Letter to the Editor

No correlation of the Lyman continuum escape fraction with spectral hardness

¹ Department of Astronomy, University of Geneva, 51 Chemin Pegasi, 1290 Versoix, Switzerland
e-mail: rui.marquescoelhochaves@unige.ch
² CNRS, IRAP, 14 Avenue E. Belin, 31400 Toulouse, France
³ Instituto de Investigación Multidisciplinar en Ciencia y Tecnología, Universidad de La Serena, Raúl Bitrán 1305, La Serena, Chile
⁴ Departamento de Física y Astronomía, Universidad de La Serena, Avda. Juan Cisternas 1200, La Serena, Chile
⁵ Institut d’astrophysique de Paris, CNRS UMR7095, Sorbonne Université, 98bis Boulevard Arago, 75014 Paris, France
⁶ School of Earth & Space Exploration, Arizona State University, Tempe, AZ 85287, USA
⁷ Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712-1205, USA
⁸ Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
⁹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’ Osservatorio, 5, 35122 Padova, Italy
¹⁰ Department of Astronomy, Oskar Klein Centre, Stockholm University, 106 91 Stockholm, Sweden
¹¹ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
¹² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹³ Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, 14b Metrolohichna str., Kyiv 03143, Ukraine
¹⁴ Astronomy Department, Williams College, Williamstown, MA 01267, USA
¹⁵ University of Michigan, Department of Astronomy, 323 West Hall, 1085 S. University Ave, Ann Arbor, MI 48109, USA
¹⁶ Department of Physics and Astronomy, Minnesota State University, Mankato, MN 56001, USA
¹⁷ Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA
¹⁸ Astronomy Department, University of Virginia, PO Box 400325 Charlottesville, VA 22904-4325, USA
¹⁹ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany

Received: 21 March 2022
Accepted: 10 May 2022

Abstract

The properties that govern the production and escape of hydrogen-ionizing photons (Lyman continuum, LyC; with energies > 13.6 eV) in star-forming galaxies are still poorly understood, but they are key to identifying and characterizing the sources that reionized the Universe. Here we empirically explore the relation between the hardness of ionizing radiation and the LyC leakage in a large sample of low-z star-forming galaxies from the recent Hubble Space Telescope Low-z Lyman Continuum Survey. Using Sloan Digital Sky Survey stacks and deep X-shooter observations, we investigate the hardness of the ionizing spectra (Q_He⁺/Q_H) between 54.4 eV (He⁺) and 13.6 eV (H) from the optical recombination lines He II 4686 Å and Hβ 4861 Å for galaxies with LyC escape fractions spanning a wide range, f_esc(LyC) ≃ 0−90%. We find that the observed intensity of He II/Hβ is primarily driven by variations in the metallicity, but is not correlated with LyC leakage. Both very strong (<f_esc(LyC)> ≃ 0.5) and nonleakers (<f_esc(LyC)> ≃ 0) present similar observed intensities of He II and Hβ at comparable metallicity, between ≃0.01 and ≃0.02 for 12 + log(O/H)> 8.0 and < 8.0, respectively. Our results demonstrate that Q_He⁺/Q_H does not correlate with f_esc(LyC), which implies that strong LyC emitters do not show harder ionizing spectra than nonleakers at similar metallicity.