PHANGS-MUSE: Detection and Bayesian classification of ~40 000 ionised nebulae in nearby spiral galaxies

Abstract

In this work, we present a new catalogue of > 40000 ionised nebulae distributed across the 19 galaxies observed by the PHANGS-MUSE survey. The nebulae have been classified using a new model-comparison-based algorithm that exploits the odds ratio principle to assign a probabilistic classification to each nebula in the sample. The resulting catalogue is the largest catalogue containing complete spectral and spatial information for a variety of ionised nebulae available so far in the literature. We developed this new algorithm to address some of the main limitations of the traditional classification criteria, such as their binarity, the sharpness of the involved limits, and the limited amount of data they rely on for the classification. The analysis of the catalogue shows that the algorithm performs well when selecting H ii regions. In fact, we can recover their luminosity function, and its properties are in line with what is available in the literature. We also identify a rather significant population of shock-ionised regions (mostly composed of supernova remnants), which is an order of magnitude larger than any other homogeneous catalogue of supernova remnants currently available in the literature. The number of supernova remnants we identify per galaxy is in line with results in our Galaxy and in other very nearby sources. However, limitations in the source detection algorithm result in an incomplete sample of planetary nebulae, even though their classification seems robust. Finally, we demonstrate how applying a correction for the contribution of the diffuse ionised gas to the nebulae's spectra is essential to obtain a robust classification of the objects and how a correct measurement of the extinction using diffuse-ionised-gas-corrected line fluxes prompts the use of a higher theoretical Hα/Hβ ratio (3.03) than what is commonly used when recovering the E(B − V) via the Balmer decrement technique in massive star-forming galaxies.

Investigating the Drivers of Electron Temperature Variations in H ii Regions with Keck-KCWI and VLT-MUSE

“…We find a negligible difference when using KCWI Hβ in place of the MUSE Hβ flux. We should add that a recent investigation has shown that correcting, or not correcting, the Balmer lines for DIG contamination can impact the measured E(B − V ) (Congiu et al 2023). For the range of E(B − V ) observed, a DIG-corrected E(B − V ) may return values of A V 0.05-0.1 mag lower than presented here.…”

“…We require the strong lines used for temperature determinations, namely Hβ, Hα, The constraints together remove 267 out of the 688 detected H II regions, leaving 421 H II regions remaining for use in future analysis. For these 421 regions, comparisons of line ratios in Figure 7 to model classifications in Figure 3 of Congiu et al (2023) suggest that this sample is consistent with their H II region classification. In order to compare the electron temperatures derived from the lines, which are critical for T e -T e comparisons, we also exclude regions with less than two significant (i.e., S/N > 3; see Section 5) detections in any auroral line.…”

“…In total, HIIphot identifies ∼688 H II regions or complexes across all of the KCWI mosaics. This number is smaller than the 2169 potential H II regions identified for the Nebular catalog (Kreckel et al 2019;Santoro et al 2022;Groves et al 2023), as well as the 2124 potential H II regions from Congiu et al (2023), inside the same KCWI footprints. This is largely due to the differences in angular resolution between KCWI and MUSE, as well as the threefold decrease in strength of Hβ emission relative to Hα.…”

“…In order to assess the emission-line properties of H II regions, we determine the H II region location and boundaries using Hβ maps constructed from the KCWI spectral data cubes and the image-segmentation software HIIphot (Thilker et al 2000). Although H II region masks have previously been constructed from the MUSE Hα maps for these galaxies (Kreckel et al 2019;Santoro et al 2022;Congiu et al 2023;Groves et al 2023), the MUSE angular resolution is higher than that of KCWI. Because of this, the H II region boundaries derived from MUSE may not fully encapsulate the spatial extent of the H II regions observed using KCWI.…”

“…Emission from Balmer transitions, [S II]λλ6716, 6731, [N II] λ6584, and other lines originating from DIG surrounding the H II regions can contaminate the H II region emission-line fluxes of the same transitions. Measuring the DIG contribution to the H II region line flux is only beginning to be explored by IFU studies (see Belfiore et al 2022;Congiu et al 2023). In order to remove regions with a large degree of DIG contamination, we calculate the contrast between H II/DIG emission following the scheme outlined in Kreckel et al (2022).…”