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REQUIEM-2D: A Diversity of Formation Pathways in a Sample of Spatially Resolved Massive Quiescent Galaxies at z ∼ 2

Akhshik, Mohammad; Whitaker, Katherine E.; Leja, Joel; Richard, Johan; Spilker, Justin S.; Song, Mimi; Brammer, Gabriel; Bezanson, Rachel; Ebeling, Harald; Gallazzi, Anna R.; Mahler, Guillaume; Mowla, Lamiya A.; Nelson, Erica J.; Pacifici, Camilla; Sharon, Keren; Toft, Sune; Williams, Christina C.; Wright, Lillian; Zabl, Johannes

REQUIEM-2D: A Diversity of Formation Pathways in a Sample of Spatially Resolved Massive Quiescent Galaxies at z ∼ 2 Thumbnail


Mohammad Akhshik

Katherine E. Whitaker

Joel Leja

Johan Richard

Justin S. Spilker

Mimi Song

Gabriel Brammer

Rachel Bezanson

Harald Ebeling

Anna R. Gallazzi

Lamiya A. Mowla

Erica J. Nelson

Camilla Pacifici

Keren Sharon

Sune Toft

Christina C. Williams

Lillian Wright

Johannes Zabl


REQUIEM-2D (Resolving Quiescent Magnified Galaxies with 2D Grism Spectroscopy) comprises a sample of eight massive strongly lensed quiescent galaxies at z ∼ 2. REQUIEM-2D combines the natural magnification from strong gravitational lensing with the high-spatial-resolution grism spectroscopy of the Hubble Space Telescope through a spectrophotometric fit to study spatially resolved stellar populations. We show that quiescent galaxies in the REQUIEM-2D survey have diverse formation histories with age gradients at the 1σ–3σ level, including examples of (1) a younger central region supporting outside-in formation, (2) flat age gradients that show evidence for both spatially uniform early formation and inside-out quenching, and (3) regions at a fixed radial distance having different ages (such asymmetries cannot be recovered when averaging stellar population measurements azimuthally). The typical dust attenuation curve for the REQUIEM-2D galaxies is constrained to be steeper than Calzetti's law in the UV and generally consistent with AV < 1. Combined together and accounting for the different physical radial distances and formation timescales, we find that the REQUIEM-2D galaxies that formed earlier in the universe exhibit slow and uniform growth in their inner core, whereas the galaxies that formed later have rapid inner growth in their inner core with younger ages than the outskirts. These results challenge the currently accepted paradigm of how massive quiescent galaxies form, where the earliest galaxies are thought to form most rapidly. Significantly larger samples close to the epoch of formation with similar data quality and higher spectral resolution are required to validate this finding.


Akhshik, M., Whitaker, K. E., Leja, J., Richard, J., Spilker, J. S., Song, M., …Zabl, J. (2023). REQUIEM-2D: A Diversity of Formation Pathways in a Sample of Spatially Resolved Massive Quiescent Galaxies at z ∼ 2. Astrophysical Journal, 943(2), Article 179.

Journal Article Type Article
Acceptance Date Nov 28, 2022
Online Publication Date Feb 7, 2023
Publication Date 2023
Deposit Date Jun 8, 2023
Publicly Available Date Jun 8, 2023
Journal Astrophysical Journal
Print ISSN 0004-637X
Electronic ISSN 1538-4357
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 943
Issue 2
Article Number 179


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Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

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