edit: ESA/Webb, NASA & CSA, J. Lee and the PHANGS-JWST and PHANGS-HST Teams.
Stars form inside dusty and dense molecular clouds where visible light cannot go through, making it challenging to study with most telescopes. In the visible light and using powerful telescopes like Hubble, we have been able to see, with exquisite detail, what is left of the cocoons that formed stars next to the dissociated molecular gas that now appears as bright atomic material, heated by the intense light produced by the new balls of fire these formed. However, how stars form and what mechanisms determine how many and where these will form have been, for a while, a source of speculation.
Learning more about these dense clouds and how they form stars requires observations in the infrared -- the light where the dust and molecular gas radiate. Furthermore, high-resolution data gathered in this type of light, near us and in other galaxies, is critical for scientists looking to understand how the physics of the gas and star formation at small scales interact and determine the galactic structure and galaxy evolution.
This is the goal of the multi-instrument and science collaboration survey PHANGS - Physics at High Angular resolution in Nearby GalaxieS[1], which includes high-resolution observations of nearby galaxies with several telescopes, including the Atacama Large Millimeter/submillimeter Array (ALMA), the Hubble Space Telescope (HST), the James Webb Telescope (JWST) and the Very Large Telescope (VLT).
The PHANGS collaboration has been building a reference data set of nearby galaxies to study the different phases of star formation and the interstellar medium (ISM) at various scales. The most recent observations of 19 nearby galaxies obtained with JWST allowed the science team to reach unprecedented scales (~5-50 pc) at infrared wavelengths. At these scales, the observations provide a detailed inventory of star formation and an accurate measurement of the mass and age of star clusters, identification of the youngest stellar populations, and characterization of the physical state of small-dust grains[2].
Credit: Janice C. Lee et al 2023].
A comparison of some of these new PHANGS–JWST imaging for NCG 628 (right panels) with data previously taken with Spitzer (left panels), illustrating the order-of-magnitude gains in resolution and sensitivity with JWST. This enables the study of individual star clusters and the identification of embedded stellar populations. It also improves the characterization of extended stellar structures and the disentangling background objects, not part of the galaxy, that in the Spitzer image are impossible to resolve (see the two galaxies appearing in the JWST F335M panel at about 7:30).
Credit: NASA/Webb, NASA & CSA, J. Lee and the PHANGS-JWST and PHANGS-HST Teams.]
All HST Credits:NASA/STScI
As we go through the views of these galaxies observed with visible and infrared light, we can pick out the differences and get an idea of how astronomers use them to understand what are the processes taking place in the spiral arms of these galaxies and how these are different from those at their centers. There are scores of information to extract when astronomers compare observations made with several filters from the various instruments on board JWST. Some filters provide information about the stellar emission, with some contribution from nebular emission and hot dust. Others trace the emission of dust or the contribution of silicate absorption. Some others capture the emission produced by Polycyclic Aromatic Hydrocarbons (PAHs, a ubiquitous component of the organic material), giving crucial information on their size and charge distributions and characterizing their evolution through the ISM, which is central to understanding the PHAs life cycle.
Credit as above
Credit:PHANGS, ALMA (PI: A.Leroy), HST* (PI: Jenkins)
The ALMA data traces the cold molecular gas.
Peering deeper into regions of star formation using JWST helps the team better measure the timescales and efficiencies of the earliest phases of star formation across the diverse galactic environments. With JWST, they can better understand how the feedback from the intense light produced by the new stars disrupts the natal cloud and could even stop or accelerate star and cloud formation. They can also build empirical models for the local ISM conditions and how these are influenced by the properties of small dust grains. Furthermore, they can try to understand better how the observed dust traces the star formation activity across the diversity of galactic environments observed[2][3].
Credit: as above.
As we compare the HST UV-optical light data with the JWST infrared, we can see that the dark brown regions appear illuminated in shades of orange and red in the mid-infrared. The structures obscuring all light within and behind it in the optical images are due to the dust that does not let light pass through. In the infrared, this emission comes mainly from the previously mentioned small dust grains, which absorbed and re-emitted the light generated by the young stars recently formed in the nearby dark molecular clouds[4].
The images also show remarkable features in the shape of filaments, shells, bubbles, and compact sources. These lie in complex networks with young stellar populations at their edges. These stars provide the feedback energy that, together with the galactic dynamics, shapes the ISM around them. Also, in these images, we can see infrared compact sources without optical counterparts, indicating the sites with the earliest stages of star formation[2].
JWST image Credit as above
The bright red starlike object at the core of some Webb images, like that of NGC 7496, is not a star but rather a bright-compact object producing diffraction spikes. Similar diffraction spikes are common in any telescope observing a bright source. In this case, the source of this intense luminosity is probably generated by a supermassive black hole, which in the visible light obtained by HST, probably obscured by dust, is just a smooth-bright region at the center of the galaxy. However, not all oversized diffraction spikes at the cores of these galaxies are caused by black holes. These sometimes appear when a slew of very bright star clusters are in the central region of images taken by Webb.
Credit: NASA/STScI HST and JWST]
In these images, the blue hue indicates light coming from concentrations of older stars, which emit light in the bluer side of Webb’s range. These, however, would appear in yellow hues in the HST images because these emit more light in the red side of the visible range. The Webb images also show newly fully formed stars as blue dots along the galaxies’ spiral arms. This is because these young stars have blown out the gas and dust around them, allowing their light, which falls within the blue range of Webb, to escape toward us. These will also appear blue in the HST images, as their light is mainly on the blue side of the visible light. There are also groups of younger stars that appear as orange stars in the Webb images. This is because these stars are infant stars, still encased in their cocoons of gas and dust, where they continue gathering material from their parent cloud -- material that will serve as the fuel to keep them alive for a long time. These would appear as purple or pink in the HST images[5].
Credits as above
Now that the PHANGS team has these detailed images, they will use them to test selection strategies, as well as to produce complete catalogs that will allow them to answer many of the questions that keep pushing astronomers to make these kinds of observations.
You can find these JWST observations at the High Level Science Products site of the Barbara A. Mikulski Archive for Space Telescopes [https://archive.stsci.edu/hlsp/phangs
[2] Lee, Janice C. et al. 2023, ApJ,944, 17