Welcome back to our five-part examination of Webb’s Cycle 4 General Observations program. In the first and second installments, we examined how some of Webb’s 8,500 hours of prime observing time this cycle will be dedicated to exoplanet characterization and the study of galaxies that existed at “Cosmic Dawn” – ca. less than 1 billion years after the Big Bang.
Today, we will look at programs focused on observing the seeds of black holes in the early Universe and galaxies as they existed roughly 2-3 billion years after the Big Bang—the period known as “Cosmic Noon.”
On March 11th, 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 broken down into eight categories that encompass Webb’s capabilities. These range from exoplanet study and characterization and observations of the earliest galaxies in the Universe to stellar science and Solar System Astronomy.
As we covered in previous installments, Webb’s unique capabilities at imaging exoplanets are allowing scientists to refine their measurements of exoplanet habitability. These same capabilities have allowed astronomers to view some of the earliest galaxies in the Universe so they can trace their evolution from “Cosmic Dawn” to the present day. By observing galaxies and supermassive black holes (SMBHs) from more recent cosmological epochs, scientists hope to understand how they evolved from the earliest times to more recent (and better-understood) cosmological epochs.
SMBHs and AGNs
In the 1970s, astronomers learned that massive galaxies have supermassive black holes (SMBHs) at their centers. Since then, research has shown that these gravitational behemoths play a vital role in galactic evolution, which includes arresting star formation later on. While decades of observations have led to a consistent theory of SMBH growth during the latter half of cosmic history (ca. the past 7 billion years), their formation and growth during the early Universe remains one of the biggest cosmological mysteries.
Observations made during the previous cycle have revealed that the “seeds” of SMBHs existed during the early Universe. However, many of the black holes observed were in the billion solar mass range, significantly larger than previous cosmological models predicted. The presence of these SMBHs was indicated by the particularly bright Active Galactic Nuclei (AGNs), or quasars, observed during this period.
Proposed explanations include the possibility that SMBHs formed directly from the collapse of massive gas clouds. This process would have been more rapid than what cosmologists previously suspected, which was that SMBH seeds formed from smaller black holes that were the remnants of the first stars (Population III) in the Universe. Another possibility was that they formed directly from primordial black holes, a hypothetical object believed to have formed shortly after the Big Bang.
Further observations of these galaxies and their SMBHs are critical to learning how cosmic structures grew and evolved during the early Universe. It will also shed light on one of the least understood periods of comic history, known as the “Epoch of Reionization” (EoR). It was during this period – which lasted from 380,000 to 1 billion years after the Big Bang – that the first stars (Population III) and galaxies formed. The ultraviolet light emitted by these stars gradually ionized the clouds of neutral hydrogen that permeated the Universe.
For example, there’s the GO 7491 program, “Probing hidden active SMBHs in the epoch of reionization: the missing link between classical quasars and faint JWST AGNs.” Led by PI Dr. Yoshiki Matsuoka, an Associate Professor at Ehime University, this program will rely on the JWST’s Near-Infrared Spectrometer (NIRSpec) to examine 30 low-luminosity AGNs at redshifts between 5.7 and 6.7 – 12.5 to 12.8 billion light years away. These active galaxies, which existed during the EoR, are known as broad H-alpha galaxies (BHaGs).
According to Webb’s previous observations, these galaxies appeared to be 10 to 100 times more numerous than what the classic quasar luminosity function (QLF) infers. The team indicates that this may imply the presence of numerous faint quasars that are missing from the existing surveys or that BHaGs represent a new population of AGNs that don’t conform to the QLF. Per their proposal:
“Discriminating between these scenarios has a huge impact on our understanding of SMBHs growing in the EoR, as well as the evolution of their host galaxies and sources of reionization. Here we propose an ambitious NIRSpec program to search for broad H-alpha in UV-luminous galaxies, in the gap between the classical quasars and BHaGs. Such galaxies are too sparse on the sky to fall in randomly chosen JWST fields, but hold the key to finding the missing link between the two AGN populations.”
Remember those “Little Red Dots” (LRDs) that Webb spotted in the early Universe that turned out to be dusty quasars? This discovery raised questions about the abundance of SMBHs during the “Epoch of Reionization” (EoR), a period when the first stars and galaxies gradually reionized all the neutral hydrogen that permeated the very early Universe. This event is what led to the Universe becoming “transparent,” or observable to astronomers today.
The goal of program GO 7076, “A comprehensive population study of Little Red Dots: Connecting early BH and galaxy growth,” will be to examine those LRDs more closely to learn more about the formation and growth of SMBHs67. Led by PI Hollis Akins, a Ph.D. student at the University of Texas at Austin, this program will rely on the Near-Infrared Spectrometer (NIRSpec) in multi-object spectroscopy (MOS) mode.
As they state in their proposal, the overall objective is to determine whether they represent a transition phase from obscured, rapidly accreting BH seeds to unobscured blue quasars.
“We propose an efficient and comprehensive NIRSpec follow-up program targeting LRDs, obtaining uniform PRISM+G395M spectroscopy for ~100 of the brightest and highest-redshift LRDs discovered by JWST, particularly those with MIRI coverage at >10 micron. With these data we will be able to disentangle the heterogeneous LRD population and:
1) Search for outflows and high-ionization lines to determine the nature of LRD obscuration
2) Measure black hole and stellar masses and examine implications for LCDM and BH seeding scenarios
3) Measure number densities of LRDs with secure redshifts, and bolometric luminosities over a large volume.”
The resulting data will facilitate a comprehensive analysis of the LRDs, test key predictions, and provide valuable insight into their role in early galaxy/SMBH growth.
Closer to home, some of Webb’s observation time will be dedicated to the GO 7532 program, “A Joint Mid-IR and X-ray Investigation of the Physics Driving Sgr A*’s Flares.” This program will conduct Medium Resolution Spectroscopy (MRS) using Webb’s Mid-Infrared Imager (MIRI) of Sagittarius A* (Sgr A*) – the SMBH at the center of our galaxy. Combined with data from NASA’s Chandra X-ray space telescope, the team’s goal is to study how matter accretes onto SMBHs by studying the one closest to us.
In the past, astronomers have spotted variable flare activity from Sgr A* that may be coming from its accretion flow or plasma jet. This variability may be the result of a tilted inner disk, gravitational lensing of bright spots in the disk, or particle acceleration. Similarly, multiple IR sources and structures have been identified that could be used to learn more about the accretion process. Their combined observations will allow astronomers to discern between emission models and study the IR sources more closely.
“Only JWST’s MIRI has the high angular resolution and mid-IR sensitivity to probe this complex and dynamic region,” they state in their proposal paper. “MIRI can detect changes in the mid-IR spectral index, which is an important diagnostic of physical conditions in the flare. The X-ray flux will constrain the energy distribution of non-thermal particles in the flare.”
Cosmic Noon
As noted, another important aspect of Webb’s mission is to trace the evolution of galaxies from the early Universe to more recent periods. Previous observations by Webb have led astronomers to theorize that early black hole growth phases are highly obscured by sources of cosmic dust. To help resolve this mystery, the GO 6827 program aims to trace the formation and growth of SMBHs across billions of years of cosmic history.
To this end, Principal Investigator (PI) Prof. Anna-Christina Eilers from the Massachusetts Institute of Technology (MIT) and her colleagues will study both luminous and heavily dust-enshrouded AGNs. In so doing, they hope to connect the unresolved mysteries of SMBH growth at Cosmic Dawn to the decade-old and seemingly well-understood results at Cosmic Noon. As they explained in their proposal:
“Using NIRCam in wide-field slitless mode as well as deep MIRI imaging in five quasar fields we propose to observe >80 (>200) unobscured (obscured) AGN and quasars across a wide range of halo mass, black hole mass, stellar mass, luminosity and redshift, in order to determine (#1) their dark matter halo masses and duty cycles, (#2) their obscuration fraction, (#3) the initial seed black hole masses, (#4) the merger rate, and (#5) their accretion rates to paint the first coherent picture of SMBH growth across cosmic time – from Cosmic Dawn to Cosmic Noon.”
Next up, there’s the program led by PI Dr. Allison Kirkpatrick, an Associate Professor at The University of Kansas (KU) and the KU Center for Research (KUCR) – GO 7957, “MEGA Spectra: Black Hole Growth and ISM Conditions at Cosmic Noon.” They propose using NIRSpec observations to search for low-luminosity AGNs in the Extended Groth Strip (EGS) field, the region of the night sky between the constellations of Ursa Major and Boötes studied by the Hubble Space Telescope (HST).
Their observations will focus on the interstellar medium (ISM) of galaxies at z=0.5-5.0 (~6 to 12.469 billion light-years distant); specifically, on star-forming regions (STRs) of 5 million Solar masses or more. These targets were selected from the MIRI EGS Galaxy and AGN (MEGA) survey conducted during Cycle 2. The near-IR spectroscopy and mid-IR photometry will be combined to create a complete census of the ISM, star formation, and black hole growth in these STRs. As they state, the primary goals of this program are to:
“1) confirm low luminosity or obscured AGN candidates (mid-IR selected) through high ionization lines such as [OIII]; 2) measure black hole masses (via H-beta) in unobscured AGN, down to M(BH) = 10^7 M(sun); 3) measure metal content via lines such as [OII], [SII], [NII] and correlate with strength of PAH features; 4) use Halpha, Paschen-alpha to calibrate PAH SFR indicators in main sequence galaxies.”
These observations, it is hoped, will accomplish two things: confirm that the LRD’s are dust-obscured AGNs and resolve the conflicting redshift estimates (i.e. determine how far away they truly are).
Webb’s investigation of the early Universe and the deepest cosmological mysteries continues! Stay tuned for our next installment, where we’ll examine how some of the JWSTs observation time will be dedicated to the study or star formation and stellar populations.
Further Reading: STScI