beyond the galaxies along the filaments of the cosmic web. The MAMMOTH-1 nebula appears to have a filamentary structure that aligns with the galaxy distribution in the large-scale structure of the protocluster, supporting the idea that ELANs are illuminated segments of the cosmic web, Cai said. "From the distribution of galaxies we can infer where the filaments of the cosmic web are, and the nebula is perfectly aligned with that structure," he said. Cai and his coauthors considered several possible mechanisms that could be powering the Lyman-alpha emission from the nebula. The most likely explanations involve radiation or outflows from an active galactic nucleus (AGN) that is strongly obscured by dust so that only a faint source can be seen associated with the nebula. An AGN is powered by a supermassive black hole actively feeding on gas in the center of a galaxy, and it is usually an extremely bright source of light (quasars being the most luminous AGNs in visible light). The intense radiation from an AGN can ionize the gas around it (called photoionization), and this may be one mechanism at work in MAMMOTH-1. When ionized hydrogen in the nebula recombines it would emit Lyman-alpha radiation. Another possible mechanism powering the Lyman-alpha emissions is shock heating by a powerful outflow of gas from the AGN. The researchers described several lines of evidence supporting the existence of a hidden AGN energizing the nebula, including the dynamics of the gas and emissions from other elements besides hydrogen, notably helium and carbon. "It has all the hallmarks of an AGN, but we don't see anything in our optical images. I expect there's a quasar that is so obscured by dust that most of its light is hidden," Prochaska said. In addition to Cai and Prochaska at UC Santa Cruz, the team includes coauthors at Steward Observatory, University of Arizona; Korea Astronomy and Space Institute; Mount Stromlo Observatory, Australia; Pontifical Catholic University of Chile; Institute for Astronomy, ETH Zurich; California Institute of Technology; Kavli Institute for Astronomy and Astrophysics, Peking University; and National Astronomical Observatory of Japan. This research was supported by the National Science Foundation and NASA.


News Article | February 24, 2017
Site: spaceref.com

Astronomers have found an enormous, glowing blob of gas in the distant universe, with no obvious source of power for the light it is emitting. Called an "enormous Lyman-alpha nebula" (ELAN), it is the brightest and among the largest of these rare objects, only a handful of which have been observed. ELANs are huge blobs of gas surrounding and extending between galaxies in the intergalactic medium. They are thought to be parts of the network of filaments connecting galaxies in a vast cosmic web. Previously discovered ELANs are likely illuminated by the intense radiation from quasars, but it's not clear what is causing the hydrogen gas in the newly discovered nebula to emit Lyman-alpha radiation (a characteristic wavelength of light absorbed and emitted by hydrogen atoms). The newly discovered nebula was found at a distance of 10 billion light-years in the middle of a region with an extraordinary concentration of galaxies. Researchers found this massive overdensity of early galaxies, called a "protocluster," through a novel survey project led by Zheng Cai, a Hubble Postdoctoral Fellow at UC Santa Cruz. "Our survey was not trying to find nebulae. We're looking for the most overdense environments in the early universe, the big cities where there are lots of galaxies," said Cai. "We found this enormous nebula in the middle of the protocluster, near the peak density." Cai is first author of a paper on the discovery accepted for publication in the Astrophysical Journal and available online. His survey project is called Mapping the Most Massive Overdensities Through Hydrogen (MAMMOTH), and the newly discovered ELAN is known as MAMMOTH-1. Coauthor J. Xavier Prochaska, professor of astronomy and astrophysics at UC Santa Cruz, said previously discovered ELANs have been detected in quasar surveys. In those cases, the intense radiation from a quasar illuminated hydrogen gas in the nebula, causing it to emit Lyman-alpha radiation. Prochaska's team discovered the first ELAN, dubbed the "Slug Nebula," in 2014. MAMMOTH-1 is the first one not associated with a visible quasar, he said. "It's extremely bright, and it's probably larger than the Slug Nebula, but there's nothing else visible except the faint smudge of a galaxy. So it's a terrifically energetic phenomenon without an obvious power source," Prochaska said. Equally impressive is the enormous protocluster in which it resides, he said. Protoclusters are the precursors to galaxy clusters, which consist of hundreds to thousands of galaxies bound together by gravity. Because protoclusters are spread out over a much larger area of the sky, they are much harder to find than galaxy clusters. The protocluster hosting the MAMMOTH-1 nebula is massive, with an unusually high concentration of galaxies in an area about 50 million light-years across. Because it is so far away (10 billion light-years), astronomers are in effect looking back in time to see the protocluster as it was 10 billion years ago, or about 3 billion years after the big bang, during the peak epoch of galaxy formation. After evolving for 10 billion more years, this protocluster would today be a mature galaxy cluster perhaps only one million light-years across, having collapsed down to a much smaller area, Prochaska said. The standard cosmological model of structure formation in the universe predicts that galaxies are embedded in a cosmic web of matter, most of which is invisible dark matter. The gas that collapses to form galaxies and stars traces the distribution of dark matter and extends beyond the galaxies along the filaments of the cosmic web. The MAMMOTH-1 nebula appears to have a filamentary structure that aligns with the galaxy distribution in the large-scale structure of the protocluster, supporting the idea that ELANs are illuminated segments of the cosmic web, Cai said. "From the distribution of galaxies we can infer where the filaments of the cosmic web are, and the nebula is perfectly aligned with that structure," he said. Cai and his coauthors considered several possible mechanisms that could be powering the Lyman-alpha emission from the nebula. The most likely explanations involve radiation or outflows from an active galactic nucleus (AGN) that is strongly obscured by dust so that only a faint source can be seen associated with the nebula. An AGN is powered by a supermassive black hole actively feeding on gas in the center of a galaxy, and it is usually an extremely bright source of light (quasars being the most luminous AGNs in visible light). The intense radiation from an AGN can ionize the gas around it (called photoionization), and this may be one mechanism at work in MAMMOTH-1. When ionized hydrogen in the nebula recombines it would emit Lyman-alpha radiation. Another possible mechanism powering the Lyman-alpha emissions is shock heating by a powerful outflow of gas from the AGN. The researchers described several lines of evidence supporting the existence of a hidden AGN energizing the nebula, including the dynamics of the gas and emissions from other elements besides hydrogen, notably helium and carbon. "It has all the hallmarks of an AGN, but we don't see anything in our optical images. I expect there's a quasar that is so obscured by dust that most of its light is hidden," Prochaska said. * "Discovery of an Enormous Ly-alpha Nebula in a Massive Galaxy Overdensity at z = 2.3," Zheng Cai et al., 2017, to appear in the Astrophysical Journal [http://apj.aas.org, preprint: https://arxiv.org/abs/1609.04021]. * "MApping the Most Massive Overdensities Through Hydrogen (MAMMOTH) I: Methodology," Zheng Cai et al., 2016 Dec. 20, Astrophysical Journal [http://iopscience.iop.org/article/10.3847/1538-4357/833/2/135 , preprint: https://arxiv.org/abs/1512.06859]. In addition to Cai and Prochaska at UC Santa Cruz, the team includes coauthors at Steward Observatory, University of Arizona; Korea Astronomy and Space Institute; Mount Stromlo Observatory, Australia; Pontifical Catholic University of Chile; Institute for Astronomy, ETH Zurich; California Institute of Technology; Kavli Institute for Astronomy and Astrophysics, Peking University; and National Astronomical Observatory of Japan. This research was supported by the National Science Foundation and NASA. Please follow SpaceRef on Twitter and Like us on Facebook.


Vlajic M.,Leibniz Institute for Astrophysics Potsdam | Vlajic M.,University of Sydney | Bland-Hawthorn J.,University of Sydney | Freeman K.C.,Mount Stromlo Observatory
Astrophysical Journal | Year: 2011

Studies of outer regions of spirals disks are fundamental to our understanding of both the process of galaxy assembly and the subsequent secular evolution of galaxies. In an earlier series of papers, we explored the extent and abundance gradient in the outer disk of NGC300 and found an extended purely exponential disk with a metallicity gradient which flattens off in the outermost regions. We now continue the study of outskirts of pure disk spirals with another Sculptor Group spiral, NGC7793. Using the Gemini Multi Object Spectrograph camera at Gemini South, we trace the disk of NGC7793 with star counts out to ∼9 scale lengths, corresponding to 11.5kpc at our calibrated distance of 3.61 ± 0.53Mpc. The outer disk of NGC7793 shows no evidence of a break in its light profile down to an effective surface brightness of ∼30 mag arcsec-2 (∼3 mag arcsec-2 deeper than what has been achieved with surface photometry) and exhibits a non-negative abundance gradient within the radial extent of our data. © 2011. The American Astronomical Society. All rights reserved.


Gwinn C.R.,University of California at Santa Barbara | Johnson M.D.,University of California at Santa Barbara | Reynolds J.E.,CSIRO | Jauncey D.L.,CSIRO | And 14 more authors.
Astrophysical Journal | Year: 2012

We present measurements of the linear diameter of the emission region of the Vela pulsar at observing wavelength λ = 18 cm. We infer the diameter as a function of pulse phase from the distribution of visibility on the Mopra-Tidbinbilla baseline. As we demonstrate, in the presence of strong scintillation, finite size of the emission region produces a characteristic W-shaped signature in the projection of the visibility distribution onto the real axis. This modification involves heightened probability density near the mean amplitude, decreased probability to either side, and a return to the zero-size distribution beyond. We observe this signature with high statistical significance, as compared with the best-fitting zero-size model, in many regions of pulse phase. We find that the equivalent FWHM of the pulsar's emission region decreases from more than 400 km early in the pulse to near zero at the peak of the pulse and then increases again to approximately 800 km near the trailing edge. We discuss possible systematic effects and compare our work with previous results. © © 2012. The American Astronomical Society. All rights reserved..


Koay J.Y.,Curtin University Australia | MacQuart J.-P.,Curtin University Australia | Rickett B.J.,University of California at San Diego | Bignall H.E.,Curtin University Australia | And 8 more authors.
Astronomical Journal | Year: 2011

The 4.9 GHz Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey detected a drop in interstellar scintillation (ISS) for sources at redshifts z ≳ 2, indicating an apparent increase in angular diameter or a decrease in flux density of the most compact components of these sources relative to their extended emission. This can result from intrinsic source size effects or scatter broadening in the intergalactic medium (IGM) in excess of the expected (1 + z)1/2 angular diameter scaling of brightness temperature limited sources resulting from cosmological expansion. We report here 4.9GHz and 8.4 GHz observations and data analysis for a sample of 140 compact, flat-spectrum sources which may allow us to determine the origin of this angular diameter-redshift relation by exploiting their different wavelength dependences. In addition to using ISS as a cosmological probe, the observations provide additional insight into source morphologies and the characteristics of ISS. As in the MASIV Survey, the variability of the sources is found to be significantly correlated with line-of-sight Hα intensities, confirming its link with ISS. For 25 sources, time delays of about 0.15-3days are observed between the scintillation patterns at both frequencies, interpreted as being caused by a shift in core positions when probed at different optical depths. Significant correlation is found between ISS amplitudes and source spectral index; in particular, a large drop in ISS amplitudes is observed at α < -0.4 confirming that steep spectrum sources scintillate less. We detect a weakened redshift dependence of ISS at 8.4 GHz over that at 4.9 GHz, with the mean variance at four-day timescales reduced by a factor of 1.8 in the z > 2 sources relative to the z < 2 sources, as opposed to the factor of three decrease observed at 4.9 GHz. This suggests scatter broadening in the IGM, but the interpretation is complicated by subtle selection effects that will be explored further in a follow-up paper. © 2011. The American Astronomical Society. All rights reserved.


Koay J.Y.,Curtin University Australia | MacQuart J.-P.,Curtin University Australia | Rickett B.J.,University of California at San Diego | Bignall H.E.,Curtin University Australia | And 8 more authors.
Astrophysical Journal | Year: 2012

The fraction of compact active galactic nuclei (AGNs) that exhibit interstellar scintillation (ISS) at radio wavelengths, as well as their scintillation amplitudes, have been found to decrease significantly for sources at redshifts z ≳ 2. This can be attributed to an increase in the angular sizes of the μas-scale cores or a decrease in the flux densities of the compact μas cores relative to that of the mas-scale components with increasing redshift, possibly arising from (1) the space-time curvature of an expanding universe, (2) AGN evolution, (3) source selection biases, (4) scatter broadening in the ionized intergalactic medium (IGM) and intervening galaxies, or (5) gravitational lensing. We examine the frequency scaling of this redshift dependence of ISS to determine its origin, using data from a dual-frequency survey of ISS of 128 sources at 0 ≲ z ≲ 4. We present a novel method of analysis which accounts for selection effects in the source sample. We determine that the redshift dependence of ISS is partially linked to the steepening of source spectral indices (α8.4 4.9) with redshift, caused either by selection biases or AGN evolution, coupled with weaker ISS in the α8.4 4.9 < -0.4 sources. Selecting only the -0.4 < α8.4 4.9 < 0.4 sources, we find that the redshift dependence of ISS is still significant, but is not significantly steeper than the expected (1 + z)0.5 scaling of source angular sizes due to cosmological expansion for a brightness temperature and flux-limited sample of sources. We find no significant evidence for scatter broadening in the IGM, ruling it out as the main cause of the redshift dependence of ISS. We obtain an upper limit to IGM scatter broadening of ≲ 110 μas at 4.9GHz with 99% confidence for all lines of sight and as low as ≲ 8 μas for sight lines to the most compact, ∼10 μas sources. © © 2012. The American Astronomical Society. All rights reserved..


Koay J.Y.,Curtin University Australia | Bignall H.E.,Curtin University Australia | MacQuart J.-P.,Curtin University Australia | Jauncey D.L.,CSIRO | And 3 more authors.
Astronomy and Astrophysics | Year: 2011

The extreme, intra-hour and >10% rms flux density scintillation observed in AGNs such as PKS 0405-385, J1819+3845 and PKS 1257-326 at cm wavelengths has been attributed to scattering in highly turbulent, nearby regions in the interstellar medium. Such behavior has been found to be rare. We searched for rapid scintillators among 128 flat spectrum AGNs and analyzed their properties to determine the origin of such rapid and large amplitude radio scintillation. The sources were observed at the VLA at 4.9 and 8.4 GHz simultaneously at two hour intervals over 11 days. We detected six rapid scintillators with characteristic time-scales of <2 h, none of which have rms variations >10%. We found strong lines of evidence linking rapid scintillation to the presence of nearby scattering regions, estimated to be <12 pc away for ∼200 μas sources and <250 pc away for ∼10 μas sources. We attribute the scarcity of rapid and large amplitude scintillators to the requirement of additional constraints, including large source compact fractions. J1819+3845 was found to display ∼2% rms variations at ∼6 h time-scales superposed on longer >11 day variations, suggesting that the highly turbulent cloud responsible for its extreme scintillation has moved away, with its scintillation now caused by a more distant screen 50 to 150 pc away. © 2011 ESO.


Van Aarle E.,Catholic University of Leuven | Van Winckel H.,Catholic University of Leuven | Lloyd Evans T.,University of St. Andrews | Ueta T.,University of Denver | And 2 more authors.
Astronomy and Astrophysics | Year: 2011

Context. The detected variety in chemistry and circumstellar shell morphology of the limited sample of Galactic post-asymptotic giant branch (AGB) stars is so large that there is no consensus yet on how the diff erent objects are linked by evolutionary channels. The evaluation is complicated by the fact that their distances and hence luminosities remain largely unknown. Aims. We construct a catalogue of the optically bright post-AGB stars in the Large Magellanic Cloud (LMC). The sample forms an ideal testbed for stellar evolution theory predictions of the final phase of low- and intermediate-mass stars, because the distance and hence luminosity and also the current and initial mass of these objects is well constrained. Methods. Via cross-correlation of the Spitzer SAGE catalogue with optical catalogues we selected a sample of LMC post-AGB candidates based on their [8] - [24] colour index and estimated luminosity. We determined the fundamental properties of the central stars of 105 of these objects using low-resolution, optical spectra that we obtained at Siding Spring Observatory and SAAO. Results. We constructed a catalogue of 70 high probability and 1337 candidate post-AGB stars that is available at the CDS. About half of the objects in our sample of post-AGB candidates show a spectral energy distribution (SED) that is indicative of a disc rather than an expanding and cooling AGB remnant. Like in the Galaxy, the disc sources are likely associated with binary evolution. Important side products of this research are catalogues of candidate young stellar objects, candidate supergiants with circumstellar dust, and discarded objects for which a spectrum was obtained. These too are available at the CDS. © ESO 2011.

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