Emergence of high-mass stars in complex fiber networks (EMERGE); II. The need for data combination in ALMA observations

Context. High-resolution images from Atacama Large Millimetre Array (ALMA) allow for the filamentary structure of the interstellar medium (ISM) to be resolved down to a few thousand astronomical units (au) in star-forming regions located at kiloparsec (kpc) distances. Aims. We aim to systematically quantify the impact of the interferometric response and the effects of the short-spacing information during the characterization of the ISM structure using ALMA observations. Methods. We created a series of continuum ALMA synthetic observations to test the recovery of the fundamental observational properties of dense cores and filaments (i.e., intensity peak, radial profile, and width) at different spatial scales. We homogeneously compared the results obtained with and without different data combination techniques and using different ALMA arrays and SD telescopes in both simulated data and real observations. Results. Our analysis illustrates the severity of interferometric filtering effects. ALMA-12 m-alone observations show significant scale-dependent flux losses that systematically corrupt (>30% error) all the physical properties inferred in cores and filaments (i.e., column density, mass, and size) well before the maximum recoverable scale of the interferometer. These effects are only partially mitigated by the addition of the ALMA ACA-7 m array, although at the expenses of degrading the telescope point-spread-function (PSF). Our results demonstrate that only the addition of the ALMA Total Power(TP) information allows for the true sky emission to be recovered down to a few times the ALMA beamsize with sufficient accuracy (<10% error). Additional tests show that the emission recovery of cores and filaments at all scales is further improved if the 7 m+TP data are replaced by additional maps obtained by a larger SD telescope (e.g., IRAM-30 m), even if the latter are noisier than expected. In particular, these observational biases affect partially resolved targets, which becomes especially critical for studies in nearby regions such as Taurus or Orion. Conclusions. Our results demonstrate the need for the use of the state-of-the-art data combination techniques to accurately characterize the complex physical structure of the ISM in the ALMA era.