Comptes Rendus
North Polar Spur/Loop I: gigantic outskirt of the Northern Fermi bubble or nearby hot gas cavity blown by supernovae?
Comptes Rendus. Physique, Volume 23 (2022) no. S2, pp. 1-24.

Radio continuum, microwave and gamma-ray images of the Milky Way reveal a conspicuous, loop-like structure that fills almost half of the northern Galactic hemisphere, called Loop I. The interior of Loop I is the most conspicuous region shining in soft X-rays, whose eastern base is a remarkably bright, elongated structure seeming to emerge from the Galactic plane, dubbed the North Polar Spur (NPS). After 40 years of debates, two very different, contradictory views of Loop I/NPS are still defended: on the one hand, the NPS is a gigantic volume of expanding hot gas that envelops and extends the northern “Fermi Bubble” (FB) known to be blown by the Galactic center, and Loop I marks the shock front; on the other hand, the NPS is totally independent of the northern FB, it is a nearby, ordinary cavity of hot gas blown by supernovae, Loop I is its shock front and both are coincidentally located in the direction of the FB. To an observer at the Sun, both can produce the same perspective view, although the former has a size comparable to the Milky Way itself, and the latter a diameter of a few hundreds parsecs. The energy involved varies by 3-4 orders of magnitude, and the solution has important consequences on the structure and history of our Galactic neighborhood, on the age of the North and South FBs and the activity at the Galactic center. Moreover, whatever are the actual shape and distance of Loop I/NPS, accurate modeling of the polarized emission associated with Loop I is important for CMB foreground removal. After a short review, I discuss recent results which have a connection with Loop I/NPS. Some of them have been used as arguments in the two opposite ways, while for others the connections with LoopI/NPS have been overlooked. They involve very different spacecraft, from a 12 Kg Cubesat (HaloSat) to major space-borne observatories (HST, Gaia, and Spektr-RG). I make use of updated 3D maps of dust and a recent massive star catalog. I distinguish arguments based on geometric similarity or dissimilarity from those derived from measurements and physical models. Considering all past and recent constraints, it is clear that there is no entirely local or entirely distant scenario that is free from contradictions with some of the data analyses or from improbable coincidences. I discuss a speculative scenario, partially inspired by recent BF and Milky Way gas models, combining near and far aspects and seeming to be able to meet the various constraints. However, new data and models are needed to bring the controversy to a close and we can still expect new episodes of this long story.

Online First:
Published online:
DOI: 10.5802/crphys.97
Keywords: Galaxy, Galactic interstellar medium, X-rays, Gamma rays, Microwave emission, Galactic Center, Galactic halo

Rosine Lallement 1

1 GEPI/Observatoire de Paris, PSL*, 5 pl. J. Janssen, 92195 Meudon, France
License: CC-BY 4.0
Copyrights: The authors retain unrestricted copyrights and publishing rights
     author = {Rosine Lallement},
     title = {North {Polar} {Spur/Loop} {I:} gigantic outskirt of the {Northern} {\protect\emph{Fermi} bubble} or nearby hot gas cavity blown by supernovae?},
     journal = {Comptes Rendus. Physique},
     pages = {1--24},
     publisher = {Acad\'emie des sciences, Paris},
     volume = {23},
     number = {S2},
     year = {2022},
     doi = {10.5802/crphys.97},
     language = {en},
AU  - Rosine Lallement
TI  - North Polar Spur/Loop I: gigantic outskirt of the Northern Fermi bubble or nearby hot gas cavity blown by supernovae?
JO  - Comptes Rendus. Physique
PY  - 2022
SP  - 1
EP  - 24
VL  - 23
IS  - S2
PB  - Académie des sciences, Paris
DO  - 10.5802/crphys.97
LA  - en
ID  - CRPHYS_2022__23_S2_1_0
ER  - 
%0 Journal Article
%A Rosine Lallement
%T North Polar Spur/Loop I: gigantic outskirt of the Northern Fermi bubble or nearby hot gas cavity blown by supernovae?
%J Comptes Rendus. Physique
%D 2022
%P 1-24
%V 23
%N S2
%I Académie des sciences, Paris
%R 10.5802/crphys.97
%G en
%F CRPHYS_2022__23_S2_1_0
Rosine Lallement. North Polar Spur/Loop I: gigantic outskirt of the Northern Fermi bubble or nearby hot gas cavity blown by supernovae?. Comptes Rendus. Physique, Volume 23 (2022) no. S2, pp. 1-24. doi : 10.5802/crphys.97.

[1] Masahiro Akita; Jun Kataoka; Makoto Arimoto; Yoshiaki Sofue; Tomonori Totani; Yoshiyuki Inoue; Shinya Nakashima Diffuse X-Ray Emission from the Northern Arc of Loop I Observed with Suzaku, Astrophys. J., Volume 862 (2018) no. 1, 88 | DOI

[2] Trisha Ashley; Andrew J. Fox; Edward B. Jenkins; Bart P. Wakker; Rongmon Bordoloi; Felix J. Lockman; Blair D. Savage; Tanveer Karim Mapping Outflowing Gas in the Fermi Bubbles: A UV Absorption Survey of the Galactic Nuclear Wind, Astrophys. J., Volume 898 (2020) no. 2, 128 | DOI

[3] M. A. de Avillez; D. Breitschwerdt Global dynamical evolution of the ISM in star forming galaxies. I. High resolution 3D simulations: Effect of the magnetic field, Astron. Astrophys., Volume 436 (2005) no. 2, pp. 585-600 | DOI

[4] E. M. Berkhuijsen A Survey of the Continuum Radiation at 820 MHz between Declinations -7° and +85°. A Study of the Galactic Radiation and the Degree of Polarization with Special Reference to the Loops and Spurs, Astron. Astrophys., Volume 14 (1971), pp. 359-386

[5] R. G. Bingham Magnetic fields in the galactic spurs, Mon. Not. Roy. Astron. Soc., Volume 137 (1967) no. 2, pp. 157-168 | DOI

[6] D. Breitschwerdt; J. Feige; M. M. Schulreich; M. A. de Avillez; C. Dettbarn; B. Fuchs The locations of recent supernovae near the Sun from modelling 60 Fe transport, Nature, Volume 532 (2016) no. 7597, pp. 73-76 | DOI

[7] W. N. Brouw; T. A. Th. Spoelstra Linear polarization of the galactic background at frequencies between 408 and 1411 MHz. Reductions, Astron. Astrophys. Supplement Series, Volume 26 (1976), pp. 129-146

[8] Jean-Marc Casandjian The Fermi-LAT model of interstellar emission for standard point source analysis (2015) (

[9] Jean-Marc Casandjian; Isabelle Grenier High Energy Gamma-Ray Emission from the Loop I region (2009) (

[10] Gerald Cecil; Alexander Y. Wagner; Joss Bland-Hawthorn; Geoffrey V. Bicknell; Dipanjan Mukherjee Tracing the Milky Way’s Vestigial Nuclear Jet, Astrophys. J., Volume 922 (2021) no. 254 | DOI

[11] Roland M. Crocker; Geoffrey V. Bicknell; Andrew M. Taylor; Ettore Carretti A Unified Model of the Fermi Bubbles, Microwave Haze, and Polarized Radio Lobes: Reverse Shocks in the Galactic Center’s Giant Outflows, Astrophys. J., Volume 808 (2015) no. 2, 107 | DOI

[12] Kaustav K. Das; Catherine Zucker; Joshua S. Speagle; Alyssa Goodman; Gregory M. Green; João Alves Constraining the distance to the North Polar Spur with Gaia DR2, Mon. Not. Roy. Astron. Soc., Volume 498 (2020) no. 4, pp. 5863-5872 | DOI

[13] Clive Dickinson Large-Scale Features of the Radio Sky and a Model for Loop I, Galaxies, Volume 6 (2018) no. 2, p. 56 | DOI

[14] Gregory Dobler; Douglas P. Finkbeiner Extended Anomalous Foreground Emission in the WMAP Three-Year Data, Astrophys. J., Volume 680 (2008) no. 2, pp. 1222-1234 | DOI

[15] Gregory Dobler; Douglas P. Finkbeiner; Ilias Cholis; Tracy R. Slatyer; Neal Weiner The Fermi Haze: A Gamma-ray Counterpart to the Microwave Haze, Astrophys. J., Volume 717 (2010) no. 2, pp. 825-842 | DOI

[16] Douglas P. Finkbeiner Microwave Interstellar Medium Emission Observed by the Wilkinson Microwave Anisotropy Probe, Astrophys. J., Volume 614 (2004) no. 1, pp. 186-193 | DOI

[17] David M. French; Andrew J. Fox; Bart P. Wakker; Colin Norman; Nicolas Lehner; J. Christopher Howk; Blair D. Savage; Philipp Richter; John O’Meara; Sanchayeeta Borthakur; Timoth Heckman The HI Column Density Distribution of the Galactic Disk and Halo, Astrophys. J., Volume 923 (2021) no. 50 | DOI

[18] Liyi Gu; Junjie Mao; Elisa Costantini; Jelle Kaastra Suzaku and XMM-Newton observations of the North Polar Spur: Charge exchange or ISM absorption?, Astron. Astrophys., Volume 594 (2016), A78 | DOI

[19] C. G. T. Haslam; W. E. Wilson; D. A. Graham; G. C. Hunt A further 408 MHz survey of the northern sky, Astron. Astrophys. Supplement Series, Volume 13 (1974), pp. 359-394

[20] Philip Kaaret; Anna Zajczyk; Daniel M. LaRocca; Rebecca Ringuette; Jesse Bluem; W. Fuelberth; H. Gulick; Keith M. Jahoda; T. E. Johnson; D. L. Kirchner; D. Koutroumpa; K. D. Kuntz; R. McCurdy; D. M. Miles; W. T. Robison; E. M. Silich HaloSat: A CubeSat to Study the Hot Galactic Halo, Astrophys. J., Volume 884 (2019) no. 2, 162 | DOI

[21] Jun Kataoka; M. Tahara; T. Totani; Yoshiaki Sofue; Ł. Stawarz; Y. Takahashi; Y. Takeuchi; H. Tsunemi; M. Kimura; Yoh Takei; C. C. Cheung; Yoshiyuki Inoue; T. Nakamori Suzaku Observations of the Diffuse X-Ray Emission across the Fermi Bubbles’ Edges, Astrophys. J., Volume 779 (2013) no. 1, 57 | DOI

[22] Jun Kataoka; M. Tahara; Tomonori Totani; Yoshiaki Sofue; Yoshiyuki Inoue; Shinya Nakashima; C. C. Cheung Global Structure of Isothermal Diffuse X-Ray Emission along the Fermi Bubbles, Astrophys. J., Volume 807 (2015) no. 1, 77 | DOI

[23] Rosine Lallement Some Observations Related to the Origin and Evolution of the Local Bubble/Local ISM, From the Outer Heliosphere to the Local Bubble: Comparisons of New Observations with Theory (J. L. Linsky; V. V. Izmodenov; E. Möbius; R. von Steiger, eds.) (Space Sciences Series of ISSI book series), Volume 31, Springer, 2009, pp. 427-436 | DOI

[24] Rosine Lallement; C. Babusiaux; J. L. Vergely; D. Katz; F. Arenou; B. Valette; C. Hottier; L. Capitanio Gaia-2MASS 3D maps of Galactic interstellar dust within 3 kpc, Astron. Astrophys., Volume 625 (2019), A135 | DOI

[25] Rosine Lallement; S. L. Snowden; K. D. Kuntz; T. M. Dame; D. Koutroumpa; I. Grenier; Jean-Marc Casandjian On the distance to the North Polar Spur and the local CO-H 2 factor, Astron. Astrophys., Volume 595 (2016), A131 | DOI

[26] M. I. Large; M. J. S. Quigley; C. G. T. Haslam A new feature of the radio sky, Mon. Not. Roy. Astron. Soc., Volume 124 (1962) no. 5, pp. 405-415 | DOI

[27] Daniel M. LaRocca; Philip Kaaret; K. D. Kuntz; Edmund Hodges-Kluck; Anna Zajczyk; Jesse Bluem; Rebecca Ringuette; Keith M. Jahoda An Analysis of the North Polar Spur Using HaloSat, Astrophys. J., Volume 904 (2020) no. 1, 54 | DOI

[28] Zhi Li; Juntai Shen; Ortwin Gerhard; Jonathan P. Clarke Gas Dynamics in the Galaxy: Total Mass Distribution and the Bar Pattern Speed, Astrophys. J., Volume 925 (2022) no. 71 | DOI

[29] Mordecai-Mark Mac Low; Richard McCray; Michael L. Norman Superbubble Blowout Dynamics, Astrophys. J., Volume 337 (1989), pp. 141-154 | DOI

[30] Eric D. Miller; Tsunemi Hiroshi; Mark W. Bautz; Dan McCammon; Ryuichi Fujimoto; John P. Hughes; Satoru Katsuda; Motohide Kokubun; Kazuhisa Mitsuda; F. Scott Porter; Yoh Takei; Yohko Tsuboi; Noriko Y. Yamasaki Suzaku Observations of the North Polar Spur: Evidence for Nitrogen Enhancement, Publ. Astron. Soc. Jpn., Volume 60 (2008) no. sp1, p. S95-S106 | DOI

[31] N. Oppermann; H. Junklewitz; M. Greiner; T. A. Enßlin; T. Akahori; Ettore Carretti; B. M. Gaensler; A. Goobar; L. Harvey-Smith; M. Johnston-Hollitt; L. Pratley; D. H. F. M. Schnitzeler; J. M. Stil; V. Vacca Estimating extragalactic Faraday rotation, Astron. Astrophys., Volume 575 (2015), A118 | DOI

[32] G. V. Panopoulou; Clive Dickinson; A. C. S. Readhead; T. J. Pearson; M. W. Peel Revisiting the distance to radio Loops I and IV using Gaia and radio/optical polarization data, Astrophys. J., Volume 922 (2021) no. 2 | DOI

[33] M. Pantaleoni González; J. Maíz Apellániz; R. H. Barbá; B. Cameron Reed The Alma catalogue of OB stars - II. A cross-match with Gaia DR2 and an updated map of the solar neighbourhood, Mon. Not. Roy. Astron. Soc., Volume 504 (2021) no. 2, pp. 2968-2982 | DOI

[34] Planck Collaboration et al. Planck 2015 results. XXV. Diffuse low-frequency Galactic foregrounds, Astron. Astrophys., Volume 594 (2016), A25 | DOI

[35] G. Ponti; M. R. Morris; E. Churazov; I. Heywood; R. P. Fender The Galactic center chimneys: the base of the multiphase outflow of the Milky Way, Astron. Astrophys., Volume 646 (2021), A66 | DOI

[36] P. Predehl; R. A. Sunyaev; W. Becker; H. Brunner; R. Burenin; A. Bykov; A. Cherepashchuk; N. Chugai; E. Churazov; V. Doroshenko; N. Eismont; M. J. Freyberg; M. Gilfanov; F. Haberl; I. Khabibullin; R. Krivonos; C. Maitra; P. Medvedev; A. Merloni; K. Nandra; V. Nazarov; M. Pavlinsky; G. Ponti; J. S. Sanders; M. Sasaki; S. Sazonov; A. W. Strong; J. Wilms Detection of large-scale X-ray bubbles in the Milky Way halo, Nature, Volume 588 (2020) no. 7837, pp. 227-231 | DOI

[37] L. Puspitarini; Rosine Lallement Distance to northern high-latitude HI shells, Astron. Astrophys., Volume 545 (2012), A21 | DOI

[38] L. Puspitarini; Rosine Lallement; J.-L. Vergely; S. L. Snowden Local ISM 3D distribution and soft X-ray background. Inferences on nearby hot gas and the North Polar Spur, Astron. Astrophys., Volume 566 (2014), A13 | DOI

[39] Zhijie Qu; Joel N. Bregman; Edmund Hodges-Kluck; Jiang-Tao Li; Ryan Lindley The Warm Gas in the MW: A Kinematical Model, Astrophys. J., Volume 894 (2020) no. 2, 142 | DOI

[40] M. Remazeilles; Clive Dickinson; A. J. Banday; M.-A. Bigot-Sazy; T. Ghosh An improved source-subtracted and destriped 408-MHz all-sky map, Mon. Not. Roy. Astron. Soc., Volume 451 (2015) no. 4, pp. 4311-4327 | DOI

[41] J.-F. Robitaille; A. M. M. Scaife; Ettore Carretti; M. Haverkorn; Roland M. Crocker; M. J. Kesteven; S. Poppi; L. Staveley-Smith Interstellar magnetic cannon targeting the Galactic halo. A young bubble at the origin of the Ophiuchus and Lupus molecular complexes, Astron. Astrophys., Volume 617 (2018), A101 | DOI

[42] Kartick C. Sarkar Possible connection between the asymmetry of the North Polar Spur and Loop I and Fermi bubbles, Mon. Not. Roy. Astron. Soc., Volume 482 (2019) no. 4, pp. 4813-4823 | DOI

[43] Kartick C. Sarkar; Biman B. Nath; Prateek Sharma Multiwavelength features of Fermi bubbles as signatures of a Galactic wind, Mon. Not. Roy. Astron. Soc., Volume 453 (2015) no. 4, pp. 3827-3838 | DOI

[44] S. L. Snowden; R. Egger; M. J. Freyberg; Dan McCammon; P. P. Plucinsky; W. T. Sanders; J. H. M. M. Schmitt; J. Trümper; W. Voges ROSAT Survey Diffuse X-Ray Background Maps. II., Astrophys. J., Volume 485 (1997) no. 1, pp. 125-135 | DOI

[45] S. L. Snowden; M. J. Freyberg; P. P. Plucinsky; J. H. M. M. Schmitt; J. Truemper; W. Voges; R. J. Edgar; Dan McCammon; W. T. Sanders First Maps of the Soft X-Ray Diffuse Background from the ROSAT XRT/PSPC All-Sky Survey, Astrophys. J., Volume 454 (1995), pp. 643-653 | DOI

[46] Yoshiaki Sofue Propagation of magnetohydrodynamic waves from the galactic center. Origin of the 3-kpc arm and the North Polar Spur, Astron. Astrophys., Volume 60 (1977) no. 3, pp. 327-336

[47] Yoshiaki Sofue; Jun Kataoka Interaction of the galactic-centre super bubbles with the gaseous disc, Mon. Not. Roy. Astron. Soc., Volume 506 (2021) no. 2, pp. 2170-2180 | DOI

[48] Yoshiaki Sofue; W. Reich Radio continuum observations of the North Polar Spur at 1420 MHz, Astron. Astrophys. Supplement Series, Volume 38 (1979), pp. 251-263

[49] Meng Su; Tracy R. Slatyer; Douglas P. Finkbeiner Giant Gamma-ray Bubbles from Fermi-LAT: Active Galactic Nucleus Activity or Bipolar Galactic Wind?, Astrophys. J., Volume 274 (2010) no. 2, pp. 1044-1082 | DOI

[50] X. H. Sun; T. L. Landecker; B. M. Gaensler; Ettore Carretti; W. Reich; J. P. Leahy; N. M. McClure-Griffiths; Roland M. Crocker; M. Wolleben; M. Haverkorn; K. A. Douglas; A. D. Gray Faraday Tomography of the North Polar Spur: Constraints on the Distance to the Spur and on the Magnetic Field of the Galaxy, Astrophys. J., Volume 811 (2015) no. 1, 40 | DOI

[51] M. Tahara; Jun Kataoka; Y. Takeuchi; T. Totani; Yoshiaki Sofue; J. S. Hiraga; H. Tsunemi; Yoshiyuki Inoue; M. Kimura; C. C. Cheung; Shinya Nakashima Suzaku X-Ray Observations of the Fermi Bubbles: Northernmost Cap and Southeast Claw Discovered With MAXI-SSC, Astrophys. J., Volume 802 (2015) no. 2, 91 | DOI

[52] J. L. Vergely; Rosine Lallement; N. L. J. Cox 3D extinction maps: inversion of inter-calibrated extinction catalogs (2022) (submitted to Astronomy Astrophysics)

[53] Matias Vidal; Clive Dickinson; R. D. Davies; J. P. Leahy Polarized radio filaments outside the Galactic plane, Mon. Not. Roy. Astron. Soc., Volume 452 (2015) no. 1, pp. 656-675 | DOI

[54] R. Willingale; A. D. P. Hands; R. S. Warwick; S. L. Snowden; D. N. Burrows The X-ray spectrum of the North Polar Spur, Mon. Not. Roy. Astron. Soc., Volume 343 (2003) no. 3, pp. 995-1001 | DOI

[55] M. Wolleben A New Model for the Loop I (North Polar Spur) Region, Astrophys. J., Volume 664 (2007) no. 1, pp. 349-356 | DOI

[56] M. Wolleben; T. L. Landecker; W. Reich; R. Wielebinski An absolutely calibrated survey of polarized emission from the northern sky at 1.4 GHz. Observations and data reduction, Astron. Astrophys., Volume 448 (2006) no. 1, pp. 411-424 | DOI

Cited by Sources:

Comments - Policy