Technical Development at the German ARC node

    As ALMA now has steady, full-scale operations, the focus is shifting to improvements in data delivery and handling and observing efficiency gains. At the same time, ALMA follows a stringent and ambitious upgrade plan throughout the 2020-2030 period that requires ongoing development. To support and complement ALMA development activities and provide users with the best possible tools and methods for data use, the European ARC nodes are engaged in numerous studies and development projects. For the past 8 years, our German node has been able to contribute to these activities thanks to the support through the BMBF-Verbundforschung and ASTRONET. Our goals are to continue to be engaging in technical developments that push the necessary innovation to let ALMA reach its full science capabilities and to strengthen German expertise in submm astronomy. The EU-ARC nodes work on self-defined technical issues that are crucial for an optimal use of ALMA. Our node concentrates on the topics described below:

Current Development

Spectral line analysis and radiative transfer tools

The Cologne Database for Molecular Spectroscopy (CDMS)

 The Cologne Database for Molecular Spectroscopy (CDMS) provides a molecular line catalogue in the millimeter and sub-millimeter frequency range, making a large database available to all users. It is part of the European project VAMDC (Virtual Atomic and Molecular Data Centre) and is maintained by the University of Cologne. The CDMS/VAMDC database is the foundation of the work we are doing in analyzing molecular data. Without molecular frequencies and line strengths, one could not derive the information we do about the physical state of molecular clouds. Even though it already contains many entries, it is and cannot be, by its very nature, something that is finished at some point. New and more precise frequencies are measured all the time and need to be fed into the database with good quality control. With the high sensitivity of ALMA, many isotopologues and vibrationally excited states that were out of reach of previous instruments actually dominate many ALMA spectra. Even if there is no intrinsic interest in them for a specific science purpose, they need to be understood since they will blend and contaminate lines of other species of interest. There is the traditional way of doing this, by fitting Hamiltonians to measured data and predict others, and this will be necessary to continue. In addition, there are alternative approaches, using laboratory data measured at different temperatures, which give all the lines of all levels, but without assignment to quantum states. This is unsatisfactory for molecular spectroscopists, but adequate for astrophysicists. The database and XCLASS infrastructure needs to be extended to handle these kinds of data, and they need to be inserted into the database to maximize the astrophysical usefulness. The data set used here needs to be consolidated with the NRAO database Splatalogue to give ALMA users a consistent view of molecular data.

EXtended CASA Line Analysis Software Suite (XCLASS)

 The eXtended CASA Line Analysis Software Suite (XCLASS) is a toolbox for the Common Astronomy Software Applications package (CASA) containing new functions for modeling interferometric and single dish data (T. Möller, C. Endres, and P. Schilke, 2017, A&A 598, A7). Among the tools is the myXCLASS program which calculates synthetic spectra by solving the radiative transfer equation for an isothermal object in one dimension, where the finite source size and dust attenuation are considered as well. Molecular data required by the myXCLASS program are taken from an embedded SQLite3 database containing entries from the Cologne Database for Molecular Spectroscopy CDMS) and JPL using the Virtual Atomic and Molecular Data Center (VAMDC) portal. Additionally, the toolbox provides an interface for the model optimizer package Modeling and Analysis Generic Interface for eXternal numerical codes (MAGIX), which helps to find the best description of observational data using myXCLASS (or another external model program), i.e., finding the parameter set that most closely reproduces the data. XCLASS has already been used in more than 30 publications.

The latest XCLASS release (which can be downloaded from our website) includes a first version of the LineIdentification function. The function analyzes a given spectrum and identifies the contained molecules automatically. Therefore, the function determines all molecules which have at least one transition within the user defined frequency range(s). In order to calculate the contribution of each molecule, the LineIdentification function performs so-called single molecule fits for each molecule. If a molecule covers a defined fraction of the spectrum the molecule is "for now" identified and the corresponding optimized molfit file is append to a so-called overall molfit file which describes the contribution of all identified molecules. After all single molecule fits are done, the LineIdentification function performs a final fit, using the overall molfit file created before to take line blending effects into account.

The development and the extension of capabilities continue: To increase the user base, we developed a XCLASS-GUI, which will be included in one of the next XCLASS releases. Future XCLASS releases will also offer the possibility to use non-LTE description of single components using the RADEX formalism, to describe radio recombination lines (in LTE and Non-LTE), to describe non-Gaussian line profiles (like Lorentz, Voigt, and Horn line profiles), to take local-overlap of neighboring lines of molecules and RRLs into account and to model dust, free-free, and synchrotron continuum contributions. In addition, the improved MapFit function will produce smoother parameter maps.

Single-dish data combination

 Most of the areas of study in astronomy, such as star formation, the study of extragalactic structures, the evolution of stars and planetary nebulae, depend on high spatial dynamic range observations that can provide information on both the diffuse, extended emission and the dense, more localized emission where astrophysical processes happen. In the last decade, facilities like ALMA aim at providing extremely high fidelity images by combining observations at high-spatial resolution (usually interferometers, or INTF) with lower-spatial resolution (usually single-dish telescopes, or SD) that can recover the extended emission. The German ALMA ARC node is leading the recently organized DataCombination team (see website here), which aims at testing different methods for the combination of high and low-spatial resolution images, with the goal of providing the community with a guide on how to combine interferometric and single-dish data.

During the last years, the German ALMA ARC node developed a GUI software tool (INTF/SD GUI) for data combination, which works on continuum images as well as position-position-frequency data cubes, offering the possibility to examine the combination of INTF and SD interactively. After beta-testing in December 2015, the INTF/SD GUI was extended to increase user input parameter flexibility with a script function that allows to save and restore past parameter settings. Activities in the improvement of the INTF/SD GUI are now under development within the DataCombination team, which include members of multiple EU, NA and EA ARC nodes.
    Three main methods are being tested for the data combination process:
  • Joint deconvolution: In this method, both the interferometer and single-dish data are combined in the so-called uv plane (or visibility space). For this, it is necessary to create visibilities for the single-dish image.
  • Feather: This method makes use of two individually created images for the single-dish and interferometer telescopes. The feathering technique combines the two images in the Fourier transform plane
  • Model-assisted cleaning: This method uses the single-dish image as a model when performing the cleaning (generation of interferometric image) of the interferometric data. The use of a model usually helps in the convergence of the interferometric image. As a second step, the single-dish image and the interferometric image (produced with the single-dish as a model) are combined using the feathering technique.

mm-VLBI with ALMA

 Staff of the German ARC node with special expertise in VLBI have supported the development and commissioning of the mm-VLBI mode at ALMA since 2012, initially by serving as translators and mediators between ALMA stakeholders and mmVLBI network operator and by helping to develop the required new policies, procedures and tools. Today, the activities focus on making the mm-VLBI mode easy to access for regular ALMA users.

Up-to-date guidelines on how to apply for time at mmVLBI networks including ALMA, scheduling and observational considerations and information on data processing and reduction can be found in our mmVLBI support section.

While the German astronomical community as a whole has been extraordinarily successful since the first Call for Proposals including mmVLBI with the ALMA phased array, the successful investigators so far almost exclusively originate from the mm-VLBI community. ALMA attracts some of the brightest minds in astronomy, yet, the mmVLBI modes seems to be difficult to access for ALMA users. In order to amend this situation, the German node has identified a number of measures that make mmVLBI more easily accessible to the regular ALMA user. The German VLBI community profits as well, by getting state-of-the-art tools that can be easily adapted to the classical VLBI and by an influx of new researchers and diverse ideas.

Currently, we are developing the simulation tool simvlbi to facilitate the planning of observations with the existing mmVLBI arrays that can include ALMA. Special care is taken to keep simvlbi easy to install and easy to use, while keeping its flexibility for expert users: The tool runs in the latest CASA releases and only requires four small files to be downloaded into the working directory. Regular ALMA users interested in trying out the new observing mode can run the code in the easy mode by only specifying the source coordinates, the total observing time and the chosen array (GMVA or EHT). In contrast, the expert mode of simvlbi allows expert users to fine-tune additional parameters like the correlator setting and the duty cycle and/or to define an interferometric array by themselves. The output of simvlbi includes diagnostic plots, the expected field of view and other useful planning parameters. Our CASA software tool simvlbi also includes a time/sensitivity calculator for the chosen interferometric array, which takes into account the rising and setting of the observed source at each telescope in the array. This newly developed tool thus exceeds the capabilities of the most commonly used sensitivity calculators in the VLBI community.

A first public release of simvlbi is foreseen for February 2020. The beta-version of the tool is already available for mmVLBI planning and simulation support at the German ARC node. Interested users are encouraged to contact the German ARC node at arc(at)astro.uni-bonn.de.


STATCONT: A statistical continuum level determination method for line-rich sources

STATCONT is a python-based tool designed to determine the continuum emission level in line-rich spectral data. The tool inspects the intensity distribution of a given spectrum and automatically determines the continuum level by using different statistical approaches. The different methods included in STATCONT are tested against synthetic data. We conclude that the sigma-clipping algorithm provides the most accurate continuum level determination, together with information on the uncertainty in its determination. This uncertainty can be used to correct the final continuum emission level, resulting in the here called 'corrected sigma-clipping method' or c-SCM. The c-SCM has been tested against more than 750 different synthetic spectra reproducing typical conditions found in astronomical sources. The continuum level is determined with a discrepancy of less than 1% in 50% of the cases, and less than 5% in 90% of the cases, when there is at least a fraction of 10% channels line free. The main products of STATCONT are the continuum emission level, together with a conservative value of its uncertainty, and data cubes containing only spectral line emission, i.e. continuum-subtracted data cubes. STATCONT also includes the option to estimate the spectral index, when providing different files covering different frequency ranges. For more information on installation, examples, etc, please see the STATCONT homepage.



Selection of Completed Projects

ARTIST

 Within the framework of the ASTRONET programme, "Common Tools for Future Large Submillimeter Facilities", we have initiated a project, "Adaptable Radiative Transfer Innovations for Submillimeter Telescopes (ARTIST)", to develop a next generation model suite for comprehensive multi-dimensional radiative transfer calculations of the dust and line emission, as well as their polarization. More information on the project can be found at the ARTIST homepage.
    The ARTIST package consists of:
  • An innovative radiative transfer code (LIME) using adaptive gridding that allows simulations of sources with arbitrary multi-dimensional (1D, 2D, 3D) and time-dependent structures, ensuring rapid convergence.
  • Unique tools for modeling the polarization of the line and dust emission, information that will come with standard ALMA observations.
  • A library of commonly used analytic/semi-analytic models as well as a comprehensive graphical user interface.

CATS

 In the ASTRONET-funded CATS program ("Coherent Set of Astrophysical Tools for Spectroscopy"), the German part consisted of MAGIX, the Modeling and Analysis Generic Interface for eXternal numerical codes, which provides a generic interface for optimization of model parameters in numerical models of observations (T. Möller, I. Bernst, D. Panoglou et al., 2013, A&A 549, A21). The MAGIX package provides the framework for an easy-to-use interface between existing codes and an iterating engine that minimizes deviations of the model results from available observational data, constraining the model parameters and providing good error estimates. MAGIX complies with the data structures and reduction tools of ALMA, but can also be used with other astronomical and with non-astronomical data. MAGIX has been used in more than 30 publications. The latest release of MAGIX (version 2.0.7) can be downloaded from the MAGIX homepage.