This is a preview of the paper I am working on at the moment, tentitively titled "Gas, Dust, and Hα/Paα Decrement Extinction Around Stellar Population in NGC 4826 with PHANGS-HST and JWST," which will be submitted in summer 2025.

I work with the Physics at High Angular resolution in Nearby GalaxieS (PHANGS) collaboration's Star Cluster and Stellar Populations working group during my post-bac at Johns Hopkins University.

Paper Outline

1. Introduction
2. Observations and Catalogs
3. Hα/Paα Decrement Extinction Estimates
4. Results
5. Discussions
6. Summary (to be written)

1. & 2. Introduction, Catalog, and Observations

If you look at a galaxy, like Figure 1 below, what do you see? Because I study stellar populations, I see structurally distributed stars.

In PHANGS, Larson et al. 2023 developed a way to study the vast stellar population in a hierachical view, the PHANGS Multiscale Stellar Associations. There are various "flavors" of the stellar association catalog, where the location and shape of these stellar association regions are determined by stars in either the NUV- or V-band, and their sizes are matched to 8 pc, 16 pc, 32 pc, or 64 pc by a watershed algorithm. Figure 2 below shows the NUV-traced watershed 16 pc stellar association regions in NGC 4826.

The galaxy we are looking at, NGC 4826, also known as Messier 64 and the Evil Eye Galaxy, features a prominent dust lane (as you can see in Figure 3) and a counter-rotating gas disk, and previous studies suggest a potential merger history.

In fact, NGC 4826 is one of the most unique galaxy among the PHANGS sample. A quick look at the stellar associations in NGC 4826 reveals that their distribution on a UBVI color-color diagram (orange markers in Figure 4) differs the average PHANGS stellar associations' distribition in other galaxies. It is possible that this is due to an unusual amount of dust in NGC 4826, but it's also possible that the stars are older. Due to the age-reddening degeneracy, it is hard to tell which is the case without additional information.

Luckily, we do have additional information! We have F657N (Hα) imaging from the HST and F187N (Paα) imaging from the JWST. As you can see in Figure 5, they look beautiful.

3. Hα/Paα Decrement Extinction Estimates

Why are Hα and Paα images helpful? Because they are both H II emission lines, and I can use them to derive dust extinction similar to the Balmer decrement.

I first produce continuum subtracted images of Hα and Paα. For Hα, I use the adjacent broadbands F555W and F814W to estimate the continuum flux, similar to the approach in Chandar et al. 2025. For Paα, we compared different broadband combinations, namely F150W & F300M and F150W & F200W. We determined using F150W & F200W yields a better result. Because F200W also contains the Paα line, we adopt an iterative process similar to that in Gregg et al. 2024.

With the continuum subtracted image products, I can make a extinction map, Figure 6.

4. Results

As my project is still ongoing, here I present some preliminary results. Figure 7 is a UBVI color-color diagram with dereddened stellar associations according to the extinction measured from the Hα/Paα decrement. Figure 8 shows the histogram of A_V from the decrement compared to SED fitting A_V. As you can see, the decrement A_V distribution is different from the SED A_V distribution, and part of it could be attributed to disentangled age-reddening degeneracy.

5. Discussions

5.1. Localization of Hα and Paα Emission

A stellar association region does not necessarily have a uniform extinction, nor a uniform Hα or Paα emission. This can make a significant impact on the extinction correction. A quantitative measure of the Hα and Paα "coverage" or "localization" would be nice.

I came up with creative way to quantify localization—by using Gini index. Extragalatic astronomers who study galaxies as a whole are already familiar with Gini index since Lotz et al. 2004, but I propose using them on these stellar association regions. Another way my mentors and I are trying is a cumulative distribution function-based (CDF) covering fraction. We find these two metrics in agreement with each other.

As a demonstration, we look at two individual stellar associations. In Figure 10, Region 292 has uniform Hα and Paα emission. Their CDF curves grows smoothly and fast. They show a low Gini index and a high CDF covering fraction.

In Figure 11, Region 286 has a bright Hα or Paα "clump," hence localized emission. Their CDF curves have abrupt growth. They show a high Gini index and a low CDF covering fraction.

As the next step, I hope to develop a visual classification system of the Hα and Paα morphology, and tie the localization metrics to the measured extinction from the Hα/Paα decrement and maybe the properties of the stellar population itself!

5.2. Dust Extinction vs. Dust Emission – PAH

By comparing Hα and Paα to polycyclic aromatic hydrocarbon (PAH) emission, we can directly link dust extinction to dust emission. We use a PAH map made from JWST's MIRI F770W according to Sutter et al. 2024. In general, H II and PAH track each other, so we aim to focus on regions where they do not, like this one in Figure 12.

We also found stellar associations residing right within an Hα, Paα, and PAH bubble, like in Figure 13, hinting at potential dust and gas clearing driven by supernova or stellar feedback.

Please stay tuned for the upcoming paper!