Constraining the Low-Mass Slope of the Star Formation Sequence at 0.5<z<2.5 (1407.1843v2)

Abstract: We constrain the slope of the star formation rate ($\log\Psi$) to stellar mass ($\log\mathrm{M_{\star}}$) relation down to $\log(\mathrm{M_{\star}/M_{\odot}})=8.4$ ($\log(\mathrm{M_{\star}/M_{\odot}})=9.2$) at $z=0.5$ ($z=2.5$) with a mass-complete sample of 39,106 star-forming galaxies selected from the 3D-HST photometric catalogs, using deep photometry in the CANDELS fields. For the first time, we find that the slope is dependent on stellar mass, such that it is steeper at low masses ($\log\mathrm{\Psi}\propto\log\mathrm{M_{\star}}$) than at high masses ($\log\mathrm{\Psi}\propto(0.3-0.6)\log\mathrm{M_{\star}}$). These steeper low mass slopes are found for three different star formation indicators: the combination of the ultraviolet (UV) and infrared (IR), calibrated from a stacking analysis of Spitzer/MIPS 24$\mu$m imaging; $\beta$-corrected UV SFRs; and H$\alpha$ SFRs. The normalization of the sequence evolves differently in distinct mass regimes as well: for galaxies less massive than $\log(\mathrm{M_{\star}/M_{\odot}})<10$ the specific SFR ($\Psi/\mathrm{M_{\star}}$) is observed to be roughly self-similar with $\Psi/\mathrm{M_{\star}}\propto(1+z)^{1.9}$, whereas more massive galaxies show a stronger evolution with $\Psi/\mathrm{M_{\star}}\propto(1+z)^{2.2-3.5}$ for $\log(\mathrm{M_{\star}/M_{\odot}})=10.2-11.2$. The fact that we find a steep slope of the star formation sequence for the lower mass galaxies will help reconcile theoretical galaxy formation models with the observations. The results of this study support the analytical conclusions of Leja et al. (2014).

• The paper finds that the SFR–stellar mass relation steepens for low-mass galaxies using UV, IR, and Hα indicators.
• It applies robust corrections for emission lines and leverages deep photometric and grism spectroscopy from CANDELS for precise redshift and mass estimates.
• The results show distinct sSFR evolution trends between low- and high-mass galaxies, challenging current galaxy formation models and feedback prescriptions.

Overview of the Star Formation Sequence Paper by Katherine E. Whitaker et al.

The paper "Constraining the Low-Mass Slope of the Star Formation Sequence at 0.5<z<2.5" by Katherine E. Whitaker et al. presents a comprehensive analysis of the star formation rate (SFR) and its relation to stellar mass across significant epochs in cosmic history. Focusing on the redshift range of 0.5 to 2.5, the paper utilizes a mass-complete sample of 39,106 star-forming galaxies derived from the 3D-HST photometric catalogs, incorporating deep photometry from the CANDELS fields. This research aims to refine our understanding of the star formation sequence, particularly examining the behavior at low stellar masses.

Key Findings and Methodology

1. Mass Dependence and Slope Variation: For the first time, the paper reports that the slope of the SFR-to-stellar-mass relation is not constant across all mass ranges but is notably steeper at lower masses. This mass dependency is observed using three different star formation indicators: UV and infrared (IR) combination, β-corrected UV SFRs, and Hα SFRs.
2. Normalization and Evolution: The paper reveals that the normalization of the star formation sequence evolves distinctly within various mass regimes. For galaxies with masses lower than $10^{10} M_{\odot}$, the specific star formation rate (sSFR) follows a self-similar evolution with $\Psi/\mathrm{M_{\star}}\propto(1+z)^{1.9}$. In contrast, more massive galaxies exhibit a stronger evolution, with $\Psi/\mathrm{M_{\star}}\propto(1+z)^{2.2-3.5}$ for $10.2 < \log(\mathrm{M_{\star}/M_{\odot}}) < 11.2$.
3. Photometric and Grism Redshift Data: The analysis leverages photometric data spanning 0.3 to 8 μm, calibrated and improved via low-resolution HST/WFC3 G141 grism spectroscopy, which provides enhanced redshift measurements and emission line diagnostics, critical for accurately estimating stellar masses and SFRs.
4. Emission Line and Stellar Mass Corrections: A significant aspect of the methodology involves correcting stellar masses by accounting for contributions from emission lines (such as Hα and [NII]) to ensure robust mass and SFR measurements across the sample. This correction is pivotal for resolving discrepancies in star formation sequences observed at different mass scales.
5. Implications and Theoretical Context: The findings suggest that the steep slope observed for low-mass galaxies challenges existing theoretical models of galaxy formation, which often predict shallower slopes for star-forming sequences. This discrepancy highlights the need to incorporate varying feedback mechanisms and potentially non-equilibrium conditions in models to reconcile theoretical predictions with observational data.

Implications and Future Prospects

The paper's insights into the mass-dependent behavior of the star formation sequence have profound implications for galaxy formation theories, particularly in refining models that account for feedback processes at varying mass regimes. The results underscore the necessity for a nuanced understanding of gas accretion rates and the processes quenching star formation in massive galaxies.

Moving forward, the research community could benefit from extending these findings through complementary observational techniques such as sub-millimeter and radio astronomy, which could offer deeper insights into dust-obscured star-forming regions. Additionally, the future direction might involve more detailed simulations that incorporate feedback effects dynamically to explain the observed evolutionary trends in the star formation sequence.

Overall, this paper by Whitaker et al. provides a significant contribution to comprehending the complexities of galaxy evolution and the drivers behind star formation across cosmic time.