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

ALMA's high-resolution images allow to resolve the filamentary structure of the ISM down to few thousand au at kpc distances. 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. We create a series of continuum ALMA synthetic observations to test the recovery of the observational properties of dense cores and filaments (i.e. intensity peak, radial profile, and width) at different scales. We compare the results obtained with and without different data combination techniques using different ALMA arrays and SD telescopes in simulated data and real observations. Our analysis illustrates the severity of interferometric filtering effects. ALMA-12m alone observations show significant scale-dependent flux losses systematically corrupting (>30%error) all the physical properties inferred in cores and filaments (i.e. column density, mass, and size) before the maximum recoverable scale of the interferometer. These effects are only partially mitigated by the addition of the ALMA ACA-7m array although degrading the telescope PSF. Our results demonstrate only the addition of the ALMA Total Power information allows to recover the true sky emission down to few times the ALMA beamsize with satisfactory accuracy (<10% error). Additional tests demonstrate the emission recovery at all scales is further improved if the 7mTP data are replaced by maps obtained by a larger SD telescope (e.g., IRAM-30m), even if the latter are noisier than expected. These observational biases particularly affect partially resolved targets, becoming critical especially for studies in nearby regions such as Taurus or Orion. Our results demonstrate the need for the use of data combination techniques to accurately characterize the complex physical structure of the ISM in the ALMA era.