A Milky Way-like barred spiral galaxy at a redshift of 3

Abstract

The majority of massive disk galaxies in the local Universe show a stellar barred structure in their central regions, including our Milky Way^1,2. Bars are supposed to develop in dynamically cold stellar disks at low redshift, as the strong gas turbulence typical of disk galaxies at high redshift suppresses or delays bar formation^3,4. Moreover, simulations predict bars to be almost absent beyond z = 1.5 in the progenitors of Milky Way-like galaxies^5,6. Here we report observations of ceers-2112, a barred spiral galaxy at redshift z_phot ≈ 3, which was already mature when the Universe was only 2 Gyr old. The stellar mass (M_★ = 3.9 × 10⁹ M_⊙) and barred morphology mean that ceers-2112 can be considered a progenitor of the Milky Way^7-9, in terms of both structure and mass-assembly history in the first 2 Gyr of the Universe, and was the closest in mass in the first 4 Gyr. We infer that baryons in galaxies could have already dominated over dark matter at z ≈ 3, that high-redshift bars could form in approximately 400 Myr and that dynamically cold stellar disks could have been in place by redshift z = 4-5 (more than 12 Gyrs ago)^10,11.

Fig. 1. Morphological modelling of ceers-2112.

a, Combined stack image of ceers-2112, with isophotal contours showing an elongated barred structure in the inner region and spiral arms departing from it. b, One-component Sérsic residuals, which highlight the bar and spiral structures (black and green contours, respectively). c, Two-dimensional bar + disk model, which shows a stellar bar of length r_Ferrers = 0.42 ± 0.03 arcsec (3.3 kpc). The bar component is shown as a red solid line and the bar + disk isophotes are shown as black dashed contours. d, Radial profiles of the relative amplitude of the odd (dashed lines) and even (solid lines) Fourier components, derived from the deprojected stack image of ceers-2112. The m = 2 mode shows a prominent bar (maximum I₂/I₀ > 0.4) with strength S_bar = 0.23 ± 0.01. Shaded region represents 1σ confidence interval for the m = 2 mode.

Fig. 2. Stellar population properties of ceers-2112.

a, Fiducial spatially resolved SFH derived with synthesizer (delayed-τ model). b, Two-dimensional mass-weighted age map of ceers-2112. c, Stellar mass-density map of ceers-2112. The isophotal contours of the stack image are superposed on the mass-weighted age and mass-density maps. Maps in b and c are 53 × 53 px², which corresponds to 1.59 × 1.59 arcsec² (12.5 × 12.5 kpc² at z = 3.03).

Fig. 3. Mass-assembly history of ceers-2112.

Mass-assembly history of Milky Way (MW) and M31 analogues (dashed red line), compared with ceers-2112 (yellow star). The observed stellar masses of Milky Way progenitors are shown as green triangles. The red shaded region shows the 25th–75th percentile range. The blue shaded region stands for the observational estimates for the stellar mass at z = 0 of the Milky Way^–. Barred galaxies at z > 1 (refs. ^,) are shown as empty squared symbols and TNG50 predictions of barred galaxies at z > 1.5 (ref. ) are shown as grey dots. Error bars show the systematic uncertainties related to the assumptions of the SFH modelling.

Extended Data Fig. 1. Multiwavelength view of ceers-2112.

Postage stamps of ceers-2112 (RA = 214.97993 degrees; DEC = 52.991946 degrees; J2000.0) in all NIRCam filters used in this work. The cutouts are 53 × 53 px², which corresponds to 1.59 × 1.59 arcsec² (12.5 × 12.5 kpc² at z = 3.03). We report the angular resolution as 2 × FHWM of the PSF and the isophotal contours (white solid lines).

Extended Data Fig. 2. Isophotal analysis of ceers-2112.

In each panel, the shaded regions mark the size of the PSF FWHM in the different bands, while error bars show the 1σ standard deviation of each point. a, Radial profiles of ellipticity derived from the isophotal analysis of ceers-2112 in the F115W (pink), F150W (blue) and F200W band (light green). b, Radial profiles of ellipticity derived from the isophotal analysis of ceers-2112 in the F277W band (dark green), F356W (orange) and F444W band (red). c, Radial profiles of ellipticity derived from the isophotal analysis of ceers-2112 in the combined stack image (all seven NIRCam filters). The region of the bar, spiral arms and outer disk are shown as black, grey and cyan datapoints. d, As panel a, but for the position angles. e, As panel b, but for the position angles. f, As panel c, but for the position angles. The inset panel shows some of the ellipses superposed to the composed stack image (1.59 × 1.59 arcsec²).

Extended Data Fig. 3. Isophotal and Fourier analysis of SW and LW stack images of ceers-2112.

a, Postage stamp of the stack SW image (F115W, F150W and F200W) with some of the ellipses superposed (1.59 × 1.59 arcsec²). b, Postage stamp of the stack LW image (F277W, F356W and F444W) with some of the ellipses superposed (1.59 × 1.59 arcsec²). c, Radial profiles of ellipticity derived from the isophotal analysis of ceers-2112 in the stack SW image (blue) and stack LW image (red). The shaded regions mark the size of the PSF FWHM in the different bands, while error bars show the 1σ standard deviation of each point. d, As c, but for the position angles. e, Radial profiles of the relative amplitude of the m = 1 (blue lines) and m = 2 (red lines; shaded regions: 1σ confidence intervals) Fourier components derived from the deprojected combined SW image (dashed lines) and the deprojected combined LW image (solid lines) of ceers-2112.

Extended Data Fig. 4. Parametric morphological modeling of ceers-2112.

a, One-component Sérsic model of ceers-2112. b, One-component Sérsic residuals, which highlight the bar and spiral structures (black and green contours, respectively) c, Two-dimensional bar+disk model, which shows a stellar bar of length r_Ferrers = 0.42 ± 0.03 arcsec (3.3 kpc). d, Two-dimensional bar+disk model residuals. The bar and spiral structures (black and green contours, respectively) are superposed to the image. The black dashed line marks the break radius of the double-exponential disk model, where the surface brightness of the model rapidly declines.

Extended Data Fig. 5. SED and redshift of ceers-2112.

Black empty squares (blue circles) denote our fiducial (model) photometry from HST/ACS + WFC3 and JWST/NIRCam instruments, respectively. The EAZYpy model spectrum is shown in blue. Error bars show the 1σ standard deviation of each point. The inset plot shows the P(z) distributions.

Extended Data Fig. 6. SFH modeling of ceers-2112.

Comparison of different model assumptions: exponentially-declining SFH (τ-model; FAST code; blue dashed line); delayed exponentially-declining SFH (delayed-τ model; Prospector; gray dashed line); two-dimensional delayed exponentially-declining SFH (2D delayed-τ model; synthesizer; red solid line); non-parametric SFH (Dense Basis and Prospector; orange and green dashed lines, respectively).