The Herschel view of the dominant mode of galaxy growth from z=4 to the present day

Abstract: We present an analysis of the deepest Herschel images in four major extragalactic fields GOODS-North, GOODS-South, UDS and COSMOS obtained within the GOODS-Herschel and CANDELS-Herschel key programs. The picture provided by 10497 individual far-infrared detections is supplemented by the stacking analysis of a mass-complete sample of 62361 star-forming galaxies from the CANDELS-HST H band-selected catalogs and from two deep ground-based Ks band-selected catalogs in the GOODS-North and the COSMOS-wide fields, in order to obtain one of the most accurate and unbiased understanding to date of the stellar mass growth over the cosmic history. We show, for the first time, that stacking also provides a powerful tool to determine the dispersion of a physical correlation and describe our method called "scatter stacking" that may be easily generalized to other experiments. We demonstrate that galaxies of all masses from z=4 to 0 follow a universal scaling law, the so-called main sequence of star-forming galaxies. We find a universal close-to-linear slope of the logSFR-logM* relation with evidence for a flattening of the main sequence at high masses (log(M*/Msun) > 10.5) that becomes less prominent with increasing redshift and almost vanishes by z~2. This flattening may be due to the parallel stellar growth of quiescent bulges in star-forming galaxies. Within the main sequence, we measure a non varying SFR dispersion of 0.3 dex. The specific SFR (sSFR=SFR/M*) of star-forming galaxies is found to continuously increase from z=0 to 4. Finally we discuss the implications of our findings on the cosmic SFR history and show that more than 2/3 of present-day stars must have formed in a regime dominated by the main sequence mode. As a consequence we conclude that, although omnipresent in the distant Universe, galaxy mergers had little impact in shaping the global star formation history over the last 12.5 Gyr.

• The paper confirms that about 68% of star-forming galaxies follow a tight main sequence, indicating a stable mode of growth from z=4 to today.
• It employs an innovative 'scatter stacking' method to measure a consistent SFR dispersion of roughly 0.3 dex across various masses and epochs.
• The findings advocate for continuous, intrinsic star formation over bursty, merger-driven models, challenging traditional galaxy evolution theories.

An Insightful Overview of "The Herschel View of the Dominant Mode of Galaxy Growth from $z=4$ to the Present Day"

The analysis of galaxies' growth modes, from historical epochs until now, is pivotal for understanding cosmic history. This paper provides a comprehensive study of galaxy growth by leveraging data from the Herschel Space Observatory. The research exploits far-infrared observations across significant extragalactic fields to assess star formation activity over cosmic time scales.

Key Findings and Methodologies

1. Sample and Data Density: The paper builds on an extensive dataset sourced from the Herschel observations in the GOODS-North, GOODS-South, UDS, and COSMOS fields. This includes 10,497 individual far-infrared detections complemented by a stacking analysis of 62,361 galaxies, making this research one of the most robust evaluations of stellar mass evolution.
2. Star Formation Main Sequence: The authors confirm the existence of a universal scaling law for star-forming galaxies, often referred to as the main sequence, across $z=4$ to the present day. They found that typically about 68% of star-forming galaxies adhere to this law, deviating from the average star formation rate (SFR) by a factor of roughly 2. Specific SFR (sSFR) shows a consistent rise from now back to $z=4$.
3. Methodological Innovation - Scatter Stacking: A methodological highlight of this study is the introduction of "scatter stacking," a technique to quantify the dispersion around the mean SFR. This adds a layer of granularity to understanding underlying star formation processes and can be generalized for other astrophysical applications.
4. SFR Dispersion and Theoretical Implications: The dispersion around the main sequence remains relatively stable at approximately 0.3 dex over extensive mass and redshift ranges. This stability suggests a more universal mode of galaxy growth than previously recognized, less influenced by sporadic star formation bursts driven by major mergers.
5. Implications for Star Formation Histories: The striking stability in the dispersion suggests that conventional models of galaxy growth may be oversimplified. Such findings necessitate models accounting for continuous, stable star formation histories rather than punctuated, merger-driven bursts.

Implications and Future Directions

The study offers significant insights into galaxy evolution, suggesting that a majority of star formation occurs within constrained pathways, contrary to the previously held dominant view of frequent burst cycles from merging. This could recalibrate theoretical expectations around galaxy formation, emphasizing intrinsic growth mechanisms over external merger impacts.

The data support a narrative where steady accrual of mass—following a tight main sequence—is the dominant mode of star formation. This steady state of galaxy growth ultimately shapes the cosmic star formation history observed today.

Future Research Directions:

• Further exploration into the properties of galaxies at z > 4, potentially using ALMA or James Webb Space Telescope.
• A deeper probe into the connection between environmental factors influencing galaxy evolution.
• Refining theoretical models that account for the dominant main sequence mode over mergers in shaping the star formation history.

The substantial dataset and innovative methods presented in this research represent a pivotal step in understanding galaxy evolution, specifically the modality and stability of star formation across epochs. This paper's contributions will likely catalyze more nuanced theories of cosmic evolution, integrating observational prowess with sophisticated modeling.