Hα in the outskirts of galaxies: Constraining the ionization sources of diffuse extraplanar gas and the outer disk initial mass function

Abstract

Abstract

Ionized gas can be found in many galaxies at great extraplanar heights and in the extreme outer disk, far away from the dense and energetic inner disk. This is in apparent conflict with the classical idea that astronomical gas is primarily ionized by the energetic radiation of young OB stars, which, due to their short lifespans, can mainly be found within newborn star clusters in the thin disk. In this thesis, I investigate this apparent conflict by studying the origin and ionization sources of extraplanar diffuse ionized gas (eDIG) in a sample of eight low-mass edge-on galaxies and by constraining the initial mass function (IMF) in the outer disk of M 83, a massive nearby face-on spiral galaxy.

My dataset includes Multi-Unit Spectroscopic Explorer (MUSE) integral field spectroscopy and Hα narrow-band imaging of a sample of eight low-mass edge-on galaxies, as well as wide-field narrow-band imaging of M 83 taken with the OmegaCAM instrument on the Survey telescope of the Very Large Telescope (VST). I also use archival Galaxy Evolution Explorer (GALEX) far and near ultraviolet (FUV and NUV) data, 3.6 µm data from the Spitzer Survey of Stellar Structure in Galaxies (S4G), and H I data from the Local Volume H I Survey (LVHIS). For the sample of edge-on galaxies, I investigate star forming rates (SFR), Hα profiles, gas kinematics, and emission-line diagnostics. I also construct detailed models of the eDIG ionization using the Cloudy photoionization code. For M 83, I count Hα-bright (including O and B stars) and blue (including B stars) clusters in the outer disk and compare the counts to stellar population models.

In my edge-on galaxy sample, I find tight correlations between disk and eDIG radial Hα profiles, a positive correlation between the specific SFR and eDIG scale height, and overall eDIG spectra consistent with OB star driven photoionization, indicating that the radiation of midplane OB stars is indeed the main driver of eDIG ionization. However, in most of the sample galaxies I also find regions of eDIG with spectra inconsistent with OB star photoionization. I find disturbed morphology and enhanced velocity dispersion in some of these regions, indicative of shock ionization driven by supernovae or stellar winds. In others, low Hα equivalent width hints at photoionization by hot low-mass evolved stars (HOLMES). I confirm with modeling that HOLMES are capable of causing the enhanced high-ionization lines in the eDIG of at least one of our sample galaxies, and produce ∼ 20% of the ionizing radiation therein (ESO 544-27).

In M 83 I find that the observed blue and Hα-bright cluster counts in the outer disk, as well as the maximum and total Hα-to-FUV flux ratios of the clusters, are inconsistent with the Kroupa universal IMF. Instead, I find that a steep IMF deficient in high mass stars is required to explain the observed properties of the outer disk star clusters.

My results demonstrate to a granular detail how local conditions determine which energetic phenomena are the most important for ionizing gas outside the classically star-forming thin disks of galaxies. These phenomena are important in understanding galaxy evolution and structure, and constraining their parameters is necessary for refining SFR diagnostics and star forming histories. Additionally, understanding star-formation and gas ionization in the outer disk and extraplanar regime of low gravitational potential has cosmological importance due to its sensitivity to baryonic feedback mechanisms.

<div lang="en" class="abs"> <p class="abs_header">Original papers</p> <ol type="I"> <li> <p>Rautio, R. P. V., Watkins, A. E., Comer&oacute;n, S., Salo, H., D&iacute;az-Garc&iacute;a, S., &amp; Janz, J. (2022). The multifarious ionization sources and disturbed kinematics of extraplanar gas in five low-mass galaxies. Astronomy &amp; Astrophysics, 659, A153. https://doi.org/10.1051/0004-6361/202142440 <a href="https://doi.org/10.1051/0004-6361/202142440">https://doi.org/10.1051/0004-6361/202142440</a></p> </li> <li> <p>Rautio, R. P. V., Watkins, A. E., Salo, H., Venhola, A., Knapen, J. H., &amp; Comer&oacute;n, S. (2024). Constraining the top-light initial mass function in the extended ultraviolet disk of M 83. Astronomy &amp; Astrophysics, 681, A76. https://doi.org/10.1051/0004-6361/202347729 <a href="https://doi.org/10.1051/0004-6361/202347729">https://doi.org/10.1051/0004-6361/202347729</a></p> <p><a href="https://urn.fi/URN:NBN:fi:oulu-202401181336">Self-archived version</a></p> </li> <li> <p>Rautio, R. P. V., Salo, H., Watkins, A. E., Comer&oacute;n, S., &amp; Venhola, A. (2024). CLOUDY modeling suggests a diversity of ionization mechanisms for diffuse extraplanar gas. Astronomy &amp; Astrophysics, 691, A16. https://doi.org/10.1051/0004-6361/202450762 <a href="https://doi.org/10.1051/0004-6361/202450762">https://doi.org/10.1051/0004-6361/202450762</a></p> <p><a href="https://urn.fi/URN:NBN:fi:oulu-202410306524">Self-archived version</a></p> </li> </ol> </div>

<div lang="fi" class="abs"> <p class="abs_header">Osajulkaisut</p> <ol type="I"> <li> <p>Rautio, R. P. V., Watkins, A. E., Comer&oacute;n, S., Salo, H., D&iacute;az-Garc&iacute;a, S., &amp; Janz, J. (2022). The multifarious ionization sources and disturbed kinematics of extraplanar gas in five low-mass galaxies. Astronomy &amp; Astrophysics, 659, A153. https://doi.org/10.1051/0004-6361/202142440 <a href="https://doi.org/10.1051/0004-6361/202142440">https://doi.org/10.1051/0004-6361/202142440</a></p> </li> <li> <p>Rautio, R. P. V., Watkins, A. E., Salo, H., Venhola, A., Knapen, J. H., &amp; Comer&oacute;n, S. (2024). Constraining the top-light initial mass function in the extended ultraviolet disk of M 83. Astronomy &amp; Astrophysics, 681, A76. https://doi.org/10.1051/0004-6361/202347729 <a href="https://doi.org/10.1051/0004-6361/202347729">https://doi.org/10.1051/0004-6361/202347729</a></p> <p><a href="https://urn.fi/URN:NBN:fi:oulu-202401181336">Rinnakkaistallennettu versio</a></p> </li> <li> <p>Rautio, R. P. V., Salo, H., Watkins, A. E., Comer&oacute;n, S., &amp; Venhola, A. (2024). CLOUDY modeling suggests a diversity of ionization mechanisms for diffuse extraplanar gas. Astronomy &amp; Astrophysics, 691, A16. https://doi.org/10.1051/0004-6361/202450762 <a href="https://doi.org/10.1051/0004-6361/202450762">https://doi.org/10.1051/0004-6361/202450762</a></p> <p><a href="https://urn.fi/URN:NBN:fi:oulu-202410306524">Rinnakkaistallennettu versio</a></p> </li> </ol> </div>