2021A&A...646A.178S

Aims. In this paper we present the detection of H₂O and OH⁺ emission in z>3 hot dust-obscured galaxies (Hot DOGs).
Methods. Using ALMA Band-6 observations of two Hot DOGs, we detected H₂O(2₀₂-1₁₁) in W0149+2350, and H₂O(3₁₂ - 3₀₃) and the multiplet OH⁺(1₁-0₁) in W0410-0913. These detections were serendipitous, falling within the side-bands of Band-6 observations aimed to study CO(9-8) in these Hot DOGs.
Results. We find that both sources have luminous H₂O emission with line luminosities of L_H2O(202_–111₎>2.2x10⁸L_☉ and L_H2O(312_–303₎=8.7x10⁸L_☉ for W0149+2350 and W0410-0913, respectively. The H₂O line profiles are similar to those seen for the neighbouring CO(9-8) line, with line widths of full width at half maximum (FWHM)∼800-1000km/s. However, the H₂O emission seems to be more compact than the CO(9-8). OH⁺(1₁-0₁) is detected in emission for W0410-0913, with a FWHM=1000km/s and a line luminosity of L_OH+⁽¹1_–01₎=6.92x10⁸L_☉. The ratio of the observed H₂O line luminosity over the IR luminosity, for both Hot DOGs, is consistent with previously observed star-forming galaxies and active galactic nuclei (AGN). The H₂O/CO line ratio of both Hot DOGs and the OH⁺/H₂O line ratio of W0410-0913 are comparable to those of luminous AGN found in the literature.
Conclusions. The bright H₂O(2₀₂-1₁₁), and H₂O(3₁₂-3₀₃) emission lines are likely due to the combined high star formation levels and luminous AGN in these sources. The presence of OH⁺ in emission, and the agreement of the observed line ratios of the Hot DOGs with luminous AGN in the literature, would suggest that the AGN emission is dominating the radiative output of these galaxies. However, follow-up multi-transition observations are needed to better constrain the properties of these systems.