Studying the ISM at ∼10 pc scale in NGC 7793 with MUSE

I. Data description and properties of the ionised gas

¹ Department of Astronomy, Oskar Klein Centre, Stockholm University, AlbaNova University Centre, 106 91 Stockholm, Sweden
e-mail: lorenza.dellabruna@astro.su.se
² Institute for Computational Cosmology, Durham University, South Road, Durham DH1 3LE, UK
³ Centre for Extragalactic Astronomy, Durham University, South Road, Durham DH1 3LE, UK
⁴ Dipartimento di Fisica G. Occhialini, Università degli Studi di Milano Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
⁵ Department of Astronomy, New Mexico State University, Las Cruces, NM 88001, USA
⁶ Instituto de Radioastronomía y Astrofísica, UNAM, Campus Morelia, Michoacan CP 58089, Mexico
⁷ Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
⁸ Sorbonne Université, CNRS, UMR7095, Institut d’Astrophysique de Paris, 75014 Paris, France
⁹ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
¹⁰ Space Telescope Science Institute and European Space Agency, 3700 San Martin Drive, Baltimore, MD 2121, USA
¹¹ Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
¹² Instituto de Astronomía, Universidad Nacional Autónoma de México, Unidad Académica en Ensenada, Km 103 Carr. Tijuana-Ensenada, Ensenada 22860, Mexico

Received: 22 November 2019
Accepted: 13 February 2020

Abstract

Context. Studies of nearby galaxies reveal that around 50% of the total Hα luminosity in late-type spirals originates from diffuse ionised gas (DIG), which is a warm, diffuse component of the interstellar medium that can be associated with various mechanisms, the most important ones being “leaking” HII regions, evolved field stars, and shocks.

Aims. Using MUSE Wide Field Mode adaptive optics-assisted data, we study the condition of the ionised medium in the nearby (D = 3.4 Mpc) flocculent spiral galaxy NGC 7793 at a spatial resolution of ∼10 pc. We construct a sample of HII regions and investigate the properties and origin of the DIG component.

Methods. We obtained stellar and gas kinematics by modelling the stellar continuum and fitting the Hα emission line. We identified the boundaries of resolved HII regions based on their Hα surface brightness. As a way of comparison, we also selected regions according to the Hα/[SII] line ratio; this results in more conservative boundaries. Using characteristic line ratios and the gas velocity dispersion, we excluded potential contaminants, such as supernova remnants (SNRs) and planetary nebulae (PNe). The continuum subtracted HeII map was used to spectroscopically identify Wolf Rayet stars (WR) in our field of view. Finally, we computed electron densities and temperatures using the line ratio [SII]6716/6731 and [SIII]6312/9069, respectively. We studied the properties of the ionised gas through “BPT” emission line diagrams combined with velocity dispersion of the gas.

Results. We spectroscopically confirm two previously detected WR and SNR candidates and report the discovery of the other seven WR candidates, one SNR, and two PNe within our field of view. The resulting DIG fraction is between ∼27 and 42% depending on the method used to define the boundaries of the HII regions (flux brightness cut in Hα = 6.7 × 10⁻¹⁸ erg s⁻¹ cm⁻² or Hα/[SII] = 2.1, respectively). In agreement with previous studies, we find that the DIG exhibits enhanced [SII]/Hα and [NII]/Hα ratios and a median temperature that is ∼3000 K higher than in HII regions. We also observe an apparent inverse correlation between temperature and Hα surface brightness. In the majority of our field of view, the observed [SII]6716/6731 ratio is consistent within 1σ with n_e <  30 cm⁻³, with an almost identical distribution for the DIG and HII regions. The velocity dispersion of the ionised gas indicates that the DIG has a higher degree of turbulence than the HII regions. Comparison with photoionisation and shock models reveals that, overall, the diffuse component can only partially be explained via shocks and that it is most likely consistent with photons leaking from density bounded HII regions or with radiation from evolved field stars. Further investigation will be conducted in a follow-up paper.