Excited OH+, H2O+, and H3O+ in NGC 4418 and Arp 220
We report on Herschel/PACS observations of absorption lines of OH +, H2O+ and H3O+ in NGC 4418 and Arp 220. Excited lines of OH+ and H2O+ with Elower of at least 285 and ∼200 K, respectively, are detected in both sources, indicating radiative pumping and location in the high radiation density enviro...
| Main Authors: | , , , , , , , , , , , , , , , , , |
|---|---|
| Format: | Journal article |
| Language: | English |
| Published: |
2013
|
| _version_ | 1797095623468515328 |
|---|---|
| author | Gonzalez-Alfonso, E Fischer, J Bruderer, S Mueller, H Gracia-Carpio, J Sturm, E Lutz, D Poglitsch, A Feuchtgruber, H Veilleux, S Contursi, A Sternberg, A Hailey-Dunsheath, S Verma, A Christopher, N Davies, R Genzel, R Tacconi, L |
| author_facet | Gonzalez-Alfonso, E Fischer, J Bruderer, S Mueller, H Gracia-Carpio, J Sturm, E Lutz, D Poglitsch, A Feuchtgruber, H Veilleux, S Contursi, A Sternberg, A Hailey-Dunsheath, S Verma, A Christopher, N Davies, R Genzel, R Tacconi, L |
| author_sort | Gonzalez-Alfonso, E |
| collection | OXFORD |
| description | We report on Herschel/PACS observations of absorption lines of OH +, H2O+ and H3O+ in NGC 4418 and Arp 220. Excited lines of OH+ and H2O+ with Elower of at least 285 and ∼200 K, respectively, are detected in both sources, indicating radiative pumping and location in the high radiation density environment of the nuclear regions. Abundance ratios OH +/H2O+ of 1-2.5 are estimated in the nuclei of both sources. The inferred OH+ column and abundance relative to H nuclei are (0.5-1) × 1016 cm-2 and ∼ 2 × 10-8, respectively. Additionally, in Arp 220, an extended low excitation component around the nuclear region is found to have OH +/H2O+ ∼ 5-10. H3O+ is detected in both sources with N(H3O+) ∼ (0.5-2) × 1016 cm-2, and in Arp 220 the pure inversion, metastable lines indicate a high rotational temperature of ∼500 K, indicative of formation pumping and/or hot gas. Simple chemical models favor an ionization sequence dominated by H+ → O+ → OH+ → H2O+ → H3O +, and we also argue that the H+ production is most likely dominated by X-ray/cosmic ray ionization. The full set of observations and models leads us to propose that the molecular ions arise in a relatively low density (≥104 cm-3) interclump medium, in which case the ionization rate per H nucleus (including secondary ionizations) is ζ > 10-13 s-1, a lower limit that is several × 102 times the highest current rate estimates for Galactic regions. In Arp 220, our lower limit for ζ is compatible with estimates for the cosmic ray energy density inferred previously from the supernova rate and synchrotron radio emission, and also with the expected ionization rate produced by X-rays. In NGC 4418, we argue that X-ray ionization due to an active galactic nucleus is responsible for the molecular ion production. © 2013 ESO. |
| first_indexed | 2024-03-07T04:30:31Z |
| format | Journal article |
| id | oxford-uuid:ce24abe3-600a-47b5-afce-4baa4c926399 |
| institution | University of Oxford |
| language | English |
| last_indexed | 2024-03-07T04:30:31Z |
| publishDate | 2013 |
| record_format | dspace |
| spelling | oxford-uuid:ce24abe3-600a-47b5-afce-4baa4c9263992022-03-27T07:33:50ZExcited OH+, H2O+, and H3O+ in NGC 4418 and Arp 220Journal articlehttp://purl.org/coar/resource_type/c_dcae04bcuuid:ce24abe3-600a-47b5-afce-4baa4c926399EnglishSymplectic Elements at Oxford2013Gonzalez-Alfonso, EFischer, JBruderer, SMueller, HGracia-Carpio, JSturm, ELutz, DPoglitsch, AFeuchtgruber, HVeilleux, SContursi, ASternberg, AHailey-Dunsheath, SVerma, AChristopher, NDavies, RGenzel, RTacconi, LWe report on Herschel/PACS observations of absorption lines of OH +, H2O+ and H3O+ in NGC 4418 and Arp 220. Excited lines of OH+ and H2O+ with Elower of at least 285 and ∼200 K, respectively, are detected in both sources, indicating radiative pumping and location in the high radiation density environment of the nuclear regions. Abundance ratios OH +/H2O+ of 1-2.5 are estimated in the nuclei of both sources. The inferred OH+ column and abundance relative to H nuclei are (0.5-1) × 1016 cm-2 and ∼ 2 × 10-8, respectively. Additionally, in Arp 220, an extended low excitation component around the nuclear region is found to have OH +/H2O+ ∼ 5-10. H3O+ is detected in both sources with N(H3O+) ∼ (0.5-2) × 1016 cm-2, and in Arp 220 the pure inversion, metastable lines indicate a high rotational temperature of ∼500 K, indicative of formation pumping and/or hot gas. Simple chemical models favor an ionization sequence dominated by H+ → O+ → OH+ → H2O+ → H3O +, and we also argue that the H+ production is most likely dominated by X-ray/cosmic ray ionization. The full set of observations and models leads us to propose that the molecular ions arise in a relatively low density (≥104 cm-3) interclump medium, in which case the ionization rate per H nucleus (including secondary ionizations) is ζ > 10-13 s-1, a lower limit that is several × 102 times the highest current rate estimates for Galactic regions. In Arp 220, our lower limit for ζ is compatible with estimates for the cosmic ray energy density inferred previously from the supernova rate and synchrotron radio emission, and also with the expected ionization rate produced by X-rays. In NGC 4418, we argue that X-ray ionization due to an active galactic nucleus is responsible for the molecular ion production. © 2013 ESO. |
| spellingShingle | Gonzalez-Alfonso, E Fischer, J Bruderer, S Mueller, H Gracia-Carpio, J Sturm, E Lutz, D Poglitsch, A Feuchtgruber, H Veilleux, S Contursi, A Sternberg, A Hailey-Dunsheath, S Verma, A Christopher, N Davies, R Genzel, R Tacconi, L Excited OH+, H2O+, and H3O+ in NGC 4418 and Arp 220 |
| title | Excited OH+, H2O+, and H3O+ in NGC 4418 and Arp 220 |
| title_full | Excited OH+, H2O+, and H3O+ in NGC 4418 and Arp 220 |
| title_fullStr | Excited OH+, H2O+, and H3O+ in NGC 4418 and Arp 220 |
| title_full_unstemmed | Excited OH+, H2O+, and H3O+ in NGC 4418 and Arp 220 |
| title_short | Excited OH+, H2O+, and H3O+ in NGC 4418 and Arp 220 |
| title_sort | excited oh h2o and h3o in ngc 4418 and arp 220 |
| work_keys_str_mv | AT gonzalezalfonsoe excitedohh2oandh3oinngc4418andarp220 AT fischerj excitedohh2oandh3oinngc4418andarp220 AT bruderers excitedohh2oandh3oinngc4418andarp220 AT muellerh excitedohh2oandh3oinngc4418andarp220 AT graciacarpioj excitedohh2oandh3oinngc4418andarp220 AT sturme excitedohh2oandh3oinngc4418andarp220 AT lutzd excitedohh2oandh3oinngc4418andarp220 AT poglitscha excitedohh2oandh3oinngc4418andarp220 AT feuchtgruberh excitedohh2oandh3oinngc4418andarp220 AT veilleuxs excitedohh2oandh3oinngc4418andarp220 AT contursia excitedohh2oandh3oinngc4418andarp220 AT sternberga excitedohh2oandh3oinngc4418andarp220 AT haileydunsheaths excitedohh2oandh3oinngc4418andarp220 AT vermaa excitedohh2oandh3oinngc4418andarp220 AT christophern excitedohh2oandh3oinngc4418andarp220 AT daviesr excitedohh2oandh3oinngc4418andarp220 AT genzelr excitedohh2oandh3oinngc4418andarp220 AT tacconil excitedohh2oandh3oinngc4418andarp220 |