Smooth(er) Stellar Mass Maps in CANDELS: Constraints on the Longevity of Clumps in High-redshift Star-forming Galaxies

Abstract: We perform a detailed analysis of the resolved colors and stellar populations of a complete sample of 323 star-forming galaxies at 0.5 < z < 1.5, and 326 star-forming galaxies at 1.5 < z < 2.5 in the ERS and CANDELS-Deep region of GOODS-South. Galaxies were selected to be more massive than 10^10 Msun and have specific star formation rates above 1/t_H. We model the 7-band optical ACS + near-IR WFC3 spectral energy distributions of individual bins of pixels, accounting simultaneously for the galaxy-integrated photometric constraints available over a longer wavelength range. We analyze variations in rest-frame color, stellar surface mass density, age, and extinction as a function of galactocentric radius and local surface brightness/density, and measure structural parameters on luminosity and stellar mass maps. We find evidence for redder colors, older stellar ages, and increased dust extinction in the nuclei of galaxies. Big star-forming clumps seen in star formation tracers are less prominent or even invisible on the inferred stellar mass distributions. Off-center clumps contribute up to ~20% to the integrated SFR, but only 7% or less to the integrated mass of all massive star-forming galaxies at z ~ 1 and z ~ 2, with the fractional contributions being a decreasing function of wavelength used to select the clumps. The stellar mass profiles tend to have smaller sizes and M20 coefficients, and higher concentration and Gini coefficients than the light distribution. Our results are consistent with an inside-out disk growth scenario with brief (100 - 200 Myr) episodic local enhancements in star formation superposed on the underlying disk. Alternatively, the young ages of off-center clumps may signal inward clump migration, provided this happens efficiently on the order of an orbital timescale.

• The paper finds that clumps drive up to 20% of the star formation rate while contributing less than 7% to total stellar mass, highlighting their transient nature.
• Researchers employed resolved SED modeling on over 600 galaxies using HST optical and NIR data to precisely map stellar populations and structural parameters.
• The findings support an inside-out growth model where dense, older cores contrast with vigorous, younger star-forming outskirts, refining galaxy evolution models.

Stellar Mass and Structure in Star-Forming Galaxies: Clump Longevity and High-Redshift Evolution

The paper presents a comprehensive analysis of the stellar mass distribution and structural properties of star-forming galaxies (SFGs) using the CANDELS survey data. This study is pivotal for understanding the longevity of clumpy structures within high-redshift galaxies, thereby elucidating the processes governing galaxy evolution during peak cosmic star formation epochs.

Key Findings

1. Sample and Methodology: The study examines two samples of SFGs: 323 at $0.5 < z < 1.5$ and 326 at $1.5 < z < 2.5$. The galaxies were selected for stellar masses above $10^{10}\ M_{\odot}$, ensuring robust statistical analyses of mass and structure. A novel approach was taken by leveraging resolved SED modeling, with data from the HST's optical and NIR bands, to derive detailed insights into the stellar populations and spatial structural parameters of galaxies.
2. Clumpy Star Formation: Clumps in these galaxies contribute significantly to star formation rates (SFRs) but relatively little to stellar mass. The upper limit contribution of clumps to the total stellar mass is found to be less than 7% despite accounting for up to 20% of the SFR, suggesting that clumps are transient features, rapidly forming stars but dissolving or migrating into the nucleus rather than building significant mass.
3. Structural Analysis: The mass profiles of galaxies are more centrally concentrated than inferred from light profiles; clumps that are prominent in UV or optical light are less defined in the mass maps. This indicates a shift in galaxy structure as function of wavelength due to variations in mass-to-light ratios with spatially resolved older and denser cores compared to younger, star-forming outskirts.
4. Inside-Out Growth: The study corroborates that galaxies grow inside-out, supported by observed trends of older stellar populations and increased dust extinction towards the nucleus. This scenario aligns with predictions from models of cold flow accretion and gravitational instability within gas-rich disks fueling star formation.

Implications

• Clump Lifetimes and Galaxy Dynamics: The brief lifetimes of galactic clumps imply rapid star formation activities potentially disrupted by feedback processes such as stellar winds and supernovae, which could prevent the long-term survival of clumps.
• Formation of Bulges: The alternative scenario of clumps migrating inwards and coalescing into a bulge still garners observational backing, although efficient migration would have to occur on orbital timescales of 100-200 Myr to fit the inferred young ages of clumps.
• Modeling Galaxy Evolution: These insights into the clumpy substructure of SFGs stress the necessity of incorporating both resolved color information and deeper NIR observations into models that predict galaxy evolution, as monochromatic studies may misrepresent the true mass distribution.

Future Directions

• High-Resolution Observations: Further high-resolution observations, particularly those provided by instruments such as JWST, will be crucial in refining these models with enhanced spatial and wavelength coverage.
• Simulation Comparisons: Efforts to reconcile observed properties with hydrodynamic simulations will prove essential in validating theoretical predictions related to clump dynamics, feedback processes, and the large-scale structural evolution of galaxies.

The paper advances our understanding of high-redshift SFGs by showcasing the complexity of their internal structures, challenging simplified assumptions of galaxy morphology and growth across cosmic time. This nuanced perspective is vital for bridging the gap between observational astrophysics and theoretical models, thereby enhancing our grasp of cosmic star formation history.