Team members: Henry Zovaro, Brent Groves, Lisa Kewley, anyone else interested!
Project description: Diffuse Ionised Gas (DIG) refers to gas ionised by radiation from “leaky” HII regions and/or hot, low-mass evolved stars, and forms a “thick disk” with a larger scale height than that of the star-forming disk. In face-on galaxies, DIG is most often identified using Halpha surface brightness cuts or HII-region finding software, both of which require very high spatial resolution. At the spatial resolution of SAMI, DIG can be detected in edge-on galaxies in the form of extraplanar gas (e.g., Ho et al. 2016); although this study provides valuable insight into the relationship between star formation and DIG, their sample size was limited by inclination requirements.
Kinematic/spectral separation potentially provides a more reliable means of separating DIG from HII emission due to their distinct kinematics: DIG resides in a thick disk with a slower rotation curve and higher velocity dispersion than the thin star-forming disk, meaning that it can theoretically be detected in galaxies at any inclination. Indeed, den Brok et al. (2020) demonstrated that DIG can be separated from HII region emission by spectrally decomposing emission line profiles into broad and narrow components in MUSE data cubes.
With its much higher spectral resolution, SAMI is really the first survey that would enable detailed analysis of DIG using this technique in a large number of galaxies. This has been confirmed via preliminary analysis of the multi-component emission line fits from DR3. Some ~100 star-forming galaxies in SAMI exhibit extended regions of spaxels with a broad emission line component that has properties consistent with those of DIG: the broad component generally exhibits rotation, with a rotational velocity slower than that of the narrow component, and has low/intermediate equivalent widths. Building upon these initial results, we aim to answer the following questions:
- what underlying galaxy properties dictate the presence/detectability of DIG?
- what are the spatially resolved properties of DIG? e.g. is it preferentially detected in nuclear regions?
- what is the relationship between the kinematic properties of DIG and star formation activity/other galaxy properties?
- how does this technique compare with other methods of identifying DIG?
The analysis is already underway:
- I have conducted a matched sample analysis, in which I compared “1-component galaxies” (star-forming galaxies with few/no spaxels with >1 component) with “2-component galaxies” (star-forming galaxies in which >20% of spaxels have multiple components), matched in quantities such as stellar mass, SFR, SFR surface density and continuum S/N. This showed that SFR surface density is the most important factor in determining the presence of spaxels with multiple components, which is consistent with previous DIG studies. It also confirmed that the presence of multiple emission line components is not an artefact of observational effects including data cube S/N, inclination or angular scale.
- I have analysed the kinematics of the broad component in these 2-component galaxies, and found it to exhibit slower rotation and higher velocity dispersion than the narrow component, which is consistent with the properties of DIG as found by den Brok et al. (2020).
- The next step is to use LZIFU to fit multi-component emission lines to stacked, rotation-subtracted spectra (similar to those used by Oh et al. 2022) to improve the S/N in the emission lines. This will enable us to obtain a better estimate of the fraction of galaxies exhibiting kinematically distinct DIG at the expense of spatial resolution.
Follow-up papers may expand upon this work, and investigate e.g. the emission line ratios/metallicity of the DIG and/or the relationship between DIG and winds/outflows.
Potential overlap: to my knowledge, this is the only study to make use of the multiple-component emission line fits to analyse DIG. The work of Ho et al. (http://sami-survey.org/paper/sami-galaxy-survey-extraplanar-emissions-ed...) laid valuable groundwork for this project in terms of looking at the relationships between star formation and extraplanar DIG, but used a different method to identify DIG, limited their analysis to edge-on systems and had different aims to the proposed project.
