Universe Today
Earlier this week, the Space Telescope Science Institute (STScI) announced the science objectives for the fourth cycle of the James Webb Space Telescope's (JWST) General Observations program - aka. Cycle 4 GO. This latest cycle includes 274 programs that will make up the JWST's fourth year of operations, amounting to 8,500 hours of prime observing time. These programs are broken down into eight categories that encompass Webb's capabilities.
This includes exoplanet study and characterization, the study of the earliest galaxies in the Universe, stellar populations and formation, and Solar System Astronomy. As we addressed in the previous installment, Cycle 4 includes many programs that will leverage Webb's extreme sensitivity and advanced instruments to observe exoplanets, characterize their atmospheres, and measure their potential habitability.
In keeping with Webb's major science objectives, many of the Cycle 4 programs will also focus on studying the earliest stars and galaxies in the Universe. These programs will build on previous efforts to observe high-redshift galaxies (those that formed shortly after the Big Bang), the first population of stars in the Universe (Population III), and examine the role Dark Matter (DM) played in their formation.
Central to this is the cosmological period known as the "Cosmic Dark Ages," which occurred between 370,000 and 1 billion years after the Big Bang. During this time, the Universe was permeated by neutral hydrogen, and there were only two main sources of photons: the relic radiation left over from the Big Bang - the Cosmic Microwave Background (CMB) - and those occasionally released by neutral hydrogen atoms.
This period is also when the first stars and galaxies are believed to have formed (ca. 13.6 billion years ago). This led to the gradual ionization of the clouds of neutral hydrogen, which led to the "Epoch of Reionization," which led to the Universe becoming "transparent" (visible to modern instruments). Cosmologists refer to the period where the first galaxies emerged from the Dark Ages as “Cosmic Dawn."
Previous instruments lacked the resolution or sensitivity to capture light from this epoch, which is shifted into parts of the infrared spectrum that are very difficult to observe. However, Webb's sensitivity and infrared optics allow astronomers to finally pierce the veil of the "Dark Ages."
High Redshift Galaxies
The earliest galaxies in the Universe are designated "high redshift," which refers to how the wavelength of their light has become elongated due to the expansion of the Universe ((aka. the Hubble-Lemaitre Constant). This causes the light to become "shifted" towards the red end of the spectrum. Light from galaxies that existed during the early Universe (more than 13 billion years ago) is redshifted to the point where it is only visible in the infrared spectrum.
This is the purpose of the GO 7208 program, titled "THRIFTY: The High-RedshIft FronTier surveY." This observation campaign will build on JWST's detection of several luminous galaxies with redshift values greater than 9 (z>9). This corresponds to galaxies that existed up to 13.5 billion years ago, one of Webb's greatest discoveries to date. The abundance of galaxies this early in the Universe and their apparent brightness was a surprise to astronomers and has led to a revision of theories on early galaxy formation.
Possible explanations include modifications to the Lambda Cold Dark Matter (LCDM) model of cosmology, the possibility that SMBHs may have been super-luminous in this period, the rapid formation of stars from abundant cold gas clouds, feedback-free starbursts, and more. However, confirmation of these theories requires more direct evidence, which the THRIFTY program hopes to address.
The program's PI is Romain Meyer, a postdoctoral researcher at the University of Geneva (UNIGE). As he and his team described in their GO 7208 program proposal, "THRIFTY will determine the true number density of ultra-luminous galaxies at z>9 by targeting a sample of 123 candidates selected from >1 million sources over a total of 0.3 square degrees (out of the Galactic plane) from all existing prime and pure-parallel JWST imaging surveys."
One of Webb's earliest discoveries from Cycle 1 was of a population of small, red-tinted galaxies during the early Universe that may have contained growing SMBHs. These "Little Red Dots" (LRDs), as they were nicknamed, were thought to be Active Galactic Nuclei (AGNs), or quasars, but many astronomers. While they were declared one of the biggest discoveries in physics in 2023, there is still no consensus on what they actually are.
Enter the GO 7404 program, titled "How I wonder what you are -- do JWST's Little Red Dots twinkle? Testing broad-line and continuum variability on week, month, and six-month." Rohan Naidu, a NASA Hubble Fellow and the Pappalardo Fellow in Physics at the Massachusetts Institute of Technology (MIT), is this program's Principal Investigator (PI). Using Webb's Near-Infrared Camera (NIRCam), they will conduct the first longwave systematic LRD monitoring campaign to determine their exact nature.
Next, there's the GO 7814 program, titled "MINERVA: Unlocking the Hidden Gems of the Distant Universe and Completing HST and JWST’s Imaging Legacy with Medium Bands." This program, led by PI Dr. Adam Muzzin of York University, will build on the deep imaging surveys conducted with the JWST Near-Infrared Camera (NIRCam). While revolutionary, these surveys were limited to broad-band observations with low spectral resolution.
For their program, they will use Webb's Mid-Infrared Instrument (MIRI) to examine the primary fields observed by the Hubble Space Telescope (HST) and the JWST. In the process, they plan to increase the surveyed area nearly by a factor of 10 compared to existing medium-band programs, leading to the discovery of rare and previously undetected populations in existing deep-field catalogs. These observations, they state, will allow them to:
"1) efficiently identify and characterize galaxies with unusual SEDs including z>12 candidates, high-redshift Balmer breaks, metal-poor extreme emission line galaxies, and extremely red/dusty sources, 2) improve stellar mass and star-formation rate density measurements at 2 < z < 10 by factors of 2-4, and 3) create resolved maps of stellar mass and star formation across 10 Gyr of cosmic time to model galaxy growth in two dimensions."
Epoch of Reionization
In addition to the earliest galaxies, one of Webb's biggest objectives is the detection of the first stars in the Universe. These Population III stars are believed to have been ultra-hot, massive, and short-lived, remaining in their main sequence phase for a few dozen million years. They also emitted tremendous amounts of ultraviolet radiation, which led to the "Epoch of Reionization" (EoR). Until the deployment of the JWST, this population of stars remained entirely theoretical.
This is the reason for programs like GO 7677, "Pushing the Faintest Limits: Extremely Low-Luminosity and Pop III-like Star-Forming Complexes in the Early Universe." Using the JWST's NIRSpec integral field unit (IFU), the team - led by Eros Vanzella, a First Researcher of the INAF Astrophysics and Space Science Observatory in Bologna - will observe two stars at z=5.663 and z=4.194, corresponding to distances of 11.7 billion and 11.425 billion light-years away. As they state in their proposal:
"This study will allow us to measure the metallicity of both sources and assess the presence of massive stars in such elusive systems by evaluating their ionizing photon production efficiency. These observations will expand (at least double) the sample of ultra-faint sources with these measurements which only JWST can perform, pushing the frontier of understanding toward Population III-like star formation conditions. The fortunate angular proximity of the two targets allows for simultaneous observation within the same IFU field of views."
There's also the GO 7436 program, "The Last Neutral Islands at the End of Reionization? Characterizing the Nature of the Longest Dark Gaps in IGM Transmission at z~5.3." During this cosmic epoch, ionized regions gradually grew and overlapped in the intergalactic medium. However, how and when it took place is still unknown, and placing accurate estimates is crucial to studying the formation of galaxies in the early Universe. It is led by PI Xiangyu Jin, a graduate student with the Stewart Observatory at the University of Arizona.
He and his team plan to use the JWST to observe galaxies with redshifts of around z=5.5, corresponding to distances of about 12.4 billion light-years away. At this point, roughly 1.4 billion years after the Big Bang, the intergalactic medium (IGM) appears highly ionized to modern instruments, but "dark gaps" have still been observed. "These long dark gaps could be the last remaining neutral islands in the IGM at the end of a highly inhomogeneous reionization process," they propose. "If confirmed, it will have a profound impact on the physics of reionization."
To this end, they propose observations using the W. M. Keck Observatory and Webb's NIRCam. While the Keck observations will probe the Lyman-alpha emissions from roughly 230 galaxies (about 75 in the "dark gap" regions), NIRCam Wide Field Slitless Spectroscopy (WFSS) will conduct redshift measurements of these galaxies. "We will also characterize the galaxy density field around long dark gaps," they added. "This joint program will allow us to directly test the ultra-late reionization model and to place robust constraints on the topology of reionization and the nature of inhomogeneous reionization."
Then there's GO 8018, titled "DIVER: Deep Insights into UV Spectroscopy at the Epoch of Reionization." Led by PI Xiaojing Lin, a graduate student with the University of Arizona Steward Observatory. , this program will build on Webb's early observations of the EoR. These revealed hard radiation fields and bursts of star formation that were sometimes accompanied by the detection of extreme conditions in the interstellar medium (ISM) and unusual chemical abundance.
According to Lin and her colleagues, high-quality rest-frame UV spectroscopy of galaxies during this period is urgently needed. The team proposes conducting a deep spectroscopic survey of over 140 galaxies in the Great Observatories Origins Deep Survey North (GOODS-N) field at redshifts of z=5 to 9 (12.469 to 13.11 billion light years away). As the team wrote, this will establish the largest and deepest UV spectral database for EoR galaxies:
"DIVER will directly (1) clock the star formation history by determining the distribution and redshift evolution of carbon abundance and (2) probe the prevalence of extremely high electron density and its connection to bursty star formation and chemical peculiarity. DIVER will also lead to various high-profile science, including the UV demographics of AGNs and massive stellar populations, and constraining the reionization history through LyA. With great legacy values, DIVER will advance our understanding of star formation and chemical enrichment history in the early Universe, providing a crucial foundation for studies of z>10 galaxies."
Dark Matter Halos
According to the Standard Model of Cosmology - the Lambda Cold Dark Matter (LCDM) model - Dark Matter (DM) played a vital role in the formation of galaxies in the early Universe. In theory, DM halos (DMHs) formed from the gravitational collapse of density perturbations after the Big Bang and provided the gravitational "wells" that allowed clouds of gas to form Population III stars and the first galaxies. Like many other aspects of the early Universe, this process has remained entirely theoretical until this point.
The purpose behind the GO 7519 program, "How do dark matter halos connect with supermassive black holes and their host galaxies?" is to address the role these played in galaxy formation. Previous observations with Webb have played an important role in measuring the mass of DMHs in high-redshift quasars, but these measurements were limited to bright quasars. Per their proposal, the team will rely on NIRCam WFSS observations to identify emission lines from doubly ionized oxygen (O III) around 12 faint quasars at distances of about 12.716 billion light-years.
"In this new effort, we will measure the average DMH mass from the cross-correlation analysis of quasars and surrounding [O III] emitters and evaluate the DMH mass probability density function for individual quasars based on cosmological simulations. This program will allow us, for the first time, to obtain a quasar sample in which the black hole mass, stellar mass, and halo mass are all measured simultaneously. This sample will reveal their lifetime and the scaling relations in the early universe, underlying the SMBH growth of SMBHs over cosmic time."
For decades, astronomers, astrophysicists, and cosmologists have had to contend with limitations on what they could see within the cosmos. Thanks to the Hubble Space Telescope, they were able to observe galaxies that existed about 1 billion years after the Big Bang. Thanks to missions like the COsmic microwave Background Explorer (COBE), the Wilkinson Microwave Anisotropy Probe (WMAP), and Planck, they were able to measure the earliest light in the Universe.
Thanks to the JWST, scientists are now able to get a look at what came in between. By observing galaxies and cosmic structures as they existed shortly after the Big Bang, we may someday be able to chart cosmic evolution all the way back to the beginning of time.
Further Reading: STScI