On the distance to the North Polar Spur and the local CO-H₂ factor

Rosine Lallement (1) , Steve L. Snowden (2) , Kip D. Kuntz (3) , Thomas M. Dame (4) , Dimitra Koutroumpa (5) , Isabelle Grenier (6) , Jean-Marc Casandjian (6)

1 GEPI - Galaxies, Etoiles, Physique, Instrumentation
2 GSFC - NASA Goddard Space Flight Center
3 Department of Physics and Astronomy [Baltimore]
4 CfA - Harvard-Smithsonian Center for Astrophysics
5 HELIOS - LATMOS
6 AIM (UMR7158 / UMR_E_9005 / UM_112) - Astrophysique Interprétation Modélisation

Most models identify the X-ray bright North Polar Spur (NPS) with a hot interstellar (IS) bubble in the Sco-Cen star-forming region at ≃130 pc. An opposite view considers the NPS as a distant structure associated with Galactic nuclear outflows. Constraints on the NPS distance can be obtained by comparing the foreground IS gas column inferred from X-ray absorption to the distribution of gas and dust along the line of sight. Absorbing columns toward shadowing molecular clouds simultaneously constrain the CO-H₂ conversion factor. We derived the columns of X-ray absorbing matter N_Habs from spectral fitting of dedicated XMM-Newton observations toward the NPS southern terminus (l^II ≃ 29°, b^II ≃ + 5 to + 11°). The distribution of the IS matter was obtained from absorption lines in new stellar spectra, 3D dust maps, and emission data, including high spatial resolution CO measurements recorded for this purpose. N_Habs varies from ≃4.3 to ≃1.3 × 10²¹ cm^-2 along the 19 fields. Relationships between X-ray brightness, absorbing column, and hardness ratio demonstrate a brightness increase with latitude that is governed by increasing absorption. The comparison with absorption data and local and large-scale dust maps rules out an NPS source near-side closer than 300 pc. The correlation between N_Habs and the reddening increases with the sightline length from 300 pc to 4 kpc and is the tightest with Planck τ₃₅₃ GHz-based reddening, suggesting a much larger distance. N(H)/E(B−V)_τ ≃ 4.1 × 10²¹ cm^-2 mag^-1, close to Fermi-Planck determinations. N_Habs absolute values are compatible with HI-CO clouds at −5 ≤ V_LSR ≤ + 25 to +45 km s^-1 and an NPS potentially far beyond the Local Arm. A shadow cast by a b = + 9° molecular cloud constrains X_CO in that direction to ≤1.0 × 10²⁰ cm^-2 K^-1 km^-1 s. The average X_CO over the fields is ≤0.75 × 10²⁰ cm^-2 K^-1 km^-1 s.

Mots clés: X-rays: ISM; radio lines: ISM; local insterstellar matter; ISM: bubbles; dust; extinction; Galaxy: center

Domaines: Astrophysique [astro-ph]

Rosine Lallement, Steve L. Snowden, Kip D. Kuntz, Thomas M. Dame, Dimitra Koutroumpa, et al.. On the distance to the North Polar Spur and the local CO-H₂ factor. Astronomy and Astrophysics - A&A, 2016, 595, pp.A131. ⟨10.1051/0004-6361/201629453⟩. ⟨insu-01399538⟩