Abstract: JWST has opened the field of infrared stellar populations in nearby galaxies with its exquisite image quality and sensitivity in the near and mid-infrared. These wavelengths are ideal to identify young stellar objects and thereby enable studies of star formation in the Large and Small Magellanic Clouds, I Zw 18 and NGC 6822 where the metallicities are subsolar. In N79, an embedded super star cluster candidate region in the Large Magellanic Cloud (0.5 Z), we have obtained spectroscopy of the forming stars identifying ice and dust features as well as atomic and molecular lines in these sources. In our imaging and spectroscopy study of NGC 346, a massive star formation region in the Small Magellanic Cloud (0.2 Z), we have identified the young stellar objects with masses less than 5 solar masses. In NGC 6822 (0.2 Z), we have a census of the young stellar objects of Spitzer I, the massive star formation region at the center of the galaxy. In I Zw 18 (0.03 Z), we identify the star formation regions and report on an early census of the dusty inhabitants that also include evolved stars.
Introduction
Most of the detailed physics of star formation derives from detailed, infrared studies of star formation regions near our Sun; e.g. Orion, Taurus, Chameleon, and Ophiuchus. However, the highest production of stars within galaxies in the Universe occurred during the peak epoch of star formation (at redshift of z=2) in conditions very different from local star formation. The metal and dust content (a.k.a. metallicity), which is critical to cooling and coalescing gas, was much lower at that point, approximately 20% of the Sun’s metallicity. The star formation intensity was significantly higher, likely producing stars in super star clusters, the remnants of which may be current-day globular clusters.
With its exquisite image quality and sensitivity in the near- and mid-infrared, the James Webb Space Telescope (JWST) enables detailed studies of star formation regions at low metallicity and high intensity in the Large Magellanic Cloud (50% solar metallicity) and Small Magellanic Cloud (20% solar metallicity) that rival Milky Way studies of star formation. In more distant, low metallicity dwarf galaxies such as NGC 6822 (30% solar metallicity) and I Zw 18 (3% solar metallicity), the studies are focused on dusty stellar populations containing both evolved stars (red supergiant and asymptotic giant branch stars) as well as young stellar objects. The JWST MIRI and NIRCam data reach up to 7 magnitudes deeper than prior surveys of these targets revealing entirely new populations. Will we find great differences that explain the cause for the peak epoch of star formation? Does the lower metal content change the initial mass function of stars produced? Is the composition of the dust and ices surrounding embedded young stellar objects different than in the Milky Way? Is the density and temperature of the gas in these star formation regions different from those found in the Milky Way?
N79: the birth of a super star cluster
In the Large Magellanic Cloud, the N79 region hosts an embedded super star cluster candidate H72.97-69.39 (Ochsendorf et al. 2017) which was discovered as the most luminous infrared source from the Spitzer SAGE and Herschel HERITAGE surveys (Meixner et al. 2006; 2013). The multi-color Mid-InfraRed Instrument (MIRI) image shows a spectacular red star emanating radiation from its embedded central region (Figure 1). Based on photometry of extracted sources from the MIRI images, we found 106 young stellar objects with masses ranging from 0.7-40 M with the most massive objects concentrated at the location of H72.97 (Nayak et al. in prep). These most massive objects saturated the MIRI image creating the brilliant star like image and the MIRI MRS spectroscopy of Nayak et al. 2024 was used to derive synthetic photometry of this most luminous region. Moreover, the Nayak et al. 2024 paper created a comprehensive catalog of identified emission and absorption line detections from young stellar objects in the region that will be used to determine the density, temperature and excitation of the gas, and the composition of the dust and ice features. This composition is important and interesting to understand the potential for planetary system formation at low metallicity.
NGC 346: discovery of low mass young stellar objects
In the Small Magellanic Cloud, the most luminous massive star formation region is NGC 346. Its young stellar populations have been well studied with HST (e.g., Sabbi et al. 2007) which found an optically visible pre-main sequence population. Spitzer SAGE survey results identified the massive young stellar objects (>8 M) in the region (Sewilo et al. 2013).
With JWST NIRCam images and photometry (Figure 2), Jones et al. (2023) discovered thousands of low mass young stellar object candidates, probably down to ~0.1 M. Candidate populations are identified with color-color and color magnitude diagrams that cleanly separate the young stellar objects and pre-main sequence populations from the old red giant branch stars and main sequence stars. Jones et al. highlighted the F200W-F444W vs. F115W-F187N color-color diagram which not only separates out the reddened young stellar objects, but also cleanly identifies the pre-main sequence stars as having Paschen-a (1.87 micron F187N filter) excess due to accretion. In the young stellar objects, the apparent Paschen-a excess is due not only to accretion but also to extinction/IR excess. The NIRCam photometry catalog has over 200,000 sources with the thousands of young stellar object candidates being redder and fainter than the other populations. Moving to the longer wavelengths of MIRI (Figure 2), the young stellar object population is prominent, perhaps even the dominant population. Habel et al. (submitted) added the MIRI photometry and identified 833 young stellar object candidates. The discovery of the low mass young stellar objects with dusty disks and envelopes indicates that the building blocks for planets (dust) are present in systems with metallicities as low as 20% solar. The photometry from both NIRCam and MIRI provide an excellent definition of these young stellar objects using the Robitaille (2017) models. The model fits define luminosity, mass and evolutionary stage and are used to identify sources for MIRI medium resolution spectroscopy and NIRSpec multi-shutter array spectroscopy. This follow-up spectroscopy will reveal the composition of the dust, ice and gas in the circumstellar environment of these young stellar objects.
NGC 6822: Spitzer I Massive Star formation region
NGC 6822 is a nearby (490 kpc), isolated, low metallicity dwarf irregular galaxy with many active, optically bright star formation regions (e.g., Hubble I, III, IV, V and X). Studies with Spitzer by Jones et al. (2019) and Hirschauer et al. (2020) reveal the infrared selected young stellar objects in these star formation regions and also found a new region with 90 young stellar objects, dubbed Spitzer I. These studies hypothesized it to be a super star cluster in formation similar to the N79 region in the Large Magellanic Cloud. Our JWST observations targeted the central bar region that contains Spitzer I in addition to the bulk of stars in this galaxy (Figure 3). The larger purpose of this data was is to study dust evolution by taking an inventory of all the dusty stellar populations (dying stars and young stellar objects) in this galaxy with the same fidelity as the SAGE project accomplished for the Magellanic Clouds. Nally et al. (2023) have analyzed the NIRCam and MIRI photometry to identify ~900,000 sources and to study the dusty stellar populations with a particular focus on the evolved star population. Lenkić et al. (2023) have focused their analysis on the Spitzer I in which there were >82,000 individual sources, mostly from the NIRCam data. Using MIRI data in combination with NIRCam data, they identified 130 young stellar object candidates in the Spitzer I region from color magnitude diagrams. Using Robitaille (2017) models to estimate masses and luminosities, they find masses and luminosities of individually identified young stellar objects lower than those identified by Jones et al. (2019) and Hirschauer et al. (2020). JWST’s superior angular resolution has resolved some of the Spitzer sources into multiple young stellar objects and in some cases found that they were background galaxies. Further analysis will be required to determine if the star formation activity is unusually high and warrants a super star cluster status.
I Zw 18: First census of dusty stellar populations
I Zw 18 is an extremely metal poor (0.03Z) dwarf galaxy, with active star formation 0.17-1 M yr-1. Located 18.2 Mpc away, I Zw 18 resides in the local volume of galaxies. Spitzer observations barely resolve I Zw 18 into a galaxy. JWST NIRCam and MIRI images of I Zw 18 have sufficient angular resolution to separate the stars from the dust and gas (Figure 4). Hence for the first time, we can study its dusty stellar populations. As can be discerned from the images, the shortest NIRCam wavelengths, F115W and F200W, have numerous sources and the numbers drop off with wavelength. We use the color magnitude diagram of F200W vs F115W-F200W to capture the source identifications. The longer wavelength NIRCam color-magnitude diagrams are used to inform which of the sources have significant infrared excess. Two populations are identified: 1. a young population with upper main sequence stars and candidate red supergiant stars and 2. a redder and older population of asymptotic giant branch, likely mostly carbon rich, stars. Younger sources tend toward the NW star-forming lobe, while older sources tend toward the SE star-forming lobe, suggesting staggered epochs of peak star formation. Mixed in with the second population are probably candidate young stellar objects. A cleaner understanding of the young stellar object population awaits a finalized MIRI catalog.
Acknowledgements:
The research was partially carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). ©2024 All rights reserved.
References:
Habel, N., Nally, C., Lenkic, L., Meixner, M., et al. ApJ, submitted, “Young Stellar Objects in NGC 346: A JWST NIRCam/MIRI Imaging Survey”.
Hirschauer, A., Crouzet, N., Habel, N., et al. ApJ, submitted, “Imaging of I Zw 18 by JWST: I. Strategy and First Results of Dusty Stellar Populations”. arXiv:2403.6980.
Jones, O., Nally, C., Habel, N., et al. 2023, Nature Astronomy, 7, 694, “JWST/NIRCam detections of dusty subsolar-mass young stellar objects in the Small Magellanic Cloud”.
Lenkic, L., Nally, C., Jones, O., et al. 2023, submitted ApJ, “A JWST/MIRI and NIRcam Analysis of Young Stellar Object Population in the Spitzer I region of NGC 6822”. arXiv:2307.15704.
Meixner, M., Gordon, K., Indebetouw, R. et al. 2006, AJ, 132, 2268, “Spitzer Survey of the Large Magellanic Cloud: Surveying the Agents of Galaxy Evolution (SAGE). I. Overview and Initial Results”.
Meixner, M., Panuzzo, P., Roman-Duval, J. et al. 2013, AJ, 146, 62, “The HERSCHEL Inventory of the Agents of Galaxy Evolution in the Magellanic Clouds, a Herschl Open Time Key Program”.
Nally, C., Jones, O., Lenkić, L. et al. 2023, MNRAS, submitted “JWST MIRI and NIRCam Unveil Previously Unseen Infrared Stellar Population in NGC 6822” eprint arXiv:2309.13521.
Nayak, O., Hirschauer, A.S., Kavanagh, P.J. et al. 2024, ApJ, 963, 94, “JWST Mid-infrared Spectroscopy Resolves Gas, Dust, and Ice in Young Stellar Objects in the Large Magellanic Cloud”.
Nayak, O., Meixner, M., Hirschauer, A., et al. 2024, ApJ, submitted, “JWST Mid-infrared Imaging Resolves 106 Protostars in N79, – Host to Super Star Cluster Candidate, H72.97-69.39”.
Ochsendorf, B., Zinnecker, H., Nayak, O., et al. 2017 Nature Astronomy, 1, 784, “The star-forming complex LMC-N79 as a future rival to 30 Doradus”.
Robitaille, T.P. 2017, A&A, 600, 11, “A modular set of synthetic spectral energy distributions for young stellar objects”.
Sabbi, E., Sirianni, M., Nota, A., et al. 2007, 133, 44 “Past and Present Star Formation in the SMC: NGC 346 and its Neighborhood”.
Sewiło, M., Carlson, L., Seale, J. et al. 2013, ApJ, 778, 15 “Surveying the Agents of Galaxy Evolution in the Tidally Stripped, Low Metallicity Small Magellanic Cloud (SAGE-SMC). III. Young Stellar Objects”.