Finding faint HI structure in and around galaxies: Scraping the barrel

Abstract

Soon to be operational H I survey instruments such as APERTIF and ASKAP will produce large datasets. These surveys will provide information about the H I in and around hundreds of galaxies with a typical signal-to-noise ratio of ∼ 10 in the inner regions and ∼ 1 in the outer regions. In addition, such surveys will make it possible to probe faint H I structures, typically located in the vicinity of galaxies, such as extra-planar-gas, tails and filaments. These structures are crucial for understanding galaxy evolution, particularly when they are studied in relation to the local environment. Our aim is to find optimized kernels for the discovery of faint and morphologically complex H I structures. Therefore, using H I data from a variety of galaxies, we explore state-of-the-art filtering algorithms. We show that the intensitydriven gradient filter, due to its adaptive characteristics, is the optimal choice. In fact, this filter requires only minimal tuning of the input parameters to enhance the signal-to-noise ratio of faint components. In addition, it does not degrade the resolution of the high signal-to-noise component of a source. The filtering process must be fast and be embedded in an interactive visualization tool in order to support fast inspection of a large number of sources. To achieve such interactive exploration, we implemented a multi-core CPU (OpenMP) and a GPU (OpenGL) version of this filter in a 3D visualization environment (SlicerAstro).

Real-time colouring and filtering with graphics shaders

“…3D array) onto a 2D plane, or image plane, producing an image to be rendered on a display device. From its ability to provide a global 3D view of the data, volume rendering plays a role in the discovery of new phenomena, unexpected relations, or previously unidentified patterns that are deemed difficult to be accomplished with automated techniques (Beeson et al 2004;Goodman 2012;Punzo et al 2016).…”

“…In general, data needs to be pre-processed before interesting features become apparent during visualisation. For low signal-to-noise data, filtering techniques have been shown to enhance manual data inspection (Oosterloo 1996;Punzo et al 2016). Such techniques are commonplace in automated segmentation methodologies like source finding and source mask generation (Whiting 2012;Serra et al 2015), but have rarely been integrated with visualisation as they are often too compute-intensive.…”

3-D interactive visualisation tools for Hispectral line imaging

“…The exact shape is, however, not known a priory, so that in practice one often uses a set of different kernels and then inspects the results to make a final decision about which one suits best. Punzo et al (2016) have investigated this problem and showed that there is an optimum filtering algorithm that works very effectively on H I data. This filter is an adaptive filter, based on the intensity-gradient driven filter of Perona & Malik (1990), which appeared to be quite effective on H I data with well chosen default parameters.…”

“…This paper gives a brief overview of the capabilities of SlicerAstro. For a detailed description see Punzo et al (2015), Punzo et al (2016) and Punzo et al (2017, in preparation).…”

“…In addition the source code must be open, modular, well documented, and well maintained. Punzo et al (2015) reviewed several existing packages and decided to develop a module for 3DSlicer (URL: https://www.slicer.org/) which adds the following characteristics to the already large number of capabilities of the current 3DSlicer system: (i) the ability to read FITS (Flexible Image Transport System, Wells et al 1981) files and handle the astronomical world coordinates system (WCS, , and Greisen et al 2006, (ii) coupled 1-D, 2-D and 3-D display of the data and fast rendering, (iii) semi-automated filtering methods to recover faint emission using the information in all three dimensions (Punzo et al 2016), (iv) 3-D selection and (v) use of 3-D modelling to characterise and select the regularly rotating H I disks in galaxies. This paper gives a brief overview of the capabilities of SlicerAstro.…”