CANDELS: The Evolution of Galaxy Rest-Frame Ultraviolet Colors from z = 8 to 4 (1110.3785v2)

Abstract: We study the evolution of galaxy rest-frame ultraviolet (UV) colors in the epoch 4 < z < 8. We use new wide-field near-infrared data in GOODS-S from the CANDELS, HUDF09 and ERS programs to select galaxies via photometric redshift measurements. Our sample consists of 2812 candidate galaxies at z > 3.5, including 113 at z = 7 to 8. We fit the observed spectral energy distribution to a suite of synthetic stellar population models, and measure the value of the UV spectral slope (beta) from the best-fit model spectrum. The median value of beta evolves significantly from -1.82 (+0.00,-0.04) at z = 4, to -2.37 (+0.26,-0.06) at z = 7. Additionally, we find that faint galaxies at z = 7 have beta = -2.68 (+0.39,-0.24) (~ -2.4 after correcting for observational bias); this is redder than previous claims in the literature, and does not require "exotic" stellar populations to explain their colors. This evolution can be explained by an increase in dust extinction, with the timescale consistent with low-mass AGB stars forming the bulk of the dust. We find no significant (< 2-sigma) correlation between beta and M_UV when measuring M_UV at a consistent rest-frame wavelength of 1500 A. This is particularly true at bright magnitudes, though our results do show evidence for a weak correlation at faint magnitudes when galaxies in the HUDF are considered separately, hinting that dynamic range in sample luminosities may play a role. We do find a strong correlation between beta and the stellar mass at all redshifts, in that more massive galaxies exhibit redder colors. The most massive galaxies in our sample have red colors at each redshift, implying that dust can build up quickly in massive galaxies, and that feedback is likely removing dust from low-mass galaxies at z > 7. Thus the stellar-mass - metallicity relation, previously observed up to z ~ 3, may extend out to z = 7 - 8.

• The paper reveals that the median β value evolves from −1.82 at z=4 to −2.37 at z=7, indicating rising dust content in early galaxies.
• The paper demonstrates a strong correlation between β and stellar mass, where more massive galaxies exhibit redder colors due to retained dust.
• The study introduces an improved SED fitting method that minimizes scatter and bias in UV color measurements, matching theoretical simulations.

The Evolution of Galaxy Rest-Frame Ultraviolet Colors from $z = 8$ to 4

The paper by Finkelstein et al. investigates the evolution of galaxy rest-frame ultraviolet (UV) colors through the epoch ranging from $z = 8$ to $z = 4$. Utilizing data from multiple high-profile surveys such as CANDELS, HUDF, and ERS, the paper focuses on changes in the UV spectral slope ($\beta$) and its implications for galaxy formation and evolution.

Main Findings

1. Evolution of $\beta$: The paper reveals a significant evolution in the median value of $\beta$ from $-1.82$ at $z = 4$ to $-2.37$ at $z = 7$. This evolution suggests an increasing dust content in galaxies over this redshift range, with low dust present at $z = 7$ escalating to a noticeable extinction at $z = 4$.
2. Galactic Colors and Dust: The paper proposes that the observed color evolution may be indicative of dust formation processes. It suggests that asymptotic giant branch (AGB) stars could play a significant role in dust production, particularly at $z < 7$, with a timeline consistent with low-mass stars reaching the AGB phase.
3. Stellar Mass Correlation: A key finding is the robust correlation between $\beta$ and stellar mass at all redshifts, asserting that more massive galaxies exhibit redder colors. This mass-dust relation hints at feedback mechanisms where massive galaxies retain dust formed in supernovae, while lower-mass galaxies may lose dust to outflows.
4. Methodological Advances: The paper utilizes a novel approach to measure $\beta$ via SED fitting, taking advantage of multiple rest-UV colors for accuracy. Simulations confirm that this method reduces scatter and bias compared to previous single-color methods.
5. Comparison with Simulations: The observed trends align well with theoretical models predicting dust production and feedback processes. The \citet{finlator11} simulations support the conclusions regarding mass-metallicity relations extending to high redshifts.

Implications

1. Dust Formation Mechanics: The insights into dust production offer avenues for understanding dust lifecycle in early galaxies. The potential role of AGB stars suggests distinct evolutionary paths for dust accumulation in different galactic environments.
2. Impact on Ly$\alpha$ Emission: The decreasing incidence of Ly$\alpha$ emission from $z = 6$ to 7 might be informed by these findings, hinting at increasing gas inflows that could obscure such emissions in otherwise blue and star-forming environments.
3. High-Redshift Galaxy Evolution: The strong $\beta$-stellar mass relation advocates for the exploration of the early mass-metallicity relation, already hinted at in these early epochs. This prompts future spectroscopic and photometric studies to refine our understanding.
4. Methodological Contributions: The SED fitting method proposed sets a new standard for measuring rest-frame UV slopes, promising more reliable results in varying observational conditions and different data sets.

Future Prospects

Future work will benefit from the new insights provided by this analysis, particularly with the advent of deeper data from facilities like JWST. Continued development of theoretical models and simulations will complement observational advancements, enabling a more comprehensive understanding of early galaxy formation and evolution.

In summary, this paper provides a detailed analysis of the evolution of UV spectral slopes and dust content in high-redshift galaxies, offering significant implications for the paper of early cosmological structures and their developmental trajectories.