[Paper Review] Resolved mass assembly and star formation in Milky Way Progenitors since $z = 5$ from JWST/CANUCS: From clumps and mergers to well-ordered disks

Vivian Yun Yan Tan, Adam Muzzin+18 more|arXiv (Cornell University)|Dec 10, 2024

Astronomy and Astrophysical Research4 citations

TL;DR

This study uses deep JWST/CANUCS photometry across 18–21 bands to resolve stellar mass and star formation rate maps of 909 Milky Way progenitor galaxies from z=5 to z=0.3. It reveals inside-out mass assembly, increasing outer sSFR up to z≈2, and a transition from chaotic, merger-dominated systems to well-ordered disks by z<3, with half-mass radii doubling and Sérsic indices remaining near n≈1.

ABSTRACT

We present a resolved study of 877 progenitors of Milky Way Analogs (MWAs) at $0.3

Motivation & Objective

• To trace the resolved mass assembly and star formation evolution of Milky Way progenitors from z=5 to z=0.3 using high-resolution photometry.
• To determine whether Milky Way analogs (MWAs) evolved through inside-out growth, as predicted by disk formation models.
• To quantify the role of mergers, clumpiness, and structural disturbances in shaping early MWA evolution.
• To assess how morphological and star formation properties evolve with redshift, particularly around cosmic noon (z≈2).
• To link observed structural changes to physical mechanisms like bursty star formation and dynamical heating from mergers.

Proposed method

• Selected 909 Milky Way progenitor candidates via abundance matching in the CANUCS survey fields, covering 0.3<z<5.
• Applied Dense Basis spectral energy distribution (SED) fitting to 18–21-band photometry from NIRCam, NIRISS, and HST to produce spatially resolved stellar mass and SFR maps.
• Constructed radial profiles of stellar mass and specific star formation rate (sSFR) to analyze inside-out growth and evolution with redshift.
• Used the Gini-M20 plane and asymmetry parameters (CAS and RMS) to classify galaxies as disturbed or undergoing mergers based on mass map morphology.
• Measured Sérsic indices and half-mass radii (Re) to track structural evolution and disk growth over time.
• Stacked sSFR and mass profiles at different redshift bins (z≈5, 4, 3, 2, 1.75, 0.5) to analyze average trends in star formation and mass distribution.

Figure 2: Top panel: Integrated stellar masses (obtained with Dense Basis) versus photometric redshift of MWA progenitors in the CANUCS fields selected by abundance matching and SMFs. The solid black line is the rolling median of the stellar mass for each redshift range, and corresponds to the solid

Experimental results

Research questions

• RQ1 How does the stellar mass assembly of Milky Way progenitors evolve with redshift, and is it consistent with inside-out growth?
• RQ2 How does the specific star formation rate (sSFR) profile evolve in space and time, particularly in inner versus outer regions?
• RQ3 To what extent do mergers and structural disturbances shape the early evolution of Milky Way analogs at z>3?
• RQ4 How do Sérsic indices and half-mass radii evolve, and what do they imply about the formation of disk-like structures?
• RQ5 What is the relationship between clumpy star formation and underlying mass distribution across cosmic time?

Key findings

• The stellar mass in the inner 2 kpc of MWA progenitors remains nearly constant at 10^9.3–9.4 M☉ from z=5 to z=2, indicating minimal inner mass growth.
• Stellar mass in outer regions (R>2 kpc) increases by 0.8 dex—from 10^7.5 M☉ to 10^8.3 M☉—between z=5 and z=2, confirming strong inside-out growth.
• The sSFR of outer regions increases with decreasing redshift, peaking at z≈2, and only begins to decline after cosmic noon (z≈1.75), while inner sSFR decreases steadily.
• Median Sérsic indices remain nearly constant at n≈1 from z=5 to z=0.3, indicating sustained disk dominance, while half-mass radii double from z=5 to z=3 and again from z=3 to z=0.3.
• At 4<z<5, 45.8±13.8% to 62.5±16.1% of galaxies are classified as disturbed, and 16.7±8.3% are classified as ongoing mergers, with both fractions decreasing toward lower redshift.
• Clumpiness in stellar mass maps is highest at z>4 (but <20% of galaxies), while SFR clumpiness increases at lower redshift, though with lower burstiness, indicating star formation becomes less tightly correlated with mass density over time.

Figure 3: The distribution of UVJ colors for the MWA progenitors. Since the quiescent fraction of MWAs is small ( $\sim 0.03\%$ ), The UVJ boundaries are defined with a bimodal distribution of 9615 galaxies in CANUCS at $0<z<5$ , with a 25% quiescent fraction (See Appendix A ). The color calibration