A massive compact quiescent galaxy at z=2 with a complete Einstein ring in JWST imaging (2309.07969v1)

Abstract: One of the surprising results from HST was the discovery that many of the most massive galaxies at z~2 are very compact, having half-light radii of only 1-2 kpc. The interpretation is that massive galaxies formed inside-out, with their cores largely in place by z~2 and approximately half of their present-day mass added later through minor mergers. Here we present a compact, massive, quiescent galaxy at $z_{\rm phot}=1.94^{+0.13}_{-0.17}$ with a complete Einstein ring. The ring was found in the JWST COSMOS-Web survey and is produced by a background galaxy at $z_{\rm phot}=2.98^{+0.42}_{-0.47}$. Its 1.54" diameter provides a direct measurement of the mass of the "pristine" core of a massive galaxy, observed before mixing and dilution of its stellar population during the 10 Gyr of galaxy evolution between z=2 and z=0. We find a mass of $M_{\rm lens}=6.5^{+3.7}_{-1.5} \times 10^{11}$ Msun within a radius of 6.6 kpc. The stellar mass within the same radius is $M_{\rm stars}= 1.1^{+0.2}_{-0.3} \times 10^{11}$ Msun for a Chabrier initial mass function (IMF), and the fiducial dark matter mass is $M_{\rm dm} = 2.6^{+1.6}_{-0.7} \times 10^{11}$ Msun. Additional mass is needed to explain the lensing results, either in the form of a higher-than-expected dark matter density or a bottom-heavy IMF.

• The paper demonstrates precise lensing mass estimates of a massive, compact quiescent galaxy at z=2 through the discovery of a complete Einstein ring in JWST imaging.
• It reports a total lensing mass of 6.5×10^11 M☉ within a 6.6 kpc radius, far exceeding the stellar mass of 1.1×10^11 M☉ and suggesting significant dark matter or a bottom-heavy IMF.
• The study contextualizes JWST-ER1g among high-redshift quiescent galaxies, emphasizing the role of dark matter profiles and IMF variations in understanding early galaxy evolution.

A Massive Compact Quiescent Galaxy at z = 2 with a Complete Einstein Ring in JWST Imaging

This research investigates a massive, compact galaxy at a photometric redshift $z_{\rm phot} = 1.94$ identified in the JWST COSMOS-Web survey. Of particular interest is an accompanying Einstein ring formed by a background galaxy at $z_{\rm phot} = 2.98$. This formation provides a model-independent measurement of the galaxy's mass enclosed within the ring, essential for understanding early galaxy evolution before the onset of significant stellar mixing and dilution over the ensuing 10 Gyr.

Key Findings

1. Galaxy and Einstein Ring Detection: The compact early-type galaxy, designated JWST-ER1g, and its ring, JWST-ER1r, were identified using JWST's NIRCam images in several filters. The diameter of the Einstein ring, found to be $1.54''$, permits an accurate estimate of the lensing mass.
2. Mass Measurements:
   • The total lensing mass within a 6.6 kpc radius is reported as $M_{\rm lens} = 6.5 \times 10^{11} M_{\odot}$.
   • The stellar mass within the radius, derived for a Chabrier IMF, is measured to be $M_{\rm stars} = 1.1 \times 10^{11} M_{\odot}$.
   • After modeling the dark matter contribution, an additional mass source is necessary to align with the lensing results.
3. Discrepancies and Interpretations:
   • The lens mass exceeds the stellar mass by a factor of approximately 5.9, prompting consideration of either anomalously high dark matter density or variations in the IMF.
   • The paper posits two potential solutions:
     • A denser-than-expected dark matter halo.
     • A bottom-heavy IMF, which might suggest a larger contribution of low-mass stars to the total mass.
4. Comparison to Other Galaxies: JWST-ER1g is contextualized within the family of quiescent galaxies at similar redshifts, noting its compact size and lack of recent star formation—a common trait for galaxies at this epoch.

Theoretical and Practical Implications

This research offers insights into the structure and evolution of early-type galaxies at high redshift, particularly those undergoing quiescence. The paper highlights the challenges present in measuring dark matter distributions and stellar population attributes in such galaxies.

• Dark Matter Profiles: The findings underscore the need to refine our understanding of dark matter halo contributions in massive galaxies at this redshift. This work bolsters the imperative for accurate dark matter references when interpreting early galaxy masses and lending greater support to theories suggesting denser dark matter halos in the early universe.
• IMF Variations: The potential for a bottom-heavy IMF aligns with some theoretical predictions and observational evidence of high-density environments and early-type galaxies with low star formation rates. This research invites further exploration into IMF variability with respect to galaxy location and formation history.

Future Directions

Further paper, potentially involving high-resolution kinematical data, could better delineate between dark matter and stellar contributions in such compact galaxies. Future spectroscopic analyses, possibly utilizing JWST's spectrograph capabilities, could refine the galaxy and source redshifts and offer more precision in characterizing their properties. Understanding the IMF variations and dark matter proclivities could vastly improve our comprehension of galaxy formation and evolution processes in the early universe.