UR: The Mystery of Orphan H II Regions: Where Are the Ionising Star Clusters?

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Jia Wei Teh

The Australian National University

Jia Wei Teh is a fourth-year (Honours year) undergraduate student majoring in Astronomy and Astrophysics at the Research School of Astronomy and Astrophysics, The Australian National University. His research interest lies primarily in the study of galaxy evolution and star formation, through the analysis of IFU data. This project is the first part of his honours year thesis, which is completed under the supervision of Dr. Kathryn Grasha, Prof. Mark. Krumholz, and Dr. Andrew Battisti.

Photo of Jia Wei Teh (author)

H II regions are visually spectacular bubbles of ionised hydrogen gas typically found surrounding newly formed star clusters. They are created by extreme UV radiation from young and massive O-type or B-type stars in the cluster. Ideally, we hope to understand the correlation between properties (i.e., mass, age, ionizing photon flux) of the young star clusters (YSCs) and their ionised regions, by matching YSCs to the H II regions they create. 

In this project, we matched YSCs available in the Legacy ExtraGalactic UV Survey (LEGUS) to H II regions in The Star formation, Ionized gas, and Nebular Abundances Legacy Survey (SIGNALS) of the nearby spiral galaxy, NGC 628 (see Figure 1). To our surprise, we found that 62% of H II regions in the combined catalogue lack corresponding young star clusters (YSC; star clusters with an age of ≤ 10 Myr).

Graph showing the positions of the star clusters with different colors for different ages, the different ages scattered throughout the graph
Figure 1: Spatial distribution of young star clusters (t 10 Myr; in diamonds) and H II regions (in grey circles). The grey circles show the true physical sizes (with respect to the axes ticks) of the H II regions. The black outline represents the LEGUS field of view . Black horizontal bar indicates the length scale of 60″, or 2.9 kpc at distance of NGC628.

We examined possible explanations for these “orphan” H II regions. One main method used to perform this analysis is the stochastic stellar population synthesis code, Stochastically Lighting Up Galaxies (SLUG), to create a library of 100,000 synthetic YSCs. For each synthesised YSC, we checked if it would be classified as star cluster in LEGUS, and then estimated its Hα luminosity via a series of conversions. The expected fraction of orphan H II regions is then the ratio of synthetic YSCs that are not detectable in LEGUS but are able to produce detectable H II regions to the total number of detectable H II regions. In the synthesised YSCs, we found that 48% of H II regions detectable by SIGNALS are powered by YSCs which would not be detected in LEGUS (see Figure 2). 

Log age vs. estimated log luminosity of the hydrogren; red vertical line on y=35 with points scattered above and below
Figure 2: The theoretical H luminosity vs age distribution of 100,000 synthesised YSCs from SLUG, showing star clusters that would be detected in LEGUS (black), and star clusters that would not (grey). The red horizontal line shows the H detection limit of SIGNALS. Orphan H II regions are represented by grey data points above the detection limit of SIGNALS.

The conclusion we drew from this analysis is that orphan H II regions in the combined catalogue likely result from star clusters that can create H II regions bright enough to be seen, yet themselves remain undetected. The orphan H II regions are not due to a flaw in our understanding of the physics that govern the link between YSCs and H II regions, but rather a result of difference in detection limits: that the sensitivity of the star cluster survey is simply not deep enough to obtain a complete catalogue of cluster−H II region associations. Our next goal is to assemble the results of this combined catalogue and investigate the physical connections between star clusters and H II regions in NGC 628.

Astrobite edited by: Haley Wahl

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  1. the regions that you detected type o and type b stars was 38%
    the regions where you detected no young stars 62% orphan regions do you expect to discover type o and type b stars here or do you expect to discover a type and f type stars here

  2. Thanks for the question, Liam!

    What we have found in this work is that in our observed data, 62% of H II regions are “orphans”. By using a library of synthetic star clusters we find that the best explanation is that the clusters that power these H II regions are themselves not detected, due to the cluster selection criteria in LEGUS. Statistically, the majority of them are indeed star clusters that are less luminous compared to the detected clusters.

    H II regions will most certainly be ionised by the O, B-type stars – the remaining stellar types (e.g., A, F, G etc.) simply will not produce sufficient ionising photons to ionise their surroundings to form H II regions.


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