The importance of radiative pumping on the emission of the H_2O submillimeter lines in galaxies

Abstract

H_2O submillimeter emission is a powerful diagnostic of the molecular interstellar medium in a variety of sources, including low- and high-mass star forming regions of the Milky Way, and from local to high redshift galaxies. However, the excitation mechanism of these lines in galaxies has been debated, preventing a basic consensus on the physical information that H_2O provides. Both radiative pumping due to H_2O absorption of far-infrared photons emitted by dust and collisional excitation in dense shocked gas have been proposed to explain the H_2O emission. Here we propose two basic diagnostics to distinguish between the two mechanisms: 1) in shock excited regions, the ortho-H_2O 3_{21}-2_{12} 75um and the para-H_2O 2_{20}-1_{11} 101um rotational lines are expected to be in emission while, if radiative pumping dominates, both far-infrared lines are expected to be in absorption; 2) based on statistical equilibrium of H_2O level populations, the radiative pumping scenario predicts that the apparent isotropic net rate of far-infrared absorption in the 3_{21}-2_{12} (75um) and 2_{20}-1_{11} (101um) lines should be higher than or equal to the apparent isotropic net rate of submillimeter emission in the 3_{21}-3_{12} (1163 GHz) and 2_20-2_{11} (1229 GHz) lines, respectively. Applying both criteria to all 16 galaxies and several galactic high-mass star-forming regions where the H_2O 75um and submillimeter lines have been observed with Herschel/PACS and SPIRE, we show that in most (extra)galactic sources the H_2O submillimeter line excitation is dominated by far-infrared pumping, with collisional excitation of the low-excitation levels in some of them. Based on this finding, we revisit the interpretation of the correlation between the luminosity of the H_2O 988 GHz line and the source luminosity in the combined galactic and extragalactic sample.

arXiv commentAccepted for publication in Astronomy & Astrophysics Letters

References in corpus (38)

• Herschel Space Observatory - An ESA facility for far-infrared and submillimetre astronomy
• The Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory
• The Herschel-SPIRE instrument and its in-flight performance
• Black hole accretion and star formation as drivers of gas excitation and chemistry in Mrk231
• Water in star-forming regions with Herschel (WISH): II. Evolution of 557 GHz 110-101 emission in low-mass protostars
• Water in star-forming regions (WISH): Physics and chemistry from clouds to disks as probed by Herschel spectroscopy
• Evidence for CO shock excitation in NGC 6240 from Herschel SPIRE spectroscopy
• [CII] absorption and emission in the diffuse interstellar medium across the Galactic Plane
• Submillimetre line spectrum of the Seyfert galaxy NGC1068 from the Herschel-SPIRE Fourier Transform Spectrometer
• H2O emission in high-z ultra-luminous infrared galaxies
• The complete far-infrared and submillimeter spectrum of the Class 0 protostar Serpens SMM1 obtained with Herschel. Characterizing UV-irradiated shocks heating and chemistry
• Water in star-forming regions with Herschel: highly excited molecular emission from the NGC 1333 IRAS 4B outflow
• Herschel observations of water vapour in Markarian 231
• Herschel-PACS Observations of Far-IR CO Line Emission in NGC 1068: Highly Excited Molecular Gas in the Circumnuclear Disk
• Water in star-forming regions with Herschel (WISH). IV. A survey of low-J H2O line profiles toward high-mass protostars
• Herschel observations of the Sgr B2 cores: Hydrides, warm CO, and cold dust
• The Herschel-PACS legacy of low-mass protostars: Properties of warm and hot gas and its origin in far-UV illuminated shocks
• Water vapor in nearby infrared galaxies as probed by Herschel
• Microwave Background Temperature at a Redshift of 6.34 from H2O Absorption
• Submillimeter H2O and H2O+ emission in lensed ultra- and hyper-luminous infrared galaxies at z ~ 2-4
• Water vapor emission reveals a highly obscured, star forming nuclear region in the QSO host galaxy APM08279+5255 at z=3.9
• Far-Infrared Spectroscopy of the Galactic Center. Hot Molecular Gas: Shocks versus Radiation near SgrA*
• Modeling the H2O submillimeter emission in extragalactic sources
• Observation of H2O in a strongly lensed Herschel-ATLAS source at z=2.3
• Far-infrared molecular lines from Low- to High-Mass Star Forming Regions observed with Herschel
• Herschel Far-Infrared Spectral-mapping of Orion BN/KL Outflows: Spatial distribution of excited CO, H2O, OH, O and C+ in shocked gas
• Shockingly low water abundances in Herschel / PACS observations of low-mass protostars in Perseus
• HIFI Spectroscopy of ${\mathrm{H}}_2\mathrm{O}$ submm Lines in Nuclei of Actively Star Forming Galaxies
• Herschel spectroscopic observations of the compact obscured nucleus in Zw 049.057
• CO, H2O, H2O+ line and dust emission in a z = 3.63 strongly lensed starburst merger at sub-kiloparsec scales
• A Far-Infrared Spectral Sequence of Galaxies: Trends and Models
• The Water Vapor Abundance in Orion KL Outflows
• A proto-pseudobulge in ESO 320-G030 fed by a massive molecular inflow driven by a nuclear bar
• Inflowing gas onto a compact obscured nucleus in Arp 299A: Herschel spectroscopic studies of H2O and OH
• Linking low- to high-mass YSOs with Herschel-HIFI observations of water
• First detection of the 448 GHz H2O transition in space
• Far infrared CO and H${}_2$O emission in intermediate-mass protostars
• Water emission tracing active star formation from the Milky Way to high-$z$ galaxies

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