The need for multicomponent dust attenuation in modeling nebular emission: Constraints from SDSS-IV MaNGA

¹ Department of Physics and Astronomy, University of Kentucky, 505 Rose Street, Lexington, KY 40506, USA
e-mail: xji243@uky.edu
² Department of Physics, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong SAR, China
e-mail: rbyan@cuhk.edu.hk
³ University of California Observatories – Lick Observatory, University of California Santa Cruz, 1156 High St., Santa Cruz, CA 95064, USA
⁴ Centro de Astronomía, Universidad de Antofagasta, Avenida Angamos 601, Antofagasta 1270300, Chile
⁵ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
⁶ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
⁷ University of Wisconsin – Madison, Department of Astronomy, 475 N. Charter Street, Madison, WI 53706-1582, USA
⁸ South African Astronomical Observatory, PO Box 9 Observatory 7935, Cape Town, South Africa
⁹ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
¹⁰ McDonald Observatory, The University of Texas at Austin, 2515 Speedway, Stop C1402, Austin, TX 78712, USA
¹¹ Department of Astronomy, Tsinghua University, Beijing 100084, PR China
¹² Apache Point Observatory, PO Box 59 Sunspot, NM 88349, USA
¹³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁴ Instituto de Física, Universidade Federal do Rio Grande do Sul, Campus do Vale, 91501-970 Porto Alegre, RS, Brazil
¹⁵ Departamento de Física, CCNE, Universidade Federal de Santa Maria, 97105-900, Santa Maria, RS, Brazil
¹⁶ Laboratório Interinstitucional de e-Astronomia – LIneA, Rua Gal. José Cristino 77, Rio de Janeiro, RJ 20921-400, Brazil

Received: 27 September 2022
Accepted: 12 December 2022

Abstract

A fundamental assumption adopted in nearly every extragalactic study that analyzes optical emission lines is that the attenuation of different emission lines can be described by a single attenuation curve, scaled by a single reddening parameter, usually E(B − V). Here we show this assumption fails in many cases with important implications for derived results. We developed a new method to measure the differential nebular attenuation among three kinds of transitions: the Balmer lines of hydrogen; high-ionization transitions (> 13.6 eV) including [Ne III], [O III], and [S III]; and low-ionization transitions (≲13.6 eV) including [O II], [N II], and [S II]. This method bins the observed data in a multidimensional space spanned by attenuation-insensitive line ratios. Within each small bin, the variations in nebular parameters such as the metallicity and ionization parameter are negligible compared to the variation in the nebular attenuation. This allowed us to measure the nebular attenuation using both forbidden lines and Balmer lines. We applied this method to a sample of 2.4 million star-forming (SF) spaxels from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. We found that the attenuation of high ionization lines and Balmer lines can be well described by a single Fitzpatrick (1999, PASP, 111, 63) extinction curve with R_V = 3.1. However, no single attenuation curve can simultaneously account for these transitions and the derived attenuation of low-ionization lines. This strongly suggests that different lines have different effective attenuations, likely because spectroscopy at hundreds of parsecs to kiloparsecs of resolution mixes multiple physical regions that exhibit different intrinsic line ratios and different levels of attenuation. As a result, the assumption that different lines follow the same attenuation curve breaks down. Using a single attenuation curve determined by Balmer lines to correct attenuation-sensitive forbidden line ratios could bias the nebular parameters derived by 0.06–0.25 dex at A_V = 1, depending on the details of the dust attenuation model. Observations of a statistically large sample of H II regions with high spatial resolutions and large spectral coverage are vital for improved modeling and deriving accurate corrections for this effect.