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A measurement of the z = 0 UV background from Halpha fluorescence

Fumagalli, Michele; Haardt, Francesco; Theuns, Tom; Morris, Simon L.; Cantalupo, Sebastiano; Madau, Piero; Fossati, Matteo

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Francesco Haardt

Sebastiano Cantalupo

Piero Madau

Matteo Fossati


We report the detection of extended Halpha emission from the tip of the HI disk of the nearby edge-on galaxy UGC 7321, observed with the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope. The Halpha surface brightness fades rapidly where the HI column density drops below N(HI) = 10^19 cm^-2 , consistent with fluorescence arising at the ionisation front from gas that is photoionized by the extragalactic ultraviolet background (UVB). The surface brightness measured at this location is (1.2 +/- 0.5)x10^-19 erg/s/cm^2/arcsec^2, where the error is mostly systematic and results from the proximity of the signal to the edge of the MUSE field of view, and from the presence of a sky line next to the redshifted Halpha wavelength. By combining the Halpha and the HI 21 cm maps with a radiative transfer calculation of an exponential disk illuminated by the UVB, we derive a value for the HI photoionization rate of Gamma ~ (6-8)x10^-14 1/s . This value is consistent with transmission statistics of the Lyalpha forest and with recent models of a UVB which is dominated by quasars.


Fumagalli, M., Haardt, F., Theuns, T., Morris, S. L., Cantalupo, S., Madau, P., & Fossati, M. (2017). A measurement of the z = 0 UV background from Halpha fluorescence. Monthly Notices of the Royal Astronomical Society, 467(4), 4802-4816.

Journal Article Type Article
Acceptance Date Feb 13, 2017
Online Publication Date Feb 17, 2017
Publication Date Jun 1, 2017
Deposit Date Mar 27, 2017
Publicly Available Date Mar 27, 2017
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 467
Issue 4
Pages 4802-4816


Published Journal Article (8.7 Mb)

Copyright Statement
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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