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Published August 2015 | Published + Submitted
Journal Article Open

Planck intermediate results. XXIII. Galactic plane emission components derived from Planck with ancillary data

Abstract

Planck data when combined with ancillary data provide a unique opportunity to separate the diffuse emission components of the inner Galaxy. The purpose of the paper is to elucidate the morphology of the various emission components in the strong star-formation region lying inside the solar radius and to clarify the relationship between the various components. The region of the Galactic plane covered is l = 300° → 0° → 60° wherestar-formation is highest and the emission is strong enough to make meaningful component separation. The latitude widths in this longitude range lie between 1° and 2°, which correspond to FWHM z-widths of 100−200 pc at a typical distance of 6 kpc. The four emission components studied here are synchrotron, free-free, anomalous microwave emission (AME), and thermal (vibrational) dust emission. These components are identified by constructing spectral energy distributions (SEDs) at positions along the Galactic plane using the wide frequency coverage of Planck (28.4−857 GHz) in combination with low-frequency radio data at 0.408−2.3 GHz plus WMAP data at 23−94 GHz, along with far-infrared (FIR) data from COBE-DIRBE and IRAS. The free-free component is determined from radio recombination line (RRL) data. AME is found to be comparable in brightness to the free-free emission on the Galactic plane in the frequency range 20−40 GHz with a width in latitude similar to that of the thermal dust; it comprises 45 ± 1% of the total 28.4 GHz emission in the longitude range l = 300° → 0° → 60°. The free-free component is the narrowest, reflecting the fact that it is produced by current star-formation as traced by the narrow distribution of OB stars. It is the dominant emission on the plane between 60 and 100 GHz. RRLs from this ionized gas are used to assess its distance, leading to a free-free z-width of FWHM ≈ 100 pc. The narrow synchrotron component has a low-frequency brightness spectral index β_(synch) ≈ −2.7 that is similar to the broad synchrotron component indicating that they are both populated by the cosmic ray electrons of the same spectral index. The width of this narrow synchrotron component is significantly larger than that of the other three components, suggesting that it is generated in an assembly of older supernova remnants that have expanded to sizes of order 150 pc in 3 × 10^5 yr; pulsars of a similar age have a similar spread in latitude. The thermal dust is identified in the SEDs with average parameters of T_(dust) = 20.4 ± 0.4 K, β_(FIR) = 1.94 ± 0.03 (> 353 GHz), and β_(mm) = 1.67 ± 0.02 (< 353 GHz). The latitude distributions of gamma-rays, CO, and the emission in high-frequency Planck bands have similar widths, showing that they are all indicators of the total gaseous matter on the plane in the inner Galaxy.

Additional Information

© 2015 ESO. Article published by EDP Sciences. Received 19 June 2014; Accepted 24 May 2015; Published online 20 July 2015. We acknowledge the use of the Legacy Archive for Microwave Background Data Analysis (LAMBDA); support for LAMBDA is provided by the NASA Office of Space Science. Some of the results in this paper have been derived using the HEALPix package The development of Planck has been supported by: ESA; CNES and CNRS/INSU-IN2P3-INP (France); ASI, CNR, and INAF (Italy); NASA and DoE (USA); STFC and UKSA (UK); CSIC, MICINN, JA and RES (Spain); Tekes, AoF and CSC (Finland); DLR and MPG (Germany); CSA (Canada); DTU Space (Denmark); SER/SSO (Switzerland); RCN (Norway); SFI (Ireland); FCT/MCTES (Portugal); and PRACE (EU). The research leading to these results has received funding from an STFC Consolidated Grant (No. ST/L000768/1), as well as the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement Nos. 267934 and 307209. A description of the Planck Collaboration and a list of its members, including the technical or scientific activities in which they have been involved, can be found at http://www.sciops.esa.int/index.php?project=planck&page=Planck_Collaboration.

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August 20, 2023
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