J/A+A/709/A152ivo://CDS.VizieR/J/A+A/709/A152CDSNigou E.Godard B.Guillard P.Pineau des Forets G.,Miville-Deschenes M.A.Lesaffre P.2026-05-19T14:14:26Z2026-05-13T07:49:43ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frinterstellar-mediumspectroscopyinfrared-astronomyA statistically significant sampling of H_2_ rotational excitation in the diffuse interstellar medium (ISM) is essential to identify its dominant excitation mechanisms and assess the importance of H_2_ in the cooling of the gas and the regulation of thermal pressure. To expand upon the statistics provided by ancillary telescopes, we conduct a search for pure rotational H_2_ emission lines in all publicly available background observations obtained with the Medium Resolution Spectrometer (MRS) aboard the JWST. The sample consists of 276 background observations acquired over the past three years. Departing from the standard pipeline, each uncalibrated MRS background file is reprocessed, enabling the analysis of H_2_ pure rotational emission. Lines of sight likely associated with star-forming complexes are excluded in order to focus on emission from the diffuse ISM. The results are compared with FUSE absorption data and are analyzed in relation to the column densities of H and H_2_ and dust emission derived from HI4PI, Planck, and WISE data. This analysis reveals widespread H_2_ emission throughout the Galaxy. We report the first detections of the pure rotational S(4), S(5), and S(7) lines in the diffuse ISM. The S(1) line is detected along 84 lines of sight, corresponding to a detection rate of 41%. Its integrated intensity decreases steeply with Galactic latitude, spanning nearly two orders of magnitude in remarkable agreement with absorption measurements. The T34 and T35 excitation temperatures are correlated, and vary between 200 and ~1000K, showing an anticorrelation with the column density of H_2_ as expected from ancillary data. All lines of sight in the sample have undergone the H-H_2_ transition, at NH>~10^+20^cm^-2^, and are partly molecular with f_H2_>~0.1. Under these conditions, the cooling rate associated with the S(1) line, expressed per hydrogen atom, is found to be remarkably constant, with a characteristic value of ~4x10^-27^erg/s/H. This study demonstrates that the high sensitivity of the JWST enables measurements that both strengthen and complement those from absorption studies. Observations collected over just a fraction of JWST's lifetime have already yielded detections along dozens of lines of sight, significantly expanding the statistical sample of H_2_ rotational excitation in the diffuse ISM.2026A&A...709A.152Nhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/709/A152CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toJ/A+A/594/A116 : HI4PI spectra and column density maps (HI4PI team+, 2016)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/709/A152https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/709/A152http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/709/A152https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/709/A152https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/709/A152http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/709/A152GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/A+A/709/A152/tablea1 (Rotational lines detections)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/709/A152/tablea1?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/709/A152/tablea1?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/709/A152/tablea1?GETtext/xml+votable180.050000true315.6947468.235630.005555555555555556Cone search capability for table J/A+A/709/A152/tableb1 (Diffuse medium Galactic properties)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/709/A152/tableb1?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/709/A152/tableb1?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/709/A152/tableb1?GETtext/xml+votable180.050000true315.6947468.235630.0055555555555555566/543 579 583 650 860 1054 1074 1076 1114 1125 1539 1703 2172 2240 2412 4099 4467 4865 5116 5263 5385 5484 5759 5909 5969 6005 6011 6803 7552 7606 7904 8217 8328 8555 8673 8724 8997 9182 9322 9407 9492 10197 10355 10856 10881 10912 10980 11027 11101 11178 11206 11355 11382 11416 11434 11557 11608 12255 12537 13511 13522 13557 13632 13995 14188 14279 14299 14655 14744 14788 14937 15172 16042 16129 16653 16755 17141 17506 17657 17686 17690 17754 18122 18707 19283 20613 20621 21215 21430 22803 22805 22854 23078 23300 23549 23566 24185 25049 25055 25083 25169 25582 25907 25958 26148 26614 26783 27199 27256 27258 27396 27409 27457 27474 27521 27765 27767 27914 28086 28254 28393 28535 29072 29092 29103 29154 29184 29286 29398 30269 30366 30519 31081 31158 31623 32396 33094 33096 33106 33318 33804 33958 34026 34273 34531 35447 35473 35935 36269 36785 36948 36956-36957 37128 37819 38139 38268 38382 38653 40221 41586 42120 42312 42446 42610 42885 42970 42981 43010 43262 43389 43874 44049 44391 44409 44660 46424 46603 46987 47137 47509 48076https://cdsarc.cds.unistra.fr/viz-bin/moc/J/A+A/709/A152?format=asciiInfrareddefaultJ/A+A/709/A152/tablea1Rotational lines detectionsrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordint_RA.icrsRight ascension (ICRS) (computed by VizieR, not part of the original data. The format may include more digits than the original data because of internal accuracy requirements in VizieR and across other CDS services)degpos.eq.raindexed_DE.icrsDeclination (ICRS) (computed by VizieR, not part of the original data. The format may include more digits than the original data because of internal accuracy requirements in VizieR and across other CDS services)degpos.eq.decindexedPIDJWST Program Identifiermeta.id;meta.mainintOIDJWST Observation Identifiermeta.id;obs.param;obsintFlag[0/1] Diffuse Medium Flag (1)meta.code.errorintGLONGalactic longitudedegpos.galactic.lonGLATGalactic latitudedegpos.galactic.latl_S(1)[<=] S(1) line detection flag (2)meta.code.errorcharS(1)? S(1) line integrated intensitymW.m**-2.sr**-1spect.line.intensitydoublenullablee_S(1)? S(1) line integrated intensity errormW.m**-2.sr**-1stat.errordoublenullablel_S(2)[<=] S(2) line detection flag (2)meta.code.errorcharS(2)? S(2) line integrated intensitymW.m**-2.sr**-1spect.line.intensitydoublenullablee_S(2)? S(2) line integrated intensity errormW.m**-2.sr**-1stat.errordoublenullablel_S(3)[<=] S(3) line detection flag (2)meta.code.errorcharS(3)? S(3) line integrated intensitymW.m**-2.sr**-1spect.line.intensitydoublenullablee_S(3)? S(3) line integrated intensity errormW.m**-2.sr**-1stat.errordoublenullablel_S(4)[<=] S(4) line detection flag (2)meta.code.errorcharS(4)? S(4) line integrated intensitymW.m**-2.sr**-1spect.line.intensitydoublenullablee_S(4)? S(4) line integrated intensity errormW.m**-2.sr**-1stat.errordoublenullablel_S(5)[<=] S(5) line detection flag (2)meta.code.errorcharS(5)? S(5) line integrated intensitymW.m**-2.sr**-1spect.line.intensitydoublenullablee_S(5)? S(5) line integrated intensity errormW.m**-2.sr**-1stat.errordoublenullablel_S(6)[<=] S(6) line detection flag (2)meta.code.errorcharS(6)? S(6) line integrated intensitymW.m**-2.sr**-1spect.line.intensitydoublenullablee_S(6)? S(6) line integrated intensity errormW.m**-2.sr**-1stat.errordoublenullablel_S(7)[<=] S(7) line detection flag (2)meta.code.errorcharS(7)? S(7) line integrated intensitymW.m**-2.sr**-1spect.line.intensitydoublenullablee_S(7)? S(7) line integrated intensity errormW.m**-2.sr**-1stat.errordoublenullablel_S(8)[<=] S(8) line detection flag (2)meta.code.errorcharS(8)? S(8) line integrated intensitymW.m**-2.sr**-1spect.line.intensitydoublenullablevLSR? mean LSR velocitykm/sphys.veloc;pos.lsrfloatnullablee_vLSR? LSR velocity errorkm/sstat.error;phys.veloc;pos.lsrfloatnullableJ/A+A/709/A152/tableb1Diffuse medium Galactic propertiesrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintPIDJWST Program Identifiermeta.id;meta.mainintOIDJWST Observation Identifiermeta.id;obs.param;obsintFlag[0,1] Diffuse Medium Flag (1)meta.code.errorintGLONGalactic longitudedegpos.galactic.lonGLATGalactic latitudedegpos.galactic.lat_RA.icrsRight ascension (ICRS) (computed by VizieR, not part of the original data. The format may include more digits than the original data because of internal accuracy requirements in VizieR and across other CDS services)degpos.eq.raindexed_DE.icrsDeclination (ICRS) (computed by VizieR, not part of the original data. The format may include more digits than the original data because of internal accuracy requirements in VizieR and across other CDS services)degpos.eq.decindexedTdustDust temperature (2)Kphys.temperaturefloatNHIAtomic hydrogen column density (3)10000m**-2phys.columnDensitydoubleNHtotTotal hydrogen column density (2)10000m**-2phys.columnDensitydoubletau353Dust opacity at 353GHz (2)phys.absorption.coeffdoubleWise12umGalactic dust emission at 12um (4)1e+20W.m**-2.Hz**-1.sr**-1phot.color.excessfloat