On Star Formation Rates and Star Formation Histories of Galaxies out to z ~ 3 (1106.5502v1)

Abstract: We compare multi-wavelength SFR indicators out to z~3 in GOODS-South. Our analysis uniquely combines U-to-8um photometry from FIREWORKS, MIPS 24um and PACS 70, 100, and 160um photometry from the PEP survey, and Ha spectroscopy from the SINS survey. We describe a set of conversions that lead to a continuity across SFR indicators. A luminosity-independent conversion from 24um to total infrared luminosity yields estimates of LIR that are in the median consistent with the LIR derived from PACS photometry, albeit with significant scatter. Dust correction methods perform well at low to intermediate levels of star formation. They fail to recover the total amount of star formation in systems with large SFR_IR/SFR_UV ratios, typically occuring at the highest SFRs (SFR_UV+IR \gtrsim 100 Msun/yr) and redshifts (z \gtrsim 2.5) probed. Finally, we confirm that Ha-based SFRs at 1.5<z<2.6 are consistent with SFR_SED and SFR_UV+IR provided extra attenuation towards HII regions is taken into account (Av,neb = Av,continuum / 0.44). With the cross-calibrated SFR indicators in hand, we perform a consistency check on the star formation histories inferred from SED modeling. We compare the observed SFR-M relations and mass functions at a range of redshifts to equivalents that are computed by evolving lower redshift galaxies backwards in time. We find evidence for underestimated stellar ages when no stringent constraints on formation epoch are applied. We demonstrate how resolved SED modeling, or alternatively deep UV data, may help to overcome this bias. The age bias is most severe for galaxies with young stellar populations, and reduces towards older systems. Finally, our analysis suggests that SFHs typically vary on timescales that are long (at least several 100 Myr) compared to the galaxies' dynamical time.

• The paper cross-calibrates multi-wavelength SFR indicators and shows that locally-calibrated MIPS 24 μm conversions overestimate L_IR for high-redshift galaxies.
• The paper validates SED modeling against UV plus IR data while noting its limitations in highly star-forming systems with saturated dust attenuation.
• The paper demonstrates that accounting for differential extinction in Hα measurements and correcting underestimated stellar ages are vital for reliable galaxy star formation history estimates.

An Exploration of Star Formation Rates and Histories in Galaxies up to Redshift 3

The paper of star formation rates (SFR) and star formation histories (SFH) is fundamental to understanding galaxy evolution across cosmic time. The paper "On Star Formation Rates and Star Formation Histories of Galaxies out to $z \sim 3$" provides a comprehensive analysis of SFR indicators using multi-wavelength data up to a redshift of 3. This work leverages a robust dataset from the GOODS-South field, amalgamating photometry from various space and ground-based observatories, including H$\alpha$ spectroscopy from the SINS survey, to explore both the continuity and discrepancies in SFR indicators over cosmic time.

Key Findings and Methodology

The research primarily focuses on:

1. Cross-Calibration of SFR Indicators: The paper presents a unique combination of data, including $U$-to-8$\mu$m photometry, MIPS 24 $\mu$m, and PACS photometry. These datasets are used to derive conversions that provide consistency across different SFR indicators. A significant finding is the discrepancy introduced when using locally calibrated conversions from MIPS 24 $\mu$m to total infrared luminosity ($L_{IR}$) at high redshifts and SFRs. The authors confirm that for the highest SFRs and redshifts, these conversions overestimate $L_{IR}$, but this can be mitigated by using a luminosity-independent conversion template.
2. Utility of SED Modeling: The paper evaluates the performance of spectral energy distribution (SED) modeling against SFRs derived using UV plus IR data. The analysis indicates that SED-modeled SFRs agree well with $SFR_{UV+IR}$ for low to intermediate star formation levels. However, SED modeling tends to underestimate SFRs in highly star-forming systems where dust attenuation saturates as a tracer of reddening.
3. H$\alpha$ SFRs and Differential Extinction: The authors provide a compelling argument for additional attenuation towards HII regions using VLT/SINFONI data. This is crucial for aligning H$\alpha$ SFRs with those derived from UV and IR methods, reflecting the necessity of correcting for differential extinction, a trend established in both local and distant galaxies.
4. Star Formation Histories and Mass Functions: One of the paper's critical insights is the comparison of observed galaxy populations and those inferred from backward evolution from lower redshifts. The authors highlight that underestimated stellar ages in SED modeling can lead to inconsistencies in galaxy continuity equations. The findings suggest that both galaxy merging and underestimated ages contribute to discrepancies in mass functions across redshifts.

Implications and Future Directions

This work has notable implications for studies of galaxy formation and evolution. The robust cross-calibration of SFR indicators, especially with the inclusion of new Herschel data, lays a foundation for future research into the mechanisms driving galaxy growth. The insights into differential extinction and its impact on SFR measurements indicate the need for detailed models accommodating complex dust geometries in starforming regions.

The paper’s exploration of SFR-SFH relations indicates a preference for star formation in galaxies to occur over prolonged periods, which has implications for models of galaxy evolution predicting rapid star formation episodes. The idea of implementing resolved SED modeling to ameliorate age estimates is promising, especially with the high-resolution data now available through instruments like the HST’s WFC3.

In future research, extending the wavelength range, particularly on the short (UV) end, and integrating spatially resolved data will refine our understanding of galaxy SFHs. Continued advancements in modeling the interplay between dust and star formation will also enhance the reliability of SFR indicators across different epochs in the universe.