JWST discovered a massive and compact quiescent galaxy

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JWST Color Image or JWST-ER1. Credit: Van Dokum et al., 2023.

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JWST Color Image or JWST-ER1. Credit: Van Dokum et al., 2023.

Astronomers have reported the discovery of a new galaxy using the James Webb Space Telescope (JWST) as part of the JWST COSMOS-Web survey. The newly discovered object, named JWST-ER1, is a massive and small quiescent galaxy. The findings are described in a paper published on September 14 in the preprint server arXiv.

Massive galaxies that have stopped forming stars (called massive quiescent galaxies) are plausible progenitors of giant elliptical galaxies. Because these objects formed stars earlier and rapidly accreted their stellar masses, they can be key to improving our understanding of the process of galaxy evolution.

Now, a team of astronomers led by Yale University’s Peter Van Tockum reports the discovery of a new galaxy of this type, named JWST-ER1. The object was identified by JWST’s Near Infrared Camera (NIRCam) as part of a wider and deeper survey of up to 1 million galaxies called COSMOS-Web. One of the most striking features of JWST-ER1 is the so-called Einstein ring – a phenomenon where light appears as a ring due to gravitational lensing.

„The galaxy and its ring were identified in JWST NIRCam observations in the context of the COSMOS-Web project, a public wide-scale survey using the F115W, F150W, F277W and F444W filters,” the researchers explained.

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NIRCam observations show that JWST-ER1 is a small early-type galaxy (JWST-ER1g) and a complete Einstein ring (JWST-ER1r) with two apparent red concentrations. The diameter of the ring’s center was measured to be approximately 1.54 arcseconds.

The new galaxy was detected at a redshift of 1.94, has a radius of about 21,500 light years and an estimated mass of 650 billion solar masses. The results point to an age of 1.9 billion years and a low star formation rate of about four solar masses per year. Thus, JWST-ER1 is a large and quiet galaxy. It is very compact, as are other galaxies at similar redshifts.

When it comes to the JWST-ER1r ring, the astronomers found that it is produced by the background galaxy at an optical redshift of 2.98. It joins the largest number of known Einstein rings, although most of them are incomplete.

The study also found that JWST-ER1 is almost perfectly round in NIRCam imaging and no clear star-forming regions, tidal tails, or other anomalies are detected.

The paper’s authors propose additional observations of JWST-ER1 to investigate whether nearby galaxies or line-of-sight structures could contribute to its mass, and whether JWST-ER1 is the central galaxy of a cluster progenitor.

More information:
Pieter van Dokkum et al, JWST imaging of a large compact quiescent galaxy at z=2 with a complete Einstein ring, arXiv (2023) DOI: 10.48550/arxiv.2309.07969

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