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- Della Bruna, Lorenza, et al.
(författare)
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Studying the ISM at ∼10 pc scale in NGC 7793 with MUSE : I. Data description and properties of the ionised gas
- 2020
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 635
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Tidskriftsartikel (refereegranskat)abstract
- Context. Studies of nearby galaxies reveal that around 50% of the total H alpha 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 similar to 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 H alpha emission line. We identified the boundaries of resolved HII regions based on their H alpha surface brightness. As a way of comparison, we also selected regions according to the H alpha/[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 similar to 27 and 42% depending on the method used to define the boundaries of the HII regions (flux brightness cut in H alpha = 6.7x10(-18) erg s(-1) cm(-2) or H alpha/[SII] = 2.1, respectively). In agreement with previous studies, we find that the DIG exhibits enhanced [SII]/H alpha and [NII]/H alpha ratios and a median temperature that is similar to 3000 K higher than in HII regions. We also observe an apparent inverse correlation between temperature and H alpha surface brightness. In the majority of our field of view, the observed [SII]6716/6731 ratio is consistent within 1 sigma 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.
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| 3. |
- Wofford, Aida, et al.
(författare)
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Candidate LBV stars in galaxy NGC 7793 found via HST photometry plus MUSE spectroscopy
- 2020
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 493:2, s. 2410-2428
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Tidskriftsartikel (refereegranskat)abstract
- Only about 19 Galactic and 25 extragalactic bonafide luminous blue variables (LBVs) are known to date. This incomplete census prevents our understanding of this crucial phase of massive star evolution which leads to the formation of heavy binary black holes via the classical channel. With large samples of LBVs one could better determine the duration and maximum stellar luminosity which characterize this phase. We search for candidate LBVs (cLBVs) in a new galaxy, NGC 7793. For this purpose, we combine high spatial resolution images from two Hubble Space Telescope (HST) programs with optical spectroscopy from the Multi Unit Spectroscopic Explorer (MUSE). By combining PSF-fitting photometry measured on F547M, F657N, and F814W images, with restrictions on point-like appearance (at HST resolution) and Ha luminosity, we find 100 potential cLBVs, 36 of which fall in the MUSE fields. Five of the latter 36 sources are promising cLBVs which have M-V <= -7 and a combination of: H alpha with a P-Cygni profile; no [O I]lambda 6300 emission; weak or no [O III]lambda 5007 emission; large [N II]/H alpha relative to HII regions; and [S II]lambda 6716/[S II]lambda 6731 similar to 1. It is not clear if these five cLBVs are isolated from O-type stars, which would favour the binary formation scenario of LBVs. Our study, which approximately covers one fourth of the optical disc of NGC 7793, demonstrates how by combining the above HST surveys with multi-object spectroscopy from 8-m class telescopes, one can efficiently find large samples of cLBVs in nearby galaxies.
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