arXiv subjects

J. G. Mangum

Publications and source records attributed to J. G. Mangum.

At least 19 recordsLinked to original sources

arXiv2026

Sulphur within the extreme environment of the central molecular zone of NGC 253: a chemical modelling approach

Sulphur(S)-bearing species are ubiquitous in Galactic star-forming regions, from dense cold cores to outflows and shocks linked to protostellar activity. A recent observational study investigated the origin of S-bearing species towards the central molecular zone (CMZ) of NGC 253 and showed that Sulphur emission is linked to the presence of forming stars. However, more extensive modelling of these observations are required to determine the exact origin of the Sulphur emission (e.g. shock or thermal evaporation). Using chemical modelling, we examine how S-bearing species behave in the more extreme environment of the starburst galaxy NGC 253, and more generally, how they can help us improve our understanding of the emission linked with the dense star-forming gas in external galaxies. We use the gas-grain time-dependent chemical model UCLCHEM to model static warm clouds and C-type shocks under the physical conditions found in the CMZ of NGC 253. We compare observed abundances and abundance ratios to the modelled output abundances. We found that depending on the model type (shock, post-shock or static cloud), the highest abundance reached by the S-bearing species varies significantly. Hence, we can use their observed abundances of S-bearing species to distinguish between shocked, post-shocked or a quiescent (non-shocked) gas. We also confirm observationally-based conclusions on their emission origins, including in the case of unresolved emission. Comparing GMC-scale observations with chemical modelling is a powerful method to investigate and constrain the origin of molecular emission towards extragalactic star-forming regions. Sulphur-bearing species are useful to distinguish between different types of ISM components (shocked/post-shocked/quiescent gas).

astro-ph.GA↗

arXiv2025

Millimeter emission from supermassive black hole coronae

Active Galactic Nuclei (AGN) host accreting supermassive black holes (SMBHs). The accretion can lead to the formation of a hot, X-ray emitting corona close to the SMBH capable of accelerating relativistic electrons. Observations in the millimetre (mm) band can probe its synchrotron emission. We provide a framework to derive physical information of SMBH coronae by modelling their spectral energy distribution (SED) from radio to far infrared frequencies. We also explore the possibilities of deriving additional information from mm observations, such as the SMBH mass, and studying high-redshift lensed sources. We introduce a corona emission model based on a one-zone spherical region with a hybrid thermal and non-thermal plasma. We investigate in detail how the corona SED depends on different parameters such as size, opacity, and magnetic field strength. Other galactic emission components from dust, ionised gas and diffuse relativistic electrons are also included in the SED fitting scheme. We apply our code consistently to a sample of radio-quiet AGN with strong indications of a coronal component in the mm. The detected mm emission from SMBH coronae is consistent with having a non-thermal relativistic particle population with an energy density that is ~0.5-10% of that in the thermal plasma. This requires magnetic energy densities close to equipartition with the thermal gas, and corona sizes of 60-250 gravitational radii. The model can also reproduce the observed correlation between mm emission and SMBH mass when accounting for uncertainties in the corona size. The mm band offers a unique window into the physics of SMBH coronae, enabling the study of highly dust-obscured sources and high-redshift lensed quasars. Gaining a deeper understanding of the relativistic particle population in SMBH coronae can provide key insights into their potential multiwavelength and neutrino emission.

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arXiv2025

Investigating the chemical link between H$_2$CO and CH$_3$OH within the CMZ of NGC 253

Formaldehyde (H$_2$CO) and methanol (CH$_3$OH) have served as traditional tracers of the star formation process for decades. Studies of the environments which produce these species, though, have pointed to significant differences in the physical environments within which each molecule resides. In this paper we investigate the physical and chemical conditions which give rise to formaldehyde and methanol emission in the nearby starburst galaxy NGC 253. We employ high spatial (1.$''$6 or $\sim28$ pc) and spectral ($\sim10$ km/s) imaging of the NGC 253 central molecular zone (CMZ) from the ALCHEMI Large Program to constrain radiative transfer models of the dense gas volume density, and temperature, molecular species column density, and source filling factor within eight giant molecular clouds (GMCs). We also measure the relative abundances of the two nuclear spin isomers of CH$_3$OH to investigate its formation history. The physical and chemical conditions derived clearly indicate that H$_2$CO and CH$_3$OH originate from distinct physical environments. H$_2$CO traces low volume density and high kinetic temperatures, while CH$_3$OH traces high volume density and low kinetic temperatures. The H$_2$CO abundances are constant, though poorly constrained, within the eight NGC 253 GMCs analyzed, while the CH$_3$OH abundance shows a radial gradient from low to high values within the NGC 253 CMZ. Our findings highlight the complex chemical and physical differentiation of CH$_3$OH and H$_2$CO in the starburst environment of NGC 253. Methanol formation appears to be influenced by warm, dynamic processes rather than cold cloud chemistry, while formaldehyde primarily forms via gas-phase reactions. These results challenge the assumption of a direct chemical link between CH$_3$OH and H$_2$CO and underscores the impact of starburst-driven shocks, turbulence, and cosmic rays on molecular gas chemistry.

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arXiv2025

Complex Organic Molecules towards the central molecular zone of NGC 253

Interstellar complex organic molecules (iCOMs) may have a link to prebiotic species, key building blocks for life. In Galactic star-forming (SF) regions, spatial variations of iCOMs emission could reflect the source physical structure or different chemical formation pathways. Investigating iCOMs in extragalactic SF regions may thus provide crucial information about these regions. As an active extragalactic SF region, the central molecular zone (CMZ) of the nearby galaxy NGC 253 provides an ideal template for studying iCOMs under more extreme conditions. We aim to investigate the emission of a few selected iCOMs and understand if a difference between the iCOMs could reflect on the source's chemical or physical structure. Using the high angular resolution ($\sim 27$ pc) observations from the ALCHEMI ALMA large program, we imaged the emission of selected iCOMs and precursors; CH$_3$CHO, C$_2$H$_5$OH, NH$_2$CHO, CH$_2$NH, and CH$_3$NH$_2$. We estimated the iCOMs gas temperatures and column densities using a rotational diagram analysis, and by performing a non-LTE analysis for CH$_2$NH.The iCOM emission concentrates mostly towards the inner part of the CMZ of NGC 253 and can be reproduced with two gas components. Different emission processes can explain iCOM emission towards the CMZ of NGC 253: at Giant Molecular Cloud (GMC) scales ($\sim 27$ pc), the iCOMs could trace large-scale shocks whilst at smaller scales (few pc), both shock and heating processes linked with ongoing star formation may be involved. Using column density correlation trends and known formation pathways, we find that more than one formation path could be involved to explain the iCOM emission. Finally, we found chemical differences between the GMCs, such as a decrease of abundance for the N-bearing species towards one of the GMCs or different excitation conditions for NH$_2$CHO and CH$_3$CHO towards two of the GMCs.

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arXiv2025

Environmental Monitoring Requirements for the ngVLA

Measurement of environmental parameters is one of the basic requirements for the proper operation of a telescope. This memo is intended to provide guidance for the measurement accuracy requirements in the context of the ngVLA. It relies on previous work for ALMA (Mangum, 2001) and EVLA (Butler \& Perley, 2008) and a review of the subject by Mangum \& Wallace (2015). The local operational environment can be broadly divided into two categories: electromagnetic and physical. Meteorological parameters (weather) primarily constitute the physical environmental component and radio frequency interference (RFI) is the essential element of the electromagnetic environment. This memo focuses on the weather component and does not address the RFI, safety and physical infrastructure components. Under weather, the relevant topics are (1) the correction to pointing arising from refraction in the atmosphere (2) the different delays in the arrival times of signals at different antennas due to propagation in the atmosphere (3) monitoring weather parameters to provide operations support, e.g. in determining prevalence of precision or normal conditions, dynamic scheduling and the choice of antennas to constitute a sub-array with a given set of characteristics, among others, and (4) archival. Here we restrict ourselves to the first two topics which impact the data obtained and its calibration.

astro-ph.IM↗

arXiv2025

Shock-induced HCNH+ abundance enhancement in the heart of the starburst galaxy NGC 253 unveiled by ALCHEMI

Understanding the chemistry of molecular clouds is pivotal to elucidate star formation and galaxy evolution. As one of the important molecular ions, HCNH+ plays an important role in this chemistry. Yet, its behavior and significance under extreme conditions, such as in the CMZs of external galaxies, are still largely unexplored. We aim to reveal the physical and chemical properties of the CMZ in the starburst galaxy NGC253 with multiple HCNH+ transitions to shed light on the molecule's behavior under the extreme physical conditions of a starburst. We employ molecular line data including results for four rotational transitions of HCNH+ from the ALCHEMI large program to investigate underlying physical and chemical processes. Despite weak intensities, HCNH+ emission is widespread throughout NGC253's CMZ, which suggests that this molecular ion can effectively trace large-scale structures within molecular clouds. Using the quantum mechanical coupled states approximation, we computed rate coefficients for collisions of HCNH+ with para-H2 and ortho-H2 at kinetic temperatures up to 500 K. Using these coefficients in a non-LTE modeling framework and employing a Monte Carlo Markov chain analysis, we find that HCNH+ emission originates from regions with H2 number densities of $\sim10^{2.80}-10^{3.55}$~cm$^{-3}$, establishing HCNH+ as a tracer of low-density environments. Our analysis reveals that most of the HCNH+ abundances in the CMZ of NGC253 are higher than all reported values in the Milky Way. We performed static, PDR, and shock modeling, and found that recurrent shocks could potentially account for the elevated HCNH+ abundances observed in this CMZ. We propose that the unexpectedly high HCNH+ abundances may result from chemical enhancement, primarily driven by the elevated gas temperatures and cosmic ray ionization rates of shocked, low-density gas in the nuclear starburst regions of NGC253.

astro-ph.GA↗

arXiv2024

An ALCHEMI inspection of sulphur-bearing species towards the central molecular zone of NGC 253

Sulphur-bearing species are detected in various environments within Galactic star-forming regions and are particularly abundant in the gas phase of outflows and shocks, and photo-dissociation regions. In this work, we aim to investigate the nature of the emission from the most common sulphur-bearing species observable at millimetre wavelengths towards the nuclear starburst of the galaxy NGC 253. We intend to understand which type of regions are probed by sulphur-bearing species and which process(es) dominate(s) the release of sulphur into the gas phase. We used the high-angular resolution (1.6" or 27 pc) observations from the ALCHEMI ALMA Large Program to image several sulphur-bearing species towards the central molecular zone (CMZ) of NGC 253. We performed local thermodynamic equilibrium (LTE) and non-LTE large velocity gradient (LVG) analyses to derive the physical conditions of the gas in which S-bearing species are emitted, and their abundance ratios across the CMZ. Finally, we compared our results with previous ALCHEMI studies and a few selected Galactic environments. We found that not all sulphur-bearing species trace the same type of gas: strong evidence indicates that H2S and part of the emission of OCS, H2CS, and SO, are tracing shocks whilst part of SO and CS emission rather trace the dense molecular gas. For some species, such as CCS and SO2, we could not firmly conclude on their origin of emission. The present analysis indicates that the emission from most sulphur-bearing species throughout the CMZ is likely dominated by shocks associated with ongoing star formation. In the inner part of the CMZ where the presence of super star clusters was previously indicated, we could not distinguish between shocks or thermal evaporation as the main process releasing the S-bearing species.

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arXiv2024

CON-quest II. Spatially and spectrally resolved HCN/HCO+ line ratios in local luminous and ultraluminous infrared galaxies

Nuclear regions of ultraluminous and luminous infrared galaxies (U/LIRGs) are powered by starbursts and/or active galactic nuclei (AGNs). These regions are often obscured by extremely high columns of gas and dust. Molecular lines in the submillimeter windows have the potential to determine the physical conditions of these compact obscured nuclei (CONs). We aim to reveal the distributions of HCN and HCO$^+$ emission in local U/LIRGs and investigate whether and how they are related to galaxy properties. Using ALMA, we have conducted sensitive observations of the HCN J=3--2 and HCO$^+$ J=3--2 lines toward 23 U/LIRGs in the local Universe (z < 0.07) with a spatial resolution of ~0.3" (~50--400 pc). We detected both HCN and HCO$^+$ in 21 galaxies, only HCN in one galaxy, and neither in one galaxy. The global HCN/HCO$^+$ line ratios, averaged over scales of ~0.5--4 kpc, range from 0.4 to 2.3, with an unweighted mean of 1.1. These line ratios appear to have no systematic trend with bolometric AGN luminosity or star formation rate. The line ratio varies with position and velocity within each galaxy, with an average interquartile range of 0.38 on a spaxel-by-spaxel basis. In eight out of ten galaxies known to have outflows and/or inflows, we found spatially and kinematically symmetric structures of high line ratios. These structures appear as a collimated bicone in two galaxies and as a thin spherical shell in six galaxies. Non-LTE analysis suggests that the high HCN/HCO$^+$ line ratio in outflows is predominantly influenced by the abundance ratio. Chemical model calculations indicate that the enhancement of HCN abundance in outflows is likely due to high-temperature chemistry triggered by shock heating. These results imply that the HCN/HCO$^+$ line ratio can aid in identifying the outflow geometry when the shock velocity of the outflows is sufficiently high to heat the gas.

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arXiv2024

Molecular isotopologue measurements toward super star clusters and the relation to their ages in NGC253 with ALCHEMI

Determining the evolution of the CNO isotopes in the interstellar medium (ISM) of starburst galaxies can yield important constraints on the ages of superstar clusters (SSCs), or on other aspects and contributing factors of their evolution. Due to the time-dependent nature of the abundances of isotopes within the ISM as they are supplied from processes such as nucleosynthesis or chemical fractionation, this provides the possible opportunity to probe the ability of isotopes ratios to trace the ages of high star forming regions, such as SSCs. The goal of this study is to investigate whether the isotopic variations in SSC regions within NGC253 are correlated with their different ages as derived from stellar population modelling. We have measured abundance ratios of CO, HCN and HCO$^+$ isotopologues in six regions containing SSCs within NGC253 using high spatial resolution (1.6",$\sim 28$pc) data from the ALCHEMI (ALma Comprehensive High-resolution Extragalactic Molecular Inventory) ALMA Large program. We have then analysed these ratios using RADEX radiative transfer modelling, with the parameter space sampled using the nested sampling Monte Carlo algorithm MLFriends. These abundance ratios were then compared to ages predicted in each region via the fitting of observed star formation tracers (such as Br$γ$) to starburst stellar population evolution models. We do not find any significant trend with age for the CO and HCN isotopologue ratios on the timescales for the ages of the SSC* regions observed. The driving factors of these ratios within SSCs could be the Initial Mass Function as well as possibly fractionation effects. To further probe these effects in SSCs over time a larger sample of SSCs must be observed spanning a larger age range.

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arXiv2023

Reconstructing the shock history in the CMZ of NGC 253 with ALCHEMI

HNCO and SiO are well known shock tracers and have been observed in nearby galaxies, including the nearby (D=3.5 Mpc) starburst galaxy NGC 253. The simultaneous detection of these two species in regions where the star formation rate is high may be used to study the shock history of the gas. We perform a multi-line molecular study using these two shock tracers (SiO and HNCO) with the aim of characterizing the gas properties. We also explore the possibility of reconstructing the shock history in NGC 253's Central Molecular Zone (CMZ). Six SiO transitions and eleven HNCO transitions were imaged at high resolution $1''.6$ (28 pc) with the Atacama Large Millimeter/submillimeter Array (ALMA) as part of the ALCHEMI Large Programme. Both non-LTE radiative transfer analysis and chemical modelling were performed in order to characterize the gas properties, and to investigate the chemical origin of the emission. The non-LTE radiative transfer analysis coupled with Bayesian inference shows clear evidence that the gas traced by SiO has different densities and temperatures than that traced by HNCO, with an indication that shocks are needed to produce both species. Chemical modelling further confirms such a scenario and suggests that fast and slow shocks are responsible for SiO and HNCO production, respectively, in most GMCs. We are also able to infer the physical characteristics of the shocks traced by SiO and HNCO for each GMC. Radiative transfer and chemical analysis of the SiO and HNCO in the CMZ of NGC 253 reveal a complex picture whereby most of the GMCs are subjected to shocks. We speculate on the possible shock scenarios responsible for the observed emission and provide potential history and timescales for each shock scenario. Higher spatial resolution observations of these two species are required in order to quantitatively differentiate between scenarios.

astro-ph.GA↗

arXiv2021

ALCHEMI: an ALMA Comprehensive High-resolution Extragalactic Molecular Inventory. Survey presentation and first results from the ACA array

We used the Atacama Large Millimeter/submillimeter Array (ALMA), covering a nearly contiguous 289 GHz frequency range between 84.2 and 373.2 GHz, to image the continuum and spectral line emission at 1.6\arcsec ($\sim 28$ pc) resolution down to a sensitivity of $30-50$ mK. This article describes the ALMA Comprehensive High-resolution Extragalactic Molecular Inventory (ALCHEMI) Large Program. We focus on the analysis of the spectra extracted from the $15''$ ($\sim255$ pc) resolution ALMA Compact Array data. We model the molecular emission assuming local thermodynamic equilibrium with 78 species detected. Additionally, multiple hydrogen and helium recombination lines are identified. Spectral lines contribute 5 to 36\% of the total emission in frequency bins of 50 GHz. We report the first extragalactic detections of C$_2$H$_5$OH, HOCN, HC$_3$HO, and several rare isotopologues. Isotopic ratios of carbon, oxygen, sulfur, nitrogen and silicon were measure with multiple species. Infrared pumped vibrationaly excited HCN, HNC, and HC$_3$N emission, originating in massive star formation locations, is clearly detected at low resolution, while we do not detect it for HCO$^+$. We suggest high temperature conditions in these regions driving a seemingly "carbon-rich" chemistry which may also explain the observed high abundance of organic species close to those in Galactic hot cores. The $L_{vib}/L_{IR}$ ratio is used as a proxy to estimate a $3\%$ contribution from proto super star cluster to the global infrared emission. Measured isotopic ratios with high dipole moment species agree with those within the central kiloparsec of the Galaxy, while those derived from $\rm^{13}C^{18}O$ are a factor of 5 larger, confirming the existence of multiple ISM components within NGC 253 with different degrees of nucleosynthesis enrichment. ALCHEMI provides a template for early Universe galaxies.

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arXiv2021

Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde IV. The ALMA view of N113 and N159W in the LMC

We mapped the kinetic temperature structure of two massive star-forming regions, N113 and N159W, in the Large Magellanic Cloud (LMC). We have used $\sim$1\hbox{$\,.\!\!^{\prime\prime}$}6\,($\sim$0.4\,pc) resolution measurements of the para-H$_2$CO\,$J_{\rm K_ aK_c}$\,=\,3$_{03}$--2$_{02}$, 3$_{22}$--2$_{21}$, and 3$_{21}$--2$_{20}$ transitions near 218.5\,GHz to constrain RADEX non-LTE models of the physical conditions. The gas kinetic temperatures derived from the para-H$_2$CO line ratios 3$_{22}$--2$_{21}$/3$_{03}$--2$_{02}$ and 3$_{21}$--2$_{20}$/3$_{03}$--2$_{02}$ range from 28 to 105\,K in N113 and 29 to 68\,K in N159W. Distributions of the dense gas traced by para-H$_2$CO agree with those of the 1.3\,mm dust and \emph{Spitzer}\,8.0\,$μ$m emission, but do not significantly correlate with the H$α$ emission. The high kinetic temperatures ($T_{\rm kin}$\,$\gtrsim$\,50\,K) of the dense gas traced by para-H$_2$CO appear to be correlated with the embedded infrared sources inside the clouds and/or YSOs in the N113 and N159W regions. The lower temperatures ($T_{\rm kin}$\,$<$\,50\,K) are measured at the outskirts of the H$_2$CO-bearing distributions of both N113 and N159W. It seems that the kinetic temperatures of the dense gas traced by para-H$_2$CO are weakly affected by the external sources of the H$α$ emission. The non-thermal velocity dispersions of para-H$_2$CO are well correlated with the gas kinetic temperatures in the N113 region, implying that the higher kinetic temperature traced by para-H$_2$CO is related to turbulence on a $\sim$0.4\,pc scale. The dense gas heating appears to be dominated by internal star formation activity, radiation, and/or turbulence. It seems that the mechanism heating the dense gas of the star-forming regions in the LMC is consistent with that in Galactic massive star-forming regions located in the Galactic plane.

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arXiv2021

CON-quest: Searching for the most obscured galaxy nuclei

Some luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs) host extremely compact and dusty nuclei. The intense infrared radiation arising from warm dust in these sources is prone to excite vibrational levels of molecules such as HCN. This results in emission from the rotational transitions of vibrationally excited HCN (HCN-vib), with the brightest emission found in compact obscured nuclei (CONs). We aim to establish how common CONs are in the local Universe, and whether their prevalence depends on the luminosity or other properties of the host galaxy. We have conducted an Atacama Large Millimeter/submillimeter Array (ALMA) survey of the rotational J=3-2 transition of HCN-vib in a sample of 46 far-infrared luminous galaxies. Compact obscured nuclei are identified in 38 percent of ULIRGs, 21 percent of LIRGs, and 0 percent of lower luminosity galaxies. We find no dependence on the inclination of the host galaxy, but strong evidence of lower IRAS 25 to 60 μm flux density ratios (f25/f60) in CONs compared to the rest of the sample. Furthermore, we find that CONs have stronger silicate features (s9.7μm) but similar PAH equivalent widths (EQW6.2μm) compared to other galaxies. In the local Universe, CONs are primarily found in (U)LIRGs. High resolution continuum observations of the individual nuclei are required to determine if the CON phenomenon is related to the inclinations of the nuclear disks. The lower f25/f60 ratios in CONs as well as the results for the mid-infrared diagnostics investigated are consistent with large dust columns shifting the nuclear radiation to longer wavelengths, making the mid- and far-infrared "photospheres" significantly cooler than the interior regions. To assess the importance of CONs in the context of galaxy evolution, it is necessary to extend this study to higher redshifts where (U)LIRGs are more common.

astro-ph.GA↗

arXiv2019

Hidden or missing outflows in highly obscured galaxy nuclei?

Understanding the nuclear growth and feedback processes in galaxies requires investigating their often obscured central regions. One way to do this is to use (sub)millimeter line emission from vibrationally excited HCN (HCN-vib). It has been suggested that the most intense HCN-vib emission from a galaxy is connected to a phase of nuclear growth that occurs before the nuclear feedback processes have been fully developed. We aim to investigate if there is a connection between the presence of strong HCN-vib emission and the development of feedback in (U)LIRGs. We collected literature and archival data to compare the luminosities of rotational lines of HCN-vib, normalized to the total infrared luminosity, to the median velocities of 119 μm OH absorption lines, potentially indicating outflows, in a total of 17 (U)LIRGs. The most HCN-vib luminous systems all lack signatures of significant molecular outflows in the far-infrared OH absorption lines. However, at least some of the systems with bright HCN-vib emission do have fast and collimated outflows that can be seen in spectral lines at longer wavelengths. We conclude that the galaxy nuclei with the highest L(HCN-vib)/L(IR) do not drive wide-angle outflows detectable using the median velocities of far-infrared OH absorption lines. It is possible that this is due to an orientation effect where sources which are oriented in such a way that their outflows are not along our line of sight also radiate a smaller proportion of their infrared luminosity in our direction. It could also be that massive wide-angle outflows destroy the deeply embedded regions responsible for bright HCN-vib emission, so that the two phenomena cannot coexist. This would strengthen the idea that vibrationally excited HCN traces a heavily obscured stage of evolution before nuclear feedback mechanisms are fully developed.

astro-ph.GA↗

arXiv2017

A hidden molecular outflow in the LIRG Zw 049.057

Feedback in the form of mass outflows driven by star formation or active galactic nuclei is a key component of galaxy evolution. The luminous infrared galaxy Zw 049.057 harbours a compact obscured nucleus with a possible far-IR signature of outflowing molecular gas. Due to the high optical depths at far-IR wavelengths, the interpretation of the outflow signature is uncertain. At mm and radio wavelengths, the radiation is better able to penetrate the large columns of gas and dust. We used high resolution observations from the SMA, ALMA, and the VLA to image the CO 2-1 and 6-5 emission, the 690 GHz continuum, the radio cm continuum, and absorptions by rotationally excited OH. The CO line profiles exhibit wings extending 300 km/s beyond the systemic velocity. At cm wavelengths, we find a compact (40 pc) continuum component in the nucleus, with weaker emission extending several 100 pc approximately along the major and minor axes of the galaxy. In the OH absorption lines toward the compact continuum, wings extending to a similar velocity as for the CO are seen on the blue side of the profile. The weak cm continuum emission along the minor axis is aligned with a highly collimated, jet-like dust feature previously seen in near-IR images of the galaxy. Comparison of the apparent optical depths in the OH lines indicate that the excitation conditions in Zw 049.057 differ from those in other OH megamaser galaxies. We interpret the wings in the spectral lines as signatures of a molecular outflow. A relation between this outflow and the minor axis radio feature is possible, although further studies are required to investigate this possible association and understand the connection between the outflow and the nuclear activity. Finally, we suggest that the differing OH excitation conditions are further evidence that Zw 049.057 is in a transition phase between megamaser and kilomaser activity.

astro-ph.GA↗

arXiv2011

Dense Gas in Nearby Galaxies: XVII. The Distribution of Ammonia in NGC253, Maffei2 and IC342

The central few 100 pc of galaxies often contain large amounts of molecular gas. The chemical and physical properties of these extragalactic star formation regions differ from those in galactic disks, but are poorly constrained. This study aims to develop a better knowledge of the spatial distribution and kinetic temperature of the dense neutral gas associated with the nuclear regions of three prototypical spiral galaxies, NGC253, IC342, and Maffei2. VLA CnD and D configuration measurements have been made of three ammonia (NH3) inversion transitions. The (J,K)=(1,1) and (2,2) transitions of NH3 were imaged toward IC342 and Maffei2. The (3,3) transition was imaged toward NGC253. The entire flux obtained from single-antenna measurements is recovered for all three galaxies observed. Derived lower limits to the kinetic temperatures determined for the giant molecular clouds in the centers of these galaxies are between 25 and 50K. There is good agreement between the distributions of NH3 and other H2 tracers, such as rare CO isotopologues or HCN, suggesting that NH3 is representative of the distribution of dense gas. The "Western Peak" in IC342 is seen in the (6,6) line but not in lower transitions, suggesting maser emission in the (6,6) transition.

astro-ph.GA↗

arXiv2007

Mechanical Measurements of the ALMA Prototype Antennas

The specifications of the Atacama Large Millimeter Array (ALMA) have placed stringent requirements on the mechanical performance of its antennas. As part of the evaluation process of the VertexRSI and Alcatel EIE Consortium (AEC) ALMA prototype antennas, measurements of the path length, thermal, and azimuth bearing performance were made under a variety of weather conditions and observing modes. The results of mechanical measurements, reported here, are compared to the antenna specifications.

astro-ph↗

arXiv2007

Study of the Dynamics of Large Reflector Antennas with Accelerometers

The Atacama Large Millimeter Array (ALMA) will consist of up to 64 state-of-the-art sub-mm telescopes, subject to stringent performance specifications which will push the boundaries of the technology, and makes testing of antenna performance a likewise challenging task. Two antenna prototypes were evaluated at the ALMA Test Facility at the Very Large Array site in New Mexico, USA. The dynamic behaviour of the antennas under operational conditions was investigated with the help of an accelerometer system capable of measuring rigid body motion of the elevation structure of the antenna, as well as a few low-order deformation modes, resulting in dynamic performance numbers for pointing stability, reflector surface stability, path length stability, and structure flexure. Special emphasis was given to wind effects, one of the major factors affecting performance on timescales of seconds to tens of minutes. This paper describes the accelerometer system, its capabilities and limitations, and presents the dynamic performance results of the two prototype antennas investigated.

astro-ph↗