Mid-IR Luminosities and UV/Optical Star Formation Rates at z<1.4

TL;DR

This study compares UV/optical and mid-IR star formation indicators in 2430 galaxies at z<1.4, revealing the significant role of intermediate-age stars in dust heating and clarifying the nature of dust-obscured star formation.

Contribution

It provides a detailed analysis of star formation rates using multi-wavelength data and Bayesian SED fitting, highlighting the importance of intermediate-age stellar populations in dust heating.

Findings

Correlation between IR luminosity and SFR increases with longer averaging timescales.

Many green valley galaxies are dust-obscured star-forming galaxies.

Some IR-luminous galaxies have little current star formation but significant dust absorption.

Abstract

UV continuum and mid-IR emission constitute two widely used star formation indicators at intermediate and high redshifts. We study 2430 galaxies with z<1.4 in the Extended Groth Strip with MIPS 24 mic observations from FIDEL, spectroscopy from DEEP2, and UV, optical, and near-IR photometry from AEGIS. The data are coupled with stellar population models and Bayesian SED fitting to estimate dust-corrected SFRs. In order to probe the dust heating from stellar populations of various ages, the derived SFRs were averaged over various timescales--from 100 Myr for "current" SFR to 1--3 Gyr for long-timescale SFRs. These SED-based UV/optical SFRs are compared to total infrared luminosities extrapolated from 24 mic observations. We find that for the blue, actively star forming galaxies the correlation between the IR luminosity and the UV/optical SFR shows a decrease in scatter when going from shorter to longer SFR-averaging timescales. We interpret this as the greater role of intermediate age stellar populations in heating the dust than what is typically assumed. This holds over the entire redshift range. Many so-called green valley galaxies are simply dust-obscured actively star-forming galaxies. However, there exist 24 mic-detected galaxies, some with L>10^11 L_sun, yet with little current star formation. For them a reasonable amount of dust absorption of stellar light is sufficient to produce the observed levels of IR. In our sample optical and X-ray AGNs do not contribute on average more than ~50% to the mid-IR luminosity, and we see no evidence for a large population of "IR excess" galaxies (Abridged).