ALBERT

Description: 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 ne 〈  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.

Description: 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 H α 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 MV ≤ −7 and a combination of: H α with a P-Cygni profile; no [O i]$, lambda 6300$ emission; weak or no [O iii]$, lambda 5007$ emission; large [N ii]/H α relative to H ii regions; and [S ii]$, lambda 6716$/[S ii]$, lambda 6731sim 1$. 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.

Description: Integral field units enable resolved studies of a large number of star-forming regions across entire nearby galaxies, providing insight on the conversion of gas into stars and the feedback from the emerging stellar populations over unprecedented dynamic ranges in terms of spatial scale, star-forming region properties, and environments. We use the Very Large Telescope (VLT) MUSE (Multi Unit Spectroscopic Explorer) legacy data set covering the central 35 arcmin2 (∼12 kpc2) of the nearby galaxy NGC 300 to quantify the effect of stellar feedback as a function of the local galactic environment. We extract spectra from emission line regions identified within dendrograms, combine emission line ratios and line widths to distinguish between ${ m H, small {II}}$ regions, planetary nebulae, and supernova remnants, and compute their ionized gas properties, gas-phase oxygen abundances, and feedback-related pressure terms. For the ${ m H, small {II}}$ regions, we find that the direct radiation pressure (Pdir) and the pressure of the ionized gas ($P_{{ m H, small {II}}}$) weakly increase towards larger galactocentric radii, i.e. along the galaxy’s (negative) abundance and (positive) extinction gradients. While the increase of $P_{{ m H, small {II}}}$ with galactocentric radius is likely due to higher photon fluxes from lower-metallicity stellar populations, we find that the increase of Pdir is likely driven by the combination of higher photon fluxes and enhanced dust content at larger galactocentric radii. In light of the above, we investigate the effect of increased pre-supernova feedback at larger galactocentric distances (lower metallicities and increased dust mass surface density) on the ISM, finding that supernovae at lower metallicities expand into lower-density environments, thereby enhancing the impact of supernova feedback.