MEGA Mass Assembly with JWST: The MIRI EGS Galaxy and AGN Survey

Bren E. Backhaus, Allison Kirkpatrick, Guang Yang, Gregory Troiani, Kurt Hamblin, Jeyhan S. Kartaltepe, Dale D. Kocevski, Anton M. Koekemoer, Erini Lambrides, Casey Papovich, Kaila Ronayne

First listed 2025-03-24 | Last updated 2025-03-26

Abstract

We present the MIRI EGS Galaxy and AGN (MEGA) survey, a four band MIRI survey with 25 pointing in the Extended Groth Strip (EGS) extragalactic field. Three of the pointings utilized only the three reddest bands (F1000W, F1500W, F2100W) while the remainder of the pointings also add a blue filter (F770W). MEGA builds upon the existing observations in the EGS field by providing MIRI imaging for 68.9% of CEERS NIRCam imaging, filling a cruciality gap in order to understand galaxy evolution by observing the obscured Universe. Here, we present the technical design, data reduction, photometric catalog creation, the first data release, and science drivers of the MEGA survey. Our data reduction starts with the standard JWST calibration pipeline, but adds additional warm pixel masking and custom background subtraction steps to improve the quality of the final science image. We estimate the image depth of the reduced mosaics and present new galaxy number counts in four MIRI bands.

Short digest

MEGA is a 67 hr, four-band MIRI imaging program in the EGS (25 pointings; 70 arcmin^2) that adds F770W–F2100W coverage over 68.9% of CEERS NIRCam, targeting obscured galaxy and SMBH growth near Cosmic Noon. The team augments the JWST pipeline with warm‑pixel masking and iterative super–background subtraction, delivering mosaics with 5σ depths of ≈0.18, 0.41, 1.26, and 4.10 µJy in F770W, F1000W, F1500W, and F2100W. They release a photometric catalog and new galaxy number counts in all four bands, extending the faint mid‑IR regime over wider area than prior JWST fields. This fills the mid‑IR gap in EGS, enabling PAH‑based SFRs and dusty AGN selection to sharpen constraints on coupled galaxy–black hole growth.

Key figures to inspect

• Fig. 1 (footprint): Verify how MEGA fills the EGS mid‑IR gap and the 68.9% overlap with CEERS NIRCam; note where MEGA extends beyond prior CEERS/MIRI tiles.
• Fig. 2 (background subtraction): Examine the improvement from 0→3 iterations—suppressed cruciform/row–column striping and a flatter sky—crucial for faint-source photometry, especially at 21 µm.
• Fig. 3 (RGB + zooms): Compare HST F606W and NIRCam F444W to MIRI F770W/F2100W to see dust‑dominated structures and potential AGN‑heated components that vanish in the optical.
• Fig. 4 (F770W flux distribution): Contrast MEGA and CEERS flux histograms to gauge depth, completeness, and the source surface density feeding the new number counts.