Herschel PEP: The star-formation rates of 1.5<z<2.5 massive galaxies

TL;DR

This study uses Herschel FIR observations to accurately measure star formation rates of massive galaxies at redshifts 1.5 to 2.5, revealing significant overestimations in previous 24 micron-based SFRs and providing insights into galaxy evolution.

Contribution

First direct FIR-based SFR measurements for individual galaxies at these redshifts, clarifying discrepancies with prior UV and 24 micron estimates and informing galaxy evolution models.

Findings

24 micron SFRs overestimate true SFRs by a factor of 4-7.5.

UV SFRs are closer to FIR-based SFRs but still show some excess.

FIR measurements resolve individual galaxies at these redshifts for the first time.

Abstract

The star formation rate (SFR) is a key parameter in the study of galaxy evolution. The accuracy of SFR measurements at z~2 has been questioned following a disagreement between observations and theoretical models. The latter predict SFRs at this redshift that are typically a factor 4 or more lower than the measurements. We present star-formation rates based on calorimetric measurements of the far-infrared (FIR) luminosities for massive 1.5<z<2.5, normal star-forming galaxies (SFGs), which do not depend on extinction corrections and/or extrapolations of spectral energy distributions. The measurements are based on observations in GOODS-N with the Photodetector Array Camera & Spectrometer (PACS) onboard Herschel, as part of the PACS Evolutionary Probe (PEP) project, that resolve for the first time individual SFGs at these redshifts at FIR wavelengths. We compare FIR-based SFRs to the more commonly used 24 micron and UV SFRs. We find that SFRs from 24 micron alone are higher by a factor of ~4-7.5 than the true SFRs. This overestimation depends on luminosity: gradually increasing for log L(24um)>12.2 L_sun. The SFGs and AGNs tend to exhibit the same 24 micron excess. The UV SFRs are in closer agreement with the FIR-based SFRs. Using a Calzetti UV extinction correction results in a mean excess of up to 0.3 dex and a scatter of 0.35 dex from the FIR SFRs. The previous UV SFRs are thus confirmed and the mean excess, while narrowing the gap, is insufficient to explain the discrepancy between the observed SFRs and simulation predictions.