arXiv subjects

J. Ott

Publications and source records attributed to J. Ott.

At least 73 records · Page 4Linked to original sources

arXiv2016

Continuum sources from the THOR survey between 1 and 2 GHz

We carried out a large program with the Karl G. Jansky Very Large Array (VLA): "THOR: The HI, OH, Recombination line survey of the Milky Way". We observed a significant portion of the Galactic plane in the first quadrant of the Milky Way in the 21cm HI line, 4 OH transitions, 19 radio recombination lines, and continuum from 1 to 2 GHz. In this paper we present a catalog of the continuum sources in the first half of the survey (l=14.0-37.9deg and l=47.1-51.2deg, |b|<1.1deg) at a spatial resolution of 10-25", with a spatially varying noise level of ~0.3-1 mJy/beam. The catalog contains ~4400 sources. Around 1200 of these are spatially resolved, and ~1000 are possible artifacts, given their low signal-to-noise ratios. Since the spatial distribution of the unresolved objects is evenly distributed and not confined to the Galactic plane, most of them are extragalactic. Thanks to the broad bandwidth of the observations from 1 to 2 GHz, we are able to determine a reliable spectral index for ~1800 sources. The spectral index distribution reveals a double-peaked profile with maxima at spectral indices of alpha = -1 and alpha = 0 , corresponding to steep declining and flat spectra, respectively. This allows us to distinguish between thermal and non-thermal emission, which can be used to determine the nature of each source. We examine the spectral index of ~300 known HII regions, for which we find thermal emission with spectral indices around alpha = 0. In contrast, supernova remnants (SNR) show non-thermal emission with alpha = -0.5 and extragalactic objects generally have a steeper spectral index of alpha = -1. Using the spectral index information of the THOR survey, we investigate potential SNR candidates. We classify the radiation of four SNR candidates as non-thermal, and for the first time, we provide strong evidence for the SNR origin of these candidates.

astro-ph.GA↗

arXiv2015

Next Generation Very Large Array Memo No. 5: Science Working Groups -- Project Overview

We summarize the design, capabilities, and some of the priority science goals of a next generation Very Large Array (ngVLA). The ngVLA is an interferometric array with 10x larger effective collecting area and 10x higher spatial resolution than the current VLA and the Atacama Large Millimeter Array (ALMA), optimized for operation in the wavelength range 0.3cm to 3cm. The ngVLA opens a new window on the Universe through ultra-sensitive imaging of thermal line and continuum emission down to milliarcecond resolution, as well as unprecedented broad band continuum polarimetric imaging of non-thermal processes. The continuum resolution will reach 9mas at 1cm, with a brightness temperature sensitivity of 6K in 1 hour. For spectral lines, the array at 1" resolution will reach 0.3K surface brightness sensitivity at 1cm and 10 km/s spectral resolution in 1 hour. These capabilities are the only means with which to answer a broad range of critical scientific questions in modern astronomy, including direct imaging of planet formation in the terrestrial-zone, studies of dust-obscured star formation and the cosmic baryon cycle down to pc-scales out to the Virgo cluster, making a cosmic census of the molecular gas which fuels star formation back to first light and cosmic reionization, and novel techniques for exploring temporal phenomena from milliseconds to years. The ngVLA is optimized for observations at wavelengths between the superb performance of ALMA at submm wavelengths, and the future SKA1 at few centimeter and longer wavelengths. This memo introduces the project. The science capabilities are outlined in a parallel series of white papers. We emphasize that this initial set of science goals are simply a starting point for the project. We invite comment on these programs, as well as new ideas, through our public forum link on the ngVLA web page https://science.nrao.edu/futures/ngvla

astro-ph.IM↗

arXiv2015

THOR - The HI, OH, Recombination Line Survey of the Milky Way - The pilot study: HI observations of the giant molecular cloud W43

To study the atomic, molecular and ionized emission of Giant Molecular Clouds (GMCs), we have initiated a Large Program with the VLA: 'THOR - The HI, OH, Recombination Line survey of the Milky Way'. We map the 21cm HI line, 4 OH lines, 19 H_alpha recombination lines and the continuum from 1 to 2 GHz of a significant fraction of the Milky Way (l=15-67deg, |b|<1deg) at ~20" resolution. In this paper, we focus on the HI emission from the W43 star-formation complex. Classically, the HI 21cm line is treated as optically thin with column densities calculated under this assumption. This might give reasonable results for regions of low-mass star-formation, however, it is not sufficient to describe GMCs. We analyzed strong continuum sources to measure the optical depth, and thus correct the HI 21cm emission for optical depth effects and weak diffuse continuum emission. Hence, we are able to measure the HI mass of W43 more accurately and our analysis reveals a lower limit of M=6.6x10^6 M_sun, which is a factor of 2.4 larger than the mass estimated with the assumption of optically thin emission. The HI column densities are as high as N(HI)~150 M_sun/pc^2 ~ 1.9x10^22 cm^-2, which is an order of magnitude higher than for low mass star formation regions. This result challenges theoretical models that predict a threshold for the HI column density of ~10 M_sun/pc^2, at which the formation of molecular hydrogen should set in. By assuming an elliptical layered structure for W43, we estimate the particle density profiles. While at the cloud edge atomic and molecular hydrogen are well mixed, the center of the cloud is strongly dominated by molecular hydrogen. We do not identify a sharp transition between hydrogen in atomic and molecular form. Our results are an important characterization of the atomic to molecular hydrogen transition in an extreme environment and challenges current theoretical models.

astro-ph.SR↗

arXiv2015

Abundant Methanol Masers but no New Evidence for Star Formation in GCM0.253+0.016

We present new observations of the quiescent giant molecular cloud GCM0.253+0.016 in the Galactic center, using the upgraded Karl G. Jansky Very Large Array. Observations were made at wavelengths near 1 cm, at K (24 to 26 GHz) and Ka (27 and 36 GHz) bands, with velocity resolutions of 1-3 km/s and spatial resolutions of ~0.1 pc, at the assumed 8.4 kpc distance of this cloud. The continuum observations of this cloud are the most sensitive yet made, and reveal previously undetected emission which we attribute primarily to free-free emission from external ionization of the cloud. In addition to the sensitive continuum map, we produce maps of 12 molecular lines: 8 transitions of NH3 -- (1,1),(2,2),(3,3),(4,4),(5,5),(6,6),(7,7) and (9,9), as well as the HC3N (3-2) and (4-3) lines, and CH3OH 4(-1) - 3(0) the latter of which is known to be a collisionally-excited maser. We identify 148 CH3OH 4(-1) - 3(0) (36.2 GHz) sources, of which 68 have brightness temperatures in excess of the highest temperature measured for this cloud (400 K) and can be confirmed to be masers. The majority of these masers are concentrated in the southernmost part of the cloud. We find that neither these masers nor the continuum emission in this cloud provide strong evidence for ongoing star formation in excess of that previously inferred by the presence of an H2O maser.

astro-ph.GA↗

arXiv2014

Properties of the Magneto-ionic Medium in the Halo of M51 revealed by Wide-band Polarimetry

We present a study of the magneto-ionic medium in the Whirlpool galaxy (M51) using new wide-band multi-configuration polarization data at L band (1-2 GHz) obtained at the Karl G. Jansky Very Large Array. By fitting the observed diffuse complex polarization $Q$+$iU$ as a function of wavelength directly to various depolarization models, we find that polarized emission from M51 at 1-2 GHz originates from the top of the synchrotron disk and then experiences Faraday rotation in the near-side thermal halo of the galaxy. Thus, the scale height of the thermal gas must exceed that of the synchrotron emitting gas at L band. The observed Faraday depth distribution at L band is consistent with a halo field that comprises of a plane-parallel bisymmetric component and a vertical component which produces a Faraday rotation of $\sim$ $-$9 rad m$^{-2}$. The derived rotation measure structure functions indicate a characteristic scale of rotation measure fluctuations of less than 560 pc in the disk and approximately 1 kpc in the halo. The outer scale of turbulence of 1 kpc found in the halo of M51 is consistent with superbubbles and the Parker instability being the main energy injection mechanisms in galactic halos.

astro-ph.GA↗

arXiv2014

An ALMA Early Science survey of molecular absorption lines toward PKS1830-211 -- Analysis of the absorption profiles

We present the first results of an ALMA spectral survey of strong absorption lines for common interstellar species in the z=0.89 molecular absorber toward the lensed blazar PKS1830-211. The dataset brings essential information on the structure and composition of the absorbing gas in the foreground galaxy. In particular, we find absorption over large velocity intervals (gtrsim 100 km/s) toward both lensed images of the blazar. This suggests either that the galaxy inclination is intermediate and that we sample velocity gradients or streaming motions in the disk plane, that the molecular gas has a large vertical distribution or extraplanar components, or that the absorber is not a simple spiral galaxy but might be a merger system. The number of detected species is now reaching a total of 42 different species plus 14 different rare isotopologues toward the SW image, and 14 species toward the NE line-of-sight. The abundances of CH, H2O, HCO+, HCN, and NH3 relative to H2 are found to be comparable to those in the Galactic diffuse medium. Of all the lines detected so far toward PKS1830-211, the ground-state line of ortho-water has the deepest absorption. We argue that ground-state lines of water have the best potential for detecting diffuse molecular gas in absorption at high redshift.

astro-ph.GA↗

arXiv2013

A Radio Survey of Galactic Center Clouds

We present a survey of molecules in a sample of Galactic center molecular clouds using the Karl G. Jansky Very large Array, which includes M0.25+0.01, the clouds near Sgr A, and Sgr B2. The molecules detected are primarily NH3 and HC3N; in Sgr B2-N we also detect nonmetastable NH3, vibrationally-excited HC3N, torsionally-excited CH3OH, and numerous isotopologues of these species. 36 GHz Class I CH3OH masers are ubiquitous in these fields, and in several cases are associated with new NH3 (3,3) maser candidates. We also find that NH3 and HC3N are depleted or absent toward several of the highest dust column density peaks identified in submillimeter observations, which are associated with water masers and are thus likely in the early stages of star formation.

astro-ph.GA↗

arXiv2013

Star Formation Sites toward the Galactic Center Region: The Correlation of CH3OH Masers, H2O Masers, and Near-IR Green Sources

We present a study of star formation in the Central Molecular Zone (CMZ) of our Galaxy through the association of three star formation indicators: 6.7 GHz CH3OH masers, 22 GHz H2O masers, and enhanced 4.5 micron emission (`green') sources. We explore how star formation in the Galactic center (l < 1.3 deg, |b| < 10', where l and b are Galactic longitude and Galactic latitude) compares with that of the Galactic disk (6 deg < l < 345 deg, |b| < 2 deg). Using an automated algorithm, we search for enhanced 4.5 micron emission sources toward 6.7 GHz CH3OH masers detected in the Parkes Methanol Multibeam Survey. We combine these results with our 22 GHz H2O maser survey of the CMZ carried out with the Mopra telescope. We find that the correlation of CH3OH masers with green sources is a function of Galactic latitude, with a minimum close to b=0 and increasing with |b| (toward the central part of the Galaxy, 6 deg < l < 345 deg, |b| < 2 deg). We find no significant difference between the correlation rate of CH3OH masers with green sources in the CMZ and the disk. This suggests that although the physical conditions of the gas are different in the Galactic center from that of the Galactic disk, once gravitational instability sets in at sufficiently high densities, signatures of star formation appear to be similar in both regions. Moreover, the detection of green sources, even at the distance of the Galactic center, shows that our technique can easily identify the early stages of star formation, especially in low extinction regions of the Galaxy. Through the association of H2O and CH3OH masers, we identify 15 star-forming sites in the CMZ. We find no coincident H2O and CH3OH masers outside the CMZ (with limited H2O maser survey coverage outside the CMZ), possibly indicating a difference in the maser evolutionary sequence for star-forming cores in the Galactic center region and the disk.

astro-ph.GA↗

arXiv2013

Candidate super star cluster progenitor gas clouds possibly triggered by close passage to Sgr A*

Super star clusters are the end product of star formation under the most extreme conditions. As such, studying how their final stellar populations are assembled from their natal progenitor gas clouds can provide strong constraints on star formation theories. An obvious place to look for the initial conditions of such extreme stellar clusters are gas clouds of comparable mass and density, with no star formation activity. We present a method to identify such progenitor gas clouds and demonstrate the technique for the gas in the inner few hundred pc of our Galaxy. The method highlights three clouds in the region with similar global physical properties to the previously identified extreme cloud, G0.253+0.016, as potential young massive cluster (YMC) precursors. The fact that four potential YMC progenitor clouds have been identified in the inner 100 pc of the Galaxy, but no clouds with similar properties have been found in the whole first quadrant despite extensive observational efforts, has implications for cluster formation/destruction rates across the Galaxy. We put forward a scenario to explain how such dense gas clouds can arise in the Galactic centre environment, in which YMC formation is triggered by gas streams passing close to the minimum of the global Galactic gravitational potential at the location of the central supermassive black hole, Sgr A*. If this triggering mechanism can be verified, we can use the known time interval since closest approach to Sgr A* to study the physics of stellar mass assembly in an extreme environment as a function of absolute time.

astro-ph.GA↗

arXiv2012

Variations in the Galactic star formation rate and density thresholds for star formation

The conversion of gas into stars is a fundamental process in astrophysics and cosmology. Stars are known to form from the gravitational collapse of dense clumps in interstellar molecular clouds, and it has been proposed that the resulting star formation rate is proportional to either the amount of mass above a threshold gas surface density, or the gas volume density. These star-formation prescriptions appear to hold in nearby molecular clouds in our Milky Way Galaxy's disk as well as in distant galaxies where the star formation rates are often much larger. The inner 500 pc of our Galaxy, the Central Molecular Zone (CMZ), contains the largest concentration of dense, high-surface density molecular gas in the Milky Way, providing an environment where the validity of star-formation prescriptions can be tested. Here we show that by several measures, the current star formation rate in the CMZ is an order-of-magnitude lower than the rates predicted by the currently accepted prescriptions. In particular, the region 1 deg < l < 3.5 deg, |b| < 0.5 deg contains ~10^7 Msun of dense molecular gas -- enough to form 1000 Orion-like clusters -- but the present-day star formation rate within this gas is only equivalent to that in Orion. In addition to density, another property of molecular clouds, such as the amplitude of turbulent motions, must be included in the star-formation prescription to predict the star formation rate in a given mass of molecular gas.

astro-ph.GA↗

arXiv2011

Spectral imaging of the Central Molecular Zone in multiple 3-mm molecular lines

We have mapped 20 molecular lines in the Central Molecular Zone (CMZ) around the Galactic Centre, emitting from 85.3 to 93.3 GHz. This work used the 22-m Mopra radio telescope in Australia, equipped with the 8-GHz bandwidth UNSW-MOPS digital filter bank, obtaining \sim 2 km/s spectral and \sim 40 arcsec spatial resolution. The lines measured include emission from the c-C3H2, CH3CCH, HOCO+, SO, H13CN, H13CO+, SO, H13NC, C2H, HNCO, HCN, HCO+, HNC, HC3N, 13CS and N2H+ molecules. The area covered is Galactic longitude -0.7 to 1.8 deg. and latitude -0.3 to 0.2 deg., including the bright dust cores around Sgr A, Sgr B2, Sgr C and G1.6-0.025. We present images from this study and conduct a principal component analysis on the integrated emission from the brightest 8 lines. This is dominated by the first component, showing that the large-scale distribution of all molecules are very similar. We examine the line ratios and optical depths in selected apertures around the bright dust cores, as well as for the complete mapped region of the CMZ. We highlight the behaviour of the bright HCN, HNC and HCO+ line emission, together with that from the 13C isotopologues of these species, and compare the behaviour with that found in extra-galactic sources where the emission is unresolved spatially. We also find that the isotopologue line ratios (e.g. HCO+/H13CO+) rise significantly with increasing red-shifted velocity in some locations. Line luminosities are also calculated and compared to that of CO, as well as to line luminosities determined for external galaxies.

astro-ph.GA↗

arXiv2011

Boosted objects: a probe of beyond the Standard Model physics

We present the report of the hadronic working group of the BOOST2010 workshop held at the University of Oxford in June 2010. The first part contains a review of the potential of hadronic decays of highly boosted particles as an aid for discovery at the LHC and a discussion of the status of tools developed to meet the challenge of reconstructing and isolating these topologies. In the second part, we present new results comparing the performance of jet grooming techniques and top tagging algorithms on a common set of benchmark channels. We also study the sensitivity of jet substructure observables to the uncertainties in Monte Carlo predictions.

hep-ph↗

arXiv2011

Water vapour at high redshift: Arecibo monitoring of the megamaser in MG J0414+0534

The study of water masers at cosmological distances would allow us to investigate the parsec-scale environment around powerful radio sources, to probe the physical conditions of the molecular gas in the inner parsecs of quasars, and to estimate their nuclear engine masses in the early universe. To derive this information, the nature of the maser source, jet or disk-maser, needs to be assessed through a detailed investigation of the observational characteristics of the line emission. We monitored the maser line in the lensed quasar MGJ0414+0534 at z = 2.64 with the 300-m Arecibo telescope for ~15 months to detect possible additional maser components and to measure a potential velocity drift of the lines. In addition, we follow the maser and continuum emissions to reveal significant variations in their flux density and to determine correlation or time-lag, if any, between them. The main maser line profile is complex and can be resolved into a number of broad features with line widths of 30-160 km/s. A new maser component was tentatively detected in October 2008 that is redshifted by 470 km/s w.r.t the systemic velocity of the quasar. The line width of the main maser feature increased by a factor of two between the Effelsberg and EVLA observations reported by Impellizzeri et al. (2008) and the first epoch of the Arecibo monitoring campaign. After correcting for the lens magnification, we find that the total H2O isotropic luminosity of the maser in MGJ0414+0534 is now ~30,000 Lsun, making this source the most luminous ever discovered.[Abridged]

astro-ph.CO↗

arXiv2010

A study of three southern high-mass star-forming regions

Based on color-selected IRAS point sources, we have started to conduct a survey of 47 high-mass star-forming regions in the southern hemisphere in 870um dust continuum and molecular line emission in several frequency ranges between 290 GHz and 806 GHz. This paper describes the pilot study of the three sources IRAS12326-6245, IRAS16060-5146, and IRAS16065-5158. To characterize the physical and chemical properties of southern massive star-forming regions, the three high-luminosity southern hemisphere hot cores were observed with APEX in five frequency setups aimed at groups of lines from the following molecules: CH3OH, H2CO, and CH3CN. Using the LTE approximation, temperatures, source sizes, and column densities were determined through modeling of synthetic spectra with the XCLASS program. Dust continuum observations were done with the Large APEX BOlometer CAmera (LABOCA) at 870um and the 3mm continuum was imaged with the ATCA. Based on the detection of high-excitation CH3CN lines and lines from complex organic species, the three sources are classified as line rich, hot core type sources. For all three, the modeling indicates that the line emission emerges from a combination of an extended, cooler envelope, and a hot compact component. All three sources show an overabundance of oxygen-bearing species compared to nitrogen-bearing species. Based on the results obtained in the three sources, which served as templates for the survey, the most promising (and feasible) frequency setups for the remaining 44 sources were decided upon.

astro-ph.SR↗

arXiv2010

Supernova Remnants and Star Formation in the Large Magellanic Cloud

It has often been suggested that supernova remnants (SNRs) can trigger star formation. To investigate the relationship between SNRs and star formation, we have examined the known sample of 45 SNRs in the Large Magellanic Cloud to search for associated young stellar objects (YSOs) and molecular clouds. We find seven SNRs associated with both YSOs and molecular clouds, three SNRs associated with YSOs but not molecular clouds, and eight SNRs near molecular clouds but not associated with YSOs. Among the 10 SNRs associated with YSOs, the association between the YSOs and SNRs can be either rejected or cannot be convincingly established for eight cases. Only two SNRs have YSOs closely aligned along their rims; however, the time elapsed since the SNR began to interact with the YSOs' natal clouds is much shorter than the contraction timescales of the YSOs, and thus we do not see any evidence of SNR-triggered star formation in the LMC. The 15 SNRs that are near molecular clouds may trigger star formation in the future when the SNR shocks have slowed down to <45 km/s. We discuss how SNRs can alter the physical properties and abundances of YSOs.

astro-ph.GA↗

arXiv2010

Physical Properties of Giant Molecular Clouds in the Large Magellanic Cloud

The Magellanic Mopra Assessment (MAGMA) is a high angular resolution CO mapping survey of giant molecular clouds (GMCs) in the Large and Small Magellanic Clouds using the Mopra Telescope. Here we report on the basic physical properties of 125 GMCs in the LMC that have been surveyed to date. The observed clouds exhibit scaling relations that are similar to those determined for Galactic GMCs, although LMC clouds have narrower linewidths and lower CO luminosities than Galactic clouds of a similar size. The average mass surface density of the LMC clouds is 50 Msol/pc2, approximately half that of GMCs in the inner Milky Way. We compare the properties of GMCs with and without signs of massive star formation, finding that non-star-forming GMCs have lower peak CO brightness than star-forming GMCs. We compare the properties of GMCs with estimates for local interstellar conditions: specifically, we investigate the HI column density, radiation field, stellar mass surface density and the external pressure. Very few cloud properties demonstrate a clear dependence on the environment; the exceptions are significant positive correlations between i) the HI column density and the GMC velocity dispersion, ii) the stellar mass surface density and the average peak CO brightness, and iii) the stellar mass surface density and the CO surface brightness. The molecular mass surface density of GMCs without signs of massive star formation shows no dependence on the local radiation field, which is inconsistent with the photoionization-regulated star formation theory proposed by McKee (1989). We find some evidence that the mass surface density of the MAGMA clouds increases with the interstellar pressure, as proposed by Elmegreen (1989), but the detailed predictions of this model are not fulfilled once estimates for the local radiation field, metallicity and GMC envelope mass are taken into account.

astro-ph.CO↗

arXiv2010

Characterizing the Low-Mass Molecular Component in the Northern Small Magellanic Cloud

We present here the first results from a high-resolution survey of the 12CO(J=1-0) emission across the northern part of the poorly-enriched Small Magellanic Cloud, made with the ATNF Mopra telescope. Three molecular complexes detected in the lower resolution NANTEN survey are mapped with a beam FWHM of ~42'', to sensitivities of approximately 210 mK per 0.9 km/s channel, resolving each complex into 4-7 small clouds of masses in the range of Mvir~10^3-4 Msol and with radii no larger than 16 pc. The northern SMC CO clouds follow similar empirical relationships to the southern SMC population, yet they appear relatively under-luminous for their size, suggesting that the star-forming environment in the SMC is not homogeneous. Our data also suggests that the CO cloud population has little or no extended CO envelope on scales<30 pc, further evidence that the weak CO component in the north SMC is being disassociated by penetrating UV radiation. The new high-resolution data provide evidence for a variable correlation of the CO integrated brightness with integrated HI and um emission; in particular CO is often, but not always, found coincident with peaks of um emission, verifying the need for matching-resolution um and HI data for a complete assessment of the SMC H2 mass.

astro-ph.GA↗

arXiv2009

Molecular and Atomic Gas in the Large Magellanic Cloud II. Three-dimensional Correlation between CO and HI

We compare the CO J =(1-0) and HI emission in the Large Magellanic Cloud (LMC) in three dimensions, i.e. including a velocity axis in addition to the two spatial axes, with the aim of elucidating the physical connection between giant molecular clouds (GMCs) and their surrounding HI gas. The CO J =1-0 dataset is from the second NANTEN CO survey and the HI dataset is from the merged Australia Telescope Compact Array (ATCA) and Parkes Telescope surveys. The major findings of our analysis are: 1) GMCs are associated with an envelope of HI emission, 2) in GMCs [average CO intensity] is proportional to [average HI intensity]^[1.1+-0.1] and 3) the HI intensity tends to increase with the star formation activity within GMCs, from Type I to Type III. An analysis of the HI envelopes associated with GMCs shows that their average linewidth is 14 km s-1 and the mean density in the envelope is 10 cm-3. We argue that the HI envelopes are gravitationally bound by GMCs. These findings are consistent with a continual increase in the mass of GMCs via HI accretion at an accretion rate of 0.05 Msun/yr over a time scale of 10 Myr. The growth of GMCs is terminated via dissipative ionization and/or stellar-wind disruption in the final stage of GMC evolution.

astro-ph.GA↗