ALMAGAL IV. Morphological comparison of molecular and thermal dust emission using the histogram of oriented gradients (HOG) method

Context. The study of molecular line emission is crucial to unveil the kinematics and the physical conditions of gas in star-forming regions. We utilize data from the ALMAGAL survey which provides an unprecedentedly large statistical sample of high-mass star-forming clumps that helps to remove bias and reduce noise (e.g. due to source peculiarities, selection or environmental effects) to determine how well molecular species trace continuum emission.Aims. Our aim is to quantify correlations in the overall emission of dense gas tracers and cold dust by comparing the morphology of integrated molecular line emission of H2CO, CH3OH, DCN, HC3N, CH3CN, CH3OCHO, SO, and SiO with that of the 1.38mm dust continuum emission at different spatial scales in the ALMAGAL sample. We included two transitions of H2CO to understand the validity of the results depending on the excitation condition of the selected transition of a molecular specie. The ALMAGAL project observed more than a 1000 candidate high-mass star-forming clumps in ALMA Band 6 at a spatial resolution down to 1000 au. A total of 916 targets from this sample have been analysed in the present paper, covering all evolutionary stages of the high-mass star-formation process and different conditions of clump fragmentation. Methods. We used, for the first time on a large statistical sample, the method of the histogram of oriented gradients (HOG) implemented in the tool astroHOG to compare the morphology of integrated line emission with the maps of 1.38 mm dust continuum emission. For each clump, we defined two masks: one that covers the extended more diffuse continuum emission and a smaller one that contains only the compact sources. We select this two masks to study if and how the correlation among the selected molecules changes with the spatial scale of emission, from extended more diffuse gas in the clumps to denser gas in compact fragments (cores). Moreover, we calculated the Pearson’s correlation coefficient and compared it with our astroHOG results.Results. Among the molecular species analyzed in this paper, only H2CO, CH3OH, and SO show emission on spatial scales comparable to the diffuse of the 1.38 mm dust continuum emission. However, from the HOG method, the median correlation of the emission of each of these species with the continuum is only ∼24-29%. In comparison with the dusty dense fragments these molecular species still have low values of correlation, on average below 45%. The low level of morphological correlation suggests that these molecular lines likely trace on average the intraclump medium or outer layers around dense fragments (in some cases possibly due to opthical depth effects) or are also at this scale tracing inner parts of outflows. On the other hand DCN, HC3N, CH3CN, and CH3OCHO show a good correlation with the dense dust fragments, above 60%. The worst correlation is seen with SiO, both with the extended continuum emission and with compact sources. Moreover, unlike the other outflow tracers, SiO in a large fraction of the sources does not cover well the area of the extended continuum emission. This and the results of astroHOG reveal that SiO and SO do not trace the same gas, as previously thought. From the comparison of the results of the HOG method and the Pearson’s correlation coefficient, the HOG method gives much more reliable results than the intensity-based Pearson’s coefficient in estimating the level of similarity of the emission morphology.