HIFI Spectroscopy of ${\mathrm{H}}_2\mathrm{O}$ submm Lines in Nuclei of Actively Star Forming Galaxies

Abstract

We present a systematic survey of multiple velocity-resolved H${}_2$O spectra using Herschel/HIFI towards nine nearby actively star forming galaxies. The ground-state and low-excitation lines (E${}_{\mathrm{up}}\le 130\mathrm{K}$) show profiles with emission and absorption blended together, while absorption-free medium-excitation lines ($130\mathrm{K}\le E_{\mathrm{up}}\le 350\mathrm{K}$) typically display line shapes similar to CO. We analyze the HIFI observation together with archival SPIRE/PACS H${}_2$O data using a state-of-the-art 3D radiative transfer code which includes the interaction between continuum and line emission. The water excitation models are combined with information on the dust- and CO spectral line energy distribution to determine the physical structure of the interstellar medium (ISM). We identify two ISM components that are common to all galaxies: A warm ($T_{\mathrm{dust}}\sim 40-70\mathrm{K}$), dense ($n(\mathrm{H})\sim 10^5-10^6\mathrm{c}{\mathrm{m}}^{-3}$) phase which dominates the emission of medium-excitation H${}_2$O lines. This gas phase also dominates the FIR emission and the CO intensities for $J_{\mathrm{up}}>8$. In addition a cold ($T_{\mathrm{dust}}\sim 20-30\mathrm{K}$), dense ($n(\mathrm{H})\sim 10^4-10^5\mathrm{c}{\mathrm{m}}^{-3}$) more extended phase is present. It outputs the emission in the low-excitation H${}_2$O lines and typically also produces the prominent line absorption features. For the two ULIRGs in our sample (Arp 220 and Mrk 231) an even hotter and more compact (R${}_s\le 100$ pc) region is present which is possibly linked to AGN activity. We find that collisions dominate the water excitation in the cold gas and for lines with $E_{\mathrm{up}}\le 300\mathrm{K}$ and $E_{\mathrm{up}}\le 800\mathrm{K}$ in the warm and hot component, respectively. Higher energy levels are mainly excited by IR pumping.

Accepted by ApJ, in press

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