The Relation Between SFR and Stellar Mass for Galaxies at 3.5 $\le z\le$ 6.5 in CANDELS (1407.6012v2)

Abstract: Distant star-forming galaxies show a correlation between their star formation rates (SFR) and stellar masses, and this has deep implications for galaxy formation. Here, we present a study on the evolution of the slope and scatter of the SFR-stellar mass relation for galaxies at $3.5\leq z\leq 6.5$ using multi-wavelength photometry in GOODS-S from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) and Spitzer Extended Deep Survey. We describe an updated, Bayesian spectral-energy distribution fitting method that incorporates effects of nebular line emission, star formation histories that are constant or rising with time, and different dust attenuation prescriptions (starburst and Small Magellanic Cloud). From $z$=6.5 to $z$=3.5 star-forming galaxies in CANDELS follow a nearly unevolving correlation between stellar mass and SFR that follows SFR $\sim$ $M_\star^a$ with $a = 0.54 \pm 0.16$ at $z\sim 6$ and $0.70 \pm 0.21$ at $z\sim 4$. This evolution requires a star formation history that increases with decreasing redshift (on average, the SFRs of individual galaxies rise with time). The observed scatter in the SFR-stellar mass relation is tight, $\sigma(\log \mathrm{SFR}/\mathrm{M}\odot$ yr$^{-1})< 0.3\ - $ 0.4 dex, for galaxies with $\log M\star/\mathrm{M}\odot > 9$ dex. Assuming that the SFR is tied to the net gas inflow rate (SFR $\sim$ $\dot{M}\mathrm{gas}$), then the scatter in the gas inflow rate is also smaller than 0.3$-$0.4 dex for star-forming galaxies in these stellar mass and redshift ranges, at least when averaged over the timescale of star formation. We further show that the implied star formation history of objects selected on the basis of their co-moving number densities is consistent with the evolution in the SFR-stellar mass relation.

• The paper confirms that star formation rates scale as SFR ∼ M^a with exponents of 0.54 at z∼6 and 0.70 at z∼4 for galaxies above 10^9 M⊙.
• It employs an updated Bayesian spectral-energy distribution fitting method to account for nebular emission, rising star formation histories, and attenuation models.
• Findings show a tight scatter (σ < 0.3–0.4 dex), supporting the role of steady gas accretion as the dominant mechanism in early galaxy evolution.

Overview of the SFR-Stellar Mass Relation for High-Redshift Galaxies

The paper "The Relation Between Star Formation Rate and Stellar Mass for Galaxies at $3.5 \leq z \leq 6.5$ in CANDELS" by Brett Salmon et al. aims to investigate the correlation between the star formation rate (SFR) and stellar mass for galaxies within the specified redshift range using data from the CANDELS and Spitzer Extended Deep Survey. This paper is significant for understanding galaxy formation as it probes epochs where the universe was only a few billion years old.

Key Findings

1. SFR-Mass Relation: The paper confirms a nearly constant SFR-stellar mass relation with redshift, following $\text{SFR} \sim M_\star^a$ where the exponent $a$ is measured as $0.54 \pm 0.16$ at $z\sim6$ and $0.70 \pm 0.21$ at $z\sim4$. This relation holds for galaxies with stellar masses above $10^9 \, M_\odot$.
2. Scatter in SFR-Mass Relation: The scatter in the SFR-mass relation remains tight, with $$\sigma(\log \text{SFR}/M_\odot \, \text{yr}^{-1}) < 0.3-0.4$$ dex. This implies that galaxies have relatively smooth star formation histories, likely tied to steady gas accretion rates.
3. Methodology: An updated Bayesian spectral-energy distribution fitting method is employed, addressing key challenges in modeling distant galaxies such as nebular line emission contributions, constant/rising star formation histories, and attenuation models.
4. Comparison with Models: The SFR-mass relation observed is consistent with predictions from semi-analytic models (SAMs), supporting the validity of smooth gas infall as the dominant mechanism for star formation at these epochs.
5. Star Formation History: The analysis indicates star formation histories that increase with time, challenging traditional models that assumed declines. This rising history helps explain the lack of evolution in the SFR-mass slope across the investigated redshifts.

Implications

The steady SFR-mass relation across a significant range of redshifts implies robust mechanisms driving star formation rates within galaxies, linked closely to gas accretion processes from cosmic streams. A tight scatter further indicates limited variations in these processes among galaxies of similar mass, suggesting redundancy in feedback mechanisms regulating star-forming activities.

Future Directions

Further research is called for to refine models predicting stellar mass assembly and SFR evolution, especially under varied initial conditions and redshift-specific environments. Extending observations to even earlier epochs with upcoming facilities like the James Webb Space Telescope will shed additional light on the formative stages of galaxies in the early universe.

Overall, this paper provides an in-depth look at the interplay between SFR and stellar mass during a critical period of the universe's history, offering insights that challenge and refine existing models of galaxy evolution.