Peeking into the Birth of Stars in Faraway Galaxies: Revolutionary view of infrared emission with JWST observations

This guest post was written by Hamid Hassani, a PhD student at the University of Alberta studying galaxy formation and evolution. He has a background in radio astronomy, having completed his MSc thesis on the topic of non-thermal radio continuum emission from the Magellanic Clouds. In his current Ph.D. studies, he is utilizing observations from AstroSat UV, JWST infrared, and HST optical to further uncover the intricacies of dust and gas in the star formation process. Hamid is a member of the PHANGS survey, which aims to study the properties of nearby galaxies.

Title: PHANGS-JWST First Results: The 21 micron Compact Source Population

Date Published: February 2023

Authors: H. Hassani, E. Rosolowsky, A. K. Leroy, et al.

First Author’s Institution: Department of Physics, University of Alberta, Canada

Status: Published in ApJ [open access]

Have you ever been curious about how astronomers unveil the secret birthplaces of stars, hidden deep within cosmic dust clouds?

One key tool astronomers use is the mid-infrared (MIR) light emitted by dust and a special type of molecule known as polycyclic aromatic hydrocarbons (PAHs), found in the space between stars, known as the interstellar medium (ISM). PAHs are fascinating molecules made up of multiple rings of carbon and hydrogen atoms, resembling tiny, flat sheets of chicken wire. They are significant in astronomy for their role in the formation of stars and planetary systems, and they give off unique infrared light signatures. Dust grains and PAHs are crucial in transforming the radiation within galaxies, turning the short-wavelength light from stars and other sources into longer-wavelength emissions in the infrared spectrum.

The mid-IR emission—- which we can observe with the James Webb Space Telescope (JWST)—is mostly generated by small dust grains, which are heated to temperatures of 100-150 K to emit photons at mid-infrared wavelengths (between 5-60 micron). PAHs emit light in specific, characteristic bands in the infrared part of the spectrum. This light is re-emitted after the PAHs have absorbed radiation from their environment. These emission bands, which range from 3 to 21 micron, correspond to the unique way the bonds in the large PAH molecules stretch and bend.

In this study, scientists used JWST observations to investigate the relationship between compact bright sources seen at 21 micron and other indicators of star formation (e.g. Ionized gas through hydrogen recombination lines) in distant galaxies. By looking at the infrared emission at different wavelengths, they were able to understand the nature of these 21 micron sources: whether they are stars, massive star-forming complexes, or background galaxies. This classification was based on the morphology of the observed sources as well as the strength of PAH emission detected within them. It was noted that star-forming complexes exhibit the strongest PAH emissions. Additionally, they were able to estimate the age and mass of the stellar population. This research provides new insights into the nature and properties of the stellar population on small scales (about 30-50 parsecs) and provides a unique view into clumps of dust and gas.

The authors used data from JWST to study the formation of stellar clusters in four distant galaxies as part of the PHANGS-JWST survey. They focused on sources detected in the MIRI 21 micron band, but also utilized data from other JWST filters and other telescopes such as ALMA, VLT, and HST to support their analysis. ALMA was used for molecular gas, VLT for Hydrogen recombination lines, and HST for optical imaging. The JWST observations of these four galaxies are shown in Figure 1.

Figure 1: RGB maps of 4 galaxies (NGC 7496, NGC 0628, NGC 1365, IC 5332) at wavelengths of 21 μm (red), 10 μm (green) and 7.7+11.3 μm (blue) with zoom-ins showing peaks, background galaxies, HII regions, stellar associations and compact stellar clusters. 1271 sources found in total, symbols shown in inset images to reduce crowding and highlight representative background galaxies in blue zoom-ins.

The authors analyzed the spectral energy distribution (SED) of each MIRI 21 micron compact source, combining data from JWST and HST and discovered that the majority of the sources they studied are likely to be regions of active star formation, associated with clusters of young stars or groups of reddened stars. These sources also exhibit bright PAH peaks at 3.3 micron, 7.7 micron, and 11.3 micron, as well as emission from Carbon Monoxide (a tracer of molecular gas) and H-alpha spectral line (a specific wavelength of light emitted by hydrogen atoms) from HII regions (See Figure 2). HII regions (where HII stands for ionized hydrogen) are clouds of gas that are ionized by young, hot stars. All of these sources are referred to as “ISM” sources, as their SEDs indicate the presence of strong PAH features, which are produced by dust mixed with ISM material.

Figure 2: A comparison of the energy emitted by star-forming complexes within galaxies (left) and confirmed background galaxies and dusty stars (right) at different wavelengths.  The grey lines represent the energy emitted by each individual source, while the coloured lines show the average energy emitted by all sources in a specific galaxy.

They also identified several sources that appear to be a specific type of old, luminous star known as ‘Extreme Asymptotic Giant Branch’ (extreme-AGB) stars. These stars, which they labeled as ‘STAR’ sources, are in one of the final stages of stellar evolution and are distinguished by their bright infrared light. The remaining sources appear to be background galaxies. These have an extended shape in F200W images and no bright PAH features, likely because the PAH features are redshifted out of the corresponding JWST bands. They refer to these as high-z or HafeZ (HZ) sources. Figure 3 shows a few of these background galaxies.

Figure 3: Scientists have uncovered over 100 previously unknown galaxies, known as HafeZ (HZ) sources, that emit brightly at 21 micron. These galaxies have been named after the Persian poet Hafez in honor of his allusive poetry shining through obscured words, similar to how these dusty galaxies are seen through other galaxies. This animation compares the observations of these galaxies captured by the JWST F200W (2 micron) and Spitzer IRAC 1 (3.6 micron) telescopes, highlighting the remarkable progress in our ability to image dusty galaxies.

The authors used the CIGALE code to estimate the age, mass and properties of the compact sources. They found that the 21 micron sources are stellar populations that are young with the ages between 2 to 25 Myrs old.

Figure 4: Scientists explore the correlation between the luminosity of the H-alpha emission line, which is attenuated by dust and the 21 micron luminosity. The plot shows the impact of dust on the emission of photons in star-forming regions galaxies, represented by color-coded age estimates and symbol sizes indicating stellar mass. The black dashed line marks the threshold for faint H-alpha sources, while the red line shows the average value. The vertical line indicates the threshold for searching for obscured, embedded sources that are relatively bright at 21 micron, but faint in H-alpha.

The study also examines the correlation between the 21 micron luminosity of a source and the luminosity of the H-alpha emission line at the same location. A significant aspect of this paper, also illustrated in Figure 4, is the finding that no bright embedded sources of star formation were found beyond the centers of galaxies (L (21 micron) > 19 W/Hz). Embedded star formation, which refers to the formation of stars in their very early stages (less than 2 Myr), can be identified by bright mid-infrared emission (21 micron in this case) and faint optical emission (H-alpha emission). This implies that embedded sources are relatively uncommon and not consistently linked with intense CO emission beyond the center of galaxies, at a physical scale of about 50 parsecs, as one might anticipate in regions where young, massive stars are forming.

This study, utilizing high-resolution mid-infrared observations from the JWST as part of the PHANGS-JWST survey, sheds light on the stellar population in distant galaxies. It reveals that the majority of studied sources are likely active star-forming regions, including obscured sources indicative of early-stage star formation, which are crucial for understanding stellar evolution. The PHANGS-JWST project, with ongoing observations and upcoming expansions, promises to further illuminate obscured star formation across various galaxy types.

Edited by Sahil Hegde
Featured image from Hassani et al. (2023)

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