Searching for the largest bound atoms in space

Abstract

Context. Radio recombination lines (RRLs) at frequencies ν < 250 MHz trace the cold, diffuse phase of the interstellar medium. Yet, RRLs have been largely unexplored outside of our Galaxy. Next generation low frequency interferometers, such as LOFAR, MWA and the future SKA, with unprecedented sensitivity, resolution, and large fractional bandwidths, are enabling the exploration of the extragalactic RRL universe. Aims. We describe methods used to (1) process LOFAR high band antenna (HBA) observations for RRL analysis, and (2) search spectra for the presence of RRLs blindly in redshift space. Methods. We observed the radio quasar 3C 190 (z ≈ 1.2) with the LOFAR HBA. In reducing this data for spectroscopic analysis, we have placed special emphasis on bandpass calibration. We devised cross-correlation techniques that utilize the unique frequency spacing between RRLs to significantly identify the presence of RRLs in a low frequency spectrum. We demonstrate the utility of this method by applying it to existing low-frequency spectra of Cassiopeia A and M 82, and to the new observations of 3C 190. Results. RRLs have been detected in the foreground of 3C 190 at z = 1.12355 (assuming a carbon origin), owing to the first detection of RRLs outside of the local universe (first reported in Emig et al. 2019). Towards the Galactic supernova remnant Cassiopeia A, we uncover three new detections: (1) stimulated C -transitions (∆n = 5) for the first time at low radio frequencies, (2) Hα transitions at 64 MHz with a FWHM of 3.1 km s −1 the most narrow and one of the lowest frequency detections of hydrogen to date, and (3) Cα at v LSR ≈ 0 km s −1 in the frequency range 55-78 MHz for the first time. Additionally we recover Cα, Cβ, Cγ, and Cδ from the -47 km s −1 and -38 km s −1 components. In the nearby starburst galaxy, M 82, we do not find a significant feature. With previously used techniques, we reproduce the previously reported line properties. Conclusions. RRLs have been blindly searched and successfully identified in Galactic (to high order transitions) and extragalactic (to high redshift) observations with our spectral searching method. Our current searches for RRLs in LOFAR observations are limited to narrow (< 100 km s −1 ) features, owing to the relatively small number of channels available for continuum estimation. Future strategies making use of a wider band (covering multiple LOFAR subbands) or designs with larger contiguous frequency chunks would aid calibration to deeper sensitivities and broader features.

The LOFAR Two-metre Sky Survey

“…In addition, the frequency resolution is high enough for wide area searches for spectral lines such as carbon radio recombination lines (e.g. Emig et al 2020, Salas et al 2019. There are also other novel uses such as examining the time-variability of the data to search exoplanet signatures (e.g.…”

“…Vedantham et al 2020;Calling-ham et al 2021) and spectral line (see e.g. Emig et al 2020, Salas et al 2019) capabilities of the data.…”

The LOFAR LBA Sky Survey

“…LoLSS data make it possible to detect the turnover from free-free emission with fairly low emission measures, probing low-density (5 cm −3 ) warm ionised gas which may be prevalent in the mid-plane of galaxies (Mezger 1978). Even though one particular statistical study using LOFAR HBA at 144 MHz hinted that free-free absorption plays a minor role (Chyzy et al 2018), the contribution from cooler (T < 1000 K) ionised gas remains largely uncertain (Israel & Mahoney 1990;Emig et al 2020). LoLSS will probe a critical turnover frequency in SED modelling that can characterise ionised gas properties and distinguish its contributions from CRe propagation effects.…”

The First Large Absorption Survey in Hi(FLASH): I. Science goals and survey design

“…The improved sensitivity of high-resolution interferometers and the large fractional bandwidths that enable deeper searches through line stacking are making RRL observations now feasible. The development of new cross-correlation techniques enables blind searches of RRLs across redshift space (Emig et al 2020).…”