CEERS Key Paper VII: JWST/MIRI Reveals a Faint Population of Galaxies at Cosmic Noon Unseen by Spitzer (2308.09750v1)

Abstract: The Cosmic Evolution Early Release Science (CEERS) program observed the Extended Groth Strip with the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) in 2022. In this paper, we discuss the four MIRI pointings that observed with longer wavelength filters, including F770W, F1000W, F1280W, F1500W, F1800W, and F2100W. We compare the MIRI galaxies with the Spitzer/MIPS 24$\mu$m population in the EGS field. We find that MIRI can observe an order of magnitude deeper than MIPS in significantly shorter integration times, attributable to JWST's much larger aperture and MIRI's improved sensitivity. MIRI is exceptionally good at finding faint ($L_{\rm IR}<10^{10} L_\odot$) galaxies at $z\sim1-2$. We find that a significant portion of MIRI galaxies are "mid-IR weak"--they have strong near-IR emission and relatively weaker mid-IR emission, and most of the star formation is unobscured. We present new IR templates that capture how the mid-IR to near-IR emission changes with increasing infrared luminosity. We present two color-color diagrams to separate mid-IR weak galaxies and active galactic nuclei (AGN) from dusty star-forming galaxies and find that these color diagrams are most effective when used in conjunction with each other. We present the first number counts of 10$\mu$m sources and find that there are $\lesssim10$ IR AGN per MIRI pointing, possibly due to the difficulty of distinguishing AGN from intrinsically mid-IR weak galaxies (due to low metallicities or low dust content). We conclude that MIRI is most effective at observing moderate luminosity ($L_{\rm IR}=10^9-10^{10}L_\odot$) galaxies at $z=1-2$, and that photometry alone is not effective at identifying AGN within this faint population.

• The paper demonstrates that JWST/MIRI can detect galaxies nearly 10 times fainter than Spitzer, revealing a previously hidden population at cosmic noon.
• It uses detailed mid-IR to near-IR color diagnostics to effectively distinguish mid-IR weak, star-forming galaxies from AGNs.
• The findings challenge existing AGN prevalence models at cosmic noon and encourage new methodologies for studying galaxy evolution.

Analysis of Faint Galaxy Populations with JWST/MIRI at Cosmic Noon

The paper detailed in this paper leverages the enhanced capabilities of the James Webb Space Telescope (JWST), specifically the Mid-Infrared Instrument (MIRI), to observe faint galaxy populations during the epoch of cosmic noon (z ∼ 1−2). The research forms part of the Cosmic Evolution Early Release Science (CEERS) program and provides a significant advancement in the depth and resolution with which these populations can be studied, offering a substantial comparison with the Spitzer/MIPS datasets.

Key Observations and Findings

The JWST/MIRI observations reveal a population of galaxies that are almost an order of magnitude fainter than those detectable by Spitzer/MIPS, even though these detections require less observational time due to the superior sensitivity and larger aperture of JWST. The core achievement of this paper is the detection of "mid-IR weak" galaxies, which showcase strong near-IR emission but weaker mid-IR emission. These galaxies mostly engage in unshielded star formation activities, providing a valuable contrast to galaxies detected in previous surveys.

Astrophysical templates to capture mid-IR to near-IR emission transition dynamics are derived, and color-color diagrams are presented for efficiently discriminating mid-IR weak galaxies and active galactic nuclei (AGN) from dusty star-forming galaxies. Interestingly, there are fewer IR AGN per MIRI pointing than might be expected, illustrating the challenge of distinguishing AGNs from inherently mid-IR weak galaxies potentially due to factors such as low metallicity or dust content.

Implications of MIRI Observations

From a theoretical standpoint, this paper makes a bold claim regarding the mid-IR weak population, showcasing a new understanding of low-luminosity galaxy characteristics at cosmic noon. The implications for understanding the evolution and properties of galaxies during this epoch are significant. The results suggest that a large component of star formation is occurring in environments not heavily obscured by dust, a fact which until now was masked in previous deep infrared surveys. Furthermore, the observed low AGN count challenges prior assumptions about their prevalence during cosmic noon, raising questions about AGN feeding mechanisms and their observational identifiers.

Future Directions

Moving forward, the findings of this paper encourage further investigation into cosmic noon galaxies. Due to the limited ability to differentiate AGNs from mid-IR weak galaxies based solely on photometry, future studies could benefit from larger MIRI surveys or complementary spectroscopic and X-ray data. Expanding these efforts may help refine our understanding of the AGN's role in galaxy evolution, especially concerning their duty cycles and feeding efficiencies.

Additionally, to better understand the intrinsic properties of the observed galaxy population, future research using JWST/MIRI could focus on verifying the AGN and mid-IR weak classifications using machine learning techniques for pattern recognition in large datasets, potentially accompanied by detailed spectroscopic campaigns to offer more precise AGN diagnostics.

Finally, the quest for enhanced far-IR observational capacity remains, as it will afford better L IR measurements, further deepening our understanding of galactic growth and evolution. Consequently, robust future planning for far-IR telescopes, aligned with feedback from JWST findings, will be instrumental in unearthing the complexities of these distant cosmic structures.

The paper henceforth serves as a cornerstone for ongoing explorations in cosmic noon phenomena, effectively utilizing the advanced functional capabilities of JWST/MIRI and laying the groundwork for refined investigations into galaxy evolution.