The SFR-M* Relation and Empirical Star-Formation Histories from ZFOURGE at 0.5 < z < 4 (1510.06072v2)

Abstract: We explore star-formation histories (SFHs) of galaxies based on the evolution of the star-formation rate stellar mass relation (SFR-M*). Using data from the FourStar Galaxy Evolution Survey (ZFOURGE) in combination with far-IR imaging from the Spitzer and Herschel observatories we measure the SFR-M* relation at 0.5 < z < 4. Similar to recent works we find that the average infrared SEDs of galaxies are roughly consistent with a single infrared template across a broad range of redshifts and stellar masses, with evidence for only weak deviations. We find that the SFR-M* relation is not consistent with a single power-law of the form SFR ~ M*^a at any redshift; it has a power-law slope of a~1 at low masses, and becomes shallower above a turnover mass (M_0) that ranges from 10^9.5 - 10^10.8 Msol, with evidence that M_0 increases with redshift. We compare our measurements to results from state-of-the-art cosmological simulations, and find general agreement in the slope of the SFR-M* relation albeit with systematic offsets. We use the evolving SFR-M* sequence to generate SFHs, finding that typical SFRs of individual galaxies rise at early times and decline after reaching a peak. This peak occurs earlier for more massive galaxies. We integrate these SFHs to generate mass-growth histories and compare to the implied mass-growth from the evolution of the stellar mass function. We find that these two estimates are in broad qualitative agreement, but that there is room for improvement at a more detailed level. At early times the SFHs suggest mass-growth rates that are as much as 10x higher than inferred from the stellar mass function. However, at later times the SFHs under-predict the inferred evolution, as is expected in the case of additional growth due to mergers.

Analyzing Star-Formation Histories from ZFOURGE Data

This paper presents a comprehensive analysis of star-formation histories (SFHs) and the star-formation rate to stellar mass ($\text{SFR}-M_*$) relation using data from the FourStar Galaxy Evolution Survey (ZFOURGE) and auxiliary data from the {\it Spitzer} and {\it Herschel} far-infrared observatories. This research focuses on measuring and interpreting the $\text{SFR}-M_*$ relation across a considerable redshift range ($0.5 < z < 4$).

Methodology and Data

The authors utilize extensive datasets obtained using deep near-infrared imaging from ZFOURGE alongside IR imaging from {\it Spitzer} and {\it Herschel} to compute the $\text{SFR}-M_*$ relation. The photometric redshifts were calculated using spectral energy distribution (SED) fitting, employing both Pegasus and Maraston models. Stellar masses were derived using FAST, a publicly available SED-fitting code, which allows for the estimation of galaxy properties such as age, metallicity, and extinction.

The authors focus on empirical data to trace the star-formation histories and mass accumulation patterns in galaxies. An analysis of infrared spectral energy distributions (SEDs) bolsters robust evaluations of obscured star-formation activity, representing a refinement over approaches that heavily rely on UV data.

$\text{SFR}-M_*$ Relation

In contradiction to simpler power-law models often assumed, the paper finds that, across the surveyed redshift range, the $\text{SFR}-M_*$ relation is not consistent with a single power-law. The slope exhibits a value of $\alpha \sim 1$ at lower masses and becomes significantly shallower at higher stellar masses above a turnover mass $M_0$. This turnover mass is shown to increase with redshift.

Empirical Star-Formation Histories

Using the evolving $\text{SFR}-M_*$ relation, the authors derive star-formation histories indicating a rise in typical SFRs early in galaxies' lifecycle, following by decline after reaching a peak. The timing of this peak is noted to occur earlier in more massive galaxies. The paper also illustrates how these SFHs correlate with mass-growth profiles calculated from the evolving $\text{SFR}-M_*$ sequence.

Implications and Future Directions

The findings suggest discrepancies in previously accepted rates of growth from star formation when compared with the evolutionary trajectory of the stellar mass function. This disagreement implies potential systematic errors in the estimation of SFRs or the evolution of stellar masses. Further explorations might include refining the methods of integrating SFHs for individual galaxies and reconciling galaxy merger activities with quantitative constraints on galaxy growth rates.

Future research could enhance the theoretical understanding by incorporating more variable approaches to SED-fitting and IMF assumptions. Moreover, exploring the implicit connections between galaxy merger rates and the discrepancies highlighted could offer new insights into galaxy evolution mechanics.

Conclusion

The paper underscores the nuanced behavior of star-formation rates relative to galaxy mass and their evolution through cosmic time, challenging simplified models with well-supported empirical data. Through its methodology, the paper provides a refined picture of galaxy growth patterns in earlier epochs and highlights areas where theoretical models must incorporate more complexity to address observed phenomena accurately. This research makes specific contributions to the understanding of galaxy evolution, though acknowledging that further inquiry is warranted in areas such as merger rates and advanced simulation comparisons.