An Extremely Compact, Low-mass Galaxy on its Way to Quiescence at z = 5.2

Abstract

We report the discovery of a low-mass z = 5.200 ± 0.002 galaxy that is in the process of ceasing its star formation. The galaxy, MACS0417-z5BBG, is multiply imaged with magnification factors ∼40 by the galaxy cluster MACS J0417.5-1154, observed as part of the CAnadian NIRISS Unbiased Cluster Survey (CANUCS). Using observations of MACS0417-z5BBG with a JWST/NIRSpec Prism spectrum and NIRCam imaging, we investigate the mechanism responsible for the cessation of star formation of the galaxy and speculate about possibilities for its future. Using spectrophotometric fitting, we find a remarkably low stellar mass of M * = 4.3 ± 0.8 0.9 × 10 7 M ⊙ , less than 1% of the characteristic stellar mass at z ∼ 5. We measure a delensed rest-UV half-light radius in the source plane of 30 ± 5 7 pc and measure a star formation rate from Hα of 0.14 ± 0.12 0.17 M ⊙ yr−1. We find that under the assumption of a double power-law star formation history, MACS0417-z5BBG has seen a recent rise in star formation, peaking ∼10–30 Myr ago and declining precipitously since then. Together, these measurements reveal a low-mass, extremely compact galaxy which is in the process of ceasing star formation. We investigate the possibilities of mechanisms that have led to the cessation of star formation in MACS0417-z5BBG, considering stellar and active galactic nuclei (AGN) feedback and environmental processes. We can likely rule out most environmental processes but leave open the possibility of a low-mass AGN that does not leave a broad-line imprint on the spectrum or that MACS0417-z5BBG could be a star-forming galaxy in the lull of a bursty star formation history.

Quiescent Low-mass Galaxies Observed by JWST in the Epoch of Reionization

“…Thanks to JWST we can now study them and investigate the role of the various feedback processes in their evolution. Looser et al (2023) recently reported the discovery of a quiescent low-mass galaxy at redshift z = 7.3 (see also Strait et al 2023 for another very low-mass quiescent system at z = 5.2) from the JADES program, JADES-GS-z7-01-QU. Following this work, we define as quiescent those galaxies with no (or negligible) star formation activity at the epoch of observation.…”

Star Formation at the Epoch of Reionization with CANUCS: The Ages of Stellar Populations in MACS1149-JD1

“…It shows properties that are consistent with other galaxies detected with JWST (e.g., Bunker et al 2023;Topping et al 2023); however, its true nature was only revealed through resolved SED fitting. While strong Balmer breaks can be present at high redshift, they are rare (Endsley et al 2023b;Laporte et al 2023;Looser et al 2023;Strait et al 2023). With the newest data for MACS1149-JD1, a strong Balmer break is excluded.…”

Can Supernovae Quench Star Formation in High-z Galaxies?

“…MACS0417-z5BBG is a z = 5.2 lensed galaxy observed by Strait et al (2023); it has a low stellar mass, M å = 4.3 × 10 7 M e . Just like JADES-GS-z7-01-QU, it exhibits a recent sudden SF drop, likely leading to a quiescent phase.…”

“…Just like JADES-GS-z7-01-QU, it exhibits a recent sudden SF drop, likely leading to a quiescent phase. Considering the SED fitting from Strait et al (2023), the (Leitherer et al 1999). Right: cooling functions (Λ) in the interstellar medium as a function of temperature (T vir ) derived with KROME (Grassi et al 2014).…”

“…The vast majority of the quiescent galaxies that have been observed to date are massive (e.g., Carnall et al 2020;Valentino et al 2020;Santini et al 2021), but the James Webb Space Telescope (JWST) is now opening a new window on the first generations of faint low-mass galaxies populating the highredshift Universe. Among these, the discoveries of the first z > 5 low-mass quiescent galaxies have been reported: the z = 7.3, M å = 10 8.6 M e JADES-GS-z7-01-QU (Looser et al 2023) and the z = 5.2, M å = 10 7.6 M e MACS0417-z5BBG (Strait et al 2023). Interestingly, both galaxies appear to be experiencing a post-starburst phase in which SF has been rapidly suppressed, implying that the quenching should be driven by internal feedback processes.…”