The Stellar-age Dependence of X-Ray Emission from Normal Star-forming Galaxies in the GOODS Fields
Woodrow Gilbertson, Bret Lehmer, Keith Doore, Rafael T. Eufrasio, Antara Basu‐Zych, W. N. Brandt, Tassos Fragos, Kristen Garofali, Konstantinos Kovlakas, Bin Luo, P. Tozzi, Fabio Vito, Benjamin F. Williams, Yongquan Xue
Abstract
Abstract The Chandra Deep Field-South and North surveys (CDFs) provide unique windows into the cosmic history of X-ray emission from normal (nonactive) galaxies. Scaling relations of normal-galaxy X-ray luminosity ( L X ) with star formation rate (SFR) and stellar mass ( M ⋆ ) have been used to show that the formation rates of low-mass and high-mass X-ray binaries (LMXBs and HMXBs, respectively) evolve with redshift across z ≈ 0–2 following L HMXB /SFR ∝ (1 + z ) and L LMXB / M ⋆ ∝ (1 + z ) 2−3 . However, these measurements alone do not directly reveal the physical mechanisms behind the redshift evolution of X-ray binaries (XRBs). We derive star formation histories for a sample of 344 normal galaxies in the CDFs, using spectral energy distribution (SED) fitting of FUV-to-FIR photometric data, and construct a self-consistent, age-dependent model of the X-ray emission from the galaxies. Our model quantifies how X-ray emission from hot gas and XRB populations vary as functions of host stellar-population age. We find that (1) the ratio L X / M ⋆ declines by a factor of ∼1000 from 0 to 10 Gyr and (2) the X-ray SED becomes harder with increasing age, consistent with a scenario in which the hot gas contribution to the X-ray SED declines quickly for ages above 10 Myr. When dividing our sample into subsets based on metallicity, we find some indication that L X / M ⋆ is elevated for low-metallicity galaxies, consistent with recent studies of X-ray scaling relations. However, additional statistical constraints are required to quantify both the age and metallicity dependence of X-ray emission from star-forming galaxies.